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Highly-crosslinked epoxies are widely used as matrix material in highperformance fibre-reinforced composites, governing several aspects of their behaviour. Yet, attempts to model their viscoplastic response have been limited, as compared to thermoplastic polymers. However, the viscoplastic response of highly cross-linked thermosets, in particular under compression, is very similar to glassy thermoplastics below their glass-transition temperature, exhibiting complex features such as strain- and temperature-sensitivity, post-yield softening followed by rehardening, and severe non-linearity upon unloading. These phenomena can in principle be captured by advanced constitutive models for glassy polymers mixing phenomenological and micromechanical elements. However, this often comes at the price of a very large number of parameters (sometimes more than 30), while the physical basis of such models remains limited. In this work, we develop a micromechanical model to describe and predict the viscoplastic behaviour of the RTM6 epoxy resin. The model relies on the concept of STZ’s (Shear Transformation Zones) as the elementary carriers of plasticity, whose activation is sensitive to the local stress, temperature and microstructural state. STZ’s interact through the (possibly polarized) elastic stress field, resulting in overall viscoplastic flow. This model involves only 5 parameters to identify, all with a physical meaning. It is rich enough to quantitatively capture all the experimental trends, even the complex rate-reversal phenomenon observed during creep tests performed after plastic deformation at intermediate stress levels. While such a model cannot replace closed-form constitutive models for the treatment of large-scale components, it provides physical insights into the small-scale mechanics and is also useful to identify the parameters in macroscopic models. |
