Characterization and modeling of polymeric matrix under multi-axial static and dynamic loading

A polymeric matrix (3501-6) used in composite materials was characterized under multi-axial loading at strain rates varying from quasi-static to dynamic. Tests were conducted under uniaxial compression, tension, pure shear and combinations of normal and shear stresses. Quasi-static and intermediate strain rate tests were conducted in a servo-hydraulic testing machine. High strain rate tests were conducted using a split Hopkinson pressure bar (Kolsky bar) system made of glass/epoxy composite bars having an impedance compatible to that of the test polymer. The typical stress-strain behavior of the polymeric matrix exhibits a linear elastic region up to a yield point, a nonlinear elastic-plastic region up to an initial peak or "critical stress," followed by strain softening up to a local minimum, plateau or saddle point stress, and finally, a strain hardening region up to ultimate failure. A general three-dimensional elastic-viscoplastic model, formulated in strain space, was developed. The model expresses the multi-axial state of stress in terms of an effective stress, incorporates strain rate effects and includes the large deformation region. Stress-strain curves obtained under multi-axial loading at different strain rates were used to develop and validate the new elastic-viscoplastic constitutive model. Excellent agreement was shown between model predictions and experimental results.