A non-isothermal fatigue crack growth law for the A356-T7 aluminum alloy

Fatigue crack growth period in cylinder heads of A356-T7 casting alloy has been found to constitute an important part of the structure's total life. Therefore, the fatigue model design should consider crack propagation life. Then, in order to find a new crack growth law, finite element simulations on a cylinder head 3D model and fatigue crack tests have been conducted as to establish the contribution of each loading parameter in governing the crack propagation. SEM, optical microscopy and other metallographic techniques have been used to examine fracture surfaces and to analyze particles cracking. It has been found that the crack growth mechanism changes gradually at the microstructural scale with the increasing of Kmax value. Performing tests at variable frequencies has shown important crack growth time dependence. Negative load ratio tests have revealed a primordial importance of the compressive load part on the crack advance. Using experimental and numerical results, a new "damage tolerant" model design based on a non-isothermal crack growth prediction law has been introduced. The law is based on a corrected linear elastic fracture mechanics stress intensity factor Kp accounting for the cumulative crack tip plasticity. The law figures all of the purely fatigue effect, the crack growth under monotonic loading condition (time dependent parameter the effect of the compressive load on the global crack advance.