Damage Accumulation Kinetics in Heat-Resistant Steels at the Plane Stress State Under Cyclic Loading at Different Temperatures

Experimental results are presented for damage accumulation in 10GN2MFA and 15Kh2MFA steels studied with the LM-hardness method at different stress states under cyclic loading at room and elevated temperatures that are consistent with the service conditions of structure elements. Damage accumulation kinetics is examined at primary and secondary creep stages in view of the multiaxial stress state and creep strain effects on the homogeneity coefficient. A higher level of initial strain on cyclic creep gives rise to more intensive damage accumulation in the metal, leading to the earlier onset of fracture in comparison with the smaller initial strain. Experimental critical homogeneity coefficients are given that correspond to the plastic strain stability loss when the stable deformation of the metal with its further fracture is affected by the reduction in the cross-section area and hardening modulus. The rate of homogeneity change in examined steels at different ratios of principal cycle stresses on cyclic creep is evaluated. The significant effect of a stress state at equilibrium between strain hardening and softening of the structure (specimen) metal at the secondary creep stage, is demonstrated. The effect of a stress state and temperature on damage accumulation kinetics of examined materials, in particular on the useful life of the metal on cyclic creep is established.