It has long been understood that acoustic nonlinearity is sensitive to many physical properties including material microstructure and mechanical damage. The lack of effective imaging has, however, held back the use of this important method. Currently, engineers are able to produce images of the interior of components using ultrasound, but can only detect large problems such as cracks.
Imaging of acoustic nonlinearity is achieved by exploiting differences in the propagation of fields produced by the parallel and sequential transmission of elements in ultrasonic arrays. Imaging acoustic nonlinearity not only provides sensitivity to smaller defects than is currently possible, but may have the potential to detect damage before macroscopic material changes occur.
This would enable intervention before cracks have even begun to form, as well as predicting the remaining life of an engineering structure. Such advances in non-destructive evaluation not only increase the safety of engineering structures but can help future design; for example, allowing the next generation of aircraft to be built thinner and lighter.