Effects of narrow bandwidth signal for coherent nonlinear array imaging

Nonlinear ultrasound has shown its sensitivity to early formation of defects such as micro-cracking and material aging. When an ultrasonic wave propagates in a nonlinear material, there is transfer of energy from the fundamental frequency to its harmonics. This energy loss can be used for spatial mapping of nonlinearity within sample using arrays. In this study, we investigate the effects of the input signal and instrument over the nonlinear measurement. To this end, a two dimensional finite difference numerical simulation has been created to model nonlinear elastic wave propagation. It was used to investigate, from a sensitivity to damage perspective, the difference between focused transmission and sequential firing of elements while focusing is performed in post processing. The model allows energy from the coherent scattered field within the material to be monitored, which is not possible in real experiments. The result shows the influence of the bandwidth of the input signal on the nonlinear measurement. This was experimentally investigated on a sample with a surface breaking crack acting as a high local nonlinearity region. Nonlinear images showed an increase in sensitivity with narrowband signals, at the expense of resolution.