Meaning
Mechanical reduction of resistance under repeated deformation characterizes polymers subjected to alternating strain amplitudes below their ultimate tensile strength. During repeated loading cycles, cyclic strain softening reduces the peak stress required to maintain a prescribed strain amplitude in ductile thermoplastics such as polyamides or polycarbonates. This response stabilizes when the polymer reaches a steady state or continues until microvoid coalescence initiates cracking.
Stress Decay
Molecular realignments under reversed shear forces drive the progressive loss of stiffness during cyclic loading. As polymer chains untangle and shear bands form within the amorphous phase, cyclic strain softening drops the required load across consecutive cycles. Virgin resins exhibit predictable decay curves based on molecular weight distribution, whereas regrind blending introduces chain scission sites that accelerate the drop in load capacity.
Tensile datasheets capture only single-cycle yield stress, which leaves moulders unaware of the rapid stiffness loss occurring in snap-fit hinges or repeated-latch components.
Hysteresis Loop
Energy dissipation per cycle shrinks as the material loses resistance under strain-controlled fatigue. In semi-crystalline polymers like polypropylene, localized necking and crystalline lamellae slip cause the hysteresis loop to narrow while shifting along the stress axis. This softening behavior alters dimensional recovery in dynamic seals, causing persistent clearance gaps or oil leaks in automotive housings.
Molded residual stresses from non-uniform cooling accentuate this effect by adding baseline mean stresses to the external cyclic load.
Tooling Boundary
Part design limits must account for load drop rather than static yield points when sizing living hinges or structural clips. Standard laboratory tests measure strain-controlled strain softening up to ten thousand cycles, beyond which thermal dissipation from internal friction can induce localized melting instead of pure mechanical softening. Wall thickness variations create localized strain concentrations that magnify the softening effect in thin-walled sections relative to nominal part geometry.
Regrind content exceeding fifteen percent typically lowers the cycle count at which mechanical failure initiates.
