Meaning
Mechanical stress magnitude variations occur within components subject to cyclical temperature fluctuations. The metric known as thermal stress amplitude quantifies the peak-to-peak mechanical strain induced by differential thermal expansion between constrained materials. The parameter governs structural fatigue life in battery interconnects, cooling plates, and busbar joints.
Scope limits cover cyclic thermal stresses without including steady-state mechanical loads.
Failure Mechanism
Mismatched coefficients of thermal expansion generate internal strain when temperature cycles. High thermal stress amplitude drives mechanical fatigue crack growth in solder joints and welded busbars. Constraint conditions prevent free expansion, converting thermal expansion into mechanical shear stress.
Repeated thermal cycling leads to joint failure and increased electrical resistance. Micro-structural displacement accumulates at structural solder interfaces during cyclic heating.
Mitigation Design
Flexible interconnect geometry reduces constrained expansion forces by absorbing displacement. Material selection matches expansion coefficients across bonded interfaces where possible. Compliance in inter-cell pads reduces thermal stress amplitude across structural frames.
Lowering operational temperature swings directly reduces cyclic stress magnitude.
Fatigue Assessment
Accelerated thermal cycling tests evaluate joint integrity under extreme temperature swings. Finite element modeling predicts thermal stress amplitude distribution across rigid weld boundaries. Passing qualification standards requires surviving specified thermal cycle counts without electrical open-circuit events.
