含微孔洞损伤的压电功能梯度矩形板热屈曲分析

研究了含微孔洞缺陷的压电功能梯度材料矩形板的热屈曲相关特性。假定功能梯度层的材料参数如热膨胀系数及弹性模量均沿厚度方向呈指数变化,且四边简支边界条件。首先推导了屈曲平衡方程,然后结合数值算例分析了功能梯度层材料性能的梯度参数、厚度及长宽比对临界屈曲温度的影响,最后分析了微孔洞损伤对临界外荷载的影响。结果表明:板的薄厚程度对屈曲特性有较大的影响,板越薄则临界屈曲温度越低;材料属性的梯度变化越大,临界屈曲温度也越低;微孔洞缺陷带来的损伤对于临界屈曲荷载的影响很小,但对于某些特殊情况,如需要更精准的传感器设计则不建议忽略,且损伤影响随着板厚增大而减小。     

二维材料曲面生长:力学与化学的邂逅

理解二维材料在曲面上的生长形态和机制具有重要的理论和应用价值,但现有关于二维材料曲面生长的力学行为及形貌演化规律的研究极为缺乏.二维材料曲面生长会导致薄膜变形及其相关的应力/应变.这类应力可引发二维材料的滑移、屈曲、褶皱、断裂和离面运动等二次力学行为,并直接与生长反应耦合,进而改变材料的生长过程.与化学能和表面扩散主导的平面生长不同,曲面生长二维材料的形貌受曲面几何形状和材料力学行为的共同影响,会产生更为复杂多样的生长形貌.通过总结国际上相关研究进展,剖析了模拟曲面生长二维材料所面临的科学问题,并论述了如何结合原子模拟(如分子动力学和蒙特卡罗模拟)与唯象理论(如相场方法)开展多尺度计算研究,再辅以实验揭示二维材料曲面生长规律.     

基于动态显式分析的复合材料盒段失效模拟

为了研究复合材料结构失效过程并为强度设计提供分析依据,对结构进行失效模拟。在静态隐式分析中,材料的失效和破坏常常导致严重的不收敛。针对复合材料多墙盒段结构,采用动态显式分析方法对弯曲载荷工况进行了试验模拟。壁板和墙均采用层壳元,在上壁板中面嵌入一层体元模拟层间失效,利用用户自定义材料(VUMAT)的方法对材料性能进行连续衰减,并进行渐进式失效分析。分析表明,动态显式分析避免了不收敛问题,能比较准确地模拟复合材料盒段的失效过程,无论是应变、屈曲载荷、破坏载荷,还是破坏形式,分析结果均与试验结果吻合较好,屈曲载荷、破坏载荷的预测值与试验结果误差在7%以内。表明本文方法可以满足工程设计和分析要求。     

热过屈曲功能梯度壁板的气动弹性颤振

功能梯度材料的宏观物理性能随空间位置连续变化,能充分减少不同组份材料结合部位界面性能的不匹配因素.功能梯度壁板用作高速飞行器的热防护结构,能有效消除气动加热带来的壁板内部热应力集中.本文考虑热过屈曲变形引入的结构几何非线性,分析功能梯度壁板的气动弹性颤振边界.基于幂函数材料分布假设,采用混合定律计算功能梯度材料的等效力学性能.根据一阶剪切变形板理论、冯·卡门应变-位移关系和一阶活塞理论,基于虚功原理建立超声速气流中受热功能梯度壁板的非线性气动弹性有限元方程.采用牛顿-拉弗森迭代法数值求解壁板的热屈曲变形,分析超声速气流对热屈曲变形的影响机理.在壁板热过屈曲的静力平衡位置分析动态稳定性,确定了壁板的颤振边界.研究表明,当陶瓷-金属功能梯度壁板的组份材料沿厚度方向梯度分布时,会破坏结构的对称性导致壁板在面内热应力作用下发生指向金属侧的热屈曲变形.超声速气流中壁板热屈曲变形最大的位置随气流速压增大向下游推移,并伴随屈曲变形量的减小.热过屈曲壁板的几何非线性效应会提高壁板的颤振边界,这种影响在高温、低无量纲速压且壁板发生大挠度热屈曲变形时表现显著.较高无量纲气流速压下由于壁板的热屈曲变形被气动力限定在小挠度范围,几何非线性效应不明显.      

粘贴压电层功能梯度材料Timoshenko梁的热过屈曲分析

研究了上下表面粘贴压电层的功能梯度材料Timoshenko梁在升温及电场作用下的过屈曲行为。在精确考虑轴线伸长和一阶横向剪切变形的基础上,建立了压电功能梯度Timoshenko层合梁在热-电-机械载荷作用下的几何非线性控制方程。其中,假设功能梯度的材料性质沿厚度方向按照幂函数连续变化,压电层为各向同性均匀材料。采用打靶法数值求解所得强非线性边值问题,获得了在均匀电场和横向非均匀升温场内两端固定Timoshenko梁的静态非线性屈曲和过屈曲数值解。并给出了梁的变形随热、电载荷及材料梯度参数变化的特性曲线。结果表明,通过施加电压在压电层产生拉应力可以有效地提高梁的热屈曲临界载荷,延缓热过屈曲发生。由于材料在横向的非均匀性,即使在均匀升温和均匀电场作用下,也会产生拉-弯耦合效应。但是对于两端固定的压电-功能梯度材料梁,在横向非均匀升温下过屈曲变形仍然是分叉形的。     

基于新修正偶应力理论的Mindlin层合板热稳定性分析

以新修正偶应力理论为基础,首次提出了机械载荷与热载荷共同作用下的微尺度Mindlin层合板热稳定性模型,该模型只引入一个材料尺度参数,通过虚功原理推导出了控制方程和边界条件,以四边简支方板为例,进行了热稳定性分析,应用纳维叶解法得到解析解。结果表明,所建模型可以捕捉到尺度效应。材料尺度参数值越大,屈曲临界温度越高;当跨厚比增大时,屈曲临界温度下降;随着板几何参数的增大,模型将退化为宏观模型;温度变化量越大,考虑热载荷作用下的屈曲临界载荷越大,尺度效应体现越显著。     

粘接层对聚甲基丙烯酸甲酯涂层刮擦破坏的影响及机理分析

为研究粘接层厚度和粘接强度对聚甲基丙烯酸甲酯(PMMA)涂层刮擦破坏的影响,对不同粘接层厚度和粘接强度的PMMA涂层开展了系统的刮擦实验.采用考虑剪切屈服和脆性断裂竞争行为的本构模型描述PMMA的力学行为,进行了PMMA涂层刮擦行为的有限元模拟,揭示了复杂刮擦破坏模式的物理机理.研究结果表明：与零厚度粘接层的涂层相比,有限厚度粘接层的变形使PMMA涂层在刮擦过程中出现局部弯曲,导致刮头下方涂层底部区域形成内部裂纹;具有强粘接强度的粘接层可限制涂层在刮擦过程中的变形,避免刮头前方可能产生的严重屈曲,进而防止涂层底部形成沿厚度方向贯穿涂层的纵向裂纹;增加涂层厚度可以提高涂层在刮擦过程中的抗弯曲和抗屈曲能力,延缓内部裂纹和纵向裂纹的形成.这些发现有助了解和改善PMMA涂层的刮擦性能.     

Progress in thermal-mechantical effects induced by laser

对激光辐照诱导的热与力学问题研究进展进行了综述,包括材料在高温、高升温速率下的本构关系,典型薄板和柱壳等结构在激光辐照下的热力破坏效应,多层材料体系的激光破坏行为等几个方面,并着重介绍了包含相变与烧蚀过程的激光破坏分析模型与机制研究,激光辐照效应的流-热-固耦合数值模拟方法,以及短脉冲激光引起的冲击与破坏机理等方面的研究新进展.     

激光辐照诱导的热与力学效应

对激光辐照诱导的热与力学问题研究进展进行了综述,包括材料在高温、高升温速率下的本构关系,典型薄板和柱壳等结构在激光辐照下的热力破坏效应,多层材料体系的激光破坏行为等几个方面,并着重介绍了包含相变与烧蚀过程的激光破坏分析模型与机制研究,激光辐照效应的流–热–固耦合数值模拟方法,以及短脉冲激光引起的冲击与破坏机理等方面的研究新进展.     

夹在两个柔性层中薄膜的正弦模态界面屈曲分析

当一层比较硬的薄膜被封闭在上下两层柔性层中,组成了三层材料结构.在压应力的作用下,这层薄膜可能会发生屈曲失稳.由可弯曲电路封装在柔性层中组成的三层材料系统能进一步提高其弯曲性能的事实得到启发,不同于广泛研究的双层材料结构表面屈曲问题,从理论上分析三层材料结构在压应力作用下的刚硬薄膜的正弦模态屈曲问题.柔性层和刚硬薄膜材料假定皆为各向同性.由于薄膜厚度远远小于柔性层厚度,柔性层在理论分析中认为是无穷厚的.刚硬薄膜可以用非线性薄板模拟,其变形为小应变但是允许有限转动,用于描述屈曲时的状态.利用线性扰动分析,得到了屈曲临界薄膜应力,波数和平衡状态下的波幅的解析表达式.结果表明,针对不同的刚硬薄膜和柔性层的弹性模量,当刚硬薄膜相对于上下柔性层越硬,就越容易发生界面屈曲.     

Micro-mechanical approximation to the stress field around an inclined fiber of FRCC

Matrix spalling or crushing is one of the important mechanisms of fiber-matrix interaction of fiber reinforced cementitious composites (FRCC). The fiber pullout mechanisms have been extensively studied for an aligned fiber but matrix failure is rarely investigated since it is thought not to be a major affect. However, for an inclined fiber, the matrix failure should not be neglected. Due to the complex process of matrix spalling, experimental investigation and analytical study of this mechanism are rarely found in literature. In this paper, it is assumed that the load transfer is concentrated within the short length of the inclined fiber from the exit point towards anchored end and follows the exponential law. The Mindlin formulation is employed to calculate the 3D stress field. The simulation gives much information about this field. The 3D approximation of the stress state around an inclined fiber helps to qualitatively understand the mechanism of matrix failure. Finally, a spalling criterion is proposed by which matrix spalling occurs only when the stress in a certain volume, rather than the stress at a small point, exceeds the material strength. This implies some local stress redistribution after first yield. The stress redistribution results in more energy input and higher load bearing capacity of the matrix. In accordance with this hypothesis, the evolution of matrix spalling is demonstrated. The accurate prediction of matrix spalling needs the careful determination of the parameters in this model. This is the work of further study.     

Identification of damage mechanism and validation of a fracture model based on mesoscale approach in spalling of titanium alloy

The subject of this paper is identification of the physical mechanisms of spalling at low impact velocities for Ti–6Al–4V alloy and determination of the macroscopic stress of spalling via meso-macro approach. Spalling is a specific mode of fracture which depends on the loading history. The aspects of the initial microstructure and its evolution during plastic deformation are very important. In order to identify the spalling physical mechanisms in titanium alloy, numerous pictures by the optical microscopy of the spall surfaces created by plate impact technique have been taken. The scenario of failure observed is in complete agreement with known physical micro-mechanisms: namely nucleation, propagation and coalescence by adiabatic shearing of micro-voids. The most interesting point in spall fracture of Ti–6Al–4V alloy is the nucleation of micro-voids. A significant amount of small micro-voids in the region of the expected spall plane has been observed. It appears that microstructural effects are important due to dual α–β phase microstructure, called Widmanstätten structure. The orientation of microstructure has a direct influence on nucleation mechanism by means of distribution of nucleation sites and decohesion between the softer particles (α-phase lamellae) and the harder lattice (β-phase). According to these observations, a fracture model has been developed. This model is based on the numerous post-mortem microscopic observations of spall specimens. The goal is to determine the macroscopic stress of spalling in function of loading time and damage level via a meso-macro approach.     

脆性材料中应力波衰减规律与层裂实验设计的数值模拟

对混凝土、岩石类脆性材料的层裂实验进行了有限元模拟,研究了应力波在此类材料中传播的衰减规律,包括两类机制:弹性波因大尺寸试样的几何弥散产生的小幅度线性衰减、与应变率相关的黏塑性波因本构关系导致的指数衰减。在此基础上,提出了包含常数项的指数型应力波峰值拟合公式。建议采用可以忽略应力波衰减影响的细长形试样进行层裂实验。混凝土类脆性材料层裂破坏模拟结果显示,有限元模拟得到的层裂片厚度与一维应力波理论得到的结果非常吻合,验证了按一维应力波理论确定层裂强度的实验方法的有效性。通过对比3种不同入射波形下层裂片的形状和净拉应力波形,发现不对称的入射波形状更有利于实验获得平直的层裂断面和较准确的层裂强度。     

A penny-shaped cohesive crack model for material damage

A novel micromechanics based damage model is proposed to address failure mechanism of defected solids with randomly distributed penny-shaped cohesive micro-cracks (Barenblatt–Dugdale type). Energy release contribution to the material damage process is estimated in a representative volume element (RVE) under macro hydrostatic stress state. Macro-constitutive relations of RVE are derived via self-consistent homogenization scheme, and they are characterized by effective nonlinear elastic properties and a class of pressure sensitive plasticity which depends on crack opening volume fraction and Poisson’s ratio. Several distinguished features of the present model are compared with Gurson model and Gurson–Tvergaard–Needleman (GTN) model, showing that the proposed model can better capture material degradation and catastrophic failure due to cohesive micro-crack growth and coalescence.     

钢纤维对陶瓷基摩擦材料摩擦学性能的影响

以钢纤维为增强材料,采用氮气保护烧结方法制备出了钢纤维增强陶瓷基摩擦材料,用XD-MSM型定速摩擦磨损试验机对比研究了不同钢纤维含量对陶瓷基摩擦材料的热衰退性能、恢复性能以及磨损性能的影响,利用扫描电子显微镜(SEM)观察磨损表面形貌,并探讨了其磨损机理.研究结果表明:钢纤维的添加量(质量百分数)为24%时,钢纤维同陶瓷基体界面结合良好,陶瓷基摩擦材料的耐磨性能有所提高,热衰退率仅为5.8%,恢复率达到了107.8%,表明此配方陶瓷基摩擦材料具有较高的抗热衰退性能和恢复性能;钢纤维的含量影响陶瓷基摩擦材料的磨损形式,当含钢纤维含量较少时,陶瓷基摩擦材料以脆性脱落和疲劳磨损混合磨损形式为主;而随添加钢纤维含量的增多,其磨损形式转变为磨粒磨损;钢纤维过量加入时,则主要磨损形式为脆性脱落和疲劳磨损,并伴有黏着磨损.     

马氏体钢表面磁控溅射类金刚石薄膜滚动接触疲劳失效机理

采用磁控溅射技术在马氏体钢基体表面制备类金刚石(DLC)薄膜,应用扫描电镜、Raman光谱仪和划痕测试仪等对薄膜进行表征. 基于对失效表面及截面微观特征的详细分析,研究了DLC薄膜在接触疲劳载荷下的失效特征和机理. 结果表明:DLC薄膜试样的滚动接触疲劳(RCF)寿命比基体的寿命显著提高,且薄膜磨损后试样的剩余寿命仍比原基体寿命长. 薄膜厚度3 μm,处于接触最大应力分布的15 μm范围内. DLC薄膜是从基体表面粗糙峰处产生微裂纹进而导致薄膜剥落,基体材料裸露,最终试样失效.      

应用微相关技术研究裂端弹塑性变形

测量裂端附近区域的弹塑性变形，对于研究材料的损伤或微裂纹的产生，获得材料早期的损伤信息，为预防宏观裂纹的产生及扩展具有十分重要的意义。本文用A3钢材料制作边缘带裂纹的显微拉伸试件，试件表面进行了金相处理。试件是用由计算机控制的显微拉伸台进行加载试验。在加载过程中，利用环境扫描电镜对裂端附近的显微结构进行原位观察以及扫描记录。应用数字散斑相关技术对扫描电子显微镜记录的显微结构图进行分析处理，求出裂端附近的弹塑性变形。借助扫描电子显微镜，不仅可以定性观察应力集中区微裂纹的萌生及扩展过程，还可以结合数字散斑相关技术，对微区内的弹塑性变形进行测试，是研究微区域内材料变形的一个有效手段。     

不同载荷下3Cr13不锈钢涂层磨损寿命研究

在T-11摩擦磨损试验机上研究了不同载荷下高速电弧喷涂3Cr13不锈钢涂层的磨损寿命.提出载荷-涂层磨损寿命耦合的关系函数方程.通过对涂层磨损形貌观察,分析了不同载荷对磨损失效机制的影响.结果表明:当载荷较低时,涂层磨损寿命分散程度大,磨损主要失效机制为点蚀和剥落;当载荷较高时,涂层磨损寿命分散程度小,分层是引发涂层磨损失效的关键机制.     

Enhanced fracture energy and effects of temperature at spalling in ceramics: continuum damage modelling

Summary Enhanced fracture energy losses at spalling and the temperature dependence of the spalling strength of alumina ceramic bars are analysed on the basis of the experimental tests conducted both in room temperature and within the temperature range up to 1500°C at strain rates of some 500 s⁻¹. The experimental method and the measurements are first shortly outlined. The mechanical response of ceramic bars is modelled then as a heterogeneous distribution of brittle-elastic mesoelements undergoing continuum damage at the known strain history, corresponding to that registered in the experiments. The mesoelements are characterised by the values of initial damage randomly fluctuating within a given band-width superposed on a deterministic distribution, which corresponds to the fabrication conditions of the ceramic bars. The model has been tested in the evaluation of room-temperature experiments, its parameters: the average value of the initial damage, Young's modulus of the undamaged material and the energy absorption capacity in continuum damage are taken from the calibration fitting the experimental data. The registered energy losses at spalling, which exceed the static values of fracture energy by almost an order of magnitude, can be explained thus by the enhancement of the dissipation due to bulk damage, which is computed on the basis of the above parameters. The temperature change of the Young's modulus of the matrix material is taken as corresponding to the measured change of the uniaxial wave velocity in the bar, and corrected by the temperature change of the mass density. The analysis of the model shows that the drop in the spalling strength of the specimens with the increase of the temperature is phenomenologically related to the falling energy absorption capacity within the continuum damage mechanism. An explanation of this phenomenon is attempted, based on the grain-size-related mechanisms of the microfracture from pre-existing intergranular flaws distributed over the bulk of ceramics. Received 7 May 1999; accepted for publication 14 June 1999     

单轴拉伸条件下脆性岩石微裂纹损伤模型研究

利用断裂力学、损伤力学和均匀化原理,对脆性岩石单轴拉伸条件下的力学特性进行分析,建立了脆性岩石的微裂纹损伤本构模型.首先对岩石内部微裂纹的统计分布规律进行分析,给出了理论分析过程中微裂纹分布的假设条件,在此基础上,参考已有研究成果,得到含细长微裂纹脆性岩石有效弹性参数的计算公式.然后,对岩石内部单一微裂纹进行断裂力学和损伤力学分析,得到了扩展裂纹尖端的应力强度因子计算公式,在一定微裂纹断裂扩展准则和断裂扩展速率的假设基础上,利用积分原理,得到了岩石整体的损伤变量和损伤演化方程,由此建立单轴拉伸条件下脆性岩石的微裂纹损伤本构模型.最后,通过一花岗岩的单轴拉伸试验结果对微裂纹损伤本构模型进行了验证.