Ultrasonic testing in an NDE method which utilises an ultrasound transducer connected to a diagnostic machine that is systematically passed over the object being inspected. The transducer is coupled to the test item by a fluid, usually grease or glycerine to enable the sound waves to be transmitted into the specimen.
There are two methods of receiving the ultrasound waveform – via reflection or 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.
Specialist LTC inspection engineers are equipped digital UT equipment. They have several advantages over older, analogue technology, including:
- A greater repeatability of results
- Quicker and more accurate calibration
- A capacity to store data – results, trace patterns, etc.
- Compatibility with computer software, permitting uploading of results for direct inclusion in reports
- Reduced weight makes the device easier to transport, whilst better materials make the devices much more robust.
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 entire thickness of the material rather than defects within it.
The instruments employed by LTC are able to measure material thickness as well as the thickness of any coating. It 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.