Fatigue Properties under Constant Stress/Variable Stress Amplitude and Coaxing Effect of Acicular Ductile Iron and 42 CrMo4 Steel

Fatigue characteristics of as-cast acicular ductile iron (ASGI) produced by alloying and controlled cooling and as-rolled 42CrMo4 steel were studied. Rotating bending fatigue tests at a stress ratio of R = -1 was employed. Tests included constant stress amplitude loading, variable stress amplitude high-low, low-high loading and coaxing. S – N curves were established and fatigue strengths were determined, (450 N/mm2 for ASGI, 460 N/mm2 for steel). The variable stress amplitude loading, high-low, showed high NT/Nf for the materials, while variable stress amplitude loading, low-high showed a decrease in NT/Nf for steel and only small improvement for ASGI (NT = total stress cycles in variable stress amplitude, Nf = stress cycles in constant stress amplitude to failure). Both materials showed coaxing effect, pronounced more in ASGI. The crack propagation rate determined using Paris equation at constant stress amplitude for ASGI was da/dN =2.0*10^-5 (ΔK)^2.6902 and for steel it was da/dN =1.0*10^-6 (ΔK)^3.5205