Experimental and numerical simulations of Type 316 stainless steel failure under LCF/TMF loading conditions

Abstract

Materials need to be designed under certain conditions to withstand high thermal gradients to operate at high temperature environments. Many advanced gas cooled reactor (AGR) power plant components with operating temperatures in the range of 500-650 °C undergo creep-fatigue loading conditions. These components may be subject to isothermal low cycle fatigue (LCF) and thermo mechanical fatigue (TMF) damages due to the cyclic operation of power plant caused by the start-up and shutdown processes and due to the fluctuation of energy demand in daily operation. Hence, the influence of these cyclic loads induced mechanically and thermally, on the different structural components need to be carefully monitored and analysed in order to prevent failure and ensure safe operating conditions of critical units. The material Type 316 SS with cast number S7646, widely used in this type of components, is investigated in this project. The aim of this research is to conduct experimental tests to obtain quality stress-strain data for the material under investigation under cyclic plasticity in isothermal and an-isothermal tests using the available testing machine systems in the University of Imperial College London. The data obtained from experimental results are then utilised to develop advanced novel finite element damage models in a creep/fatigue loading environment in order to predict the cyclic behaviour under LCF conditions. Finally, the results of cyclic data derived from isothermal tests were used to predict the thermo mechanical fatigue behaviour for this alloy. The LCF-TMF testing unit, Instron 8801 with a temperature uniformity of less than ±10°C within the gauge section of the specimens were employed to conduct the experimental tests. Fully-reversed, strain-controlled isothermal tests were conducted at 500°C and 650°C for the strain ranges of ∆ɛ=±0.4%, ±0.8%, ±1.0% and ±01.2%. Strain-controlled in-phase (IP) thermo-mechanical fatigue tests were conducted on the same material and the temperature was cycled between 500°C and 650°C. Additionally, the creep-fatigue interactions were investigated with the introduction of symmetrical hold time at maximum strains in tension and compression under both LCF-TMF tests. From the investigation and the analysis of the experimental stress-strain data, three phases are observed when the cyclic stress responses are plotted; cyclic hardening, stabilisation and damage evolution. In the final stage of the behaviour of the material, a nonlinear decrease of the peak stress level was observed which was initiated by the presence of micro-crack and the failure occurred as the crack propagated. The evolution of inelastic strain energy density, ∆w, against the number of cycles, N, was used to determine the number of cycles at which the material stabilised, N_sta , the damage initiated, N_i and the failure occurred,N_f. The introduction of the hold time in both tension and compression strains in the LCF and TMF tests, produced an increase in the plastic strain range which subsequently increased the inelastic strain energy density and slightly reduced the peak flow stress when compared with the continues cyclic tests. The stress relaxation was observed when the hold time was introduced. The amount of stress relaxation was dependent on the test temperature and the imposed strain amplitude and the same trend was found when different strain ranges were examined. The cyclic behaviour of the Type 316 steel was further studied by analysing and performing microstructural investigations using the scanning electron microscope (SEM). The metallographic and the fractographic studies revealed that in all LCF-TMF tests the cracks mostly initiated in transgranular mode and propagated in either transgranular (under continuous cyclic loading) or in a mixed mode (under symmetric dwell period). The comparison of the metallographic and the fractographic studies of the LCF and TMF tests under both conditions (i.e. with and without dwell period) highlighted that the proportion of intergranular cracking increases with decrease in frequency, i.e. from 0.01Hz to 0.001Hz. Furthermore, the transgranular fatigue process dominates at high frequencies whereas the intergranular time dependent mechanism governs at low frequencies, low imposed mechanical strain amplitude and they both act together at intermediate frequencies and imposed mechanical strain amplitude. A constitutive model based on isotropic and nonlinear kinematic hardening rules was used to replicate numerically the cyclic structural behaviour of the material. A user-defined subroutine was developed and implemented in the finite element software, ABAQUS to predict the cyclic hardening, the stress relaxation during hold time and finally to demonstrate the damage evolution once the damage initiated. The final stage of the material behaviour (i.e. failure) was simulated numerically for both LCF and TMF tests conducted with and without hold time where for the tests with continuous cyclic loading (without hold time) a hysteresis energy-based phenomenological model was implemented in a USDFLD subroutine. Further, this model in combination with the creep damage model based on the time-fraction law were employed simultaneously to replicate the experimental results in which the hold time was introduced. In the end, the FE results were compared with the experimental results and the minor deviations observed in e.g. the first and stabilised hysteresis loops under TMF conditions or in the FE hysteresis damages, could be minimised by conducting further isothermal tests to define additional material properties at intermediate temperatures and performing tests at various strain ranges respectively.