FRACTURE BEHAVIOUR OF Al12wt.%Si0.35wt.%Mg(0–0.02)wt.%Sr CASTING ALLOYS UNDER FATIGUE TESTING

A systematic study of the fatigue crack growth characteristics and mechanisms in Al–Si–Mg and A356 casting alloys was carried out. Compact tension specimens, prepared from modified and unmodified alloys were tested at different stress ratios and stress intensity factor range values, and a study of the mechanistic role of the silicon particles in influencing the fracture behaviour during fatigue crack propagation was made, employing both optical and scanning electron microscopy. The results indicated that the fatigue crack growth behaviour of the alloys is affected by the stress ratio, stress intensity level and the size, shape and distribution of the eutectic silicon particles. The particle characteristics also determine the fracture mode of the alloy. Fracture Characteristics observed include decohesion of the silicon particles from the aluminum matrix; silicon particle cleavage/cracking; and striations in the aluminum phase, particularly at high stress ratios.