The prediction of formation and early growth of microstructurally small fatigue cracks requires use of constitutive models that accurately estimate local states of stress, strain, and cyclic plastic strain. However, few research efforts have attempted to systematically consider the sensitivity of overall cyclic stress-strain hysteresis and higher order mean stress relaxation and plastic strain ratcheting responses introduced by the slip system back-stress formulation in crystal plasticity, even for face centered cubic (FCC) crystal systems. This paper explores the performance of two slip system level kinematic hardening models using a finite element crystal plasticity implementation as a User Material Subroutine (UMAT) within ABAQUS (Abaqus unified FEA, 2016) [1], with fully implicit numerical integration. The two kinematic hardening formulations aim to reproduce the cyclic deformation of polycrystalline Al 7075-T6 in terms of both macroscopic cyclic stress-strain hysteresis loop shape, as well as ratcheting and mean stress relaxation under strain- or stress-controlled loading with mean strain or stress, respectively. The first formulation is an Armstrong-Frederick type hardening-dynamic recovery law for evolution of the back stress [2]. This approach is capable of reproducing observed deformation under completely reversed uniaxial loading conditions, but overpredicts the rate of cyclic ratcheting and associated mean stress relaxation. The second formulation corresponds to a multiple back stress Ohno-Wang type hardening law [3] with nonlinear dynamic recovery. The adoption of this back stress evolution law greatly improves the capability to model experimental results for polycrystalline specimens subjected to cycling with mean stress or strain. The relation of such nonlinear dynamic recovery effects are related to slip system interactions with dislocation substructures.
Advances in higher resolution experimental techniques have shown that metallic materials can develop fatigue cracks under cyclic loading levels significantly below the yield stress. Indeed, the traditional notion of a fatigue limit can be recast in terms of limits associated with nucleation and arrest of fatigue cracks at the microstructural scale. Although fatigue damage characteristically emerges from irreversible dislocation processes at sub-grain scales, the specific microstructure attributes, environment, and loading conditions can strongly affect the apparent failure mode and surface to subsurface transitions. In this paper we discuss multiple mechanisms that occur during fatigue loading in the high cycle fatigue (HCF) to very high cycle fatigue (VHCF) regimes. We compare these regimes, focusing on strategies to bridge experimental and modeling approaches exercised at multiple length scales and discussing particular challenges to modeling and simulation regarding microstructure-sensitive fatigue driving forces and thresholds. We conclude by discussing some of the challenges in predicting the transition of failure mechanisms at different stress and strain amplitudes.
This paper examines the mechanical response of a simple bolted joint, the Brake–Reuß beam, under shock loading. This is done by creating a high-fidelity finite element model of the beam and subjecting it to a quasi-static bolt load followed by a dynamic shock load. The influence of several parameters on the beam’s response is studied, which include impact force, impact duration, impact location, and residual stress. The results indicate that when the energy input into the beam is held constant, the most influential parameter is the shock’s frequency and that increasing its frequency significantly increases dissipation. The next most influential parameter is the impact location, though its effect is frequency dependent and becomes stronger for higher frequencies. Finally, the results show that while residual stresses can significantly modify the contactpressure distribution, they have minimal influence on the energy dissipated due to friction resulting from shock loading.
Fatigue crack initiation in the high cycle fatigue regime is strongly influenced by microstructural features. Research efforts have usually focused on predicting fatigue resistance against crack incubation without considering the early fatigue crack growth after encountering the first grain boundary. However, a significant fraction of the variability of the total fatigue life can be attributed to growth of small cracks as they encounter the first few grain boundaries, rather than crack formation within the first grain. Our paper builds on the framework previously developed by the authors to assess microstructure-sensitive small fatigue crack formation and early growth under complex loading conditions. Moreover, the scheme employs finite element simulations that explicitly render grains and crystallographic directions along with simulation of microstructurally small fatigue crack growth from grain to grain. The methodology employs a crystal plasticity algorithm in ABAQUS that was previously calibrated to study fatigue crack initiation in RR1000 Ni-base superalloy. Our work present simulations with non-zero applied mean strains and geometric discontinuities that were not previously considered for calibration. Results exhibit trends similar to those found in experiments for multiple metallic materials, conveying a consistent physical description of fatigue damage phenomena.
The number of cycles required to form and grow microstructurally small fatigue cracks in metals exhibits substantial variability, particularly for low applied strain amplitudes. This variability is commonly attributed to the heterogeneity of cyclic plastic deformation within the microstructure, and presents a challenge to minimum life design of fatigue resistant components. Our paper analyzes sources of variability that contribute to the driving force of transgranular fatigue cracks within nucleant grains. We also employ crystal plasticity finite element simulations that explicitly render the polycrystalline microstructure and Fatigue Indicator Parameters (FIPs) averaged over different volume sizes and shapes relative to the anticipated fatigue damage process zone. Volume averaging is necessary to both achieve description of a finite fatigue damage process zone and to regularize mesh dependence in simulations. Furthermore, results from constant amplitude remote applied straining are characterized in terms of the extreme value distributions of volume averaged FIPs. Grain averaged FIP values effectively mitigate mesh sensitivity, but they smear out variability within grains. Furthermore, volume averaging over bands that encompass critical transgranular slip planes appear to present the most attractive approach to mitigate mesh sensitivity while preserving variability within grains.