Ultrasonic Testing

This method uses the ability of high frequency sound waves, typically above 2 MHz (2,000,000 CPS), to pass through materials.

A probe is used which contains a piezo electric crystal to transmit and receive ultrasonic pulses. Ultrasound hitting any air interface, or an interface with a different material density, which is perpendicular to the ultrasonic beam, is reflected back and displayed on a cathode ray tube (CRT)*. The actual display relates to the time taken for the ultrasonic pulses to travel the distance to an interface and back, i.e. the longer the time, the further away the interface.

An interface could be the opposite side of the plate, therefore, wall thickness measurements can easily be made.

Lamination checks are easily carried out using ultrasonic methods (opposite to radiography). Welds can be tested using angle type probes, although this requires more operator skill to apply and interpret results. Defects in welds usually can be located but the type of defect is sometimes difficult to identify.

To detect a linear defect with radiography, the defects must have depth in line with the radiation beam; the opposite is true for ultrasonic flaw detection, i.e. when using ultrasonic testing the defects should ideally have their major face at 90° to the axis of the ultrasonic beam.

For the ultrasound to enter a material a couplant must be introduced between the probe and the specimen, e.g. grease, oil, glycerine or water, because ultrasound does not travel very well through air.

Ultrasonic equipment is quite portable, but one major disadvantage with most of the equipment used is that no permanent record of results is produced. Equipment that is able to record results is currently expensive.