Meaning
Time-dependent reduction in stress or force within a material held under constant strain. Creep relaxation occurs in the structural foam, compression pads, and plastic housings of battery packs over extended operating lifetimes. The phenomenon causes a gradual loss of the initial clamping force holding cells together.
Material Behavior
Polymeric materials exhibit viscoelastic behavior that allows molecular chains to rearrange and relieve internal stress over time. Elevated operating temperatures accelerate this rearrangement, leading to faster force reduction than observed at ambient conditions. Metals used in tie rods also undergo this process, though at a much slower rate.
Structural Implication
Lower clamping force allows individual pouch or prismatic cells to expand more freely during charging. Excessive swelling damages cell internal structures, leading to delamination and increased contact resistance. Maintaining a minimum residual force is therefore necessary to prevent premature cell failure.
Sourcing Evaluation
Test specifications for compression pads require long-term stress relaxation profiling under thermal cycling conditions. Procurement teams utilize these relaxation curves to compare the longevity of different elastomeric materials from suppliers. Higher-grade materials maintain their elastic rebound longer, extending the useful life of the pack.
Selecting a material with a low relaxation rate ensures that the cells remain securely constrained throughout the vehicle warranty period.
