In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over the object being inspected. The transducer is typically separated from the test object by a couplant (such as oil) or by water, as in immersion testing.
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 from 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 traveling 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.
Use Case for Ultrasonic Testing
One of the primary uses for ultrasonic gauges is the measurement of wall thickness in pipes and tanks that may be corroded. These measurements can be carried out quickly without requiring access to the internal area of the pipe or for it to be emptied. Ultrasonic testing can locate and measure a range of cracks, inclusions, and other issues that impact structural integrity. The range of detectable flaw size is dependent on the type of material being tested and the wavelength utilized.
How Ultrasonic Testing is Employed
To measure the thickness or internal structure of a test piece, ultrasonic NDT employs high frequency sound waves that surpass the upper limits of human hearing (within the range of 500kHz to 20,000 kHz). The sound waves will reflect off of the flaws in material in certain predictable ways. An ultrasonic flaw detector both generates and processes ultrasonic signals that are then displayed as a graphical waveform that can be interpreted by a trained operator. The nature of high frequency sound waves is that they are highly directional and will travel through material such as steel or plastic until they hit another medium, at which point they disperse and some of the waves reflect back to the detector.
An operator is able to compare characteristic reflection and dispersion patterns from a baseline ‘good’ part to the patterns of a potential ‘flawed’ test piece. The analysis of these reflections can be used to determine thickness of a test piece as well as find cracks and other flaws.
Advantages of UT
Using generated high frequency sound waves, ultrasonic testing is entirely nondestructive. Test pieces do not need to be cut, sectioned for samples, or exposed to chemical agents. Testing can be done by approaching one side of the subject, as compared to other measurement approaches such as micrometers. It is also very safe and poses no threat or health hazards to operators or the environment. The tests are highly replicable and have a high-degree of reliability.