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Research Papers

Grain Level Dwell Fatigue Crack Nucleation Model for Ti Alloys Using Crystal Plasticity Finite Element Analysis

[+] Author and Article Information
Kedar Kirane, Mike Groeber

Computational Mechanics Research Laboratory, Department of Mechanical Engineering, The Ohio State University, Columbus, OH 43210

Somnath Ghosh1

Computational Mechanics Research Laboratory, Department of Mechanical Engineering, The Ohio State University, Columbus, OH 43210ghosh.5@osu.edu

Amit Bhattacharjee2

Computational Mechanics Research Laboratory, Department of Mechanical Engineering, The Ohio State University, Columbus, OH 43210

1

Corresponding author.

2

Also at Materials Science and Engineering, The Ohio State University.

J. Eng. Mater. Technol 131(2), 021003 (Mar 06, 2009) (14 pages) doi:10.1115/1.3078309 History: Received December 31, 2007; Revised November 02, 2008; Published March 06, 2009

Abstract

A microstructure sensitive criterion for dwell fatigue crack initiation in polycrystalline alloy Ti-6242 is proposed in this paper. Local stress peaks due to load shedding from time dependent plastic deformation fields in neighboring grains are held responsible for crack initiation in dwell fatigue. An accurately calibrated and experimentally validated crystal plasticity finite element (FE) model is employed for predicting slip system level stresses and strains. Vital microstructural features related to the grain morphology and crystallographic orientations are accounted for in the FE model by construction of microstructures that are statistically equivalent to those observed in orientation imaging microscopy scans. The output of the finite element method model is used to evaluate the crack initiation condition in the postprocessing stage. The functional form of the criterion is motivated from the similarities in the stress fields and crack evolution criteria ahead of a crack tip and dislocation pileup. The criterion is calibrated and validated by using experimental data obtained from ultrasonic crack monitoring techniques. It is then used to predict the variation in dwell fatigue lifetime for critical microstructural conditions. The studies are extended to field experiments on $β$ forged Ti-6242. Macroscopic aspects of loading are explored for their effect on dwell fatigue life of Ti-6242.

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Figures

Figure 1

Fractograph of a faceted initiation site for a failed Ti-6242 dwell fatigue sample

Figure 2

(a) OIM scan of the critical primary crack initiation site in the MS1 microstructure and (b) scale

Figure 3

Microtexture distributions (a) from OIM scan of critical region and (b) in the critical FE model of MS1

Figure 4

FE model for polycrystalline Ti-6242, which is statistically equivalent to the OIM scan of the critical region of microstructure MS1. Also shown is the contour of the c axis orientation distribution (radians).

Figure 5

Distribution of local variables: (a) loading direction stress (σ22) and (b) local plastic strain along a section parallel to the x-axis at the end of 1000 s for a creep simulation on the two models of microstructure MS1 with two different mesh densities

Figure 6

Distribution of local variables: (a) loading direction stress (σ22) and (b) prominent prismatic Schmid factor along a section AA parallel to the x-axis at the end of 1 dwell cycle and 300 dwell cycles for a dwell fatigue simulation of the MS1 model

Figure 7

Evolution of the maximum R over number of cycles for the FE models of microstructures (a) MS1, (b) MS2, and (c) MS3

Figure 8

Variation in predicted number of cycles to crack initiation by the criterion with (a) Schmid factor mismatch, (b) size of soft grain, and (c) level of microtexture

Figure 9

FE model for polycrystalline Ti-6242, which is statistically equivalent to the OIM scan of α/β forged Ti-6242. Also shown is the contour of c axis orientation distribution (radians).

Figure 10

Evolution of the maximum R over the number of cycles for (a) dwell fatigue simulation and (b) normal fatigue simulation on β forged Ti-6242

Figure 11

Effect of loading constraints on (a) plastic strain accumulation and (b) local maximum equivalent stress at the end of a creep simulation for 10,000 s

Figure 12

Effect of hold time on (a) plastic strain accumulation by cycle and (b) local maximum stress in the loading direction at the end of 100 cycles

Figure 13

Effect of stress ratio on (a) plastic strain accumulation by cycle and (b) local maximum stress in the loading direction at the end of 100 cycles

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