TOFD Presentation

March 8, 2019 | Author: nirmalmthp | Category: Ultrasound, Diffraction, Waves, Welding, Applied And Interdisciplinary Physics
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“Application of Time-Of -Flight Diffraction (TOFD) for Weld Inspections” Michael Moles

Presentation Outline History  What is TOFD?  Advantages of TOFD  Limitations of TOFD  Typical images  Defect depth measurement  Codes  Some TOFD examples  Summary 

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 other industries 

DDT Defect Sizing using Ultrasonics

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

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.

TOFD: Typical Setup

Transmitter

Receiver   Lateral wave Upper tip Lower tip

Back-wall reflection

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 (or L-wave) diffracted by the edge of a defect.

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. Set-up independent of weld configuration.

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.



Low signal-to-noise ratio.

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.  Complex inspections, e.g. nozzles. 

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

Defect Depth S

S Receiver 

Transmitter 

t0

t0

d

Signals NOT linear; either correct, or remember

2

 c  2 2 d  =   • (t  − 2t 0 ) − S   2 

Defect Analysis with Cursors A-scan d1 h

d1 t1

t2

Cursors Build-in calculator l t1,t2 ⇒ d1, d2 and h are automatically calculated

P D-scan

Use cursors on top and bottom of defect (note phase changes) Use calculator or math for analysis.

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.  ASME CC 2235 permits TOFD 

Examples – Inside Surface-breaking Defect 1

Transmitter 

Lateral wave

Receiver 

2 3

Back wall echo blocked LW

Tip

No back  wall echo

No, or reduced, backwall echo Signal from tip for detection and sizing

Example – Near-Surface Breaking Defect

Transmitter 

Lateral wave is blocked

Receiver  1 2

Back-wall reflection B W No Lateral wave

Crack  tip

Lateral wave blocked Sizing by measuring crack tip

Example – Midwall Defect No break in lateral wave or backwall Top and bottom signals visible (if defect deep enough) Can measure lengths using hyperbolic signals

1 2 3 4

Typical Defect for TOFD

TOFD is very “strong” on midwall defects where pulse echo is weak  Strengths of TOFD complement PE, and

vice versa

Example – Lack of Root Penetration

1

•Sometimes see break in backwall signal; should see perturbation

2 3

•Defect can be sized using timeof-arrival •Similar to other root defects

Example – 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.

Example - Porosity

1

2

3

Multiple small reflectors, each with hyperbolic tails. Usually can characterize, but sizing difficult.

Example – Transverse Cracks

1 2

3

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

Example – Interpass Lack of Fusion

Lateral wave

Transmitter 

Receiver 

Reflected signal Back wall reflection B W

L W Reflection echo

1 2

3

Strong signal, but defect benign. Cannot size depth.

TOFD – 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  – Noise interference

TOFD Equipment 

 New portable system reduces TOFD cost significantly



 Needs encoded data e.g. handscanner or  automated system

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

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 



Finally we have a TOFD code!

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 with “company specs”.

Thank you

Any questions?

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