Meaning
Directional hardening behaviour causes polycrystalline metals to exhibit a lower yield stress under reversed loading than under continuing forward deformation. Plastic prestrain in tension shifts the yield locus, leading to early micro-yielding when the material experiences compression. The bauschinger effect originates from internal backstresses generated at dislocation pile-ups against grain boundaries or precipitate interfaces.
Unloading locks in microscopic residual stresses, which aid dislocation movement in the reverse direction. This phenomenon ceases to govern material responses once total reversal strain exceeds the primary prestrain regime and isotropic hardening dominates the microstructure.
Tooling Consequence
Cyclic mechanical loads inside high-pressure injection moulds induce asymmetrical plastic flow across sliding surfaces. When mould cores experience bending during cavity filling, the bauschinger effect accelerates core deflection on clamp release. Ejector pins subject to forward drive force yield prematurely under retraction friction.
Tool steels like AISI P20 display early micro-plasticity under high-tonnage cycling. This mechanism lowers expected fatigue resistance in core inserts subject to alternating hydraulic pressures.
Dimensional Drift
Progressive micro-yielding modifies the cavity geometry across long production runs. Part wall thickness drifts beyond specification tolerances when core supports undergo cumulative reverse-yield deformation. Virgin resins run under constant injection pressures suddenly display flashing or localized sink marks as cavity boundaries move.
Regrind mixtures with higher melt viscosity generate larger peak pressure swings, exacerbating tooling deflection. Process technicians often misdiagnose this mechanical mould wall movement as resin thermal shrinkage variation.
Microstructural Recovery
Heat treatment cycles and thermal soaking mitigate internal backstresses within stressed mould inserts. Annealing allows entangled dislocation arrays to climb and annihilate across subgrain boundaries. Once the backstresses dissolve, reverse yield limits return to virgin alloy baseline numbers.
Tool maintenance procedures for high-cavitation moulds track cumulative cycles to schedule stress-relief treatments before dimensional loss occurs. Unchecked cycling without thermal intervention leads to permanent tool clearance degradation.
