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Influence of the temperature on the course of strain energy accumulation in GX12CrMoVNbN9-1 cast steel under low cycle loading conditions

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Recent years have brought a particular development of research within the energetic description of low cycle fatigue. It is mostly due to the fact that energy-based approach to fatigue is more complete than the strain- or stress-based description, as it considers mutual interactions of both: stresses and strains [1]. There are many hypotheses and concepts of energy-related approach to the description of low cycle fatigue process. Currently there are two basic approaches to describing the fatigue of metals from the energetic point of view. The first approach involves total cumulated energy in the fatigue process, and it is comparable to the energy absorbed during the static tensile tests. Proposal of such an approach can be found inter alia in the work [2]. The second approach is based on the accumulation of the energy dispersed in the material in particular cycles of changing load. In this approach there are three distinguishable groups, concerning respectively: -- only the energy of plastic strain ΔWpl [3÷5], -- the sum of elementary energy of plastic strain ΔWpl and elastic strain ΔWs [6, 7], -- elementary energy of elastic-plastic strains ΔW [8, 9]. Schemes graphically illustrating the most frequently occurring descriptions of the criterion quantity in the energy depiction are shown in Figure 1. This paper focuses on the low cycle fatigue description in which only the energy of plastic strain ΔWpl is taken into account. Fatigue graph using this quantity criterion is plotted on the basis of research carried out in the conditions of controlled strain (εac = const) or stress (σa = const). The value of quantity criterion ΔWpl within one loading cycle corresponds to the area of hysteresis loop (Fig. 1a). The loop area is determined on the basis of the analysis of measurement signals of the force burdening the test sample and its strains. These signals, after converting force into stress, al[...]

Niskocyklowa trwałość zmęczeniowa w temperaturze 550 oC wyżarzonego staliwa GX12CrMoVNbN9-1

  W pracy przedstawione zostały wyniki badań trwałości zmęczeniowej w zakresie niskocyklowym staliwa GX12CrMoVNbN9 - 1 (GP91) w temperaturze 550 oC. Badaniu poddane zostało staliwo w stanie wyjściowym (po obróbce cieplnej) oraz po 8 000 godzinach wyżarzania w temperaturze 600 oC. Przeprowadzone badania wykazały, że staliwo GP91 podlega silnemu cyklicznemu osłabieniu i nie wykazuje wyraź- nego okresu stabilizacji. Wykazano także, że proces wyżarzania wpływa na podstawowe parametry pętli histerezy, m.in. amplitudę odkształ- cenia całkowitego εac, amplitudę odkształcenia plastycznego εap i amplitudę naprężenia σa oraz trwałość zmęczeniową Nf. Wyżarzanie bada- nego staliwa powoduje wyraźne obniżenie jego trwałości zmęczeniowej, której wielkość zależy od wartości odkształcenia całkowitego εac. The paper presents the results of research on low cycle fatigue life of GX12CrMoVNbN9 - 1 (GP91) cast steel at the temperature of 550 oC. The cast steel subject to investigation was in the as-received condition (after heat treatment) as well as after 8 000 hours of ageing at the temperature of 600 oC. The tests carried out have proved that the GP91 cast steel is subject to strong cyclic softening and does not show a clear period of stabilization. It has also been demonstrated that the process of annealing influences the basic parameters of hysteresis loop, such as the amplitude of total strain εac, the amplitude of plastic strain εap and the amplitude of stress σa, as well as fatigue life. Annealing in the examined cast steel causes an evident decrease in its fatigue life, which depends on the value of total strain. Słowa kluczowe: staliwo, niskocyklowe zmęczenie, podwyższona temperatura Key words: cast steel, low - cycle fatigue, elevated temperature.1. Wprowadzeni[...]

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