Effects of a Fracture on Ultrasonic Wave Velocity and Attenuation in a Homogeneous Medium

Nondestructive ultrasonic testing is commonly used to assess damage in infrastructure mostly based on elastic wave velocity. This study focuses on understanding the effects of a thin fracture not only on ultrasonic elastic wave velocity but also on attenuation. Experiments are performed to quantitatively assess the effects of a thin fracture within polymethylmethacrylate (PMMA) specimens. Wave velocity and attenuation are measured across the width of these homogeneous specimens using the ultrasonic pulse velocity method. Seventeen specimens are tested for three different conditions (intact, with a hole, and with a fracture) for two different thicknesses. First, specimens made of two PMMA blocks with an intact fused interface are tested; then, specimens with a small hole (created for generating stress concentration) perpendicular to the interface and milled ends are tested; and, finally, specimens with an induced fracture at the fused interface are tested. Four additional specimens, two with fused (but weak) interfaces between blocks and two solid blocks, are tested during fracture growth under uniaxial strain-controlled test conditions. In fact, wave attenuation can cause the first arrival to be undetected and overestimated by up to 10 %. This error in the selection of the first arrival could be misinterpreted as a change in wave velocity when fractures are present in the material. Although wave velocity shows marginal reduction, less than 4 %, when a thin fracture is present, wave energy attenuates by up to 60 %. This work demonstrates quantitatively that wave attenuation measurements from selected frequency bands in the Fourier spectra can be used to identify the presence of thin fractures using ultrasonic testing.