9.Welding Bonding Permanent Joints
Short Description
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Description
Chapter 9
Welding, Bonding, and the Design of Permanent Joints ME
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
9–1
Welding Symbols
9–2
Butt and Fillet Welds
9–3
Stresses in Welded Joints in Torsion
9–4
Stresses in Welded Joints in Bending
9–5
The Strength of Welded Joints
9–6
Static Loading
9–7
Fatigue Loading
9–8
Resistance Welding
9–9
Adhesive Bonding
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Overview
- A weldment is fabricated by welding together a collection of metal shapes, cut to particular configurations. - During welding, the several parts are held securely together, often by clamping or jigging. - The welds must be precisely specified on working drawings, and this is done by using the welding symbol as standardized by the American Welding Society (AWS).
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Welding Symboles
- The arrow of this symbol points to the joint to be welded. - The body of the symbol contains as many of the following elements as are deemed necessary:
• Reference line • Arrow • Basic weld symbols as in Fig. 9-2 • Dimensions and other data • Supplementary symbols • Finish symbols • Tail • Specification or process
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Welding Symboles
- Figures 9–3 to 9–6 illustrate the types of welds used most frequently by designers. - For general machine elements most welds are fillet welds, though butt welds are used a great deal in designing pressure vessels. - The parts to be joined must be arranged so that there is sufficient clearance for the welding operation.
- Since heat is used in the welding operation, there are metallurgical changes in the parent metal in the vicinity of the weld. Also, residual stresses may be introduced because of clamping or holding or, sometimes, because of the order of welding. - Usually these residual stresses are not severe enough to cause concern; in some cases a light heat treatment after welding has been found helpful in relieving them. When the parts to be welded are thick, a preheating will also be of benefit.
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Welding Symboles
Fillet welds. (a) The number indicates the leg size; the arrow should point only to one weld when both sides are the same. (b) The symbol indicates that the welds are intermittent and staggered 60 mm along on 200-mm centers.
The circle on the weld symbol indicates that the welding is to go all around.
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Welding Symboles
Butt or groove welds: (a) square butt-welded on both sides
(b) single V with 60o bevel and root opening of 2 mm
(d) Single bevel
(c) double V
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Welding Symboles
Special groove welds: (a) T joint for thick plates;
(b) U and J welds for thick plates;
(c) corner weld (may also have a bead weld on inside for greater strength but should not be used for heavy loads);
(d) edge weld for sheet metal and light loads.
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Welding Symboles
9–1
Welding Symbols
9–2
Butt and Fillet Welds
9–3
Stresses in Welded Joints in Torsion
9–4
Stresses in Welded Joints in Bending
9–5
The Strength of Welded Joints
9–6
Static Loading
9–7
Fatigue Loading
9–8
Resistance Welding
9–9
Adhesive Bonding
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Overview
Figure 9–7a shows a single V-groove weld loaded by the tensile or compression force F, the average normal stress is
F hl
where h is the weld throat and l is the length of the weld. - Note that the value of h does not include the reinforcement. The reinforcement can be desirable, but it varies somewhat and does produce stress concentration at point A. - If fatigue loads exist, it is good practice to grind or machine off the reinforcement.
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Butt and Fillet Welds
The average stress in a butt weld due to shear loading (Fig. 9–7b) is
F hl
- For a typical transverse fillet weld as shown, from a free body at angle θ the forces on each weldment consist of a normal force Fn and a shear force Fs. - Summing forces in the x and y directions gives Fs F sin Fn F cos
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Butt and Fillet Welds
- Using the law of sines for the triangle in the figure yields t h 2h sin 45 sin 135 cos sin
- Solving for the throat length t gives
h t cos sin
- The nominal stresses at the angle θ in the weldment, τ and σ , are Fs Fs F sin cos sin F sin cos sin 2 A tl hl hl Fn Fn F cos cos sin F cos2 sin cos A tl hl hl
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Butt and Fillet Welds
- The von Mises stress σ′ at angle θ is
3 2
2 12
2 2 12 F 2 2 cos sin cos sin sin cos hl
- The largest von Mises stress (differentiating the equation (f)) occurs at θ = 62.5° with a value of
2.16F hl
- The corresponding values of τ and σ are τ = 1.196F/(hl) and σ = 0.623F/(hl)
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Butt and Fillet Welds
- The maximum shear stress can be found by differentiating the equation (d) with respect to θ and equating to zero. - The stationary point occurs at θ = 67.5° with a corresponding
τmax = 1.207F/(hl) σ
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and
= 0.5F/(hl)
Butt and Fillet Welds
- A model of the transverse fillet weld of Fig. 9–8 is easily constructed for photoelastic purposes and has the advantage of a balanced loading condition. - Norris constructed such a model and reported the stress distribution along the sides AB and BC of the weld. An approximate graph of the results he obtained is shown as Fig. 9–10a. Note that stress concentration exists at A and B on the horizontal leg and at B on the vertical leg. Norris states that he could not determine the stresses at A and B with any certainty. - Salakian presents data for the stress distribution across the throat of a fillet weld (Fig. 9–10b).
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Butt and Fillet Welds
Stress distribution in fillet welds: (a) stress distribution on the legs as reported by Norris; (b) distribution of principal stresses and maximum shear stress as reported by Salakian.
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Butt and Fillet Welds
- The most important concept here is that we have no analytical approach that predicts the existing stresses. - The geometry of the fillet is crude by machinery standards, and even if it were ideal, the macrogeometry is too abrupt and complex for the methods. - The approach has been to use a simple and conservative model, verified by testing as conservative. The approach has been to • Consider the external loading to be carried by shear forces on the throat area of the weld. By ignoring the normal stress on the throat, the shearing stresses are inflated sufficiently to render the model conservative. • Use distortion energy for significant stresses. • Circumscribe typical cases by code.
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Butt and Fillet Welds
- For the model, the basis for weld analysis or design employs
F 1.414 F 0.707hl hl which assumes the entire force F is accounted for by a shear stress in the minimum throat area. - Note that this inflates the maximum estimated shear stress by a factor of 1.414/1.207 = 1.17. - Further, consider the parallel fillet welds shown in this Figure, where, as in Fig. 9–8, each weld transmits a force F. However, in the case of this Figure, the maximum shear stress is at the minimum throat area and corresponds to Eq. (9–3).
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Butt and Fillet Welds
Under circumstances of combined loading we • Examine primary shear stresses due to external forces.
• Examine secondary shear stresses due to torsional and bending moments. • Estimate the strength(s) of the parent metal(s).
• Estimate the strength of deposited weld metal. • Estimate permissible load(s) for parent metal(s).
• Estimate permissible load for deposited weld metal.
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Butt and Fillet Welds
9–1
Welding Symbols
9–2
Butt and Fillet Welds
9–3
Stresses in Welded Joints in Torsion
9–4
Stresses in Welded Joints in Bending
9–5
The Strength of Welded Joints
9–6
Static Loading
9–7
Fatigue Loading
9–8
Resistance Welding
9–9
Adhesive Bonding
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Overview
τ r
- The figure illustrates a cantilever of length l welded to a column by two fillet welds. - The reaction at the support of a cantilever always consists of a shear force V and a moment M. - The shear force produces a primary shear in the welds of magnitude
V A
where A is the throat area of all the welds. - The moment at the support produces secondary shear or torsion of the welds, and this stress is given by the equation
Mr J
where r is the distance from the centroid of the weld group to the point in the weld of interest and J is the second polar moment of area of the weld group about the centroid of the group.
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Stresses in Welded Joints in Torsion
- When the sizes of the welds are known, these equations can be solved and the results combined to obtain the maximum shear stress.
- Note that r is usually the farthest distance from the centroid of the weld group. - The figure shows two welds in a group. The rectangles represent the throat areas of the welds. - Weld 1 has a throat thickness t1 = 0.707h1, and weld 2 has a throat thickness t2 = 0.707h2. Note that h1 and h2 are the respective weld sizes. The throat area of both welds together is
A A1 A2 t1d t2b
- This is the area that is to be used in equation of the primary shear.
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Stresses in Welded Joints in Torsion
- The x axis in the figure passes through the centroid G1 of weld 1. The second moment of area about this axis is
t1d 3 Ix 12 - Similarly, the second moment of area about an axis through G1 parallel to the y axis is
dt13 Iy 12 - Thus the second polar moment of area of weld 1 about its own centroid is
t1d 3 dt13 J G1 I x I y 12 12
- In a similar manner, the second polar moment of area of weld 2 about its centroid is
JG2
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bt23 t2b3 12 12
Stresses in Welded Joints in Torsion
- The centroid G of the weld group is located at A1 y1 A2 y2 A1 x1 A2 x2 x y A A
- From the figure, we see that the distances r1 and r2 from G1 and G2 to G, respectively, are 12
2 r1 x x1 y 2
2 12
r2 y2 y x2 x 2
- Using the parallel-axis theorem, we find the second polar moment of area of the weld group to be J J G1 A1r12 J G 2 A2 r22 - This is the quantity to be used to calculate secondary shear. The distance r must be measured from G and the moment M computed about G.
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Stresses in Welded Joints in Torsion
- The reverse procedure is that in which the allowable shear stress is given and we wish to find the weld size. The usual procedure is to estimate a probable weld size. - The quantities t13 and t23 which are the cubes of the weld thicknesses, are small and can be neglected. - Setting the weld thicknesses t1 and t2 to unity leads to the idea of treating each fillet weld as a line.
- The resulting second moment of area is then a unit second polar moment of area Ju. - The advantage of treating the weld size as a line is that the value of Ju is the same regardless of the weld size.
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Stresses in Welded Joints in Torsion
- Since the throat width of a fillet weld is 0.707h, the relationship between J and the unit value is J 0.707hJ u in which Ju is found by conventional methods for an area having unit width.
- Table 9–1 lists the throat areas and the unit second polar moments of area for the most common fillet welds encountered. The example that follows is typical of the calculations normally made.
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Stresses in Welded Joints in Torsion
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Stresses in Welded Joints in Torsion
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Stresses in Welded Joints in Torsion
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Stresses in Welded Joints in Torsion
b = 56 mm d = 190 mm
= 10.4 mm = 95 mm
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Stresses in Welded Joints in Torsion
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Stresses in Welded Joints in Torsion
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Stresses in Welded Joints in Torsion
τA rA τB rB τC rC
τD rD
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Stresses in Welded Joints in Torsion
= (‘2 + ‘‘2)1/2
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Stresses in Welded Joints in Torsion
9–1
Welding Symbols
9–2
Butt and Fillet Welds
9–3
Stresses in Welded Joints in Torsion
9–4
Stresses in Welded Joints in Bending
9–5
The Strength of Welded Joints
9–6
Static Loading
9–7
Fatigue Loading
9–8
Resistance Welding
9–9
Adhesive Bonding
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Overview
- A rectangular cross-section cantilever welded to a support at the top and bottom edges. - The shear force produces a primary shear in the welds of magnitude
V A
where A is the total throat area. - The moment M induces a throat shear stress component of 0.707τ in the welds. - Treating the two welds of Fig. 9–17b as lines we find the unit second moment of area to be bd 2 Iu 2
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Stresses in Welded Joints in Bending
- The second moment of area I, based on weld throat area, is bd 2 I 0.707hI u 0.707h 2
- The nominal throat shear stress is now found to be
Mc Md 2 1.414M I 0.707hbd 2 2 bdh
- The model gives the coefficient of 1.414, in contrast to the predictions of Sec. 9–2 of 1.197 from distortion energy, or 1.207 from maximum shear. - The second moment of area is based on the distance d between the two welds. If this moment is found by treating the two welds as having rectangular footprints, the distance between the weld throat centroids is approximately (d + h). This would produce a slightly larger second moment of area, and result in a smaller level of stress.
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Stresses in Welded Joints in Bending
- This method of treating welds as a line does not interfere with the conservatism of the model. It also makes Table 9–2 possible with all the conveniences that ensue.
Bending Properties of Fillet Welds
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Stresses in Welded Joints in Bending
Bending Properties of Fillet Welds (continued)
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Stresses in Welded Joints in Bending
Bending Properties of Fillet Welds (continued)
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Stresses in Welded Joints in Bending
9–1
Welding Symbols
9–2
Butt and Fillet Welds
9–3
Stresses in Welded Joints in Torsion
9–4
Stresses in Welded Joints in Bending
9–5
The Strength of Welded Joints
9–6
Static Loading
9–7
Fatigue Loading
9–8
Resistance Welding
9–9
Adhesive Bonding
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Overview
- The matching of the electrode properties with those of the parent metal is usually not so important as speed, operator appeal, and the appearance of the completed joint. The properties of electrodes vary considerably, but this Table lists the minimum properties for some electrode classes. - It is preferable, in designing welded components, to select a steel that will result in a fast, economical weld even though this may require a sacrifice of other qualities such as machinability. Minimum Weld-Metal Properties
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The Strength of Welded Joints
- All steels can be welded, but best results will be obtained if steels having a UNS specification between G10140 and G10230 are chosen. All these steels have a tensile strength in the hot-rolled condition in the range of 410 to 480 MPa. - One of the best standards to use is the American Institute of Steel Construction (AISC) code for building construction.
- The permissible stresses are based on the yield strength of the material instead of the ultimate strength, and the code permits the use of a variety of ASTM structural steels having yield strengths varying from 230 to 340 MPa. - Provided the loading is the same, the code permits the same stress in the weld metal as in the parent metal. For these ASTM steels, Sy = 0.5Su.
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The Strength of Welded Joints
- This Table lists the formulas specified by the code for calculating these permissible stresses for various loading conditions. - The factors of safety implied by this code are easily calculated. - For tension, n = 1/0.60 = 1.67. For shear, n = 0.577/0.40 = 1.44, using the distortion-energy theory as the criterion of failure. Stresses permitted by the AISC code for weld metal
*The
factor of safety n has been computed by using the distortion-energy theory. †Shear stress on base metal should not exceed 0.40S of base metal. y
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The Strength of Welded Joints
- It is important to observe that the electrode material is often the strongest material present. - If a bar of AISI 1010 steel is welded to one of 1018 steel, the weld metal is actually a mixture of the electrode material and the 1010 and 1018 steels. - The weld metal is usually the strongest, do check the stresses in the parent metals. - The fatigue stress-concentration factors listed in Table 9–5 are suggested for use. These factors should be used for the parent metal as well as for the weld metal.
Fatigue Stress-Concentration Factors, Kfs
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The Strength of Welded Joints
Allowable steady-load information and minimum fillet sizes.
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The Strength of Welded Joints
9–1
Welding Symbols
9–2
Butt and Fillet Welds
9–3
Stresses in Welded Joints in Torsion
9–4
Stresses in Welded Joints in Bending
9–5
The Strength of Welded Joints
9–6
Static Loading
9–7
Fatigue Loading
9–8
Resistance Welding
9–9
Adhesive Bonding
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Overview
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Static Loading
Fillet weld 50 mm long on both sides
1015 hot rolled base metal
Table 9-4
h = 10 mm l = 50 mm
Eq. 9-2
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Static Loading
Eq. 9-1
Table 9-4
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Static Loading
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Static Loading
Table A-20
Table 9-4
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Static Loading
75*4
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Static Loading
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Static Loading
Eq. 8-56
Table 9-3 Weld metal: E70, Sut = 70 kpsi
o
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Static Loading
Weld metal Eq. 9-3: assumed that F is accounted by a shear stress in the minimum throat area
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Static Loading
Base metal
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Static Loading
Table 9-4
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Static Loading
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Static Loading
Stresses in welded joints in bending
r = d/2
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Static Loading
Eq: 5-21
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Static Loading
AISI 1018 HR steel
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Static Loading
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Static Loading
9–1
Welding Symbols
9–2
Butt and Fillet Welds
9–3
Stresses in Welded Joints in Torsion
9–4
Stresses in Welded Joints in Bending
9–5
The Strength of Welded Joints
9–6
Static Loading
9–7
Fatigue Loading
9–8
Resistance Welding
9–9
Adhesive Bonding
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Overview
- In fatigue, the Gerber criterion is best; however, the Goodman criterion is in common use. - Recall, that the fatigue stress concentration factors are given in Table 9–5. For welding codes, see the fatigue stress allowable in the AISC manual.
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Fatigue Loading
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Fatigue Loading
ka = aSutb
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288
Fatigue Loading
completely reversed
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Fatigue Loading
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Fatigue Loading
Type 1 Table 9-1
repeatedly applied
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Fatigue Loading
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Fatigue Loading
9–1
Welding Symbols
9–2
Butt and Fillet Welds
9–3
Stresses in Welded Joints in Torsion
9–4
Stresses in Welded Joints in Bending
9–5
The Strength of Welded Joints
9–6
Static Loading
9–7
Fatigue Loading
9–8
Resistance Welding
9–9
Adhesive Bonding
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Overview
- The heating and consequent welding that occur when an electric current is passed through several parts that are pressed together is called resistance welding.
(a) Spot welding; (b) seam welding.
- Spot welding and seam welding are forms of resistance welding most often used. - The advantages of resistance welding over other forms are the speed, the accurate regulation of time and heat, the uniformity of the weld, and the mechanical properties that result. In addition the process is easy to automate, and filler metal and fluxes are not needed. - Seam welding is actually a series of overlapping spot welds, since the current is applied in pulses as the work moves between the rotating electrodes.
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Resistance Welding
- Failure of a resistance weld occurs either by shearing of the weld or by tearing of the metal around the weld. - Because of the possibility of tearing, it is good practice to avoid loading a resistance-welded joint in tension. - Thus, for the most part, design so that the spot or seam is loaded in pure shear.
- The shear stress is then simply the load divided by the area of the spot. - Because the thinner sheet of the pair being welded may tear, the strength of spot welds is often specified by stating the load per spot based on the thickness of the thinnest sheet. - Somewhat larger factors of safety should be used when parts are fastened by spot welding rather than by bolts or rivets, to account for the metallurgical changes in the materials due to the welding.
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Resistance Welding
9–1
Welding Symbols
9–2
Butt and Fillet Welds
9–3
Stresses in Welded Joints in Torsion
9–4
Stresses in Welded Joints in Bending
9–5
The Strength of Welded Joints
9–6
Static Loading
9–7
Fatigue Loading
9–8
Resistance Welding
9–9
Adhesive Bonding
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Overview
- The use of polymeric adhesives to join components for structural, semi-structural, and nonstructural applications has expanded greatly in recent years as a result of the unique advantages adhesives may offer for certain assembly processes and the development of new adhesives with improved robustness and environmental acceptability. - The increasing complexity of modern assembled structures and the diverse types of materials used have led to many joining applications that would not be possible with more conventional joining techniques.
Diagram of an automobile body showing at least 15 locations at which adhesives and sealants could be used or are being used.
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Adhesive Bonding
- Adhesives are also being used either in conjunction with or to replace mechanical fasteners and welds. Figure 9–24 illustrates the numerous places where adhesives are used on a modern automobile. Indeed, the fabrication of many modern vehicles, devices, and structures is dependent on adhesives.
- In well-designed joints and with proper processing procedures, use of adhesives can result in significant reductions in weight. - Eliminating mechanical fasteners eliminates the weight of the fasteners, and also may permit the use of thinner-gauge materials because stress concentrations associated with the holes are eliminated.
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Adhesive Bonding
- The capability of polymeric adhesives to dissipate energy can significantly reduce noise, vibration, and harshness (NVH), crucial in modern automobile performance. - Adhesives can be used to assemble heat-sensitive materials or components that might be damaged by drilling holes for mechanical fasteners. - They can be used to join dissimilar materials or thin-gauge stock that cannot be joined through other means
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Adhesive Bonding
Types of Adhesive - There are numerous adhesive types for various applications. They may be classified in a variety of ways depending on their chemistry (e.g., epoxies, polyurethanes, polyimides), their form (e.g., paste, liquid, film, pellets, tape), their type (e.g., hot melt, reactive hot melt, thermosetting, pressure sensitive, contact), or their load-carrying capability (structural, semi-structural, or nonstructural). - Structural adhesives are relatively strong adhesives that are normally used well below their glass transition temperature; common examples include epoxies and certain acrylics. Such adhesives can carry significant stresses, and they lend themselves to structural applications. - Contact adhesives, where a solution or emulsion containing an elastomeric adhesive is coated onto both adherends, the solvent is allowed to evaporate, and then the two adherends are brought into contact. Examples include rubber cement and adhesives used to bond laminates to countertops.
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Adhesive Bonding
Types of Adhesive - Pressure-sensitive adhesives are very low modulus elastomers that deform easily under small pressures, permitting them to wet surfaces. When the substrate and adhesive are brought into intimate contact, van der Waals forces are sufficient to maintain the contact and provide relatively durable bonds. Pressure-sensitive adhesives are normally purchased as tapes or labels for nonstructural applications, although there are also double-sided foam tapes that can be used in semi-structural applications. - As the name implies, hot melts become liquid when heated, wetting the surfaces and then cooling into a solid polymer (the glue guns in popular use).
- Anaerobic adhesives cure within narrow spaces deprived of oxygen; such materials have been widely used in mechanical engineering applications to lock bolts or bearings in place.
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Adhesive Bonding
This Table presents important strength properties of commonly used adhesives.
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Adhesive Bonding
Stress Distributions - Good design practice normally requires that adhesive joints be constructed in such a manner that the adhesive carries the load in shear rather than tension. Bonds are typically much stronger when loaded in shear rather than in tension across the bond plate. - Lap-shear joints represent an important family of joints, both for test specimens to evaluate adhesive properties and for actual incorporation into practical designs. - Generic types of lap joints that commonly arise are illustrated in Fig. 9–25. - The simplest analysis of lap joints suggests the applied load is uniformly distributed over the bond area.
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Adhesive Bonding
Common types of lap joints used in mechanical design:
(a) single lap (b) double lap (c) Scarf
(d) Bevel (e) Step (f ) butt strap
(g) double butt strap (h) tubular lap.
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Adhesive Bonding
Joint Design • Design to place bondline in shear, not peel. Beware of peel stresses focused at bond terminations. When necessary, reduce peel stresses through tapering the adherend ends increasing bond area where peel stresses occur, or utilizing rivets at bond terminations where peel stresses can initiate failures. • Where possible, use adhesives with adequate ductility. The ability of an adhesive to yield reduces the stress concentrations associated with the ends of joints and increases the toughness to resist debond propagation. • Recognize environmental limitations of adhesives and surface preparation methods. Exposure to water, solvents, and other diluents can significantly degrade adhesive performance in some situation, through displacing the adhesive from the surface or degrading the polymer. Certain adhesives may be susceptible to environmental stress cracking in the presence of certain solvents. Exposure to ultraviolet light can also degrade adhesives.
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Adhesive Bonding
Joint Design • Design in a way that permits or facilitates inspections of bonds where possible. A missing rivet or bolt is often easy to detect, but debonds or unsatisfactory adhesive bonds are not readily apparent. • Allow for sufficient bond area so that the joint can tolerate some debonding
before going critical. This increases the likelihood that debonds can be detected. Having some regions of the overall bond at relatively low stress levels can significantly improve durability and reliability. • Where possible, bond to multiple surfaces to offer support to loads in any direction. Bonding an attachment to a single surface can place peel stresses on the bond, whereas bonding to several adjacent planes tends to permit arbitrary loads to be carried predominantly in shear. • Adhesives can be used in conjunction with spot welding. The process is known as weld bonding. The spot welds serve to fixture the bond until it is cured.
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Adhesive Bonding
This Figure presents examples of improvements in adhesive bonding.
Design practices that improve adhesive bonding.
(a) Gray load vectors are to be avoided as resulting strength is poor.
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Adhesive Bonding
Design practices that improve adhesive bonding.
(b) Means to reduce peel stresses in lap-type joints.
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Adhesive Bonding
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