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Research Papers

Micromechanics Study of Fatigue Damage Incubation Following an Initial Overstrain

[+] Author and Article Information
Yibin Xue1

Mechanical and Aerospace Engineering, Utah State University, Logan, UT 84322anna.xue@usu.edu

Amanda M. Wright

 Lockheed Martin Corporation, Houston, TX 77058

David L. McDowell

GWW School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405

Mark F. Horstemeyer, Kiran Solanki, Youssef Hammi

Center for Advanced Vehicular Systems, Mississippi State University, Mississippi State, MS 39762

1

Corresponding author.

J. Eng. Mater. Technol 132(2), 021010 (Feb 19, 2010) (8 pages) doi:10.1115/1.4000227 History: Received March 19, 2009; Revised September 08, 2009; Published February 19, 2010; Online February 19, 2010

Understanding and quantifying the effects of overloads/overstrains on the cyclic damage accumulation at a microscale discontinuity is essential for the development of a multistage fatigue model under variable amplitude loading. Micromechanical simulations are conducted on a 7075-T651 Al alloy to quantify the cyclic microplasticity in the matrix adjacent to intact or cracked, life-limiting intermetallic particles. An initial overstrain followed by constant amplitude cyclic straining is simulated considering minimum to maximum strain ratios of 0 and 1. The nonlocal equivalent plastic strain at the cracked intermetallic particles reveals overload effects manifested in two forms: (1) the cyclic plastic shear strain range is greater in the cycles following an initial tensile overstrain than without the overstrain and (2) the initial overstrain causes the nonlocal cumulative equivalent plastic strain to double in subsequent tensile-going half cycles and triple in subsequent compressive-going half cycles, as compared with cases without an initial tensile overstrain. The cyclic plastic zone at the microdiscontinuity corresponds to that of the maximum strain during the initial overstrain and the nonlocal cyclic plastic shear strain range in the matrix near the intact or cracked inclusion is substantially increased for the same remote strain amplitude relative to the case without initial overstrain. These results differ completely from the effects of initial tensile overload on the response at a macroscopic notch root or at the tip of a long fatigue crack in which the driving forces for crack formation or growth, respectively, are reduced. The micromechanical simulation results support the incorporation of enhanced cyclic microplasticity and driving force to form fatigue cracks at cracked inclusions following an initial tensile overstrain in a fatigue incubation model.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

The computational configuration and boundary conditions, finite element meshes, and meshes around an isolated inclusion

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Figure 2

The accumulation of the nonlocal equivalent plastic strain as a function of loading cycles at the interface of an intact Fe-rich particle for a remote uniaxial normal strain with an amplitude of 0.7%

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Figure 3

The maximum equivalent plastic strain at the interface of an intact Fe-rich particle calculated after cyclic saturation for a remote uniaxial strain amplitude in the range of 0.6–0.8%: (a) at the centroid of the element (point-wise measure) of maximum equivalent plastic strain and (b) averaged (nonlocal) maximum equivalent plastic strain over four elements of mesh I

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Figure 4

Nonlocal micronotch-root maximum plastic shear strain at the interface of a fractured Fe-rich particle under remote uniaxial strain amplitudes ranging from 0.25% to 0.7% for Rε=−1.

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Figure 5

Microplasticity at three typical kinds of intermetallic particles with Young’s modulus of 50 GPa, 120 GPa, and 134 GPa under an applied cyclic strain amplitude of 0.7%: (a) nonlocal maximum plastic shear strain range and (b) the plastic zone size normalized by the particle diameter

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Figure 6

The microplasticity around Fe-rich particles in 7475-T651 Al matrix under remote strain amplitude ranging from 0.25% to 1%: (a) nonlocal maximum plastic shear strain range and (b) the plastic zone ratio

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Figure 7

Nonlocal equivalent plastic strain at Fe-rich particle for CA strain of 0.3% and for the case of an initial OL corresponding to 0.7% amplitude overstrain for one cycle and CA of 0.3% thereafter using micromechanical simulation

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Figure 8

The increments of the nonlocal equivalent plastic strain at each loading step: (a) for constant CA of 0.3%, (b) for an initial OL strain of 0.7% for one cycle and 0.3% thereafter, and (c) comparison of increments of equivalent plastic under CA and OL loading with strain ratio of Rε=−1

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Figure 9

The nonlocal maximum plastic shear strain as a function of applied strain with overstrain ratio, Rε=0 and −1, followed by a constant amplitudes of 0.25%, 0.3%, and 0.5%: (a) for a first cycle 1% overstraining and (b) for first cycle 0.7% overstraining

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Figure 10

Nonlocal maximum plastic shear strain range versus applied strain amplitude for first cycle 0.7% and 1% overstraining followed by a constant strain amplitudes of 0.25%, 0.3%, and 0.5%: The load ratio for all cases is Rε=0 or −1

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Figure 11

Correlation of initial overstrain effects on the nonlocal maximum plastic shear strain range for a load ratio of Rε=−1

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