Fatigue Properties And Damage Mechanism Of China Low Activation Martensitic Steel At High Temperature

Fusion energy has the potential to provide a sustainable solution to global energy crisis for safe,"clean and permanent" characteristics. The blanket is one of the most critical and technically challenging components in International Thermonuclear Experimental Reactor (ITER) to achieve tritium fuel proliferation, energy conversion and output. In the present design, ITER-TBM (Test Blanket Module) will be operated at about450℃with long-pulse mode, so the temperature and stress of ITER-TBM will be changed with operation time. Furthermore, earthquake and unstable operation will increase the number of the change of temperature and stress during its operation. Thus, the fatigue property of structural material is very important for design. China Low Activation Martensitic (CLAM) steel developed by FDS team has been selected as the primary candidate structural materials for Chinese ITER TBM. And the fatigue property is also one of the key properties for CLAM steel code qualification and China TBM design.Thus, the objective of this thesis is to study the high cycle fatigue (HCF) and low cycle fatigue (LCF) behaviors of CLAM steel at450℃and550℃. The research mainly focuses on:1) temperature effect on the relationship between fatigue life and strain range;2) the mechanism of cyclic softening;3) development of a material constitutive model to predict the cyclic loading behaviour at high temperature.Firstly, the HCF tests were conducted with stress controlled with a frequency of100Hz in air. The test results showed that the fatigue strength of CLAM specimen decreased with increasing of the temperature, and the condition fatigue limits (N=107) were275±5MPa and235±5MPa at450℃and550℃, respectively. The corresponding fatigue limits were reduced to lower values at550℃compared to those obtained at450℃. The fatigue lifetime prediction module and engineering S-N curves were established with the results in the finite life resign. The fractograph results indicated that the fractures were mainly initiated from the surface of the specimen.Secondly, LCF tests of CLAM steel were carried out in air using a fully reversed push-pull strain controlled triangular wave with the strain rate of0.8%/s. The fatigue life at550℃was slight decreased compared with that at450℃when the life was plotted against the total strain range. Three distinct stages of cyclic softening have been identified from the full cycle data of the cyclic tests. The evolution process of hysteretic curve and elasticity modulus were studied during the fatigue test, it was found that elasticity modulus is an effective factor to describe the fatigue damage process of CLAM steel. The coarsening of the subgrains and the decreasing of dislocation density are the mainly reason for the nonlinear decrease of the peak stress level.Finally, a unified viscoplasticity model, which includes combined isotropic softening and kinematic hardening with a viscoplastic flow rule for time-dependent effects was used to model the LCF behavior of CLAM steel. The constants in the viscoplasticity model were determined from the cycle stress-strain data and the maximum stress evolution during tests. The simulation results of cycle stress-strain curve showed a good consistence with the experimental data.