Effect of moisture on mixed-mode TSR on a glass/epoxy interface

Ferreira Vieira de Mattos, Daniel
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The understanding of interfacial failure in adhesively bonded structures is important for several sectors including transportation and infrastructure. This problem has motivated studies for several decades. Adhesives are polymeric and, as such, present time, temperature, strain rate and moisture dependence. The effect of moisture on interfacial adhesion and fracture is still an open problem and demands a deep multi-disciplinary study considering nonlinear viscoelasticity, fracture mechanics, diffusion, chemistry and surface science. This is justified through the mechanisms through which moisture can affect interfacial adhesion. The presence of moisture can degrade the interface integrity. The absorbed moisture also modifies the mechanical properties of the bulk adhesive as well as its interactions with substrates, which introduces changes in the response of the adhesively bonded structure as it is subjected to an external load. An additional complication for interfacial cracks constrained to grow along the interface is that crack growth is governed by the tensile and shear stresses at the interface as well as the interfacial interactions (adhesion energy, strength and range) embodied in traction separation relations and giving rise to the term mixed-mode fracture. This research investigates the effect of moisture on interfacial fracture for different mode-mixes. The content is developed in four parts. First, the adhesive is experimentally characterized via the following tests: mechanical loading, water diffusion, thermal and hygral expansion. These results introduce the second part: a nonlinear viscoelastic model calibrated considering all those measured properties. This model captures the effect of time, temperature, strain rate and moisture on the mechanical behavior of the adhesive. The third part deals with the fracture behavior of a glass/epoxy interface over a range of mode-mixes and moisture conditions. This is complemented by analyses including optical profile measurements of the fractured surfaces and extraction of traction and separation relations and toughness. Finally, a significant emphasis was placed on the numerical analysis which was required for each of the three components outlined above.