Standard Uniaxial Testing Deficiencies in Amorphous Polymer Characterization
Uniaxial tensile tests misrepresent multi-axial yield, physical aging, and strain-rate sensitivity in amorphous polymers, causing structural failure in real parts.
Drucker-Prager Yield Criterion
Meaning
Pressure-dependent plastic yield formulations modify classical von Mises stress models by incorporating hydrostatic stress components into the failure envelope. Applying the drucker-prager yield criterion accounts for the observed asymmetry where polymers exhibit higher yield strength in compression than in tension. The criterion models structural plastic deformation in mineral-filled, reinforced, and bulk amorphous polymers, but fails to predict yield in highly anisotropic or oriented fiber composites.
Stress Modification
Hydrostatic compression squeezes polymer chains together, which elevates internal friction and increases yield resistance. Tensile hydrostatic stress expands interchain volume and promotes earlier yield initiation. Standard isotropic metal criteria ignore this pressure dependence, leading to unsafe part designs under complex multiaxial stress fields.
Engineers calibrate the model using matching tensile and compressive yield test data.
Yield Surface
Yield surface representation forms a smooth cone in principal stress space rather than the uniform cylinder of metal plasticity. Polymer yield behavior shifts outward along the hydrostatically compressed axis, providing structural margin in compressive load paths. Injection molded parts containing high molded-in compressive stresses benefit from this strength enhancement.
Excess regrind degrades matrix cohesion, lowering the shear yield parameters calibrated in the constitutive equations.
Formulation Boundary
Linear yield cones overpredict strength at extreme hydrostatic pressures.