OmniSX_MX2_Training_16J TOFD Flaw Sizing and Characterization

OmniScan SX \ MX2 Training ProgramTOFD Flaw Sizing and Characterization

Chris Magruder

OmniScan SX \ MX2 TOFD Flaw Sizing Overview

TOFD flaw sizing is performed with the TOFD readings group selected that allows

the length, depth, and height of a flaw to be displayed and recorded after the

operator has identified the flaw extremities on the UT axis (Depth and height) and the

scan axis. (Length)

Prior to flaw analysis and recording, the TOFD data will have been corrected for

lateral wave straightening if necessary and calibrated with one of the wizard options:

Wedge delay.

WD and PCS.

Velocity and WD.

Again, the most common and

useful option is the WD and

PCS option.

It is highly recommended that

either the data base velocity is

used or a 1X velocity

measurement using pulse echo

technique is made and recorded

for future inspections of the

same series of welds.

A flaws extremities are defined for depth, height and length by the box created with

the cursors on the scan and UT axis of the TOFD B-scan.

After TOFD calibration the UT axis ruler and cursors are displayed with the A-scan

time value in usec and linearized depth value in mm.

Additionally, the scan axis parabolic cursors are available and used to assist in length

sizing accuracy.

The overwhelming trend among entry level TOFD operators is to oversize flaws.

Be conservative and practice.

OmniScan SX \ MX2 TOFD Flaw Sizing Overview cont.

During the group set up wizard the TOFD readings for flaw sizing and recording were

automatically selected and displayed in the OmniScan header.

Readings lists are organized by application and contain 8 readings each that will be

displayed in the header and recorded in the indication table and OmniScan report.

The list of 8 readings is displayed in two groups of 4 readings each that can be

toggled by touching the readings area of the OmniScan header.


The TOFD group readings will display the position of each cursor on the B-scan and

the delta between them to identify the flaw depth, height, and length for recording.

Any of the 8 individual readings can be changed or put in another order by a touch

and hold or right mouse click on the reading, select reading, and replace from the

available options.


Select reading list = a group of

8 readings relative to the

application.

Select reading = replacing 1 of

the current 8 readings

displayed.

Measure

cursor

TOFD flaw length sizing is accomplished with the vertical cursors on the horizontal

scan axis of the B-scan. (Scan axis ruler is blue and represents the probe

movement)

S(r) = Scan axis position of the red reference cursor. ( Flaw start position = 265mm)

S(m) = Scan axis position of the green measure cursor. (Flaw end position = 283mm)

S(m-r) = Delta between the scan axis reference and measure cursor. (Flaw length = 18mm)

OmniScan SX \ MX2 TOFD Flaw Length Sizing

Reference

cursor

D

a

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a

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r

s

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r

S(r) S(m)

Slag

Measure

cursor

Again, precision placement of the cursors on the scan axis of the B-scan for flaw

length sizing is dependent on the operator doing it accurately and greatly improved

by use of the parabolic cursors.

The overwhelming trend of flaw length sizing by junior TOFD inspectors is to over

size flaws. Be conservative and validate the TOFD sizing technique on EDM notches

and flaws of known size in weld samples.

OmniScan SX \ MX2 TOFD Flaw Length Sizing cont.

Reference

cursor

D

a

t

a

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r

There is no mathematic or

software solution that will

predict results or validate

TOFD system hardware,

probes, and setup better than

experimentation on EDM

notches of known length and

depth in the same weld bevel

and process as the welds to be

inspected.

Measure

cursor

Question: What is the flaw length sizing accuracy in a typical TOFD inspection?

Answer: Normally the best case scenario is approximately +/- 2mm. (.080 inches)

In >Scan>Area>Scan Resolution the default value is 1mm. This parameter

determines the interval that the TOFD A-scan is recorded in the data file as the

scanner is moved. The position is represented by the data cursor on the blue

horizontal ruler on B-scan.


Reference

cursor

D

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With a 1mm inspection

resolution, a 1mm error is

possible on both sides of the

flaw resulting in a +/- 2mm

length sizing tolerance by

expert user.

Measure

cursor

Question: What is the minimum inspection resolution that can be entered in

>Scan>Area>Scan Resolution? (OmniScan default 1mm)

Answer: Theoretically the minimum resolution is the distance equivalent to 1 step of

the encoder.

If using a standard Olympus quadrature encoder with a 12 step resolution, that

distance would be .08mm. (1mm \ 12 steps = .08mm)


Reference

cursor

D

a

t

a

c

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r

s

o

r

However 1mm scan resolution

is sufficient for most codes and

standards including ASME, EN

and API.

Use of .5mm resolution will

provide slightly better results

and double the size of the data

file.

Measure cursor

TOFD (m)

TOFD flaw depth sizing is accomplished with the horizontal cursors on the UT axis of

the A-scan and B-scan. (UT axis is magenta and represents the UT range)

TOFD(r) = UT axis position of the red reference cursor. (Flaw start = 19.89mm)

TOFD(m) = UT axis position of the green measure cursor. (Flaw end position = 21.38mm)

TOFD(m-r) = Delta between UT axis reference and measure cursor. (Flaw height = 1.5mm)

OmniScan SX \ MX2 TOFD Flaw Depth Sizing cont.

Reference cursor

TOFD (r)

D

a

t

a

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r

s

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r

Slag

From any data view such as the A-scan or B-can, a touch and hold on the touch

screen or right mouse click in any area free of gates or cursors allows cursor to be

repositioned.


Cursors can be displayed or hidden in the options menu on the interactive title bar.

Tap or click the cursor to activate it and the current position is displayed in the upper

left corner of the OmniScan. Use the Rotary knob or mouse wheel to reposition.

In the example below, the green UT axis measure cursor is active, flashing, and its

position of 11.65 usec is displayed in the upper left corner of the display.


When the TOFD group is calibrated the UT axis cursor values are displayed in both

usec and TOFD corrected true depth. The process is called TOFD A-scan

linearization.

The cursor values can be displayed or hidden by selecting the overlay option in

>Display>Overlay>Cursor>Values.


Question: What is the smallest defect that can be detected using a 10MHz TOFD probe?

The minimum detectable defect in any ultrasound application including TOFD is generally

defined as a defect in size equal to the wave length of the probe.

A flaw in size equal to the wave length will be exposed to the peak sound pressure of either

the negative or positive pulse cycle at least 1X resulting in detection.

Distance = Velocity X Time (5890 meters\second X 50 nanoseconds = .3mm)

Answer: .3mm is theoretically the smallest defect a 10MHz probe can detect under favorable

conditions.


.3mm in carbon steel

(10MHz probe)

The smallest detectable defect is

also limited by factors that affect

signal to noise ratio such as steel

quality, weld process, weld

procedure, flaw location in

relation to beam focus, up-front

engineering to include UT

optimization and TOFD validation

using EDM notches and

embedded flaws of known size.

Question: What is the smallest defect that can be detected using a 5MHz TOFD

probe?

the wave length of a 5MHz probe is 100 nanoseconds.

Distance = Velocity X Time (5890 meters\second X 100 nanoseconds = .6mm)

Answer: .6mm is the theoretical smallest defect that can be detected by a 5MHz

probe under favorable conditions.

Actual results depend on many other factors specific to the weld bevel and process.


.6mm in carbon steel

(5MHz probe)

Question: What is the smallest defect that can be detected by a 2.25MHz TOFD

probe?

the wave length of a 2.25MHz probe is 222 nanoseconds.

Distance = Velocity X Time (5890 meters\second X 222 nanoseconds = 1.3mm)

Answer: 1.3mm is the theoretical smallest defect that can be detected by a 2.25MHz

probe under favorable conditions, and the more cycles that are exposed to the flaw,

the better the probability of detection and sizing accuracy.


1.3mm in carbon steel

(2.25MHz probe)

Flaw characterization is defined as the ability to identify the flaw type based on failure

mechanism or weld flaw type.

TOFD flaw characterization can be divided into two general groups:

Planer flaws. (Lack of fusion, inadequate penetration, cracks, etc.)

Volumetric flaws. (Slag, porosity, tungsten inclusion, excessive penetration, etc.)

TOFD detects sharp reflectors or emitters best. Higher probability of detection and more precise

sizing accuracy is possible on planer flaws.

Planer flaws such as lack of fusion are most likely to be missed by RT, and best detected by

TOFD or pulse echo UT.

Volumetric flaws such as porosity are most likely to be missed by TOFD or pulse echo UT, and

best detected by RT.

OmniScan SX \ MX2 TOFD Flaw Characterization

Planer flaw - Lack of fusion detected in the lateral wave Volumetric flaw - Slag detected in the weld volume

Flaws that are connected to the inner or outer surface are considered more critical

and in some codes such as ASME have a different acceptance criteria.

A flaw connected to the inner surface will disrupt the back wall signal.

OmniScan SX \ MX2 TOFD Flaw Characterization cont.

Lateral wave

Back wall

Shear wave

Planer flaw crack connected to the inside pipe diameter

A flaw connected to the outer surface will disrupt the lateral wave signal.

Flaws connected to the outside of the pipe or vessel are more difficult to detect than

flaws connected to the inside surface and when multi zone TOFD inspections are

configured, the cap group will use 70 degree 10-15MHz probes as close to the weld

as possible for a shallow focus to optimize near surface detection.


Planer flaw EDM notch on outside vessel diameter weld toe disrupts the lateral wave


1 2 3

Planer flaws EDM notch and embedded flaws to validate TOFD weld cap coverage

A flaw connected to the outer surface will disrupt the lateral wave signal.

Flaws connected to the outside of the pipe or vessel are more difficult to detect than

flaws connected to the inside surface and when multi zone TOFD inspections are

configured, the cap group will use 70 degree 10-15MHz probes as close to the weld

as possible for a shallow focus to optimize near surface detection.

The TOFD Data files used to in this presentation can be loaded on the OmniScan or

OmniPC.

TF_25mm_TOFD.Opd has a straightened lateral wave and no calibration. Upon

successful calibration using the PCS and WD wizard, the PCS should be 58mm +/-

1mm and the WD 5.9usecs +\- .25usec.

TF_25mm_TOFD_Calibrated.Opd has a straightened lateral wave and is calibrated

for PCS and WD.

OmniScan SX \ MX2 TOFD Companion Data File TF_25mm_TOFD.Opd

1

23 4

No factor more than location should weigh heavier for characterizing flaws. Flaw 1

pictured below occurs at 12mm of a 25mm double V weld indicating inadequate root

penetration with clear upper and lower flaw extremity detection and phase inversion

in the B-scan.

Note that although the flaw is in the middle of the weld at 12mm deep it appears in

the upper 1/4th of the TOFD B-scan.

OmniScan SX \ MX2 TOFD Companion Data File Flaw #1

Flaw start position (Sr): 112mm

Flaw stop position (Sm): 132mm

Flaw length (Sm-r): 20mm

Flaw upper tip (TOFDr): 11.43mm

Flaw lower tip: (TOFDm): 13.96mm

Flaw height (TOFDm-r): 2.53mm

ID\OD\Embedded?: Embedded

Detected in shear wave?: Yes

Flaw type: Inadequate penetration

Accept\reject?: Reject

TF_25mm_TOFD_Calibrated.Opd Flaw #1 Inadequate penetration

No factor more than location should weigh heavier in characterizing the flaw. Flaw 2

pictured below occurs in the lower 1/3 of the weld where probability of detection and

sizing accuracy is highest.

Without the use of supplemental pulse echo UT is not possible to know in the TOFD

data on which side of the weld the lack of fusion occurs.


Flaw start position (Sr): 197mm








Flaw type: Side wall lack of fusion


TF_25mm_TOFD_Calibrated.Opd Flaw #2 Side wall lack of fusion

The flaw is contained within one weld pass of a SMAW weld rod.

There is no distinguishable upper and lower tip as in the previous planer flaws.

Multiple diffracted signals without clear phase inversion emitted from the sharp

reflectors of the slag line.

The flaw was confirmed with phased array to be not connected to the ID.


TF_25mm_TOFD_Calibrated.Opd Flaw #3 Slag Flaw start position (Sr): 265mm



Flaw upper tip (TOFDr): 20mm





Flaw type: Slag


The flaw upper extremity is detected well and sized with precision.

There is little supporting data in the TOFD group that the flaw is connected to the

inside diameter of the pipe.

Supplemental phased array inspection was performed to confirm crack is connected

to the ID and on the skew 90 side of the weld line just off the weld toe.


TF_25mm_TOFD_Calibrated.Opd Flaw #4 ID crack Flaw start position (Sr): 323mm




Flaw lower tip: (TOFDm): ID connected


ID\OD\Embedded?: ID connected


Flaw type: ID crack


Phased array inspection confirmed the crack is connected to the ID and on the skew

90 side of the weld line just off the weld toe.

TOFD is normally a complementary technique that will improve depth, height, and

length sizing but does not have any easily comparable standard based on amplitude

for flaw acceptance.

TOFD is a pure TOF (Time of flight) application and unlike pulse echo UT or phased

array, the amplitude of the flaw cannot be trusted in determining its severity or size.

OmniScan SX \ MX2 TOFD Companion Data File Flaw #4 cont.

The crack disrupts the lateral wave and the lower tip is detected.

OmniScan SX \ MX2 TOFD Flaw Characterization OD Toe Crack

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1

22

1

The crack disrupts the lateral wave and the lower tip appears in the A-scan.

Note the upper and lower diffracted signals from the flaw extremities

Because it is one continuous flaw, the two signals are out of phase.(+-+, -+-)

In phase signals (+-+, +-+) would typically indicate separate non-connected

flaws.

OmniScan SX \ MX2 TOFD Flaw Characterization Lack of Root Fusion

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3

4

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Inadequate penetration is detected above the back wall signal.

Note the phase inversion between the lateral wave (-+-) and the inadequate

penetration signal above the back wall. (-+-)

OmniScan SX \ MX2 TOFD Flaw Characterization Inadequate Penetration

1

2

3

1

2

3

Shallow side wall lack of fusion is detected.

Note the upper extremity of the lack of fusion signal is in the lateral wave.

Note two separate out of phase signals typical of side wall lack of fusion.

OmniScan SX \ MX2 TOFD Flaw Characterization Side Wall Lack of Fusion

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4

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Cluster porosity is detected in the weld volume resulting in multiple diffracted signals.

Porosity is typically the weld flaw most likely to be missed by UT pulse echo or TOFD

and use of higher frequency probes will improve the probability of detection and

sizing accuracy.

OmniScan SX \ MX2 TOFD Flaw Characterization Cluster Porosity

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3

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2

Note separation in back wall signal without phase inversion.

OmniScan SX \ MX2 TOFD Flaw Characterization Concave Root

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1

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Interpass non fusion typically does not have sufficient height for a clear upper and

lower tip extremity in the TOFD data.

Interpass non fusion is not typically detected by pulse echo UT or RT and can

generate many small low amplitude indications in the TOFD data that are difficult to

characterize with certainty.

OmniScan SX \ MX2 TOFD Flaw Characterization Inter pass Non Fusion

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OmniScan SX \ MX2 TOFD Flaw Characterization Movie

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OmniSX_MX2_Training_16J TOFD Flaw Sizing and Characterization