Influence of Mean Strains on the Isothermal Fatigue Behaviour of the Cast Aluminium Alloy AlSi10Mg Reinforced with 15 Vol.- percent Al_2O_3 (Saffil) Short Fibers

摘要

Isothermal, strain controlled fatigue tests with constant total mean strains in the range of- 1.6 percent<=epsilon_(m,t)<=0.4 percent were carried out on the cast aluminium alloy AlSi10Mg reinforced with 15 Vol.- percent discontinous Al_2O_3 (Saffil) fibers. The test temperatures were T=150 deg C and 300 deg C, respectively. Only a small influence of the mean strain on the stress- and plastic strain amplitudes was observed. At T-300 deg C the initial mean stresses relax to values near zero within less than 10 percent of the number of cycles to fracture. Contrarily, at T=150 deg C only a partial relaxation of the initial mean stresses is observed until the end of the test. Compared with epsilon_(m,t)=0, increasing positive mean strains lead to a decrease of the lifetime which is less pronounced at T=150 deg C than at 300 deg C. Negative mean strains down to -1.6 percent only have a weak influence on the number of cycles to fracture N_f at T=300 deg C, whereas at T=150 deg C a change of the mean strain from 0 percent to -0.4 percent results in an increase of N_f by a factor near five. At T=150 deg C the influence of mean strains from, epsilon_(m,t)<=0.2 percent on the lifetime behaviour can be assessed properly using the Smith-Watson-Topper damage parameter, whereas this parameter does not give even a rough estimation of the dependence of N_f on epsilon_(m,t) at T=300 deg C.