Thermo-Mechanical Degradation of EPDM Seals in EV Inverter Environments-A Study of Mullins Effect and Viscoelastic Relaxation

Abstract

Electric Vehicle (EV) inverters are essential components in the powertrain of electric vehicles, converting direct current (DC) from the battery into alternating current (AC) to power the electric motor. The seals within EV inverters are critical for maintaining system integrity, preventing contamination, and ensuring the proper functioning of inverter components. This study focuses on the performance of High Voltage Direct Current (HVDC) Press-In-Place (PIP) seals made from Ethylene Propylene Diene Monomer (EPDM) rubber, a Hyperelastic material known for its large elastic deformation and incompressibility.

The research investigates the mechanical performance of EPDM PIP seals for three material conditions: Least Material Condition (LMC), Nominal Material Condition (NMC) & Maximum Material Condition (MMC). The EPDM PIP seals were evaluated at different temperatures up to 1000hrs and under Thermal Cyclic loading.

A combined simulation approach incorporating the Mullins effect and viscoelastic relaxation was adapted to model stress softening over multiple thermal cycles and up to a large duration of 1000hrs. Results indicate that the PIP (Press-in-place) seals lose contact pressure during thermal cycling due to Stress Relaxation & Mullin’s effect occurring due to expansion & contraction of seals inducing cyclic Thermal strains. The results also reveal significant stress-softening in early cycles due to change in polymer chain structure, followed by gradual stabilization. Viscoelastic effects contribute to stress decay during hold periods, impacting long-term sealing performance. The study provides insights into the interplay between material behaviour and thermal loading offering design direction for improving seal reliability in EV inverter systems.