Nonlinear ultrasonic evaluation of damage to bonding interface under cyclic temperature fatigue

Adhesively bonded structures possess various industrial applications, such as safety-critical structures in the aerospace and automotive industries. With the increasing using of adhesive joints, corresponding methods of evaluating and testing the structural integrity and quality of bonded joints have been widely investigated and developed for the structural health monitoring. Studies show that the damage and degradation of material are closely related to the nonlinearity of ultrasonic waves propagating within the material. In this paper, for the evaluating of the damage to bonding interface under cyclic temperature fatigue, acoustic nonlinear parameters (ANPs) of specimens made of aluminum alloy 6061 and modified acrylate adhesive are measured experimentally by using the nonlinear ultrasonic technique; and thus the variations of the ANPs with the fatigue time under high and low cyclic temperature are obtained for the bonded specimens. The study shows that the ANP, which serves as an indicator of material properties, remains nearly unchanged in the initial stage of high temperature cyclic fatigue test, and the ANP obviously increases with temperature cyclic time increasing. For low temperature cyclic fatigue test, the ANP increases rapidly with the increase of temperature cyclic time in the initial stage, and its value growth slows down in the later stage. Further discussion shows that the increase of third order elastic constant is the main reason for the change of ANP for high temperature cyclic fatigue, and that the change of the tensile stiffness of the bonding interface is the main source for the change of the ANP for low temperature cyclic fatigue. It is shown that the ANP based on the theoretical model increases consistently with the experimentally measured values. The present research is expected to provide a promising way of characterizing and monitoring the damage to bonding interface under cyclic temperature fatigue.