混凝土拉压强度尺寸效应的细观非均质机理

为了从材料细观非均质角度揭示混凝土强度尺寸效应机理,建立了混凝土细观单元等效非均质力学模型,开展了立方体抗拉、抗压强度尺寸效应细观数值模拟研究。研究结果表明：混凝土强度尺寸效应根源于材料细观非均质性,随着模型尺寸的增加,混凝土材料细观单元弹性模量变异系数增大,材料细观非均质性增强,大尺寸模型内部存在更多的低强度单元或缺陷,导致混凝土立方体抗拉、抗压强度降低,极限应变减小,脆性增大;混凝土损伤破坏由少量集中区域,发散扩展形成多条非贯通的裂纹带;数值模拟结果与尺寸效应实验数据相吻合。     

Characteristics of Brittle Failure and Stress Drop Under Triaxial Loading and Unloading

The characteristics of deformation failure and stress brittle drop of granite under loading and unloading conditions are studied respectively based on two unloading stress path cases and conventional triaxial compression experiments. The experiments show that the primary deformation of rock is rebound or tension in the (main) unloading direction, but plastic deformation in the non or sub unloading direction is not significant. And the curves of stress strain after peak strength present obvious brittle. Whilst the primary deformation of rock under loading is axial pressure deformation, and plastic deformation increases with the raise of confining pressure. The fracture is characterized by strong tensile, and all tensile cracks are fully developed. Furthermore, when the rock samples are unloaded in two directions, and even normal to unloading direction, sub unloading annular tensile cracks are produced. The shear fractures is obvious comparatively when confining pressure becomes higher, and it develops following tensile cracks with more micro tension cracks formed in two sides along shear plane. But the fracture is characterized by shear under conventional triaxial compression with little tensile cracks; the coefficients of brittles stress drop of granite is increased with the growth of confining pressure under conventional triaxial compression and reduced under unloading. When the initial confining pressure is the same, the coefficients of brittles stress drop under unloading is less than that under loading. In SchemeⅡ, the coefficients of brittles stress drop reaches negative when initial confining pressure is up to 30 MPa, and the one （R）is RⅢ>RⅠ>RⅡ in three test cases.     

市场经济条件下农民的素质与能力分析

农民是市场经济的主体,其素质与能力结构直接影响社会主义新农村建设。本文通过思想道德素质、科技文化素质、经营管理能力和民主政治意识四个方面的现实分析,提出相应的对策建议。     

小麦合理价格测定与分析——以河北省为例

小麦市场价格简称小麦价格,是小麦的实际价格。而小麦的合理价格是根据小麦价格构成的有关因素和作价原则,用理论公式计算出来的小麦价格,是一种测算性质的价格。小麦合理价格作为理论意义的价格,虽然不能马上付诸实践,但它能作为衡量实际的小麦市场价格是否合理的标准,因此,对小麦合理价格进行测定具有现实的意义。以小麦生产大省河北省为例,通过测定2004年河北省小麦的生产成本,分析全国不同地区间小麦合理价格,得出了当前小麦的合理价格范围为1.21~1.45元/kg。     

长输管道定量风险评价方法研究

油气管疲乏的定量风险评价技术是一门新型的管理技术。介绍了定量风险评价技术的内容，以及国外在定量风险评价技术方面所取得的成绩。在我国进行管道的定量风险评价技术具有重要意义，管道管理者可以根据评价结果了解管道的完好状况和提前掌握管道的危险地段情况，以便合理地分配维护资金，交管道的盲目被动维修为预知主动维修，提高管道的运行安全性。     

我国柑橘市场竞争力比较分析

从品质、价格、成本、土地生产率、品种结构、经营方式以及产业链方面比较了我国柑橘市场竞争力与国外的差距,并测算了"入世"后我国柑橘的国际市场竞争力。研究表明,宽皮橘、柑橘罐头、柑橘果酱具有很强的出口竞争力,而橙、柠檬、柑橘汁等完全没有优势。研究得出,提高柑橘市场竞争力需要科技支撑。     

Using global organic markets to pay for ecologically based agricultural development in China

The traditional command and control approach and the more recent free market have proven inadequate for promoting ecological agricultural development in China. Organic certification represents a regulated market mechanism with the potential to stimulate ecologically based agricultural research, extension, and investment. Recent linkages between the global organic food industry and local agricultural development in China provide an opportunity to test this potential. The article examines Chinas two largest organic certification systems for their potential to promote the adoption of integrated pest management (IPM) as a key component of ecological agriculture. Organic certification is providing a format for research, extension, and implementation of IPM principles and practices, and has the potential to do much more. However, possible contradictions between ecological and market rationality, inherent in organic certification and marketing systems, may be exacerbated by the authoritarian political economy of rural China.Paul Thiersis an assistant professor of Political Science at Washington State University Vancouver where he teaches comparative politics, political economy, and environmental policy in the Program in Public Affairs. His research focuses on the environmental, social, and political dynamics of globalization in rural China, with a particular emphasis on Chinas integration into global food trade and food standards.     

腐蚀管道的失效概率和剩余寿命预测方法

以腐蚀管道失效压力和腐蚀速率分析为基础,建立了腐蚀管道可靠性极限状态函数和管道剩余寿命预测模型.采用改进的一次二阶矩法对腐蚀管道的可靠性指标和失效概率进行了分析和计算.结合工程实例,求解了某含腐蚀缺陷天然气管道不同服役年限的可靠性指标和失效概率,预测了该腐蚀管道的剩余寿命,计算和分析了工作压力、腐蚀速率、腐蚀深度和长度等参数对腐蚀管道可靠性的影响.     

市场经济与高校办学自主权

市场经济要求转变高等教育行政主管部门的职能,赋予高校一定的市场主体资格,以解决高校教育资源供求之间的矛盾。同时,高校已经具备一定的驾驭市场经济的经验和能力,社会、政府以及法律的监管约束机制也已基本形成。当前可以从融资、专业和学科的创设、招生的方式与途径、收费或定价等方面考虑进一步扩大高校办学自主权。     

绿色产品市场“柠檬效应”分析

绿色产品环境质量的隐蔽性使得消费者和厂商之间产生信息不对称现象,这将影响绿色产品市场的正常运行。通过建立数学模型对此进行了分析。结果表明:在信息不对称条件下,如果进入壁垒较低,则市场中将出现“柠檬效应,”而提高产品的环境标准可以缓和或避免这种现象。