Sensitivity of resonance-frequency shifts to defect location in adhesive-bonded joints

Abstract

Techniques based on frequency shifts and mode-shape analysis are being investigated to determine their feasibility for characterizing defects in adhesive-bonded joints in automotive structures. It is well known (Rayleigh–Ritz derivation) that introduction of a crack-like defect into a structure reduces its stiffness and results in a corresponding downward shift in resonance frequencies. Experimental and modeling results show that analysis of resonance-frequency shifts is much more complicated for bonded joints where the defect consists of a gap in the adhesive layer. Structures containing a defective joint sometimes exhibit higher-resonance frequencies than a structure with an undamaged joint. Such results have been observed in laboratory experiments on aluminum plates with adhesive-bonded T joints, and in finite-element simulations for a variety of structures. In all cases, the defects studied are gaps in the adhesive layer of the joints. While the reduction in mass associated with a gap in the adhesive layer is very small, the mass effect overwhelms the frequency-decreasing effect of the reduced stiffness when the defect is located in a low-stress region of the joint. Thus, the direction and magnitude of frequency shifts depend on the resonance mode and the location of the defect.