(normalized K gradient set at -0.10mm-1) was used to gradually decrease crack growth rates to the near-threshold level. The fatigue threshold stress-intensity range (ΔKth) was defined at a maximum growth rate of 10-10 m/cycle. For determine the Paris’ constants c and m in Paris’ equation (da/dN=mΔKc). Constant amplitude technique (Δ K-decreasing) had been used. Data recorded during the tests were analyzed to obtain da/dN-ΔK curves. 3. Results 3.1 Fatigue crack growth Fig. 2 shows the variation of the fatigue crack growth rates, da/dN, as a function of the stress intensity factor range, ΔK, for the unreinforced alloy and the two composites. It is obvious that the curve for the 4.5μm SiC reinforcement composite is below the other two curves for the 20μm and the Al-7Si alloy. Therefore, the 4.5μm SiC/Al-7Si composite has the best fatigue crack growth resistance. The threshold stress intensity factor range ΔKth of 3.878 a m ΜΡ • for the 4.5μm SiC reinforcement composite is larger than the 20μm(3.630 a m ΜΡ • ) and the Al-7Si alloy (3.605 a m ΜΡ • ). The 20μm SiC particulates reinforced composite shows lower FCG rate than the unreinforced alloy at ΔK < 14.88 a m ΜΡ • , but exhibits lower fatigue crack growth rate at ΔK > 14.88 a m ΜΡ • . This is to say, the values of the FCG rate for the 20μm are lower than the matrix alloy in the near-threshold regime and Paris regime, however, it become higher than those of Al-7Si alloy at the fast fracture state. Comparison of the FCG curve of the 4.5μm SiC with 20μm gives a clear indication that the increase in particle size results in an obvious higher FCG rate in the near-threshold regime and the fast fracture state. However, in the Paris regime, i.e. between 10-6 and 10-4 mm/cycle, the composite reinforced with 20μm SiC particulates exhibits a slighter smaller slope than the 4.5μm SiC. The results depicted in Fig.2 for the particulate-reinforced spray-formed aluminum-matrix composite are similar to that reported of composite [10], where reinforcements cause marker improvements in resistance to fatigue fracture, especially in the near-threshold regime. In general, in the Paris region, there is almost no effect of SiC particle size on fatigue crack growth, while in the near-threshold regime and fast fracture state, the SiC particle size has great influence. This is possibly attributed to the different failure mechanism of crack growth caused by the actions of SiC size, shape and distribution.
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