31. Effect of Water and Mechanical Stress on Durability

Adhesive joints with structural adhesives are weakened significantly in air at high humidity, and the rate of decline is controlled by water diffusion into the adhesive. There appears, however, to be a critical relative humidity, and only if this is exceeded are joints significantly weakened; evidence is that this is about 65%. The most harmful effect of water is potentially at the interface between the adhesive and adherend; however, surface treatment of metallic adherends can greatly improve water durability. Accelerated aging using elevated temperatures and humidities can provide information on the environmental resistance of different systems but does not easily correlate to aging in natural conditions. The entry of water into the adhesive layer can conform to the simple Fickian model, which just depends on two parameters (diffusion coefficient and solubility coefficient), but in some cases it is non-Fickian. Current state-of-the-art environmental degradation modeling of bonded joints involves multi-physics finite element analysis combined with progressive damage modeling. Essentially, this involves three main steps. The first step is modeling moisture transport through the joint in order to determine the moisture concentration distribution in the joint as a function of time. The second step involves evaluation of the transient mechanical-hygro-thermal stress–strain state resulting from the combined effects of hygro-thermal effects and applied loads. The final step involves the incorporation of a damage or failure criterion in order to model the progressive failure of the joint and hence enable the residual strength or lifetime of a joint to be predicted.