The role of the polymer/metal interphase and its residual stresses in the critical strain energy release rate (Gc) determined using a three-point flexure test

The critical strain energy release rate (G(c)), the residual stresses (sigma), Young's modulus (E), and the practical adhesion, characterized by ultimate parameters (F-max or d(max)), of organic layers made of DGEBA epoxy monomer and IPDA diamine hardener were determined. The prepolymer (DGEBA-IPDA) was deposited both as thick coatings and as a mechanical stiffener onto degreased aluminum alloy (5754) or chemically etched titanium alloy (Ti-6Al-4V). During the three-point flexure test used as a practical adhesion test [this test is also called the double cantilever adhesion test (DCAT)], the failure may be regarded as a special case of crack growth. To understand the real gradient properties of the interphase, substrate, and bulk polymer properties, a three-layer model was developed for quantitative determination of the critical strain energy release rate (G(c)). The particular characteristic of this model was to consider the residual stresses developed within the entire three-layered system, leading to an intrinsic parameter representing the practical adhesion between a polymer and a metallic substrate. Moreover, to determine the residual stresses generated in such three-layer systems, the gradient of interphase mechanical properties was considered. The maxima of residual stress intensities are found at the interphase/substrate interface, leading to an adhesional (interfacial) failure that is experimentally observed. The determination of the critical strain energy release rate by the three-point flexure test (DCAT) shows that residual stresses cannot be neglected. A comparison between the results obtained from the three-point flexure test (DCAT) and those obtained by the tapered double cantilever beam (TDCB) test is presented.