金属材料中低加载速率下的动态韧脆转变及断裂韧性测量

金属材料在冲击下的韧脆转变现象和动态断裂韧性的测量是金属材料冲击力学性能研究的重要组成部分.针对金属材料在冲击下的韧脆转变现象认识不足和韧性材料在较低加载率下动态$J$-$R$阻力曲线难以测量的现状,提出了采用高速材料试验机, 设计专用试验夹具,测量15MnTi钢和11MnNiMo钢在不同加载速率下的韧脆转变过程,以及裂尖约束对其动态韧脆转变速率变化的影响.在高速材料试验机上采用上夹具辊刹车,通过调节压缩杆长度改变试验中裂纹扩展量的试验方法,测量了15MnTi钢三点弯曲试样件在较低加载率下的动态断裂韧性.试验发现15MnTi钢CT试验件加载速率低于0.025 m/s时呈现出韧性断裂的特点,加载速率在0.1,$\sim$,0.5 m/s时为韧脆结合型断裂,加载速率高于0.5 m/s后进入脆性断裂区; 11MnNiMo钢CT试验件加载速率大于1.5 m/s后,分层断裂过程中出现先脆断后韧段的现象;发现15MnTi钢和11MnNiMo的动态韧脆转变速率受裂尖约束的影响非常明显,面内约束和面外约束的升高都会导致材料动态脆断速率出现明显降低;还发现三点弯曲试验中, 15MnTi钢在8788 MPa$\cdot$mm/s加载率内断裂韧性随加载率的提升呈现出缓慢下降的趋势.      

685均质钢静动态断裂韧性实验研究

为了解685均质钢的裂纹在静态加载和动态加载下的裂纹起裂和扩展情况,分别在静态试验机和基于Hopkinson杆技术改进的动态加载装置上采用三点弯曲试样对685均质钢的静、动态断裂韧性进行了研究。685均质钢中存在的少量孪晶马氏体组织对其断裂韧性造成了不利影响,可以适当降低碳含量来改善685均质钢的断裂韧性性能。当加载率KI≤1.8778×106MPa（m）^1/2/s时,685均质钢的动态断裂韧性值都随加载率的增加而下降。当加载率KI>1.8778×106MPa（m）^1/2/s时由于裂纹尖端热软化效应的影响,使得该材料的动态断裂韧性值又上升。采用高速摄影技术记录了裂纹的起裂和扩展,测得了裂纹的扩展速率。发现用裂纹嘴张开位移（Crack Mouth Opening Displacement,CMOD）法计算得到的动态断裂韧性值与用电阻应变片计算的结果相一致。     

爆炸加载下混凝土表面的裂纹扩展

为深入研究内爆加载下岩土类材料的破坏机理，提出了一种新的爆炸裂纹检测算法，采用数字图像相关方法测量表面位移场和应变场，建立了裂纹扩展和扩张模型，并通过混凝土内爆试验观测裂纹扩展过程，研究了裂纹长度扩展与宽度扩张规律。结果表明，裂纹长度扩展是应力波和爆生气体共同作用的结果，裂纹最大扩展速度为225.95 m/s，平均速度为122.27 m/s，裂纹总长159.92 mm，长度扩展止于1.75 ms；裂纹的张开由气体主导，最大宽度1.59 mm，作用时间长达4.5 ms；拉应变集中区先于裂纹出现，其形状决定了裂纹的走向和趋势，爆炸加载下断裂过程区长度为骨料粒径的8～9倍。     

低速冲击下混凝土少筋梁断裂性能

动态加载下,混凝土中钢筋的阻裂性能一直是冲击动力学研究领域的难点之一。利用落锤试验机对含缺口的混凝土少筋梁进行三点弯曲试验,分析了不同加载速率下梁的冲击力、跨中挠度、混凝土起裂应变率和钢筋应变。实验结果表明:在一定加载速率范围内（0.885～1.252 m/s）,混凝土预制裂缝尖端的裂纹起裂应变率、冲击力最大值、跨中挠度峰值与加载速率呈线性增长关系,当加载速率增至1.771 m/s时,增长趋势减弱;冲击力卸载时,钢筋部分弹性变形恢复导致裂纹产生闭合,裂纹嘴张开位移逐渐减小至恒定值,对裂纹嘴张开位移峰值前的部分曲线进行拟合后得到裂纹嘴张开位移率,结果表明裂纹嘴张开位移率随加载速率的提高而线性增大。     

缺陷介质切槽爆破断裂行为的动焦散线实验

利用数字激光动态焦散线实验系统,对含缺陷介质在切槽爆破和普通炮孔爆破中爆生裂纹的断裂行为进行对比研究。结果表明,切槽爆破中沿切槽方向起裂的主裂纹比非切槽方向早10 μs,有利于能量优先沿切槽方向释放;切槽方向主裂纹的起裂韧度为0.58 MN/m3/2,其裂纹扩展的平均速度为277 m/s,分别是普通爆破时主裂纹相应值的54%和86%;当切槽方向主裂纹与缺陷介质贯通后,为爆生气体提供了足够的膨胀空间,诱导爆生气体向预制裂纹两端释放,翼裂纹起裂以Ⅰ型拉伸破坏为主,并在裂纹扩展的60~250 μs内,Ⅰ型动态应力强度因子保持在0.6~0.8 MN/m3/2,形成了明显的平台,延缓了翼裂纹扩展速度的衰减,最终较普通炮孔翼裂纹扩展时间和扩展长度分别增加了22.7%和17.8%。     

冲击载荷下扩展裂纹尖端动态能量释放率分布的焦散线分析

借助高速摄影捕捉裂纹瞬态扩展过程，利用动态焦散线研究了含有裂纹的三点弯曲梁在冲击载荷作用下扩展裂纹尖端的动态能量释放率分布规律；综合分析了裂纹扩展时间、长度、速度，以及扩展裂纹尖端动态应力强度因子与它的变化关系，表明了动态能量释放率在裂纹扩展过程中的驱动作用。     

基于DIC的爆炸加载下脆性材料裂纹扩展规律的试验研究

通过数字图像相关法（DIC），应用PMMA对爆炸加载条件下脆性材料的裂纹扩展规律进行了试验研究。基于对称性试验模型，实现了裂纹尖端位置和应变场信息的同步记录。以此为基础，通过对比分析获知，主应变场应变值最大点不能作为裂纹尖端的判断依据。并以动态裂纹扩展速度为参量，应用断裂动力学和最小二乘牛顿迭代法，计算出了考虑惯性效应的Ⅰ-Ⅱ混合型裂纹的应力强度因子:K_Ⅰ和K_Ⅱ值会随着裂纹扩展方向改变而发生突变；K_Ⅰ最大值为2.63 MPa·m1/2，最小值为0.89 MPa·m1/2；其整体变化趋势表明，爆炸加载条件下脆性材料裂纹扩展随能量积聚和释放呈循环阶梯式递减发展。     

涂层式裂纹监测系统中基体裂纹穿越行为研究

建立了智能涂层的两相模型与三相模型,基于能量准则分别用这两种模型研究了基体裂纹达到涂层界面后的穿越/偏转行为. 用有限元法分析了相对裂纹扩展长度、弹性错配参数及界面层厚度对偏转裂纹与穿越裂纹能量释放率之比的影响,结果表明当基体裂纹到达驱动层与基体界面时,能量释放率之比不仅与基体和驱动层之间的弹性错配相关,而且当驱动层较薄时对驱动层与传感层之间的弹性错配亦有较强的依赖性. 此外,随着驱动层厚度的增加,能量释放率之比对驱动层与传感层之间的弹性错配的依赖性逐渐降低. 通过与实验结果相比,建立的模型能够较好的解释基体裂纹在界面的扩展行为,可用于智能涂层裂纹传感器的优化设计.      

深埋椭圆形片状裂纹的偏折扩展

基于无限大弹性基体深埋椭圆形片状裂纹的变形场,推导了椭圆形片状裂纹的能量释放率,采用能量平衡方法建立了椭圆形片状裂纹承受拉应力和剪应力时的复合断裂准则. 考虑裂纹在拉-剪应力作用下的偏折扩展,分析了裂纹的偏折方向,提出了椭圆形片状裂纹发生偏折扩展时的初始偏折位置的确定方法.      

弯曲疲劳短裂纹扩展规律研究

本文对３５ＣｒＭｏ钢和４２ＣｒＭｏ钢带穿透短裂纹（初始裂纹长度ａ＝０．１～０．５ｍｍ）的试件进行了弯曲疲劳试验研究，得到了在弯曲疲劳加载下短裂纹扩展速率的计算表达式以及应力比，试件厚度对短裂纹扩展的影响。     

Dynamic fracture instabilities in brittle crystals generated by thermal phonon emission: Experiments and atomistic calculations

Dynamic cleavage fracture experiments of brittle single crystal silicon revealed several length scales of surface and path instabilities: macroscale path selection, mesoscale crack deflection, and nanoscale surface ridges. These phenomena cannot be predicted or explained by any of the continuum mechanics based equations of motion of dynamic cracks, as presumably critical energy dissipation mechanisms are not fully accounted for in the theories. Experimentally measured maximum crack speed, always lower than the theoretical limit, is another phenomenon that is as yet not well understood.We suggest that these phenomena depend on velocity dependent and anisotropic material property that resists crack propagation. The basic approach is that the bond breaking mechanisms during dynamic crack propagation vibrate the atoms at the crack front to generate thermal phonon emission, or heat, which provides additional energy dissipation mechanisms. This energy dissipation mechanism is a material property that resists crack propagation. To evaluate this property, we combined the continuum based elastodynamic Freund equation of motion with molecular dynamics atomistic computer “experiments”.We analyzed the above experimental dynamic fracture instabilities in silicon with the obtained velocity dependent and anisotropic material property and show its importance in cleavage of brittle crystals.     

厚度与加载速率对铁素体钢断裂韧性影响的SP压杆法实验研究

采用SP压杆实验方法,在常温下研究小圆薄片断裂韧性的厚度效应及加载速率对断裂特性的影响.实验结果表明,随着厚度的增加,断裂变形能增加,断裂部分的外表面因双向应力状态表现出微突起,微突起四周存在微小裂纹;随着加载速度的增加,断裂变形能增加,剪切断裂表面表现出从密集韧窝到韧窝连接成片特征.考虑试件变形过程中不同部分的能量耗散,从SP试件的整体断裂变形能得到试件的断裂韧性的宏观表达,断裂韧性随着厚度的增加而增加,随着加载速度的增加而减少.采用临界塑性断裂应变作为裂纹起裂判据,单位面积的能量耗散率作为裂纹扩展和失效判据的断裂模型,用有限元方法对SP压杆实验进行模拟,得到与实验结果比较相符的模拟结果.     

Dynamic behaviors of mode III interfacial crack under a constant loading rate

In an earlier study on intersonic crack propagation, Gao et al. (J. Mech. Phys. Solids 49: 2113–2132, 2001) described molecular dynamics simulations and continuum analysis of the dynamic behaviors of a mode II dominated crack moving along a weak plane under a constant loading rate. The crack was observed to initiate its motion at a critical time after the onset of loading, at which it is rapidly accelerated to the Rayleigh wave speed and propagates at this speed for a finite time interval until an intersonic daughter crack is nucleated at a peak stress at a finite distance ahead of the original crack tip. The present article aims to analyze this behavior for a mode III crack moving along a bi-material interface subject to a constant loading rate. We begin with a crack in an initially stress-free bi-material subject to a steadily increasing stress. The crack initiates its motion at a critical time governed by the Griffith criterion. After crack initiation, two scenarios of crack propagation are investigated: the first one is that the crack moves at a constant subsonic velocity; the second one is that the crack moves at the lower shear wave speed of the two materials. In the first scenario, the shear stress ahead of the crack tip is singular with exponent ?1/2, as expected; in the second scenario, the stress singularity vanishes but a peak stress is found to emerge at a distance ahead of the moving crack tip. In the latter case, a daughter crack supersonic with respect to the softer medium can be expected to emerge ahead of the initial crack once the peak stress reaches the cohesive strength of the interface.     

Effects of vacancy defect on the tensile behavior of graphene

Graphene is the strongest material but its performance is significantly weakened by vacancy defects. We use molecular dynamics simulations to investigate the tensile behavior of a graphene which contains a single vacancy defect. Our results suggest that because of the single vacancy, the fracture strength of graphene losses about 17.7%. The stress concentration around the vacancy defect leads to the destruction of nearby six-member rings structure, which forms the initial crack. The propagation direction of this crack in defective graphene is at an angle of 60° to the tensile direction initially, but then becomes perpendicular to the tensile direction.     

Fracture Properties of Concrete–Concrete Interfaces Using Digital Image Correlation

The mode I and mode II fracture toughness and the critical strain energy release rate for different concrete–concrete jointed interfaces are experimentally determined using the Digital Image Correlation technique. Concrete beams having different compressive strength materials on either side of a centrally placed vertical interface are prepared and tested under three-point bending in a closed loop servo-controlled testing machine under crack mouth opening displacement control. Digital images are captured before loading (undeformed state) and at different instances of loading. These images are analyzed using correlation techniques to compute the surface displacements, strain components, crack opening and sliding displacements, load-point displacement, crack length and crack tip location. It is seen that the CMOD and vertical load-point displacement computed using DIC analysis matches well with those measured experimentally.     

Anti-plane analysis of semi-infinite crack in piezoelectric strip

Using the complex variable function method and the technique of the conformal mapping, the fracture problem of a semi-infinite crack in a piezoelectric strip is studied under the anti-plane shear stress and the in-plane electric load. The analytic solutions of the field intensity factors and the mechanical strain energy release rate are presented under the assumption that the surface of the crack is electrically impermeable. When the height of the strip tends to infinity, the analytic solutions of an infinitely large piezoelectric solid with a semi-infinite crack are obtained. Moreover, the present results can be reduced to the well-known solutions for a purely elastic material in the absence of the electric loading. In addition, numerical examples are given to show the influences of the loaded crack length, the height of the strip, and the applied mechanical/electric loads on the mechanical strain energy release rate.     

Multiscale modeling of crack initiation and propagation at the nanoscale

Fracture occurs on multiple interacting length scales; atoms separate on the atomic scale while plasticity develops on the microscale. A dynamic multiscale approach (CADD: coupled atomistics and discrete dislocations) is employed to investigate an edge-cracked specimen of single-crystal nickel, Ni, (brittle failure) and aluminum, Al, (ductile failure) subjected to mode-I loading. The dynamic model couples continuum finite elements to a fully atomistic region, with key advantages such as the ability to accommodate discrete dislocations in the continuum region and an algorithm for automatically detecting dislocations as they move from the atomistic region to the continuum region and then correctly “converting” the atomistic dislocations into discrete dislocations, or vice-versa. An ad hoc computational technique is also applied to dissipate localized waves formed during crack advance in the atomistic zone, whereby an embedded damping zone at the atomistic/continuum interface effectively eliminates the spurious reflection of high-frequency phonons, while allowing low-frequency phonons to pass into the continuum region.The simulations accurately capture the essential physics of the crack propagation in a Ni specimen at different temperatures, including the formation of nano-voids and the sudden acceleration of the crack tip to a velocity close to the material Rayleigh wave speed. The nanoscale brittle fracture happens through the crack growth in the form of nano-void nucleation, growth and coalescence ahead of the crack tip, and as such resembles fracture at the microscale. When the crack tip behaves in a ductile manner, the crack does not advance rapidly after the pre-opening process but is blunted by dislocation generation from its tip. The effect of temperature on crack speed is found to be perceptible in both ductile and brittle specimens.     

分子动力学模拟含不同初始缺陷单晶镍的断裂行为与应力演化

断裂是一个跨尺度复杂的物理过程，对宏观尺度的断裂行为已有深入的研究和发展，然而对微观尺度的断裂行为及断裂过程中应力场的变化缺乏深入的理解。本文通过分子动力学模拟，研究了具有不同初始缺陷(尖锐裂纹、钝裂纹和孔洞)的单晶镍的断裂行为和应力分布特征。结果表明，不同的初始缺陷导致了不同的断裂机制、断裂强度和抗断裂性能。含初始孔洞的单晶镍样品有最高的断裂强度和最强的抗断裂性能，这与孔洞扩展过程中堆积层错的形成密切相关。其次是含初始钝裂纹的样品，在裂纹扩展过程中出现由[100]超位错发射引起的裂尖钝化；含尖锐裂纹的样品表现为脆性断裂，裂尖原子没有出现微结构的变化，其强度和抗断裂性能最低。此外，不同的初始缺陷也会导致断裂过程中应力分布的变化，对含有尖锐裂纹的脆性断裂试样，高应力(拉伸应力、平均应力和米塞斯应力)总是出现在扩展裂纹的裂尖。而对于含有钝裂纹或孔洞的韧性断裂试样，高应力不仅分布在裂尖，也分布在位错发射和堆积层错形成的区域，在裂纹/孔洞扩展之前，应力随着加载时间的增加而迅速增加，而一旦裂纹或孔洞开始扩展，应力增加非常缓慢或几乎不增加，但拉伸应力值始终大于平均应力和米塞斯应力值。这表明，在I型...     

Brittle fracture dynamics with arbitrary paths III. The branching instability under general loading

The dynamic propagation of a bifurcated crack under arbitrary loading is studied. Under plane loading configurations, it is shown that the model problem of the determination of the dynamic stress intensity factors after branching is similar to the anti-plane crack branching problem. By analogy with the exact results of the mode III case, the energy release rate immediately after branching under plane situations is expected to be maximized when the branches start to propagate quasi-statically. Therefore, the branching of a single propagating crack under mode I loading should be energetically possible when its speed exceeds a threshold value. The critical velocity for branching of the initial single crack depends only weakly on the criterion applied for selecting the paths followed by the branches. However, the principle of local symmetry imposes a branching angle which is larger than the one given by the maximum energy release rate criterion. Finally, it is shown that an increasing fracture energy with the velocity results in a decrease in the critical velocity at which branching is energetically possible.