延性金属层裂自由面速度曲线特征多尺度模拟研究

以延性金属钽为研究对象,对钽在平板撞击下的层裂行为进行了多尺度下的数值模拟研究,从微观视角对自由面速度曲线上的典型特征进行了新的解读。在宏观尺度,对比分析了光滑粒子流体动力学法（smootfied particle hydrodynamics, SPH）与Lagrange网格法以及几种本构模型的模拟结果及其适用性。通过与实验数据的对比表明,Steinberg-Cochran-Guinan本构模型在层裂模拟中与实验数据吻合较好,通过改变加载条件获得了不同应变率下的自由面速度曲线,分析了不同应变率下的自由面速度曲线中的典型特征。在微观尺度,采用分子动力学方法获得层裂区域内损伤演化情况,揭示了宏观尺度自由面速度曲线典型特征所蕴含的物理内涵。分析表明,层裂表现为材料内部微孔洞形核、长大和聚集的损伤演化过程,自由面速度曲线上的典型特征与层裂区域的损伤演化过程存在密切关联。Pullback信号是层裂区域内微孔洞形核的宏观表征;自由面速度曲线的下降幅值在一定程度上反映了微孔洞的形核条件,由此计算得到的层裂强度实际上是微孔洞的形核强度。此外,Pullback信号后的速度回跳速率反映了微损伤演化的速率。     

修正金属本构模型在超高速撞击模拟中的应用

为更加准确地计算93钨合金弹超高速撞击Q345钢板问题,构建了修正的金属本构模型。引入GRAY三相物态方程描述材料相态变化,采用Johnson-Cook强度模型描述撞击后期材料的力学行为。结合封加波损伤演化模型以及Johnson-Cook失效模型描述不同应力三轴度下材料的拉伸、剪切失效行为;引入曹祥提出的断裂演化模型,描述材料失效后应力归零的过程。通过对比超高速撞击数值模拟结果与实验结果,验证了本构模型的适用性,并进一步分析了典型弹靶撞击条件下破片群的空间分布特征。研究结果表明:基于修正金属本构模型获得的超高速撞击靶板穿孔直径、弹体侵蚀长度、破片群扩展速度结果与实验结果一致;GRAY三相物态方程能够相对准确地给出弹体撞击首层靶板以及剩余弹体、破片群撞击第2层靶板时弹靶材料的熔化情况;封加波损伤演化模型能够准确判断超高速撞击过程中靶板是否产生层裂破坏;综合封加波损伤演化模型、Johnson-Cook失效模型以及曹祥提出的断裂演化模型后,数值模拟获得的破片群撞击后效靶板的穿孔面积与累积数量的统计曲线结果与实验结果一致;获得了典型条件下的柱形93钨弹体超高速撞击Q345靶板破片群空间分布结果,破片群的前端具有较高的质量、轴向动量以及横向动量（绝对值）。     

延性金属层裂模型比较

在平面一维弹塑性流动有限差分计算程序中加入4种延性金属层裂模型，对平板撞击层裂实验进行数值模拟。结果表明:简单最大拉伸应力模型和简单损伤累积模型能定性反映层裂的物理现象，由于忽略损伤对本构的影响，计算结果和实验有偏差，但模型要求参数较少，对于一些精度要求不是很高的工程问题，可以采用；从材料损伤断裂物理本质出发，采用微损伤统计方法得到的NAG模型和封加波损伤度函数模型，能很好地再现实测的自由面速度剖面,数值计算结果与实验吻合很好。     

冲击加载下K9玻璃中预制缺陷对样品后界面速度影响的数值模拟

用数值模拟方法研究K9玻璃中的预制缺陷对自由面粒子速度的影响。K9玻璃以317m/s速度碰撞带有缺陷的K9玻璃靶板,自由面粒子速度历史曲线表明没有破坏波出现,计算结果和实验结果符合较好。同时模拟研究了缺陷位置和缺陷大小对自由面粒子速度的影响,计算结果表明,不同位置和大小的缺陷对自由面粒子速度影响较大。     

炸药爆炸驱动下铜板的层裂现

在进行炸药驱动铜板实验中,用激光速度干涉仪(VISAR)记录铜板自由面速度,观察到了铜板层裂现象,并用非线性有限元方法对爆炸载荷下铜板层裂过程进行数值模拟分析,得到了与实验结果吻合的计算结果。     

改进SPH方法在陶瓷材料层裂数值模拟中的应用

为了探讨铝飞片撞击陶瓷材料时的层裂现象,采用改进SPH方法模拟应力波在陶瓷材料中的传播。结果表明,当离散粒子分布不均匀时,数值模拟计算的自由面速度时程曲线与实测曲线吻合良好。对比CSPM方法,改进SPH方法的精度更高。提出适用于数值模拟的陶瓷材料损伤演化方程,对脉冲载荷下陶瓷/钢层合板层裂的破坏过程进行数值模拟,结果表明,由于陶瓷的波阻抗高于钢的,且抗压强度远高于抗拉强度,因此拉应力引起的层裂破坏是主要的。即使在材料内部传播的只是弹性压缩波,当弹性波到达材料界面时,由界面反射引起的卸载波也能导致陶瓷发生层裂破坏。     

平板撞击下梯度镍的层裂损伤研究

为研究高应变率下梯度镍的层裂损伤行为,对均匀镍材进行准静态扭转,制备出硬度和晶粒尺寸沿径向规则变化的圆柱形梯度镍.运用一级轻气炮对梯度镍进行平板撞击加载,利用多通道激光多普勒测速仪测量梯度镍的自由面粒子速度,计算层裂强度,并结合金相显微镜观察梯度镍回收试样的层裂损伤形貌.实验结果表明:平板撞击加载下,梯度镍的层裂损伤以...     

冲击荷载作用下滤波混凝土的动态响应与层裂损伤数值研究

借鉴局域共振材料的工作机制，通过在混凝土基体中嵌入滤波单元，设计出具有应力波衰减特性的滤波混凝土。通过将滤波混凝土结构简化为质量弹簧力学系统来分析滤波混凝土对应力波的衰减机制。采用数值模拟方法，对比研究了冲击荷载作用下普通混凝土模型和滤波混凝土模型中应力波的传播特性和层裂破坏模式。通过参数分析，研究了滤波单元的材料和几何属性对其储能效果的影响。研究结果表明：滤波单元有效降低了混凝土基体中应力波的传播速度和应力峰值；滤波单元的储能机制有效降低了混凝土基体中的能量；金属球的质量越大，滤波单元的储能效果越好，但弹性层的弹性模量和厚度需要通过适当分析进行设计以实现滤波单元的储能最大化；滤波混凝土基体的局部损伤耗散了荷载中的大量能量，有效降低了结构自由面附近的破坏程度。     

一种微裂纹型拉伸损伤模型在混凝土材料中的应用

将包含众多不同规则微裂纹的混凝土材料视为等效微裂纹系统,结合细观微裂纹动力律与波动理论,确定了裂纹演化与应力波传播的内在关系;根据有核长大思想和能量耗散理论,建立了一种便于工程应用的混凝土类准脆性材料拉伸损伤演化方程,并由混凝土单轴拉伸实验确定了相关材料参数;进行了混凝土平板撞击和内爆炸所引起的波传播和层裂问题的数值计算。结果表明:混凝土靶板的自由面速度时程曲线呈现明显周期性震荡,震荡周期与应力波在裂片中往返一次的时间基本相同;混凝土靶板损伤空间分布和损伤云纹图与已有实验结果一致,验证了本文所建立的微裂纹型拉伸损伤模型的科学实用性。     

钢筋混凝土柱破坏过程扩展有限元数值模拟

为描述钢筋混凝土柱的破坏过程,分别建立了由三维实体单元和三维杆单元构成的精细化模型以及结合纤维梁单元的纤维梁与实体单元耦合模型。耦合模型中,钢筋混凝土柱下段混凝土和钢筋分别用三维实体单元与三维杆单元建模,通过耦合连接实现精细单元与粗糙单元之间的变形协调。基于混凝土弹塑性本构关系,运用扩展有限元方法模拟钢筋混凝土柱的荷载-位移曲线以及开裂过程,并进行比较。数值模拟与试验结果的比较表明,模拟得到的裂缝位置以及荷载-位移曲线与试验结果吻合较好。扩展有限元具有无需重划网格、无需预设裂纹的优点,能有效地模拟筋混凝土柱的开裂过程。耦合模型具有计算效率高的优势,且能较好地模拟构件的破坏模式。     

Cohesive zone modeling of interfacial delamination in PZT thin films

Previous experimental investigations [Shang, F., Kitamura, T., Hirakata, H., Kanno, I., Kotera, H., Terada, K., 2005. Experimental and theoretical investigations of delamination at free edge of interface between piezoelectric thin films on a substrate. International Journal of Solids and Structures 42 (5–6) 1729–1741] have demonstrated that multilayered Cr/PZT/PLT/Pt/Ti thin films deposited on single-crystal silicon substrates are delaminated along the interface between Cr and PZT layers in a brittle manner. This study starts with a model based on the cohesive zone concept and carries out numerical simulations to check the fracture behavior of this interfacial delamination. Three types of cohesive zone models (CZMs) are adopted, including the exponential, bilinear, and trapezoidal models. Characteristic CZM parameters are extracted through comparisons with experimental results. The simulation results show that (i) cohesive strength and work of separation are the dominating parameters in the CZMs; (ii) the bilinear CZM more suitably describes this brittle interfacial delamination; and (iii) in comparison with typical several mm-thick film/coating materials, the fracture energy of this weak Cr/PZT interface is quite low. Our study demonstrates the applicability of CZM in characterizing the interface fracture behavior of film materials with micrometer thicknesses.     

Analysis of crack growth and crack-tip plasticity in ductile materials using cohesive zone models

Crack initiation and crack growth resistance in elastic plastic materials, dominated by crack-tip plasticity are analyzed with the crack modeled as a cohesive zone. Two different types (exponential and bilinear) of cohesive zone models (CZMs) have been used to represent the mechanical behavior of the cohesive zones. In this work, it is suggested that different forms of CZMs (e.g., exponential, bilinear) are the manifestations of different micromechanisms-based inelastic processes that participate in dissipating energy during the fracture process and each form is specific to each material system. It is postulated that the total energy release rate comprises the plastic dissipation rate in the bounding material and the separation energy rate within the fracture process zone, the latter is determined by CZMs. The total energy release rate then becomes a function of the material properties (e.g., yield strength, strain hardening exponent) and cohesive properties of the fracture process zone (e.g., cohesive strength and cohesive energy), and the form of cohesive zone model (CZM) that determines the rate of energy dissipation in the forward and wake regions of the crack. The effects of material parameters, cohesive zone parameters as well as the form/shape of CZMs in predicting the crack growth resistance and the size of plastic zone (SPZ) surrounding the crack tip are systematically examined. It is found that in addition to the cohesive strength and cohesive energy, the form (shape) of the traction–separation law of CZM plays a very critical role in determining the crack growth resistance (R-curve) of a given material. It is further observed that the shape of the CZM corresponds to inelastic processes active in the forward and wake regions of the crack, and has a profound influence on the R-curve and SPZ.     

基于相似材料的岩爆模型实验及其能量释放机制

岩体的卸载破坏和加载破坏有本质区别,岩爆是高地应力区地下工程开挖卸载产生的地质灾害现象.针对处于高静水压力状态下的岩体,采用松香模型实验研究径向瞬时卸载引起动力破碎型“岩爆”;通过分析动力破碎过程中的速度峰值、卸载波作用时间特征,推导了.应变能及剥落(破碎)块体动能及速度计算公式.结果表明:文中给出的速度峰值和动能计算方法是可行的,剥落(破碎)块体动能仅占可恢复应变能很小部分,大部分能量最终以不同形式耗散掉;距自由面不同距离处的卸载波作用时间大致相等,远大于卸载波的扰动时间;破碎波阵面在介质中推进速度大致为匀速,也远小于卸载波扰动速度.     

复合材料T型接头冰雹高速撞击损伤的数值模拟

利用高速空气炮进行了冰雹撞击复合材料T型接头的实验,研究了不同撞击速度下结构的损伤情况。同时,采用光滑质点流体动力学方法与黏聚区模型相结合的方法,对冰雹撞击复合材料T型接头进行了数值模拟。通过与实验结果的对比,验证了数值模拟模型的有效性。在此基础上,利用数值模型研究了影响复合材料T型接头冰雹撞击损伤的各种因素。计算结果表明:冰雹撞击对复合材料T型接头造成的损伤主要为分层损伤;冰雹的撞击速度、尺寸、入射角等,都对T型接头的损伤程度有很大影响;T型接头的分层面积与冰雹的撞击能量之间呈近似线性关系,分层面积随着撞击能量的增大而增大;冰雹的入射角越大,分层面积与撞击力峰值也越大。     

初始应力状态对材料层裂破坏特性影响研究

通过对球面飞片加载条件下的应力/应变状态分析表明采用施加径向应变方法可以近似模拟球面加载的受力过程. 采用过盈配合的热装配方法对平面样品施加了径向预应变,一维平面应变气炮实验结果显示初始预应力(变)明显降低了LY12铝层裂强度. 从空洞长大基本原理出发分析了各向异性受力条件下空洞长大的路径和所消耗能量不同于各向同性应力加载.通过数值模拟对含损伤的材料本构模型进行分析,得到了材料层裂强度与其外部宏观应力场密切相关,也间接的与构型相关的结论.      

Ductile–brittle transitions in the fracture of plastically-deforming,adhesively-bonded structures. Part I: Experimental studies

Rate effects for adhesively-bonded joints in steel sheets failing by mode-I fracture and plastic deformation were examined. Three types of test geometries were used to provide a range of crack velocities between 0.1 and 5000 mm/s: a DCB geometry under displacement control, a wedge geometry under displacement control, and a wedge geometry loaded under impact conditions. Two fracture modes were observed: quasi-static crack growth and dynamic crack growth. The quasi-static crack growth was associated with a toughened mode of failure; the dynamic crack growth was associated with a more brittle mode of failure. The experiments indicated that the fracture parameters for the quasi-static crack growth were rate independent, and that quasi-static crack growth could occur even at the highest crack velocities. Effects of rate appeared to be limited to the ease with which a transition to dynamic fracture could be triggered. This transition appeared to be stochastic in nature, it did not appear to be associated with the attainment of any critical value for crack velocity or loading rate. While the mode-I quasi-static fracture behavior appeared to be rate independent, an increase in the tendency for dynamic fracture to be triggered as the crack velocity increased did have the effect of decreasing the average energy dissipated during fracture at higher loading rates.     

An Experimental Technique for Spalling of Concrete

The spalling strength of concrete is measured by examining the strain wave profiles in a polymer buffer bar behind the slender concrete bar specimen placed between a large diameter (Φ100 mm) Hopkinson bar and the buffer bar. The experimental results indicate that the spalling strength is related to not only the compressive strength of concrete but also the impact velocities (the loading rates). The rate effect of spalling strength mainly results from the different cracking paths in concrete under different impact velocities. However when the input compressive stress to specimen exceeds the threshold required to trigger the compressive damage, the spalling strength decreases due to the evolution and cumulation of compressive damage in concretes. The repeated impact loading experiments indicate that damage plays an important role in the spallation process of concrete. The high speed video of the spalling fracture process shows that multiple spalling fractures may occur in the scab and damage accumulation resulting from stress wave propagation in scab is the main reason for the producing of multiple spallations.     

内聚力模型裂纹问题分析的解析奇异单元

内聚力模型已经被广泛应用于需要考虑断裂过程区的裂纹问题当中,然而常用的数值方法应用于分析内聚力模型裂纹问题时还存在着一些不足,比如不能准确的给出断裂过程区的长度、需要网格加密等。为了克服这些缺点,论文构造了一个新型的解析奇异单元,并将之应用于基于内聚力模型的裂纹分析当中。首先将虚拟裂纹表面处的内聚力用拉格拉日插值的方法近似表示为多项式的形式,而多项式表示的内聚力所对应的特解可以被解析地给出。然后利用一个简单的迭代分析,基于内聚力模型的裂纹问题就可以被模拟出来了。最后,给出二个数值算例来证明本文方法的有效性。     

A simple cohesive zone model that generates a mode-mixity dependent toughness

A simple, mode-mixity dependent toughness cohesive zone model (MDG_c CZM) is described. This phenomenological cohesive zone model has two elements. Mode I energy dissipation is defined by a traction–separation relationship that depends only on normal separation. Mode II (III) dissipation is generated by shear yielding and slip in the cohesive surface elements that lie in front of the region where mode I separation (softening) occurs. The nature of predictions made by analyses that use the MDG_c CZM is illustrated by considering the classic problem of an elastic layer loaded by rigid grips. This geometry, which models a thin adhesive bond with a long interfacial edge crack, is similar to that which has been used to measure the dependence of interfacial toughness on crack-tip mode-mixity. The calculated effective toughness vs. applied mode-mixity relationships all display a strong dependence on applied mode-mixity with the effective toughness increasing rapidly with the magnitude of the mode-mixity. The calculated relationships also show a pronounced asymmetry with respect to the applied mode-mixity. This dependence is similar to that observed experimentally, and calculated results for a glass/epoxy interface are in good agreement with published data that was generated using a test specimen of the same type as analyzed here.     

A thermomechanical cohesive zone model for bridged delamination cracks

The coupled thermomechanical numerical analysis of composite laminates with bridged delamination cracks loaded by a temperature gradient is described. The numerical approach presented is based on the framework of a cohesive zone model. A traction-separation law is presented which accounts for breakdown of the micromechanisms responsible for load transfer across bridged delamination cracks. The load transfer behavior is coupled to heat conduction across the bridged delamination crack. The coupled crack-bridging model is implemented into a finite element framework as a thermomechanical cohesive zone model (CZM). The fundamental response of the thermomechanical CZM is described. Subsequently, bridged delamination cracks of fixed lengths are studied. Values of the crack tip energy release rate and of the crack heat flux are computed to characterize the loading of the structure. Specimen geometries are considered that lead to crack opening through bending deformation and buckling delamination. The influence of critical mechanical and thermal parameters of the bridging zone on the thermomechanical delamination behavior is discussed. Bridging fibers not only contribute to crack conductance, but by keeping the crack opening small they allow heat flux across the delamination crack to be sustained longer, and thereby contribute to reduced levels of thermal stresses. The micro-mechanism based cohesive zone model allows the assessment of the effectiveness of the individual mechanisms contributing to the thermomechanical crack bridging embedded into the structural analysis.