SAW velocity spectroscopy has been long considered to be one of the leading candidates for nondestructive characterization of surface-treated metals because of its ability to probe the material properties at different penetration depths depending on the inspection frequency. This research effort is directed towards the use of a highly accurate laser-ultrasonic technique to study the feasibility of SAW dispersion spectroscopy for residual stress assessment on shot-peened metals. Unfortunately, surface acoustic waves are sensitive to spurious parameters, which are byproducts of the surface treatment, i.e., surface roughness and cold work.
The experimental results obtained on rough surfaces were compared to both theoretical and computational simulations for surface wave induced dispersion from the literature. It was found that experimental results were consistent with the numerical simulations, but neither of them showed conclusive evidence of the high frequency positive dispersion suggested by earlier models. Also, we studied the effect of gradually relaxed shot-peened aluminum specimens on the SAW velocity. Finally, we investigated the individual contribution of residual stress by determining the acoustoelastic constants of the material and we present numerical predictions of the effect of texture on the surface wave dispersion.
We detail that the dispersion of the surface wave arises from three different sources, namely, (a) there is an apparent dispersion on smooth surfaces due to the diffraction of the surface acoustic wave as it travels over the surface of the specimen, (b) there is a real but spurious dispersion caused by SAW scattering on the rough surface, and (c) there is the principal dispersion caused material effects of the surface treatment, including the primary compressive residual stress effect and the secondary cold work effect.
The results of this investigation revealed some important aspects of SAW propagation on surface treated metals and confirmed that in aluminum SAW dispersion is mainly sensitive to cold work effects and much less to residual stress effects. It still can be a very useful NDE tool since quantitative assessment of the level and distribution of cold work in surface-treated metals is of primary importance from the point of thermo-mechanical stability of the beneficial residual stresses.
|School:||University of Cincinnati|
|Department:||Engineering : Aerospace Engineering|
|School Location:||United States -- Ohio|
|Source:||DAI-B 79/10(E), Dissertation Abstracts International|
|Keywords:||Nondestructive testing, Shot peening, Spectroscopy, Ultrasonic surface waves|
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