Depth Measurements of Short Cracks in Perspex with the Scanning Acoustic Microscope

Abstract

By using a scanning acoustic microscope (SAM) combined with the time-of-flight diffraction technique, the depths of short cracks in perspex have been measured. Perspex was chosen for the work because it is a transparent, isotropic, and acoustically slow material; therefore, it enables one to measure the crack geometry in the light microscope, and it eliminates the involvement of the Rayleigh surface waves and the influence of wave speed anisotropy on the acoustic measurements. Short cracks of different geometries (depth to length ratios of 0.72 and 0.25) were initiated in commercial perspex by bending the specimens slightly and adding a little acetone to the surface bearing a tensile stress while at 20°C and 50°C, respectively. The SAM was operated in a short pulse mode capable of time resolved acoustic measurements. While one scans the lens across a crack, a narrow acoustic pulse (<20ns wide) is sent into the specimen, and the intensities of the signals scattered from the specimen are recorded in a plot of time-of-flight 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 breaking crack. They are (1) specular reflection of incident waves from the specimen surface; (2) diffraction of incident waves at the crack mouth edges; (3) reflection of longitudinal lateral surface waves from the crack mouth; (4) conversion of incident waves into longitudinal lateral surface waves at the crack mouth edges or the other way round; (5) propagation of longitudinal lateral surface waves in the specimen surface; (6) reflection of longitudinal waves from the crack face below the surface, if the crack is oblique; and (7) diffraction of longitudinal waves at the subsurface crack tip. As in the conventional time-of-flight diffraction technique, the crack tip diffracted signals were used to measure the crack depth. In order to enhance the contrast of the crack tip diffracted signals in an s(t,y) plot, the specimens were bent during the acoustic measurements, and a substraction algorithm was used to remove the much stronger specular reflections. The resulting acoustic depth measurements agreed with direct light microscope measurements within 93%, thus demonstrating the ability of the acoustic microscopic to measure the depth of short cracks in perspex. The prospect of using the acoustic microscope to measure the depth of short cracks in opaque materials is apparent.