NDT / NDT Exclusives

On-Site Ultrasonics

April 4, 2012
KEYWORDS ndt / ultrasonics
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Embedded modeling tools provide on-site inspection of welds using phased array and conventional UT.



Stand-off positions and beam angles are shown in the scan plan. The weld profile is defined by simply selecting an existing weld geometry and adjusting the appropriate parameters as shown here. Source: Sonatest

The wider use of multi-technique ultrasonic instruments combining phased array, time of flight diffraction (TOFD) and conventional UT has increased the number of parameters to be controlled by the ultrasonic technician. Visualization and modeling are generally of great help to ensure that all settings are in accordance with the inspection technique and code requirements. These tools are now available on portable phased array flaw detectors and can be applied to inspections.

The ASTM E2700 Standard Practice for Contact Ultrasonic Testing of Welds Using Phased Arrays is used as an example of typical code requirements. This practice refers to the use of angle beam inspection with either sectorial scan (S-scan) or linear scan (L-scan), also called electronic scan (E-scan). The practice is based on its equivalent ASTM E164 for conventional UT.

A typical setup workflow for weld inspection will include these most common steps that an inspector must go through before going on-site. The scan plan would include: define inspection parameters, choose probe and wedge, define part being inspected, define focal laws, position probes on part, and set up the encoder. The final step would be calibration.



A weld geometry definition is shown here. Source: Sonatest

The user interface of the phased array flaw detector discussed here was designed to match this intuitive workflow as closely as possible. One important aspect to notice is that most of these steps refer to the examination procedure and scan plan requirements. Once the inspection technique has been established, which essentially include probes and scans, quantity selection as well as the type of scan, the details of the scan plan can be defined. This approach is in accordance with the standard practice, which state that the phased array scanning procedure for welds shall be established using a scan plan that indicates the stand-off positions for the probes and the appropriate beam angles. These requirements are part of the setup workflow defined previously. The embedded software of the phased array flaw detector includes the facilities to visualize in one glance the appropriate standoff distance and beam coverage. The sectorial scan shows up as it will be seen on the screen of the device. In other words, beams are not reflected off the bottom part of the plate but projected. Various information such as the focal plan, near field length and individual focal laws position can also be displayed whenever appropriate.



This is an example of butt weld inspection using two S-scans perpendicular to the weld axis. Source: Sonatest

Where possible, the standard practice states that welds have to be inspected from both sides. Moreover, if any cross cracking is suspected, it is recommended to add transducers essentially parallel to the weld centerline. These additional requirements obviously increase the complexity of the whole setup and at the same time raise the risk of mistakes. This is where the use of embedded modeling tools can provide the appropriate balance between capability and simplicity. For example, consider a butt weld inspection combining the use of two sectorial scans covering the whole weld volume with a second pair of transducers positioned at 15 degrees from the weld centerline to detect any transverse indications. An experienced operator can define the whole setup from scratch in less than 10 minutes. When preparing such a configuration, each probe and wedge definition can be loaded from a database or manually entered. The weld profile is defined by simply selecting an existing weld geometry and adjusting the appropriate parameters.

The scan tab allows the operator to set regular ultrasonic testing (UT) settings and all parameters used to calculate focal laws while the geometry tab is used to set the probe/wedge positions. The result is a scan plan that can be visualized in either 2-D or 3-D, providing a powerful tool to ensure appropriate beam coverage and probe positioning.



This is an example of two S-scan to detect transverse indications. Source: Sonatest

When thicker plates have to be inspected or when a unique standoff position cannot be used, raster scanning with either semi-automated or automated motion can be an option. In the later case, it becomes mandatory to define a datum position. The datum acts as the weld reference point from where encoded positions are recorded. This is an essential parameter that needs to be part of the examination report. By defining a two axis encoder, the embedded software will automatically create the raster scan pattern which can afterwards be adjusted with the proper index and scan axis offset. It is important to mention that indication locations will be relative to the datum point along the scan axis while on the index axis measurements can either be relative to the wedge front or weld centerline. Once again, clear illustration of advanced setups can be made available directly from the phased array flaw detector itself. Once the actual inspection starts, field results can differ from the theory, therefore requiring on the fly adjustments. If the root location appears away from its expected position, the weld geometry needs to be adjusted. This change could be made directly on the unit without the need of a laptop computer.

As a final example, consider a TOFD scan with a -12 decibel (dB) beam divergence, which in this particular case shows a lack of coverage near the weld cap. Again, the 3-D view could provide an instant answer without the need for complex calculations.



This demonstrates some of the capabilities of the new software tools that are now made available directly on phased array flaw detectors. These tools can be used to meet requirements relative to scan plan development. They provide advantages not only during the preparation process, but also while performing the inspection on-site.



Tech Tips

Common steps that an inspector has to go through before going on-site:

Define inspection parameters

Choose probe and wedge

Define part being inspected

Define focal laws

Position probes on part

Set up the encoder

Calibration

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