PSI - Issue 61

Orhun Bulut et al. / Procedia Structural Integrity 61 (2024) 3–11

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O. Bulut et al. / Structural Integrity Procedia 00 (2024) 000–000

Bache (2003). Following this, the fatigue life reduction due to dwell period has become a significant design criterion in engine component design. Dwell induced crack initiation has been experimentally observed to occur around α grains with their c-axis oriented close to the loading axis (see Sinha and Ghosh (2006)), due to their interactions with ’softer’ adjacent grains. The general consensus links dwell susceptibility to the formation of large commonly oriented regions of α -phase, referred to as microtextured regions (MTRs) or macrozones within the material (see Le Biavant et al. (2002)). Crack nucleation and propagation are governed through time-dependent creep deformation occurring at relatively low temperatures, which is why this behavior is referred to as cold dwell fatigue, with the life debit observed to decrease at higher temperatures. The fracture mechanisms in dwell fatigue have been observed to di ff er from traditional low cycle fatigue. Subsurface crack initiation is a characteristic feature of the dwell fatigue in contrast to low cycle fatigue where cracks generally initiate at the surface (see e.g. Evans and Bache (1994); McBagonluri et al. (2005)). Experimental studies of the dwell fatigue phenomenon have proved imprecise, as certain critical micro-structural features may not be present in a representative sample simply due to the greater scale of material volume in practical application. This at times has led to an overestimation of the fatigue life and endurance limit under lab conditions (see e.g. Ozturk et al. (2019); Ghosh et al. (2007)). Time dependent behavior has considerable dependence on the grain orientation due to the low symmetry of the HCP α phase. A combination of local crystallographic orientations with respect to the loading direction causes stress redistribution revealed by crystal plasticity simulations as shown in Hasija et al. (2003); Dunne et al. (2007). During the dwell period, the stress rises in the hard grain adjacent to soft grains, a phenomenon referred to as load shedding or load redistribution, which is considered to be one of the main causes of crack nucleation in the polycrystalline titanium microstructure. In order to study such phenomena at the scale of individual grains, researchers utilize crystal plasticity finite element (CPFE) models. These models o ff er a granular perspective, detailing how plastic behavior emerges based on the orientation of each grain and the specific slip systems that are relevant to it. This approach allows for a nuanced understanding of how microscopic structures influence material behavior (see e.g. Venkatramani et al. (2007); Bulut et al. (2022); Acar et al. (2022); Bulut et al. (2023); Gu¨nay et al. (2023)). The e ff ects of various parameters on dwell fatigue have been explored through crystal plasticity simulations, such as temperature (Zheng et al. (2017)), multiaxial loading (Cuddihy et al. (2017)) and microstructure (Liu et al. (2021)). Failure with CPFE method is usually simulated using cohesive zone elements (see Yalc¸inkaya et al. (2019, 2021)) for intergranular cracking or with stress and plastic strain based uncoupled failure criteria (see Aydiner et al. (2024)) for intragranular failure. For the prediction of dwell fatigue life, a couple of studies used CPFE in tandem with fatigue crack nucleation models (see e.g. Anahid et al. (2011)). There are a limited number of studies on the coupling of the CPFE method with a failure model specifically for dwell fatigue. In this context, the CPFE technique coupled with a phase field fracture model is used to illustrate the crack initiation and propagation in Ti representative volume elements (RVE) in the current study. The phase field paradigm provides a computationally robust and coupled, non-local damage model which is able to capture both crack nucleation and propagation (see Francfort and Marigo (1998); Bourdin et al. (2000)). Initially developed for brittle fracture (Miehe et al. (2010)), it has since been adapted to far more complex cases such as dynamic fracture (Borden et al. (2012)), fatigue fracture (Waseem et al. (2024)), anisotropic fracture (Teichtmeister et al. (2017)) and ductile fracture (Ambati et al. (2015); Borden et al. (2016)). Existing phase field ductile fracture frameworks have been adapted to crystal plasticity (see e.g. De Lorenzis et al. (2016); Cheng et al. (2020)) with a recent example focused on studying crack propagation in Ti polycrystals (see Maloth and Ghosh (2023)). In the current work, various combinations of microstructures are generated in representative volume elements and their influence on the crack initiation and propagation is studied. To investigate the interactions between misoriented grains, a local crystal plasticity model coupled with phase field fracture model is utilized. Energy equivalent contribu tions of elastic and plastic deformation influence the evolution of the phase field parameter. A bi-crystal RVE structure is prepared to explore the nature of hard-soft grain interactions on the evolution of phase field following the work in Maloth and Ghosh (2023). Furthermore, a polycrystal HCP structure is analyzed under static and dwell fatigue load to assess the model’s capability in predicting load shedding between hard and soft grains and subsequent crack initiation.