DEPTH MEASUREMENTS OF SHORT CRACKS IN PERSPEX WITH THE SCANNING ACOUSTIC MICROSCOPE

By using a scanning acoustic microscope (SAM) combined with the time-o f-flight diffraction technique, the depths of short cracks in perspex have been measured. Perspex was chosen for the work because it is a tr ansparent, isotropic, and acoustically slow material; therefore, it en ables one to measure the crack geometry in the light microscope, and i t eliminates the involvement of the Rayleigh surface waves and the inf luence of wave speed anisotropy on the acoustic measurements. Short cr acks of different geometries (depth to length ratios of 0.72 and 0.25) were initiated in commercial perspex by bending the specimens slightl y and adding a little acetone to the surface bearing a tensile stress while at 20-degrees-C and 50-degrees-C, respectively. The SAM was oper ated in a short pulse mode capable of time resolved acoustic measureme nts. While one scans the lens across a crack, a narrow acoustic pulse (<20ns wide) is sent into the specimen, and the intensities of the sig nals scattered from the specimen are recorded in a plot of time-of-fli ght versus lens position, called an s(t,y) plot. Ray theory provides a useful description concerning the significant contributions to the s( t,y) plot from perspex when the lens is scanned across a surface break ing crack. They are (1) specular reflection of incident waves from the specimen surface; (2) diffraction of incident waves at the crack mout h edges; (3) reflection of longitudinal lateral surface waves from the crack mouth; (4) conversion of incident waves into longitudinal later al surface waves at the crack mouth edges or the other way round; (5) propagation of longitudinal lateral surface waves in the specimen surf ace; (6) reflection of longitudinal waves from the crack face below th e surface, ff the crack is oblique; and (7) diffraction of longitudina l waves at the subsurface crack tip. As in the conventional time-of-fl ight diffraction technique, the crack tip diffracted signals were used to measure the crack depth. In order to enhance the contrast of the c rack tip diffracted signals in an s(t,y) plot, the specimens were bent during the acoustic measurements, and a subtraction algorithm was use d to remove the much stronger specular reflections. The resulting acou stic depth measurements agreed with direct light microscope measuremen ts within 93%, thus demonstrating the ability of the acoustic microsco pic to measure the depth of short cracks in perspex. The prospect of u sing the acoustic microscope to measure the depth of short cracks in o paque materials is apparent.