MODELS FOR ULTRASONIC CHARACTERIZATION OF ENVIRONMENTAL DEGRADATION OF INTERFACES IN ADHESIVE JOINTS

In this paper we discuss two models of environmental degradation of ad hesive joints developed from experimental observation of the joint fai lure mode. It is found that after severe degradation, failure is domin ated by the interfacial mode, i.e., by failure at the interface betwee n adhesive and adherend. The fraction of failure in the interfacial mo de was found to be related to the joint strength and to be proportiona l to the frequency shift of a minimum in the spectrum of the reflected ultrasonic signal. One model considers an interface as an interphase in the form of a nonhomogeneous layer composed of two phases: ''soft'' which is viscoelastic (degraded part of the interphase) and ''stiff'' corresponding to the nondamaged interphase. Increase of the ''soft'' phase fraction corresponds to the process of degradation in the interp hase. The second model describes degradation in a form of disbonds fil led by adsorbed water at the interface. The disbonded interface is mod eled by transverse spring boundary conditions, with the complex spring stiffness representing the quality of the bond. The influence of diff erent disbond growth scenarios is considered. Advantages and drawbacks of these models are discussed.