Evaluating Interfacial Foam Compression Set Effects on Module Stress Distribution Profiles
Foam compression set collapses baseline clamping force, shifting planar pressure to rigid edges, elevating thermal impedance, and risking tab fatigue.
Kohlrausch-Williams-Watts Model
Meaning
Viscoelastic relaxation in polymeric materials is often represented by a stretched exponential function rather than a simple exponential decay. This mathematical formulation, known as the Kohlrausch-Williams-Watts model, predicts the long-term stress relaxation of polyurethane foam pads used in battery modules. It allows designers to estimate how much force a foam pad will exert against expanding cells after years of constant compression.
Mathematical Framework
The relaxation of stresses is described by a stretched exponential function that incorporates a characteristic relaxation time and a stretching parameter. In the Kohlrausch-Williams-Watts model, the stretching parameter varies between zero and one, representing the distribution of relaxation times within the polymer network. By fitting experimental test data to this model, engineers can predict the residual pressure of the cushion at any future point in the cell’s lifetime.
This mathematical representation provides a reliable alternative to running multi-year tests for every new foam formulation.
Polymer Mechanics
Molecular chain reorientation in cross-linked polymers is a time-dependent process that is accelerated by high temperatures. The Kohlrausch-Williams-Watts model captures this behavior by accounting for the non-linear decay of internal forces under constant strain. Sourcing teams use this data to verify that the compression pad will maintain the minimum required load to prevent cell movement, even after prolonged exposure to the high temperatures found in active battery modules.
Sourcing Application
Material suppliers provide these modeling coefficients to help cell pack designers run structural simulations. These models ensure that the selected foam pads prevent cells from shifting while avoiding excessive forces that could deform the module plates. Sourcing teams utilize this information to compare foam compounds from different chemical suppliers.