Nonlinear ultrasonic transmission measurements and destructive tensile tests on thin adhesive bonds

Samples consisting of aluminum plates joined together by thin epoxy adhesive layers are investigated by nonlinear ultrasonic transmission measurements. In order to allow both ultrasonic transmission experiments and tensile loading, the ultrasonic set-up was integrated into a small laboratory scale tensile-test stage. The ultrasonic transmission data are related to destructive tensile tests of the adhesive bonds in order to extract convenient ndt parameters for bond-quality evaluation. The absolute strain amplitudes were determined by using an optical interferometer and a capacitive transducer for calibration. A threshold behavior of the transmitted harmonics was observed. Their amplitudes depend on the excitation following the power series expansion of a quasi-static interaction force at low amplitude excitation, and the phases of the signals vary little. Exceeding the threshold causes a considerable change in the dynamic behavior of the interface. The description of thin bonded interfaces only by binding forces without taking into account explicitly the material properties of the adhesive layer was used to determine interaction forces in the interface. It turned out that the phase of the transmitted wave at the excitation frequency and the interaction forces in the interface correlate with the tensile strength rather than the second or the third-order nonlinearity parameter and the distortion factor.