Links between Surface Changes with Strain whitening, Shear Banding and Damage in a Toughened Epoxy Adhesive

Abstract

Owing to the increased use of toughened epoxy adhesives in current transportation light weighting efforts, it is critical that the damage mechanisms observable as strain whitening in these materials are understood and quantified. Quantification of damage is needed for finite element constitutive models used in structural design; however, thin bond lines in adhesive joints limit direct observation of the adhesive. In this study, microscope observations of bulk material specimens subjected to tensile loading were linked to strain whitening and damage in a toughened epoxy adhesive. Cracks on the surface were observed to open during loading, with strain whitening at the crack tips and with the initiation and propagation of shear bands. The stresses approximated at the crack tips suggested that particle cavitation could be occurring in these regions. Image analysis showed that strain whitening was present at crack tips and that these areas served to initiate the shear-bands. Changes in tensile specimen stiffness and strength were evaluated during load-unload and reload testing, and were linked to the presence of crack growth, as well as the formation of shear bands. Considering changes in strength, the predicted damage level before failure (D~18%) was lower than that predicted using traditional load-unload stiffness (D~35%), attributed to short-term viscoelastic effects; however, damage calculated from load-reload material stiffness (D~19%) was in good agreement with the damage estimated from changes in strength. A new approach, calculating damage from direct image analysis of strain whitening on the free surface (D~21%) was in good agreement with damage quantified by changes in strength and stiffness, with the benefit of quantifying damage over the loading history of the test sample and identifying areas of damage localization.