On the driving force for fatigue crack formation from inclusions and voids in a cast A356 aluminum alloy |
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Authors: | Ken Gall Mark F Horstemeyer Brett W Degner David L McDowell Jinghong Fan |
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Affiliation: | (1) Materials & Engineering Sciences Center, Solid & Material Mechanics Department, Sandia National Laboratories, Livermore, CA 94550, USA;(2) GWW School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA |
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Abstract: | Monotonic and cyclic finite element simulations are conducted on linear-elastic inclusions and voids embedded in an elasto-plastic matrix material. The elasto-plastic material is modeled with both kinematic and isotropic hardening laws cast in a hardening minus recovery format. Three loading amplitudes (![Delta](/content/pl448346101p28h7/xxlarge916.gif) /2=0.10%, 0.15, 0.20%) and three load ratios (R=–1, 0, 0.5) are considered. From a continuum standpoint, the primary driving force for fatigue crack formation is assumed to be the local maximum plastic shear strain range, ![Delta](/content/pl448346101p28h7/xxlarge916.gif) max, with respect to all possible shear strain planes. For certain inhomogeneities, the ![Delta](/content/pl448346101p28h7/xxlarge916.gif) max was as high as ten times the far field strains. Bonded inclusions have ![Delta](/content/pl448346101p28h7/xxlarge916.gif) max values two orders of magnitude smaller than voids, cracked, or debonded inclusions. A cracked inclusion facilitates extremely large local stresses in the broken particle halves, which will invariably facilitate the debonding of a cracked particle. Based on these two observations, debonded inclusions and voids are asserted to be the critical inhomogeneities for fatigue crack formation. Furthermore, for voids and debonded inclusions, shape has a negligible effect on fatigue crack formation compared to other significant effects such as inhomogeneity size and reversed loading conditions (R ratio). Increasing the size of an inclusion by a factor of four increases ![Delta](/content/pl448346101p28h7/xxlarge916.gif) max by about a factor of two. At low R ratios (–1) equivalent sized voids and debonded inclusions have comparable ![Delta](/content/pl448346101p28h7/xxlarge916.gif) max values. At higher R ratios (0, 0.5) debonded inclusions have ![Delta](/content/pl448346101p28h7/xxlarge916.gif) max values twice that of voids. |
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Keywords: | Bonded cracked and debonded inclusions fatigue crack formation finite element method local plastic strain monotonic and cyclic voids |
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