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.
Stress State Triaxiality
Meaning
Mathematical stress ratios quantify the relationship between hydrostatic stress and von Mises equivalent stress at a specific material point. Calculating stress state triaxiality identifies regions prone to void nucleation, micro crazing, and sudden brittle fracture in structural polymer parts. The parameter dictates fracture toughness under multiaxial loading, deep notches, and thick structural sections, but decreases to low values under pure uniaxial tension or pure shear.
Hydrostatic Ratio
High triaxiality ratios indicate dominant hydrostatic tension relative to equivalent shear stress. Tensile hydrostatic stress expands interchain volume and promotes void growth while suppressing plastic shear yielding. Ductile thermoplastics exhibit sudden brittle failure when triaxiality values exceed critical material thresholds.
Structural finite element analyses map triaxiality hot spots to locate vulnerable radii and thick rib intersections.
Fracture Transition
Notches and sharp corners raise local triaxiality by constraining lateral plastic deformation. Elevated triaxiality prevents shear band formation, forcing matrix failure through void growth and fibril rupture. Molders reduce local triaxiality by adding fillets, softening wall thickness transitions, and eliminating sharp internal re-entrant corners.
Contaminated regrind resin introduces internal void initiation sites that accelerate brittle failure under high triaxiality fields.
Constraint Severity
High hydrostatic tension suppresses plastic shear flow and promotes brittle cracking.