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Non-linear ultrasonic imaging (Research in progress)

Most traditional NDE techniques for damage detection are based on linear elastodynamic theory, and rely on measuring the reflection and scattering of primary waves at the material heterogeneities and discontinuities. The presence of defects changes the phase or amplitude of the measured signal, but the frequency of the input waveforms remains the same. Although these techniques work well in the presence of a significant impedance contrast, when the impedance mismatch is less pronounced – eg micro-damage, nonlinear effects lead to transformation of some of the incident energy into frequency harmonics. In principle, imaging of the nonlinear waveforms can provide information about a range of  material properties such as early stage fatigue damage.

Several RCNDE projects have investigated the potential of nonlinear imaging through modelling and experimental  work. Recent work at Bristol has looked in particular at imaging defects in the presence of geometric features (e.g. fastener holes), for which the nonlinear response can be significant due to contact-acoustic nonlinearity, localised plasticity and friction. This is a technique of enormous potential benefit, capable of imaging defects transparent to conventional linear acoustic techniques or where defect signals are swamped by signals from the geometrical features as illustrated below.

Imaging a fatigue crack growing from a fastener hole. The non-linear image (a) clearly shows the crack growing from the left hole, while the crack signal in a conventional image (b) is  swamped by geometric reflections.

Imaging a fatigue crack growing from a fastener hole. The non-linear image (a) clearly shows the crack growing from the left hole, while the crack signal in a conventional image (b) is swamped by geometric reflections.