TOFD - By Nicholas Bublitz - USA

TOFDTime of Flight Diffraction

By: Nick Bublitz

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Diffraction

Each point of the defect generates new elementary spherical waves called diffraction

The incoming wave vibrates the defect.

Based on Huygens’ principal

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Waves

FLAW

Diffractedwaves

Diffractedwaves

Incidentwave

Reflectedwave

All directions

Low energy

Independent of incidence angle

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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

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How it Happens

defined exit point60 deg

beam spread in shoe

beam spread in material

center beam

shear wave beam spread

shear wave component appro. ½ long. center beam

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Beam SpreadConventional 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.

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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)

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Beam Spread in Material-60 degree (refracted)

2 MHz- 29.4-90 deg3 MHz- 38.3-90 deg 4 MHz- 43-90 deg5 MHz- 45.9-90 deg

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Modes of travelSome waves may go all the way as LWSome will go part way as LW, part way as SWSome 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.

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The display

LW

volume of material

BW

mode-converted area

mode converted bw

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A-Scan SignalsTransmitter Receiver

Lateral wave

LW

Upper tip Lower tip

Back-wall reflection

BW

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The Lateral WaveThe 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.

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The LW BackwallCombination of reflected/diffracted energy.Strong signal

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Mode Converted signalsOccur after LW Backwall due to slower speed.Strong signals typicallyNot used for measurement of defects as velocity is uncertain.Near surface defects may be more resolvable here due to more spatial resolution.

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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).

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The EffectIf the wave starts in a positive cycleand hits a vertical defect-1. 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 changeand remains like the lateral wave (+)

Slag and porosity are often too thin to produce separate top and bottom signals.

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Data Visualization

D-scan

Uppersurface

Back wall

A-scanLW

BW

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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.

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Data Visualization

White+

Black-

Amplitude

Time

Time

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

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Typical TOFD Scans2 typical TOFD scans used-– nonparallel scans-

ultrasonic beam is perpendicular to the scan axis-most common

– parallel scan-ultrasonic beam is parallel to scan axis

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Non-parallel scan

SDHNotch

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

ViewScan axis

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Non-Parallel

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Non-parallel ScansLocate flawsDetermine depthlength in scan axisnot the highest precision for TW height measurementsrapid, easiest to employ, especially with weld capsProbes usually centered around area of interest in index axis- weld etc

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Parallel Scan

Notch top of plate

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Parallel Scansprecision TW height determinationwidth assessmenttiltlateral positioning-amplitude will be greatest when flaw is directly center of probes

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The View-B-scan

View Scan axis

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Other Types of ScansDouble Skip-used when there are problems resolving near surface defects-skipping off backwall

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Other Types of ScansOff-axis Scans-Non-parallelIf 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.

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Other Types of ScansManual scanning- unencodedUsed only when is only resort typicallyNegatives-

1.Sampling interval not constant-operator left to try to match PRF - works best

in teams with area marked out at intervals-no better than +/- 5mm accuracy should be expected

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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 scanningDetection and sizing almost orientation independent.Based on TOF so avoid common amplitude technique sizing errorsTOFD has a potential through-wall dimension accuracy of ±1 mm and monitoring capability of ±0.3 mm Setup virtually independent of weld configurationWide coverage area

TOFD Advantages

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Blind areas :– near surface 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 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 = TrainingLack 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

TOFD Limitations

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Ellipse of Equal Time

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

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Lateral Positioning

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Defect Position Uncertainty

Transmitter ReceiverS S

t2t1

dmin dmax

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.

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Missed off-axis flaws

missed or obscured bylarge bwsignal

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The Non-Linearity of DepthWith 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.

LW time 50us

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3

4

Total Time S to R.99beam 1=50.99us2.85beam 2= 53.85us4.45beam 3= 58.3us5.73beam 4= 64.03us

S R Depth in Time

5 us10 us

15 us

20us

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Main Fact-the deeper in thematerial=less error

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Recommended Solution

TOFD: YESBUT: do not forget the good things offered by the standard Pulse-Echo techniqueSOLUTION: do both TOFD and PE simultaneously, without reducing the scan speedPulse Echo channels can focus on the cap and root, TOFD’s weak spot!

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Recommended Solution : PV-100

The system allows for simultaneous acquisition and analysis (inTomoview

only) of TOFD and PE

TOFDPE 45° SW PE 60° SW

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PV100-Tomoview

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PV-100

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Multiple Tofd

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Scanner/Fixture NecessitiesAbsolute-

1. good contact with surface2. control of PCS3. able to scan straight lineGood features1. magnetic wheels for ferrous mat.2. preamp3. couplant feeds4. rulers for adjustment of PCS5. adjustable for curved surfaces6. umbilical wiring7. probes/wedges individually sprung and gimballed8. laser or other guides

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Single Tofd Pair

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Multiple Tofd or Tofd/PE

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Tofd and Phased Array

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

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

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AnalysisNormal characterization-

1. position in scan axis2. length3. depth and height4. type- (surface/bottom breaking or

embedded)

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

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

Flaw

diff. signals

signal

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

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Parabolic CursorsTo acquire defect length the cursors are often placed to fit the natural curve of the defect to eliminate the effectsof beam spread. Fromthe cursors defect startand length can bedetermined.

position of ref.cursor in scan axis-flaw start

dist. between two cursors-length in scan axis

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Parabolic CursorsUsing phase information, the cursors can also be positionedat 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

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

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

Some Typical Defects-non-parallel scans

Upper surface breaking crackBack Wall breaking crackHorizontal planar defect

TOFD Typical Defects

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

Transmitter ReceiverLateral wave

LW

Back wall reflection

BW

Reflection echo

Reflected signal

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Upper Surface Breaking CrackTransmitter Receiver

Crack tip

Back-wall reflection

BW

Lateral wave is blocked

No Lateral wave

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Back Wall Surface Breaking CrackTransmitter Receiver

Lateral wave

LW

Tip

Back wall echo blocked

No back wall echo-not always the case!

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Near Surface Crack

The crack blocks the Lateral WaveAnd the lower tip appears on the A-scan

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1

2

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Incomplete Root Penetration

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Note the two signals from the top & bottom

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3

4

1 2 3 4

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Lack of Root Penetration

Note the inverted phase between LW and defect

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1

23

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Lack of Fusion - Side Wall

Note the two signals from the top & bottom

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4

1

2

3

4

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Porosity

Porosity may image in many forms whether individual or cluster

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3

1

2

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Transverse Crack

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

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2

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1

2

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Concave Root

Distortion of back-wall echo

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1

2

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Lack of Fusion - Interpass

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Setting up TOFD

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Determine PCS2 d Tan ( theta ) to focus at a determined point4/3 d Tan (theta) to focus at 2/3 dd= depth of focus or part thicknesstheta = refracted angle of wedgeex: 25 mm butt-weld-focus at center using 70 degree wedges

2 (12.5) (Tan 70)= 69mm PCSfocus at 2/3 in the material-4/3 (25) (Tan 70)= 92 mm PCS

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PCS-Probe Center SeparationDistance between exit points of send and receive probes

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How is It Determined?

4/3 D x Tan theta

General rule- for linear joints focus at 2/3 total T.D= total part T

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T

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

D=where want to focus

2D Tan θ

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Effect of PCSIn general a wider PCS results in more coverage ofthe material but can lead tomissed areas- impropercalculations can be detrimental to coverage!

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Effect of Angle

From Charlesworth & Temple

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.

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

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Calculator

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

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

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Choosing Probes-Frequency and depth resolutionmore cycles within the time frame between LW and BW=better depth resolutiongeneral rule- aim for 20 or better cycles (30 and over optimum)positive-increased frequency= increased cyclesnegative-beam spread reduced, attenuation/scatter increased

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T in mmfocus 2/3 T

LW-BW time us

1 MHz# cycles

3 MHz# cycles

5 MHz# cycles

10 MHz# cycles

20 MHz# cycles

10 1.25 1.3 3.8 6.3 12.5 25.1

25 3.13 3.1 9.4 15.7 31.3 62.7

50 6.265 6.3 18.8 31.3 62.7 125.3

100 12.53 12.5 37.6 62.7 125.3 250.7

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Probe examplesPCS 84 mm

PCS 84 mm

narrow bs

big beam spread

10 MHz 15mm dia

3 MHz 6mm dia

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Effect of Frequency on Beam Spread

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Effect of Crystal Diameter on Beam Spread

3mm dia

6mm dia

12mm dia

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Effect of Wedge Angle

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Comparing Wedge Angles

45 60 70In general, the higher thewedge angle, the morethe time scale will becompressed.In general a higher wedge angle givesmore coverage.

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Setting FiltersGeneral ruleHigh pass- ½ FrequencyLow pass- 2x Frequency

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RangeGeneral Rule-leave at least 1 us before LWleave 1 us after mode-converted back wall

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Digitization FrequencyHigher = better if possibleAbsolute minimum 2x Frequency (10mhz x2=20mhz)

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

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PRFLook for ghost echoesLook at trace before lateral wave, should be flat or near flat or prf may be to highOn Omniscan-optimum is usually adequate

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Sampling interval/Encoder Resolution

1mm typical- 2 or 3mm to reduce data file sizes if necessary for long scans

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Averaging32 maximum realistic value8 or 16 usually betterAveraging can increase SNR

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Pulse WidthGeneral Rule- LW = 1 ½ cycles, maximum 2

On OmniScan, optimum is usually adequate

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Sensitivity/gainGeneral rulesLW to 20- 50% FSH

backwall to 100% FSH +10dbSensitivity 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

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Using Calibration Blockusing 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.

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

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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-

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Reference Good A-scanCursor positioned in area free of flaws for reference LW and BW.

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Input Test Parameters

1.PCS2.Thickness3.Geometry

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Input and Train Top and BottomMany 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

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Input and Train Top and Bottom

measure cursor positioned at –peak of BWsoftware told this is this thickness

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Calibration performed

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No Signal?Check gainCheck couplant in and under shoesCheck Cables-undamaged, connected correctlyCheck orientation- wedges facing each other Check flaw detector circuitsCheck preamp (if used)- make sure on and connected correctly-check battery

By: Nick Bublitz

Tofd Demo on Sonaspection Plate 2.0 SW OmniScan

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Required EquipmentOmniScan (16:16,16:64,16:128, 2/4/8 channel) w/ Tofd option and 2.0R2 SWTofd 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 batteryBNC-Lemo adapters (2) as demonstratedCouplant- water for plate, grease for wedge as demonstratedEncoder- Old mini-encoder used

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Setting up Equipment

turn on

receive probe

send probeencoder

preamplifier

flaw detector

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Boot up and Select UTTurn the OmniScan on, start the SW and if necessary switch from PA to UT from the menu

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Access the Wizard

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Start the Group Wizard

choose modifyand Next

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Set Part Parameters

Flatpart T

Material

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Connection Type

Tofd inspection

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Choose Probe

angle beam type

then probe 1

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Choose Wedge

Tofd type

then wedge

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2 3

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Position

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

choose non-parallel

set PCS- distancepin to pin of wedges-general demo on .5”29-35 mmas cap allows

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Finish!

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Set rangecouple wedges flat on base metal-can remove encoderto aid

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

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3

2

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

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Overlays

if necessary turn off gates and cursors

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Adjust Gain

adjust gain so LW is20-50% FSH

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2

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Adjust Pulser

if necessary adjust voltageand PRF/PW- signal before LWshould be relatively flat

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Adjust Receiver

1

2

choose filtering ifneeded- auto or 5MHz probably sufficient

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Setup Encoder

1

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scan length and resolutionone line/encoder 1

3 5

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Set Polarity

move direction want to scan and make sure getting larger

change here if needed

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Calibrate Encoder

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Set Origin

position at 0 point andhit next

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Set Distance

set distance going to move, move that distancethen hit calibrate

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Accept or Restart

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

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Scan the PlateApply couplant over the plateApply the scanner in proper orientationhit start acquisition scan the plate and freeze data

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Turn Analyze Aids onTurn Cursors back on

Turn readings on

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Calibrate

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Select Reference A-scan

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

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Input Parameters-As needed

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2

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Set Cursor Position and Depth

1 move ref. cursor tofirst + cycle of LW

2-input 0 fortop surface

3 move meas. cursor tofirst - cycle of BW

4-input T forback

surface5

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Accept or Restart

1

view vel /delay

2 Accept or restart