Low Cycle Fatigue Life Prediction Model of 800H Alloy Based on the Total Strain Energy Density Method

Abstract

In this paper, a high-temperature low-cycle fatigue life prediction model, based on the total strain energy density method, was established. Considering the influence of the Masing and non-Masing behavior of materials on life prediction, a new life prediction model was obtained by modifying the existing prediction model. With an 800H alloy of the heat transfer tube of a steam generator as the research object, the high-temperature and low-cycle fatigue test was carried out at two temperatures. The results show that the predicted and experimental results are in good agreement, proving the validity of the life prediction model. Keywords: 800H alloy; strain control; low-cycle fatigue; total strain energy density method; life prediction model IntroductionLife prediction is an important process that needs to be considered when using metal materials. It can effectively avoid catastrophic failures in nuclear power plants, pipeline systems, offshore platforms, petrochemicals, the aerospace industry, and other related structures. At present, there are many life prediction methods, including the linear cumulative damage method [1], plastic exhaustion method [2], Coffin-Manson correction method [3], strain range division method [4], strain energy range division method [5], metallography method [6], and the stress relaxation range method [7]. Although scholars have studied and furthered the development of these models, equipment damage due to creep-fatigue damage still persists, indicating that accuracy of the existing life prediction models is not sufficient to meet the life-span analysis conditions of different equipment or materials. In response to this problem, scholars have made corrections to existing models, resulting in newer ones. For example, Rao Jin [8], through transformation of the load spectrum based on the linear cumulative damage theory, proposed a functional relationship between the load-holding time and lifetime. Yongqin Wang [9] proposed a new method to predict average plastic strain life based on the energy method. According to the creep fatigue load, the material generates irreversible micro-defects (dislocations, voids, micro-cracks) with the increase of internal energy. The corresponding energy law formula is listed. Runzi Wang [10,11], based on the linear cumulative damage rule, combined with the time fraction method and the plastic exhaustion method, obtained a new strain energy density depletion model by substituting the strain energy constitutive equation.The above life prediction models were validated for relevant alloy materials and showed their practicability. Incoloy alloy has excellent high-temperature mechanics, corrosion resistance, and oxidation resistance. It is often used in the petrochemical and nuclear power industries, especially for heat transfer tubes in nuclear power plant steam generators. Periodic loading with high-temperature

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“…The comparison between the fatigue life of specimens tested with two different load modes (kinematic and dynamic) required the selection of the correct fatigue parameter. Among many models found in the literature [32,[34][35][36][37], five common energetic models were used:…”

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“…Jhansale and Topper (1971) 10 proposed the method of master curve construction by translating the hysteresis loops (Figure 3A). Ellyin and Kujawski 6 extended the master curve approach 10 and proposed a relationship 6 to calculate the CPSED for non‐Masing (Type‐I) behavior of the material as follows: where is the strain hardening exponent of the master curve and is the change in proportional stress limit of stable hysteresis loops at half‐life 137 . The change in loop shape or the non‐Masing behavior is taken into account by measuring the extra energy absorbed by the material compared with its Masing behavior.…”

“…where n à is the strain hardening exponent of the master curve and δσ 0 is the change in proportional stress limit of stable hysteresis loops at half-life. 137 The change in loop shape or the non-Masing behavior is taken into account by measuring the extra energy absorbed by the material compared with its Masing behavior. Conventionally, the measurement of the extra energy is accomplished by measuring the change in the linear elastic portion of the hysteresis loops with strain amplitude.…”

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