In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over the object being inspected. The transducer is coupled to the test item by a fluid, usually grease or glycerine, this provides for sound transmission into the specimen as sound at the frequencies used will not propagate in air.

There are two methods of receiving the ultrasound waveform, reflection and attenuation. In reflection (or pulse-echo) mode, the transducer performs both the sending and the receiving of the pulsed waves as the "sound" is reflected back to the device. Reflected ultrasound comes from an interface, such as the back wall of the object or an imperfection within the object. The diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance, representing the arrival time of the reflection. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after travelling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted, thus revealing their presence.

We have been using digital ultrasonic equipment for the past 21/2 years. The advantages over the older, analogue technology being: -

i   Greater repeatability of results

ii  Quicker, more accurate calibration

iii  Memory capacity, allowing storage of results, trace patterns etc.

iv  Compatibility with computer software, permitting uploading of results for direct inclusion in reports

v Less weight, more robust.

 

An example of a defect in a shaft, as located by ultrasonic testing

Trace patterns such as this can be included in reports etc .

 

Ultrasonic Thickness Measurement.

Ultrasonic methods are also used to measure material thicknesses to detect or monitor corrosion. The principles used are the same as for defect location but utilising echoes from the through thickness of the material rather than defects within it.

For the past year we have used digital ultrasonic thickness measurement. The instrument employed by LTC is able to measure material thickness as well as the thickness of any coating.

The instrument can also save “profiles” of material under inspection, giving a representation of any corrosion present.

This allows us to retain a database of thickness reading for a particular project, which can be uploaded to a computer for reporting or analysis.