Of all the industrial applications for ultrasonic testing, material flaw detection is the oldest and the most common. Since the 1940s, the laws of physics that govern the propagation of sound waves through solid materials have been used to detect hidden cracks, voids, porosity and other internal discontinuities in metals, composites, plastics and ceramics.
In ultrasonic testing, high-frequency sound waves are reflected from flaws in predictable ways, producing distinctive echo patterns that can be detected, displayed and recorded. Its ease of use and non-destructive nature of makes it a very useful tool for the quality control of engine and transmission components, particularly cast or fabricated components. In this month’s article we will take a closer look at the details behind it.
Ultrasonic testing relies on the fact that sound waves travelling through a medium will be reflected or transmitted in different ways if the composition of the medium varies. They can therefore be used to identify flaws or voids in materials. Using an ultrasound transducer to generate an ultrasonic sound wave, a wide range of materials can be tested for integrity.
There are two methods of receiving the reflected ultrasound waveform: reflection and attenuation. Using the reflection (or pulse-echo) method, a single transducer performs both the sending and receiving of the pulsed waves, as the process relies on the sound waves that are reflected back from the material being tested. The reflected ultrasound that is received by the transducer from an impervious obstacle such as the back wall of the object or an imperfection within it. These results are then displayed in the form of a signal with an amplitude representing the intensity of the reflection and the distance from the sensor, which is represented by the time difference between sending and receiving. As the transducer passes over a flaw, the signal displayed will change from that produced by homogenous material.
With the 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 test sample. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of ultrasound transmitted, thus revealing their presence. This type of testing is used to check large-volume components.
There are several features of ultrasound testing that make it particularly attractive for use in motorsport. As mentioned, it is non-destructive, meaning valuable parts do not need to be chopped up to check their integrity. Ultrasonic testers are also portable, so they can be easily used trackside for checking components, and this portability means they can also be used to examine large assemblies. They can also be used for tasks ranging from checking welds to assessing castings for porosity. Overall, ultrasonic testing is an exceptionally useful technique that can provide an invaluable capability in any race ’shop.
Written by Lawrence Butcher