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引言谷粒受到环境变化的影响时,内部会产生裂缝,谷壳会破裂。场土变干,运输,以及人工干燥时发生的气温和空气相对湿度条件的变化,都会导致谷粒的物理损伤。可以推测,由于温度梯度和水分梯度,在谷粒各部分引起程度不同的膨胀和收缩,将在谷粒内部产生应力。因为热传递速率比水分传递的速率高得多,温度梯度和水分梯度的综合效应不好确定。Hcndcrson观察了大米内部的裂痕沿短轴从中心向边上扩展的过程。他指出,在变干的早期阶段,其内应力主要是由温度梯度引起的。Strcmmer 相似文献
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Cu-Al-Be形状记忆合金湿磨粒磨损性能研究 总被引:1,自引:2,他引:1
采用销-盘式二体磨损试验研究了CuAlBe形状记忆合金的湿磨粒磨损行为.结果表明,CuAlBe形状记忆合金的磨损性能不完全取决于材料硬度,具有热弹性马氏体组织的A合金的抗磨性优于高硬度B合金.在湿磨粒磨损条件下,A合金具有β′1 β双相组织,在磨粒的交变应力作用下,由于β相应力诱发马氏体相变、β′1相马氏体变体择优取向并产生形状记忆效应,使应变弹性回复,并钝化裂纹尖端,使得A合金具有优良的抗磨粒磨损性能. 相似文献
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汽缸结构上下缸接触的有限元分析 总被引:3,自引:0,他引:3
采用有限元软件MSC/NASTRAN计算分析了汽轮机汽缸结构上下缸接触状态的应力分布和变形.分析了汽缸在温度场作用下以及温度场同内压联合作用下的应力分布情况,重点分析上下半缸的螺栓连接面——中分面上的应力和变形情况.建立了气缸三维实体有限元分析模型,并对每根连接螺栓均建立了模拟模型.分析结果表明,同内压引起的应力相比,热应力是缸体中应力的主要成分.当内外壁温差达到100℃时,缸体中最大应力为1230MPa,出现在约束处应力集中部位,缸体绝大部分应力水平在600-700MPa;汽缸外壁温度为250℃时,缸体中最大应力为1080MPa,缸体绝大部分应力水平在100MPa,得出减小汽缸内外壁的温差能有效减小缸体中应力的结论.分析表明,缸体轴向最大伸长量为2.55mm,横向最大变形为2.02mm.Z向最大位移为1.24mm.中分面有分离,但分离程度较小,分离值均在10^—3mm量级上. 相似文献
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针对两种不同类型的滚子,研究了大型三排滚柱回转支承承载能力和疲劳寿命.首先,通过理论推导滚子和滚道之间的变形量,并数值模拟轴承滚子和滚道之间的接触应力分布,从而推导滚子的寿命.计算结果显示,圆柱滚子的端部应力较大,而异形修正滚子最大应力出现在接触中心处,端部应力在数值上远小于圆柱滚子.通过等效应力的计算,在载荷较低时,圆柱滚子的疲劳寿命高于修正滚子;当载荷较大时,异形修正滚子的疲劳寿命明显高于圆柱滚子. 相似文献
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谷粒的热——湿应力分析 总被引:2,自引:0,他引:2
针对谷物烘干问题,本文研究弹性及粘弹性圆球和椭球的热,湿应力。对圆球分析表明,热应力峰值远比湿应力峰值出现时间早,在假定材料为热,湿流变简单的Maxwell粘弹性体条件下,本文导出了圆球的热,湿应力分布公式,并应用有限元法研究了椭球的热,湿应力,文中以无量纲形成给出了圆球中心和表面应力随时间的 变化情况,以及椭球中心轴向应力峰值随椭球长,短半轴之比的变化情况。 相似文献
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为探究应力波界面透反射行为对分层黏弹性颗粒材料细观动力响应的影响规律,推导了界面透反射系数的理论表达,并建立了单频应力波入射和多频应力波入射两类条件下分层黏弹性颗粒材料的细观动力响应模型.典型算例分析结果表明:单频应力波入射时,上层颗粒动力响应形成“驻波”现象;双频应力波入射时,上下层颗粒动力响应均形成“行波”现象;两类条件下,分层界面处相邻颗粒的位移振幅均出现明显突变.以高速铁路无砟轨道路基基床为应用场景,分析基床填料刚度、阻尼系数及密度等参数对界面透反射特性的影响规律,探讨分层颗粒材料动力变形的调控思路,即增加基床表层填料的刚度、阻尼系数及密度,或减小基床底层填料的密度等措施可实现增益透射、抑制反射目标.此外,动力变形调控时还需控制路基颗粒材料固有频率远离动载激振特征频率. 相似文献
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针对嵊州硅藻土的高结构性及地基应力场的复杂多变,应用GDS 动三轴仪进行不同围压和不同循环应力比下的不排水循环加载试验,分析应力-应变滞回曲线演化、应变累积特性、回弹特性及孔压特性.研究表明:循环应力比及围压对原状硅藻土应变和孔压的发展规律影响明显,随着循环应力比的增加,滞回曲线由线性状态转变为非线性状态,且逐渐向x 轴倾斜,累计应变和回弹应变都随着循环应力比的增加而略微增加,但总应变基本小于2 %;残余孔压比随循环应力比增大而增大,范围为0.1~0.5.并且存在一个临界循环应力比,其值随有效围压的增大而减小,范围为0.8~2.2,当循环应力比大于该值时,试样内部产生一个破裂面迅速发生破坏.在相同循环应力比下,试样的应变随着有效围压的增大而增大. 相似文献
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Accelerated aging of unidirectional hybrid composites under the long-term elevated temperature and moisture concentration 总被引:2,自引:0,他引:2
N. BoualemZ. Sereir 《Theoretical and Applied Fracture Mechanics》2011,55(1):68-75
Despite their high performances, composites with polymer matrix are very sensible to the increase in temperature and moisture concentration. During long years of services, both phenomena cause a critical transient hygrothermal transverse stresses, particularly at first-ply; i.e. at two edges of the composite plates. Therefore, significant degradation of hygrothermal characteristics and ultimate strengths of materials are occurred. To get an explicit relation between the durability and the damage probability of the composite, quadratic failure criterion in stress space is used. This criterion enables us to find a direct relation between transient hygrothermal stresses produced by the increase in temperature and moisture concentration and the ultimate strengths. It is necessary to calculate the strength ratio R from initial to saturation time for each condition imposed of temperature and moisture concentration. The strength ratio gives a point of view on the damage probability of the composite plates, where the rupture occurs if R = 1. In order to limit the consequences of simultaneous effects of temperature and moisture concentration, unidirectional hybrid composites in graphite epoxy was proposed. To reach this aim, hygrothermal transverse stresses are calculated through the thickness of unidirectional hybrid plate. Finally, the strength ratio was evaluated along of the plate with a gradual increase in temperature and moisture concentration. 相似文献
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The main objective of this research is to investigate the hygroelastic behavior of a non-homogeneous circular plate made up of porous metamaterial resting on an auxetic material plate. The mechanical properties of the main plate, as well as moisture concentration, vary as an exponential function in the transverse direction. Poisson's ratio is constant. The elastic supporting medium is developed by considering the structurestructure coupling. Based on the linear hygroelasticity theory, the governing state equations in terms of displacements and moisture concentration are acquired. At first, the Fickian equation is solved to compute the nonlinear distribution of moisture through the plate thickness, and then the state equations are semi-analytically solved using the statespace(SS) method and the differential quadrature(DQ) rule to predict the elastic field quantities. A comprehensive parametric analysis is accomplished to elucidate the effects of key parameters on the steady-state response of the plate under the mechanical and hygral loads. 相似文献
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The field of moisture concentration and the field of temperature are the two factors that induce stresses in dried materials. The main aim of this paper is to estimate the influence of these two factors on the formation of drying-induced stresses. The considerations are based on the model elaborated by the authors and the analysis of the drying-induced stresses is carried out on a convectively dried prismatic bar. 相似文献
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吸收湿气对微电子塑料封装影响的研究进展 总被引:5,自引:0,他引:5
微电子塑料封装中常用的聚合物材料因易于吸收周围环境中的湿气而对器件本身的可靠性带
来很大影响, 本文回顾了封装材料中湿气扩散、湿应力及蒸汽压力的产生这3个互相联系过
程的研究情况, 从理论分析、特征参数描述及其实验测定、有限元模拟分析的角度来分别予
以介绍.从已有的理论分析与实验结果中可以看出, 塑封材料吸收湿气会给器件的可靠性带
来诸多影响, 湿气的吸收、扩散、蒸发等过程, 实验测量, 以及由湿气带来的其它相关问题正
成为微电子封装可靠性研究领域中的新热点, 受到越来越多的关注与重视. 相似文献
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《International Journal of Solids and Structures》2005,42(24-25):6261-6276
The creep of paper is accelerated by moisture cycling. This effect is known as mechano-sorptive creep. It is assumed that this is an effect of transient stresses produced during moisture content changes in combination with non-linear creep behaviour of the fibres. The stresses produced by the moisture content changes are often much larger than the applied mechanical loads. If this is the case, the mechanical loads are only a perturbation to the internal stress state, and it will appear as if the mechano-sorptive creep is linear in stress. It is possible to take advantage of this feature. In the present report the pure moisture problem is first solved. The mechanical load is then treated as a perturbation of the solution to the moisture problem. Using this strategy, it is possible to linearize a non-linear network model for mechano-sorptive creep and to formulate a continuum model. As a result, the number of variables in the model is reduced. This is a significant improvement as it will be possible to use the linearized model to describe the material in a finite element program and solve problems with complicated geometries. 相似文献
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In this paper, we propose a new fractal model to determine the moisture effective diffusivity of porous membrane such as expanded polytetrafluorethylene membrane, by taking account of both parallel and perpendicular channels to diffusion flow direction. With the consideration of both the Knudsen and bulk diffusion effect, a relationship between micro-structural parameters and effective moisture diffusivity is deduced. The effective moisture diffusivities predicted by the present fractal model are compared with moisture diffusion experiment data and calculated values obtained from other theoretical models. 相似文献
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降雨入渗容易造成边坡滑坡自然灾害,含水率是影响土体抗剪强度的关键因素之一。本文以滑坡区红黏土为研究对象,制作了5组不同含水率的土样,并在不同垂直压力下进行快速剪切实验,得到每组对应的剪切强度参数,研究了含水率与剪切强度参数之间的关系。利用实验所得到的土工参数进行高地下水位边坡稳定性数值模拟,拟合得到边坡坡顶含水率与边坡安全系数的关系。结果表明:含水率从10%增加到25%,红黏土强度指标粘聚力、内摩擦角约分别下降50%和30%;含水率与粘聚力和内摩擦角间均呈半对数关系;边坡坡顶含水率增加导致边坡安全系数近似成二次曲线下降趋势。 相似文献
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Buckling and Postbuckling of Laminated Thin Cylindrical Shells under Hygrothermal Environments 总被引:1,自引:0,他引:1
沈惠申 《应用数学和力学(英文版)》2001,22(3):270-281
IntroductionInrecentyears,fiber_reinforcedcompositelaminatedshellstructuresarewidelyusedintheaerospace ,marineindustry ,automobileindustryandotherengineeringapplications.Duringtheoperationallife ,thevarianceoftemperatureandmoisturereducestheelasticmoduli… 相似文献