Papers by Author: Chen Bao

Abstract: In the fracture mechanics, the failure of mode-III crack belongs to the anti-plane shear (also called out-plane shear or longitudinal shear). Fatigue crack growth (FCG) behavior under mode-III reversed load (load ratio R=-1) was studied by using 30Cr2Ni4MoV rotor steel with types of circumferential notches. Under the remote cyclic torsional load, the precrack process was applied to specimens with notch in the round bar, and the ideal sharp-crack was obtained. Then, mode-III FCG test under the remote cyclic torsional load and the axial constant small load was carried out until to fatigue failure. The axial constant small load is aimed at reducing the cracks interaction in the mode-III FCG experiment. Due to experimental data of 30Cr2Ni4MoV rotor steel, the mode-III FCG rate in the range of KIII from 12MPa·m1/2 to 40 MPa·m1/2 and the threshold value were got. Furthermore, a prediction model for mode-III FCG is proposed with considering the small scale yield of plane stress crack tip zone and the plastic strain energy density failure criteria. The predicted mode-III FCG rate is found to agree well with the experimental curves of 30Cr2Ni4MoV rotor steel.

Abstract: The inherent law between fatigue behaviors of shear-type representative volume element and mode-II fatigue crack growth is found in the range of cycle plastic zone near the crack tip. Prediction model for mode-II fatigue crack growth rate is then proposed by utilizing shear-type low cycle fatigue properties, plastic strain energy criterion, and effective cycle stress-strain field. Experimental data of two Aluminum alloys, 2024-T351 and 7075-T6, are used for the model verification. Good agreement between experimental and theoretical results is obtained.

Abstract: For front-force compact tension specimen (FFCT), based on the refined results of the relationship between crack tip opening displacement and load line crack opening displacement from Finite Element Analysis (FEA), the influences of material properties and plastic deformation near the crack tip have been analyzed. A simplified and accurate transform formula for FFCT specimens is presented in this paper, and the error analysis is conducted.

Abstract: Based on a series of tests and elastic-plastic contact finite element analyses, a sharp indentation method for predicting the constitutive relationship of metallic materials was presented. Dependent on the dimensionless linear relationship which is independent of the hardening index n, a forecasting formula of constitutive parameters can be gotten and its form is much terser. The test results and finite element calculations show that, the predicted stress-strain curves are in agreement with the uniaxial tensile test results. The validity and precision of this method was certified by in-dentation tests on kinds of different materials.

Abstract: Abstract: Based on the Miner′s linear accumulative damage theory and the low cycle fatigue damage parameter that is relative to the total strain amplitude and the mean stress of the plastic zone near crack tip, a new numerical simulation method to predict the fatigue crack growth (FCG) of materials is presented. The numerical FCG results for middle-cracked tension (MT) specimens of Cr2Ni2MoV steel, TA12 and TC4 alloys by ANSYS are closed to those by tests. Therefore, the new method has capability to give good FCG prediction accuracy for different materials. Furthermore, the Paris FCG models of TA17 and TC11 alloys were predicted by this method and can be referred to the key engineerings.

Abstract: In this work, a nondimensional load separation conforming to similarity principle was proposed. Based on this new principle, similarity simulation of the physical phenomenon between the prototype sample and the dimension-changed model sample can be implemented. Then, a modified separation parameter S_pb method was developed, which can absolutely eliminate the effect of referencing blunt cracked specimen on the instantaneous crack length estimation of the sharp cracked specimen, and the forced blunt-corrected initial crack length and the final crack length are recommended as the calibration points for J-resistance curve estimation. Finally, the modified S_pb method is successfully applied in the J-resistance curve estimation of two steels with CT and SEB specimens, respectively.

Abstract: One of tasks of fracture mechanics analysis is to get J resistance curves and fracture toughness of ductile materials. Based on the dimensionless theory, a modified Spb method from the load separation method was proposed. In order to apply load separation method to analysis the fracture toughness of a material from its test data, the testing application J-LSS (J resistance curve-Load Separation Software) has been developed by using visual basic (VB) language and was applied to obtain the fracture toughness values of Cr2Ni2MoV rotor steel and 316L stainless steel. Additionally, the differences between the traditional normalization method and the modified S^pb method in analyzing fracture toughness through J-LSS were discussed. The modified S^pb method presented in the paper is more available and can be recommended to renew the current material fracture toughness standard test.

Abstract: Based on low cycle fatigue critical damage, a numerical procedure for predicting stable fatigue crack propagation behavior of materials and structures was developed, which was named as LFF (LCF: Low Cycle Fatigue + FCP: Fatigue Crack Propagation + FEA: Finite Element Analysis) in this work. In the LFF method, stress and strain amplitude of nodes located at the plastic zone in the crack growing direction and fatigue damage of these nodes were firstly computed from a finite element code ANSYS. Then, by assuming a discontinuous step of propagation in the process of fatigue crack propagation, the stable fatigue crack propagation behavior of materials and structures could be estimated. Experiments on low cycle fatigue and fatigue crack propagation for Cr2Ni2MoV, TA12 and TC4 alloys were employed to certificate the applicability and validity of the LFF method. The results show that the LFF method is suitable for the estimation of stable fatigue crack propagation of a CT specimen with a straight crack under mode Ⅰloading and with a slant crack under mixed mode Ⅰ and Ⅱ loading.

Abstract: Thin plates and small thin-walled tubes of zircaloys are used as fuel cladding materials of nuclear reactors. In order to prevent buckling of plate or tube specimens of zircaloys under cyclic strain loading, a set of self-invented clamps were used for strain fatigue tests of small thin-walled tubes of Zr-4 and Zr-1Nb at elevated temperature. Depending on axial control strain, a new strain fatigue test method named as (equivalent strain)-SF(strain fatigue) was developed for thin funnel-like plate specimens of N18 zircaloy with double side notches. A series of monotonic tension tests for the three kinds of zircaloys at different temperatures and strain fatigue tests for Zr-4 and Zr-1Nb at 375°Cand for N18 at 20°C,300°Cand 500°C were carried out. Monotonic and cyclic constitutive relations and Manson-Coffin life-estimating expressions for these zircaloys are given. ES-SF method has no Poisson effect and has better accuracy to predict strain fatigue behavior.

Abstract: Based on compliance testing of a straight-notch compact tensile (SN-CT) or a single edge bending (SEB) specimen, this paper present a formula with increment form to calculate plastic part of J-integral, two formulas for a SN-CT specimen and a SEB specimen to transform crack mouth opening displacement (CMOD) v0 to the crack opening displacement (COD) q along load line, and simplified formulas to estimate crack length a and effective young’s modulus E. Furthermore, the relation between v and q of the SN-CT specimens of 45 steel and SEB specimens of 30Cr steel were investigated. The results show that the formulas to describe the relation between the ratio v/q and the dimensionless crack length a/w of the specimens accord with the testing results better.