Calibration and evaluation of nonlinear ultrasonic transmission measurements on thin bonded interfaces

Interfaces in bonded structures significantly influence the mechanical behavior of components and often limit their load capacity. This requires nondestructive testing techniques allowing one to investigate the interaction forces in adhesive joints and to evaluate the quality of bonds. To this end the nonlinear stress-strain relations of adhesives and adhesive interfaces, which cause a nonlinear modulation of ultrasonic waves in reflection as well as in transmission, may be exploited. Bonded interfaces which are much thinner than the ultrasonic wavelength can approximately be described only by the binding forces, without explicitly taking into account the material properties of the adhesive layers. These may be measured by the amplitudes and phases of ultrasonic waves transmitted through the interface. Measurements are presented on aluminum plates joined together by thin epoxy adhesive layers. A threshold behavior of the harmonics generated in the adhesive layer has been observed. Their amplitudes depend on the excitation following the power series expansion of a quasi-static interaction force curve, and their phases vary little for low amplitude excitation. Exceeding the threshold causes a change in the response of the interface. The input and output ultrasonic amplitudes in the interface are calibrated interferometrically to obtain the absolute interaction force. The ultrasonic transmission data are related to destructive tensile tests of the adhesive bonds. Contact address: Dr. Sigrun Hirsekorn