R-CURVE MODEL OF CERTAIN CERAMIC COMPOSITES

Abstract— The influence of crack-bridging on the R -curve for certain types of ceramic composites is modeled. Small scale bridging conditions are assumed. A recurrence procedure is used to solve the governing coupled integral equations, resulting in R -curves (stress intensity factor versus crack growth) that depend on well defined microstructural parameters. In addition, the bridging stresses and the crack opening diplacements were computed for all stages of the R -curve development. The numerical scheme is quite general and can be used for a variety of bridging laws. The convergence of the solution method is uniform, and the accuracy is a priori controlled. The results are normalized so that they can be applied for a large variety of composites with different micromechanical parameters. R -curves were computed for different bridiging laws, and were found to be sensitive to the specific bridging law used each time. Some strength-toughness relations are also discussed. The results are particularly suitable for slow crack growth analysis.     

六相感应电机直接转矩驱动系统的建模和仿真研究

研究了六相感应电机直接转矩控制的新策略.根据直接转矩控制技术,一个小型(2kW)两极六相感应电机驱动系统在电流数字滞环控制下通过Matlab仿真和实验实测得到证实,理论计算结果与实验结果基本一致.仿真和实验结果表明,电磁转矩和转矩电流在MMF平衡条件下呈线性关系而与负载的大小无关,这种转矩控制策略可使系统具有良好的动态性能.     

梯形波相电流驱动六相感应电机建模与控制

针对交流电机的气隙磁链定向控制具有气隙磁通能被直接测量的优点,但要实现气隙磁链定向控制必须采用派克变换算法,使控制系统复杂化和运算量大等缺点,提出了一种气隙磁链定向控制策略:即六相感应电机梯形波相电流控制。该梯形波相电流由励磁电流和转矩电流组成,在该梯形波相电流驱动下六相定子绕组可以分为励磁绕组和转矩绕组,模拟直流电机实现励磁磁场和转矩的直接控制而不需要复杂的派克变换。采用有限元分析,研究六相电机的主要性能,建立六相电机定子相电路模型。整个六相电机驱动系统的性能通过实验完成。结果表明该种控制方案可以真正实现磁链和转矩的独立控制,使控制器结构简单,具有便于实现特点。     

Water Vapor Permeability of an Edible, Fatty Acid, Bilayer Film

A bilayer film consisting of stearic and palmitic acids as one layer and hydroxypropyl methylcellulose as the other was prepared. The permeability of this film to water vapor was tested under a variety of conditions involving film composition, temperature and relative humidity. This film exhibited permeability values (g · mil w m⁻²· day⁻¹· mm Hg⁻¹) of 0.5 at 40°C 0.3 at 25°C 1.7 at 5°C, and 6.0 at −19°C. This film would be expected to perform well at relative humidities below 90% and temperatures within the range of 40°C to -19°C.     

Use of an Edible Film to Maintain Water Vapor Gradients in Foods

An edible, biiayer film consisting of a layer of stearic-palmitic acid and a layer of hydroxypropyl methylcellulose was situated between two food components of markedly different water activities to determine the film's ability to retard equalization of water activity. Tomato paste or salted tomato paste was used as the high-moisture food and ground crackers were used as the low-moisture food. Compared to a filter paper control, the bilayer film substantially slowed transfer of water from the salted tomato paste to the crackers during 14 days at 25°C and 21 days at 5°C. During 70 days at -20°C, the film essentially stopped the transfer of water from tomato paste to the crackers.     

转炉副枪测量与成分预报技术

梅钢炼钢厂采用贺利氏副枪新技术和自主开发成分预报模型相结合的办法,对转炉冶炼过程进行控制.结果表明,过程碳、终点碳含量的预测精度提高,误差分别降低到0.023 %和0.006 %,同时也使转炉模型预测终点磷含量精度的准确性提高到96.47 %.     

基于梯形波相电流驱动的六相交流感应电动机性能

为了实现六相感应电动机的励磁磁场和电磁转矩直接控制,提出了梯形波相电流来驱动六相交流感应电动机,并对其性能进行研究.首先采用理论分析和有限元分析对气隙磁场和电动机的电磁转矩进行了计算.其次,利用所构建的六相电动机的实验装置对理论分析结果进行了佐证.该六相电动机控制策略的最大优点是可以实现励磁磁场和转矩电流的直接控制而不需要派克变换,另一个优点是通过六相定子分布,合成的磁场电流可以产生一个近似的方波气隙磁场,使电磁转矩的输出得到极大的提高.     

六相感应电机直接转矩控制的新策略

为直接控制电机的气隙磁通和电磁转矩,提出了特殊的定子电流波形.从理论上对这种特殊电流波形激励的六相感应电机内部气隙的磁场强度性能进行了分析,并推导出电磁转矩的方程式.根据所提出的直接转矩控制技术,对一个小型（2 kW）两极六相感应电机驱动系统在电流数字滞环控制下进行实验研究.实验结果表明,电磁转矩和转矩电流在MMF平衡条件下呈线性关系而与负载的大小无关.采用电流波形驱动六相感应电机的优点是气隙磁场类同于直流电机的一个方波,并且可以实现电磁转矩的直接控制,省去了繁琐复杂的派克变换和逆变换.这种转矩控制新策略也可使系统具有良好的动态性能.     

Water Vapor Permeability of Edible Bilayer Films

Edible films composed of a water soluble, carbohydrate layer (hydroxypropyl methylcellulose) and various kinds of lipid layers were tested for resistance to water vapor permeability. Films were tested at 25°C and a relative humidity differential of 85%. Films containing solid lipids, such as beeswax, paraffin, hydrogenated palm oil or stearic acid yielded permeabilities of 0.2 g · mil · day⁻¹· mmHg⁻¹ or less which is a smaller value than that for low density polyethylene.