Research And Application Of Fatigue And Fracture Properties Test Method Of Non-Conventional Samples

With the development of civil engineering, nuclear reactor, aerosapce and biomechanics, and with the improvement of the manufacturing techonology of automobile, high-speed railway and major ship, the design and safe assessment of engineering structures need a large number of accurate and reliable fatigue and fracture properties of materials. However, it's difficult to test low cycle fatigue performance of materials based on the thin plate and thin-walled short tubes. And fracture toughness is difficult to be tested for high toughness materials and small diameter rod. The main works performed in this thesis are listed as follows:(1) A new method to obtain cyclic constitutive relationship for a material obeying Masing law was proposed, and experiments on low cycle fatigue of a turbine rotor steel B65A-S at 300℃were employed to verify the validity of this method. According to finite element analysis on the basis of calibrated cyclic constitutive relationship, two types of transforming models from mean stress of thin funnel-like plate to local axial stress at the root of the funnel and from measuring strain to local axial strain at the root of the funnel for the two materials were set up. A series of constant amplitude low cycle fatigue tests for thin funnel-like plates of N18 zircaloy and SAPH440 steel were carried out. Based on the principle of fatigue damage equivalent, Manson-Coffin model of strain fatigue life prediction and the equation of cyclic stress-strain relationship for the two materials were established. Finally, low cycle fatigue tests for straight round bars of SAPH440 steel were completed, and the accuracy of the new test method for low cycle fatigue of thin funnel-like plates was completely certificated.(2) One pair of the chuck for adjusting center were designed. A series of fracture toughness tests for 16Mn cylindrical specimens with different diameters and different crack depths were carried out. Based on finite element analysis, unified compliance formulas for ring cracked round bar were found. The differences in existing relevant formula and the equivalent energy method for ring cracked round bar were studied. Double extensometer measured axial displacement; photograph and equivalent area method to calculate the final crack length, the impact of crack growth eccentricity were solved. The finite element calculation proof load separation theory applied to the sample. Crack geometry function G(a/R) expression was presented. Changing curves of J integral and crack propagation Aa were obtained by using the normalization method. Fracture toughness tests for standard SEB specimens of 16Mn steel were completed, and the accuracy of the new test method for low cycle fatigue of thin funnel-like plates was completely certificated. The results of the two ways are close.(3) The author deduces the Chaboche constitutive model which derives from the Ramberg-Osgood model. Derivative formula was used in the ANSYS calculation. Relationship between J integral and crack tip opening displacement was obtained. A more accurate validity criterion which contains material constitutive relation parameters was derived.(4) By using a set of self-invented clamps and thin-walled tubular specimens with small diameter, a series of monotonic tension tests at different temperatures and low cycle fatigue tests at 375℃for zircaloy-4 and Zr-1Nb alloys were carried out. Temperature and strain rate influence on tensile properties and cycling performance of materials under different temperature have been studied. Based on the test data of the two alloys, monotonic and cyclic constitutive relations and Manson-Coffin life-estimating models were given.(5) A series of monotonic tension tests and low cycle fatigue tests at room temperature and 300℃for STM528 rotor material were carried out. Fatigue crack propagation and quasistatic fracture toughness tests were also completed. Monotonic relationships and Manson-Coffin life-estimating expressions for STM528 steel were given. And the Paris's laws for the stable fatigue crack propagation and quasistatic fracture toughness of STM528 were also abtained.