Crystal Plasticity Based Fe Model for Understanding Microstructural Effects on Creep and Dwell Fatigue in Ti-6242

Abstract

This paper is aimed at identifying key microstructural parameters that play important roles in the failure initiation of polycrystalline Ti-6242 subjected to creep and dwell loading. A finite element model, incorporating rate dependent elastocrystal plasticity, is developed for analyzing evolving variables in material microstructure. The crystal plasticity parameters are characterized by a combination of microtesting, orientation imaging microscopy, computational simulations, and minimization process involving Genetic algorithms (Ga). Accurate phase volume fractions and orientation distributions that are statistically equivalent to those observed in orientation imaging microscope scans are incorporated in the computational model of polycrystalline Ti-6242 for constant strain rate, creep, and dwell tests. The computational model is used for the identification of possible microstructural variables that may result in local crack initiation. Basal normal stress, equivalent plastic strain, and stress in loading direction are considered as candidate parameters, of which the former is chosen as most probable from results of creep and dwell experiments and simulations. Creep induced load shedding phenomena is observed to lead to high value stresses that cause failure. The role of grain orientation with respect to the loading axis and misorientation with its neighbors, in causing load shedding and stress localizations is explored.

Effects of crystallographic orientation and grain morphology on crack tip stress state and plasticity

“…Cold dwell fatigue [1,[4][5][6][7][8][9][10] refers to a failure mode observed in titanium alloy components due to a stress hold (dwell) at peak stress during cyclic loading at ambient temperatures. This issue, first recognised in the early 1970s after the uncontained failure of two titanium alloy aero-engine fan discs [1], remains a serious concern for all engine manufacturers [4].…”

Dual-time scale crystal plasticity FE model for cyclic deformation of Ti alloys

“…Subsequently, a constrained Voronoi tessellation of the 3D region is executed, where the tessellation process is biased by statistics of the generated distributions. Crystallographic orientations are subsequently assigned to the grains using the orientation probability assignment method (OPAM), misorientation probability assignment method (MPAM0 and micro-texture probability assignment method (MTPAM) algorithms discussed in [8,9]. Such a synthetic microstructure is generated and used to study the local response under cyclic loading.…”

“…The hcp crystals consist of 5 different families of slip systems, namely the basal a , prismatic a , pyramidal a , first order pyramidal c + a and second order pyramidal c + a with a total of 30 slip systems while the bcc crystal system consists of different slip families, 111 110, 111 112 and 111 123 with a total of 48 slip systems. The details of material modeling and validation for Ti6242 using crystalline plasticity can be found in [7][8][9]24]. A brief description of the material model is presented here.…”

“…Crystal plasticity models with explicit grain structures are effective in predicting localized cyclic plastic strains [4][5][6][7][8]. The recent years have seen significant efforts in modeling cyclic plasticity and fatigue with considerations of microstructural stress-strain evolution [9][10][11][12][13][14]. Most studies represent the 3-d microstructure in a simple way using cubic, dodecahedron or tessellated grains with statistically equivalent orientation distributions.…”

The Influence on Microstructure and Microtexture on Fatigue Crack Initiation and Growth in Alpha + Beta Titanium

“…The sample volume and shape of microtextured regions 6) have recently been implicated in dwell fatigue life variability in laboratory specimens making the development of transfer functions for components difficult. Modeling efforts [7][8][9][10] have revealed the effects of microtexture at the grain level in creating non-uniform stress-strain distributions, the accumulation of plastic slip near grain boundaries and the propensity for crack nucleation.…”