Damage Accumulation Kinetics in Heat-Resistant Steels at the Plane Stress State Under Cyclic Loading at Different Temper
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DAMAGE ACCUMULATION KINETICS IN HEAT-RESISTANT STEELS AT THE PLANE STRESS STATE UNDER CYCLIC LOADING AT DIFFERENT TEMPERATURES F. F. Giginyak,* P. O. Bulakh,1
UDC 620.178.15/179.119
and O. M. Maslo 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. Keywords: heat-resistant steel, cyclic loading, elevated temperature, plane stress state, LM-hardness method, homogeneity coefficient, metal damage, plastic strain stability loss. Introduction. A load-induced stress state in the operation of many products, in particular energy equipment, exerts great influence on the behavior of structural materials. The effect of a stress state on their cyclic creep and durability is detailed in several studies [1–3]. The damage kinetics of 10GN2MFA and 15Kh2MFA heat-resistant steels influenced by different ratios of principal cycle stresses was analyzed at 20°C under cyclic loading, which demonstrates their importance for life computations of heavily loaded structure elements [2, 4, 5]. The creep of heat-resistant steels under cyclic loading at the multiaxial stress state in view of metal damage was evaluated with the modified viscoelastoplastic deformation model [5]. The above would require a detailed analysis of experimental results under incremental cyclic loading; with this, the assumption of certain physical similarity of creep and active prior deformation parameters may be used [2]. So cyclic creep completed with quasistatic fracture gives rise to the substructure similar to that, arising in the same material at large residual strains in active tension [6]. * Deceased. Pisarenko Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kyiv, Ukraine ( [email protected]). Translated from Prob
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