Meaning
Progressive structural damage occurring when a component undergoes repeated fluctuations in temperature constitutes this failure mode. Thermal cyclic fatigue results from the internal stresses generated as the material expands and contracts against constraints or its own uneven geometry. Over many cycles, these micro-stresses lead to crack initiation and eventual part failure.
Coefficient Variation
Differences in the rate of expansion between a polymer and an overmoulded metal insert create local stress concentrations. These points are the first to develop cracks during temperature swings.
Strain Accumulation
Hysteresis in the polymer means that not all deformation is recovered when the temperature returns to its baseline. Each swing adds a small amount of permanent damage to the molecular structure. This effect is particularly pronounced in crystalline polymers where the degree of crystallinity might change slightly with each cycle.
Accelerated aging tests simulate years of service in a few weeks of environmental chamber exposure.
Process Stability
Uniform wall thickness and consistent cooling during the moulding process minimize the initial internal stresses that worsen fatigue. Parts with high molded-in stress are more susceptible to early failure under thermal loads. Ensuring a stable melt temperature helps produce a more homogenous part that distributes these thermal stresses more evenly.
