Thermo-mechanics-damage-coupled Crystal Plasticity Model And Algorithm Investigation For Sheet Metal

The processing and service performance of advanced material have an important influence on the development of both vehicle engineering and aerospace engineering.The warm forming and hot stamping technique have been widely used in the manufacturing of structural components in vehicle engineering.Aluminum alloy and boron steel are the mostly applicable materials in these techniques to obtain higher strength and to reduce spring back.In aerospace engineering,the titanium alloy is generally used in the structural components of the engineering of hypersonic aircraft because of its high strength,low density,the unique resistance of high or low temperature and corrosion.The material microstructure is the key to increase the forming limit in the manufacturing and the service performance in the hypersonic loading.Under high temperature and different loading conditions,the current microscope technique is not sufficient to track the real-time mechanical responses and the micro structural change in the material.The crystal plasticity finite element method gives a prediction of the microstructural and thermo-mechanical responses in the perspective of the crystallography.This thesis proposed an advanced physical-based crystal plasticity method coupled with dislocation density,temperature field and damage mechanism to model the material behavior under different loading and environmental conditions.The main work including:1.In the single crystal plasticity,the thermal activation mechanism is introduced to propose the energy-based shear strain rate function for both finite and small strain frameworks.Dislocation density based hardening rule is considered to inform the forest and debris dislocation pinning mechanism inside the grain.The damage mechanism controlled by shear strain is coupled in the computation of mechanical responses to describe the stress softening during the post-necking region.Based on the Newton-Raphson method and forward Euler methods,implicit and explicit calculation procedures are derived for the stress updating in the advanced constitutive model.2.The introduction of the current multi-scale method application in crystal plasticity and the eigen-strain-based homogenization method used in the structural-scale simulation.The influence functions and the coefficient tensors are applied to obtain the macroscopic responses informed by the microstructural properties.The multi-part method could be used to increase the accuracy of describing stress or strain distributions inside the grains.3.In the representative volume element,the mesh size,mesh type and grain number are discussed for aluminum alloy,boron steel and titanium alloy.Based on the optimization method,a single set of physical-based parameters could be calibrated to capture the mechanical responses for one material at different temperatures and strain rates.The thermal activated material parameters are captured by using the strain rate and temperature jump tests.4.In the thermal tensile and shear tests of aluminum alloy and boron steel,reasonable agreement is obtained between experimental and numerical results for different specimens,temperature conditions and materials.This approach simultaneously captures the strain hardening rate,damage softening,non-linear post-necking and fracture strain.Microstructural effects on ductile fracture are tracked and investigated including dislocation density and crystallographic orientation.The results show that local dislocation density rise is associated with damage initiation.Different fracture morphologies and necking paths are caused by distinct initial misorientation distributions in comparison with experimental observation of 7075 aluminum alloy.Local misorientations are investigated and critical misorientation ranges are computed for promoting void growth in zigzag and straight fracture morphologies.Schmid factor is computed as not necessary variable to trigger void growth.5.In the small strain framework,the proposed approach is used to investigate the prediction of crack nucleation in titanium alloys in the context of a multiscale computational methodology that allows the effective upscaling such as grain level information to the structural scales.The eigenstrain-based reduced-order homogenization(EHM)approach has been extended to account for the presence of HCP(primary a phase)and BCC(? phase)grains,within which the deformation process is modeled using a dislocation density-based crystal plasticity formulation.The generalized crystal plasticity EHM implementation has been thoroughly verified to assess the accuracy of the reduced order model in capturing local and global behavior compared with direct CPFE simulations.A new and simple microstructural fatigue crack nucleation metric is proposed,numerically verified and validated against experimental observations in Ti-6242S subjected to pure fatigue and dwell fatigue.The proposed fatigue nucleation metric tracks the total sessile dislocation density and models the effect of dislocation pile-up near the hard-soft grain boundary that ultimately leads to facet formation.A structural scale study is performed to characterize the spatial distribution of fatigue crack nucleation sites in the context of a large structural component analysis.