Experimental and computational investigations on strain rate sensitivity and deformation behavior of bulk materials made of epoxy resin structural adhesive

The deformation behavior and strain rate sensitivity of an epoxy resin structural adhesive and a CTBN (carboxyl-terminated butadiene-acrylonitrile)-modified epoxy resin adhesive are experimentally investigated using an INSTRON-type material testing machine and a split Hopkinson pressure bar apparatus. The experimental results show some fundamental features of a typical compressive stress-strain behavior of amorphous glassy polymers with linear elastic and nonlinear inelastic deformation stages. In the inelastic deformation, a peak stress and a strain-softening stage after the peak can be observed in the entire range of strain rate from 10(-4) to 10(3) s(-1). In addition, it can be found that the relationship between the peak stress and strain rate on a semi-logarithmic plot is linear in the range of low strain rate. However, the slope of the curve changes at a high strain rate, and the nonlinear behavior of the peak stress can be obtained against the strain rate. In order to describe such a nonlinear peak stress-strain rate relationship and the deformation behavior of the structural adhesives in a wide range of strain rate on the basis of the experimental results, a plastic shear strain rate is formulated. Then, a three-dimensional constitutive model is derived based on a four-elements model with an elastic series element by considering the adhesive to be a glassy polymer. The plastic deformation rate tensor is expressed by the effective stress, which is the difference between the total stress and back stress, and the plastic shear strain rate proposed here. The stress tensor can be obtained by solving nonlinear simultaneous equation. The formulated constitutive model is implemented into the commercial FE code, ABAQUS/Explicit, and then a computational simulation is performed. As a result, the validity of the proposed model is shown by comparing the experimental result