Apparent interface bond strength

Apart from the elastic stress transfer at the perfectly bonded interface, another important phenomenon that must be taken into account is the stress transfer by friction, which is governed by the Coulomb friction law after the interface bond fails. Furthermore, matrix yielding often takes place at the interface region in preference to interfacial debonding if the matrix shear yield strength, Xm is significantly smaller than the apparent interface bond strength, tb. It follows thus... 

Furthermore, the absorption of water by the interphasal polymer can reduce its yield strength below the interfacial bond strength. Thus, the apparent interfacial shear strength will be reduced, and a yield front, rather than a debond, will propagate along the fibre interface modifying the stress transfer micromechanics at a fibre break. A consequence is that the stress concentrations in adj acent fibres to the fibre break will be reduced, and the probability of the formation of a flaw of critical dimensions is also reduced. The number of interacting fibre breaks associated with a flaw of critical dimensions will increase. 

In order to have a durable joint, it is necessary that the interfacial region maintain its mechanical integrity upon exposure to moisture. Davis and Venables have shown that the primary mechanism for degradation of strength of an aluminum-epoxy joint during moisture aging is the conversion of the oxide to boehmite, AlOOH, which apparently is weakly bonded to the base metal. AES analysis of joints broken after exposure to high humidity has shown that fracture occurs at or near the boehmite-metal interface. ... 

AU the test methods have different stress concentration states and the apparent shear strength from different test methods shouidn t be compared with each other. None of the test methods in this study induced exclusive shear failure in the joint bonding layer. With a high strength joint materiai, the joint layer and the joint/substrate interface are likely stronger than the substrate, making valid shear failure in the joint layer unachievable. 

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