Durability of bonded assemblies - Predictive approach

To assess the reliability of a homogeneous, isotropic material, fatigue tests are commonly used to determine the material's damage threshold through its endurance limit. If failure in bonded joints were always localized within the adhesive (cohesive), the concepts of continuum mechanics and fracture mechanics would be sufficient to dimension bonded structures through the determination of failure criteria. Unfortunately, as we mentioned in the first part of our study of durability , bonded joints in hot or humid environments delaminate as the interface weakens. The phenomena governing the establishment of the interface are not yet fully understood, so the laws governing the temporal evolution of substrate/adhesive interfaces have not yet been established.

However, several approaches can provide "indicators" of the durability of the bonded assembly within its environment. The thermodynamic approach, for example, proposes a stability criterion for the adhesive system. The experimental approach, based on mechanical tests, predicts the behavior of bonded joints from accelerated laboratory tests and empirical or semi-empirical laws. The output data are either reductions in mechanical strength or changes in fracture energies. In this case, the experimenter is usually in a "who can do more can do less" scenario. Other promising approaches have been developed in recent years. They are based on numerical calculation and the use of statistical and probabilistic methods as predictive tools.