On the driving force for fatigue crack formation from inclusions and voids in a cast A356 aluminum alloy

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 epsilon /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 gamma (max), with respect to all possible shear strain planes. For certain inhomogeneities, the Delta gamma (max) was as high as ten times the far field strains. Bonded inclusions have Delta gamma (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 gamma (max) by about a factor of two. At low R ratios (-1) equivalent sized voids and debonded inclusions have comparable Delta gamma (max) values. At higher R ratios (0, 0.5) debonded inclusions have Delta gamma (max) values twice that of voids.