Tool Part Interaction and Residual Stress Evolution in Constrained Post Cure Dwells
Constrained post cure dwells reduce residual stresses only when dwell temperatures exceed glass transition points to enable shear stress relaxation across tools.
Corner Locking Shear
Meaning
Deforming a dry woven fabric over a double-curvature mould forces crossing yarns to rotate until adjacent threads press against each other and resist further planar movement. High values of corner locking shear occur when local shear angles exceed the mechanical limit of the textile weave, causing the fabric to buckle out of plane. The resistance governs fabric drape capability across tight radius geometry up to the onset of ply wrinkling.
Prototype preforms evaluated on automated draping cells reveal whether specific tool radii induce excessive local stress. Unintended yarn jamming during automated placement creates severe thickness variations and localized resin-rich pockets during subsequent infusion.
Kinematic Limit
Yarn rotation angle determines the transition from smooth shear deformation to structural instability. When dry preforms undergo corner locking shear, lateral contact forces between warp and weft yarns rapidly escalate, locking the lattice grid. Pilot trials measure this angle using optical strain tracking to identify geometric features that demand darting or ply splitting.
Defect Formation
Out-of-plane buckling occurs when the applied forming force exceeds the critical shear stiffness of the jammed fabric. In regions dominated by corner locking shear, plies lift away from tool surfaces and form crests that prevent full mould closure. Production scrap rates increase when tooling radii are designed tighter than the natural drape limit of the fabric architecture.
Shear Behavior
Material response transitions from low-stiffness yarn rotation to high-stiffness yarn compression once the critical shear angle is breached. Measuring corner locking shear under bi-axial tension establishes the exact boundary between stable fabric shearing and localized wrinkling. Forming operations that respect this limit produce consistent component cross-sections with predictable fiber volume fractions.