Micro-Crack Initiation and Propagation in 304 Stainless Steel Plain Specimen under Fatigue-Oxidation Interaction at Elevated Temperature

Abstract

Strain controlled time-dependent fatigue tests were carried out on plain specimens of 304 stainless steel at 550°C in air and the initiation and propagation behavior of main cracks were observed. The strain wave shapes were widely varied, including one day hold at tensile peak strain.
The slow strain rate and long hold time in tensile strain enhanced both crack initiation and propagation, while those in compressive strain did not affect them, because the time-dependent fatigue in the test condition was dominated not by creep damage but by environmental oxidation. A good correlation existed between the period of tensile straining in one cycle τ_t and the fatigue-oxidation interaction characteristics under various strain wave shapes. The number of cycles to initiate 0.05mm long crack N_c0.05, the number of cycles to failure N_f and the fatigue crack growth rate were proportional to the power of τ_t when τ_t was longer than about 100sec.
The value of N_c0.05 was frequently less than 10% of N_f and the crack propagation behavior governed the failure cycles. Although the fatigue crack growth rate da/dN was large when the crack was smaller than a grain diameter, 0.05mm, it was proportional to the effective crack length a_e for large crack sizes at each test condition. The normalized fatigue crack growth rate k[=(da/dN)/a_e] can be obtained from N_f of a plain specimen through the relation expressed as k=10/N_f.