Abstract
A fatigue fracture mechanics method was developed, implemented and used to characterize the crack kinetics of laminated electrical steel. Tests were carried out on double cantilever beam specimen based on electrical steel sheets and an epoxy adhesive layer with a thickness of just 10 µm. Two different crack length measurement methods based on optical measurement or compliance calculation were evaluated. As validation method specimens with different artificial initial crack length were used. The accuracy is limited to thin and stiff adhesives ensuring a small plastic zone size compared to the crack length. The accuracy of the crack length evaluation methods was higher for the compliance based approach meeting standard specifications at low to intermediate crack length values. The compliance based method was applied to investigate a specimen series varying the pre-curing temperature of the epoxy adhesive systematically. The fatigue tests were performed at ambient condition and 60 °C. At ambient temperature, laminates based on coated electrical steel pre-cured at low to intermediate temperatures revealed rather slow crack propagation rates in comparison to laminates made from coated steel pre-cured at 250 °C. The differences in the stable crack propagation rate of the investigated laminates at ambient conditions were up to two orders of magnitude higher. At 60 °C, the worst performance was also obtained for laminates based on coated steel sheets pre-cured at 250 °C. The failure mode was similar for laminates tested at 23 and 60 °C. While interfacial failure was ascertained for laminates based on coated steel sheets pre-cured at 190 and 250 °C, the better performing laminates made from sheets with epoxy coating pre-cured at 210 or 230 °C exhibited mainly cohesive failure. Hence, adequate pre-curing conditions are of utmost importance for the crack kinetics of adhesively bonded electrical steel.
Highlights
Bonded rotor and stator cores made of stacked electrical steel sheets are gaining importance especially in the auto motive and wind energy industry
The overall objective of this paper is to develop a methodology for testing and evaluation of the fatigue crack growth behaviour of epoxy/electrical steel laminates with adhesive layer thickness of 10 μm
That the operating temperatures of stacked electrical steel laminates are well above ambient, a further aim of this paper is to develop a testing system allowing to perform fatigue crack growth tests at elevated temperatures
Summary
Bonded rotor and stator cores made of stacked electrical steel sheets are gaining importance especially in the auto motive and wind energy industry. The stacked structure of cores is necessary to reduce the eddy current losses and to raise the effi ciency of electric machines. Conventional joining methods like interlocking or welding have serious disadvantages. Stress concentrations next to local mechanical connections are avoided by bonding of stacked electrical steel sheets. Received 31 January 2021; Received in revised form 4 October 2021; Accepted 6 October 2021.
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