PSI - Issue 57

Sanjay Gothivarekar et al. / Procedia Structural Integrity 57 (2024) 487–493 S. Gothivarekar et al./ Structural Integrity Procedia 00 (2023) 000 – 000

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1. Introduction The increasing demand for versatile electrical transportation means has promoted research in the performance of electrical drive systems. Here, an important design objective is to improve the service life of the electrical motor. Nomenclature 0

Initial length Central length Width 1 Principal stress ∆ 1 E Young’s modulus ′ Fatigue strength coefficient ′ Fatigue ductility coefficient Fatigue strength exponent Fatigue ductility exponent Cycles Principal strain range

Within an electric motor, the rotor core plays a central role in transferring electromagnetic energy into mechanical torque at high speeds. Due to the continuous dynamic loading conditions, fatigue cracks can start to initiate in the thin plates that make up the core. To this end, a growing interest has been found in the fatigue fracture behavior of thin electrical steels [1]. However, a recent study on the fatigue cracking characterization of 30WGP1600 electrical steel reported a significant amount of scatter in the fatigue test data [2]. Micrographs were investigated of initial cracks at the edge of the minimum cross-section, indicating that the preferred sites for initiation are found near crystal grains, inclusions and surface grain boundaries. Another study on thin tensile steel specimens confirmed a similar form of slip band crack initiation near surface grain boundaries, implying an increased microstructure sensitivity of thin metals [3]. To understand the fatigue behaviour and the associated amount of scatter on the experimental data, several fatigue damage models have been developed [4]. These models combine constitutive modelling and polycrystalline grain structures to understand the microstructural effects on fatigue. In addition, the implementation of Voronoi diagrams in finite element analysis (FEA) software can be used to represent polycrystalline models, thereby simulating multiple grains with different orientations and interactions [5, 6, 7]. As a result, microscopic variation was found in the local deformation as opposed to the idealistic uniform deformations found in isotropic continuum models. Another recent article explored the impact of Voronoi partitioning and microstructure inhomogeneity on the fretting fatigue simulation of S45C steel [7]. A significant impact on the stress distribution was reported, where the variance in Young ’ s modulus resulted in a more irregular stress distribution. Fatigue life estimation was then performed using continuum damage mechanics (CDM) and the theory of critical distances (TCD). The current study adopts a similar approach for modelling the grain structure and investigating the influence on the fatigue life estimation. 2. Numerical modelling 2.1. Reference model A numerical framework is developed to investigate the influence of microstructure topology and elastic heterogeneity on the fatigue life. A loop is generated that copies a reference model and introduces a new region with unique Voronoi partitioning and material assignment. The reference model consists of a 2D-deformable planar shell part that represents the tensile fatigue specimen. In Figure 1, the basic fatigue specimen geometry is shown that was adopted from literature [2]. Using symmetry boundary conditions, only half of the specimen was modelled as shown