| Casting aluminium alloys, particularly A356 and A357, are widely used to manufacture machine components to replace cast iron and forged steel in aviation and automotive industry because of weight and costing saving.Fatigue crack propagation behaviors of A357-T5 and A357-T6 casting aluminium alloys at stress ratio R=0.05 and R=0.7 are studied, and the effects of stress ratio and heat treatment on crack propagation rate of A357 casting Aluminum Alloys are discussed in this paper. The crack deflection model is selected according to observing crack paths by option microscope and scanning electron microscope. The fatigue crack propagation curves are amended by crack deflection model, and the influence of crack deflection on fatigue crack propagation rate are analyzed. The research results show that fatigue crack propagation rate da/dN is faster at R=0.7 than that of R=0.05 at T5 and T6 conditions because of crack closure due to crack surface roughness at R=0.05, which reduced the driving force of crack tip. Fatigue crack grows along boundary of dendrite cells because of debonding eutectic Si particles to cause remarkable crack deflections. The decohesion of eutectic Si particles and the holes appeared on the crack surface also show that fatigue crack grows along dendrite boundary. The effective stress intensity factor ranges of a deflected crack with same projection length are less than those of an undeflected crack at the same crack growth rates at stress ratio of R=0.05 and R=0.7 for both A357-T5 and A357-T6 casting Aluminum Alloys. The effective driving force of deflected crack are less than that of undeflected crack at the same crack growth rates when the stress intensity factor of modelâ…¡crack at a deflected crack tip was ignored.
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