Study on inhomogeneous cooling behavior of extruded profile with unequal and large thicknesses during quenching using thermo-mechanical coupling model

The interfacial heat transfer coefficient between hot profile surface and cooling water was determined by using inverse heat conduction model combined with end quenching experiment. Then, a Deform-3D thermo-mechanical coupling model for simulating the on-line water quenching of extruded profile with unequal and large thicknesses was developed. The temperature field, residual stress field and distortion of profile during quenching were investigated systematically. The results show that heat transfer coefficient increases as water flow rate increases. The peak heat transfer coefficient with higher water flow rates appears at lower interface temperatures. The temperature distribution across the cross-section of profile during quenching is severe nonuniform and the maximum temperature difference is 300 °C at quenching time of 3.49 s. The temperature difference through the thickness of different parts of profile first increases sharply to a maximum value, and then gradually decreases. The temperature gradient increases obviously with the increase of thickness of parts. After quenching, there exist large residual stresses on the inner side of joints of profile and the two ends of part with thickness of 10 mm. The profile presents a twisting-type distortion across the cross-section under non-uniform cooling and the maximum twisting angle during quenching is 2.78°.     

Design and validation of a grinding wheel optical scanner system to repeatedly measure and characterize wheel surface topography

This paper describes the design and validation of an upgraded grinding wheel scanner system that controls the position of a Nanovea CHR-150 Axial Chromatism sensor along the x- and y-directions of the wheel surface to measure and characterize wheel surface topography. The scanner features a novel homing system that enables the wheel to be removed from the scanner, used on a grinding machine and then re-mounted and re-homed so that the same location on the wheel surface can be repeatedly measured and monitored. The average standard deviation for homing was 27.6 μm and 19.3 μm in the x- and y-directions, respectively, which is more than adequate for typical area scans of 25 mm². After homing, the scanner was able to repeatedly measure features that were similar in size to an abrasive grain (∼200 μm diameter) with an average error of 9.3 μm and 5.9 μm in the x- and y-directions, respectively. The resulting topography measurements were compared with Scanning Electron Microscope images to demonstrate the accuracy of the scanner. A custom particle filter was developed to process the resulting data and a novel analysis technique involving the rate of change of measured area was proposed as a method for establishing the reference wheel surface from which desired wheel topography results can be reported such as the number of cutting edges, cutting edge width and cutting edge area as a function of radial depth.     

平面折展机构平面弹簧的设计与分析

针对常规平面弹簧变形过程中运动端容易产生周向旋转从而造成振动和磨损的问题,设计了一种基于LEMs的平面弹簧,该弹簧由平面薄板加工成形且能实现平面外运动,具有体积小、易加工,结构简单及变形过程中运动端不发生周向旋转等优点。首先,基于悬臂梁模型和伪刚体模型分别推导了LEMs平面弹簧的刚度计算公式。其次,为验证所推导公式的正确性,建立了LEMs平面弹簧的ANSYS仿真模型,并将有限元分析结果与上述两种理论模型的计算结果进行了对比。结果表明,在变形较小的情况下由悬臂梁模型所推导的公式计算精度更高,在变形较大的情况下使用由伪刚体模型所推导的公式更为合适。最后,通过大量实例分析推导了两个公式的适用范围,并用一组数值算例证明了其正确性。     

Modeling study of the hydrodynamic arc breaking mechanism in BEAM

Blasting erosion arc machining (BEAM) utilizes thermal effect of the arc rather than spark to remove workpiece material. This paper investigates the expansion and distortion rules of the discharge channel based on the analysis of high speed photography and the micrographs of craters to study mechanism of BEAM. Based on these investigations, a thermal finite element model is developed. Then the temperature distributions in workpieces, both with and without flushing, are obtained. The comparisons of the obtained temperature distributions explain the formation of the trailing crater and the effect of hydrodynamic arc breaking mechanism on the efficiency of BEAM.     

Improving austenitic stainless steel resistance spot weld quality using external magnetic field

A ring-shaped permanent magnet is applied in resistance spot welding to improve the weld quality of austenitic stainless steel. Under the action of an external magnetic field, the profile of the weld nugget became peanut-shaped instead of ellipsoidal. The crystal orientation near the faying surface was less directional, and equiaxed grains were formed in weld nugget centre. Moreover, the shrinkage cavities tendency in traditional resistance spot welds was reduced. The relatively slow cooling speed could cause element segregation and thus change solidification mode and significantly affect weld microstructures. The mechanical performance of the welds was finally improved by the applied external magnetic field in terms of microhardness and lap-shear strength.     

Prediction of cutting forces in three and five-axis ball-end milling with tool indentation effect

Simulation of multi-axis ball-end milling of dies, molds and aerospace parts with free-form surfaces is highly desirable in order to optimize the machining processes in virtual environment ahead of costly trials. This paper presents a mechanics model that predicts the cutting forces in feed (x), normal (y) and axial (z) directions by modeling the chip thickness distribution, and cutting and indentation mechanics. The shearing forces are based on commonly known cutting mechanics models. The indentation of the cutting edge into the work material is modeled analytically by considering elasto-plastic deformation of the work material pressed by a rigid cutting tool edge with a positive or negative rake angle. The distribution of chip thickness and geometry of indentation zone are evaluated by considering five-axis motion of the tool along the toolpath. The proposed model has been experimentally validated in plunge indentation, as well as in three and five-axis ball-end milling of free-form surfaces. The prediction of axial (z) cutting forces is shown to be improved significantly when the proposed indentation model is integrated into the mechanics of ball-end milling.     

Thermal Postbuckling Behavior of 3D Braided Rectangular Plates

Thermal postbuckling characteristics of 3D braided rectangular plates are investigated. A 3D braided composite may be treated as a cell system and the geometry of each cell is deeply dependent on its position in the cross-section of the plate. Based on Reddy's higher-order shear deformation plate theory, a perturbation technique is employed to determine buckling temperatures and postbuckling equilibrium paths of simply supported 3D braided rectangular plates. The results reveal that the temperature-dependent properties, geometric parameter, fiber volume fraction and braiding angle have a significant effect on thermal postbuckling behavior of braided composite plates.     

Tidal effect compensation system for point absorbing wave energy converters

Recent studies show that there is a correlation between water level and energy absorption values for the studied wave energy converters: the absorption decreases when the water levels deviate from average. The situation appears during tides when the water level changes significantly. The main objective of the paper is to present a first attempt to increase the energy absorption during tides by designing and realizing a small-scale model of a point absorber equipped with a device that is able to adjust the length of the rope connected to the generator. The adjustment is achieved by a screw that moves upwards in the presence of low tides and downwards in the presence of high tides. Numerical results as well as experimental tests suggest that the solution adopted to minimize the tidal effect on the power generation shows potential for further development.     

EMG and EPP-integrated human-machine interface between the paralyzed and rehabilitation exoskeleton

Although a lower extremity exoskeleton shows great prospect in the rehabilitation of the lower limb, it has not yet been widely applied to the clinical rehabilitation of the paralyzed. This is partly caused by insufficient information interactions between the paralyzed and existing exoskeleton that cannot meet the requirements of harmonious control. In this research, a bidirectional human-machine interface including a neurofuzzy controller and an extended physiological proprioception (EPP) feedback system is developed by imitating the biological closed-loop control system of human body. The neurofuzzy controller is built to decode human motion in advance by the fusion of the fuzzy electromyographic signals reflecting human motion intention and the precise proprioception providing joint angular feedback information. It transmits control information from human to exoskeleton, while the EPP feedback system based on haptic stimuli transmits motion information of the exoskeleton back to the human. Joint angle and torque information are transmitted in the form of air pressure to the human body. The real-time bidirectional human-machine interface can help a patient with lower limb paralysis to control the exoskeleton with his/her healthy side and simultaneously perceive motion on the paralyzed side by EPP. The interface rebuilds a closed-loop motion control system for paralyzed patients and realizes harmonious control of the human-machine system.     

《剩余质量阵和剩余刚度阵的近似计算方法》

在模态综合法中，剩余质量阵和剩余刚度阵的计算通常依赖于子结构的质量阵和刚度阵，由于几乎不可能通过实验方法得到子结构的质量阵和刚度阵，因此很难将实验数据应用于理论模态综合法中。针对这一困难，通过分析剩余质量阵和刚度阵的表达式，推导出它们的近似计算公式，这一近似计算公式中不含质量阵和刚度阵，亦即不需要事先知道质量阵和刚度阵就可以计算剩余质量阵和剩余刚度阵，从而回避实验辨识质量阵和刚度阵这一极具挑战的动力学反问题，克服实验模态综合法应用中的困难。数值计算表明，所提的剩余质量阵和剩余刚度阵的近似计算方法切实可行，且模态综合结果具有较高的精度。