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Abstract A history dependent and physically-motivated Internal State Variable (ISV) constitutive model is presented that simultaneously accounts for the effects of static recrystallization, dynamic recrystallization, and grain size with respect to the mechanical behavior under different strain rates, temperatures, and pressures. A unique aspect of our ISV constitutive model is that grain size and recrystallized volume fraction can be directly included along with its associated rate of change under deformation and time in a coupled manner. The present ISV constitutive model was calibrated to several metals (oxygen-free high conductivity copper, AZ31 magnesium alloy, pure nickel, and 1010 low carbon steel) and geological materials (olivine and clinopyroxene). The model calibration shows good agreement with the experimental stress-strain behavior and average grain size data. Validation of the ISV constitutive model was accomplished by applying complex and history sensitive thermomechanical problems once the model was calibrated: i) sequential transitions of different loading conditions and ii) a multistage tubing process. The history dependence naturally provided by ISVs enabled the present model to effectively capture the complex boundary value problems with changing boundary conditions. Highlights • An ISV-based model for recrystallization and grain size evolution is formulated. • The model unifies static and dynamic recrystallization with average grain size. • The model is successfully validated for polycrystalline metals and minerals. • The model captures history sensitive sequential boundary value problems. [ABSTRACT FROM AUTHOR]

The microstructure sensitive multistage fatigue model captured the fatigue life of a powder metal FC-0205 steel alloy. Uniaxial strain controlled fatigue data and microstructure information from sets of high and low porosity specimens calibrated the model. Strain-life behaviour depicted that above the plastic strain limit of 0-002 mm mm-1 in the low cycle fatigue regime, where ubiquitous plasticity occurred, the different porosity levels gave distinct, visibly different results. However, specimens tested below the plastic limit in the high cycle fatigue regime, where failure was dominated by local cyclic microplasticity, showed unclear fatigue lives at different porosity levels. Fractography using scanning electron microscopy showed no clear presence of striations; however, asserted striations in powder metal specimens were similar to geometrical features observed on fracture surfaces of monotonically loaded specimens. The experimental and microstructure data calibrated a fatigue model that allowed for satisfactory prediction of the varying porosity specimen strain-life curves. [ABSTRACT FROM AUTHOR]

A microstructure-based internal state variable (ISV) plasticity-damage model was used to model the mechanical behavior of a porous FC-0205 steel alloy that was procured via a powder metal (PM) process. Because the porosity was very high and the nearest neighbor distance (NND) for the pores was close, a new pore coalescence ISV equation was introduced that allows for enhanced pore growth from the concentrated pores. This coalescence equation effectively includes the local stress interaction within the interpore ligament distance between pores and is physically motivated with these highly porous powder metals. Monotonic tension, compression, and torsion tests were performed at various porosity levels and temperatures to obtain the set of plasticity and damage constants required for model calibration. Once the model calibration was achieved, then tension tests on two different notch radii Bridgman specimens were undertaken to study the damage-triaxiality dependence for model validation. Fracture surface analysis was performed using scanning electron microscopy (SEM) to quantify the pore sizes of the different specimens. The validated model was then used to predict the component performance of an automotive PM bearing cap. Although the microstructure-sensitive ISV model has been employed for this particular FC-0205 steel, the model is general enough to be applied to other metal alloys as well. [ABSTRACT FROM AUTHOR]

Abstract: This study is a first to analyze the microstructure and associated plasticity and fracture characteristics under finite deformations for quantifying different applied stress state, strain rate, and temperature effects on a porous powder metal steel (FC-0205). The complexities of the inelastic deformation from the coupled plasticity-damage evolution induced stress-state differences (compression, tension, and torsion) up to 20% at just 2% strain when examining the stress–strain behavior. As far as the temperature effect, when the temperature increased, the yield stress and work hardening rate decreased as is typical in steel alloys. As the strain rate increased, the yield stress displayed an increase. The yield stress did exhibit an increase as strain rate increased for both porosity levels. However, when the strain rate increased, the work hardening did not increase as is typical in dense steel alloys. It might be that the competitions of the microbuckling and dislocation emission and interaction under compression have different rates of change, and hence the applied strain rate differences could induce these non-intuitive results. The fracture process was dominated by the stereological statistics of the porosity. The pore volume fraction (porosity) ranged from 9 to 19%, the average pore diameter was approximately 6μm with a maximum of 160μm, and the average nearest neighbor distance was approximately 10μm with a maximum of 60μm. Since the nearest neighbor distance was close to the pore size, the stress interaction between pores was almost immediate upon loading thus causing a coalescence interaction that was dominant for the tensile fracture process. Some minor cleavage planes were also observed in tension. The greater the initial porosity, the lower the elastic modulus, work hardening rate, and strain to failure. For compression, because of the large amounts of porosity, microbuckling of ligaments between pores was a key inelastic deformation mechanism along with dislocation emission and movement. For torsion, cleavage planes were very prevalent on the fracture surfaces. [Copyright &y& Elsevier]

• A historical review on the formulation and developments of the MSF model are given. • A detailed mathematical summary of the MSF model is provided. • Revisions to the model are proposed to eliminate fundamental physics mistakes. This work reviews the history and development of the MultiStage Fatigue (MSF) model since the inaugural work of McDowell et al. [1] in 2003 and presents a revised model that integrates the improvements and additions made over the past two decades. The MSF model is a physically based model that combines the multiscale, heterogeneous microstructures with the considered boundary conditions of a structural component in order to predict fatigue life performance. As such, a hierarchy of fatigue sensitivities can be determined for the microstructures of a given material, providing insight to the material's structure–property relations. Given that microstructures are a direct result of processing methods, the MSF model is part of a greater paradigm to design performance-specific components following a chemistry-process-structure–property-performance logical flow. Since McDowell et al. [1] , the MSF model has been used for a broad range of materials, processing methods, and life-cycle performance environments with some key enhancements and abstractions added by various authors all contributing to the model in its final form as it is presented herein. Therefore, this review also serves to consolidate the changes made over the past two decades and present a refined mathematical framework and nomenclature. [ABSTRACT FROM AUTHOR]

Abstract: The fatigue behavior of resistance spot welding (RSW) in aluminum 6061-T6 alloy (AlMg1SiCu per International Standard Office nomenclature) was experimentally investigated. Three welding conditions, denoted as “nominal”, “low” and “high”, were studied to determine the microstructure of the weld nuggets. The process optimization included consideration of the forces, currents and times for main weld and post-heating. By successive iterations and “witness samples” collected, the optimum welding parameters were determined. Load control cyclic tests were then conducted on single weld lap-shear joint coupons to study the microstructure and fatigue life properties. These tests were used to characterize the fatigue behavior in spot welded specimens to elucidate the influence of the process parameters. This work revealed that the welding process parameters have a great influence in the microstructure and fatigue life of the 2mm-thick aluminum sheet resistance spot welded joints. Different fatigue failure modes were observed at several load ranges and ratios for a constant frequency and three welding currents. [Copyright &y& Elsevier]

Abstract: In this paper, we illustrate a formal calibration, validation, and verification process that includes uncertainty in an internal state variable plasticity-damage model that is implemented in a finite element code. The physically motivated continuum model characterizes damage evolution by incorporating material uncertainty due to microstructural spatial clustering. The uncertainty analysis is performed by introducing material variation through model validation and verification. The effect of variability in microstructural clustering and boundary conditions on the sensitivities and uncertainty of the plasticity-damage evolution for the 7075 aluminum alloy is characterized. The results show the potential of this methodology in the evaluation of material response uncertainty due to microstructure spatial clustering and its effect on damage evolution. For damage evolution, we have shown that the initial isotropic damage evolved into an anisotropic form as the deformation increased which is consistent with experimentally observed behavior for 7075 aluminum alloy in literature. Also, the sensitivities were found to be consistent with the physics of damage progression for this particular type of material. Through the sensitivity analysis, the initial defect size and number density of cracked particles are important at the beginning of deformation. As damage evolves, more voids are nucleated and grow and the sensitivity analysis illustrates this as well. Then, voids combine with each other and coalescence becomes the main driver, which is also confirmed by the sensitivity analysis. This work also shows that the microstructurally based damage evolution equations provide an accurate representation of the damage progression due to large intermetallic particles. Finally, we illustrate that the initial variation in the microstructure clustering can lead to about ±7.0%, ±8.1%, and ±9.75% variation in the elongation to failure strain for torsion, tensile, and compressive loading, respectively. [Copyright &y& Elsevier]