Material Nonlinearity Detected by Spatial Evolution of Laser-Generated Broadband Rayleigh Waves with Focus on Residual Strain

The structural integrity of additively manufactured engineering components is a critical concern, as additive manufacturing is increasingly regarded as a promising method for fabricating complex geometries. Material evaluation that is nondestructive, noncontact, and in-situ is of great importance to prevent the production of sub-standard components. In this regard, laser ultrasound is well suited for in-situ process monitoring. Laser generated broadband Rayleigh waveforms evolve as they propagate due to material nonlinearity, making them ideal for evaluating microstructure without interference from system-induced nonlinearities. We thermally aged IN718 samples that were additively manufactured to change the microstructure and increase material nonlinearity. The spatial evolution of the Rayleigh waveforms was quantified in ex-situ tests before and after thermal aging. Thermal aging promoted the precipitation of γ^′,γ^′′, and MC carbides as well as the development of residual stress, all of which contribute to enhanced material nonlinearity and thus spatial waveform evolution. To isolate the effects of phase transformations and residual stress on waveform evolution, we estimated residual strain and stress using shifts in X-ray diffraction patterns. However, it was not possible to quantitatively distinguish the effects of phase transformation and residual strain. Therefore, the effect of residual strain on material nonlinearity is investigated through atomistic-scale simulations. The results start to answer the question of how features of the microstructure are associated with material nonlinearity as detected through nonlinear elastic wave propagation.