受冲击玻璃破坏波传播过程分析

分析平板冲击压缩下玻璃材料中破坏阵面的传播特性。结合已有实验结果,认为破坏波的传播为因试件表面微裂纹在冲击压缩下启裂、累积并向材料内部渗透过程,具有宏观扩散属性。建立玻璃材料的动态损伤本构关系及以表征材料损伤与破坏的微裂纹浓度N为传播特征的扩散方程,描述破坏波的传播过程。数值模拟冲击压缩下K8玻璃中横向应力历程,并与实验数据进行比较,结果显示两者吻合良好。     

冲击压缩下准脆性材料含微裂纹损伤的本构模型

基于准脆性材料中翼型拉伸裂纹的成核准则,运用细观损伤理论推导了翼型裂纹损伤对材料弹性模量的弱化作用.考虑裂纹扩展对材料动态断裂的滞后效应,建立了动态裂纹扩展准则,并给出损伤演化方程,在此基础上建立了准脆性材料单轴冲击压缩下的动态损伤本构模型.结合氧化铝陶瓷材料独特的力学响应和破坏特性,讨论了模型中微裂纹成核参数、微裂纹尺寸对动态断裂强度的影响,并用该模型计算了单轴压缩下氧化铝陶瓷的应力应变曲线,数值结果与实验结果吻合良好.     

爆破冲击荷载作用下宏观孔隙混凝土损伤演化的近场动力学模拟

针对静载、冲击荷载条件下宏观孔隙混凝土的孔隙参数对混凝土损伤演化过程的影响问题,采用MATLAB程序结合蒙特卡罗方法生成随机的孔隙结构物理模型,通过自编近场动力学程序模拟了单轴压缩下宏观孔隙混凝土的应力应变过程,并制备了以聚苯乙烯小球高温融化为孔隙体的混凝土砂浆试件,对宏观孔隙率0%～40%的混凝土试件进行了单轴压缩,实验验证了模型计算结果的准确性,随后模拟了爆破冲击作用下宏观孔隙混凝土的裂纹萌发与扩展。结果表明：孔隙率对宏观孔隙混凝土材料力学性能的影响显著,随着孔隙率的增加,试样的密度、抗压强度、弹性模量以及纵波波速不断降低,当孔隙率达到30%后,孔隙率对轴线抗压强度影响减弱,并且出现峰前应力跌落现象。孔隙率为零的混凝土试样在无围压冲击荷载作用下具有明显的压剪破坏形式,而含孔隙的混凝土由于大量的孔隙存在导致孔隙间混凝土骨架的拉伸或剪切贯通破坏。在相同冲击荷载下不同孔隙率混凝土的破坏情况不同,孔隙率大的孔隙混凝土试样破坏严重,表明孔隙率对冲击载荷作用下混凝土试件能量耗散起着重要作用。     

冲击荷载作用下橡胶混凝土的损伤研究

针对冲击荷载作用下橡胶混凝土的损伤问题,利用自动冲击球压仪对橡胶混凝土的动态冲击损伤行为进行研究,并应用激光共聚焦扫描显微镜(LSCM)和扫描电子显微镜(SEM)分析了其冲击损伤形貌及机理特征。结果表明:冲击荷载下橡胶混凝土抗冲击能力优于基准混凝土,且随橡胶粒径的减小抗冲击能力增强;随着冲击荷载的不断增加,橡胶混凝土的压痕增长速率小于基准混凝土,极限荷载值却大于基准混凝土,而橡胶粒径的减小,使压痕增长速率呈减小趋势,极限荷载值增大,表面抗塑性变形能力及试样表面动态力学性能增强;冲击荷载下材料表面形成球冠状小坑,压痕边缘出现不同程度的材料堆积,80目的橡胶在混凝土中分布更加均匀,且损伤区较为连续无明显微裂纹。研究结果为冲击荷载作用下对橡胶混凝土材料损伤研究提供依据。     

非均质混凝土材料破坏的三维细观数值模拟

在随机骨料模型的基础上,采用特征单元尺度对网格进行剖分,建立了非均质混凝土材料损伤破坏及宏观力学特性研究的三维细观单元等效化模型。对单轴拉伸、单轴压缩条件下湿筛混凝土试件的破坏过程及宏观力学性能进行了模拟分析;研究了混凝土梁的三分点弯拉力学特性,并与平面模型结果作了对比。研究表明：1) 与平面计算模型相比,三维模型更真实地模拟混凝土材料在外荷载作用下的损伤破坏过程,更准确的描述非均质混凝土材料的宏观力学性能,且与实验结果吻合;2) 骨料空间分布形式基本不影响混凝土材料的宏观弹性模量及强度,但影响其破坏过程和破损路径;3) 与随机骨料模型等细观力学方法相比,该方法具有高效性。     

TiCp/W复合材料热冲击损伤行为的数值模拟

为了揭示TiC颗粒增强的钨基复合材料（TiCp/W)高温下的失效规律,采用有限元方法从宏观和微观两个方面对该复合材料在氧乙炔冲击中的损伤行为进行了数值模拟,模拟结果表明,复合材料试样宏观损伤行为是裂纹在试样周边萌生,沿径向向心部扩展,微结构损伤行为是微裂纹在TiCp/W界面附近产生,而生在基体中扩展,TiC颗粒含量越高,复合材料超易损伤。TiC颗粒没有阻止裂纹扩展的作用,在基体中增加TiC反而会降低材料的抗热冲击性能。复合材料非稳态温度场的模拟结果,材料的宏观与微观损伤行为的模拟结果都与实验结果吻合。     

冲击荷载作用下混凝土缺陷模型细观损伤演化定量分析

通过对传统数字混凝土模型的改良,建立了带有内部缺陷的数字混凝土模型,在对内部缺陷分区的基础上,结合双折线损伤演化模型,对试件在三种速率冲击荷载作用下的细观损伤、应力和应变的分布规律进行了讨论,并对结果进行了定量描述,建立了混凝土材料细观损伤与宏观裂纹演化之间的联系,为混凝土材料的物理试验和在工程中的应用起到了辅助作用。     

基于显式动力学的焊点冲击失效分析

组件级高速剪切测试是用来研究芯片封装中Sn-Ag-Cu焊点冲击可靠性问题的一个重要手段。实验研究表明：随着冲击速度的增加,焊点封装结构的失效会由焊锡母材的韧性破坏向界面金属间化合物(IMC)的脆性断裂过渡;同时,其荷载-位移响应曲线形态也会发生显著的改变。为了能够更详细地了解封装结构的冲击失效行为,并进一步改进其结构设计,该文提出结合焊锡材料应变率相关的动态硬化特性,利用渐进损伤模型来模拟其动态损伤过程;同时,引进一种能够有效表征复合型裂纹扩展的内聚力模型来模拟IMC的脆性动态断裂。与实验结果的对比表明：该文提出的方法能够较为有效地表征焊点封装结构在不同冲击速度下的失效行为。     

碾压混凝土HJC动态本构模型修正及数值验证

研究高应变率冲击爆炸荷载作用下水工碾压混凝土大坝结构的动力响应,离不开对筑坝材料动态力学特性和本构关系的深入认识。参考实际水工混凝土大坝筑坝材料的配合比和施工方式,制备碾压混凝土试样,分别开展了静态压缩试验和分离式霍普金森压杆(SHPB)试验,以探求碾压混凝土的动态力学特性。基于静、动态力学试验结果,对目前多用于描述混凝土类材料高应变率下力学行为的HJC模型的强度面、应变率增强效应和破坏准则进行了修正,并利用有限元计算手段,建立SHPB试验的数值模型,以验证修正HJC模型的有效性。结果表明:碾压混凝土在高应变率冲击荷载下的动态力学特性表现出明显的应变率效应,动态压缩强度随应变率增加而提高,且与试样尺寸有关。基于试验数据的改进HJC模型有效预测了碾压混凝土在高应变率冲击荷载作用下的动态力学行为,数值计算得到的重构应力——应变曲线基本与SHPB试验结果吻合,采用最大主应变失效准则模拟得到了与SHPB试验加载过程中接近的试样损伤破坏模式,研究成果可用于碾压混凝土结构的抗冲击爆炸设计中。     

考虑反复加载时混凝土损伤积累的RC平板及剪力墙的有限元分析模型的开发

反复荷载作用下的混凝土、钢筋的本构模型是评价和分析钢筋混凝土(RC)结构的承载力-变形特性关键,旨在开发适用于反复荷载作用下RC平板、剪力墙的2维非线性有限元分析的材料本构模型,探讨并推出了反复荷载作用下在混凝土的本构模型中考虑混凝土损伤积累的方法;并对RC平板和剪力墙进行了反复加载的2维有限元模拟。分析结果与实验结果的比较表明,采用所建议的考虑损伤积累的2维非线性有限元分析模型由加载至最大承载力前能很好的模拟反复加载时RC平板、剪力墙的非线性特性。     

Effect of micro-cracks on plastic zone ahead of the macro-crack tip

From the macroscopic point of view, the plastic zone (PZ) is obtained based on the distributed dislocation technique (DDT) and von Mises yield criterion. From the microscopic point of view, PZ is determined by the DDT model. The effect of micro-cracks on PZ of the macro-crack tip is analyzed. The results show that the micro-crack has a little amplification effect on PZ of the macro-crack tip when it locates in front of PZ. As the micro-crack is close to the macro-crack tip, PZ of the macro-crack tip and the micro-crack tip will join together. When the micro-crack enters into PZ of the macro-crack tip, it has an obvious shielding effect on PZ. When the micro-crack is behind the macro-crack tip, the width of PZ decreases while the height increases. The dislocation distribution in PZ is in the form of inverse pileup. The amplification and shielding regions are divided into five strip-shaped regions, and they appear alternately. The results can provide useful information to predict plastic behaviors near crack tip. The analysis of amplification and shielding effect is important to materials design.     

Interaction of antiplane cracks with elastic waves in transversely isotropic materials

Summary The interaction of plane time-harmonic SH-waves with micro-cracks in transversely isotropic materials is investigated. Elastic wave scattering by a single micro-crack is first analyzed. The scattered displacement is expressed as a Fourier integral containing the crack opening displacement. By using this representation formula and by invoking the traction-free boundary condition on the faces of the crack, a boundary integral equation for the unknown crack opening displacement is obtained. Expanding the crack opening displacement into a series of Chebyshev polynomials and adopting a Galerkin method, the boundary integral equation is converted into an infinite system of inear algebraic equations for the expansion coefficients which is solved numerically. Numerical results are presented for the elastodynamic stress intensity factors, the scattered far-field and the scattering cross section of a single crack. Then, propagation of plane time-harmonic SH-waves in a transversely isotropicmaterial permeated by a random and dilute distribution of micro-cracks is investigated. The effects of the micro-crack density on the attenuation coefficient and the phase velocity are analyzed by appealing to a simple energy consideration and by using Kramers-Kronig relations.     

铸钢表面镍基合金渗层的热疲劳行为

采用铸渗技术在铸钢ZGCr5Mo试样表面制备了镍基合金渗层,渗层的厚度为0.6—1.2 mm,考察了镍基合金渗层的热疲劳行为,用扫描电镜(SEM)、X射线衍射(XRD)对热疲劳循环后的渗层表面进行了形貌观察与成分分析。结果表明:在热循环次数低于20次时,表面仅发生了氧化现象,当热循环次数超过90次时,渗层表面出现微裂纹,随着循环次数的增加,在渗层与基体的侧表面上出现贯穿渗层与基体的微裂纹以及在渗层与基体的界面处的表面氧化膜层出现了平行于渗层表面以及发散的微裂纹;随着镍基合金渗层厚度的增加,出现微裂纹的热循环次数略有降低。表面的氧化膜层主要为镍、铬的氧化物以及镍铬的复杂氧化物。     

Near-tip field of an anti-plane crack in a micro-cracked power-law solid

Asymptotic near-tip field is investigated for an anti-plane (mode III) crack in a power-law solid permeated by a distribution of micro-cracks. The micro-crack location is assumed to be random, while the micro-crack orientation is taken to be non-random. The anisotropic nature of this kind of damage gives rise to anisotropic constitutive equations for the overall macroscopic strains and stresses. The structure of the asymptotic field at a macro-crack tip is analyzed by solving a nonlinear eigenvalue problem. It is shown that under the assumptions made in this analysis the asymptotic crack tip field of the damaged solid has the same structure as the mode III HRR-field of the undamaged solid. Numerical results are presented for the angular functions, the contours of constant effective shear stress, the normalization constant arising in the near-tip field, and the crack opening displacement. By means of these results, the effects of the micro-crack density and orientation on the crack-tip field will be explored.     

微裂纹扩展的非共线超声混频定位研究

金属结构在早期服役过程中易出现微小裂纹,对早期微小裂纹扩展方向的检测与跟踪对避免结构失效而导致的突发性灾难具有重要的现实意义。基于非共线超声混频方法,开展金属材料微裂纹扩展的定位与表征研究。研究中选择两列横波相互作用产生混频纵波的非共线混频模式,通过实验测量验证了其混频效应和信号的传播性。选用铝合金7075-T6中预制的垂直和倾斜微裂纹为研究对象,提取每个测量点的超声混频非线性参数并进行归一化处理绘制成扫查成像图,从而表征微裂纹的长度与扩展方向。最后,将扫查成像图与光学显微镜的金相观察结果进行对比分析。研究表明,超声混频方法能够有效定位和跟踪铝合金材料中不同扩展方向的微裂纹,可为金属结构中微裂纹扩展提供检测方法。     

Effect of stacking sequence on micro-cracking in a cryogenically cycled carbon/bismaleimide composite

An apparatus was developed to thermally cycle coupon-sized mechanical test specimens to −196 °C. Using this device, IM7/5250-4 carbon/bismaleimide cross-ply ([0/90]_2S and [90/0/90/0/90/0/90/0/90]) and quasi-isotropic ([0/45/−45/90]_S) laminates were submerged in liquid nitrogen (LN₂) and returned to room temperature 400 times. Ply-by-ply micro-crack density (transverse cracks), micro-crack span, laminate modulus, and laminate strength were measured as a function of thermal cycles. The composite micro-cracked extensively in the surface plies followed by sparse micro-cracking of the inner plies. The tensile strength of the two blocked lay-ups (lay-ups with adjacent plies of the same orientation) decreased by 8.5% after 400 cycles. Sectioning of the samples revealed that the micro-cracks in the surface plies spanned the full width of the sample while many of the micro-cracks observed on the edge of the inner plies did not extend to the center of the samples, implying that a rectangular specimen with exposed free edges may result in a significantly different micro-crack density than a sample without free edges.     

新老混凝土粘结面断裂损伤过程区研究

在新老混凝土粘结面断裂性能试验的基础上,建立了新老混凝土粘结面断裂破坏模式,分析讨论了界面层特性及界面微裂缝扩展对新老混凝土粘结面断裂性能的影响.建立了新老混凝土粘结面带状微裂缝断裂过程区模型,根据该模型可求出断裂过程区尺寸.结合裂缝过程区的荷载~应变试验结果,用临界断裂韧度对新老混凝土粘结断裂韧度进行了断裂过程区影响修正.     

低速碰撞下复合材料叠层板的微裂纹演化及其对弹性模量的影响

采用双线性特性破坏模型研究了复合材料叠层板各层内部开裂裂纹的演化;通过引入弹性模量的裂纹影响系数表示,推导出裂纹影响系数与应变及应变率之间的微分关系,并得到裂纹耗散功率与裂纹影响系数变化率之间的关系。通过计算不同初始碰撞速度下复合材料叠层板的应变、应变率响应以及裂纹影响系数的演化,得到整个冲击过程中各层内任意点附近裂纹开裂情形及其对弹性模量的影响;通过检查界面各点处的裂纹影响系数是否发生改变,预测了碰撞完成之后复合材料叠层板中各层内微裂纹的分布区域位置与大小;并将该预测结果与其他破坏准则计算结果进行了比较。计算结果表明,在碰撞过程中各层内任意点处的应力值超过其屈服强度后,该点附近的弹性模量开始发生衰减,衰减大小随铁球初始碰撞速度的增大而增大。在四边夹支的边界条件下,复合材料叠层板的裂纹分布区域同样最先出现在碰撞点及边界中点位置,区域面积随初始碰撞速度的增大不断扩大     

Computational Prediction of Micro-crack Induced Ultrasound Attenuation in CFRP Composites

A computational study is presented of ultrasound attenuation arising from linear elastic scattering by matrix micro-cracking in carbon fiber reinforced polymer (CFRP) composites. The model considers ultrasound propagation in a unidirectionally reinforced laminate containing randomly distributed matrix micro-cracks, configured such that neither the cracked laminate nor wavefield display dependence on the spatial direction of fiber reinforcement. The resulting 2D scattering formulation is solved using the boundary element method (BEM). Scatter-induced ultrasound attenuation is computed using a fully-interacting scattering matrix, as well as approximate multiple scattering formulations. Through comparison to the fully-interacting computation, the validity of level 1 scattering (independent scattering approximation), level 2 scattering (single neighbor interaction), and level 3 scattering (two neighbor interaction) is noted as a function of micro-crack density. A single parameter estimation of attenuation dependence on micro-crack density is obtained by fitting exponential dependence to the fully-interacting scattering results. Additionally, this single parameter is shown to be extracted equally well by fitting over the limited validity range of the independent scattering formulation. When uniform crack morphology is assumed, the exponential fit to the independent scattering approximation yields a practical estimation of micro-crack induced attenuation over the full range of micro-crack density through consideration of forward scattering by a single micro-crack. Comparison of results to limited experimental data, and to an independent alternate computational approach, lends plausibility to the study’s conclusions.