TOFD - By Nicholas Bublitz - USA

TOFD Time of Flight Diffraction By: Nick Bublitz

Diffraction Based on Huygens’ principal

The incoming wave vibrates the defect. Each point of the defect generates new elementary spherical waves called diffraction 2

Waves Incident wave

Diffracted waves All directions Low energy

Reflected wave

FLAW

Diffracted waves 3

Independent of incidence angle

Diffraction – – – – – – – – –

Modification or deflection of sound beam Sound striking defect causes oscillation Ends of defect become point sources Not related to orientation of defect Weaker signal than reflected – needs higher gain or amplification (preamp on receive side) Sharp defects provide best emitters Tip signals are located accurately Time of flight of tip signals used to size Wide beam longitudinal

4

How it Happens beam spread in material beam spread in shoe

defined exit point

60 deg

shear wave component appro. ½ long. center beam

center beam

shear wave beam spread

5

Beam Spread ‹ Conventional

theory only focuses on dominant frequency. ‹ In reality differences in wavelength occur due to the range of frequencies produced by a single probe. (bandwidth) ‹ Beam spread can be recalculated using lower frequency component. ‹ The presence of a lateral wave for 45 degree and above can be justified. 6

Rough Examples 60 Degree Wedge (2.7mm/us, 6mm crystal @ 5 MHz

Beam spread in wedge- center beam 23.3 deg. (incident) 2MHz +/- 10.3 deg. (13-33.6deg) 3 MHz +/- 6.8 deg. (16.5-30.1 deg) 4 MHz +/- 5.1 deg. (18.2-28.4 deg) 5 MHz +/- 4.1 deg. (19.2-27.1 deg)

7

Beam Spread in Material-60 degree (refracted)

‹2

MHz- 29.4-90 deg ‹ 3 MHz- 38.3-90 deg ‹ 4 MHz- 43-90 deg ‹ 5 MHz- 45.9-90 deg

8

Modes of travel ‹ ‹ ‹ ‹

Some waves may go all the way as LW Some will go part way as LW, part way as SW Some will go all the way as SW This is why we use LW!- LWs are about twice as fast as SW so they are sure to get to the receive probe first. We never use the mode-converted area for depth measurements as we are unsure of how those waves traveled.

9

The display

LW BW mode converted bw

volume of material

mode-converted area

10

A-Scan Signals Transmitter

Receiver

Lateral wave

Back-wall reflection BW

LW

Upper tip

Lower tip 11

The Lateral Wave ‹ The

lateral wave- compression velocity, arrives first, for curved surfaces will travel straight across the metal. Not a true surface wave, but a bulk wave generated at the edge of the wide beam generated by the send transducer. Frequency content tends to be lower than the center of the beam. Becomes weaker with increased PCS.

12

The LW Backwall ‹ Combination ‹ Strong

of reflected/diffracted energy.

signal

13

Mode Converted signals ‹ ‹ ‹

‹

Occur after LW Backwall due to slower speed. Strong signals typically Not used for measurement of defects as velocity is uncertain. Near surface defects may be more resolvable here due to more spatial resolution.

14

Why RF? ‹ To

observe phase shifts to observe “tips” ‹ A wave traveling in a higher acoustic impedance material will shift 180 deg in phase when it is reflected at an interface of lower impedance. (ex: steel to air).

15

The Effect If the wave starts in a positive cycle and hits a vertical defect1. the wave from the top tip acts like energy reflected off the bw and changes phase 180 deg. (-) 2. the wave from the bottom of the defect acts like it “runs around” the bottom without a phase change and remains like the lateral wave (+) Slag and porosity are often too thin to produce separate top and bottom signals. 16

Data Visualization LW

A-scan

D-scan

Upper surface 17

BW

Back wall

Why Grayscale? ‹ Typical

pulse echo techniques often associate a full color scale based on % FSH amplitude with red being highest amplitude because it is a natural attention getter for people. ‹ Since TOFD does not rely on amplitude but rather TOF, we want to take out an natural preference of color and view each defect individually. 18

Data Visualization Amplitude +

White

Time

-

Black

Time One A-scan picture is replaced by one gray-coded line 19

Typical TOFD Scans ‹2

typical TOFD scans used– nonparallel scansultrasonic beam is perpendicular to the scan axis-most common – parallel scan-ultrasonic beam is parallel to scan axis

20

Non-parallel scan

SDH

Notch

21

The View-D-scan (Omniscan=B-scan)

a c S

is x na

View

22

Non-Parallel

23

Non-parallel Scans ‹ Locate

flaws ‹ Determine depth ‹ length in scan axis ‹ not the highest precision for TW height measurements ‹ rapid, easiest to employ, especially with weld caps ‹ Probes usually centered around area of interest in index axis- weld etc 24

Parallel Scan

Notch top of plate

25

Parallel Scans ‹ precision

TW height determination ‹ width assessment ‹ tilt ‹ lateral positioningamplitude will be greatest when flaw is directly center of probes

26

The View-B-scan

Scan axis

w e i V

27

Other Types of Scans ‹ Double

Skip-used when there are problems resolving near surface defects-skipping off backwall

28

Other Types of Scans ‹ ‹

Off-axis Scans-Non-parallel If a near surface flaw lies close to one probe in a normal centered non-parallel scan, the signal from the defect can have a short time delay from the LW, causing poor resolution. Resolution can often be improved by performing off-axis scan. Depth measurement may become less accurate depending on positioning.

29

Other Types of Scans ‹ Manual

scanning- unencoded ‹ Used only when is only resort typically ‹ Negatives1.Sampling interval not constantoperator left to try to match PRF - works best in teams with area marked out at intervalsno better than +/- 5mm accuracy should be expected 30

TOFD Advantages ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾

Permanent data recording with B-scan type imaging (side view) Accurate sizing capability (height)-through-wall height most critical for fracture mechanics. Technique allows for rapid scanning Detection and sizing almost orientation independent. Based on TOF so avoid common amplitude technique sizing errors TOFD has a potential through-wall dimension accuracy of ±1 mm and monitoring capability of ±0.3 mm Setup virtually independent of weld configuration Wide coverage area

31

TOFD Limitations ‹

Blind areas : – near surface Æ Width of the lateral wave and timing error (can be reduced by reducing PCS, using higher frequency, using highly dampened broadband probes, and software tools (lateral wave removal). – back wall Æ Large signal from reflected energy

‹ ‹ ‹ ‹ ‹ ‹

Off axis flaws at the back wall can be missed (can be reduced by off center scans and wider PCS) Flaw classification limitation (some cases)-no simple amplitude criteria Sensitive to grain noise (frequency and material flaws) If not experienced user, analysis can be difficult = Training Lack of index positioning (non-parallel scans: can be compensated by parallel scans or pulse echo prove up) Parent material indications can be perceived as weld indications 32

Ellipse of Equal Time

Ellipse of Equal Time- defects at either location plot the same time-exists for anywhere on the ellipse 33

Lateral Positioning

34

Defect Position Uncertainty S

S Receiver

Transmitter

dmin dmax

t1

t2

In practice: Maximum error on absolute depth position lies below 10 %. Error on height estimation of internal (small) defect is negligible. Caution for small defects situated at the back wall. 35

Missed off-axis flaws

36

missed or obscured by large bw signal

The Non-Linearity of Depth With a constant 5us change in depth, (expressed here in time), we can observe the total time from transmit to receive does not hold constant. This causes a distortion of depth on our display, indications appear to be much closer to the surface then were they lie in reality. Calibrations for true depth determination are critical.

Total Time S to R .99 beam 1=50.99us 2.85 beam 2= 53.85us 4.45 beam 3= 58.3us 5.73 beam 4= 64.03us

S

LW time 50us 1 2

R

Depth in Time 5 us 10 us

3 15 us 4 20us

37

Main Factthe deeper in the material=less error

38

Recommended Solution ‹ TOFD:

YES ‹ BUT: do not forget the good things offered by the standard Pulse-Echo technique ‹ SOLUTION: do both TOFD and PE simultaneously, without reducing the scan speed ‹ Pulse Echo channels can focus on the cap and root, TOFD’s weak spot!

39

Recommended Solution : PV-100

PE 45° SW

TOFD

PE 60° SW

The system allows for simultaneous acquisition and analysis (inTomoview only) of TOFD and PE 40

PV100-Tomoview

41

PV-100

42

Multiple Tofd

43

Scanner/Fixture Necessities Absolute1. good contact with surface 2. control of PCS 3. able to scan straight line Good features 1. magnetic wheels for ferrous mat. 2. preamp 3. couplant feeds 4. rulers for adjustment of PCS 5. adjustable for curved surfaces 6. umbilical wiring 7. probes/wedges individually sprung and gimballed 8. laser or other guides ‹

44

Single Tofd Pair

45

Multiple Tofd or Tofd/PE

46

Tofd and Phased Array

47

Couplant Considerations ‹ Water

usually best- uniform ‹ Couplant feeds allow even application while scanning-holes bore in bottom of wedge for flow- IHC (irrigation, holes, carbides)

48

Wedge Considerations ‹ To

prevent wear, carbide pins useful on bottom of wedge (gap scanning) ‹ typical- .2mm ‹ Gaps of ¼ and ½ wavelength should be avoided to avoid interference effects

49

Analysis ‹ 1. 2. 3. 4.

Normal characterizationposition in scan axis length depth and height type- (surface/bottom breaking or embedded)

50

Defect Shape ‹ Due

to beam spread, many defects often have a curved look.

51

Flaws Parallel to the Surface ‹ Flaws

relatively parallel to the surface will have a minimum time delay when the probes are directly in line with the flawparabolic shape will be seen as probes approach and leave the flaw as seen previously

Flaw diff. signals signal 52

Parabolic Cursors To aid in measuring the defect as if the beam is a point source, parabolic (curved) cursors are often used. After calibration these are available to fit the parabolic shape of the flaw to eliminate/reduce the effect of the beam spread in the scan axis.

53

Parabolic Cursors To acquire defect length the cursors are often placed to fit the natural curve of the defect to eliminate the effects of beam spread. From the cursors defect start and length can be determined. position of ref. cursor in scan axis-flaw start 54

dist. between two cursors-length in scan axis

Parabolic Cursors Using phase information, the cursors can also be positioned at the highest amplitude response from each tip- for depth and through wall height information. position of ref. cursor in ut axis (depth to top of flaw)

distance between two cursors in ut axis- height 55

Surface Breaking Defects ‹ Since

only one tip will be present in surface breaking defects, the back wall or LW can be used as second reference point.

56

Parabolic Cursors ‹ Indications

that exactly fit the curve of the parabolic cursors are normally regarded as having little to no length-small gas pores, grain responses, etc

57

Some Typical Defects-nonparallel scans ‹Upper

surface breaking crack ‹Back Wall breaking crack ‹Horizontal planar defect

TOFD Typical Defects

Horizontal Planar Defect (Lack of Inter-Run Fusion, Laminations) Receiver

Transmitter

Lateral wave Reflected signal

Back wall reflection BW

LW

Reflection echo 60

Upper Surface Breaking Crack Transmitter

Lateral wave is blocked

Receiver

Back-wall reflection BW No Lateral wave

Crack tip 61

Back Wall Surface Breaking Crack Receiver

Transmitter

Lateral wave

Back wall echo blocked LW

Tip 62

No back wall echonot always the case!

Near Surface Crack

1 2

2

1

The crack blocks the Lateral Wave And the lower tip appears on the A-scan 63

Incomplete Root Penetration 1

2 3 4

1

2

4 2

1 3

Note the two signals from the top & bottom 64

Lack of Root Penetration 1

1 2 3

2 3

Note the inverted phase between LW and defect 65

Lack of Fusion - Side Wall 1 1

2

2 3

3

4

4

Note the two signals from the top & bottom 66

Porosity 1 1

2

2

3

Porosity may image in many forms whether individual or cluster 67

Transverse Crack 1 1 2 1

2 3

2

4

3

3

In the LW we can observe the wide beam effect on the crack

68

Concave Root 1 1

2 2 3

3

Distortion of back-wall echo 69

Lack of Fusion - Interpass

1 2 3

70

Setting up TOFD

Determine PCS ‹2

d Tan ( theta ) to focus at a determined point ‹ 4/3 d Tan (theta) to focus at 2/3 d ‹ d= depth of focus or part thickness ‹ theta = refracted angle of wedge ‹ ex: 25 mm butt-weld‹ focus at center using 70 degree wedges 2 (12.5) (Tan 70)= 69mm PCS focus at 2/3 in the material4/3 (25) (Tan 70)= 92 mm PCS 2

PCS-Probe Center Separation ‹ Distance

between exit points of send and receive probes

3

How is It Determined? General rule- for linear joints focus at 2/3 total T. D= total part T 4/3 D x Tan theta 2 3 T

4

Focus at Point of Interest 2D Tan θ

‹ Used

when expected indications are at predetermined location (ex: root) or multiple Tofd pairs to cover large volumes.

‹ D=where

want to focus

5

In general a wider PCS results in more coverage of the material but can lead to missed areas- improper calculations can be detrimental to coverage!

Effect of PCS

6

Effect of Angle The amplitude response from the bottom and top tip of a vertical crack varies as a function of beam angle. The amplitude has the greatest response around 65 degrees. Between 45 and 80 degrees the amplitude difference is less than 6 db. Notice around 38 deg. the signal from the bottom crack tip drops off in amplitude greatly. 7

From Charlesworth & Temple

Optimum Upper tip q ≈ 64° Optimum Lower tip q ≈ 68°

Calculator

TOFD probe separation can be calculated with basic mathematical formula or Excel calculator tools 8

Choosing Probes- Give and Take-Overview ‹

‹

‹

‹

Highly dampened- to reduce ring time and increase near surface resolution Frequency- lower frequency will give more beam spread for detection with less resolution. Higher frequency probes attenuate faster. Increased frequency= more cycles occur= better depth resolution Diameter- smaller crystals will create more beam spread, but again beam intensity is less. A number of guides are available

9

Choosing Probes-Frequency and depth resolution ‹

‹

‹

‹

more cycles within the time frame between LW and BW=better depth resolution general rule- aim for 20 or better cycles (30 and over optimum) positive-increased frequency= increased cycles negative-beam spread reduced, attenuation/scatter increased 10

1.25

1 MHz 3 MHz 5 MHz 10 MHz # # # cycles cycles cycles # cycles 1.3 3.8 6.3 12.5

20 MHz # cycles 25.1

25

3.13

3.1

50 100

T in mm focus 2/3 T 10

LWBW time us

9.4

15.7

31.3

62.7

6.265 6.3

18.8

31.3

62.7

125.3

12.53 12.5

37.6

62.7

125.3 250.7

11

Probe examples PCS 84 mm

10 MHz 15mm dia

narrow bs

PCS 84 mm

3 MHz 6mm dia big beam spread 12

Effect of Frequency on Beam Spread

13

Effect of Crystal Diameter on Beam Spread

3mm dia

12mm dia

6mm dia 14

Effect of Wedge Angle

15

Comparing Wedge Angles In general, the higher the wedge angle, the more the time scale will be compressed. In general a higher wedge angle gives more coverage.

45

16

60

70

Setting Filters ‹ General

rule ‹ High pass- ½ Frequency ‹ Low pass- 2x Frequency

17

Range ‹ General

Rule‹ leave at least 1 us before LW ‹ leave 1 us after mode-converted back wall

18

Digitization Frequency ‹ Higher

= better if possible ‹ Absolute minimum 2x Frequency (10mhz x2=20mhz) ‹ Ideal

minimum 5x frequency (10 MHz x 5 = 50 MHz) ‹ 100 MHz if possible 19

PRF ‹ Look

for ghost echoes ‹ Look at trace before lateral wave, should be flat or near flat or prf may be to high ‹ On Omniscan-optimum is usually adequate

20

Sampling interval/Encoder Resolution ‹ 1mm

typical- 2 or 3mm to reduce data file sizes if necessary for long scans

21

Averaging ‹ 32

maximum realistic value ‹ 8 or 16 usually better ‹ Averaging can increase SNR

22

Pulse Width ‹ General

Rule- LW = 1 ½ cycles, maximum 2

‹ On

OmniScan, optimum is usually adequate 23

Sensitivity/gain ‹ General

rules ‹ LW to 20- 50% FSH ‹ backwall to 100% FSH +10db ‹ Sensitivity not related to size of defect-FBH and SDH should be avoided-signal from FBH is simply related to area and signal from SDH will give two signals-reflection from top of hole and creeping wave that runs around bottom of hole.-stnds /guides differ on the proper targets to use for calibration 24

Using Calibration Block ‹ using

series of thin slots and setting response from bottom (BS 7706) Slot must be upper surface breaking- signal from top largely reflected, while bottom gives response similar to fatigue crack. Slots often 1/3 and 2/3 T. Block should be similar material and T. ‹ Using slots at varying depths covering material T. Gain set on deepest slot so signal is 60% FSH. 25

Using grass/noise ‹ Setting

gain on calibration block, then increasing gain until noise is 5% FSH in between LW and BW.

26

Velocity and Wedge Delay Calibration ‹ Unlike

most UT inspections, the calibration of velocity and wedge delay are performed after acquisition. ‹ Normally carried out by telling the inspection system three things-

27

Reference Good A-scan ‹ Cursor

positioned in area free of flaws for reference LW and BW.

28

Input Test Parameters

1.PCS 2.Thickness 3.Geometry

29

Input and Train Top and Bottom Many conventions- key is consistency! One popular method is to use first + peak of lateral wave and - peak of long. BW

ref. cursor positioned on first + peak of LW software told this is 0 or Top 30

Input and Train Top and Bottom

measure cursor positioned at – peak of BW software told this is this thickness 31

Calibration performed

32

No Signal? ‹ Check

gain ‹ Check couplant in and under shoes ‹ Check Cables-undamaged, connected correctly ‹ Check orientation- wedges facing each other ‹ Check flaw detector circuits ‹ Check preamp (if used)- make sure on and connected correctly-check battery 33

Tofd Demo on Sonaspection Plate 2.0 SW OmniScan By: Nick Bublitz

Required Equipment ‹ ‹ ‹ ‹ ‹ ‹ ‹ ‹ ‹ ‹

OmniScan (16:16,16:64,16:128, 2/4/8 channel) w/ Tofd option and 2.0R2 SW Tofd scanner (HST-X04) (2) 60 degree wedges (ST1-60L-IHC) (2) 5MHz probes- (C543-SM) Weld (Sonaspection .5”) Cables- (2) Lemo-MD, (1) Lemo-Lemo as demonstrated) Preamp- (5682), 9 volt battery BNC-Lemo adapters (2) as demonstrated Couplant- water for plate, grease for wedge as demonstrated Encoder- Old mini-encoder used 2

Setting up Equipment

encoder

flaw detector

send probe

3

receive probe

preamplifier

turn on

Boot up and Select UT ‹ Turn

the OmniScan on, start the SW and if necessary switch from PA to UT from the menu 1

2 4

Access the Wizard

2

1

5

Start the Group Wizard

1 2 choose modify and Next

3 6

Set Part Parameters

Material Flat

7

part T

Connection Type

Tofd inspection

2

8

1

Choose Probe

2 3

angle beam type

then probe 9

1

Choose Wedge

2

3

Tofd type then wedge 10

1

Position

choose non-parallel

leave 0 for demo or input distance from side of plate start and scan start

set PCS- distance pin to pin of wedgesgeneral demo on .5” 29-35 mm as cap allows 11

Finish!

12

tip- if you can’t see signals, set start at 0 and range to 30us, then start adjusting start/range back and forth until dialed in

Set range couple wedges flat on base metalcan remove encoder to aid 1 2

3 adjust start and range until LW,BW, and mode converted BW are on screen with at least 1 us 13 before LW and after Mode BW

Overlays

1

2 if necessary turn off gates and cursors

14

3

Adjust Gain

1

adjust gain so LW is 20-50% FSH

2 15

Adjust Pulser

1

2

if necessary adjust voltage and PRF/PW- signal before LW should be relatively flat

16

3

Adjust Receiver choose filtering if needed- auto or 5MHz probably sufficient

1

2

17

Setup Encoder

1 2 4 3 one line/encoder 1

5

scan length and resolution 18

Set Polarity

2

move direction want to scan and make sure getting larger

1

change here if needed 19

Calibrate Encoder

1

2

4 3

20

Set Origin

1

3

2 21

position at 0 point and hit next

Set Distance set distance going to move, move that distance then hit calibrate 2

3

1 22

Accept or Restart

1

to verify: move back to zero point and look at axis position

2

23

Scan the Plate ‹ Apply

couplant over the plate ‹ Apply the scanner in proper orientation ‹ hit start acquisition ‹ scan the plate and freeze data

3 1

2 24

4

Turn Analyze Aids on ‹ Turn

Cursors back on

‹ Turn

readings on

1

2 3

4

7

5

25

6

Calibrate

1

2

3

4 26

Select Reference A-scan

Show the OmniScan a good area with no flaws, LW/BW only 2

1

27

Input Parameters-As needed

1

2

28

Set Cursor Position and Depth

1

move ref. cursor to first + cycle of LW

3

move meas. cursor to first - cycle of BW

2-input 0 for top surface

5

29

4-input T for back surface

Accept or Restart

2 Accept or restart

1 view vel /delay 30