冻土破坏过程的微裂纹损伤区的计算分析

对冻土破坏过程进行了试验研究,冻土破坏的过程是微裂纹损伤与临界串接的演化过程,确认了冻土微裂纹损伤区的存在,并分别用莫尔准则和最大拉应力准则建立了微裂纹损伤区计算模式。将微裂纹损伤区按缺陷处理并简化为当量裂纹,进行了裂纹尺寸的定量计算,结果表明其尺寸与冻土中初始裂纹尺寸相当,将计算结果与观测结果相比较,误差不超过10％。     

原状冻土Ⅱ型非线性断裂韧度实验测试研究

由于原状冻土处于自然状态下,其本构关系及水分、温度的分布符合实际状态,而且其实际破坏形式往往是非线性的剪切破坏,所以论文采用了非线性冻土断裂实验模型,对自然状态下的原状冻土进行了断裂破坏实验,着重对Ⅱ型断裂(剪切)破坏的四点弯曲直裂纹试样进行了实验研究.实验过程中改造了四点弯曲实验台,采用着色法测量预制裂纹尺寸.实验原理采用基于能量平衡的方法,利用数据采集系统分别测出了原状冻土四点弯曲试样加力点处位移与力的关系曲线以及相应的非线性参数,推导出裂纹扩展非线性能量释放率计算公式,当试样达到承载极限状况时测试出Ⅱ型断裂试样非线性断裂韧度.同时,提出了修正因子计算非线性断裂韧度的方法,将能量方法测试结果与修正因子方法结果进行了对比,二者基本是一致的.以上提出的冻土Ⅱ型断裂非线性断裂韧度测试方法,及获得的非线性断裂韧度测试结果,为非线性理论研究和工程应用提供了依据.     

动载作用下复合型裂纹扩展的近场动力学模拟

构建改进的非局部近场动力学模型和粒子系统动力加载算法,开展复合型裂纹动力扩展过程模拟。在常规近场动力学微极模型中引入能反映非局部长程力尺寸效应的核函数修正项,以提高计算精度和收敛稳定性。通过模拟动载作用下含复合型裂纹混凝土三点弯梁的裂纹扩展与破坏过程并与试验结果对比,验证了本文模型和算法的可靠性。在此基础上,通过分组模拟分析了动载作用下,初始裂纹的位置、长度和方向对含复合型裂纹三点弯梁破坏模式、起裂时间与承载能力的影响规律。     

基于扩展有限元法的混凝土细观断裂破坏过程模拟

扩展有限元法（XFEM）是分析不连续力学问题（特别是断裂问题）的一种有效的数值方法。在常规的有限元位移模式中,基于单位分解的思想加入一个跳跃函数和渐进缝尖位移场来对不连续体附近的节点自由度进行局部加强,从而反映了位移的不连续性。介绍了扩展有限元的基本原理,给出了扩展有限元进行混凝土开裂及裂纹扩展的分析方法,最后采用扩展有限元法模拟了湿筛混凝土单轴拉伸作用下及WinklerL-型混凝土板的细观断裂破坏过程。分析了混凝土裂纹萌生、扩展的过程及破坏形态,数值结果与实验结果吻合良好。研究表明：扩展有限元法通过特定的位移模式,使裂纹两侧不连续位移场的表达独立于网格划分,能有效地模拟混凝土材料细观断裂破坏过程。     

CTS试样Ⅰ/Ⅱ混合型断裂特性数值计算研究

采用CTS试样研究Ⅰ/Ⅱ混合型断裂特性计算裂纹前缘应力强度因子时可采用解析公式,一旦裂纹发生扩展,解析公式便不再适用.文中采用有限元法研究紧凑拉伸剪切(CTS)试样在Ⅰ/Ⅱ平面混合型加载下的裂纹扩展行为.采用ANSYS建立CTS试样Ⅰ/Ⅱ混合型测试系统有限元模型,为模拟真实受力状态,在CTS试样-销-扇型夹具以及扇型夹具-销-U型夹具之间分别建立接触对进行接触力学分析.通过与解析公式结果进行对比验证了该数值方法的可靠性.采用最大环向应力准则(MTS),模拟了CTS试样不同加载角度下的裂纹扩展路径,获得了裂纹扩展路径中应力强度因子随裂纹长度的变化曲线,解释了裂纹扩展路径不与外载荷方向垂直的原因.结合文中计算结果,在CTS试样Ⅰ/Ⅱ混合型裂纹扩展速率实验测得裂纹长度与寿命的关系曲线a-N的基础上,便可得到材料Ⅰ/Ⅱ型混合型裂纹扩展速率曲线.     

复合型裂纹断裂的新准则

以Ⅰ-Ⅱ复合型裂纹为研究对象,对裂纹尖端的塑性区分布规律进行了理论分析.引入两组评价裂纹尖端应力场对裂纹扩展影响的参数,考虑裂纹尖端存在的局部塑性变形,并采纳如下两个假设,(1)裂纹沿最短路径穿过塑性区向弹性区扩展,(2)当在扩展方向上的弹塑性边界极半径r大于其临界极半径rc时,裂纹开始扩展.在此基础上,导出了新的复合型裂纹断裂准则,并与现有部分断裂准则及实验结果进行了对比.结果表明:新建立的复合裂纹断裂准则与实验结果吻合程度非常高.文中还分析了裂纹尖端应力场对复合断裂的影响机制,阐述了以单一KⅠc或KⅡc建立的复合裂纹扩展准则的局限性以及考虑裂纹尖端应力场的必要性.     

无限冰介质中爆炸裂纹扩展机理与数值模拟

根据冰体力学性能,提出了无限淡水冰介质在爆炸冲击载荷下的压碎区及裂隙区的理论计算方法.利用大型有限元软件ANSYS/LS-DYNA模拟了爆炸载荷下无限区域冰体内裂纹扩展过程,模拟结果与实验结果基本一致,表明采用的损伤断裂模型能够较准确的反映无限冰介质动态力学性能及裂纹成形过程.数值模型可近似计算冰体中裂隙区的半径.数值计算值与理论计算值误差不超过5%,数值模拟结果与实验结果吻合,从而确定了无限区域冰体在-15℃时的基本破坏特征及范围.     

微波照射下脆性岩石裂纹扩展临界条件及断裂过程研究

基于热力学定律及Griffith断裂理论,建立了包含初始裂纹的脆性岩石理论模型研究微波破岩过程,推导了微波照射下均匀脆性岩石内部初始微裂纹的临界扩展条件。得到微波照射参数与裂纹扩展半径的关系呈V形曲线,且V形曲线上方为裂纹非稳定区域,下方为裂纹稳定区域。将V形曲线左半部分对应的裂纹定义为"初始短裂纹",断裂开始后,裂纹发生动态扩展,此时裂纹扩展行为不能由V形曲线完全描述,动态扩展的最终半径由能量守恒定律推导得到;当输入微波能量达到新的临界值之后,"初始短裂纹"则继续沿V形曲线发生准静态扩展。将V形曲线右半部分对应的裂纹半径定义为"初始长裂纹",对于初始长裂纹,输入的微波能量达到临界值之后,裂纹始终沿V形曲线以准静态方式扩展。根据裂纹扩展行为可以预测微波照射下岩石强度折减规律,其结果与已有实验结果吻合较好,对于指导微波辅助机械破岩工程实践具有重要意义。     

基于扩展有限元的地质聚合物混凝土断裂过程研究

扩展有限元法是基于常规有限元框架分析裂纹等不连续力学问题的一种有效数值方法,在常规的有限元位移表达式中,增加了能够反映位移不连续性的跳跃函数和渐进缝尖位移场函数来对不连续结构附近的节点自由度进行局部加强。本文介绍了扩展有限元法及粘聚力模型的基本原理,给出了基于扩展有限元法的地质聚合物混凝土断裂过程分析方法。分别采用四种不同的软化曲线对I型缺口地质聚合物混凝土梁从裂纹萌生、扩展直至断裂破坏的全过程进行了模拟,并基于双K断裂准则分析了其断裂韧性。结果表明,Petersson模型与试验结果吻合较好,最后基于模拟结果进一步揭示了断裂过程区的演化过程。     

破片撞击下YAG透明陶瓷复合靶的破坏特性

YAG透明陶瓷兼具有优秀的透光性能和抗冲击破坏性能,是武器装备透明部分的优秀防护材料,在军事装备、航天等国防领域具有良好的应用前景。冲击载荷下材料的加载响应特性对掌握材料破坏机制至关重要,能为透明复合靶设计提供依据。为获得YAG透明陶瓷多层复合靶的冲击破坏特性,利用内径9 mm的气体驱动发射平台进行了碳化钨球形破片在20～310 m/s速度下撞击YAG透明陶瓷复合靶的实验,通过高速摄影捕捉的陶瓷表面损伤演化过程,计算了典型径向、环向裂纹扩展速度。通过观测回收的靶体和YAG碎片的宏细观破坏特征,分析了撞击速度与靶体破坏特征之间的联系。结果表明,YAG陶瓷层径向裂纹和环向裂纹扩展速度均随着时间的延长线性降低,且裂纹扩展速度几乎不受撞击速度影响。陶瓷层中心粉碎区面积随撞击速度的提高而增大,且中间玻璃层破坏区域面积与陶瓷锥底面积相关联,陶瓷锥角与撞击速度关联性不强。同时,观察到陶瓷层在冲击破坏过程中出现了裂纹簇,获得了裂纹簇数量与破片撞击速度之间的关系,分析了裂纹簇的特征及其成因。裂纹变向、应力波作用会显著影响细观断面破坏特征。径向、环向和锥裂纹中沿晶断裂的比例均随着裂纹扩展距离的增大而增加,且穿晶比例随着撞击速度的提高而增加。     

航空结构铝合金Ⅰ/Ⅱ复合型断裂的三维弹塑性数值分析

在LC4CS铝合金三维Ⅰ/Ⅱ复合型断裂实验的基础上,对Ⅰ/Ⅱ复合型加载模型进行了细致的三维弹塑性有限元计算,系统分析了不同复合度φ和厚度B对裂纹端部应力场和起裂角的影响.结果表明,对于不同φ和B的试样,周向应力极大值和三维离面约束因子极小值出现在相同的角度;根据最大周向应力准则,在Ⅰ型载荷占主导(βeq＞65.45°)...     

FEM analysis on mixed-mode fracture of CSM-GRP

A FEM analysis for studying mixed-mode fracture problem of chopped strand mat glass fibre reinforced polyester laminate is presented. The analysis is formulated on the basis of 8-node quadrilateral isoparametric element. The collapsed triangular quarter-point singular elements were used for calculating stress intensity factors K_Ι and K_Ⅱ.The crack propagation process was computed by implementing constraint release technique. Three different approaches to the solution of stress intensity factors K_Ι and K_Ⅱ were compared. The effect of constraint condition imposed upon the displacement of the three collapsed nodes of the crack tip elements on the K_Ι and K_Ⅱ results was evaluated. The mixed-mode critical stress intensity factors K_ΙC and K_ⅡC were estimated for CSM-GRP through the consideration of K_Ι and K_Ⅱ calculated and the measured failure load and critical crack length in the experiment.     

混凝土复合型裂纹扩展的非局部近场动力学建模分析

基于非局部近场动力学理论,构建了修正的能反映混凝土宏观拉压异性和断裂特征的近场动力学本构模型,开发了相应的离散、加载和时间积分算法,实现典型混凝土构件中复合型裂纹扩展过程模拟。在物质点对尺度上定义局部损伤并考虑物质点对的相对转动,通过求解时空微-积分方程实现裂纹的自然萌生与扩展,避免裂尖不连续带来的求解奇异性、网格依赖性和网格重构以及常规近场动力学本构模型的泊松比限制。通过含单边和双边初始裂纹四点剪切混凝土梁裂纹扩展破坏全过程模拟,得到破坏形态、破坏荷载以及完整的荷载-裂纹开口滑移曲线,并与试验和其他数值模拟结果对比,验证了模型的精确性和算法的稳定性。     

A unified potential-based cohesive model of mixed-mode fracture

A generalized potential-based constitutive model for mixed-mode cohesive fracture is presented in conjunction with physical parameters such as fracture energy, cohesive strength and shape of cohesive interactions. It characterizes different fracture energies in each fracture mode, and can be applied to various material failure behavior (e.g. quasi-brittle). The unified potential leads to both intrinsic (with initial slope indicators to control elastic behavior) and extrinsic cohesive zone models. Path dependence of work-of-separation is investigated with respect to proportional and non-proportional paths—this investigation demonstrates consistency of the cohesive constitutive model. The potential-based model is verified by simulating a mixed-mode bending test. The actual potential is named PPR (Park-Paulino-Roesler), after the first initials of the authors’ last names.     

The strain energy criterion of mixed-mode brittle fracture

In this paper, we proposed a new cr tenon or mixea-moae brittle fracture, i.e., the strain energy criterion, which can be stated as (K_Ⅰ/K_Ⅰc)² +(K_Ⅰ/K_Ⅰc)²+(K_Ⅱ/K_Ⅱc)²=1. This criterion is shown to be in good agreement with known experimental data.In this paper, an experimental criterion:(K_Ⅰ/K_Ⅰc)^m+(K_Ⅱ/K_Ⅱc)ⁿ=1, 1≤_n^m≤2.is also proposed.     

Experimental evaluation of mixed-mode fracture in adhesive bonds

Previous tests have shown that the mode II and mode III fracture energies of adhesive bonds coincide, with the nominal value dictated by post-yield shear deformation in the interlayer. This suggests that the complete mixed-mode fracture behavior may be dellineated by determining the interaction curve in either theG _I -G _II orG _I -G _III plane. A DCB-type specimen capable of delivering the entire opening versus shearing toughness spectrum in a single test was used. A brittle and a ductile epoxy resin were evaluated, with the adhesive thickness varying from a few micrometers up to 0.6 mm. Excluding very thin bonds, the mixed-mode fracture curve was approximately bilinear; when the applied energy release rate in shear,G _s , was relatively small, the total fracture energy equaledG _IC but otherwise, the fracture curve decreased essentially linearly with increasingG _S . In the case of the ductile adhesive, the transition in trends occurred whenG _S was approximately 55 percent of the shearing fracture energy. When the bond thickness was decreased to a few micrometers, the mixed-mode curve displayed a concave shape, with mode interaction occurring promptly. SEM analysis and analytical considerations suggest that this change in mixed-mode behavior was due to the development of a triaxial state of stress in the interlayer. Based on previous fracture studies of the individual fracture modes in adhesive bonds and laminated composites, the present results should be also applicable to mixed-mode interlaminar fracture of laminated composites.Herzl Chai, formerly associated with Polymers Division, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899     

Study of mixed-mode oscillations in a nonlinear cardiovascular system

Nonlinear Dynamics - Problems related to cardiovascular dynamics and its representation by responses of electric circuit models with a novel estimation method for continuously tracking cardiac...     

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.     

Study of pure and mixed-mode fracture of a brittle material

In order to study the behavior of a crack in a linear-elastic material in plane mixed mode (modes I+II), a specimen's shape and loading have been specially adapted. The specimens are first precracked in mode I and then subjected to monotonic loading until instability is reached by an original device which makes it possible to control the nature of the mixed mode applied and which is adjustable from pure mode I to pure mode II. After the specimens are fractured, the lengths of the initial cracks and the kinking angles were measured in the plane-strain area. Then the stress-intensity factorsK _I andK _II at the moment when crack instability appeared were calculated. This made it possible to apply two criteria: maximum principal stress and maximum stressintensity factorK ₁ ^* at the onset of kinking. From comparing the calculated values with the experimental values we may note that there is good agreement with respect to the crackkinking directions. However, for the limit load values considerable divergences have been recorded which are analyzed.