Measurement and evaluation of nonlinear ultrasonic transmission on adhesive bonds

The experimental and theoretical research work on Nonlinear Elastic Wave Spectroscopy (NEWS) at IZFP is, so far, focused to investigations of nonlinear ultrasonic transmission through bonded interfaces to assess the local bond quality. The generation of higher harmonics by a quasi-monochromatic excitation is evaluated. Compressional waves are insonified perpendicularly to a bonded interface by a narrow-band transducer. The transmitted ultrasonic signals are detected by a broad band transducer, recorded, and Fourier transformed. A phase-sensitive detection unit was added to measure the phase of the ultrasonic signals relative to the rf-carrier which excites the transducer. The resulting amplitude and phase spectra were stored in dependence on the transmitting power. For thin bonded interfaces with a thickness much less than the ultrasonic wave length a description only by binding forces without taking into account explicitly the material parameters of the adhesive is used. This description renders possible the determination of local interaction forces in the bond by the amplitudes and phases of the ultrasonic waves transmitted through the interface. This contribution reports on nonlinear ultrasonic transmission measurements on bonded samples provided by the aircraft industry and the evaluation of the experimental results. Local second and the third-order nonlinearity parameters, distortion factors, interface interaction forces and also the phases of the transmitted waves are investigated and discussed in order to ascertain the significance of the different parameters for the quality assessment of adhesive bonds. The techniques shall also be applied on damaged samples containing microcrack fields, cracks and/or delaminations, because those defects form contacts similar to thin weak bonds, and thus, similar measuring effects can be expected.