Crack growth characterisation of A356-T7 aluminum alloy under thermo-mechanical fatigue loading

This paper presents results and interpretations of several long crack thermo-mechanical fatigue tests on A356-T7 cast aluminum alloy used in automotive cylinder heads. These tests were conducted on CT16 and SEN specimens for different positive and negative load ratios, temperatures, frequencies and stress intensity factor values in order to establish the contribution of each parameter governing the fatigue crack propagation. It was shown that the decrease of the frequency causes significant increase of the crack growth rate especially at high temperatures and load ratios. The performing of negative load ratio tests had shown a primordial importance of the compressive load part of fatigue on the crack advance. In addition, SEM, optical microscopy and other metallographic techniques were used to examine fracture surfaces and to analyze particles cracking. For positive load ratios tests, it was shown that in stage II (Paris regime) and stage III (high crack growth rates) the number of cracked particles increases with Kmax and gradual changes in fracture surface appearance occur. These changes were associated with different crack growth mechanisms at the microstructural scale. The amplitude of the inelastic deformations and the extent of the plastic zone ahead of the crack tip were used to explain the observed changes in crack growth mechanisms. The crack growth under negative load ratio loads is especially analyzed, and a way of deriving this rate is given and shown to be relevant even for nonisothermal loading.