Defocus cracks

This gives the contrast in an imaging acoustic microscope due to a crack in a specimen, for any defocus and lens position. Once again a great advantage of... 

Fig. 12.5. A crack from an indent in glass (a) z = 0 (b) z = — 3.8 pm (c) z = —5.2 pm ELSAM, 1.5 GHz. The experimental line-scans superimposed on the images can be compared with the plots calculated using two-dimensional theory (eqns (12.2), (12.13), and (12.14)) with elastic constants from Table 6.3 and values of defocus (a) z = 0 (b) z = —4.2 pm (c) z = —6.8 pm. The values of z in the calculations were chosen for best fit the reason for the discrepancy is not known, though no doubt there are the usual uncertainties associated with thermal drift, the measurement of z, and the frequency and pupil function used (Briggs etal. 1990).

Fig. 12.8. Short incipient fatigue cracks in stainless steel (a) negative defocus, z = —20 (tm (b) positive defocus, z = +32 m 0.37 GHz (Rowe et al. 1986).

A clue to this is where the fringes from a crack in the matrix stop. The best way to understand it is to calculate the contrast. Figure 12.9(b) shows the results of such a calculation, plotted as a line scan with the vertical axis at the interface. The material constants of the matrix and the fibre were used to calculate the Rayleigh parameters from (12.46) V(x) was then calculated for the frequency and defocus used in Fig. 12.9(a), and plotted with the same horizontal scale for direct comparison. In the calculated curve, not only do the mean levels... 

When a specimen includes an elastic discontinuity, such as an edge, a step, a crack, or a joint interface, an acoustic image of the elastically discontinuous and peripheral portions visualized by the SAM shows unique contrast such as fringes or black stripes when the lens is defocused toward the specimen. As a model shown in Fig. 12, this type of contrast appears as an interference effect of surface acoustic waves incident on and reflected from elastic discontinuities. Since the thickness of the film is about 100 nm, the same effect may not be remarkable at the operating frequency ranging from 0.6 to 1 GHz for the discontinuities. However, this technique is useful for enhancing existence of surface microcracks that conventional microscopes may not be able to visualize... 

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