014-TOFD

Time of Flight Diffraction (TOFD)

TOFD UT Technique









Overview Calibration Applications Summary & Limitations

TOFD UT Technique









Overview Calibration Applications Summary & Limitations

TOFD Overview 



Forward Diffraction Technique First described by Silk in 1977 • Using diffracted signals from crack tips



Traditional grey scale TOFD presentation

 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

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

Diffraction The incoming wave vibrates the defect. Each point of the defect generates new elementary spherical waves.

Waves Incident wave

Diffracted waves All directions Low energy

Reflected wave

FLAW

Diffracted waves

Independent of incidence angle

Signals 

Signals Received • Lateral wave • Subsurface • Back-wall echo • Mode converted (shear wave) echo

 

Define top and bottom of part Note phase change

Signals +

White

    e      d     u      t      i      l     p     m      A

A-scan Time -

Black

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

Time

Signals Lateral Wave

Back-wall Echo

Calculator

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

Calculator

Calculator -80

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

5 -5    )  . -15   m   m    (   s -25    i   x   a      Z

-13.6

-28.9

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

7MHz 100mmPCS 1.5 cycles assumed Note volume coverage less than ideal at this PCS (missing upper third). -12dB beam transmit only used for coverage calculation

60

70

80

Calculator -80

-70

-60

-50

-40 -30

-20

-10

0

10

20

30

40

50

60

5 -5    )  . -15   m   m    (   s -25    i   x   a      Z

-6.6

-23.3

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

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

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

TOFD - SAFT

• Pre-SAFT processing

• Post-SAFT Processing

Calibration 

Place the TOFD configuration on the calibration block

Calibration 

Adjust probe distance to be 2 times the part thickness

Calibration 

Under Probe/Part >Select>Wedge • Select Wedge

Calibration



Under Probe/Part >Select>Wedge • Select Probe

TOFD Setup 

Editing probe properties

Calibration 

Under UT menu>Mode >TOFD

Calibration 

Under UT General>Set Gain>Set Range>

Calibration





If there is a lot of noise, averaging can be used UT>Receiver >Averaging

Scanning

Save Options



Save TOFD setup

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

Near Surface Crack

1 2

2 The1crack blocks the Lateral Wave and the lower tip appears on the A-scan

Incomplete Root Penetration 1

2 3 4

1

2

1

3

4

2

Note the two signals from the top & bottom

Lack of Root Penetration 1 1

2 3

2 3

Note the inverted phase between LW and defect

Lack of Fusion - Side Wall 1 1

2

2 3

3

4

4

Note the two signals from the top & bottom

Porosity 1

1

2

2

3

Porosity may image in many forms whether individual or cluster

Transverse Crack 1 1 2 1

2 3

2 3

4 3

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

Concave Root 1

1 2

2 3 3

Distortion of back-wall echo

Lack of Fusion - Interpass

1 2

3