Nonlinear Ultrasonic Phased Array Measurements

      The ultrasonic signal could be divided in two parts: the early part of the signal containing coherent backwall reflections, and a later noise-like backscattered part containing information of microstructural features within the bulk of the material. This later part is generally called diffuse field or diffuse coda wave. Even though it has an appearance of random noise, its power spectrum shows that it is highly repeatable and sensitive to small changes in the microstructure. The spatial and temporal incoherence of the diffuse field originates in the vast number of scattering events occurring while the elastic waves propagated back and forth between the front surface and the back surface of the studied material. We have observed that minute nonlinear defects of the microstructure such as fatigue damage precursors, e.g. lattice dislocations, slip bands, veins, micro-voids, micro-cracks, subharmonics and higher harmonics within the diffuse field. Nonlinear metrics that are ratios of high harmonics to subharmonic spectral energies are strongly correlated to the increase of fatigue damage precursors within the bulk of the material under cyclic stress.
      While diffuse field measurements can be carried out with single element probes, phased array ultrasonic systems have the ability to perform faster scans of the structural components.