FATIGUE OF CAST-ALUMINUM ALLOYS UNDER CONSTANT AND VARIABLE-AMPLITUDELOADING

Three cast aluminium materials, Al 206, Al 319 and Al 390, were fatigu e tested under constant- and variable-amplitude loading. A stress rati o of -1 was used for the constant-amplitude tests. The variable-amplit ude load history consisted of underloads followed by constant-amplitud e small cycles. The stress ratio and the number of the constant-amplit ude small cycles following an underload were adjusted so that the crac k did not close and remained fully open for all the small cycles. Unde rloads reduced the fatigue strength of the alloys by 66-77%. A crack g rowth analysis based on a fracture mechanics approach was used to mode l the fatigue behaviour of the cast aluminium material under constant- and variable-amplitude loading. The crack growth analysis was based o n an effective strain-based intensity factor, elastic and plastic notc h strain calculations based on Neuber's formula, and a reference crack growth rate curve obtained during closure-free crack growth. In model ling the fatigue life behaviour of cast aluminium the flaws were model led as circular notches having the same diameter as the flaws. The not ches were assumed to be edge notches, which represents the flaw locati on with the most detrimental effect on fatigue life. Fatigue life pred ictions were in good agreement with experimental results. The fatigue life behaviour for an ideal cast material free of flaws was also predi cted using the same model for an unnotched specimen. The cast material was then hipped (the material was subjected to a high pressure at hig h temperature and then slowly cooled) to eliminate internal flaws. The hipped material showed an increase in fatigue strength of almost 40% under constant-amplitude loading with R = -1, which was in good agreem ent with the results predicted by the model.