基于泛形理论的裂纹扩展路径模拟

材料断裂面的泛形特征是由于材料内部不均匀造成的．本文利用纳米压痕实验测得的弹性模量随机样本，得到了表示材料非均匀特性的Weibull统计分布参数；对含裂纹的HT250试件的裂纹扩展过程进行了基于扩展有限元法的数值模拟，在此结果上计算了裂纹扩展路径的泛形复杂度，模拟结果与试验结果吻合较好；分析了铝合金7075不同均质度对非均匀模型裂纹扩展的影响．研究结果表明，灰口铸铁的Ⅰ型裂纹扩展路径具有泛形特征，裂纹的泛形复杂度依赖于材料的非均匀性且呈负相关关系．该研究方法也适用于其他应力应变呈单值关系材料的裂纹扩展分析．     

基于反向传播神经网络的疲劳裂纹扩展分析

材料发生疲劳断裂时往往会引起重大安全事故,而基于传统数值模拟方法求解疲劳裂纹扩展问题时模 型复杂、计算量大．本文基于包含多隐层的反向传播神经网络分析金属材料疲劳裂纹扩展行为,计算了裂纹扩 展过程中的 von Mises应力场和位移场,并与数值解和实验解进行对比,误差分析结果表明其求解精度高．并 基于该神经网络有效预测了裂纹扩展中裂纹长度及裂纹扩展速率的变化过程,预测精度高．该神经网络分析方 法可为材料剩余寿命和疲劳强度预测提供研究基础．     

双拉试样裂纹扩展过程中的应力强度因子计算

论文将使用一种界面单元来解决二维裂纹的静态扩展问题.这种界面单元基于虚拟裂纹闭合法,利用商业有限元软件ABAQUS的用户自定义单元UEL功能,发展为界面断裂单元,计算应变能释放率(GⅠ和GⅡ).在裂纹尖端的两个节点间设置一个特殊刚度的弹簧,并引入哑节点计算裂纹尖端后面的张开位移和裂纹尖端前面的虚拟裂纹扩展量.采用这种单元计算应变能释放率时不需要使用奇异单元或折叠单元,不会出现收敛问题,也不需要复杂的后续处理.因此,采用这种断裂单元分析二维裂纹扩展问题是方便的、高效率的,而且也能得到可靠的精度.     

冲击载荷下镁铝合金裂纹动态扩展过程的数值模拟

采用基于黏聚裂纹模型的扩展有限元方法，开展了镁铝合金结构冲击破坏过程的数值模拟研究。通过镁铝合金三点弯曲试样冲击实验，获得了不同子弹撞击速度下试样的冲击破坏模式。在此基础上，建立了实验结构的扩展有限元模型，并采用最大主应力准则，以及含损伤型的本构关系模拟材料的冲击断裂行为。对于裂纹尖端附近区域，采用黏聚裂纹模型模拟裂纹的断裂过程。对子弹速度分别为12.2、15.1、26.3 m/s的3种工况下镁铝合金试样的动态破坏过程进行了数值模拟研究，获得了与实验相一致的断裂模式。计算结果表明，试样以Ⅰ型断裂模式为主，裂纹沿初始预制裂纹方向扩展。当裂纹扩展到一定程度后，在试样韧带区域被撞击端附近，由于应力波及边界效应导致该区域处于复杂应力状态，试样出现复合型断裂模式，裂纹偏离原扩展路径，与本文实验结果相吻合。     

两相材料界面裂纹的起裂方向及断裂参量研究

采用FRANC2D软件研究了两相材料含界面裂纹的断裂特性。通过在材料界面利用CASCA手动生成界面裂纹并在裂纹尖端附近设置1/4奇异等参元,得到了界面裂纹的复应力强度因子;数值模拟得到了界面裂纹的起裂方向,并分析了界面裂纹的破坏特征。计算结果表明:1/4奇异等参元很好地描述了裂纹尖端场的1/r(1/2)奇异性,FRANC2D软件能够模拟界面裂纹的扩展方向,可以得到界面裂纹尖端的应力场和复应力强度因子,为界面裂纹的断裂特性的进一步分析提供依据。     

岩石材料动态破坏的近场动力学方法数值模拟

裂纹的聚集、扩展、分叉是岩石等脆性材料破坏失效的关键因素，本文在验证了近场动力学方法在研究岩石类材料裂纹动态扩展方面的有效性之后，采用近场动力学方法分别对冲击载荷作用下含有双裂纹岩石材料和单轴压缩作用下含单斜裂纹的岩石材料进行数值模拟．结果表明，对于双垂直裂纹，其裂纹扩展路径大致与预制裂纹成70°夹角；对于单裂纹，裂纹的扩展路径随裂纹倾角的变化而变化，最终导致构件的整体破坏．数值模拟结果表明近场动力学方法可以很好地模拟岩石等脆性材料的裂纹扩展直至破坏的过程，反映裂纹扩展的物理机理；其作为一种新的基于非局部理论的数值方法，在地下岩体工程方面及页岩气的开采方面会有很好的发展前景．     

界面裂纹萌生与扩展的分子动力学模拟

运用分子动力学模拟方法研究了裂纹在界面端处萌生与沿界面扩展的临界条件. 模拟考虑了一双相材料的3种模型，即构成90°/90°和 90°/180°夹角的两个界面端和一个界面裂纹. 模拟采用了包含原子区域与连续区域的并发型多尺度模型，即在界面端尖端和裂纹尖端附近 采用分子动力学(MD)方法，MD区域之外则按照线弹性有限元方法分析. 结果表明，在断裂启动时刻，3个模型沿界面的最大应力均达到界面理想强度；而且，其界 面能恰好足以克服界面材料的本征内聚能. 因此，界面端裂纹萌生与沿界面扩展的断裂条件可以通过界面理想强度和内聚能联系起来. 并基于模拟计算结果提出了界面断裂启动的统一准则.     

高韧性管道动态断裂的气体减压模式和材料韧性研究

天然气管道上动态裂纹扩展包含气体、结构和断裂的相互作用．因此，分析射流场分布特性及其与管壁开裂变形的相互作用是数值模拟过程的关键问题．随着钢管韧性等级的迅速提升和气体压力的不断提高，原有的经验公式乃至算法多数不再适用，亟需通过理论、试验和数值模拟给出新的扩展与止裂判据，以控制裂纹在管道上扩展的速度和距离．本文通过一系列的韧性试验校正了现行的管材韧性判定办法，并对不同工况下的裂纹动态扩展以及超声速射流场进行了数值仿真，以建立一套工程适用的评价体系。     

岩石偏心圆孔单裂纹平台圆盘的动态裂纹扩展与止裂

针对平台圆环构型的优点, 提出偏心圆孔单裂纹平台圆盘(cracked eccentrically holed flattened disc, CEHFD), 该试样具有更长的断裂路径。利用霍普金森压杆加载系统, 径向冲击CEHFD试样, 完成Ⅰ型动态断裂实验。砂岩试样表面粘贴应变片和裂纹扩展计, 用于监测裂纹动态起裂、扩展和止裂的全过程。实验表明, 在整个断裂过程中, 裂纹非匀速扩展, 裂纹扩展速度在裂纹起裂后加速上升, 在裂纹止裂前有明显的减速, 与地震时断层的动态破裂全过程完全吻合。采用实验-数值-解析法得到动态应力强度因子, 其时间历程呈现先增大后减小的趋势。根据断裂过程不同时刻, 得到相应的动态起裂韧度、扩展韧度及止裂韧度。在动态断裂全过程中, 动态扩展韧度为速度的函数, 变化趋势与速度一致, 随着时间先增大后减小; 动态起裂韧度大于动态止裂韧度, 止裂韧度随着裂纹最大扩展速度的增大而降低, 并且有较大的离散性。     

基于圆弧底试件的动态裂纹扩展及止裂规律研究

为了研究脆性材料的动态裂纹扩展及止裂规律，设计了一种带圆弧形底边的梯形开口边裂纹（trapezoidal opening crack with arc bottom，TOCAB）构型的试件。在落锤冲击设备加载下，对圆心角为0°、60°、90°和120°的TOCAB试件进行了冲击实验，并采用裂纹扩展计(crack propagation gauge，CPG)监测裂纹起裂和扩展时间，从而获得裂纹扩展速度。采用有限差分软件AUTODYN对落锤冲击设备和试件进行数值模拟，研究了裂纹的动态扩展过程及止裂规律。还基于实验和数值方法，计算了裂纹的临界动态应力强度因子。实验和数值结果均表明:3种弧度的TOCAB试件都可以实现运动裂纹止裂，该构型可用于研究动态裂纹止裂问题；数值计算的裂纹扩展路径与实验结果基本一致，验证了数值模型的有效性；裂纹起裂和止裂时刻的临界动态应力强度因子大于裂纹动态扩展过程中的临界动态应力强度因子。     

Atomistic/continuum simulation of interfacial fracture Part II: Atomistic/dislocation/continuum simulation

Coupled atomistic/dislocation/continuum simulation of interfacial fracture is performed in this paper. The model consists of a nanoscopic core made by atomistic assembly and a surrounding elastic continuum with discrete dislocations. Atomistic dislocations nucleate from the crack tip and move to the continuum layer where they glide according to the dislocation dynamics curve. An atoms/continuum averlapping belt is devised to facilitate the transition between the two scales. The continuum constraint on the atomic assembly is imposed through the mechanics atmosphere along the overlapping belt. Transmissions of mechanics parameters such as displacements, stresses, masses and momenta across the belt are realized. The present model allows us to explore interfacial fracture processes under different mode mixity. The effect of atomistic zigzag interface on the fracture process is revealed: it hinders dislocation emission from the crack tip, especially under high mode mixity. The project supported by the National Natural Science Foundation of China     

Dynamic crack kinking from a PMMA/Homalite interface

The behavior of a pre-existing, dynamically loaded, interfacial crack kinking away from the interface separating two materials was experimentally investigated under dynamic loading conditions. Dynamic fracture experiments were performed on pre-cracked bimaterial panels of PMMA bonded with Homalite-100 impact loaded using a high-speed gas gun. By varying the location of impact, a large range of mixed mode loading at the crack tip was produced. Information about the stress state surrounding the crack tip was obtained through use of the lateral shearing interferometer of coherent gradient sensing in conjunction with high-speed photography. The high-speed interferogram corresponding, to the time of crack initiation was analyzed in each case to find the preinitiation mode mixity at the crack tip. Measurement of both the local initiation mode mixity and the crack kink angle allows for possible extension of existing quasi-static interface crack kinking criteria, such as maximum opening stress or maximum energy release rate, to the case of dynamic loading. It was found that for bimaterial systems with small material property mismatch, such as the PMMA/Homalite system, the maximum opening stress criterion accurately predicts the relation between crack tip mode mixity and resulting kink angle for small initial crack kinking speeds.     

Interfacial mixed-mode fracture of adhesive bonds undergoing large deformation

Interfacial fracture of adhesive bonds undergoing large-scale yielding is studied using a combined experimental/finite-element approach. The full range of in-plane mode mixity is produced over bond thickness ranging from 30 to 500 μm using the scarf and the ENF joint geometries. Novel techniques for introducing pre-cracks and surface decoration, together with in situ observations, facilitate accurate determination of the bond-average and the local shear strains at the crack tip during the onset as well as the rest of the crack propagation event. The crack generally grew along one of the two interfaces of the bond, although the failure was always fully cohesive. The local shear strain at the crack tip is independent of the bond thickness, and, under quasi-static conditions, it remains constant throughout the growth, which make it a viable fracture parameter. This quantity strongly depends on the mode mixity, the sign of the phase angle (i.e., shearing direction) and the crack speed, however.A finite-element analysis is used to obtain the crack tip deformation field for an interface crack in adhesively bonded scarf and ENF joints. Large-strain and quasi-static conditions are assumed. A distinct material model in the fracture process zone that allows for volume change in the post-yield regime is incorporated into the analysis. The local deformation is characterized by a pair of bond-normal and tangential displacements corresponding to the nodal points adjacent to the crack tip. The critical values of these quantities are obtained when the FEM bond-average shear strain at the crack tip becomes equal to its experimental counterpart. The so defined critical local displacements, after an appropriate normalization, seem to conform to a single-valued, linear type interrelationship over the entire range of mode mixity. The fact that this relationship is independent of the bond thickness, and furthermore it encompasses both cases of positive and negative phase angles, makes it a viable candidate for characterizing mixed-mode interfacial fracture under large-deformation conditions.     

Mode mixity and nonlinear viscous effects on toughness of interfaces

This paper examines steady-state crack growth at interfaces between polymeric materials and hard substrates under quasi-static conditions. The polymeric material is taken to be an elastic nonlinear viscous solid while the substrate is treated as a rigid material. Void growth and coalescence in the rate-dependent fracture process zone is modeled by a nonlinear viscous porous strip of cell elements. In the first part of this paper, the polymeric background material surrounding the process zone is assumed to be purely elastic. Under fixed mode mixity, the computed interface toughness is found to be a monotonically increasing function of crack velocity; toughness also increases rapidly with higher rate sensitivity. This behavior can be explained in terms of voids growing in a strain-rate strengthened process zone. In the second part of the paper, the background material is also treated as an elastic nonlinear viscous solid. The competition between work of separation in the process zone and energy dissipation in the background material leads to a U-shaped toughness–crack velocity curve. Effects of mode mixity, initial porosity, rate sensitivity, as well as the initial yield strain on toughness are studied. The simulations produce trends that agree with interface toughness vs. crack velocity data reported in experimental studies for rubber toughened epoxy-paste adhesive and urethane acrylate adhesive.     

Delamination in patterned films

When the dielectric constant of an insulator in an interconnect is reduced, mechanical properties are often compromised, giving rise to significant challenges in interconnect integration and reliability. Due to low adhesion of the dielectric an interfacial crack may occur during fabrication and testing. To understand the effect of interconnect structure, an interfacial fracture mechanics model has been analyzed for patterned films undergoing a typical thermal excursion during the integration process. It is found that the underlayer pattern generates a driving force for delamination and changes the mode mixity of the delamination. The implications of our findings to interconnect processes and reliability testing have been discussed.     

Atomistic/continuum simulation of interfacial fracture part I: Atomistic simulation

The phenomenon of interfacial fracture, as manifested by atomistic cleavage, debonding and dislocation emission provides a challenge for combined atomistic-continuum analysis. As a precursor for fully coupled atomistic-continuum simulation^[1] of interfacial fracture, we focus here on the atomistic behavior within a nanoscopic core surrounding the crack tip. The inter-atomic potential under Embedded Atom Method is recapitulated to form an essential framework of atomistic simulation. The calculations are performed for a side-cracked disc configuration under a remoteK field loading. It is revealed that a critical loading rate defines the brittle-to-ductile transition of homogeneous materials. We further observe that the near tip mode mixity dictates the nanoscopic profile near an interfacial crack tip. A zigzag interface structure is simulated which plays a significant role in the dislocation emission from an interfacial crack tip, as will be explored in the second part of this investigation. The project supported by the National Natural Science Foundation of China     

An inclined surface crack subject to biaxial loading

The elastic–plastic stress fields and mode mixity parameters for semi-elliptical surface cracks on biaxial loaded plates have been investigated using detailed three-dimensional finite element calculations. Different degrees of mode mixity are given by combinations of the far-field stress level, biaxial stress ratio and inclined crack angle. These analyses were performed for different surface flaw geometries to study the combined load biaxiality and mode mixity effects on the crack-front stress fields and the size and shape of the plastic zones. It is clear from considering the local stress distributions along the crack front that the elastic crack tip singularities have been derived for several particular cases of mixed mode biaxial loading. By theoretical analysis, the new formulae have been introduced for both the elastic and plastic mode-mixity parameters, accounting for ratios between the I/II, II/III and III/I modes. Particular attention was paid to the strong variations of the mode-mixity parameters along the semi-elliptical surface crack front. The mixed-mode behavior of the crack growth direction angle along the semi-elliptical crack front for different combinations of biaxial loading and inclination crack angles was also determined. It was done using methods based on the maximum tangential stress and the strain energy density criteria.     

纤维复合材料损伤过程的数值模拟

利用界面断裂力学和有限元法数值模拟纤维增强复合材料的细观损伤过程，研究各种主要破坏模式之间的相互转变和影响，指出以断裂能和混合度表示的界面性能是控制复合材料损伤过程的主要细观参数。分析了界面韧度对破坏性能的影响，探讨了基于破坏模式控制的复合材料韧度设计的新途径。     

Interface crack growth for anisotropic plasticity with non-normality effects

A plasticity model with a non-normality plastic flow rule is used to analyze crack growth along an interface between a solid with plastic anisotropy and an elastic substrate. The fracture process is represented in terms of a traction-separation law specified on the crack plane. A phenomenological elastic–viscoplastic material model is applied, using an anisotropic yield criterion, and in each case analyzed the effect of non-normality is compared with results for the standard normality flow rule. Due to the mismatch of elastic properties across the interface the corresponding elastic solution has an oscillating stress singularity, and with conditions of small scale yielding this solution is applied as boundary conditions on the outer edge of the region analyzed. Crack growth resistance curves are calculated numerically, and the effect of the near-tip mode mixity on the steady-state fracture toughness is determined. It is found that the steady-state fracture toughness is quite sensitive to differences in the initial orientation of the principal axes of the anisotropy relative to the interface.