On the numerical modeling of nucleation and growth of microstructurally short cracks in polycrystals under cyclic loadin
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On the numerical modeling of nucleation and growth of microstructurally short cracks in polycrystals under cyclic loading Martin Boeff1, Hamad ul Hassan1,a)
, Alexander Hartmaier1
1
Interdisciplinary Centre for Advanced Materials Simulation, Ruhr-Universität Bochum, Bochum 44801, Germany Address all correspondence to this author. e-mail: [email protected]
a)
Received: 14 May 2019; accepted: 21 August 2019
In the scope of this work, a micromechanical model based on the crystal plasticity finite element method is proposed and applied to describe the nucleation and growth of microstructurally short fatigue cracks in polycrystalline materials under cyclic loads. The microstructure is generated in the form of a representative volume element of a polycrystalline material with equiaxed grains having columnar structure along thickness and random crystallographic texture. With this model, we investigate the influence of loading amplitude on the crack growth behavior. It is shown that for smaller strain amplitudes, a single crack nucleates and propagates, while for larger strain amplitudes several independent crack nucleation sites form, from which microcracks start propagating. It is also observed that the global plastic strain amplitude decreases from the initial to the final cycle, during total strain-controlled loading. However, this can even increase the crack growth rate because the crack advance is governed by the local plastic slip which accumulates at the crack tip over the number of cycles. With this work, it is shown that micromechanical modeling can strongly improve our understanding of the mechanisms of short-crack nucleation and growth under fatigue loading.
Introduction There is a continuously growing demand regarding lightweight material design in the aeronautical or automotive industry to reduce material costs and carbon emissions. To cope with these requirements, structural components that undergo cyclic loading are usually designed to be damage tolerant based on the expected number of load cycles during their fatigue life. To follow this design philosophy, a profound knowledge of the fatigue data of the material is necessary, and reliable lifetime prediction models are required [1]. In the engineering components subjected to repeated cyclic loading, fatigue is caused by progressive damage accumulation; i.e., permanent microstructural and topological changes, which are caused by repeated and partially irreversible cyclic microstrains, take place. This accumulation of microstrains over the increasing number of cycles ultimately leads to fatigue damage [2]. Hence, for the accurate lifetime assessment and prediction of a material, it is imperative to properly model crack initiation and growth under cyclic loading. For a realistic coupling of these two
ª Materials Research Society 2019
phenomena causing crack initiation and propagation, it is necessary to develop a deep understanding of plasticity and fatigue damage [3]. The total fatigue life of a component is primarily composed of crack initiation (
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