Fatigue strength, crack initiation and small crack growth behaviour in two kinds of squeezecast aluminium alloys, AC8A-T6 and AC4C-T6 were investigated using smooth specimens subjected to rotatary-bending fatigue at room temperature. Fatigue resistance of these alloys was almost the same as that of the wrought aluminium alloys because of their fine microstructure and of the decrease in defect size due to squeeze-casting. Fatigue crack initiation sites were at the eutectic silicon particles on the surface of specimens or at internal microporosity in the specimens. Crack initiation life, dehed as a crack length of 50 pm on the specimen surfam, was successfully estimated from an evaluation of initiation sites using fracture mechanics and the statistics of extrema. Small fatigue crack growth in the two kinds of alloys obeys the relation proposed by Nisitani et al., namely that d(2c)/dN= C(aa/a,)".(2c), where C is a constant and C T~ is the ultimate tensile strength. It is pointed out that an improvement in fatigue strength of cast aluminium alloys can be expected by refining the eutectic silicon rather than by an increase in static strength.
NOMENCLATUREare&, = maximum projection area of silicon-particle =maximum projection area of microporosity C,, n = material constants 2c = surface crack length 2c,, 2c, = initial and final crack length d(2c)ldN =crack growth rate F ( x ) = cumulative probability K,__ = maximum stress intensity factor Nf = number of cycles to failure Ni = number of cycles to crack initiation Np = crack growth life So = standard inspection area Sr = ratio of the sum of crack lengths propagated within the cluster of Si-particles to the total crack length Sro = Sr when crack growth is independent of microstructure and is along a straight path T(x) = return period ua = stress amplitude CT, = ultimate tensile strength u0.* = 0.2% offset yield strength