2. Yield Criterion for Polymeric Matrix Under Static and Dynamic Loading

A polymeric matrix (3501-6) used in composite materials was characterized under multi-axial quasi-static and dynamic loading at varying strain rates. 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 system built for the purpose. This SHPB system was made of glass/epoxy composite (Garolite) bars having an impedance matching the test polymer closer than metals. The typical stress–strain behavior exhibits a linear elastic region up to a yields 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. It was observed that under multi-axial loading, yielding is governed by one characteristic property, the yield strain under uniaxial tension. Furthermore, it was found that the yield point varied linearly with the logarithm of strain rate. A general three-dimensional elasto-viscoplastic model was formulated in strain space expressed in terms of an effective strain and its yield point. A unified yield criterion was proposed to describe the onset of yielding under any state of stress and at any strain rate.