A polymeric matrix (3501-6) used in composite materials was characterized under multi-axial loading at strain rates varying from quasi-static to dynamic ones. Tests were conducted under uniaxial compression, tension, pure shear and combinations of compression and shear. 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 built for the purpose. This SHPB system was made of a glass/epoxy composite (Garolite) bars having an impedance that is closer to that of the test polymer than metals. The typical stress-strain behavior of the polymeric matrix exhibits a linear elastic region up to a yield point, a nonlinear elastoplastic region up to an initial peak or "critical stress," followed by a strain softening region up to a local minimum and finally, a strain hardening region up to ultimate failure. A general three-dimensional elasto-viscoplastic model was developed incorporating strain rate effects, and including the large deformation region. The model was formulated in strain space unlike most models in the literature. The stress-strain curves obtained were used to develop and validate the new elasto-viscoplastic constitutive model.