12- Theory Tofd Fcb

TOFD Time of Flight Diffraction

TOFD Technique 

Overview



Calibration

 Applications 

Summary & Limitations

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TOFD Technique 

Overview



Calibration

 Applications 

Summary & Limitations

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TOFD Overview 

Forward Diffraction Technique



First described by Silk in 1977  – Using diffracted signals from crack tips



Traditional grey scale TOFD presentation

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 Advantages of TOFD Technique  – Wide coverage area using a pair of transducers  –  Accurate flaw sizing; amplitude-independent  – Sizing technique using time-of-flight information  – On-line volume inspection - very fast scanning  – Setup independent of weld configuration  – Sensitive to a variety of defects  – No sensitivity to defect orientation  –  Amplitude-insensitive - acoustical coupling less critical

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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  – Sharp defects provide best emitters  – Tips signals are located accurately  – Time of flight of tip signals used to size

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Diffraction

The incoming wave vibrates the defect. Each point of the defect generates new elementary spherical waves called diffraction 6/15/2013

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Waves Incident wave

Diffracted waves All directions Low energy

Reflected wave

FLAW

Diffracted waves 6/15/2013

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Independent of  incidence angle

Conventional Use of Diffraction Tip

diffraction method (satellite-pulse observation technique)

Amplitude

1 Angle

2 1

2

Tip diffraction Slot or crack  Corner reflection

Time TOF 6/15/2013

TOF, Angle and velocity

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Height

Signals  Signals

Received

 – Lateral wave  – Subsurface  – Back-wall echo  – Mode converted (shear wave) echo  Define

top and bottom of part  Note phase change

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Basic Principles of the TOFD Technique

TOFD: Typical Setup

Transmitter

Receiver  Lateral wave Upper tip Lower tip

Back-wall reflection

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 A-Scan Signals Receiver 

Transmitter 

Lateral wave

Back-wall reflection BW

LW

Upper tip 6/15/2013

Lower tip 12

Some Typical Defects Upper

surface breaking crack

Back

wall breaking crack

Horizontal

planar defect

Upper Surface Breaking Crack Transmitter 

Lateral wave is blocked

Receiver 

Back-wall reflection BW

No Lateral wave 6/15/2013

Crack tip 14

Back Wall Surface Breaking Crack Receiver 

Transmitter 

Lateral wave

Back wall echo blocked LW

Tip 6/15/2013

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No back  wall echo

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

Transmitter 

Lateral wave

Reflected signal

Back wall reflection BW

LW

Reflection echo 6/15/2013

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Data Visualization Amplitude +

White

Time

-

Black 

Time One A-scan picture is replaced by one gray-coded line 6/15/2013

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Data Visualization LW

A-scan

D-scan

Upper surface 6/15/2013

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BW

Back wall

Calibration Tools

A-scan

PCS LW T

BW

c

Parameters: PCS, Thickness, velocity, Probe delay, Lateral wave or Back wall Not all of the parameters need to be known 6/15/2013

D-scan 19

Measurement Tools d1 h

A-scan d1 t1

t2

Cursors Build-in calculator l

t1,t2  d1, d2 and h are automatically calculated

P D-scan

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Defect Position Influence S

S Receiver 

Transmitter 

t0

t0

d

x

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Defect Position Uncertainty S

S Receiver 

Transmitter 

dmin dmax

t1

t2

Constant time locus (t1+t2=ct)

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. 6/15/2013

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Transverse Scan Time will be minimum when probes are symmetrically positioned over the defect

Lateral wave

Upper surface

Back-wall B-scan This type of scan yields a typical inverted parabola

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What do TOFD scans really look like?

TOFD images show the lateral wave and backwall, plus SW signals after and reflections from all defects

Lack of Fusion

Porosity

Incomplete Penetration Slag

Source: Ginzel

What do TOFD scans really look like?

Lateral wave is clearly seen in a good TOFD scan. Typically used for calibration. On clean material, defects show up well. Backwall is always strong. Watch for perturbations.

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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

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Typical Requirements for TOFD and PE Applications 

Small, lightweight, 1 to 16 channels



PE and TOFD software



Lateral wave straightening



Real-time averaging



Multi-channel data acquisition and display

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Linearization for true depth on flat or cylindrical surfaces



Processing (data compression,..)

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Calibration

A-scan PCS LW

T

BW

c

PCS, Thickness, velocity, Probe delay, Lateral wave or Back wall

Typical multi-channel UT instrument is very user friendly and guides you with a software Wizard 6/15/2013

D-scan 28

Weld 1 (PL4882)

Toe crack 

Porosity

Lack of side wall fusion

12.5 mm

Lack of root fusion

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Weld 1 (plate 4882) with Pulse-Echo TOFD and PE clearly show the defects embedded in this weld: Lack of Fusion (root) •

Lack of Sidewall Fusion •

Porosity

•

Toe Crack 

•

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Parallel Scan LW

A-scan

D-scan

Upper surface 6/15/2013

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BW

Back-wall

Recommended Solution

PE 45 SW

TOFD

PE 60 SW

The system allows for simultaneous acquisition and analysis (inTomoview only) of TOFD and PE 6/15/2013

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Linearized Lateral Wave

Receiver 

Transmitter  Lateral wave

Couplant thickness variation Change in time of flight

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Linearized Lateral Wave

Receiver 

Transmitter  Lateral wave

Misalignment variations Change in time of flight

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Linearized Lateral Wave

Transmitter

Receiver  Lateral wave

Small mechanical variations of probe separation Change of time of flight

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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

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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  Not easy to interpret 

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 A Few Final Words on TOFD 

Is the best defect sizing technique available when correctly set-up



Use in conjunction with pulse-echo for code and PoD reasons

Signals Lateral Wave

Back-wall Echo 6/15/2013

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Calculator 

TOFD probe separation can be calculated with basic mathematical formula or Excel calculator tools 6/15/2013

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Calculator 

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Calculator  -80

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

5 -5 -13.6

   )  . -15   m   m    (   s -25    i   x   a      Z

-28.9

-35 -45 -55 X-axis (mm.)

7MHz 100mmPCS 1.5 cycles assumed

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Note volume coverage less than ideal at this PCS (missing upper third). -12dB beam transmit only used for coverage calculation 48

60

70

80

Calculator  -80

-70

-60

-50

-40 -30

-20

-10

0

10

20

30

40

50

60

5 -5

-6.6

   )  . -15   m   m    (

-23.3

  s -25    i   x   a      Z

-35 -45 -55 X-axis (mm.)

Increase refracted angle to 65 ° improves coverage without compromising resolution (for these specific conditions) PCS remains the same. 6/15/2013

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70

80

Flaw Tip



Flaw lengths parallel to the surface can be measured from the TOFD image by fitting hyperbolic cursors similar to SAFT correction but SAFT post-processes the data

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TOFD - SAFT

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 – Post-SAFT Processing

Pre-SAFT processing

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Limitations of TOFD  Cannot

detect all defects

 Limited

coverage results from two potential dead zones  – Dead zone near the surface » as a result of the lateral wave

 – Dead zone at the backwall » resulting from the width of the backwall reflection

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Other Typical Defects

Near Surface Crack

1 2

2

1

The crack blocks the Lateral Wave  And the lower tip appears on the A-scan 6/15/2013

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Incomplete Root Penetration 1

2 3 4

1

2

1

4 2

3

Note the two signals from the top & bottom 6/15/2013

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Lack of Root Penetration 1

1

2 3

2 3

Note the inverted phase between LW and defect 6/15/2013

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Lack of Fusion - Side Wall 1 1

2

2 3

3

4

4

Note the two signals from the top & bottom 6/15/2013

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Porosity 1 1

2

2

3

Porosity may image in many forms whether  individual or cluster  6/15/2013

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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

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Concave Root 1 1

2 2 3

3

Distortion of back-wall echo 6/15/2013

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Lack of Fusion - Interpass

1 2

3

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Choosing an Angle » Optimum Upper tip q ≈ 64°

» Optimum Lower tip q ≈ 68°

 Angle selected is a compromise for depth May require selecting several “zones” for best results

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From Charlesworth & Temple

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TOFD Advantages  Diffracted

rather than reflected signals

 Longitudinal  B-scan

type imaging (side view)

 Accurate  Fast

waves

sizing capability (height)

scanning

 Interpretation  Less

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of defects

sensitive to defect orientation 64

TOFD Limitations  Blind

area - near surface, backwall

 Weak  Flaw

signals

classification limitation

 Interpretation  Sensitive

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of defects

to grain noise

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Codes and Standard  British

Standard

 European  ASME

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TOFD Guides Developed  BS

7706 (1993) Guide to calibration and setting-up of the ultrasonic time-of-flight diffraction (TOFD) technique for detection, location, and sizing of flaws. British Standards Institute 1993.

 pr

EN 583-6 (1995) Nondestructive testingultrasonic examination - Part 6: Time-offlight diffraction technique as a method for  defect detection and sizing.

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ASME Adaptations to TOFD  ASME

VIII Code Case 2235 (2000 Edition)

 Ultrasonic

examination to be in accordance with ASME Section V, Article 4

 “Alternatively,

for techniques that do not use amplitude recording levels, …”.

 This

has opened the door for TOFD to be used on Section VIII pressure vessels

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Root Crack

Root crack defect

TOFD technique

X-Ray

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TOFD technique

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Porosity

Porosity defect

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TOFD technique

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Slag inclusion

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Slag inclusion defect

Lack of Root Fusion TOFD technique

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Lack of root fusion defect

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TOFD technique

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Root Concavity

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Root Concavity defect

Incomplete Root Penetration Incomplete root penetration defect

TOFD technique

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Over Penetration TOFD technique

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Over penetration defect

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Irregular Root Penetration TOFD technique

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Irregular root penetration defect

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