Degradation mechanisms in brittle materials investigated by ultrasonic scanning

An experimental study of degradation mechanisms and patterning in brittle materials is reported. Specimens of a cast-in brittle material were subjected to external mechanical load. At various pre-peak load levels, through transmission ultrasonic measurements were taken at several locations of the sample. The damage evolution and its patterning were studied through analysis of the attenuation of the transmitted wave. For monotonically increasing load significant attenuation was observed at low load levels, while for unloading the attenuation changes were relatively small. The test results indicate random and non-symmetric distribution of attenuation within the sample. However, in general, higher attenuation was observed close to the load-free surfaces than in the interior of the sample. This observation signifies the so-called skin effect. Occasionally, at a few locations of various samples the energy of the received wave increased with increasing external load. This may indicate unloading of partially formed cracks. Measured surface strains are compared to the overall ones and the degradation patterning within the samples. The ultrasonic measurements are examined with respect to estimating the internal material length. The relation between ultrasonic wave attenuation and mechanical dissipated energy is examined.