基于拉伸压破试验的人体升主动脉内膜力学特性

为获取人体升主动脉内膜力学性质,利用单轴拉伸系统,设计升主动脉内膜压破及拉伸试验方案,获取人体升主动脉内膜力学性质.研究结果表明:人体升主动脉内膜最大压破力与厚度正相关;内膜的周向与纵向的应力-应变均呈非线性曲线关系,并存在各向异性,升主动脉内膜的纵向最大拉伸断裂应力大于周向,初始弹性模量分别为1 104、787 k Pa.因此,在对主动脉内膜破裂风险评估及建立主动脉壁本构方程时应考虑厚度;在夹层、动脉瘤扩张初期,原发破口易沿纵向继续撕裂.

大豆脱粒过程籽粒力学特性试验研究

为了更准确地研究机械收获中大豆籽粒与机器脱粒分离部件之间的作用力、揭示大豆籽粒力学性能变化规律,采用万能材料试验机对5个大豆品种进行了压破、剪切和顶破试验。选取含水率作为试验因素,针对不同的受力方向、加载速度、刀片角度、钢针锥度、压入深度等试验方式,获得了不同试验条件下籽粒压破力、极限剪切力及硬度的变化规律,并对其函数关系进行了拟合。结果表明:大豆籽粒压破力与受力方向有很大关系,压破力随加载速度的增加而降低,随含水率的升高呈现先升高后降低的趋势;极限剪切力随刀片角度的增加而增大,随加载速度的增加而降低,随含水率的升高而降低;压入深度在0.2~1mm范围内,硬度与压入深度无明显关系,硬度随钢针锥度的增加而增大,随含水率的升高而降低。研究结果可为改进大豆脱粒和输送装置、确定大豆最佳机械收获时间和其它工艺参数提供理论依据和参考。

Management of portal hypertension in children

Portal hypertension can be caused by a wide variety of conditions.It frequently presents with bleeding from esophageal varices.The approach to acute variceal hemorrhage in children is a stepwise progression from least invasive to most invasive.Management of acute variceal bleeding is straightforward.But data on primary prophylaxis and long term management prevention of recurrent variceal bleeding in children is scarce,therefore prospective multicenter trials are needed to establish best practices.

Numerical investigation on the sensitivity of jointed rock mass strength to various factors

The mechanical properties of jointed rock masses, such as strength, deformation and the failure mechanism, can be understood only by studying the sensitivity of jointed rock mass strength (both the peak and residual strengths) to the factors that affect it. An orthogonal design of uniaxial compression tests was simulated on eighteen groups of jointed rock specimens having different geometric and mechanical properties using RFPA2D (Rock Failure Process Analysis) code. The results show that the peak strength is controlled by the geometric parameters of the joints, but that the residual strength is controlled by the mechanical prop- erties of the joint interfaces. The failure mode of jointed rock specimens is mainly shear failure. Joint quantity, or density, is the most important index that affects jointed rock mass strength and engineering quality.

Experimental study on the type change of liquid flow in broken coal samples

A test system of the permeability of broken coal samples mainly consists of a CMT5305 electronic universal test machine, crushed rock compaction containing cylinder and a self-designed seepage circuit, which is composed of a gear pump, a reversing valve, a relief valve and other components. By using the steady penetration method, the permeability and non-Darcy flow β factor of broken coal samples under five different porosity levels were measured, the grain diameters of the coal samples were selected as 2.5-5 mm, 5-10 mm, 10-15 mm, 15-20 mm, 20-25 mm and 2.5-25 ram, respectively. After measuring the permeability under each porosity, the overfall pressure of the relief valve continuously increased until the coal sample was broken down. In this way, the flow type of liquid inside the broken coal samples changed from seepage to pipe flow. The correlation between breakdown pressure gradient （BPG） and porosity was analyzed, and the BPG was compared with the pressure gradient when seepage instability occurred. The results show that, ①the non-Darcy flow β factor was negative before broken coal samples with six kinds of diameters were broken down; ②the BPG of coal samples with a grain size of 2.5-25 mm was lower than that of the others; ③ the BPG of coal samples with a single diameter under the same porosity increased as the grain size increased; ④ the BPG could be fitted by an exponential function with porosity, and the exponent decreased as the grain size increased for coal samples with a single diameter; ⑤ the BPG was slightly less than the seepage instability pressure gradient. The change in liquid flow type from seepage to pipe flow could be regarded as the performance of the seepage instability.

Experimental study on the compressive strength of grouted concrete block masonry based on nondestructive detection methods

Existing nondestructive detection methods were adopted to test the compressive strength of grouted concrete block masonry,i.e.the rebound method,pulling-out method and core drilling method were employed to test the strength of block,mortar and grouted concrete,respectively.The suitability of these methods for the testing of strength of grouted concrete block masonry was discussed,and the comprehensive strength of block masonry was appraised by combining existing nondestructive or micro-destructive detection methods.The nondestructive detection test on 25 grouted concrete block masonry specimens was carried out.Experimental results show that these methods mentioned above are applicable for the strength detection of grouted concrete block masonry.Moreover,the formulas of compressive strength,detection methods and proposals are given as well.

Bounding surface model for ballast with additional attention on the evolution of particle size distribution

Particle size distribution significantly influences the mechanical response of the ballast under low confining pressure.However,particle breakage usually occurs and unfavorably degrades the particle size distribution of the ballast when sufficient load is applied.To model the triaxial stress-strain behavior and its associated evolution of particle size distribution of the ballast,a specific bounding surface model is proposed.The proposed model is based on the traditional bounding surface plasticity and a modified particle breakage index,which correlates the initial gradation and the ultimate gradation together with the current gradation.Simulation of the experimental results from the triaxial compression tests shows that the proposed model can predict the strain softening and volumetric expansion of the ballast under relatively lower confining pressure.It is also able to simulate the strain hardening and volumetric compression of the ballast under relatively higher confining pressure.Most importantly,the proposed approach was observed to have a great potential in predicting the evolution of the particle size distribution of the ballast.