Modified Life Prediction Model Research Articles

Creep properties and life model of anisotropic Ni-based single crystal superalloys over a wide temperature range

The current study is centered on the creep anisotropy of a nickel-based single crystal superalloy across a wide temperature range and with orientations of [001], [011], and [111]. Based on continuous damage mechanics, a new damage evolution law of anisotropic nickel-based single crystal materials is proposed. Based on the crystal slip theory, creep resistance is innovatively introduced and the modified creep constitutive model of nickel-based single crystal superalloy is presented. Meanwhile, in order to facilitate engineering application, an anisotropic creep life prediction model for single crystal superalloy was established based on continuous damage mechanics and crystal plasticity theory. By establishing the relationship between deformation gradient and shear strain rate on the sliding system, the combination of macroscopic creep deformation and microscopic crystal slip is realized. According to the working state of single crystal blade, a wide range of temperature creep tests at different load points were carried out. The creep parameters A (temperature dependent parameter), n (apparent stress index), Q (apparent activation energy) and τres(α) (creep resistance) are obtained by fitting on the basis of the slip theory of finite deformation crystals and test results. The creep behavior of anisotropic Ni-based single crystal superalloy is simulated by using a modified life prediction model considering creep resistance in finite element framework, and the numerical results are compared with the experimental results. The results show that the constitutive formula proposed in this paper can be used to simulate the mechanical properties of Ni-based single crystal superalloys at different orientations in the wide temperature range, and the constitutive formula also can accurately predict the creep life and creep deformation with different temperatures and directions. The life prediction error is in the range of 1.2 times.

Low cycle fatigue and fatigue-creep interaction effects on softening behavior and life prediction of cast aluminum alloy at elevated temperature

In this study, a series of strain-controlled low-cycle-fatigue (LCF) and low-cycle-fatigue-creep (LCFC) experiments were conducted on a cast Al-Si-Cu alloy. LCF and LCFC experiments were performed at 250 ℃, 300 ℃ and 350 ℃ with three different strain amplitudes. The results showed significant softening behavior, with temperature affecting both the softening behavior and fatigue life. Interestingly, coupling creep at 250 ℃ was found to increase fatigue life. Microstructure evolution was investigated by using scanning electron microscopy, and it was found that the introduction of strain-dwell at the tensile peak caused the interaction of fatigue and creep, resulting in a large amount of plastic deformation that affected fatigue life. Two fracture models were proposed for LCF and LCFC tests at different temperatures, based on the microstructure evolution. Using the experimental data, a modified life prediction model was proposed, which demonstrated a better prediction ability than other traditional models in fatigue-creep life prediction under different temperature and strain amplitudes.

Thermal aging effect on the tensile and fatigue properties of the narrow-gap TIG welded joints in offshore floating nuclear power plants

In offshore floating nuclear power plants, the narrow-gap tungsten-inert gas (TIG) welding method is commonly applied to connect various geometries of pipes due to its high efficiency welding under the low heat input of the welding process. However, the narrow-gap TIG welds have a duplex structure and are sensitive to thermal aging mechanism. In this paper, the narrow-gap TIG joints were subjected to a series of accelerated thermal aging tests at 400 °C for 0, 2000, 5000, 13,000 and 30,000 h, respectively, to investigate the tensile and fatigue properties at different locations. The test results indicated that the yield strength and the ultimate tensile strength of the narrow-gap TIG welded joints increased with the increasing thermal aging time. Fatigue life increased along with the thermal aging duration. A modified life prediction model was proposed.

The role of anelastic recovery in the creep-fatigue interaction of 9–12% Cr steel at high temperature

Fatigue tests of 9–12% Cr ferrite steel with various hold times at both peak and valley points were conducted under stress control at 898K. The role of anelastic recovery in the creep-fatigue interaction was discussed systematically. Results indicate that anelastic strain increases primarily with the cycle increase and then decreases slowly until the specimen failed during the unloading process. By contrast, the kinetics of anelastic recovery decreases during the unloading process while is insensitive to the peak stress and hold time at the early stage. In addition, the creep-fatigue (cyclic creep) interaction is much decelerated due to the anelastic recovery compared to the static creep state. A modified life prediction model is developed accordingly under the frame of ductility exhaustion concept by incorporating the effect of anelastic recovery. A good agreement with the experimental data is achieved and thus provides us confidence in the use of the modified model in practice.

The fatigue failure analysis and fatigue life prediction model of FV520B-I as a function of surface roughness in HCF regime

Abstract

Characterization of Low Cycle Fatigue Performance of New Ferritic P92 Steel at High Temperature: Effect of Strain Amplitude

Low cycle fatigue (LCF) tests were performed at various strain amplitudes ranging from 0.2 to 0.8% to investigate the effects of strain amplitude on cyclic softening behavior and LCF lifetime of new ferrtic P92 steel. LCF tests were conducted under strain controlled in fully reversed manner with strain rate of 1.0 × 10−3 s−1 at high temperature of 650 °C. A novel fatigue life end criterion was adopted in this analysis. The effects of strain amplitude on the cyclic softening behavior, the evolution of plastic strain amplitude, hysteresis loop area, and the fatigue life were discussed. Similar softening curves were achieved except for the strain amplitude of 0.2%. The evolution of hysteresis loop areas at the smaller strain range amplitude (less than 0.4%) is contrary to the larger ones. A modified life prediction model was proposed based on Coffin–Manson model and energy‐based model, comparison of predicted lifetime and experimental data shows that the model gives a good estimation. In order to better understand the basic stress‐strain behavior, time‐independent cyclic plasticity model was used to represent the cyclic mechanical behavior of this steel. Comparison of the simulated and experimental results shows that the proposed model can give a reasonable prediction of stress‐strain hysteresis loop for P92 steel at high temperature.