Non-Destructive Testing (NDT) Inspections: Ultrasonic Testing

Inspection of welds using ultrasound (Ultrasonic Testing) is a leading-edge technique for finding in-weld flaws. It is important that those involved with NDT inspection/welding fabrication have specific knowledge of the method and its application. The basic ultrasonic equipment consists of a power source with CRT screen probes and calibration blocks. Ultrasonic waves are generated by applying electric current from the power source to a probe. The probe contains piezoelectric crystals, such as quartz.

The piezoelectric effect allows conversion of electrical energy to mechanical energy, which is then transmitted through the material under test. Any energy that is reflected is returned to the probe, converted back to electrical energy, and is then transmitted to the ultrasonic equipment to transmit the sound energy into the material. The air gap between the probe and the metal must be eliminated. This task is done by using a couplet, usually oil, grease, or water.

The calibrated CRT screen displays the history of the transmitted ultrasonic waves. Principally, these are the distance that sound travels of time-base and the amplitude of any reflector. Before testing a weld, all other components casting the equipment must be calibrated on a block, which is of the same material and contains known dimensions and hole sizes here. The horizontal sweep on the CRT time base can be calibrated as a multiple of 25 millimeters—the thickness of the block

There are two types of ultrasonic beam modes usually employed in the ultrasonic testing of welds. These are longitudinal or normal waves, and shear or angle waves. The direction of the longitudinal wave is normal to the surface being inspected and that of a shear wave at an angle. When a change occurs in a material, such as a void caused by a flaw, an acoustic mismatch occurs and some of the sound energy is reflected back to the ultrasonic probe, converted into an electrical signal. This is then electronically displayed on the CRT screen.

It is usual to investigate a weld using a shear wave probe as the angle allows the operator to stand off to the side of the world. It avoids the weld buildup and allows the operator to select a probe angle that will strike the fusion line of the weld preparation at approximately 90 degrees. The fusion line being a likely location for in weld laws, such as lack of fusion or trapped inclusions. By reference to the calibration carried out on the test block, the position, length, and depth of the Florina weld can be analyzed from the data given on the CRT screen.

Accurate sizing and positioning of flaws is one of the great advantages of non-destructive testing when using ultrasound. This helps with decisions on how to facilitate a weld repair and also defect sizing plays a critical role in the analysis of structures using fitness for purpose techniques.

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