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Dai Q.,Tsinghua University | Liang Z.,Tsinghua University | Chen G.,Tsinghua University | Meng L.,CSR Sifang Locomotive and Rolling Stock. Co. | Shi Q.,Tsinghua University
Materials Science and Engineering A

Fatigue crack propagation (FCP) at different locations of friction stir welded AA6N01 joint was investigated. FCP experiments were performed to study crack propagation rates in the Paris region. The highest FCP rate was in the stir zone (SZ) which composed of much finer grain compared with the other areas of joint. Fracture surfaces at different locations were examined. More brittle fracture characteristic was observed in SZ. To further investigate the mechanism of high FCP rate in SZ, differential scanning calorimetry (DSC) analysis was conducted. The results showed that there was relatively high energy stored in SZ. After releasing the stored energy, the resistance of FCP in SZ improved significantly while the grain size kept the same, the fatigue fractographs had changed from quasi-cleavage to plastic fatigue striation fracture. These proved that the stored energy affected fatigue crack propagation in FSW joint significantly. © 2013 Elsevier B.V. Source

Dai Q.,Tsinghua University | Meng L.,CSR Sifang Locomotive and Rolling Stock. Co. | Liang Z.,Tsinghua University | Wu J.,Tsinghua University | Shi Q.,Tsinghua University
Hanjie Xuebao/Transactions of the China Welding Institution

Fatigue crack propagation (FCP) behavior at different locations of friction stir welded (FSW) and metal inert-gas welded (MIG) A6N01 joints were studied. The fatigue fractures and microstructures of the joints were analyzed. For FSW and MIG joints, the fatigue crack propagation rate (FCPR) in nugget (weld) zone was the highest, while the FCPR in heat-affected zone (HAZ) was lower than that in the nugget (weld) zone. The base metal had the highest fatigue crack propagation resistance. The FCPR had no significant difference when crack propagated in the same regions of FSW and MIG joints. However, the fatigue crack growth threshold (ΔKth) of crack propagating in the nugget zone of FSW joint was higher than that of MIG joint, in other words, the resistance to fatigue crack propagation in the nugget zone of FSW joint was superior to that of MIG joint. The fracture surface of fatigue crack propagating in the nugget (weld) zone of FSW and MIG joints revealed brittle fracture, while that in heat-affected zone was mainly plastic fatigue striation. © 2015, Harbin Research Institute of Welding. All right reserved. Source

Chu J.-K.,Dalian University of Technology | Li R.-H.,Dalian University of Technology | Li Q.-Y.,Dalian University of Technology | Li Q.-Y.,CSR Sifang Locomotive and Rolling Stock. Co. | Wang H.-Q.,Dalian University of Technology
International Journal of Automation and Computing

Inspired by human behaviors, a robot object tracking model is proposed on the basis of visual attention mechanism, which is fit for the theory of topological perception. The model integrates the image-driven, bottom-up attention and the object-driven, top-down attention, whereas the previous attention model has mostly focused on either the bottom-up or top-down attention. By the bottom-up component, the whole scene is segmented into the ground region and the salient regions. Guided by top-down strategy which is achieved by a topological graph, the object regions are separated from the salient regions. The salient regions except the object regions are the barrier regions. In order to estimate the model, a mobile robot platform is developed, on which some experiments are implemented. The experimental results indicate that processing an image with a resolution of 752 × 480 pixels takes less than 200ms and the object regions are unabridged. The analysis obtained by comparing the proposed model with the existing model demonstrates that the proposed model has some advantages in robot object tracking in terms of speed and efficiency. © 2010 Institute of Automation, Chinese Academy of Sciences and Springer-Verlag Berlin Heidelberg. Source

Gao H.,Southwest Jiaotong University | Dai H.-Y.,Southwest Jiaotong University | Ni P.-T.,CSR Sifang Locomotive and Rolling Stock. Co.
Tiedao Xuebao/Journal of the China Railway Society

Elastic deformation of wheelset should be considered in research of wheelset dynamics. In this paper, a new wheel-rail contact point searching algorithm used for flexible wheelset simulation was developed on the basis of the conventional wheel-rail contact line method. Normal vectors of a point on each rolling circle and its projection on rail were calculated to find out possible contact points, which formed the wheel-rail contact line. The final contact point was determined by the minimum vertical distance between the contact line and rail section profile. The rigid-flexible coupling dynamic equations of the single wheelset were established and dynamic simulation was made. The impact of wheelset deformation on the wheel-rail contact position and creepage was discussed. Comparison of rigid and flexible wheelsets shows that the new method is effective to solve flexible wheelset and rail contact problems. Source

Yan Z.,Harbin Institute of Technology | Chen S.,CSR Sifang Locomotive and Rolling Stock. Co. | Shang Z.,Beijing Aerospace Xinfeng Mechanical Equipment. Co. | Liu X.,Harbin Institute of Technology | Fang H.,Harbin Institute of Technology
Hanjie Xuebao/Transactions of the China Welding Institution

The welding joint and repair welding joint of A7N01 aluminium alloy are analyzed in this paper, the mechanical properties of the welding joint before and after repair welding were studied by welding residual stress measurement, tensile test, micro-hardness test, metallographic observation and fracture toughness test, respectively. Based on the fracture toughness theory, the critical crack length of the structure before and after repair welding is calculated. With the initial crack length determined by metallographic observation, the residual fatigue life before and after repair welding is calculated using Paris formula. The result shows that although the repair welding degrade the welding joint property to some extent, it is still an efficient method to make sure the structure with flaw has enough load-carrying capacity. Source

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