弹体侵彻混凝土的临界跳弹

为了保证钻地战斗部打击防护层目标时不发生跳弹,需要对弹体侵彻目标的临界跳弹角度进行分析和估算。开展了一定大长径比弹体斜侵彻混凝土的跳弹实验,分析了在250~430 m/s速度下弹体侵彻30和60 MPa钢筋混凝土的临界跳弹角度,给出了弹体临界跳弹角度包络线。当靶板强度相同时,随着侵彻速度的增加,弹体的临界跳弹倾角增大,增大的趋势逐渐变缓;在相同侵彻速度下,随着靶板强度的增加,弹体的临界跳弹倾角减小;经验公式分析得到的弹体临界跳弹倾角偏低于实验,但偏差基本在3°以内。     

柔性基底上含过渡层纳米薄膜在双向拉伸下断裂损伤的实验研究

含过渡层的柔性基底薄膜的力学性能对现代电子元器件的广泛应用至关重要,对其力学性能的深入研究变得极为迫切。本文针对其在双向拉伸载荷下的断裂损伤进行了实验研究。对125μm聚酰亚胺上沉积的不同薄膜结构进行不同加载比下的双轴拉伸实验,通过光学显微镜观察裂纹演化过程与饱和裂纹形态。根据最小应变能密度因子理论对裂纹演化的开裂角度进行了理论分析,用有限元分析了加载比例和过渡层泊松比对结构各层双向应力比传递的影响,并在不同薄膜结构和加载比下,对裂纹演化应变进行了比较。研究结果表明,在双向拉伸载荷作用下,薄膜裂纹呈现出网状分布,裂纹演化角度与加载比、裂纹初始角度相关;一级裂纹萌生的临界应变随着加载比降低而降低,二级及以上裂纹萌生的临界应变与各级裂纹的饱和应变随加载比降低而升高;在等双轴拉伸下,不同薄膜结构的含过渡层柔性基底薄膜一级裂纹的临界应变基本一致,二级及以上裂纹的临界应变显现明显差异。     

环境温度对熔铸炸药圆柱体装药热安全性的影响

根据热爆炸理论的热平衡方程,在Thomas边界条件下,数值计算了热爆炸临界体系温度、临界环境温度和临界装药尺寸的关系。在不同的温度范围,环境温度对临界尺寸的影响不同:在80℃以上,温度的影响比较显著;而在70℃以下,温度的影响不太显著;环境温度为105℃时,环境温度对临界尺寸的影响结果也就是FrankKamenetskii热量平衡方程在稳态条件下的处理结果,且是最保守的估计。作为示例列出了一种EAK(Ethylenediaminedinitrate Ammomiumnitrate Totassiumnitrate)基的分子间炸药的热爆炸临界条件的计算结果。     

考虑道路信息和转向灯的可换道BML模型

车辆换道是司机为获得更好行驶条件而采用的常见措施, 而转向灯对车辆换道行为有重要的指导作用. 本文在BML (Biham-Middleton-Levine)模型的基础上加以改进, 提出了综合考虑道路信息和前车转向灯影响的可换道BML模型. 当车辆无法前行时, 如满足换道条件, 则将道路信息(车道密度及平均速度)和转向灯影响量化为车辆换道概率, 确定车辆是否可以换道. 通过数值模拟, 研究了周期边界条件下车辆换道行为对有、无交通灯控制的两种BML模型发生相变的临界密度以及系统通行能力的影响. 模拟结果表明对于无交通灯BML模型, 引入换道规则可以明显提高系统发生相变的临界密度, 在较小尺度下该临界密度接近有交通灯BML模型, 换道效果明显, 并发现了一种新的局部拥堵和自由流的共存相, 讨论了该共存相的生成和演化机制. 在较高密度下局部阻塞将演化为全局拥堵; 对于有交通灯BML模型, 引入换道规则对系统发生相变的临界密度没有明显的影响, 但相变的过渡区域更窄. 这表明有交通灯时, 换道虽然可以改变局部交通特征, 但难以显著影响交通系统的全局特征.      

疲劳过程中的能量耗散和疲劳寿命的预测

试验测量了A3钢和铝合金LY12CZ在疲劳过程中的耗散能密度与应力幅的关系和破坏时的临界累积耗散能密度。通过一系列不同加载频率和应力比的比较试验,结果表明耗散能密度与应力幅的关系和临界累积耗散能密度在不同加载频率下变化不大,但是受应力比的影响较大。本文还建立了临界累积耗散能密度疲劳寿命预测判据,并用此判据进行了带中心孔板条构件的疲劳寿命预测,取得了较好的结果（误差在25％以内）。这种方法对于构件局部的疲劳主要由一个方向应力控制的工程问题,使用简便有效。     

切应力协同下受热过冷层流液膜的破断特性

针对界面切应力协同下受热过冷层流液膜流动的破断过程, 建立了不同气液流向下的临界液膜厚度和最小润湿量的理论模型, 分析了不同驱动力作用下, 接触角、流体温度、界面切应力和壁面热流密度对液膜破断特性的影响. 研究表明: 临界液膜厚度和最小润湿量均随壁面热流密度的增加而增大; 重力驱动下的接触角影响在不同热流密度下有所不同, 流体温度在不同驱动力下对最小润湿量的影响截然相反; 同向切应力驱动下临界液膜厚度和最小润湿量随切应力增加而减小; 在重力和切应力协同驱动下, 同向切应力对最小润湿量的影响与重力和切应力所起作用的相对大小有关, 反向切应力使得临界液膜厚度和最小润湿量有所增大.      

HMX粒度与爆轰波传播性能的关系研究

采用楔形装药,用临界截面厚度来表征爆轰波传播特性,分别对两种密度条件下HMX粒度对其自身和以HMX为主体的混合炸药HMX/F2641（wHMX/wF2641=95∶5）爆轰波传播性能的影响作了实验研究。结果表明:HMX粒度对爆轰波传播特性有显著的影响,随着HMX粒度的减小,临界截面厚度变小,即爆轰波传播性能增强;同一HMX粒度下, HMX/F2641爆轰波传播性能优于HMX;装药密度的增加有利于爆轰波传递。     

砂土中锚板抗拔性能试验研究

运用自行研制开发的试验装置和数据采集系统,对锚板在砂土中的抗拔性能进行了试验研究。分析了锚板在松砂和密砂两种情况下抗拔力和位移的关系曲线特征,研究了不同埋深率下抗拔力和破坏系数的变化规律,并根据抗拔力与破坏系数随埋深率的变化趋势得到了松砂和密砂状态下的临界埋深率。试验结果表明,密度对砂土中锚板的抗拔性能有非常大的影响,增加砂土的密实度可以大幅度提高锚板的抗拔承载力并显著减小锚板的位移变形。增加锚板的埋置深度同样也可以大幅提高锚板的抗拔承载力,但抗拔承载力的增加幅度受临界埋深率的限制,临界埋深率随密度增加有增大的趋势。该试验结果可为锚板上拔预测模型的建立及设计提供参考依据。     

在冲击载荷作用下结构大变形的挠度估计

参照Martin和Ponter关于冲击载荷作用下塑性结构大变形的最终挠度的上限定理,从能量平衡的角度提出了一种在冲击载荷作用下结构挠度的估计方法。对刚塑性材料制成的圆板梁和圆环,给出了最终挠度的估计表达式,同已有的解作了比较,讨论了支承条件的影响,从而说明了这是一种在工程上十分实用的良好近似方法。     

纤维增强层合板在单双面压缩下屈曲的优化设计

1979年,日本宇航研究所的Hirano发表了一篇纤维增强层合板铺层角度优化的研究论文。他把与x轴成α_k和—α_k(k表示某一层)方向所铺设的纤维同种、数目相等、基体同一的一个层片定义为一个单层。各个单层之间可以不完全满足单层所定义的条件,这样就考虑到了由N个单层迭合起来的层合板沿厚度方向异质的情况。同时,他还限定各个单层厚度是相等的。每个单层可以取任意的角度。给出层合板的层数,寻求一组铺层角度——各个单层的角度,使临界载荷值最大。由于与迭代起始值有关,他一般是给出8种起始值,然后再从8组结果中挑选临界载荷最大者为最优铺层角度。     

Growth of crack caused by temperature gradients with change in surface insulation

This work is concerned with thermoelastic stress and failure analysis of a centrally cracked panel subjected to temperature gradients while the insulation on the crack surface is varied. The corresponding temperature and thermoelastic stress fields are obtained by application of the finite element method. According to the strain energy density criterion, the crack grows incrementally when the maximum of the minimum strain energy density function reaches a critical value for a given material. Crack growth resistance curves involving plots of the strain energy density factor S versus the half crack length a are developed for crack surfaces with varying degree of heat resistance. The resulting curves are straight lines satisfying the condition dS/da = const. and useful for determining combined influence of thermal loading and structural geometry that lead to global instability.     

Crack growth prediction of an inclined crack in a half-plane thermopiezoelectric solid

The strain energy density theory is used to determine the direction of crack propagation in the case of an inclined crack in a half-plane thermopiezoelectric sheet subjected to uniform heat flow. Based on the Stroh's formulation and the thermoelectroelastic Green's function, a system of singular integral equations for the unknown thermal analog of dislocation density defined on the crack faces is presented and used to calculate the corresponding strain energy density. The direction of crack extension is then determined by way of the minimum strain energy density criterion. The study shows that the criterion is easy-to-use for thermal fracture problems.     

Analytical solutions of thermal stress distribution in plastic encapsulated integrated circuit packages

IntroductionElectronicpackagesarecomplexcompositestructure.Theyhavemyriadcornersandinterfacesthatcanactaspotentialfailuresitesduringfieldoperation .Atpresenttime ,plasticencapsulatedintegratedcircuits (ICs)arewidelyusedinengineering .AplasticencapsulatedICpackageconsistsofasilicondie,dieattachment,passivation ,wireinterconnects,aleadframe ,andplasticmoldingcompound .Thermalexcursionduringpackagequalificationfromthermalcycling thermalshock ,orpowercyclingduringnormaloperationcansetupthermomec…     

Kinetics of crack formation and growth on the material interface

A model of thermal fluctuation crack formation in aweakened bond region on amaterial interface is proposed. The weakened bond region is modeled by a bridged crack whose properties vary in time according to the thermal fluctuation mechanism. It is assumed that at least one of the materials is a polymer and the crack part occupied by bridges (the end region) is not small compared with the crack length. The stresses in the bridges and the kinetic dependence of the bond density in the crack end region are determined by solving a system of singular integrodifferential equations. The condition for the crack-defect nucleation is the decrease to the critical value of the average bond density on the corresponding part of the weakened bond region. Numerical results permitting one to estimate the crack nucleation time and the typical levels of external loads for the chosen material parameters are presented.     

Thermoelastic buckling of steel columns with load-dependent supports

The in-plane elastic buckling of a steel column with load-dependent supports under thermal loading is investigated. Two elastic rotational springs at the column ends are used to model the restraints which are provided by adjacent structural members or elastic foundations. The temperature is assumed to be linearly distributed across the section. Based on a nonlinear strain–displacement relationship, both the equilibrium and buckling equations are obtained by using the energy method. Then the limits for different buckling modes and the critical temperature of columns with different cases are studied. The results show that the proposed analytical solution can be used to predict the critical temperature for elastic buckling. The effect of thermal loading on the buckling of steel columns is significant. Furthermore, the thermal gradient plays a positive role in improving the stability of columns, and the effect of thermal gradients decreases while decreasing the modified slenderness ratios of columns. It can also be found that rotational restraints can significantly affect the column elastic buckling loads. Increasing the initial stiffness coefficient α or the stiffening rate β of thermal restraints will increase the critical temperature.     

刚性圆形压头作用下热电材料半平面的无摩擦接触问题

热电材料可以将热能转化为电能,反之亦然,这一优良的性质将有助于研发更具成本效益的设备和器件。本文研究了刚性圆形压头作用在热电材料半平面的无摩擦接触问题。假定压头为电导体、热导体,且压头压入深度及与材料的接触区域宽度未知。首先求解电场和温度场,利用傅里叶变换得到了电势函数、温度、电流密度和能量通量的解析表达式。然后求解弹性场,利用积分变换和边界条件,将该热弹性接触问题转化为第一类奇异积分方程并数值求解。数值结果讨论了压头半径和热电载荷对法向接触应力、电流强度因子和能量通量强度因子的影响。结果表明,对于圆压头,热电材料的法向电流密度、法向能量通量在接触边缘表现出奇异性,而表面法向接触应力在接触边缘为零。本文建立的研究模型有助于更深层次的了解热电材料的接触行为。     

负梯度闭孔泡沫金属的力学性能分析

运用三维Voronoi技术生成闭孔梯度泡沫模型,结合有限元分析方法模拟负梯度闭孔泡沫金属在不同冲击速度下的力学行为。结果表明,随着冲击速度的提高,得到了与均匀泡沫一样的三种变形模式：准静态模式,过渡模式和冲击模式。通过对名义应力应变曲线和变形模式的研究,提出了一种新的定义局部密实化应变的方法,并研究了相对密度和密度梯度对它的影响。分别建立了相对密度和密度梯度与冲击速度的变形模式图。通过引入密实化因子,确定了三种变形模式对应的临界冲击速度。最后讨论了不同冲击速度下,密度梯度大小对泡沫材料能量吸收能力的影响。结果表明,在高速冲击的变形初期,密度梯度的绝对值越大,泡沫材料的能量吸收能力越强。     

热-力联合作用下柱壳结构变形的计算分析

本文研究了激光连续加热和恒内压作用下柱壳结构的变形规律。以4340钢材料作为研究对象，其本构方程选用热粘塑性本构模型（Johnson-Cook模型）；选取某一应变率为临界值（本文中临界应变率取为1s^-1)。考察了激光功率密度，预载荷大小，激光作用时间等对结构变形规律的影响。主要结论有：结构失稳是导致激光辐照下充内压柱壳破坏的重要原因；激光功率密度和内压越高，结构破坏时间越短；根据辐照时间的长短，预测到了结构急速破坏，延迟破坏和不破坏三种模式。本工作对于深入认识激光作用下预载结构的热－力联合破坏有一定意义。     

Crack trajectories influenced by mechanical and thermal disturbance in anisotropic material

This work is concerned with the application of the volume energy density criterion for predicting the crack trajectories as influenced by mechanical and thermal disturbance in an anisotropic material. Two-dimensional linear thermoelasticity is employed in conjunction with the well-known complex potentials such that a linear relationship is obtained for the boundary conditions across the crack or line of discontinuity. Boundary collocation is then used to determine the unknown coefficients from which the contours of the volume energy density in the cracked plate can be obtained. The crack path is assumed to coincide with the loci where dilatation would dominate. This corresponds to the locations of relative minimum energy density which can be found by visual inspection. An equal and opposite mechanical stress and thermal gradient are applied on the cracked plate. The former and latter enhance symmetric and asymmetric crack growth, respectively. They would complete depending on the magnitude of the mechanical and thermal load. Numerical results are presented for three (3) different cases of a plate whose principal axes of material symmetry are tilted to the crack plane. The influence of anisotropy on crack path is found to be secondary.