Chen Bao

IMPROVED NORMALIZATION METHOD FOR DUCTILE FRACTURE TOUGHNESS DETERMINATION BASED ON DIMENSIONLESS LOAD SEPARATION PRINCIPLE

This study successfully deals with the inhomogeneous dimension problem of load separation assumption, which is the theoretical basis of the normalization method. According to the dimensionless load separation principle, the normalization method has been improved by introducing a forcible blunting correction. With the improved normalization method, the J-resistance curves of five different metallic materials of CT and SEB specimens are estimated. The forcible blunting correction of initial crack size plays an important role in the J-resistance curve estimation, which is closely related to the strain hardening level of material. The higher level of strain hardening leads to a greater difference in JQ determined by different slopes of the blunting line. If the blunting line coefficient recommended by ASTM E1820-11 is used in the improved normalization method, it will lead to greater fracture resistance than that processed by the blunting line coefficient recommended by ISO 12135-2002. Therefore, the influence of the blunting line on the determination of JQ must be taken into full account in the fracture toughness assessment of metallic materials.

Strain Fatigue Behavior of Thin-Walled Tubes of Zr-1Nb and Zr-4 and Thin Plates of N18 at Elevated Temperatures

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.

The Dimensionless Load Separation Method Used for Fracture Toughness Test and its Application

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 Spb method in analyzing fracture toughness through J-LSS were discussed. The modified Spb method presented in the paper is more available and can be recommended to renew the current material fracture toughness standard test.

Investigation on Spb Method Based on Nondimensional Load Separation Principle for SEB and CT Specimens

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 Spb 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 Spb method is successfully applied in the J-resistance curve estimation of two steels with CT and SEB specimens, respectively.

Estimation ofJ-resistance Curves for CT Specimen Based on Unloading Compliance Method and CMOD Data

The unloading compliance method is one of the most widely used methods in laboratory testing for J-resistance estimation. A specimen configuration including compact tension (CT) is recommended by almost all the standard test procedures for evaluating fracture toughness, such as ASTM E1820-11, ISO 12135-2002, and GB/T 21143-2007. In the unloading compliance method, key techniques including instantaneous crack size measurement, correction of specimen rotation, and J-integral computation can be used for a CT specimen when using load line displacement (LLD) but not when using crack mouth opening displacement (CMOD) from those standard test procedures. However, a CT specimen with CMOD measurement is also used for J-resistance estimation under certain severe environments because of the convenience of measuring displacement. This study proposes a simplified linear formula for crack size calculation for a CT specimen with CMOD data, discusses the rotation correction in detail based on the equivalence of work, and develops reasonable incremental expressions for J-integral computation and a CMOD-to-LLD conversion equation. By measuring the LLD and CMOD data of a CT specimen simultaneously, we carried out experiments on J-resistance curve estimation for four different types of materials. According to the developed procedure of the unloading compliance method in this study, J-resistance curves for all materials determined from LLD and CMOD data of the CT specimen are clearly exhibited.

Mode-II Fatigue Crack Growth Model from Shear-Type Low Cycle Fatigue Properties

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.

A Prediction Model for Mode-III Fatigue Crack Growth

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.

Experimental Estimation of J Resistance Curves From Load-CMOD Record for FFCT Specimens

Fracture toughness of ductile materials is often characterized by J resistance curves. It is generally required to adopt load line crack mouth opening displacement (CMOD) records in load line compact tension (LLCT) specimens to calculate the J integral according to ASTM E1820-13e1. However, in some cases, for example in nuclear engineering, due to material size limitations, a standard test specimen size is not achievable, and attaching or mounting the displacement transducer to measure the load line CMOD (LLD) of a smaller compact tension specimen is very challenging, and an alternative approach is required. The present study is aimed at evaluating the use of front face CMOD (FFD) records in front face compact tension (FFCT) specimens to estimate J resistance curves. Results are compared with those using LLD-based J calculations for P92 steel. It is shown that J resistance curves determined from the two approaches are in good agreement.

The Fined COD Transform Formula for CT Specimens to Investigate Material Fracture Toughness

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.

Simulation for Fatigue Crack Propagation Behavior Based on Low Cycle Fatigue Critical Damage

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.