CRACK INITIATION AND SMALL FATIGUE CRACK GROWTH BEHAVIOUR OF SQUEEZE‐CAST Al‐Si ALUMINIUM ALLOYS

Abstract

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

Microstructure-based fatigue modeling of cast A356-T6 alloy

“…This form for U is consistent with FE calculations and results of [4]. The exponent n % 4:8 in the work of [28] is much greater than the usual LEFM value due to the local cyclic plasticity associated with interactions of the crack tip with microstructure heterogeneities. Furthermore, da=dN is linear with crack length in the MSC/PSC regimes early in the HCF life and does not follow LEFM relations.…”

“…The threshold value of cyclic crack tip displacement, DCTD th , is taken on the order of the Burger's vector of the Al-rich matrix; this threshold provides a nonpropagating crack limit which is generally applicable at higher stress levels than the intrinsic threshold limit for cyclic microplasticity associated with incubation. This value is just slightly above the minimum cyclic crack growth rates measured for squeeze cast Al-Si alloys [27,28]. Plumtree and Schafer [27] reported the minimum crack growth rate for cracks blocked at dendrite cell triple points.…”

“…Here, C II is a coefficient intended to apply to the MSC and PSC regimes for crack lengths ranging from a few microns to the millimeter range (cf. [14,28]), as long as the crack tip cyclic plastic zone is substantially less than the DCS. The factor U addresses mean stress effects on propagation, which are influenced strongly by interdendritic particle interactions ahead of and in the wake of the crack; U ¼ 1=ð1 À RÞ for R < 0, and U ¼ 1 for R P 0, where R (stress ratio) is based on the maximum principal stress.…”

“…The first term is based on the correlations of Shiozawa et al [28] for cracks in the MSC/PSC regime under HCF loading conditions, with an additional influence of average void volume fraction (porosity) u u via the function f ð u uÞ to be discussed later. The second term is added to describe elastic-plastic crack propagation in the limit plasticity regime and represents gross plastic strain driven growth, with C I as the leading coefficient; da=dN is essentially independent of the crack length in this regime, with the macroscopic maximum plastic shear strain amplitude, Dc p max =2 macro , as the driving force.…”

“…The DCTD accounts for the effects of microstructure heterogeneity. As shown by [28], the propagation of small cracks (in the range of 0.1-1 mm) in two squeeze cast Al-Si alloys (crack formation controlled by Si particles, minimum porosity) for R ¼ r min =r max ¼ À1 followed an EPFM law that was valid for steels as well [26,35], i.e.,…”