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

Monotonic and cyclic finite element simulations are conducted on linear-ela stic inclusions and voids embedded in an elasto-plastic matrix material. Th e elasto-plastic material is modeled with both kinematic and isotropic hard ening laws cast in a hardening minus recovery format. Three loading amplitu des (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 shea r strain range, Delta gamma (max), with respect to all possible shear strai n 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 inc lusions. 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 crac k formation. Furthermore, for voids and debonded inclusions, shape has a ne gligible effect on fatigue crack formation compared to other significant ef fects such as inhomogeneity size and reversed loading conditions (R ratio). Increasing the size of an inclusion by a factor of four increases Delta ga mma (max) by about a factor of two. At low R ratios (-1) equivalent sized v oids and debonded inclusions have comparable Delta gamma (max) values. At h igher R ratios (0, 0.5) debonded inclusions have Delta gamma (max) values t wice that of voids.