Materials and Hardware

Module 6 Materials and Hardware

CONTENTS

Page Definitions Ferrous metals Allay steels Non ferrous metals Aluminium alloys Chemical abbreviations Identification of metals Practical tests Heat treatments - plain carbon steels Case hardening Heat treatments - alloy steels Heat treatment - aluminium alloys Destructive testing of metals Corrosion Non metallic materials Plastics Fibre reinforced plastics Resins Cores Manufacture of composite components Adhesives - general Destructive testing of composites Degradation of composites Sealants and bonding agents

MATERIALS - GENERAL You should have a good knowledge of the following terms which are used throughout this book, so, using a piece of paper define each of their meanings. Take no more than about 30 minutes. The answers are given below.

* Stress * Density

* Fatigue * Toughness * Brittleness * Hardness * Softness * Ductility * Malleability * Resistivity (p)

Stress. Defined as force per unit area. In the SP system it is the Pascal (Pa) and is defined as a Newton per square meter N/m2. In the imperial system it is pounds per square inch (psi).The higher the stress levels a material can take the better. Note that stress units are the same a s pressure units, Density. This is the amount of "substance" in a material. It is defined as "mass per unit volume" ie, Density = kgs/m3. Dense material such as lead is said (incorrectly) to be heavy. A kg of lead is no heavier than a kg of feathers. As an example aluminium h a s a density of 2700 kg/m3 and steel is 7900 kg/m3. For aircraft the less dense a material is the better - provided is retains those desired properties such as high strength etc. Fatigue. Fatigue is associated with cyclic stress. All materials should be resistant to fatigue. Fatigue is serious and has been the cause of many aircraft accidents. Normally the stress level that causes fatigue failure is well below that required to cause the part to fail under normal tensile stress. Toughness. This is the ability of a material to absorb an impact load. Rubber is tough - ordinary glass is not. Toughness is a good quality, without it metals would fracture at the slightest knock. Brittleness. The opposite to toughness. Hardness. The ability to resist scratching and indentation. Glass is hard, wood is not. Bearings and piston rings for example should be hard so as to resist wear. Softness. The opposite to hardness. When two surfaces are in rubbing contact with each other, such as some bearings then one is usually made softer than the other so it will wear first - usually the easier one to replace.

Ductility. The ability of a material to be permanently deformed by the application of a tensile load. Wire is drawn into shape by being pulled through a series of dies and is said to be ductile (Drawn - Dies - Quctile). Malleability, The ability of a metal to be permanently deformed by the action of a compressive load - hammering for example. Rivets are malleable as they are formed by compression.

Resistivity. This gives the resistance of a body in terms of its dimensions. It is called (p) rho. The resistance of a n object can be found from the equation

Where p is in ohm metres, L is length in metres and A is cross sectional area in m2. Copper has a resistivity of 1.7 ohm metres whereas steel has a resistivity of 15 ohm metres. Copper is a better conductor than steel. METALS Metals can be divided into two main groups - ferrous and non ferrous.

FERROUS

NONFERROUS

Fig. 1 METALS Ferrous (Fe) Metals These metals have an iron base and include all the plain carbon steels, allc steels, cast irons and wrought iron. A plain carbon steel is a steel which contains only iron (Fe) and carbon (C) between about 0.15% and 1.4% C.

WROUGHT IRON

LOW CARBON STEEL

I

I

I

0

0.02

0.1 5

HIGH CARBON STEEL

CAST IRON

I

I

1.4 % CARBON

Fig. 2 PERCENTAGE CARBON IN STEELS & IRONS

4.5

.--

Fe metals can be divided into 3 main groups - irons, plain carbon steels and alloy steels. Fe METALS

IRONS

PLAIN CARBON STEELS

ALLOY STEELS

Fig. 3 Fe METALS The following pages contain tables relating to properties and uses of metals used on aircraft. Some metals are almost never found on aircraft - such a s cast iron - but they have been included because they are found in aircraft related engineering.

TABLE 1 - FERROUS METALS MATERIAL

PROPERTIES

USES

Cast iron u p to 4.5% C

Brittle, weak, casts well, resists crushing. Good anti-friction properties, self lubricating. Good vibration damping qualities. Density 7700kglm3.

Machine beds, frames and details. General castings, bearings. Pistons, Piston rings.

Wrought iron 0.02% C

Ductile, malleable, soft, easily magnetised, easily welded. Density 7800kg/m3.

Cores of dynamos, lifting chains, crane hooks.

.................................................................................................

Mild steel (low carbon) 0.15 to 0.3% C

Ductile, less malleable. Stronger and harder than wrought iron. Easily forged, welded, machined or stamped to shape. Density 7800kg/m3. p = 15ohmm.

Bolts and nuts. General workshop machined components. Girders, forgings, car body panels.

Medium carbon steel 0.3 to 0.5% C

Higher strength than mild steel and responds readily to heat treatments to increase its toughness and hardness.

Leaf springs, wire ropes general tools, axles, crankshafts. Used in high strength areas fuselage joints, bolts, hinge pins etc.

TABLE I CONTINUED High carbon steel. 0.5% to 1.4% C

More expensive than medium carbon steel. Tougher and harder.

Cutting tools. Coil springs.

Alloy steels (See table "Alloy Steels")

By adding other elements the properties of plain carbon steel can be altered.

Chromium increases hardness - ball bearings Nickel increases strength and toughness also resistance to fatigue. Tungsten helps the steel to retain its hardness at high temperatures.

Alloy Steels The main difficulty when studying alloy steels is that there is such a wide range of alloys that, trying to commit the details to memory, or even a small part of them, would be difficult. For this reason the included table is of the more commonly used elements used in steels to produce particular properties.

TABLE 2 - ALLOY STEELS ELEMENT

Yo

QUALITIES

USES

.................................................................................................

NICKEL (Ni)

3-5 27 36

Increased hardness without loss of ductility. Non-magnetic almost non-corrodible. Non-magnetic. Has a low co-efficient of linear expansion.

Case hardened parts. Easily worked.

Precision instruments. "Invar" steel.

.................................................................................................

CHROMIUM (Cr)

3 Great hardness. 12- 17 Nearly non-corrodible.

MANGANESE (Mn) 1.5 12

Greater strength than 5% nickel and harder than 3% chromium. Very tough.

Ball and roller bearings. Welds easily a purifier.

-

acts a s

Parts exposed to "wear and tear".

TABLE 2 CONTINUED - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - -- -- - - - - - - - - - - - - - -.- - - - - - - - - - - - -- - - - - - - - - - - -- - - .-- - - - - - - - -

TUNGSTEN (W)

U p to Very hard up to 600°C. 20

14% tungsten is used in high speed steel drills. Work a t higher speeds and temperatures.

COBALT (Co)

12

With tungsten.

35

Easily magnetised.

Used in drills etc working at temperatures higher than 600°C. Permanent magnets.

20

Increase strength without loss of ductility.

VANADIUM (V)

Chrome-vanadium steels for valves and other springs.

MOLYBDENUM (Mo) 2-4 Similar effect to tungsten. .................................................................................................

NICKEL & CHROMIUM

1-2 Stainless steel 18-8 Stainless steel 3-5 Great strength toughness

Magnetic. None magnetic. Gears, crankshafts engine and airframe parts. -

INVAR

Contains 36% Ni, and 64 Fe. Has a low co-efficient of linear expansion (0.9). (Mild steel has a co-efficient of 15.0).

Precision instruments and gauging systems.

STAINLESS STEEL

Almost zero rate of Structures - where heat corrosion. Typically resistance is required. contains 18% Cr & 8% Ni, Pipelines. though other grades of "non corrodible steel" are available.

AUSTENITIC STEELS & IRONS

There are several Same uses as above. austenitic steels but most are based on 18:8 stainless steel. Besides the qualities of stainless steel they are non magnetic.

TABLE 2 CONTINUED VALVE STEELS

For aero engines, usually contain 13%Ni, 13% chromium and 3% tungsten. Good resistance to scaling at dull red heat temperatures.

Valves.

HIGH SPEED STEELS

Typically contain 18% tungsten, 4% chromium and 1% vanadium. Will work at higher temps. than high carbon steel without affecting the temper.

Drills. Hacksaw blades.

PERMANENT MAGNET STEELS

May contain u p to 35% Cobalt. Various trade names are available eg Columax contains 8%Al, 14% Ni, 23% Co and 3% Cu.

Permanent magnets.

-------------------------------------------------------------------------------------------------

HIGH PERMEABILITY STEELS (Those that can be magnetised and de-magnetised easily)

Soft iron was used but metals such as Permalloy (78% Ni) and Mumetal (75% Ni) are now more common.

Transformer cores.

TABLE 3 - NON-FERROUS METALS MATERIAL

PROPERTIES

USES

................................................................

TITANIUM ALLOYS

High strengthlweight ratio. Good physical properties and corrosion resistance. Density 4500kg/m3. p = 2.6 ohm m. Tensile strength u p to 1300MPa. Works at temp. u p to 480°C.

Used to replace steel with a saving in weight. Used for compressor and fan blades in turbine engines. Fire proof bulkheads. Heat shields.

TABLE 3 CONTINUED NICKEL

Hard, ductile, Corrosion resistant. Withstands high temps.

Anti corrosive.

NICKEL ALLOYS

Good strengthlweight ratio. Corrosion resistant at high temperatures. Monel tensile strength u p to 1170MPa. Some alloys contain 80% Ni 20%Cr.

Turbine blades and hot end fittings.

MAGNESIUM

Soft. Poor corrosion resistant.

Bombs and flares. Light alloys.

MAGNESIUM ALLOYS

Cast well. Prone to corrosion. Alloyed to give it strength as pure magnesium is weak and soft. Density 1800kg/m3. Will burn in under some conditions, particularly when in powder or swarfe form.

Aircraft wheels, and airframe structures.

COPPER

Tough, ductile, malleable. High thermal and electrical conductivity. resistant to corrosion Solders well. Density 8900kg/m3. p = 1.7ohmm Weak - about 200 to 400MPa. Different coppers classified by CDA (Copper Development Association).

Tubing. Electrical conductors. Used as a base for brass and bronze.

BRASS

Contains copper, zinc, tin, manganese, lead, nickel, aluminium, and silicon. Good wearing, anti-friction and corrosion resistant. Density 8500kgIm3. Some brasses have a tensile strength u p to lOOOMPa

Lightly stressed castings, pipe fittings, tubing, filter elements, bushes, electrical contacts.

TABLE 3 CONTINUED BRONZE

Copper, tin, nickel and Lead dloy. Similar properties to brass.

Bearing bushes

PHOSPHOR BRONZE

Copper, tin and phosphorous. Stronger and good in compression.

Bearing bushes.

.................................................................................................

TUNGUM

Contains copper, zinc, aluminium, nickel, silicon. Resistance to fatigue and corrosion. Strong and ductile.

Pipe lines. Radiator matrix. Not in common use.

---

LEAD

Soft, weak, ductile. Density 11300kg/m3.

Counter balance a n ~ mass balance weigh. , . Alloyed to make solder.

TIN

Soft, ductile corrosion resistant.

Used for tin plating. Alloyed to lead to make solder.

SOLDER

Tin and lead. Low melting point. Density 9000kg/m3.

Soft soldering.

ZINC

Soft. Good corrosion resistance. Density 7 1OOkg/m3.

Protection of steel parts.

.................................................................................................

ZINC ALLOYS

Low cost low melting point castings.

Inexpensive comme* :a1 small parts.

DEPLETED URANIUM

Hard. Density 19,00Okg/m3.

M a s s balance weights, now removed for safety reasons.

GOLD

Soft. Density 19,300 kg/m3. p = 2.4 o h m m Good corrosion resistance.

Used for plating some electrical contactors. Gold film windscreens.

MONEL METAL

Contains 70% Ni, and 30% Cu. Resistant to corrosion.

Some structural uses and tucker pop rivets.

TABLE 3 CONTINUED CADMIUM

Corrosion resistant.

- -- - -- - -- - ---- - - - - - -

ELEKTRON

-,

-. .

Anti corrosive plating.

- - - - - - - - - - - - - - - -.- -. - - - - .-- - - - - - - - -. - - - - - -. - - - - - - - - - - - - - - - - -.-- - - - - - - - - - - - - - - -. -,

Magnesium, Aluminium 1 I.%, Zinc 3.5%, Manganese 2.5%. Requires heat treatment. May be cast or wrought.

Wheels, crankcases

----------

------------------------------------------------------

ALUMINIUM and its alloys.

See table 4

..

Wheels, castings, aircraft structure

Super Alloys - also non-ferrous This class of metals is mainly based on nickel and cobalt (Inconal for example) with strengths u p to 1450MPa. They are expensive, difficult to form and machine but meet the needs for strength and operating conditions

Aluminium Alloys - non-ferrous These are supplied in the wrought or cast form and may be heat treatable or non heat treatable. The British Standards cover: BS 1470 to 75 - Wrought BS 1490 - Cast BSL Series - Aircraft DTD Specifications. - Aircraft (DTD

=

Directorate of Technical Development).

An American coding system for wrought alloys is based on the main alloying element a s follows: CODE

MAIN ALLOYING ELEMENT

None - 99% pure aluminium Copper Manganese Silicone Magnesium Magnesium 8r, silicon Zinc Other

The first digit indicates the main group, the second digit indicates any modification to the original alloy and the last two digits indicates the actual alloy in the group or the impurity level. Example 1. Duralumin and any suffix after the forth digit would indicate, for example: 20 17.- 0 - T2 - T6

Annealed wrought Duralumin. = Annealed cast Duralumin. = Solution treated and artificially aged Duralumin. =

Example 2. 2025-H4 indicates aluminium copper alloy (2xxx), original alloy (xOxx),with 4.5% copper, 0.6% manganese and 1.5% magnesium (xx24) and strain hardened (xxxx-H4). Strain hardening is not used much on A1 alloys used on aircraft. To modify +lie properties of A1 alloys heat treatments are used. 7 series alloys have a strength approaching that of steel and are widely used on aircraft. Aluminium-lithium alloys are now being developed that have a 10% lower density (lighter) and are u p to 20% stronger than existing A1 alloys. (this would make a weight saving on the construction of a Boeing 747 for example of about 14,000 Ibs (6400kg). The codings are specified on the metal specification (packets for rivets), drawings etc and details of what they mean found in tables. Some A1 alloys will increase their strength with time after heat treatment (age hardening), others require precipitation heat treatment to bring on the process and some alloys will not age harden at all. (Refer to the section on Heat Treatments in this book). A1 alloys generally have the following properties:

* -k

* * * * *

Good strengthlweight ratio. Fatigue limited (see the section in this book "Testing of Metals"). Notch sensitive (a small scratch is liable to develop into a crack). Less corrosion resistant than aluminium. Less malleable and ductile than aluminium. Good thermal and electrical conductivity (p = 5). Up to 8 times stronger than aluminium with little or no increase in weight. (Density = 2800 kg/m3, aluminium = 2700 kg/m3).

TABLE 4 ALUMINIUM & ITS ALLOYS -. - - - -. - - - .-- - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - .--- - - - .-- -- - - - - .-- - - - - -.- - - - - - - - - ,-

MATERIAL

PROPERTIES

- - - - - - - - - - - - - - .- - - - - - - - - -. - -

USES

ALUMINIUM

Soft, malleable, corrosion resistant. High electrical and thermal conductivity. Little strength. Density 2700kg/m3. (Compare mild steel at 7800kgIm3).

Used in light alloys a s a base material, and used for cladding. Used as a conductor.

DURALUMIN (Wrought)

Nearly as strong as mild steel. Density about 113 that of steel. Must be heat-treated. "Age Hardens". Aluminium. Copper 4.5% Magnesium 0.7% Manganese 0.7% Silicon 0.7%

Structural parts. Sheets, rivets, tubes.

ALCLAD (Wrought)

Dural sheet with coating of aluminium. Corrosion resistant Heat treatment as above.

ALPAX (Cast)

Aluminium. Silicon 13.0% Iron 0.6% Manganese 0.5% Zinc 0.1% Good for casting. Strong. Low thermal expansion. Fair corrosion resistance.

Intricate castings. aircraft and engine parts.

"Y" ALLOY (Cast)

Aluminium Copper 4.5% Nickel 2.3% Magnesium 1.7% Resistant to corrosion and fatigue. Must be heat treated. "Age hardens". Withstands relative high temperatures.

Pistons and cylinder heads,

HIDUMINIUM

Copper 2.5% Nickel 1.5% Magnesium 1.2% Iron 1.5% Silicon 1.0% Strong as mild steel. Requires heat treatment and low temp. heat treatment to age harden.

Aircraft structures. Pistons and cylinder heads.

2000 SERIES Al ALLOYS

Damage tolerant.

Used in critical structural areas.

7000 SERIES

Main alloying element - zinc.

Used in strength critical areas.

TABLE 4 CONTINUED ---- ----------------

LITHIUM BASED A1 ALLOYS

Improved strengthlweight ratio.

Several types being produced for newer aircraft to replace both the 2000 and 7000 series.

Chemical Abbreviations These are used extensively within the industry and while you need not remember them specifically you should have some knowledge of the more commonly used terms eg: Aluminium Carbon Cadmium Cobalt Chromium Copper Iron Magnesium Manganese Molybdenum Nitrogen Nickel Oxygen Lead Tin Titanium Vanadium Tungsten Zinc

IDENTIFICATION MARKINGS ON METALS The CAA specifies that materials used in the manufacture of aircraft parts shall comply with a t least one of the following specifications:

* British Standard Aerospace Series (BSAS) Specifications. * DTD Specifications. * Specifications approved by the CAA. * Specifications prepared by an organisation approved by the CAA.

BSAS and DTD specifications make provision for the material to be marked by the inspector as well a s other markings to ensure full identification. Marlung Methods Materials during manufacture should be marked as soon as possible during their production run with one or more of the following methods: (a) (b) (c) (d)

Metal stamp marking (not usually on titanium). Markings produced by a die or mould used in the shaping of the metal. Marking by rubber stamp, roller, or printing machine. Using a colour scheme.

The marking should not be easily removed and should not damage the metal. Stamp marking should not be used on:

* * * *

Stressed parts where the stamp might cause stress concentration. Thin section metals. Metals of hard surface finishlspecial surface finish. Parts or materials machined to close tolerances.

Standard Colour Scheme A widely used system for the identification of metals is the standard colour scheme. The scheme is additional to any identification requirements laid down in the various specifications. If the colour scheme has not been applied by the manufacturer then it should be applied by the operator before the metal is placed in bonded store.

An alternative method to colour coding is overall marking. The metal - usually in sheet form - is printed all over with the material specification eg, BSL 72 (L72). The metal must, of course, be printable and the metal must not be affected by the print. The colours may be applied as a band or bands across the corner of sheet metal bearing the identification stamp. On some sheet metals the bands may be painted near one edge of the metal and at right angles to it. Strip material will have the bands painted on one end, or in some cases on both ends. Some sheet metals have a coloured disc 3" (76mm) diameter painted on them with additional colours added a s concentric rings 1.5" (38nlm) wide. For material in coil form the colours will be marked a t intervals.

Protective Film Treatments All metals are required by regulation to be capable of storage without deterioration. This means that if there is any chance of corrosion etc occurring during storage then the metal must be given an anti-corrosive treatment sufficient to protect it during the expected storage life. This means that most metals are required to have an additional protective film treatment applied a s soon a s possible after production. This may be a clear film, but if it is coloured, such as red lanolin resin, then a n additional band of black paint is added to the colour scheme and the protective film is added u p to the black band. Some metals which differ only in surface condition or intended usage but are the same basic material are given the same colour code. Metals with the same specification but with different heat treatment conditions or properties havc different colour codes. When the specification number of a material is changed eg, from a DTD number to a BS number, then the colour code will not be changed unless there is a significant change in the material itself.

Colours Current colours used are: black, blue, brown, green, red, white, yellow (and violet for aluminium rivets).

Heat Treated Material Material that is released in a heat treated state other than that stated in th specification must be marked in red with the appropriate term to denote thp condition. (See Heat Treatment Symbols in the book entitled "Drawing" in ~ l l i s series). The Approved Certificate must also be annotated. Examples of the terms used. (a) (b) (c)

AS ROLLED ANNEALED - material in its softest condition. NOT AGED - material solution treated and requires precipitation treatment. (See heat treatments).

The Identification Marking The marking should contain the following information: A A

* A

*

The specification number. The inspection stamp (where necessary). The manufacturer. Batch number (and cast number where appropriate). Test report.

General Always use material specification as laid down in the aircraft maintenance/repair manual (SRM). If the correct material is not available check the alternative spares list, and if that does not help contact the aircraft manufacturer. 2.

Always positively identify the material from the colour coding /specification numbers. If in the stores also check specification with the Approved Certificate/EASA Form 1 and/or other documents from the manufacturers.

3.

If the material has to be cut (sheet or strip) and used in smaller piecesthen always cut from the sidelend furthest from the identification. This does not apply to "all over marking" material.

4.

If in doubt about the identification of a piece of metal then it is not to be used on aircraft.

5.

The following two tables are practical workshop tests for the identification of metals and are not to be used for the identification of metals to be used on aircraft.

blank

PRACTICAL TESTS TABLE 5 - PRACTlCAL TESTS FE METALS

METAL

Grey Cast Iron

NOTE WHEN DROPPED ON ANVIL

BEHAVIOUR WHEN CHIPPED

APPEARANCE OF FMCTURE

TYPES OF SPARK FROM A GRINDlNG WHEEL

No ring

Chips easily

Dark grey crystals of uniform size

Dull red nonbursting.

............................................................................................

--

Wrought Iron

Low pitch ring.

Easily chipped. Chips bend.

Course fibrous grain.

Bright yellow nonbursting.

Low Carbon Steel

Medium pitch ring.

as above

Bright silvery large crystals

Bright yellow few carbon bursts.

High Carbon Steel

High pitch ring.

Harder to chip Pale grey, fine than low carbon crystals. steel, but chips bend.

Bright yellow, all bursting.

Tungsten Steel

Very high pitch ring.

Will not chip.

Red no1 burstin. following the wheel.

Austenitic Steels

Non magnetic

Silky blue grey fine crystals.

.-----

Stainless Steel

Copper is not deposited when copper sulphate solution is applied.

TABLE 6 - PRACTICAL TESTS NON FE METALS METAL

TEST

Aluminium

White in colour, light and mon-magnetic. Soft and bends easily. Caustic soda turns the surface white.

Alclad

More springy than aluminium. Caustic soda turns the surface white and the edge black.

Duralumin

The same properties a s Alclad except that the application of caustic soda turns the surface black.

Magnesium Alloy

White in colour, lighter than aluminium and non magnetic. Fillings ignite in a flame. Copper sulphate causes effervescence and the surface to turn black.

Solder

White, heavy and soft. Non-magnetic and melts with a soldering iron. Will mark on paper and crackles when bent.

Titanium

Lighter than steel. White sparks when held against a grinding wheel.

HEAT TREATMENTS Metals may have their properties changed by alloying. Alloying can give a metal:

* A

*

Better anti-corrosive properties. Better strength and fatigue resistant properties. Better macbineability, casting and heat treatable properties.

The heat treatment of a metal normally involves heating the metal to a specific temperature and then cooling at a specific rate. Heat treatments can produce the following properties:

*

* * * JC

Increase Increase Increase Increase Increase

strength. hardness. softness. toughness. "springiness".

Some heat treatments can affect the anti-corrosive properties of a metal though they are not normally heat treated for this reason.

HEAT TREATMENT OF PLAIN CARBON STEELS QUESTION What does "plain carbon" mean? (2 mins)

ANSWER

A steel containing Fe and C only

When steel is heated its temperature increases steadily until it is momentarily checked a t the "critical" or "arrest" point. At this point the metal absorbs heat and changes occur in the structure of the metal, without temperature rise. After this period has passed the temperature continues to rise as before. If steels having different carbon contents are heated in this way and the "arrest" points plotted o n a graph, and if all these points are joined an Iron Carbon Equilibrium Diagram is produced (figure 4).

Line AEB of figure 4 represents the lower arrest points and is called the Low.,r Critical Point (LCP),and line DEC represents the higher arrest points and i, called the Upper Critical Point (UCP). Most of the heat treatments that are carried out on plain carbon steels relate to the LCP and UCP temperatures on the iron carbon equilibrium diagram.

0

0.2

0.4

0.6

0.8

1.O

1.2

1.4

1.6

% CARBON

Fig. 4 IRON CARBON EQUALIBRIUM DIAGRAM Micro Structure of Steels. When viewed under the microscope the micro structure of plain carbon steel looks similar to the views shown in figure 5. The ferrite is pure iron and cementite is iron carbide, Pearlite gets its name from its pearl-like appearance (under the microscope) and is made up of fine plates of cementite and ferrite. When heated to just above the LCP (line AEB on the graph) about 700°C, the pearlite changes to austenite. The ferrite and cementite does not change.

AT 0.87% C

ALL PEARLITE

CEMENTITE

ABOVE 0.87% C

Fig. 5 MICROSTRUCTURE OF STEELS

When heated to higher t h a n the UCP (line DEC on the graph) the metal goes into what. i s called Solid Solution where the whole structure becomes austenite. Hardening. Produces a hard brittle steel. Heat to just above the UCP for steel u p to 0.87% C, and just above the LCP for steels with a higher carbon content. Quench in water. Slower quenching produces a tougher (and not so hard) steel. Tempering. Relieves the brittleness in a hardened steel. Reheat a hardened steel to between 2 0 0 to 300°C and cool or quench. (The cooling rate is not critical). Some steels are heated to 600°C which produces a high strength steel, tough and with good ductility. In general the higher the temperature the less the hardness and the greater the toughness. Examples :

* -k J;

Some structural steels 600°C (Tough). Springs 300°C. Drills, taps and dies 240°C. (Hard)

Annealing. This will refine the structure of the steel and convert it to its softest possible state. Heat to the same temperature as for hardening but cool a s slowly as possible by leaving the part in the ashes or furnace and allowing the furnace to cool naturally. Normalising. This process allows the structure to be refined back to its normal condition after working. When the metal is cold worked internal stresses are set up which make it weak and brittle, to relieve this condition normalisirig is carried out. The steel is heated to its annealing temperature and allowed to cool naturally in still free air. On low carbon steel Low Temperature Normalising may be carried out using a temperature about 500°C.

Refining. Prolonged heating above the UGP can cause the grain structure to coarsen (the grains to get bigger and the structure more brittle), so refining will reduce the size of the crystalline structure, reduce the brittleness and increase the toughness. This process is usually carried out on steels that have been case hardened. Heat to about 900°C a n d quench. Repeat the process 2 or 3 times but with a lower temperature each time.

CASE HARDENING Applied to low carbon steels to produce a hard wearing "outer skin" whilst still retaining a tough inner core. The process is normally carried out in the following sequence: 1. 2. 3. 4. 5.

Carburising (eg introducing extra carbon into the "outer skin" cthe metal). Annealing - slow cooling from the carburising temperature. Refining (as described above). Hardening. Tempering - as necessary.

Methods of Carburising Open Hearth. Hezt the part to a cherry red and dip in a box of carburising compound (Kasenite). Repeated 3 or 4 times to give a "case" of about 0.005in (0.13mm) thick. Box Process. The parts are packed in Kasenite (a carbon rich compound) in sealed metal box and heated u p to 900°C. Four hours a t this temperature produces a case thickness of 0.040in (1.02mm). Cvanide Hardening. The part is placed in molten sodium cyanide at 920°C to produce a case of about 0.0 loin (0.25mm). Nitriding. Used on certain alloy steels containing aluminium and chromium called Nitriding Steels. The parts are heated to 500°C in a box through which is passed ammonia gas. Produces a case thickness of 0.030 in (0.76mm) after 90 hours. The low temperature and the fact that there is no quenching required means that there is less likelihood of distortion.

HEAT TREATMENT OF ALLOY STEELS There is such a wide range of alloy steels that it is difficult, if not impossible in a single book, to describe all the heat treatments that may be carried out. The following table attempts to give some idea a s to the range of alloy steels and the Hardening and Tempering heat treatments, TABLE 7 - HEAT TREATMENTS OF ALLOY STEELS METAL

APPROXIMATE COMPOSITION

HARDENING COOLING TEMPERING ("c) TEMP ("C)

Pearlitic Manganese Steel

1.5% Mn 0.4% C 0.5% Mo

840

OIL

Austenitic 14% Mn Manganese Steel 1% C

1000

WATER

Pearlitic Nickel Steel

5% Ni 0.4% C

860

OIL

600

Pearlitic Chromium Steel (En 3 1)

810

OIL

150

1.4% Cr 1% C

Low Alloy Nickel Chrome Steel

5% Ni 1.5% Cr 0.35% C

850

OIL

600

Stainless Steel

13% Cr 0.4% C

950

OIL

600

650

.................................................................................................

Stainless Steel (S80)

18% Cr 2% Ni 0.1% C

950

OIL

650

Stainless Steel (1818)

18% Cr 8% Ni

950- 1075

OIL

100-750

High Speed Steel

18% W 4% Cr 1% V, 0.6% C

1320

OIL

550

Again, the heat treatment is carried out using an Equilibrium Diagram which is more complex than the Iron Carbon Equilibrium Diagram shown above.

Note. Nickel-chrome steels are prone to a defect known a s "temper brittleness" when being tempered through the range 250°C to 400°C. The problem - which is not fully understood - causes a marked reduction in the toughness of the metal and, to make things more difficult, can only be revealed by destructive testing of test pieces after the heat treatment process. HEAT TREATMENT OF ALUMINIUM ALLOYS The heat treatments that can be carried out to A1 alloys are as follows: (a)

(b) (c)

Solution Treatment. Initially makes the metal soft but allows the process of age hardening to occur. Precipitation Treatment. Only carried out after solution treatment and accelerates the process of hardening. Annealing. Makes the metal soft for working.

The process of heat treatment requires the metal to be heated for a specifie~ time at a specified temperature then cooling or quenching in a specific way. IMPORTANT NOT ALL ALUMINIUM ALLOYS CAN BE HEAT TREATED AND THOSE THAT CAN MUST BE HEATED TO SPECIFIC TEMPERATURES WITHIN SPECIFIC TOLERANCES. TO HEAT TREAT A PARTICULAR ALUMINIUM ALLOY REFERENCE MUST ALWAYS BE MADE TO THE APPROPRIATE SPECIFICATION/PROCESS DOCUMENTS. Example: To heat treat L'72 refer to British Standards BSL72. It will specify treatments, temperatures and cooling methods. For example, Solution Heat Treat to 495°C 5°C. The soaking time will be specified a s well as the quenching process. Note the temperatures here are quite specific, with most ferrous metals, temperatures may be approximate to 30" or so.

+

Solution Treatment

NOTE. The term has n0thin.g to do with putting the metal into a salt bath or any other type of solution - other than for cooling/quenching. The metal maq be heat treated in a salt bath but it is more convenient to use an electric oven. This will soften the metal for a short period only and will allow the metal to age harden - with a n increase in strength. The metal is heated to a specific temperature usually within the range 460°C to 540°C for a period of time then quenched in cold or boiling water. Rivets so treated must be formed (used) within 2 hours of treatment. They will attain their design strength in 2 to 4 days (see graph). Metals can be lightly fabricated/ bent within this period.

PRECIPITATION TREATED

STRENGTH

t TIME

--t

Fig. 6 GRAPH OF STRENGTH AGAINST TIME Precipitation Treatment This process, where specified, will greatly accelerate the rate of age hardening. The metal may attain its design strength within 2 to 20 hours. After precipitation the strength of the metal is greater than if it is allowed to age harden naturally (see figure 6). Precipitation heat treatment temperatures are low, usually within the range 100°C to 200°C and so&ng times may be u p to 20 hours. Cooling may be by quenching in cold water or cooling in still air.

Annealing This permanently softens the metal for working (unless heat treated further). In many cases it also makes the metal more prone to corrosion. In general the metal is heated to a specific temperature within the range 360°C to 420°C and after the soaking time, allowed to cool in still air.

Refrigeration To slow down the process of age hardening the metal may be refrigerated immediately after solution treatment. For example, rivets previously solution treated can be kept in a cold storage cabinet next to where the work is being carried out. Rivets removed from the cabinet must be used within 2 hours. Storage time will depend on temperature eg, minus 20°C, storage time up to 150 hours. I t is common in the industry to use a domestic freezer

Doing it this way means that when doing a big repair a large quantity of rivets can be heat treated in the heat treatment shop, which may be the other side of the airfield. The rivets can then be put in the freezer close to where the work is being carried out and used, 2 hours worth, at a time. QUESTION If a rivet h a s to be heat treated, can you work out what sort of heat treatment would be carried out? (10 mins). ANSWER

Lets analyse the wrong answers first. We cannot heat treat rivets that have already been formed. Made u p parts, riveted plates etc, must not be heat treated because they will warp due to contraction/expansion. This means we cannot anneal the rivet so as to make it soft for working as it would be required to solution treat after forming - and that's not possible. If we precipitated the rivet it would make it too hard to form. So the only treatment we can carry out - if allowed by the specification - is solution treatment. And the rivet must be used within 2 hours - or put in a refrigerator straight away. It must be used within 2 hours of removal from the refrigerator.

Soaking Times This is the time the part is kept in the over/salt bath a t the specified temperature. In general the larger the part the ionger the soaking time - but do not over soak. Times, for exaxple are:

26 SWG sheet (0.18" 0.0457mm thick) rivets 16 SWG sheet (0.64" 0.1163mm thick)

10 mins 15 mins 25 mins

Quenching Always quench or cool in accordance with the specificatian. The quenching methods listed below start with the fastest method first. 1. 2. 3. 4. 5. 6.

Brine (salt water) Cold water (not warmer than 20°C). Hot water. Oil. Still air. Warm oven.

Most cooling/quenching for aluminium alloys is 2 or 5 above.

Methods of Heating 1. 2. 3.

Thermostatically controlled electrically heated ovens. Air heated furnaces. Salt baths, These use salts that melt at high temperatures and have significant safety issues attached to their operation.

Limitations on Heat Treatments Clad aluminium alloy sheet should not be heat treated more than 3 times. Riveted up, bolted and joined sections should not be heat treated. Only heat treat A1 alloys where it is laid down in the specification for that metal.

Cleaning [t is most important that parts treated in a salt bath should be thoroughly cleaned after treatment (the salts are highly corrosive). The parts should also be thoroughly cleaned prior to putting in a salt bath because a dirty part can cause a violent reaction with the molten salts (effectively a small explosion). If splashed with molten salts wash off immediately and seek medical advice. Some salts can be u p to 600°C and will cause severe burns. Also parts quenched in brine must be thoroughly washed and dried as it is also corrosive.

Rivets These are usually placed in a wire basket for treatment. If any treatment is dlowed for a specific rivet it will be solution treatment.

Identification Of Heat Treated Conditions Immediately after the material h a s been heat-treated, it should be marked with the appropriate symbol denoting the treatment to which it h a s been subjected. Rivets should be put in a bag and labelled. There are two identification systems in general use in the UK ie, that recommended in British Standards 1470 to 1477 and that recommended by in SP4089.

Identification System Recommended in British Standards 1470 - 1477 Material in the annealed condition. Material in the "as-manufactured"condition, e.g. a s rolled, a s extruded, straight and/or drawn to size, or a s forged, without subsequent heat treatment of any kind. Material which has been annealed and lightly drawn (at present applicable only to rivet, bolt and screw stock). Material which has been solution-treated and requires no precipitation treatment. Material which has been solution-treated and will respond effectively to precipitation treatment. Material which has been solution-treated and precipitationtreated. Material which has been drawn after solution treatment (at present only applicable to wire). Material which has been precipitation-treated only.

TESTING OF METALS Various tests are carried out on metals (and other materials) to ascertain the material's properties in terms of strength, toughness, hardness, etc. The te: are normally destructive in that they damage the metal in some way. Each process normally tests for one property. The tests are carried out in a laboratory with special test equipment and qualified personnel.

-

THE TENSILE TEST An accurately machined test piece is placed in a machine and stretched under a tensile load until it breaks. This test provides data on: A J;

x

*

Ultimate tensile strength (UTS). Yield point. Elastic limit. Modulus of elasticity etc.

When the test piece is stretched, during the early stages it behaves elastically. In other words if the load were removed the test piece would return to its original length. In the later stages it behaves plastically - in other words it takes on a permanent stretch (or permanent set) so that if the load were removed the test piece would stay at its "new" length. A graph is plotted of load (stress) against extension (strain),and from this graph certain facts can be ascertained. For mild steel the elastic limit is well defined, a s is the yield point where the metal takes on a permanent set. Of course in normal use the part will not be loaded past its elastic limit. The ultimate tensile strength of the test piece is shown where the graph is at its highest. This is the highest load the metal will take before it breaks.

I LOAD OR STRESS

I

-PLASTIC DEFORMATION (PERMANENT DEFORMATION)

ULTIMATE TENSILE STRENGTH

YIELD POINT

ELASTIC EXTENSION The test piece will return to its original length when the load is removed

EXTENSION OR STRAIN

Fig. 7 GRAPH OF STRESS AGAINST STRAIN FOR MILD STEEL Proof Stress Some metals, when tested, do not show a marked elastic limit and yield point, therefore it is difficult to compare the test results of one specimen with another. For this reason values are recorded of Proof Stress. Proof stress is that stress that is required to produce an elongation of the test piece by 0.1% of its original length (0.1% Proof Stress). For 0.2% Proof St-ress the change in length is 0.2%.

0.1% OF GAUGE LENGTH

Fig. 8 METHOD OF DETERMINING 0.1%PROOF STRESS

HARDNESS TESTING QUESTION Define Hardness (2 mins). ANSWER

Hardness is the ability to resist scratching and indentation.

There are several different test methods available, and they all rely on indenting the surface of the metal with a n "indentor" and measuring the indentation size or depth.

The Brinell Hardness Test This uses a hard steel ball and is covered by British Standards 240. A special machine presses a small steel ball into the surface of the test piece for a period of 10 - 15 seconds with a certain force and the Brinell Hardness Number (HB) is found from the formula (there is no need to remember it):

where

F D d

-

the force in kg. diameter of the ball in mm. diameter of the indentation in mm. (measured using a graduated microscope)

Because different values can be obtained by using different diameter balls on the same test piece, it is usual when quoting the HB number to quote the ball diameter as well as the force applied eg:

where

HB 10 3000

=

= =

Brinell Hardness Number. 10mm ball. force in kg.

For very hard materials, ball deformation becomes a problem, and it is better to use another method such as the Vickers Hardness Test. The Vickers Hardness Test This is covered by British Standards 427 and uses a diamond head. This shallow pyramid shaped head is pushed into the surface of the material for a period of 1 5 seconds. A force is applied of between 5 to 120kg. The diagonals of the indentation are measured and the Vickers Hardness Number (HV) is either calculated or found from tables. When quoting the HV number it is usual to specify the load used eg:

where

HV 30 650

= = =

Vickers Hardness Number 30 kg force hardness value

The Rockwell Hardness Test rhis is covered by British Standard 89 1 and unlike the others it measures the depth of indentation of a standard indentor. Nine scales of hardness are available and the amount that the indentor moves into the metal is measured by a Dial Test Indicator (DTI)fiied to the test equipment. The test value would be found by calculation or from tables and quoted as: HRB = HR = B 60 =

60 Rockwell Test Scale B Hardness number

The Shore Scheroscope Test This involves the dropping of a small diamond pointed hammer onto the surface to be tested and measuring the height of the re-bound. The height of the rebolxnd is measured against a special graduated scale - the higher the rebound the harder the metal. This test leaves no visible impression.

TOUGHNESS TESTING QUESTION Define toughness. (2 mins) ANSWER

This is the ability of a material to absorb an impact load. It is opposite to brittleness.

Most tests involve hitting the test piece with a mass of known energy and ascertaining how much energy is used to break the test piece. A heavy pendulum is supported at a set height by a latch a n the impact testing machine. The amount of energy in the pendulum is known a s a function of Potential Energy (PE) in Joules.

where

PE m g h

= -

-

Potential Energy (Joules). the mass in kg. acceleration due to gravity (9.81m/s2). datum height in metres.

When the pendulum is released it swings down and breaks the test piece clamped in a special jaw a t the bottom of the swing arc. The test piece must break for the test to be valid. The pendulum will continue on its swing to reach a certain height on the other side of the test machine. The height that it would have reached had there not been a test piece in the way is already known, so the height that it reaches after striking a test piece is an indication of the amount of energy taken out of the swinging pendulum to break the test piece.

PENDULUM RELEASE CATCH

SCALE 8 POINTER

BRAKE TO STOP PENDULUM AFTER AFTER TEST

1;

Fig. 9 IMPACT TESTING MACHINE The lzod Test This uses a notched test piece supported vertically in a vice like jaw. The end is broken off in the test.

The Charpy Test This uses a notched test piece laid across a gapped jaw. This test piece is snapped in the middle by the swinging pendulum.

CREEP TESTING Clreep is the slow plastic deformation of metal, subjected to prolonged loading often a t high temperatures. It is a problem with:

*

*

J e t engine turbine blades. Structures subject to aerodynamic heating during high speed flight.

Creep is tested for by using several test pieces (of the same metal) and subjecting each test piece to a particular load and temperature. Each test will normally produce a graph of Creep Strain against Time (figure 10).

CREEP STRAIN

TIME

Fig. 10 GRAPH OF CREEP STRAIN AGAINST TIME During primary creep the metal is "settling in" and hardening is occurring. Secondary creep occurs over the life time of the component and is generally very slow. Tertiary creep is dangerous because it can lead rapidly to lose of appropriate clearances and component failure.

FATIGUE TESTING QUESTION What is fatigue? (5 mins) ANSWER

It is the cyclic stressing of a part. The stress level is normally well within the elastic limit level and therefore it could be considered to be harmless. IT IS NOT. Failure can occur due to fatigue at strt , levels well within the design maximum normal stress.

All fatigue testing involves the loading and unloading of a test piece a number of times until it breaks. The test cycles (N) are then recorded against the load (stress) on a graph. The test machine can vary but a common method is to use a rotating test piece loaded downwards so that one revolution of the test piece will produce one load reversal. A bar (test piece) of circular cross section is clamped in a chuck which is

rotated by an electric motor. A bearing is fitted at the free end of the test piece with a mass carrier fitted to the bearing.

REV COUNTER BEARING

\

TEST PIECE

C

\

/ -

\

0

MOTOR

-

MASS CARRIER

'I

'

CUT-OFF SW'TCH

POWER

SUPPLY

Fig. 11 FATIGUE TESTING MACHINE The mass carrier always hangs vertically downwards so that when the motor rotates the test piece, the test piece is put through one complete cyclic loading for each revolution. Effectively being bent u p and down once every revolution. A heavy mass is placed on the mass carrier. The cage over the machine is put in place and the motor switched on. The test piece rotates. When the test piece fails, the mass carrier falls down, contacts the cut-off switch and stops the motor. The rev counter on the motor shows the number of revs (and hence cyclic loads) that has occurred to failure. This value (N) is plotted on a graph against stress (o).

Another (identical) test piece is fitted, the mass is reduced slightly and the whole process is repeated. The result is a n increased value for N. This is also plotted on the same graph. After many tests, each with a slightly reduced load on the mass carrier, all the points on the graph are joined up, and a graph, as shown in figure 12 is produced.

Tested in a corrosive atmosphere producing corrosion fatigue

STRESS

IT

0 0

CYCLES (N)

Fig. 12 GRAPH OF STRESS AGAINST CYCLES (PLAIN CARBON STEEL)

As you can see, for plain carbon steels, if the stress level is kept low enough then failure will not occur under normal conditions. If the stress is raised too high then failure will occur. - the higher the stress the sooner the failure.

Fatigue Limit Some metals do not exhibit a fatigue limit and no matter how low the stress level fatigue failure will occur at some time. With structures made of metals with no fatigue limit then special inspection/ tests are carried out on the structure at regular intervals whilst in service. The structure might be "lifed" and after a certain life span withdrawn from service. Figure 13 shows the graph produced by metals that do not have a fatigue 1jlB,it.

0

CYCLES (N)

Fig. 13 GRAPH OF STRESS AGAINST CYCLES (Non Ferrous Metals & Austenitic S t e e l )

CORROSION Corrosion results from the fact that most metals will try to revert to their natural or more stable state. Generally metals are inherently unstable in their commercial form and fairly readily combine with other elements to degrade the metal. For example, metals react with oxygen to form oxides, acids and alkalis combine with metals to form salts, hydroxides etc. Some metals, however, are very stable and strongly resist corrosion eg, gold, platinum, titanium, silver etc.

Although there are a large number of reactions that may occur between metals and their environments, the reactions may be broadly divided into two:

*

k

Oxidation or "dry" corrosion. The reaction between a metal and its environment without the intervention of an electrolyte. Electrochemical or "wet" corrosion. Requires a n electrolyte such as impure water, water vapour, or some other electrically conducting liquid.

Oxidation This term is applied to corrosion where no electrolyte is present. It can occur where metals are in contact from combustion products from internal combustion engines, gas turbine engines, etc.

OXIDE THICKNESS

t TIME --+-

Fig. 14 OXIDATION RATE COMPARED TO TIME WHEN TEMPERATURE VARIES The oxide film that forms on metals generally tends to protect them from further corrosive attack. Oxygen reacts instantly with bare metal to form a film that adheres to the metal surface. This forms a barrier (for some metals) that prevents further attack by the oxygen on the metal. The rate of oxidation depends on the environment and the nature of the oxide film. Some films are more permt:able than others and some adhere more strongly to the metal than others. It h a s been noted that the rate of oxidation falls sharply with increase in film thickness. A general curve of oxidation rate with time is shown in figure 14. Jt can be seen that as temperature increases so does the oxidation rate.

Electrochemical Corrosion This is the most commonly met with category of corrosion. It can take many forms but usually always takes place in the presence of water or water vapour with traces of other substances. A pd (potential difference measured as voltage) exists between two surfaces, or two areas within the same surface. One of the areas or surfaces becomes anodic (+) a n d the other becomes cathodic (-). The anodic area usually corrodes while the cathodic area has material added to it. The electrolyte provides the current path.

CURRENT FLOW

Fig. 15 SIMPLE CORROSION CELL The corrosion cell, a s shown in figure 15, is minute in size but will join with other cells to attack large areas, or form deep pits, or follow grain boundaries inside the metal. The main factor affecting the rate of corrosion attack is the pd between the two joined metals or between two areas of the same metal. The pd between two metals can be measured with a sensitive voltmeter and recorded and a list drawn u p of all metals, known a s the galvanic series.

The Galvanic Series The Galvanic Series lists metals in pd order with the least noble a t one end and the most noble at the other. It is usual to specify the electrolyte used with the table (the most common being seawater). This means that, when joining any two metals together, the metal that is likely to corrode out of the two can be found by reference to the Galvanic Series. For example, joining copper t.o low carbon steel would result in the low carbon steel corroding if corrosion started. (The table shows low carbon steel to be less noble than copper).

The least noble end may be called the Active end and the most noble end may be called the Passive end. Those metals marked with a r ~asterisk (") may be found in more than one position in the table depending on their actual composition.

TABLE (PART) OF THE GALVANIC SERIES IN SEA WATER MOST NOBLE END (pd = +0.2V) Graphite Platinum Ni-Cr-Mo Alloy Titanium (pd = OV) Stainless Steels* Ni-Cu Alloys Silver Nickel Ni-Cr Alloys Lead Bronzes Brass* Copper Tin Cast Iron Low Carbon Steel Cadmium Al Alloys Zinc Magnesium (pd = -1.6V)

LEAST NOBLE END

Types of Corrosion Corrosion rarely occurs in one form only, since one type of corrosion invariably leads to another, often more serious. Surface Corrosion. Appears as a reddish brown rust on steels, a whitish grey powder on aluminium and its alloys and magnesium alloys, and a s a green discoloration on copper and its alloys. It occurs on the surface of metals but can develop into pitting corrosion. Pitting Corrosion. The pd locally in the metal causes the corrosion to develop into the metal forming pits, sometimes very deep. Often starts with surface corrosion, can be very serious and may develop into Stress Corrosion and Fatigue Corrosion.

PITTING CORROSION CORROSION

SURFACE CORROSION

/

STRESS CONCENTRATION

Fig. 16 STRESS CORROSION

Stress Corrosion. Metals under stress generally corrode more rapidly than unstressed metals. Stress will be increased by cracks and Pitting Corrosion because they reduce the amount of good metal left which is capable of taki- , the load. With the development s f a pit the stress level a t the end of the pit increases. With a n increase in the depth of the pit the amount of remaining metal is reduced so increasing the stress level which will open u p the pit more to allow further corrosive attack. This is turn leads to a deepening pit and even higher stress levels. The process is a continuous cycle (a form of positive feedback) that will eventually lead to the failure of the part. Corrosion Fatigue. This is similar to stress corrosion but the loads are cyclic. The definition of fatigue is "Cyclic stressing of a part - often a t stress levels well below the ultimate stress level the part will fail at". For many metals fatigue will eventually cause failure but with corrosion present in the pit failure occurs significantly earlier. Of course, it goes without saying that, if stress/fatigue corrosion is found iL, ~ t s early stages then appropriate rectification (usually replacement of the part) will prevent failure. Identification of these types of corrosion is not easy so Non Destructive Techniques (NDT) are used such a s X-rays etc. Galvanic Corrosion. Can develop where metals are in contact. The main areas of attack are the faying surfaces (contact surfaces), so the corrosion may not be readily visible externally. Though it may be seen around the faying edges. Can occur between two different metals in contact (see the Galvanic Series) or between two identical metals having had different heat treatments.

CORROSION

/ LESS NOBLE

NOBLE METAL

Fig. 17 GALVANIC CORROSION Where metals have to be joined, always try to join metals that are the same material, and, ideally having had the same heat treatment. But a t any rate always u s e the correct jointing compound (check the Aircraft Maintenance Manual -- AMM, or the Structure Repair Manual - SRM). Signs of the corrosion should be looked for along the faying edges of skin panels, around bolt heads, rivets, metal to metal joints etc. Intercrystalline Corrosion. This is a most serious form of corrosion a s it is very difficult to detect. It usually occurs between the grain boundaries of alloys and is within the metal. It may develop close to the surface, in which case a crack or small blisters may become visible. On the other hand it may not develop near the surface and external indications may never appear - until it is too late - when the part fails. Internally it can be detected by using X-rays or ultrasonic testing - if it is suspected that it is there in the first place.

GRAINS

ROSlON

OR

CRYSTAL

Fig. 18 HIGHLY MAGNIFIED SECTION SHOWING GRAIN STRUCTURE AND INTERCRYSTALLINE CORROSION

Fretting Corrosion. Occurs in bolted joints, riveted joints and other assemblies subject to fretting (slight rubbing movement between the joined parts). The most usual cause of fretting is vibration and this can be induced into the airframe or components by the engines, electric/hydraulic/pneumatic motors/pumps and it can also be induced aerodynamically by propellers, rotor blades a n d flutter. If assemblies are not attached securely enough to each other and are subject to vibration then fretting corrosion can occur. The heat a n d friction developed promotes oxidation which is rubbed into a powder called "cocoa" powder. The combined action of the corrosion process and the fretting will cause rapid deterioration/wear of the joined parts locally

Joints should be correctly and securely assembled with jointing compound a s specified in the AMM (and correctly lubricated for splined shafts etc), and assemblies should be checked for signs of cocoa staining. Crevice Corrosion. Occurs in crevices and areas where a lack of ventilation prevents the metal maintaining its natural protective oxide film. Also the areas remain damp longer than open areas. CORROSION

Fig. 19 CREVICE CORROSION

Filliform Corrosion. Corrosion penetrates the outer layer (cladding) of the metal either via a damaged area, pitting or rivet holes, and spreads radially along the boundary of the cladding and the parent metal. May be impossible to see unless it becomes severe. Affects alclad Al alloys. Exfoliation Corrosion. This corrosive attack occurs along the grain boundaries within the metal. It is found in rolled A1 alloys and tends to follow the direction of the rolling. The effect of severe exfoliation corrosion is to produce a quilt like texture to the surface of the metal, hence the name for the condition (figure 20) - Quilting or Pillowing.

Fig. 20 T H E RESULT O F EXFOLIATION CORROSION

a0

-

Microbiological Corrosion. Occurs in aircraft fuel tanks d u e to the growth of micro-organisms which require the water content of kerosene fuels for their development. They will give off corrosive substances such a s ammonia, sulphides, a n d acids. The growth collects as slime on the tank walls affecting the electrolyte concentration locally. These areas become anodic and a corrosive attack begins. The slime can also affect the operation of the fuel system comporlents by clogging fuel filters etc. AcidIAlkali Corrosion. Caused by spilt acids and alkalis and will cause serious damage unless quickly neutralised. Mercury spillage also causes rapid and serious chemical change in aluminium alloys which will normally require replacement. Erosion Corrosion When corrosion occurs in the presence of a fast moving fluid the rate of corrosive attack may be much higher than would occur in a slow moving or still environment. The fluid may be in the form of a powder, liquid or gas. Metal may be removed from the material surface either as dissolved ions or as solid particles. Commonly found on propeller leading edges, rotor blade leading edges, compressor and turbine blades and aerofoil leading edges. The initial action on most of these components is the removal of the protective/outer layer by the abrasive action of the air - compounded if the air contains particles such as water droplets or dust particles - a s happens when a n aircraft flies through a d u s t cloud - part of a sand storm or a cloud thrown u p by a volcano. Prevention/darnage reduction on engine components is usually achieved by the use of hard, erosion resistant coatings. On propellers a n erosion strip may be fitted. The best preventative measures are the identification and frequent inspections of suspect areas and prompt rectification of any damage found. Cavitation Corrosion. In certain fluid systems cavitation can occur within the fluid. This is caused by a sudden drop in pressure which allows gas bubbles to form. My happen occasionally because of rapid fluid system pressure drop or may be nearly continuous a t positions in the system such as spur gear pumps where the teeth inter-mesh. The result can be that material is worn away (of the gear teeth) and if the atmosphere is corrosive then corrosion will occur. The combined effect of cavitation erosion and corrosion can cause rapid metal removal with decreased machine efficiency and eventual failure.

Anti-Corrosive Treatments The following is a list of anti-corrosive treatments, most of which are applied by the manufacturer/overhaul facility only. For anti-corrosive treatments, repairs and anti-corrosive measures applied/taken by the maintenance engineer you are referred to the appropriate book on the subject in EASA module 7. PROCESS

APPLlCATION

The surface of the part is covered with a thin layer of metal by being exposed to a solution of a metallic salt which is decomposed by electrolysis. The part is placed in an electrolyte bath and a current is passed through. Copper, nickel, chromium, lead, cadmium, tin, zinc, a n precious metals are used for plating. Cadmium platin: is used extensively for steel parts on aircraft. The part is immersed in a bath of molten metal thereby Hot Dipping acquiring a covering of that metal. Plating metals for this process have relatively low melting points, eg tin (tinning) and zinc (galvanising). The part is coated with a plating metal by being heated Cementation whilst in contact with a dust or powder of that metal, eg aluminium (calorising) and zinc (sherardising). Heated particles of the plating metal are sprayed onto the Metal Spraying part (like paint spraying). The particles impinge upon the work to form a n adherent coating. Aluminium, brass, copper, nickel, and zinc are used as spraying metals. The part is immersed in a bath of boiling acid phosphate Phosphating solution. The solution reacts with the surface of the metal to form a metallic phosphate which is highly anticorrosive. The process is applied to ferrous metals a n c may be known by various names eg, parkerising, walterising, etc. A surface conversion process. Anodic Oxidation Usually called anodising but may be known by other names. The part is placed on the anode bar of an anodising bath and immersed in the electrolyte. With current flowing the surface of the part is chemically converted to a n oxide layer. This layer prevents corrosive attack in service. Used extensively on aluminium and its alloys. A surface conversion process.

Electro - Plating

continued

Alodising

An anti-corrosive treatment for A1 alloys which also increases the paint bonding qualities. The metal is cleaned with a n acid, washed in clean water and then given a coating of Alodine (a propriety chemical similar to Alochrome). This turns the surface a greenish colour. The metal is again washed in clean water and then given a rinse in a Deoxylyte bath (also a propriety chemical solution). Used a t user unit level.

Chromate treatment

The part is placed in a bath of chromating solution which produces a protective chromate film on its surface. Applied to magnesium alloys and zinc exposed to humid atmospheric conditions. A surface conversion process. A mechanical process of rolling one metal onto another eg, a thin layer of aluminium is rolled onto both sides of duralumin sheet to produce alclad. May consist of protective compounds held in suspension in a suitable liquid (eg chromates in primers) which dries out after application. Applied by brushing, spraying, dipping or rolling and are often used as additional protection to those listed above. Used a t user unit level. Oils, greases, lanolin, jellies etc are often used a s temporary, or semi permanent processes and sometimes as a n additional process to those listed above. Used a t user unit level.

Cladding

Paints, enamels, etc

Organic treatments

NON METALLIC MATERIALS

In this book we deal with the following materials:

* Cloth * Wood * Plastics

* Rubber * Fibre reinforced composites Fibre reinforced composites are covered to a greater depth than the other materials. Cloth, wood, plastics, and rubber are mentioned briefly because they have their uses in the aircraft industry.

CLOTH Used in aircraft construction for the covering of some light aircraft and for furnishings. Fabric used for aircraft covering may be:

* IJnbleached Irish linen * Madapollam. Madapollan. * Polyester cloth. Unbleached Irish linen and Madapollam/ Madapollan are tautened by doping whilst polyester is tautened by the application of heat. Cloth used for the covering of aircraft seats and berths is usually made from man made fibres and must conform to current fire and smoke blocking regulations.

WOOD Used extensively in older aircraft for all parts of the structure and in the manufacture of propellers. Still used in some composite constructions. Used on some comparatively modern aircraft eg, the fuselage of the de Havilland jet fighter - the Vampire.

TABLE 8 - WOOD COMPARED TO AL ALLOY MATERIAL

DENSITY (kg/rn3)

LONGITUDINAL TENSILE STRENGTH (GPa)

Wood (Spruce)

600

0.05

A1 Alloy

2700

70

Its strength and density can vary considerably depending on the type of woud selected and, of course, it can rot and be attacked by insects, fungus etc. It is easily worked and repaired. Wood is stronger in tension along the grain than across it. For more details on wood and wooden structures (for the mechanical person) see the book in this series entitled "Wood and Fabric Structures".

PLASTICS Strictly speaking plastics should be called polymers. Polymers can be man made or natural. Natural polymers include rubber (from trees) and shellac (the excrement from a South American ant).

Man made polymers can be divided into two main groups Thermosetting Plastics.

-

Thermoplastics and

Thermoplastics. These soften on heating and harden on cooling, and the process is capable of repetition. Examples include Perspex (polymethyl methacrylate) and Nylon. Fibre reinforced thermoplastics include PPS, PEEK etc. Thermosetting Plastics. These become plastic on initial heating but become permanently set on cooling. They can not be softened again by further heating. A good example is Bakelite (phenol formaldehyde), Formica, Ebonite and Epoxy resins. The term thermosetting also includes those polymers which set by the addition of a curing agent and do not necessarily need heating eg, epoxides.

Fibre reinforced thermosetting plastics include polyesters, vinlyesters, epoxy 2tc. Various fillers can be added during manufacture, for example:

* Asbestos - resistant to high temperatures. * Carbon - improves conductivity. Colour Various pigments and dyes can be added to plastics in the production stage to give a n "all through" colour.

Rubber A naturally occurring thermosetting plastic obtained from the sap of trees. Natural rubber is normally vulcanised with sulphur to produce a tough elastic material. Used in anti vibration mountings; drive belts; shock absorbers (simple bungee cord types) and of course tyres. It can be made electrically conductive by adding carbon.

FIBRE REINFORCED COMPOSITES These are increasingly being used in the aircraft industry for structures and components because they exhibit a high Specific Strength (SS) (strength/density). Example: the tensile SS of carbon fibre is 4 to 6 times the tensile SS of A1 alloy or steel. (Airbus have already tested a complete airliner size wing in carbon fibre).

Additional advantages include: Does not corrode. Easy to shape - double curves etc. High level of integration possible with other structures, eg skin to stiffeners, formers, frames etc. High fatigue limits with load cycles much higher than with metals. High resistance to chemical attack and weathering. Can be made radar transparent. High impact resistance. Can be made a s a insulator; a conductor and a dielectric. Good thermal properties and a fire retardant. In this section we will deal with the materials and all general aspects of composites and should more information be required, particularly on structures, the reader is directed to the book in this series entitled "Aircraft Structures". GFRP

AFRP = Aramid fibre reinforced plastic (Kevlar). CFRP = Carbon fibre reinforced plastic. GFRP = Glass fibre reinforced plastic.

/A

GFRPRADOME

1

FAIRING

\

CFRP

57

k

k

v-

7-

&

CFRP SPOILERIAIRBRAKES

a

CFRP

TRACK FAIRINGS

Fig. 21 USE OF COMPOSITES ON THE A 3 1 0 Types of Composites k

* * k

Fibre reinforced plastics - polyesters, PPS etc. Sandwich structures with the outer layers of metal or fibre, and the core using honeycomb made of nomex, A1 alloy, carbon etc. Fibre metal laminates such as ARALL (Kevlar fibres) and GLARE (Airbus A 3 8 0 ) . Metal Matrix Composites (MMCs) using aluminium, titanium etc.

* k

* *

Glass Matrix Composites. Ceramic Matrix Composites. Ceramic Ceramic Composites. Carbon Carbon Composites.

A note on GLARE

This is a new material and is used in the construction of large parts of the Airbus A380 fuselage. GLARE is made u p of alternating layers of aluminium foil and glass fibre polymer prepreg layers and, size for size, is 25% stronger than A1 alloy and 20% lighter. It can be made a s sheets or complete structures (with stringers, frames bulkheads etc) in a n autoclave. It is inspected in the normal way for external defects and requires a specialist .qDT ultrasonic technique for the detection of internal defects. Fibres Various fibres are used as reinforcing elements within a resin for fibre reinforced plastics. They include: Glass. These are continuous glass filaments 6 to 15pm in diameter (0.000,006 to 0.000,O 15m) [This is called a micrometre or sometimes, incorrectly, mu-metres. Mu is the Greek letter p]. The fibres are usually coated with a lubricant to improve handling and may have other coatings to improve bonding etc. The fibres are supplied in different forms: A glass; C glass; S glass etc, E glass is currently the most popular. Aramid Fibre. This is a n organic fibre. Supplied a s Kevlar (Du Pont). Kevlar 29 used for cordage and ropes. Kevlar 49 is supplied for plastics reinforcement. Carbon Fibre (HT and NM). There are some 12 manufacturers world-wide making over 40 different carbon fibres. The fibre is manufactured as a tow and the finer tows have up to 12,000 filaments in each tow. Carbon fibres are strong in tension and are often coated to improve handling and bond strength.

Hybrid Fibres. Hybrid fibres can be made u p in many different forms and can include: (a)

(b)

Two or more different types of fibres layered together within a resin matrix. A mixture of two or more different types of fibre weave within a resin mix.

The following table shows the comparisons of density and strength of different materials used on aircraft with properties of some fibres given in the next table,

TABLE 9 - COMPARISON TABLE, STRENGTH & DENSITY MATERIAL

DENSITY (kg/m3)

Wood (example) A1 Alloy Aramid Fibre Glass Fibre E VHM Carbon Fibre*

800 2700 1380 2000 1690

*

VVHM

=

LONGlTUDINAL TENSILE STRENGTH (GPa)

Very High Modlllus

TABLE 10 - PROPERTIES OF FIBRES MATERIAL

E Glass Carbon Aramid

SPECIFIC TENSlLE STRENGTH (GPa) 0.54 1 .O 0.83

FATIGUE FAILURE STRESS @ 106 CYCLES (MPa) 260 860 980

RELATIVE COST

(£1

CHARPY IMPACT TEST (kg/ m2

1 40 20

Although aramid fibres have good fatigue strength, aramid reinforced composites don't. This is because of bond fracture between the resin and the fibre and is causing trouble in service because of the moisture absorption by the microcracks.

Make u p of Fibre Cloth Layers of fibre cloth are layered u p within a resin so that when cured the resultant structure is solid with good strength properties. Fibre cloth may be supplied untreated or pre-preg in a variety of grades and weaves to BS 3369 or MIL-C-9084 standards. Glass cloth may be supplied in one of three basic forms:

* * *

Chopped strand mat. The yarns are in a random direction and of comparatively short length. Not common. Weftless weave. Continuous yarns only going one way, with a n occasional yarn going at right angles to keep all the others together. May be called Unidirectional weave. Plain weave or weft and warp weave. With continuous yarns going u p and down and, weaved in between, yarns going from left to right.

Zarbon fibre is made u p into sheets of varying thicknesses either unidirectional or plain, pre-preg or untreated. Most cloths are made u p of yarns going u p and down (warp yarns) and weaved between them yarns going from left to right (weft yarns) (weft to right). With a weftless weave the fabric is stronger along the yarns than across them, so if a composite is made u p using this type of fabric weave and if it was to fail it would be more likely to fail along the fibres (ie cracks along the fibres) that across them. Fabric with a weft and warp weave is likely to tear 'one yarn at a time' so the tear will propagate usually in the form of a n L shape. WARP YARNS

SUPPORT YARNS

/

PLAIN WEAVE

WEF YAR

UNIDIRECTIONAL WEAVE

Fig. 22 WEAVES

Now try the following questions to see if you have understood the information in the tables.

QUESTION What does GPa mean? (5 mins) ANSWER

Pa means Pascal and is the unit of stress and pressure. It is small (nearly 7000 to lpsi). G means giga ie, lo9, or in other words 1,000,000,000. GPa is spoken a s Giga Pascal.

QUESTION What do you understand by the term "Fatigue failure stress @ 106 cycles"? (15 mins) ANSWER

Fatigue is the cyclic stressing of a part and if the stress level is low enough (for some materials) the part will never fail. For most materials if the stress level is raised then failure occurs sooner. In the table above the part is put through 106 cycles (1,000,000).If the applied stress is low enough failure does not occur. If the stress level is raised so that the test specimen fails at exaptly 1 million cycles then we have a comparison of the materials resistance to fatigue failure. You can see from the table that while aramid fibre is not as strong a s carbon fibre it is significantly better when it comes to fatigue resistance (as a fibre only).

RESINS

The fibres (like string) are very good in tension but poor (very poor) in compression. To make them more rigid and able to withstand bending and compressive loads they are bonded together using resin. Various resins are available for bonding laminates and as an adhesive for the adhesive bonding of metal to metal, metal to wood, metal to polymer etc. A few are described below. Unsaturated Polyester Resins. Used with glass reinforced plastics (GRP).Tb-y have good strength and chemical resistance. They tend to shrink on curing and do not like temperatures above 150°C. Vinyl Ester Resins. These are similar to the unsaturated polyester resins above. Phenolic Resins. Used for aircraft interiors because of their low smoke emission properties. Epow Resins. These are a thermosetting resin. They are versatile, have a low shrinkage rate with high strength and good chemical resistance. They are used widely in engineering and are usually supplied as a two part mix. Polyamide Resins.. These are suitable for use up to 300°C and are available in films, varnishes, powders, laminates etc.

General Hardening occurs through the reaction of curing agents, hardeners, catalysts or activators and some epoxylhardener combinations will cure a t ambient temperatures - while others will require heat to cure (refer to the manufacturers literature). When dealing with the mixing of resins for composites or adhesives the catalyst/accelerator is added and mixed into the resin to start the cherrlical reaction process. Once mixed it will have a "pot life" which will be shortened if the ambient temperature is high. Once the composite is "laid up" a curing time is required which will be shorter if heat is applied. Some resins are cold cured and do not require the application of heat whilst others must be heated to allow the bonding process to reach its full strength. Pot life is stated in the manufacturer's literature.

TABLE 11 - PROPERTIES OF SOME RESINS RESIN

Unsaturated Polyester EPOXY Vinyl Ester Polyarnide

TENSILE STRENGTH (MPa) Up to 9 0

TEMP. LIMIT ("C) 180

WATER RESISTANCE

SOLVENT RESISTANCE

GOOD

FAIR

105 85 120

220 180 400

GOOD GOOD LOW

GOOD FAIR ------

The choice of resin is important as a n incorrect resin can have a n adverse effect on the material it is being used with. It may not be strong enough or fail due to heat or age. When using resins it is important to maintain strict cleanliness during the mixing and bonding process as any dirt, dust etc will seriously adversely affect the joint strength. Always follow the resin manufacturer's instructions.

CORES With all structures subject to bending it is the outer layers (actually called fibres) of the structure that take most of the stress (compressive and tensile). Figure 2 3 shows a cantilever beam (cantilever = supported at one end only) but the same principle applies to non-cantilever structures such a s floor panels, skin panels etc.

The centre portion of the beam takes very little stress (except for shear) and in a uniform monolithic structure this centre is almost so much "dead weight". The drawing shows a beam and the principle applies to all structures whether it is a main spar of an aircraft, the skin of the airframe, a helicopter rotor blade etc. I FORCE

LOW DENSITY CORE

OUTER FIBRES

Fig. 23 BENDING

In each case the outer fibres of the material take all the tensile and compressive stress with the centre fibres taking very little. Many compositt structures are therefore designed having a low strength, low density core to reduce the weight of the overall structure, with most of the stress being taken by the outer fibres. The core may be made of honeycomb, foam, or some other low density material while the outer fibres are made of metal, fibre composites etc. The core then is of low density, designed mainly to resist shear and compressive loads and include the following: Balsa Wood

Not used much these days but was used as a core on several aircraft including the de Havilland Mosquito (plywood/ balsa wood/plywood sandwich fuselage skin).

Honeycorrlb

Used extensively a s core material in aircraft floors, structures, control surfaces, helicopter blades etc. Can l-made of aluminium, glass fibre or composite.

Foam

(Polyvinyl chloride) PVC is used as the core of some composite structures.

Micro balloons

Within a resin mix.

TABLE 12 - COKE MATERIALS MATERIAL Balsa Wood Nomex Honeycomb Aluminium Honeycomb Foam (PVC)

DENSITY k/m3 96

COMPRESSIVE SHEAR STRENGTH (MPa) STRENGTH (MPa) 5.2 1.3

64

2.9

1.7

118 100

7.6 1.4

5.2 1.1

Note. Nomex is made from aramid fibres bonded with phenolic resins. MANUFACTURE OF COMPOSITES COMPONENTS Several methods are used to manufacture composite components and this section is included for interest only. There is no need to commit this to memory, although some of the general principles are used when repairing composite aircraft structures.

Compression Moulding Usually uses pre-preg fibres (fibres impregnated with resin) in sheet, tape or woven form. Individual plies of pre-preg are laid one on top of the other to produce the required thickness. This "preform" is then laid in the bottom half of a mould. The top half is then closed and secured and heat applied. The heat and pressure allows the resin in the pre-preg to flow and bonds the ?lies into a single structure to the shape of the mould. On cooling the mould is opened and the item removed. Trimming and finishing is then carried out. Vacuum Bag/Autoclave Moulding The most common method is to use a pre-preg lay-up similar to that used for compression moulding. Once the lay-up is completed a vacuum bag is placed over the complete assembly and evacuated of air. Thus atmospheric pressure produces the necessary force to push the plies together. For components/structure repairs where a bag cannot be placed over the complete assembly a plastic sheet is used - with a suitable valve attached which is sealed with special adhesive tape around the edges of the repair area.

Heat may be obtained by: (a) (b) (c)

Placing the assembly in a n oven (autoclave)at 30Q°C and 1.4MPa pressure (about 200psi). Using a heater blanket. Using heaters within the mould.

To allow the resin to flow an absorbent membrane is placed between the vacuum bag and the lay-up material. Temperature sensing bulbs are usually placed inside the vacuum bag close to the laid-up material to automatically control the temperature of the heater elements.

Mandrel Wrapping Involves wrapping a mandrel with layers of pre-preg material. After heating curing the mandrel is removed. Used for tubes and hollow sections.

~d

Pultrusion This is a continuous process for the production of rod, tubes and long sections. The fibre (glass, Kevlar or carbon) is drawn from a spool through: FIRST THEN THEN TO

a resin impregnation tank, through a pre forming die, through a curing die (heated), emerge as a continuous section to be cut to length as required.

Filament Winding Separate filaments are accurately wound onto a mandrel of the appropriate shape after first being impregnated with resin (or pre-preg is used). The complete assembly is heated to cure the resin then the mandrel is collapsed/dismantled and removed. In some cases the mandrel may be left in place and form an integral part of the component. Used in the manufacture of pressure vessels.

Adhesive Bonding Used in the process of metal to metal joining; metal to composite joining; composite to composite joining and honeycomb joining. To manufacture a cored composite structure the two "skins" are manufactured either by compression moulding or auto-clave moulding.

The two "skins"are then bonded either side of the core by using resin adhesives. The structure is heated in a pressurised aut.0--clave. The resins may be a two part mix resin (epoxy)or it may be supplied in film form. This process, in general, produces a strong structure, without any stress risers (such a s rivets, bolts etc) with a good strength/weight ratio. However, i t is difficult to know if the bond joint is satisfactory. With a riveted joint, for example, the formed rivets can be inspected for shear, correct forming etc. With a n adhesive bonded joint there is no sign that the joint is satisfactory .- it. looks the same after the bonding process as before. This means that special checks must be carried out. These include:

* *

A

Complete cleanliness and scrupulous attention to detail when preparing the materials and carrying out the process. A thorough inspection of the joined parts to see if there has been any relative movement and to check any visible bond lines for signs of the bonding agent. The destructive testing of test pieces manufactured at the same time using the same materials and techniques a s employed with the original work.

ADHESIVES - GENERAL Many theories exist as to why adhesives work. Why does the adhesive "stick" to the surface (the adherent)? Several theories have been suggested including chemical reactions, intermolecular forces (absorption)and intermolecular electrical forces. Text books differ on the subject. The advantages of adhesive bonding include: T! .

J;

* *

No holes to weaken the material, No high temperatures involved during the manufacturing process, unlike welding. Smooth surfaces. Ideal for external aircraft skins. The adherends are sealed.

The disadvantages include:

* *

* * *

Long curing times. Careful joint preparation required. Some materials are dangerous to handle. Difficult to inspect the finished joint. Joints not suitable for high working temperatures.

[

.

ADHESIVE

ADHEREND

I

< ADHEREND

Fig. 24 ADHERENDS

Classification of Adhesives Adhesives can be classified as either organic or inorganic, with the organic range split into two - synthetic and natural. Synthetic adhesives can be div;*bd into thermoplastic, elastomeric and thermosetting.

s c ADHESIVES

INORGANIC

ORGANIC

SYNTHETIC

NATURAL

L

'THERMQPLASTIC

ELASTOMERIC

THERMOSETTING

Fig. 25 ADHESIVE CLASSIFICATION

Adhesives Inorganic. Such a s sodium silicate based. Not used for metal bonding. Natural. Rubber (from trees), shellac (from an ant), cellulose etc. Used for things like paper and wood. Thermoplastic. Made from thermoplastic resins. Are softened by heating which can be repeated. Used were great strength is not required though hot melt thermoplastics can have a strength up to 18MPa. Elastomeric. Rased on synthetic rubber they produce and instant stick when the two adherends are brought together. They set by the evaporation of the solvents. For structural work thermoplastic and thermosetting resins are added.

U r m o s e t t i n g . Includes epoxide and urea resins. Provides a strong joint and used in the manufact.ure of structural components. The process of making the joint usually involves a curing agent. When the resin and agent are brought together curing takes place which involves a chemical reaction. Testing The Joint After a joint is bonded and after the appropriate curing time the test specimen should be tested. Depending on the materials and the type of joint made these tests can include a Tensile Test, a Shear Test, a Peel Test and a Cleavage Test (for thicker materials). Special testing machines are provided that provide a calibrated load and this can be plotted on a graph against extension/deformation/breaking of the test piece. i'he cleavage test would only be suitable for thicker non-flexible test pieces, whilst the peel test would only be suitable for thinner flexible material.

FORCE

TENSILE TEST

SHEAR TEST

'

CLEAVAGE TEST

PEEL TEST

Fig. 26 JOINT TESTING DESTRUCTIVE TESTING OF COMPOSITES This section deals with testing of composites. Many of the tests are similar to those used on metals, but many composites can prove difficult to test and get valid results. Remember, with destructive testing of metals the results are only meaningful if the test piece is destroyed during the test. For non destructive testing (NDT)refer to the book Non Destructive Testing in module 7 in this series.

Testing of any material/joining process can be divided into: 1.

2. 3, 4.

Visual examination (a form of NDT). Conventional NDT methods. Destructive methods - workshop. Destructive methods - laboratory or manufacturer

Items 1, 2, and 3 will be dealt with later and item 4 (for metals) has already been covered, but the laboratory destructive testing of composites has produced its own problems because their properties do not lend themselves readily to the "standard" methods of testing used on metals. Some of the tests are similar, though the results may not be as good or a s definitive as one would like. Where the tests are similar reference will be made to the section on The Testing of Metals. When evaluating the results of tests of composites it is important that care taken because the results can vary. This variation can be caused by:

* Minor variations in the batch being tested.

* Fine variations in the preparation of the test specimen. * Small variations in the actual test method. Testing is carried out to British Standards (BSI) and to standards set by the American Society of Testing of Materials.

Safety Considerable energy can be stored u p in a test piece during the test. This energy can be released in explosive form and can be very dangerous. All testing must be carried out behind safety screenslshatter proof guards, and by qualified staff.

Flexural Test This measures centre point deflection as a function of load. Tests may involve a three point test or a four point test with the load increased in increments and at each stage the amount of deflection measured. A graph is then plotted of load against deflection.

Tensile Test This is carried out in a similar way to tensile testing of metals ie, the test piece is "stretched" in a tensile testing machine and its extensionlbreaking point is measured against the load applied.

FORCE

O , TEST

SPAN

1

-----cFORCE

Pig. 27 THREE POINT FLEXURAL TEST

C

FORCE

SPAN

Fig. 28 FOUR POINT FLEXUFWL TEST

The load is progressively increased and at intervals the value of the load is recorded and the extension of the test piece measured. At the end of the test a graph is drawn of load against extension. 'Test pieces have to be thin because of their high tensile strengths and it is often very difficult to satisfactorily attach the test specimen to the machine due to its plasticity - the test piece deforms and slips out of the chuck 01-jaws.

Compressive Test 'The test piece is placed in a similar machine to the tensile test machine but the machine is selected to "squash" the test piece under a compressive force. Like all compressive test pieces it has to be of a reasonable diameter to prevent buckling. If it buckles the test is invalid. Again, this test has its problems as failure often occurs due to "transverse delamination" - not what is being testing for. As with the tensile test measurements are taken regularly of load and size of deformation and will all the readings obtained and a graph is plotted of load against reduction in size.

Shear 'T'est (figure 29) [Jsually applled to tubes and round sections and difficult to test for. In general the test is as follows: X i

%

Clamp the test piece at one end to a torque measuring device. Rotate the free end slowly (about half a radian per minute - about I complete revolution in 12 minutes) and note the torque (Nm) a t the fixed end at regular time intervals. The relationship between the indicated torque a t the fixed end and the rotated amount at the free end is a n indication of the amount of s h e w stress in the test piece.

r\

FIXED END

1

ROTATING END

TEST PIECE

Fig. 29 SHEAR OR TORSION TEST

Impact Testing The following tests are used but none have proved totally satisfactory (a) (b) (c) (d) (e)

Izod pendulum test. Charpy pendulum test. Drop weight test. Ballistic impact test, Slow bend test.

For details of (a)a n d (b) refer to the section in book 1 on The Testing of Metals. The other tests have been listed for reference only.

DEGRADATION OF COMPOSI'I'ES LJnlike metals, composites do not corrode but they do have their problems.

Galvanic Corrosion Galvanic corrosion can occur to A1 alloys a n d cadmiurn plated steel if attached r o CFC (Carbon Fibre Caxnposite).

The pd (potential difference) can be as high as 1 volt. Special jointing cornpounds are used a s is the use of epoxy paint treatment.

Surface 0xidat.ion This is not an important factor with cornposjtes though surface changes can occur when combined with uv light and rain.

Frost Will damage any composites where water h a s ingressed into the material. When water gets into a composite and then freezes it expands - this causes delamination and de -bonding.

UV Radiation

This will degrade glass more than carbon - but at any rate - uv absorbing additives should be used on the outer surfaces of all composites. Aramid fibres are seriously affected and must be protected.

Erosion This can come from many sources but with aircraft it is usually airstream driven rain and debris (insects, dust etc). It affects wing leading edges, engine compressor blades, engine intakes, rotor blades, etc. Glass fibres are more resistant to this sort of damage than carbon and boron.

Lightening Strikes/ Static Electricity Carbon epoxy resins are 3 times more insulative than A1 alloys - this leads to very high field levels on the surface of the material. Various processes have been tried to reduce the problems including metal meshing within the composite. Aluminium surface foiling is used on carbon composites.

Fire Inorganic resins will not withstand high temperatures and soon give off inflammable gases and thick black smoke. To reduce this problern the o u t e r lays of the composite should be glass fibre and the surface should be treated with a fire retardant coating - particularly cabin furnishings.

SEAIAN'I'S, BONDING AGENTS & COMPOUNDS A wide range of non- metallic materials is used for the maintenance, repair and

overhaul of aircraft. 'They include: compounds, greases, oils, detergents, fillers, jo~ntingcompounds, cleaning agents, pre-treatments, anti corrosive agents, paints, paint strippers, fuels, fuel additives, hydraulic fluids, anti-ice fluids, lacquers, adhesive tapes, bonding adhesives, disinfectants, storage preservatives, powders, et c. The AMM for each aircraft type will have a comprehensive list of these "consumables". This is published in chapter 20-3 1-00. It is important that you consult this chapter before using any compounds from oils to paints, to greases and speed tapes. 'The AMM will list all the compounds that can be used on the aircraft, with their specifications (eg, British, US, German, NATO code, etc) if applicable. Some compounds may be listed as "local purchase" whilst others may be supplied by specific manufacturers. Some may be listed under a trade narr,,, eg Loctite. Where fuel additives are listed the actual percentages may be quoted. In some cases the ratios are stated a s "ppm" (parts per million).

For large aircraft the tables in chapter 20-3 I list literally hundreds of nonmetallic materials. Below, are tables of some of the materials that are available. They are for reference only and not included are:

* Fuels. * Fuel additives. * Hydraulic fluids. * De-icing fluids. * Paints and paint strippers. * Extinguishants. These will be dealt with in the appropriate book in the LBP series covering that particular topic. The information under the SPEC column includes those countries that have local specifications to meet that required by the equipment manufacturer, and/or a brand name product. There should be no need to corrirnit the details to memory but you should have a some knowledge of the more c:ornmon sealants and bonding agents used.

blank

-

62 -

TABLE 13 - GREASES

/

DESIGNATION

I SPEC

Mineral based

USA

High temp Bushes, roller & ball bearings

Graphited, mineral based

IJK USA France UK USA France

General purpose 5% graphite

Graphited, thread compound

1 USES

Anti seize grease for threads. 50% mineral jelly 50% graphite

Synthetic, high pressure -

-

-

Silicon

USA

Vaseline or petroleum jelly

UK USA France UK USA

Lubrication for metal and rubber in pneumatic systems

--

Anti fretting

/ Mineral

--

General use at normal temperatures

USA

Used a s a corrosion prevention layer

-- -

--

UK

1----

- -.

---

Used as a n anti fretting compound

UK USA France

USA Lubricant O2 systems

---- --

--

Synthetic rubber seals Electrical bonding faying surfaces

--

Silicon, insulating &, sealing

--

Used in engine fuel and oil systems

USA France

Corrosion preventative

-

For certain applications. Temp range - 54"C to 12 1'

Fuel & oil resistant

-----

1

- --.

-

--

--

--

- -

-

Metal to metal sealing against moisture ingress .-

USA

Thread lubricant for oxygen systems

TABLE 14 - LUBRICANTS DESIGNATION

SPEC

Rust inhibitor

Dinatrol USA Germany

Solid film

Molycote USA

Air drying solid film lubricant

General

Various Grades

Used as a n assembly aid during component overhaul and a t lubrication points of aircraft systems.

Anti seize

USA

Prevents locking of screwthreads

SPEC

USES

USES

TABLE 15 -. LACQUERS DESIGNATION -

.-

-

.-

. .

1

- -. .

Clear epoxy varnish with catalyst

---

Astral

1

-

-

Electrical lacquer

Transparent lacquer

Sikkens

Corrosion preventative

Rustban395 Corrosion preventative USA lacquer

TABLE I. 6

-

For covering metal labels such as landing gear labels

BONDING AND AUF-IESIVES SPEC

USES

General purpose adhesive honeycomb filler

Araldite 106 USA Germany

Composite repairs

General purpose dimethacrylate compound

Loctite270 USA

Permanent thread compound

-- .-

.

--- - .-

High temperature sealant

-

--

Thread locking compound (occasional removal)

I

TABLE 16 - BONDING AND ADHESIVES cont IJSES .

--

-- -- .--

-

/

/

--- -

--

---- -

- -.

--.---. -.

--

-

- -. .--- . - --.

1 ~ ~ ~ + 7 3 For 2 toilets and galleys Primer USA Con tact adhesive

Sealant

1

Solvent based nitril rubber adhesive

I----

. ---

--

1 Adhesive film

-

-

--

-

--

- --

--

Adhesive for PTFE cloth

Araldjte

Two-part epoxy adhesive

I FM73-M-06 I Structural adhesive -. .

Scotch425 Germany USA Scotch36 1

Self adhesive aluminium tape Glass fibre tape

-

I-

I

bonding -- - --Temporary protective cover

- -

-

--

- - -- -

Temporary repair of cargo hold fire proof panels

-.

Sound damping tape

Permacel Germany USA

Polytetrafluoroethylene antiseize tape

121 USA

Aluminium backed cotton tape for sound & thermal insulation For use on liquid & gaseous oxygen systems -

--

High temperature adhesive

AF 143

Metal to metal - honeycomb to metal bonding

TABLE 17 - SEALANTS ----

-

-..

Polysulfide brush consistency

PR1422A2 UK USA

Brush on, fuel tank and pressure cabin fuselage sealant

Polysulfide fillet consistency

PR1422B2 UK USA Various UK USA Germany

Fuel tank and pressure fuselage fillet sealer

Polysulfide sealants

-

general

I

-- --

-

I

-

---

--

- -- -

-

Various different sealants supplied for sealing (a) along edges of joined structures (b) individual nut and bolt assemblies, & ( c )applying to faying surfaces prior to assembly

I

TABLE 18 -- CLEANING AGENTS --

--

-

1 DESIGNATION I--

.---

-

-

SPEC -

--

-

Liquid detergent concentrate -----

-.

-----

-

---- -.

General purpose aircraft exterior cleaner

Aircraft exterior

.

- ..---

Varsol/white spirit

Ardrox6025 USA UK USA

Cleaner and stain remover

Genklene USA

Trichloroethane (Methyl chloroform)

Cleaning solvent Cleaning oxygen system pipe lines

Trich lorotrifluoroethane

--

/ General cleaning

Air3660 France USA

Isopropyl alcohol

--.

-

-

-- - -

. ---.-

Altupol

Safety solvent

USA

Carpet & fabric cleaner . - - . ... --

A USA

I USA

Cabin window cleaner Plastic polishing compound (fine grade)

!

-

--

Rain repellent cleaner

-

----- -.

-

Cleaning solvent for mechanical parts

Cleaning rain repellent off windscreens Odour free solvent cleaning agent

- -. ---

PP-560 USA

Paste for polishing Plexiglas

Alglas V

Anti static flight-deck Visual Display Unit (CRT) screen cleaner

--

VDU cleaner

TABLE 19 - MISCELLANEOIJS --

Hydraulic fluid removal powder Microballoons

Aluminium metal polish - - --

---

---- - ---

- -- -

1

-- - - -

1 ~ 7 0

-.

-

--

Removal of Skydrol fluid spillage

Used as a filler when carrying out composite repairs Abrasjve polish for polishing out scratches in aluminium

TABLE 19

-

--- - -

-

inhibitor -.

.--. .

--

r g---e- n leak detector .

.

IEi1-2 I

MISCELLANEOUS cont.

.--. ....

.-

1

---

.--.-. .

Toilet deodorant

Corrosion preventative Moisture repellent

WD4Q USA

-

- .- . ..- . -. - .. .

AMS 1476

-- .. - .

. a

. -

i

-.

--

--

..

Non formaldel~ydebased toilet deodorant

-..-.

Drinking water system disinfectant

I

.. . -~

1

. -.

- --.-- ---

Calcium hypochlorite disinfectant for the potable water system

I

~tis not possible (within the confines of this publication) to specify the storage conditions for all the materials listed above. But in general the following points should be noted. Keep all containerised materials in their original sealed containers. 2. Open slatted shelving is recommended. 3 . Follow the storage instructions on the container and/or in the material manufact urers5literature. 4. Keep records of materials in store - batch numbers; date of receipt; manufacturer etc. File all manufacturers' documentation, Release Certificates1 EASA form 1s. 5. Rotate stock - first in - first out. 6. Note any storage life/use-by-date. Discard any out-of-date material in accordance with manufactures' jnstructions/local regulations. 7. Store inflammable materials in non-combustible lockers/buildings away from workshops, hangars and aircraft. 8 . In general storage areas should be clean, dry, secure, and frost free. The materials should not be in direct sunlight. The area should be well ventilated and the temperature should be kept a s even a s possible. 9. Specific temperatures may be specified for certain materials by the manufacturer, eg -20°C for pre-preg carbon fibres; 7 to 23°C for paints and dopes, etc. 10. Specific (m,wimum) relative humidity levels may also be spec~liedfor certain materials. 1.

For storage details of specific materials (eg batteries, paints etc) refer to the appropriate book in the LBP series.

CONTENTS

Page Glass fibre repairs Carbon fibre repairs 'rspection of composite structures

1 5 1'7

COMPOSITE REPAIRS Repairs to composite structures is generally considered to be more difficult than repairs to metal structures. Of course, all repair information, cornposlte and metal, is given in the repair manual (SRM) a n d most operators will use a "composites" qualified person to carry out repairs, However, a s a licensed engineer you are required to know how this is done as the composites person will report to you on completion of the repair. GLASS FIBRE

It is most important when carrying out a repair to follow the repair manual instructions. This usually specifies that the same type of core is fitted to that which has been removed during the repair process. Remember on radomes/dielectric covers the repair should be 'radar transparent'. 'The repair should also follow, as far as possible, the original contour and shape of the rigirlal component.

Preparation and Mixing of Resins In general always: (a) (b) (c)

(d) (e)

Wear protective clothing, including goggles. Work in a well ventilated area. Mix the chemicals in accordance with the manufacturer's instructions. Wash the area thoroughly if chemicals come in contact with the skin. Irrigate the eyes immediately with water if the chemicals come into contact with the eyes - and seek medical advice.

Mixing The ingredients should be stored (normal maximum time 12 months) at temperatures less than 10°C and be allowed to come to room temperature before mixing and all materials, working areas, tools and utensils must be kept thoroughly clean and dry. The resin and additives should be carefully measured into a glass container in the correct proportions a s specified in the manufacture's instructions. These proportions may be specified a s percentages by weight. The catalyst should be thoroughly mixed into the resin before adding the accelerator and any additional material such a s fillers etc.

GLASS CLOTH PATCHES

CORE

Fig. 1 TYPICAL REPAIR TO CRACKED SKIN

Repatrs to be at feast 10" (25mm) apart with dimensions A and Bat a maximum of 2.5 to 7" (63.5mm to 117.8mm) depending on type of repair (round or square).

\ GLASS CLOTH PATCHES GLASS CLOTH

Fig. 2 TYPICAL REPAIR TO DELAMINATED SKIN

Pot Life Once mixed the resin begins to cure and may have a pot life of between a few minutes and several hours before it begins to gel. Always ensure the resin is used well within it's pot lifetjme. Djscard (in accordance with local regulations) all time expired materials.

Curing Most mixed resins will cure a t room temperature within a few hours, but may take several days to cure completely It may be necessary to use heat to cure the resin, sa check the Repair Manual (SRM) for details. Heating may be carrled by the use of lamps, electric heaters, electric blankets or ovens. Temperature control may be by a thermostat or by marking the part with a special pencil that changes colour at a specific temperature. Film Adhesives Some adhesives are supplied in film form and the amount required is simply cut from a large sheet. They are generally easier to apply than liquid or powder adhesives, but once the protective backing is removed it is most important that the adhesive film is not touched a s this will severely affect its adhesive properties, Each patch is 0.7" larger than the next one

GLASS CLOTH PATCHES

OUTER SKIN PATCH PLATE thickness as inner skin)

Dimensions A 8 B are a max of 1 to 5" (25.4 to 127mm) whether the repair is round or sguare

t--"--i SKIN SECTION

INNER SKIN

I11 11 11 11 11 1 I

Fig. 3 TYPICAL REPAIR WHERE BOTH SKINS & CORE ARE DAMAGED Figure 1 shows an exarnple of a patch repair to a crack on the outer skin. The ends of crack are stop drilled using a 31 1 6 t h (4.8mm) twist drill. Glass cloth patches (3)are cut as per SRM and using the mixed resin bonding agent are cemented into position. Pressure is applied and this can be done using a vacuum sheet stuck with double sided sticky tape to the skin. A parting layer is used between the patches and the vacuum sheet and vacuum is applied from a vacuum pump via a valve in the vacuum sheet.

Figure 2 shows a typical repair where the outer skin is damaged and has to be repaired by insertion. The skin is cut away without damaging the core using a router (not easy a s the skin arld core are bonded together). Two glass cloth inserts are cut'and (using the rnixed resin) placed into position. The glass cloth patches are placed in the same way. Again, pressure is applied a s before. Figures 3 , 4 and 5 shows repairs where the core has been damaged and requires replacement. A s with the other repairs a router is used for material removal, and sometimes wood chisels and the like are used to remove old resin -- which is difficult to do. Limits are specified in the SRM a s to the maximum length of crack/size of damage, the minimum distance between repairs and the minimum distance from the repair to the edge of the panel.

OUTER SKIN

MIN LAND 0.5" (13mrn)

Fig. 4 EXAMPLE OF CORE REPAIR 3" DIA MAX General Repair Considerations 1.

Ascertain the exact extent of the damage and classify the repai using the repair manual (negligible- repairable - replacement .,,c).

2.

Support or jury rig the structure if necessary.

3.

Check the effect of the repair on radar transparency

4.

Mix and use the resins in a warm dry atmosphere (min 20°C).

5.

Remove resins from store and allow to attain room temperature for at least 24 hours.

6.

Remove paint from the area by sanding, then clean with acetone or MEK and allow to dry.

7.

Cut out the damage to a regular shape, stepped or otherwise, a s per the S R M dimensions.

-

if applicable.

8.

Sand area if specified in the SRM.

9.

Lay u p the repair using cloth and resins in accordance with the repair manual. Cloth plies normally in the same direction as the original lay.

10.

Apply pressure to the repair using weights, clamps ox vacu ~rxn bags.

11.

Use a mould, for more complex shapes, made fi-om wood or other similar material.

12.

Use a parting agent on the mould to prevent the resins from adhering to the mould.

13.

Remove all traces of parting agent from the repair.

14.

Inspect the repair, repaint and carry out functional check to check for radar transparency.

15.

If a control surface check weight a n d mass balance and carry out control system check plus an independent check.

16.

Record all work done and clear Log Book.

All materials & dimensions similar to the previous drawing

Fig. 5 EXAMPLE OF SMALL CORE REPAIR (1.5"BIA MAX) CARBON FIBRE COMPOSITES (CFCs)

There is an increasing use of CFCs in the construction of aircraft. The advantages of this material over conventional metals are many and include: k .k

k

A

Good strength/weight ratio. Resistance to impact damage - often difficult to detect if it has sustained damage. Non-corrodible. Easily moulded to complex shapes.

9,

*

1s not aff'ect.ed by hydraulic and other fluids. Does not suffer from cracking and has vexy good fatigue strength.

Like GRP it is made u p of layers of fibre but carbon and not glass. It may be pre-preg (already pre-impregnated with resin) or may be carbon fibre material requiring a bonding agent between the layers. Once the layers are made u p the resin is allowed to cure - usually using heat and pressure (vacuum bags). Materials (a)

(b) (c)

Resins and other chemicals. Stored at -18°C usually has a shelf life of 12 months - refer to manufacturers literature. CFC and Kevlar material stored in a dark room in their original plastic containers. Kevlar is affected by uv light. CFC pre-preg is stored at - 18°C and again may have a shelf life nf 1 2 months. May have a life of one month out of cold store.

All materials should be allowed to reach room temperature before being worked on. This usually means keeping at room temperature for a period of 24 hours.

Types of Structure Sandw&Construction. Not unlike the sandwich construction of GRP. It is designed to have a light, reasonably weak centre with strong outer fibres. The outer fibres being in tension or compression with the centre being in shear. Many combinations of composite (metal and non metal) can be used. Figure 6 is a typical example. The sandwich is usually made u p of a honeycomb centre with multiple plies of composite pre-preg cloths laid at different angles to each other and cured under pressure in an autoclave. HONEYCOMB CORE

Fig. 6 TYPICAL SANDWICH STRUCTURE jVJonolithic Structux. Structural components such as sheet skin, angles, ribs, frames, top hat sections etc are made from monolithic material in a similar way to the build up of the outer layers of the sandwich structure.

Figure 7 shows an A310 spoiler made from glass fibre reinforced plastlc (GFRP) skln panels and ribs. With the fittings being made from metal. Mixed Structure. Figure 17 shows the construct~onof a n A320 flap. It 1s a mixed structure with some monolithic and some sandwich components

Fig. 7 MONOLITHIC STRUCTURE

SANDWICH STRUCTURE

MONOLITHIC PANELS

Fig. 8 EXAMPLE OF MIXED STRUCTURE

Like GRP, damage that does occur may be difficult to detect. It is therefi~re important that if damage is suspected then a thorough investigation is c arried out aver the whole area. The cfarrlage is usually associated with impact and the inspection procedure is similar to that used with GRP.

AREA OF DELAMINATION

IMPACT I I

SPREAD OF

-\

/----BROKEN INNER LAYERS

Fig. 9 IMPACT 'SPREAD' O N CFC SKIN X--raysmay be used to check for internal darnage/delamination on sandwich structures and ultra-sonics may be used on monolithic structures. When using ultra-sonics a couplant must be used between the probe and the part being tested (oil or grease on metals). For CFCs a rubber tyred wheel or wat js used. Thermal Pulse Thermography (TPT) may be used. This process involves the use of a high intensity thermal pulse and the rate of diffusion is measured. An image of the thermal pattern is then displayed on a screen and a change in the pattern will indicate a defect,. Modern TPT systems will involve the use of computers for storage and analysis of data.

Repair

The repair process is similar to that which is employed with GKP structures. Equipment The equipment will vary depending on the type and level of the repair beincarried out, but the following is a typical list of the equipment required:

* * * * k

A

*

A CFC bay with everything kept scrupulously clean. Repair heaters - electrical heater mats thermostatically controlled. Vacuum pressure bags - to put the repair under pressure when curing. Temperature probes - to monitor the temperature of the repair when curing. Cold storage equipment. Various tools including diamond coated saw blades and diamond tipped drill bits. Breathing equipment and a dust extraction plant. CFC particles and dust are dangerous if breathed in and fumes from the chemicals are toxic.

Repair Methods These will be laid down in Chapter 51 of the SRM and may involve the use of infill, metal patching, GRP lay-up, CFC lay-up, core replacement etc, Damage (and the repair) can be divided into three main groups:

* *

k

Negligible damage. May be repa~red/modifiedfor cosmetic reasons or to stop the damage getting worse. Structural damage. Has to be repaired to maintain the integrity of the structure. May be a standard repair in the manual, or may require the approval of the aircraft manufacturer. Replacement damage. Severe damage that requires the replacement of the component.

When assessing the damage always inspect an area much larger than the 'obvious' damage a s the impact shock can travel through the material and .how u p some distance away. For example - if it is damage to a panel, check for security and damage a t the panel attachments and check for transmitted shock into the surrounding structure. Of course, all these types are damage are laid down in the SRM, a s are the repair schemes. In general the repair materials should be the same a s the original component unless specified otherwise. General Repair Procedure Clean and dry the repair area. Remove the paint (by sanding) in the area taking care not to damage the fibres. Remove all traces of dust. Remove the damage. Check that all the damage h a s been removed. Scarf the edges a s specified in the manual. The scarfed edge may have a taper of 20: 1. The core is removed by the use of a router. Check the repair limitations in the repair manual. The fibre layers are laid u p by hand and usually involve the use of pre-preg material. This may be laid up at 0°, 45"and 90" Use might be made of 'in-fill', an insert, blind rivets, bolts, metal patches etc. Allow to 'cold cure7- use a vacuum bag or heat in a n autoclave. Inspect the repair and repaint if necessary. Depending on what has been repaired check the system and sign for all the work done.

,BLIND RIVETS,

COMPOSITE DOUBLER

Fig. 10 SKIN REPAIR USING RIVETS ADHESIVE & DOUBLER

COVER PLY PLY 6 PLY 5 PLY 4 PLY 3 PLY 2 PLY 1

ADHESIVE FILM

Q

0 STEPPED

-C-)-

CUT-BUTS

Completed lay-up before hot bonded cure

Fig.. 11 SKIN REPAIR HOT CURE USING PRE-PREG

Two basic methods of repair: (a)

'Cold Cure'. Using room temperature (20°C min) or heater blankets. Curing can take up to 7 days but with heater blankets using temperatures of about 80°C the time can be reduced to less than an hour - depending on materials, type of repair etc.

(b)

'Hot Cure'. This process uses an autoclave with temperatures u p to 180°C and curing times as short as 45 minutes, again depending on materials and type of repair.

Repairs to Sandwich Structure The damaged core is usually removed and the void filled with a mixture (of adhesive and thickening agent), or a core plug of honeycomb is bonded into position. The skin is then repaired in the same manner as already described.

COVER PLY

PLY 6 PLY 5 PLY 4 PLY 3 FLY 2

-

-

PLY 1

Completed wet lay-up before cure

Fig. 12 SKIN REPAIR USING COLD CURE WET LAY-UP

ADHESIVE

COMPOSITE DOUBLER /

Fig. 13 COLD CURE REPAIR USING DOUBLER AND VOID FILLER

Delamination and Debonding Delamination occurs when two or more plies become separated frorn each other often due to impact. They may be repaired by layering or by injecting adhesive through the rivet holes (drilled iaw the repair drawing) and riveted u p using blind rivets.

-

,-------COMPOSITE DOUBLER

--PLUG

ADHESIVE,

HONEYCOMB

Fig. f 4 HOT BONDING USING A HONEYCOMB INSERT Debonding occurs when the honeycomb core separates from the outer skin. Repair can be carried out by injecting adhesive into the honeycomb through holes drilled in the skin. Pressure should be applied to the skin to ensure a good bond between the skin and the core material.

DELAMINATION

BLIND RIVETS ADHESIVE INJECTED THROUGH RIVET HOLES

Fig. 15 DELAMINATION REPAIR Metal Patching The metal patch may be bolted or bonded into position. Metal patching does not attempt to restore the structure to its original strength or contour but is a quick method af repairing small cracks or limited damage to non-primary structures. -, *',

., ,'., ...-...'

*,

, \ ,

\

HYPODERMIC

b,

\,

*, ' *, ,.-*\

\

a,\.':..*-'~,,

a s torclue is applied, t h e shaft moves axially (as it rotates). This axial movement pushvs on a small piston t h u s producing pressure in a n o ~ l e dfilled d a s h pot. The oil pressure is transduced into a n electrical signal for fllght deck indicators r e a d l ~ i gtorque in Nm.

SPUR GEAR PRE CYL

DRIVING GEAR)

-

AXIAL THRUST WHEN TORQUE APPL-IED

HELICAL GEAR

OUTPUT SHAFT

Fig. 28 GEAR BOX SHOWING HELICAL GEAR A S TORQUETRANSDUCER

S p u r gears a r e found in gearboxes; in epicyclic reduction gear trains; accessory drive trains, a n d in gear-type oil p u m p s - for engine oil systems and some older hyclra~~lic systems (giving low pressure/ high flow rates). For internal s p u r gears, the positions of t h e adderldum a n d dcclendum are rcvcrsc.ci from those of t h e external gear but are still relatetl to I l ~ root e a n d tip. This I-csults i11 a different tooth :\ction a n d less s l ~ p p a g ethari with a n equivalent external s p u r . 'I'hc. iiitcrnal gear makes it suited to closer centre distances t1iat-I could IJ(> used ivith :ti1 external gear of the same size When it is necessary to r r ~ c t i ~ ~ t tatie in s a m e s e n s e of rotation for two parallel s l ~ a fst , the, 1nte1-nttlgc1:11 i s t,spec-~:~ily desi~ :t t ~ l ebecatisc it eljmi~i;~tes thy nced for an idler- gcar 'I'1ic:sc. c.orlditio[ls rnnltc- the internal gear highly adaptable tto ey~icyrlicand pl;t~l(:t;t~ y ~c;\J tr~~lns

11s r r i e ~ ~ t ~ o; t rt xl ~~\ l~t ~l~, C I I C ; I I gears