Background

Transmit-Receive Longitudinal (TRL) conventional probes are advantageously used for the inspection of dissimilar metal welds and other acoustically noisy material. Dual matrix array (DMA) probes combine the advantages of TRL probes with the advantages of phased-array technology allowing electronic steering, skewing and focusing of acoustic beams.

Problematic

Dissimilar and austenitic welds are very difficult to inspect with ultrasound, especially when the inspection has to be done from only one side since the ultrasonic beam is scattered due to the anisotropy of the weld material.

A TRL is basically a dual element probe with one emitter and one receiver separated by acoustic insulation. The squint and roof angles configuration allows focusing at one point in the part and creates a pseudo-focalization. These probes eliminate the interface echo, have no dead zones due to wedge echoes, reduce the backscattering signals and permit the use of higher gain.

As a TRL probe has a fixed refracted angle and pseudo-focal point, a typical inspection can require many of these probes to cover a range of different configurations.

Solutions

Dual matrix arrays (DMA) are the logical evolution of this technology. Composed of two matrix arrays, the beam can typically be swept from 30 to 70 degrees (LW or SW), the pseudo-focal point can be adjusted at different depths, the beam size can vary by selecting different aperture sizes and the beam can be skewed looking for oblique defects. Used in conjunction with an OmniScan MX and its imaging, a dual matrix array is more flexible that a TRL probe as it can adapt to different configurations electronically.

Equipment

The equipment used for the inspection is comprised of:

Pictures of the dual matrix array mounted on a wedge

Dual matrix array schematic representation

The dual matrix array used is the 1.5DM7X4-19X12-A17-P-2.5-OM-DUAL28 mounted on a wedge. Full apertures of 28 elements were used at the emission and reception using the full active surface of 19 mm x 12 mm (each matrix).

The OmniScan MX 32: 128 PR was programmed with 3 different sectors with beam steering from 30° to 70° LW for each of them but with different skew angles of -15°, 0° and 15°. All beams were set to focus at the ID of the pipe.

Results

The picture below shows the results on a pipe-to-pipe stainless steel weld. The dual matrix array was programmed with a sector scan from 30° to 70° LW and a skew of 0°. An ID connected crack of 1.1 (long) in x .3 in (height) is clearly detected.

The picture below shows the results on a pipe-to-pipe stainless steel weld. The dual matrix array was programmed with a sector scan from 30° to 70° LW with 3 different skew angles (-15, 0 and 15 degrees). In this case, an ID connected crack of 1.5 (long) in x .4 in (height) is better detected with a skew angle of -15 degrees. In this configuration, even the crack tip is visible.

Conclusion

Dual matrix arrays can be advantageously used during the inspection of noisy material. When used with a portable phased-array instrument like the OmniScan, these probes are more flexible than conventional TRL probes.