Papers by Author: Xu Chen

Abstract: A modified normalization (NM) method to determine J-R curves using clamped single edge notched tension (SENT) specimens was proposed. To validate and quantify the modified NM method, the J-R curves of X80 pipeline steel obtained by NM method are compared with those determined by the unloading compliance (UC) method for SENT specimens. The comparison shows that modified NM method is obvious better than unmodified NM method for SENT specimens. The modified NM method has great agreements with UC method, and is a valid and cost-effective tool to be applied to obtain J-R curves of API X80 steel using SENT specimens with shallow cracked depth to deep cracked depth.

Abstract: Effects of thermal aging on tensile and Charpy impact properties in 16MND5 steel was investigated, which were aged at 500°C for 0 h, 1000 h, 3000 h, 5000 h. A significant decrease in the yield stress and ultimate tensile strength was observed after thermal aging, while the elongation exhibited a slight decrease follow by an increase aged for 5000 h. What's more, the ductile-to-brittle transition temperature (DBTT) showed a remarkable increase with the prolongation of thermal aging duration. These facts indicate thermal aging caused embrittlement of the steel, which was further investigated by microstructure observation of SEM. The results show cleavage fracture after thermal aging. Furthermore, experimental results at 350°C thermal aging temperature originated from the previous literature were used to analysis the effect of thermal aging temperature. Thus, thermal embrittlement should be taken seriously.

Abstract: The study attempts to present a new finite element method to implement the direction prediction of crack extension for 2D and 3D cracks based on the maximum tensile stress (MTS) criterion by identifying the first principal stress direction. Especially in the 3D crack propagation, this paper gave a detailed and innovative research method to overcome the challenge of direction prediction on crack extension. The maximum principal stress of the node of crack tip and its neighboring nodes were calculated to determine the increment and the direction of crack tip respectively. Some examples of complex crack propagation were performed under a variety of mixed fracture modes based on linear elastic analysis by ANSYS software. A reasonable evaluation was executed and the advantages of this method were analyzed in detail.

Abstract: In this paper, the loading path effects on the plane strain elastic-plastic crack-tip stress field are investigated computationally. Three different loading sequences include one proportional loading and two non-proportional loading paths are applied to the modified boundary layer (MBL) model under small-scale yielding conditions. For the same external displacement field applied at the outer boundary of the MBL model, the mode I K field and T-stress field combined as the different loading paths are applied to investigate the influence of the nonproportional loading. The results show that for either the compressive or tensional T-stress, the loading path which applied K field followed by T field generates the lower crack-tip constraint. There is only slightly difference between the proportional loading path and that with the T-stress field following by K field. The results show that it is very important to include the load sequence effects in fracture analysis when dealing with nonproportional loading conditions.

Abstract: Aluminium alloys are widely used in the fields of automobile, machinery and naval construction. To investigate the effect of non-proportional loadings and corrosive environment on the fatigue resistance of 6061-T6 aluminum alloy, a set of uniaxial and multiaxial low cycle fatigue tests were carried out. Firstly, the results of uniaxial tests showed that the alloy exhibited cyclic hardening then cyclic softening. With the increase of stress amplitude the cyclic softening became pronounced. The increasing of plastic deformation was basically cyclically stable with small plastic strain amplitude accumulation when the stress amplitude was lower than 200MPa ,while it was increasing rapidly when the stress amplitude was higher than 220MPa. Secondly, it was observed that non-proportional cycle additional hardening of 6061-T6 aluminum alloy was little. While the fatigue life was badly affected by the loading paths. Thirdly ,the fatigue corrosion interactions were also talked about in details by performing the tests under the same loading conditions with corrosive environment. The experiment proved that the seawater corrosion has huge impact on fatigue life under pH 3. Finally, a multi-axial fatigue life prediction model was used to predict the fatigue life with or without the corrosive environment which showed a good agreement with experimental data.

Abstract: Ratcheting deformation is studied on elbow pipe made of Z2CND18.12N by FEM software. The simulation is conducted by ANSYS. Chen-Jiao-Kim (CJK) kinematic hardening model is added in ANSYS for the study. The elbow pipe is subjected to internal pressure and reversed in-plane bending. Internal pressure can be constant or cyclic. Many different loading paths are used in the study. Ratcheting deformations of under different ways are studied. The result shows that ratcheting deformation occurs mainly in the circumferential direction. Ratcheting deformation at the crown and intrados of elbow pipe is more notable because of higher stress. Tensile or compressed load can influence the position of dangerous point. It is found that ratcheting deformations under different paths with same peak load are different.

Abstract: Bismuth telluride-based alloys are the most widely used thermoelectric materials, which can obtain the conversion between heat energy and electricity. As a kind of lead-free solder alloy, Sn96.5Ag3.0Cu0.5 (SAC305) shows great potential in soldering between the thermoelectric element and electrode in thermoelectric couples. Cu/ SAC305 /Ni-plated Bi_0.5Sb_1.5Te₃ (n-type) and Cu/ SAC305/Ni-plated Bi_1.8Sb_0.2Se_0.15Te_2.85 (p-type) couples were connected at 250°C by reflow soldering to investigate effects of temperature and thermall aging on the efficiency and reliability of thermoelectric device. Effects of temperature and environmental aging on shear force of thermoelectric couples were studied respectively in the temperature range of 20°C to 180°C. At room temperature of 20°C, n-type thermoelectric couples showed higher shear force than p-type thermoelectric couples. N-type thermoelectric couple showed higher shear reliability than p-type thermoelectric couples after synchronized environmental aging. The shear force of both types of thermoelectric couples decreased with the growth of thermal aging cycles. Finally, the fracture surfaces of two types of thermoelectric couples were investigated respectively using SEM to further analyse the failure mechanism.

Abstract: Uniaxial ratcheting behaviors of Z2CN18.10 austenitic stainless steel under both tensile pre-strain (TP) and compressive pre-strain (CP) were experimentally studied at room temperature. The experimental results show that: TP restrains ratcheting strain accumulation of subsequent cycling with positive mean stress; lower level of CP is found to accelerate ratcheting strain accumulation while higher level of CP retards the accumulation. Based on the Ohno-Wang II kinematic hardening rule, rate-independent model, viscoplastic model, isotropic hardening model and a modified model were constructed to describe the ratcheting behaviors under various pre-strain conditions. All the four models gave fairly good prediction on ratcheting strains for various TP. The isotropic hardening model and modified model predicted acceptable ratcheting strain though still showed slight tendency of over prediction.

Abstract: With the considerable use of high-grade pipeline steel in onshore and offshore project, welded joints are recognized as the weak link in pipeline because of the non-uniform microstructural regions induced by welding heat input. At first, the microstructural of different regions in API X80 pipeline welded joints was characterized and quantified by SEM, which indicate that the pipeline steel is a typical acicular ferrite steel. In this paper we investigated the J-integral resistance curve (J-R curve) in different locations of API X80 pipeline welded joints through low constraint SENT specimens with side grooves at room temperature. The effect of notch orientation (longitudinal-radial (L-R) and transverse-radial (T-R)) on resistance curve were investigated in base metal, which reveal the orientation almost have no effect on resistance curve. As the welded joints adopted in this study is two-pass steel arc welds, so the J-R curves of the inner surface, the outer surface and through-thickness surface notches specimens in the weld metal were investigated. The inner surface sample have the highest toughness through three samples because of the effect of second pass welding process. The effect of constraint on resistance curve was conduct between low constraint SENT specimen and high constraint SENB specimen, which found that the lower constraint corresponding to the higher resistance curve. After finishing the test, crack advancing plan of different positions were etched and observed by OM to demonstrate that the crack path always in the region which we would like to test.

Abstract: In this study, a series of multiaxial ratcheting tests under combined cyclic axial and inner pressure were conducted on M5 zirconium alloys tubes used as nuclear fuel cladding in Pressurized Water Reactors (PWRs) for the purpose to investigate the multiaxial ratcheting behavior of M5 and the factors of loading history may influence it. The experiment was stress-controlled and designed in a special loading path that all tubes in actually were under axial symmetrical cyclic load and constant inner pressure. A set of new patent multiaxial clamp was designed and applied to make sure the tubes can withstand inner pressure and axial load at the same time. A dynamic closed-loop multiaxial testing controller was used to manipulate two actuators simutaneously. Biaxial strain gauges were adopted to detect the signals of axial and hoop strain during the tests. The experimental results show that axial ratcheting strain decreases obviously within the original cycles reaching to a stop in certain cycles, the axial ratcheting increases with the cycles going. Loading history has obvious influence on the ratcheting behavior in axial direction. Higher stress amplitude level after loading history with lower stress amplitude level leads to an abnormal ratcheting behavior. The axial ratcheting shows a high sensitive to the complex loading history, especially when the stress amplitude is high. The hoop ratcheting strain accumulates continuously with the cycles going. Loading history has no obvious influence on hoop ratcheting strain and its ratcheting when the stress amplitude is low. The ratcheting strain and its rate shows more sensitive to the loading history when the stress amplitude increases to a certain value.