Non-destructive evaluation of gradient properties of hardened steel by surface acoustic waves measurements

Measurements of velocity dispersion on hardened steel were achieved by means of an absolute method for surface acoustic waves (SAW) based on the Knife-edge technique. SAW are generated and transmitted to the surface sample by interdigital transducers (barium titanate, quartz or lithium niobate). A laser beam sent on this surface is deflected by the ripple produced on it by the SAW and the deviation of the beam is detected by a photomultiplier with a knife in front of it. The steel sample is modelled by a two media system (layer-substrate). Numerical calculations of the Rayleigh wave velocity dispersion are achieved for various thickness values of the surface layer (perturbed steel). Measurements and calculations agree well for a particular value of the surface layer thickness. A confirmation of this value is obtained by similar measurements on the other face of the sample on which a chromium layer of 21 μm was previously deposited. This face is modelled by a 3 media system (Chromium-perturbed steel-bulk steel) and presents a measured dispersion that agrees well with the dispersion calculated using the previously determined parameters of the perturbed steel