Internal reflections within ultrasonic transducers are a source of false signals that remain static on the screen of the ultrasonic testing machine, causing obstruction and hindrance in the detection and sizing of defect signals that are close to the scanning surface. Internal reflections of ultrasonic transducers should be monitored by the NDT technician before the first use of the transducer, and also periodically in regular intervals, so the detrimental effects of IRs can be avoided. Depending on the source of IRs, NDT technicians need to perform conditioning of the transducer face to remove any surface irregularities and control the IRs of the ultrasonic transducers. Sources of Internal reflections can typically be generated for the following reasons:
1) Transducer manufacturing fault: Internal reflections arising within the transducer crystal wedge because of improper alignment or geometry of the transducer crystal can be considered as a permanent transducer fault, which usually prohibits any rectification by an NDT technician in the field. Such transducers should be identified as damaged and not used.
2) Transducer wear and tear: Internal reflections arising within the acrylic transducer wedges because of wear, tear, and occasional scratch marks on the transducer wedge faces caused by the scanning on metallic surfaces can be repaired and controlled by the NDT technicians. These types of deformities on the transducer wedge faces cannot be avoided because metallic surfaces found in the field often have irregularities.
3) Transducer re-wedging: Internal reflections arising within the acrylic transducer wedge because of improper re-wedging can be repaired and controlled by the NDT technicians. Transducer re-wedging is an essential criterion when transducer wedge height becomes reduced or sloped because of frequent scanning on metallic surfaces, which cause substantial wear on the transducer assembly and changes the wedge height. Internal reflections can occur if replaced transducer wedges have undesirable geometric features or the transducer wedge is not in full contact with the transducer base.
Ultrasonic Testing
The ideal procedure should be per AWS D1.1, Internal Reflections Procedure, which states to calibrate the equipment, so the signal from the 0.060” diameter hole of an IIW calibration block is set at the reference level of approximately 50% screen height. Subsequently, removing the transducer from the calibration block and keeping all the equipment adjustments unaltered, the calibrated gain is to be increased to 20 dB higher than the reference level. The screen area beyond the 0.5” sound path and above reference level height should be free of any indications.
The implication of the AWS D1.1 instruction can be interpreted to mean that any indication beyond the 0.5” sound path and above reference level height should be considered to be an internal reflection of the transducer, which needs to be investigated and controlled.
Visual Examination
When IRs are detected on a brand-new transducer or a transducer that has seldom been used, the cause of the IRs may be attributed to a transducer manufacturing fault, as mentioned earlier. In this case, the transducer should not be used. When IRs are detected on a transducer that is in regular and frequent use, the cause of the IRs can be attributed to transducer wear and tear caused by scratch marks on the transducer wedge face that need to be examined and fixed. Critical visual testing refers to the close observation of a transducer wedge face under enhanced illumination, for example, using an LED flashlight and a clear, 10X magnifying glass.
Visual Discontinuities
The identification procedure consists of finger pressing on the visible scratch marks with oil drops on the transducer wedge face and simultaneously observing the screen of the ultrasonic equipment for any damping of the IR signal. Wherever the damping of the IR signal results, the area of the transducer wedge face under the finger should be identified as a possible cause.
Remedial Mechanical Surface Conditioning
Once the surface deformities and scratch marks causing IR signals are identified, the appropriate surface conditioning should be performed by mechanical means so that IR signals are removed as per code requirement. The removal process is done by sanding the acrylic surface of the transducer wedge face with sandpaper of various grit sizes. This amount of sanding depends on the severity of the deformities and the height of the IR signals they produce on the screen. The direction of the sanding should be relative to the orientation of the scratch marks to ensure proper removal of such deformities. The abrasive filing action should be transversely to the orientation of the scratch, not in parallel, which could result in propagation of the scratch rather than removal. It is essential to perform this procedure correctly to eliminate IR signals effectively.
Periodic Monitoring
Controlling the quality of ultrasonic testing significantly depends on controlling the IRs of ultrasonic transducers, which requires frequent periodic monitoring. AWS D1.1 requires that the maximum frequency of monitoring internal reflections of ultrasonic transducers should be on every 40 hours of use. Refurbished transducer wedges may not be entirely free of IRs signals, but they should conform to the code requirements.
Conclusion
Controlling internal reflections of ultrasonic transducers should be an essential measure that needs to be monitored on all ultrasonic testing job environments where it is evident the surface condition is a variable factor, even within tolerance levels. This should apply to all industries wherever ultrasonic testing is a mandatory requirement.