Ultrasonic response to material fatigue

In many engineering materials micro-cracks develop when the materials are subjected to repeated loading or fatigue loading. These micro-cracks increase the material attenuation and alter the ultrasonic wave speed. Careful experiments show that as a material is subjected to a greater number of fatigue cycles, its higher order (also known as the nonlinear) acoustic properties change significantly. The percentage change in the nonlinear acoustic properties is greater than the percentage changes in the wave speed and attenuation. However, experimentally it is very difficult to measure these nonlinear acoustic properties. Materials like concrete develop a large number of micro-cracks under fatigue loading and show measurable changes in its attenuation. However, some other materials, such as Plexiglas, apparently do not develop micro-cracks under fatigue loading. They look same, equally transparent, before and after the repeated loading up to the point of its failure. Its longitudinal wave speed and attenuation also do not change by any measurable amount due to the fatigue loading. Changes in the nonlinear ultrasonic properties are too difficult to measure. Can there be any relatively robust ultrasonic measurement to capture the property changes in such materials due to fatigue loading? It is investigated in this paper. It is found that some guided wave propagation characteristics change with fatigue loading. These changes are strong enough to be detected by ordinary ultrasonic measurements without taking help of any highly precise ultrasonic measuring instrument in the experimental setup.