No abstract

Abstract. This research presents the nonlinear mixing technique of two co-directionally travelling Rayleigh surface waves generated and detected using laser ultrasonics. The optical generation of Rayleigh waves on the specimen is obtained by shadow mask method. In conventional nonlinear measurements, the inherently small higher harmonics are greatly influenced by the nonlinearities caused by coupling variabilities and surface roughness between the transducer and specimen interface. The proposed technique is completely contactless and it should be possible to eliminate this problem. Moreover, the nonlinear mixing phenomenon yields not only the second harmonics, but also the sum and difference frequency components, which can be used to measure the acoustic nonlinearity of the specimen. In this paper, we will be addressing the experimental configurations for this technique. The proposed technique is validated experimentally on Aluminum 7075 alloy specimen.

This work proposes that laser pulses can generate finite amplitude Rayleigh waves for process monitoring during additive manufacturing. The noncontact process monitoring uses a pulsed laser to generate Rayleigh waves, and an adaptive laser interferometer to receive them. Experiments and models in the literature show that finite amplitude waveforms evolve with propagation distance and that shocks can even form in the in-plane particle velocity waveform. The nonlinear waveform evolution is indicative of the material nonlinearity, which is sensitive to the material microstructure, which in turn affects strength and fracture properties. The measurements are made inside a directed energy deposition additive manufacturing chamber on planar Ti-6Al-4V and IN-718 depositions. By detecting the out-of-plane particle displacement waveform, the in-plane displacement and velocity waveforms are also available. The waveform evolution can be characterized (i) for one source amplitude by reception at different points or (ii) by reception at one point by applying different source amplitudes. Sample results are provided for intentionally adjusted key process parameters: laser power, scan speed, and hatch spacing.

No abstract

Rayleigh waves are very useful for ultrasonic nondestructive evaluation of structural and mechanical components. Nonlinear Rayleigh waves have unique sensitivity to the early stages of material degradation because material nonlinearity causes distortion of the waveforms. The self-interaction of a sinusoidal waveform causes second harmonic generation, while the mutual interaction of waves creates disturbances at the sum and difference frequencies that can potentially be detected with minimal interaction with the nonlinearities in the sensing system. While the effect of surface roughness on attenuation and dispersion is well documented, its effects on the nonlinear aspects of Rayleigh wave propagation have not been investigated. Therefore, Rayleigh waves are sent along aluminum surfaces having small, but different, surface roughness values. The relative nonlinearity parameter increased significantly with surface roughness (average asperity heights 0.027–3.992 μm and Rayleigh wavelengths 0.29–1.9 mm). The relative nonlinearity parameter should be decreased by the presence of attenuation, but here it actually increased with roughness (which increases the attenuation). Thus, an attenuation-based correction was unsuccessful. Since the distortion from material nonlinearity and surface roughness occur over the same surface, it is necessary to make material nonlinearity measurements over surfaces having the same roughness or in the future develop a quantitative understanding of the roughness effect on wave distortion.