Zhang Yunlong

Strength and fracture toughness of MgO-modified glass infiltrated alumina for CAD/CAM.

OBJECTIVE This study was conducted to investigate the effect of MgO additive to Al2O3 on the flexural strength, fracture toughness of glass infiltrated alumina for CAD/CAM application. METHODS Alumina blanks with additive of 0.5 wt% MgO were prepared via isostatic pressing and sintering at 1400 degrees C for 2h, and then alumina-glass composites were fabricated by infiltrating the molten glass into the partially sintered alumina compact. Flexural strength and fracture toughness were determined using three point bending methods and a single edge notched beam method. The mechanism of crack propagation was observed under a field emission scanning electron microscope. RESULTS The three-point flexural strength and fracture toughness of partially sintered alumina and alumina-glass composite were 210 MPa, 1.86 MPam(1/2), and 432.2 MPa, 5.12 MPam(1/2), respectively, and they were free of shrinkage during the processing of glass infiltration. The field emission SEM micrograph indicated that indentation caused a non-planar crack propagation including crack deflection and crack bowing. SIGNIFICANCE MgO was used as an additive to alumina to improve the strength and fracture toughness of partially sintered alumina and alumina-glass composite. The high strength and toughness are related to toughening by the distribution of alumina with uniform particle sizes, crack bowing, crack deflection and the beneficial wetting properties of the particle surface.

The Shock Wave Refraction in Supersonic Planar Mixing Layers

We analyze the shock wave refraction in a spatially developing shocked mixing layer by means of direct numerical simulation. Both regular and Mach reflections can occur depending on the relative strength of the induced shock wave over the vorticity of interacting vortex in the mixing layer. The stronger incident shock wave frequently refracts Mach reflection. The shock polar diagram is used to determine the shock wave refraction patterns. Moreover, the vortices are deformed and compressed by the shock wave, and their vorticities are increased. The interaction of shock wave and coherent structure can be helpful to enhance the mixing process.

Idle Speed Control of Gasoline Engine for Hybrid Electric Vehicle

Hybrid Electric Vehicles use electric throttle instead of mechanical throttle as to distribute power and optimize performance of gasoline engine. At present, in order to keep oil pump and compressor working normally, idle speed control is still needed for some HEV gasoline engines. This article introduces the method we used in the development of idle speed control system based on electric throttle. Besides, the construction and working principle of idle speed control system are given in detail. The software function validation and the preliminary calibration, which are fulfilled in the bench test, indicate that electric throttle has the ability to control idle speed and the method developed by us could control idle speed with high stability and precision.

Effects of Density Stratification on Mixing Enhancement by Shock Wave in Supersonic Shear Layer Flows

We analyze the response of passive mixing to shock wave in a density-stratified supersonic shear layer by means of direct numerical simulation with a high-order hybrid compact-WENO scheme. The coexistence gradients of density and pressure have a complex influence on generation of baroclinic vorticity, depending on the gradient directions. The growth rate of mixing layer thickness is increased by the presence of shock wave, but the mixing layer thickness is not always increased. The passive mixing can be enhanced in a thickened shear layer by the incident shock wave if the homodromous vortexes are induced by the baroclinic torque. The mixing efficiency is also enhanced by increasing the density difference of two streams. The present study can be helpful to take measures to strengthen the mixing process in supersonic shear flows.