OmniScan Tip of the MonthTip Diffraction or dB Drop for Sizing of Weld Flaws?

Chris Magruder

Jun 13, 2014

V4.1R8

� Question: Precision sizing of weld flaws using phased array inspection is best

achieved by which sizing technique?

1. Shear wave tip diffraction.

2. Amplitude drop sizing.

OmniScan SX \ MX2 \ OmniPC Tip – dB Drop or Tip Diffraction?

� The answer to this question depends on the flaw type, size, and orientation.

� The intent of this presentation is to provide a general overview of length, depth, and

height sizing of common flaw types and under which condition a dB drop or tip

diffraction sizing technique is most beneficial.

OmniScan SX \ MX2 \ OmniPC Tip – dB Drop or Tip Diffraction?

� Pictured below is a two sided phased array inspection of a 25mm carbon steel weld

using 5MHz .6mm pitch, 32 element phased array probes. An optimum inspection

for precision sizing of weld flaws.

� The first indication in the C-scan is a high amplitude flaw that is detected from both

sides of the weld and connected to the inner surface of the pipe. (B0)

� Based on the flaw location in the weld and the signal characterization of the A-scan

and S-scan it can be identified with certainty as lack of fusion.

OmniScan SX \ MX2 \ OmniPC Tip – dB Drop for Length Sizing

� Length sizing is performed on the C-scan and B-scan using an amplitude drop sizing

technique regardless of flaw type.

� If upfront engineering is performed on EDM notches or flaws in the same welds, the

sizing can be refined to take into account all aspects of the inspection including

probe frequency, beam size, focus, and pipe diameter.

� Where that is not necessary or practical, a -6dB, -3dB, or rapid dB drop technique is

typically used.

Flaw Length Sizing

OmniScan SX \ MX2 \ OmniPC Tip – dB Drop for Length Sizing cont.

� Length sizing is performed on the C-scan scan by placing the scan axis cursors on

the extremities of the flaw using the color palette for a -6dB drop.

� The start and stop position are identified for a total flaw length of 29mm.

OmniScan SX \ MX2 \ OmniPC Tip – dB Drop for Length Sizing cont.

� Manipulation of the OmniScan color palette can assist in precision dB drop

measurements as well.

� In the example below all data <50% amplitude in gate A was excluded from the C-

scan resulting in a -6dB drop measurement of the flaw length.

� The result as the previous slide.

OmniScan SX \ MX2 \ OmniPC Tip – dB Drop for Height and Depth Sizing

� Height sizing is performed on the S-scan and A-scan by placing the cursors on the

extremities of the flaw.

� Again, because the flaw is larger than the beam size and there is near perpendicular

orientation, a dB drop sizing technique for lack of fusion flaws will result in precision

depth and height sizing accuracy.

Flaw Height Sizing

OmniScan SX \ MX2 \ OmniPC Tip – dB Drop for Height and Depth Sizing cont.

� The vertical purple scale on the UT axis ruler is angle corrected for true depth.

� The green UT cursor is placed at 25mm as the flaw is determined to be connected to

the ID . (25mm thickness at B0)

� The red UT cursor is placed at the flaw upper extremity based on -6dB drop sizing

using the color palette or manipulating the data cursor in the S-scan.

Flaw Height Sizing

OmniScan SX \ MX2 \ OmniPC Tip – dB Drop for Height and Depth Sizing cont.

� The upper extremity of the lack of fusion is identified with the cursor for a through

wall dimension of 5mm measured from the pipe ID.

� The -6dB drop technique is precise for a lack of fusion measurement because of the

flaw size and perpendicular orientation of the beam to the flaw.

Flaw Height Sizing

OmniScan SX \ MX2 \ OmniPC Tip – dB Drop for Height and Depth Sizing cont.

� The DA reading displays the true depth of the maximum amplitude signal in gate A

for the selected beam.

� Placing the S-scan blue data cursor on the flaw extremity updates the trigonometry

readings that are automatically calculated based on gate max amplitude position.

� Using the gate data from the 64.5 degree beam, the true depth position of the upper

extremity of the lack of fusion is 19.99mm deep measured from the part surface.

Flaw Height Sizing

OmniScan SX \ MX2 \ OmniPC Tip – dB Drop for Height and Depth Sizing cont.

� Manipulation of the OmniScan color palette can assist in precision dB drop

measurements as well.

� In the example below all data <50% amplitude in gate A was excluded from the S-

scan resulting in a -6dB drop precision measurement of the flaw height.

OmniScan SX \ MX2 \ OmniPC Tip – Tip Diffraction for Crack Sizing

� Crack tips like pictured below are most accurately sized with a diffraction technique.

� In this example from the same inspection, a mid wall thermal fatigue crack is

detected from both sides of the weld. Lack of a corner trap signal at end of 2nd leg

indicates that the crack is not connected to the OD pipe surface and is determined to

be embedded.

OmniScan SX \ MX2 \ OmniPC Tip – Tip Diffraction for Crack Sizing cont.

� The flaw start and stop position for length sizing were detected from different sides of

the weld for a total length of 50mm.

� Although length sizing of cracks is also performed using the amplitude color palette,

cracks will vary in amplitude, depth, and height along the weld line.

� For this reason a rapid dB drop technique on the flaw extremities is used regardless

of flaw maximum amplitude detected along weld line.

OmniScan SX \ MX2 \ OmniPC Tip – Tip Diffraction for Crack Sizing cont.

� Using the same 5MHz focused phased array probe, the shear wave tip diffracted

signal from the crack ligament will be smaller than the beam size and dB drop sizing

of the A-scan amplitude and S-scan color palette will play no part in the sizing.

� Cursor placement is made based on the center of energy of the crack ligament.

� The color palette is used to determine the center of the energy in the tip diffraction

signal but is not used for amplitude drop to determine flaw extremity.

Before palette adjustment

After palette adjustment

Palette adjustment

OmniScan SX \ MX2 \ OmniPC Tip – Tip Diffraction for Crack Sizing cont.

� Cursor position is at the center of energy of deepest and shallowest tip signals from

the crack ligaments.

� In this case the deep and shallow tip extremities were detected from different sides of

the weld with different probes.

Deepest crack ligament from skew 90 probe Shallowest crack ligament from skew 270 probe

OmniScan SX \ MX2 \ OmniPC Tip – Tip Diffraction for Crack Sizing cont.

� The embedded crack has a through wall dimension of 10.78mm, the deepest

ligament at 13.29mm and it is not connected to the outside diameter pipe surface.

OmniScan SX \ MX2 \ OmniPC Tip – Tip Diffraction for Crack Sizing cont.

� The blue cursor on the C-scan displays the S-scan slice of the weld at that position.

� After height sizing the flaw, the C-scan data cursor is moved along the scan axis to

ensure the crack is wholly contained within the area defined by the UT axis cursors.

OmniScan SX \ MX2 \ OmniPC Tip – Shear Wave Probes for Sizing

� The accuracy of depth and height sizing is dependent on the beam exit, angle

accuracy, and material velocity in the OmniScan software.

� Velocity error, angle error, and calibration procedure will all effect results.

� Using clock scans on common UT calibration blocks like the IIW pictured below

allows rapid check of component velocity, wedge velocity, beam angle, beam exit

from wedge, and trigonometry from gate readings.

OmniScan SX \ MX2 \ OmniPC Tip – Shear Wave Probes for Sizing

� The flaw sizing examples in this presentation were carbon steel weld flaws inspected

by shear wave S-scans that most expert OmniScan customers would put at +/- .5mm

accuracy for 5Mhz probes and a little better than that with 10MHz probes.

� The sizing accuracy of any UT or phased array inspection is dependent on the

frequency of the probe, PA and UT configuration, and inspection strategy.

� Pictured below is a clock scan of an IIW block’s 15mm deep X 1.5mm SDH for

verification of beam size and quality, and trigonometry reading accuracy.

OmniScan SX \ MX2 \ OmniPC Tip – Advanced Probes for Austenitic Sizing

� Austenitic sizing is more challenging due to the increased grain structure size,

material attenuation, and decreased SNR of flaws.

� Low frequency specialized probes like the Olympus 2.25Mhz A17 dual matrix array

(DMA) utilize a refracted longitudinal beam in pitch-catch configuration for austenitic

and inconel dissimilar metal welds not inspectable with shear wave beams.

� A clock scan over a SDH using the OmniScan envelope to measure vertical beam

size is a direct indicator of expected sizing accuracy for one line scan inspections

where the flaw cannot be optimized by moving the probe in and out from the weld.

25mm deep X 1.5mm SDH in SS304

OmniScan SX \ MX2 \ OmniPC Tip – Supplemental TOFD for Sizing

� TOFD is also a diffraction technique not dependent on flaw orientation and uses only

1 of the 256 A-scans available in an OmniScan MX2 inspection.

� Use of TOFD for supplemental detection and sizing is highly recommended when

possible.

� Upfront engineering to include experimentation with the probe frequency and focus

on side drilled hole and notches in the same piping welds will optimize results.

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Thank you and Good luck!Questions or comments contact

Chris.magruder@Olympusndt.com