TOFD Presentation

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“

Application of Time

-

---

Of

-

---

Flight

Diffraction (TOFD) for Weld

Inspections

”

Michael Moles Michael Moles

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Presentation Outline

  HistoryHistory 

 What is TOFD?What is TOFD?



 Advantages of TOFDAdvantages of TOFD



 Limitations of TOFDLimitations of TOFD



 Typical imagesTypical images



 Defect depth measurementDefect depth measurement



 CodesCodes



 Some TOFD examplesSome TOFD examples



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History

 Developed in UKAEA Harwell in ~70’s  Manufactured commercial Zipscan

 Used very effectively in nuclear PISC II and

DDT trials

 => Showed good detection and excellent

sizing

 Recently “adopted” by petrochemical and

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DDT Defect Sizing using Ultrasonics

Comparison of measured vs. actual defect sizes: left, all UT techniques; right, TOFD only. (DDT Plate 1)

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What is TOFD?

 Time-Of-Flight Diffraction (TOFD) relies

on the diffraction of ultrasonic energies from 'corners' and 'ends' of internal

structures (primarily defects) in a component being tested.

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TOFD: Typical Setup

Transmitter Receiver

Lateral wave

Upper tip Lower tip

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TOFD Waves

 The Lateral wave: A sub-near-surface

longitudinal wave generated from the wide beam of the transducer.

 The Backwall reflection: A longitudinal wave

reflected from the back wall

 The Reflected wave: A longitudinal wave

reflected by a lamellar planar defect

 The Tip Diffracted wave: A circular longitudinal

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Advantages of TOFD

 Good midwall defect detection.

 Accurate sizing of defects using the time of

arrivals of diffracted signals.

 Defect detection even if defects are mis-oriented

or located away from the weld centreline.

 Very rapid linear scanning (raster scanning not

required)

 Non-amplitude scanning and detection.

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Limitations of TOFD

 Dead zone at top surface (OD).

 Dead zone at bottom surface (ID).

 Sensitive to very small defects with a risk of

false calls if not combined with pulse echo.

 Analysis can be difficult; specialist

interpretation required.

 Some sizing errors possible from lateral position

of defect.

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Alternative Diffraction Arrangements

 Shear-shear diffraction

 Longitudinal-shear diffraction

 Single transducer diffraction (called “back

diffraction” or the “tip echo method” in Japan)

 Twin transducer TOFD with both

transducers on the same side of the defect/weld.

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Typical TOFD Display

•Gray scale and rf for phase info. •OD and ID visible •Defects detectable in middle •L-wave display only (usually)

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Defect Depth

Transmitter Receiver S _S d t₀ t₀

(

)

2 2 0 2

2

S t t c d



• −

−











=

Signals NOT linear; either correct, or remember

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Defect Analysis with Cursors

A-scan D-scan Cursors t₁ t₂ l P

t₁,t₂⇒d₁, d₂and h are automatically calculated

d₁

d₁ h

Build-in calculator

Use cursors on top and bottom of defect (note phase changes)

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TOFD Codes

 Two European “guidelines”: BS7706 and

EN583_6.

 ASTM E-2373-04 now available

 ASME working on TOFD codes – due out

soon

 ASME TOFD Interpretation Manual

(Nonmandatory App. N) due out very soon.

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Examples

Examples –

– Inside Surface

Inside Surface--breaking Defect

breaking Defect

Transmitter _{Lateral wave} Receiver

LW

Tip

Back wall echo blocked

No back wall echo

1

2 3

No, or reduced, backwall echo

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Example

Example –

– Near

Near--Surface Breaking Defect

Surface Breaking Defect

Transmitter Receiver Crack tip Back-wall reflection B W Lateral wave is blocked No Lateral wave 1 2

Lateral wave blocked

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Example

Example –

– Midwall Defect

Midwall Defect

1 2

3 4

No break in lateral wave or backwall

Top and bottom signals visible (if defect deep enough) Can measure lengths using hyperbolic signals

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Typical Defect for TOFD

TOFD is very “strong” on midwall defects where pulse echo is weak

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Example

Example –

– Lack of Root Penetration

Lack of Root Penetration

1 2 3 •Sometimes see break in backwall signal; should see perturbation •Defect can be sized using time-of-arrival

•Similar to other root defects

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Example

Example –

– Lack of Sidewall Fusion

Lack of Sidewall Fusion

1 2 3 4 •Should see no perturbations in lateral wave or backwall

•In this case, top signal is “buried” in lateral (OD) wave

•Can size if signals clear.

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Example

Example -- Porosity

Porosity

1 2

3

Multiple small reflectors, each with hyperbolic

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Example

Example –

– Transverse Cracks

Transverse Cracks

1 2

3

Transverse cracks are rare, and similar to porosity No perturbation of lateral or backwall

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Example

Example –

– Interpass Lack of Fusion

Interpass Lack of Fusion

Transmitter Lateral Receiver

wave L W Back wall reflection B W Reflection echo Reflected signal 1 2 3

Strong signal, but defect benign. Cannot size depth.

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TOFD

TOFD –

– What can go wrong?

What can go wrong?

 TOFD scans can go wrong, just like any

NDE inspection. Some reasons:

– Incorrect gain levels (too high or too low) – Incorrect gate positions

– Incorrect transducer separation – Poor coupling

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TOFD Equipment

 New portable system

reduces TOFD cost significantly

 Needs encoded data

e.g. handscanner or automated system

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TOFD Advantages

 Excellent PoD for mid-wall defects

 Good detection of mis-oriented defects  Can characterize surface-breaking defects

 Excellent sizing for defects in transverse TOFD

mode, especially with signal processing

 Tolerable sizing for defects in linear mode

 Works very well in conjunction with pulse-echo  Rapid (and relatively low cost) inspections

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TOFD Limitations

 Dead zone of ~3mm at outer surface  Potential dead zone at inner surface  Prone to “noise”

 Over emphasizes some benign defects, e.g.

porosity, laminations, interlamellar LoF

 Hard to interpret

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A Few Final Words on TOFD

 Good for detection, especially midwall  Much the best defect sizing technique

available when correctly set-up

 Always use in conjunction with pulse-echo

for code and PoD reasons

 Well worth including in inspections, even

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