| Low cycle fatigue (LCF) and fatigue crack propagation (FCP) properties of materials are basis for structure safety design and structure integrity assessment. With the initiating of Chinese "large commercial-aircraft project", "strategy of high-speed railway going out" and a large number of nuclear power projects, structure design and safety assessment are increasingly important, the new test methods are needed to get the material properties for torsional LCF at high temperature and FCP using small size specimens. As there exist not better methods for torsional LCF test at high temperature and for FCP test using small size specimens, the innovated methods on torsional LCF test using funnel-type specimens and FCP test with C-ring small specimens were proposed, and the new methods were used to investigate fatigue behavior of aeronautical material C250 maraging steel. The main works performed in this thesis are listed as follows:(1) A series of axial tension tests for C250 martensite steel at room temperature,150℃, 200℃ and 350℃ were carried out. The axial stress-strain curves of the material at four different temperatures were obtained. For high temperature torsional LCF, there are some problems for traditional straight round bar. Thus, the small curvature funnel-type specimens were designed for high temperature torsional LCF. In the present study, high temperature torsional LCF tests were completed, which contain two different load ratio, R=-1 and R=0.1, and four different temperatures, room temperature,150℃,200℃ and 350℃. Finally, the relationship between the torque and the nominal torsional angle (T-(φn) was established.(2) A new test method is proposed for torsional LCF. Based on the ANSYS software, a 3D funnel-type torsional FEA was established by using the rigid beam loading technique. By comparing the experimental T-φn curves of funnel-type specimens for C250 steel, the iterative FAT(Finite-element-analysis Aided Testing) analysis was employed to get the effective cyclic constitutive relationships, in which the axial tensile stress and strain curves were set as the initial constitutive relationship. According to the effective cyclic stress-strain and the new 3D FEA under torsional loading, the transforming formulae of shear strain and nominal torsional angle (γ-φn), shear stress and torque (τ-T) were acquired.(3) Based on the transforming formulae, the true stress and strain distributions at the root of the funnel-type specimen of C250 steel were given. Thus, torsional LCF properties of the material were also obtained. Comparing with Manson-Coffin model, stretching hysteresis energy damage model and three-parameters power model (TPM) model, the results prove that the TPM model can better describe torsional LCF properties of C250 steel, furthermore, torsional LCF life prediction equations with two kinds of load ratio and at three kinds of temperature for C250 steel were found.(4) For the requirement of fracture properties assessment of small scale components, a type of C-ring specimen with outer radial crack (CRO) and the corresponding loading device were designed. Based on 2D FEA, formulae for crack length prediction and stress intensity factor of CRO specimen were obtained.(5) 5083-H112 aluminum alloy was used to verify the validity of the new FCP test method by using CRO specimens. Based on the new method, FCP tests were completed for C250 steel and the FCP properties were got. |
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