Fabrication of sintered wick in micro -grooved pipe
The sintering heat pipe is an ideal component in solving the heat dissipation of high heat flux chips. The heat pipe with sintered wick structure in micro-grooved pipe was featured with low heat resistance, high bond strength and hard to dryout. According to the sintering theory, the manufacturing process of sintered wick could be divided into four phases: reduction phase, sintering starting phase, sintering preservation phase and sintering cooling phase. The sintering temperature, sintering time and sintering atmosphere about fabrication of the innovative sintered wick were also inspected. The result was discoveried that 100 mesh spherical copper powders had high porosity and low radial shrinkage at the sintering temperature of 950°C for 3 hours and the mandrel could be easily pulled out. It was also found that the oxidation reduction process could manufacture new micro structure—the segmentation cracks structure on the surface of copper powders.
Experimental study of oil-filled high-speed spin forming micro-groove fin-inside tubes
Abstract This paper presents an oil-filled high-speed spinning process of manufacturing micro-groove fin-inside tubes. In spinning, a high-pressure oil-film, which squeezes the tube outer-wall, is formed as the spinning speed increases. A floating mandrel extrudes the tube inside wall to form the micro-grooves/fins continuously. By experiments of spinning micro-groove fin-inside tubes, this paper analyzes the influences of each process parameter on the formation quality, and optimizes each parameter. The experimental results show that the dynamic pressure can significantly improve the stability of spinning and the quality of finished surface; the minimum thickness of oil-film is decided by the spinning speed and the oil viscosity directly. In the case of full oil dynamic lubrication, there is no obvious scrape on tube surfaces. The main factors affecting the groove depth and the surface roughness are the ratio ( α ) of the wall thickness of the tube before drawing to the finished one, the material plasticity, the ratio ( β ) of drawing to spinning, the spinning speed ( n ), and lubricating state, etc. The axial drawing force mainly depends on the friction between the floating mandrel and the tube internal wall, the friction between the reducing mold and the outer surface, and plastic deformation resistance in the reducing mold area and spinning area as well.
Numerical analysis of surface crack propagation in flexible pavements using XFEM and cohesive zone model
Surface initiated (top-down) cracking in the longitudinal direction of pavements has become a predominant mode of failure in thick asphalt pavements. Most previous researchers studied crack propagation using the traditional Linear Elastic Fracture Mechanics (LEFM) method by calculating the stress intensity factor around the crack tip. In this paper, Extended Finite Element Method (XFEM) was used to study the key factors causing surface initiated crack propagation under surface tension and/or shear. A two-dimensional (2-D) finite element (FE) model for a two-lane flexible pavement was built to predict pavement responses and crack propagation potential under various scenarios. It was found that surface tension is the major driving force for crack propagation when the crack is at some distance away from tire loading. The results show that the distance from the location of crack to tire loading, the depth and direction of the crack, and the modulus ratio between the hot-mix asphalt (HMA) layer and the base layer have significant influences on crack propagation under surface tension. On the other hand, this study shows that shear stress can induce surface initiated crack propagation near the loading area as the shear resistance of material becomes low at the high temperature.