Nondestructive evaluation of thermal aging of adhesive joints by using a nonlinear wave mixing technique

This paper describes the use of a wave mixing technique to monitor thermal aging of adhesive joints. This newly developed technique measures the acoustic nonlinearity parameter of the adhesive in the joint. One of the significant features of this technique is that it requires only one-side access to the adhesive joint being measured, which significantly increases the technique's utility in field applications. To demonstrate the effectiveness of the new technique, measurements are conducted on adhesively joined samples that are thermally aged under different heating conditions, i.e., different temperature profiles and aging times. The results show that the measured acoustic nonlinearity parameter of the adhesive is very sensitive to thermal aging. On the other hand, linear reflection from the adhesive layer remains almost a constant among all the aging conditions investigated in this study. It is therefore concluded that the acoustic nonlinearity parameter is a much more sensitive measure of thermal aging of adhesive joints.