Nonlinear ultrasonic evaluation of the fatigue damage of adhesive joints

An experimental method based on the nonlinear ultrasonic technique is presented to evaluate fatigue damage of an adhesive joint. In this paper, specimens made from AZ31 magnesium-aluminum alloy bonded through an epoxy layer are subjected to a fatigue load. The ultrasonic harmonics generated due to damage within the adhesive layer are measured; and the acoustic nonlinearity parameter (ANP) based on the fundamental and second harmonics is determined. The results show that the normalized ANP increases with the fatigue cycles. Furthermore, a theoretical model with different interfacial compression and tension stiffness is proposed to interpret the generation of second harmonics. (C) 2014 Elsevier Ltd. All rights reserved.