Multiaxial Kitagawa analysis of A356-T6

Experimental Kitagawa analysis has been performed on A356-T6 containing natural and artificial defects. Results are obtained with a load ratio of R = −1 for three different loadings: tension, torsion and combined tension–torsion. The critical defect size determined is 400 ± 100 μm in A356-T6 under multiaxial loading. Below this value, the microstructure governs the endurance limit mainly through Secondary Dendrite Arm Spacing (SDAS). Four theoretical approaches are used to simulate the endurance limit characterized by a Kitagawa relationship are compared: Murakami relationships [Murakami Y. Metal fatigue: effects of small defects and nonmetallic inclusions, Elsevier; 2002], defect-crack equivalency via Linear Elastic Fracture Mechanics (LEFM), the Critical Distance Method (CDM) proposed by Susmel and Taylor [Susmel L, Taylor D. Eng Fract Mech 2008;75:15] and the gradient approach proposed by Nadot and Billaudeau [Nadot Y, Billaudeau T. Eng Fract Mech 2006;73:1]. It is shown that the CDM and gradient methods are accurate; however fatigue data for three loading conditions is necessary to allow accurate identification of an endurance limit.