Improvement of the environmental fatigue resistance of epoxy-based electrical steel laminates by pre-treatment of the substrate

Electrical generators or engines are based on full surface bonded steel plies. Temperature control is of high relevance to achieve reduced heat dissipations along with improved efficiency. This paper evaluated the effect of service relevant operating conditions (i.e., cooling agents, elevated service temperature and fatigue stresses) on the delamination behavior of electrical steel laminates on specimen and lab level. Therefore, a linear elastic fracture mechanics method and a set-up suitable for superimposed environmental and cyclic fatigue testing was implemented and employed. Two electrical steel laminate configurations were investigated. Pre-treated and un-treated sheets were double side coated with an epoxy varnish. Further, a catalyst was applied on both sides of the coated sheets made from the pre-treated substrate. Double cantilever beam specimens were manufactured by stacking and laminating 6 coated substrates. The fractured surfaces of the fatigue tested specimen were characterized by laser confocal microscopy and infrared spectroscopy in hemispheric reflectance mode. Threshold strain energy release rate values (Gth) ranging from 16 to 62 J/m2 were obtained depending on the investigated coated substrates and the environmental conditions. Laminates produced with pre-treated sheets and catalyst exhibited notably enhanced Gth and decreased rates of crack propagation within the stable crack growth regime. A similar crack growth behavior was ascertained in hot air or oil-based cooling agent. The crossover point of the temperature dependent Gth values was indicative for the glass transition temperature of the adhesives. The fractured surfaces presented mainly cohesive failure at higher strain energy release rate values followed by mixed cohesive and interfacial failure when approaching to the threshold regime.