The Effects of Elevated Temperature and Laminating on the Quasi-Static and Fatigue Properties of Electrical Steel

Abstract

Electrical steel is an alloy of steel that can have on average 3% silicon. These alloys are known to possess great magnetic properties, and this makes them an ideal material choice for the application of electric motors as they are used in the rotor and stator construction. Rotors and stators are comprised of laminated stacks of thin electrical steel sheets. As the automotive industry moves towards electrification of automobiles, it is crucial to comprehend the impact of laminating electrical steel and exposing it to higher temperatures on both the quasi-static and fatigue properties.

An electric motor can reach high temperatures under a heavy load, and it is important to understand the combined effect of temperature and load on the electrical steel’s performance to ensure the durability and safety of electric vehicles. This research investigated the quasi-static and fatigue strength and failure behavior of stamped 0.27mm thick electrical steel sheets as well as laminated electrical steel samples that were comprised of 8 individual sheets. Stress-controlled fatigue tests were performed at both room temperature and 150°C. The quasi-static test results showed a decrease in mechanical strength at the higher temperature in terms of the elastic modulus, ductility, yield strength and ultimate tensile strength. The single sheet samples had a decrease in yield strength by 21% and a decrease in elastic modulus by 5%. The laminated samples had a decrease in yield strength by 18% and a decrease in elastic modulus by 6.7%. The cyclic test results showed a decrease in the fatigue life of the samples at the elevated temperature compared to room temperature, in both the LCF (Low Cycle Fatigue) and HCF (High Cycle Fatigue) regimes. For the single sheet samples a decrease in life of 21% was observed at 0.51 normalized stress and 55% at 0.40 normalized stress. For the laminated samples a decrease in life of 38% was observed at 0.51 normalized stress and 56% at 0.40 normalized stress. Examination of the fracture surface of the single sheet samples tested at room temperature showed some inter-granular cleavage facets along with predominant trans-granular facets in the crack initiation zone and transitioned to only trans-granular cleavage facets in the crack propagation zone, at both the low- and high-cycle regimes. In contrast, the high temperature samples showed a smaller fatigue damage zone, and outside of this zone, the main failure mechanism was severe necking for both the low- and high-cycle samples. The laminated samples behaved similarly to the single sheet samples and possessed very similar quasi-static and fatigue properties. Regardless of test condition and load level, the crack always initiated from the breakaway zone on the stamped edge of the samples. The higher temperature adversely affected the overall strength, as the higher temperature releases residual stresses and annihilates dislocation density induced during the sheet manufacturing and sample preparation, resulting in shorter fatigue life.