In the context of non-destructive testing, nonlinear acoustics (NLA) tests structures – components, assemblies or finished items – using sound or ultrasound.
It differs from the field of nonlinear acoustics in physics, which looks at the behaviour of sound waves with very large amplitudes.
In non-destructive testing we excite the structure with a linear stimulus. We look for a nonlinear response, which is indicative of crack-like flaws.
There are several different techniques we can employ in NLA, depending on the nature of the structure and on the requirements of the test.
If we wish to discover detail about any flaws – location, size, etc. – we can use a scanning technique.
If we are only interested in the integrity of the structure – whether it is safe to use and fit for purpose – we can employ a faster, triaging approach. Here, we test the whole structure rapidly but do not gain detailed information about the flaws.
Pulse Inversion (PI) uses two pulses of ultrasonic energy. The first is transmitted into the structure and the response of the structure is captured. After the structure has returned to equilibrium, a second pulse is transmitted. The second pulse is a phase-inverted (180 degrees out-of-phase) replica of the first pulse. The response of the structure is captured for a second time.
These two responses are then summed to provide the residual response of the structure. If the structure is linear, the residual response will be zero. If the structure is nonlinear, the residual will be non-zero, containing harmonics of the original pulse frequency.
Nonlinear wave mixing uses two simultaneous pulses of energy at two discrete frequencies. If the structure is nonlinear it will cause the two waves to mix and generate intermodulation products.
This approach offers advantages where there are sources of nonlinearity in the measurement chain in addition to flaws in the test structure. Nonlinear wave mixing has been shown to work in situations where conventional ultrasonic techniques are inadequate.
Theta uses nonlinear resonance (NLR) for NDT triaging.
NLR exploits the phenomenon known as the “cracked bell” principle: if we strike an intact bell twice, gently and violently, it will resonate at the same frequency in both instances. If we strike a cracked bell in the same manner, the resonance frequency will be lower in the latter strike.
This is a nonlinear phenomenon, which we call ‘strain-dependent resonance’. It is possible to assess a structure very rapidly using this method – usually in less than a minute – which makes it a very attractive quality assurance approach for manufacturing environments.
At Theta we are continually pushing the boundaries of non-destructive testing. To talk further, please get in touch.