Calibration and Evaluation of Nonlinear Ultrasonic Transmission Measurements of ThinBonded Interfac.pdfVIP

403 25 Calibration and Evaluation of Nonlinear Ultrasonic Transmission Measurements of Thin-Bonded Interfaces S. Hirsekorn, A. Koka, S. Kurzenhäuser, and W. Arnold Abstract Interfaces in bonded structures influence the mechanical behavior of compo- nents significantly, and often limit their load capacity. This requires nondestruc- tive testing techniques allowing one to investigate the interaction forces in adhe- sive joints and to evaluate the quality of bonds. To this end the nonlinear stress–strain relationships of adhesives and adhesive interfaces, which cause a nonlinear modulation of ultrasonic waves in reflection as well as in transmis- sion, may be exploited. Bonded interfaces which are much thinner than the ul- trasonic wavelength can be approximately 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 alumi- num plates joined together by thin epoxy adhesive layers. A threshold behavior of the harmonics generated in the adhesive layer has been observed. Their am- plitudes depend on the excitation following the power series expansion of a qua- si-static interaction force curve, and their phases vary little for low-amplitude ex- citation. Exceeding the threshold causes a change in the response of the inter- face. The input and output ultrasonic amplitudes in the interface are calibrated interferometrically to obtain the absolute interaction force. The ultrasonic trans- mission data are related to destructive tensile tests of the adhesive bonds. 25.1 Introduction The interfaces in bonded structures influence the mechanical behavior of com- ponents significantly. Therefore, an