Compressive damage detection in concrete by a nonlinear ultrasonic testing procedure

The presence of cracks or damage phenomena in concrete elements is strictly correlated to the occurrence of nonlinear effects in their elastic response to an ultrasonic excitation. The Fourier analysis is often inadequate to pinpoint such effects, since the signal-to-noise ratio of higher order harmonics is usually very low. In order to overcome this drawback, we suggest an alternative procedure, denoted as Scaling Subtraction Method (SSM), to extract nonlinearity indicators from the recorded ultrasonic signals, based on the dependence of the response of the system on the excitation amplitude. The SSM is first described and then used to analyze the evolution of nonlinearity due to progressive crack formation induced by quasi-static compressive loads in concrete core-drilled cylinders. Our approach allows to distinguish the initial micro-damage formation from the semi-stable progression and the pre-rupture phases.