Damage measurements in epoxy structural adhesives using microhardness

The design of adhesively joined components requires the ability to predict and model the joint response under expected operating conditions, including crash events for vehicle structures. Specifically, quantifying adhesive material damage accumulation from static and dynamic loading is essential to predict the response of bonded components in such scenarios. In this study, Vickers microhardness measurements were used as a forensic technique to quantify damage in bulk tensile samples for three structural epoxy adhesive materials: an untoughened epoxy; a toughened epoxy; and a high toughness epoxy. The samples were tested to failure over a range of strain rates (0.002-100 s−1), and hardness measurements were taken post-test along the gauge length. In general, for toughened epoxies the damage extended over much of the sample gauge length, while the un-toughened epoxy demonstrated damage localization at the failure location. The hardness data support the contention that mechanisms such as crazing and shear banding play a role in microhardness changes in toughened epoxies. Increments in strain rate led to an increase in the damage localization. Microhardness measurements were a valuable tool to quantify damage, with the limitation that the magnitude of change in hardness could be adhesive-specific, hypothesized to be related to competing damage mechanisms. The benefits of this approach include the ability to spatially quantify damage, to detect strain rate effects and to carry out measurement of damage post-test in support of constitutive modeling and failure analysis.