地表潜在断错位移的概率评价方法

目前跨活动断层的线状工程的抗断设防一般是以断层位错确定性评价为基础 ,考虑的是最大位错量 ,与抗御灾害的风险设计的实际要求不相符。本文将走滑断层上最大位错点的位置分布及最大位错点两侧的位错展布 ,与可产生地表断错位移的强震复发模型联合 ,构造出评估走滑活动断层各部位地表潜在断错位移的概率性评价方法。最后 ,以鲜水河走滑断裂中带西北部的松林口 -乾宁段为例 ,对其未来 10 0年潜在断错位移的危险性做出定量评估 ,可给出断层段上各点不同超越概率水平下的潜在位移。这一研究结果 ,可为跨断裂的线形工程进行抗御地表潜在断错位移的风险设计提供科学依据.     

裂尖曲率对裂纹前缘塑性区的影响

考虑尖端为圆弧形的钝头裂纹模型，在外围取线弹性无裂纹体的解，应用线场分析方法。形成一套估计钝头裂纹前缘塑性区尺寸的方法。对含径向裂纹和圆弧形裂尖的圆盘受均匀张力作用情况，给出了塑性区的裂纹前缘尺寸与裂纹尖端曲率的关系。得到的结论是，塑性区的裂纹前缘尺寸与裂纹尖端曲率有关；对于给定的塑性区的裂纹前缘尺寸，载荷反比于外缘尺寸的平方。前一结论说明了塑性区的裂前尺寸作为裂纹失稳扩展判断的局限性；后一结论说明了裂纹体强度失效的尺寸效应规律：抗断强度与总体线尺寸的平方成反比。     

弯载下中长圆柱壳表面裂纹的线弹簧模型解

针对含环向表面裂纹的中长圆柱壳,基于薄壳半膜力理论和线弹簧模型,导出了其在弯载作用下的解析解,并给出了相应的表面裂纹前缘的应力强度因子的计算公式以及表面裂纹存在对整个圆柱壳柔度的影响的表达式。研究表明,对于中长圆柱壳中的较长裂纹,裂纹前缘最深点处的应力强度因子对裂纹前缘的形状并不敏感;相应的数值计算结果表明,本文的解与有限元结果的误差不超过3%。     

三维疲劳多裂纹扩展数值模拟的计算方法及程序设计

研究了含多条初始裂纹的紧固孔件的裂纹扩展模拟及裂纹扩展寿命。紧固孔形式为直通孔和沉头(有锪窝)孔,裂纹形式为孔边角裂纹,将裂纹前缘形状简化为二次曲线形式。采用更新网格法追踪裂纹扩展,并人为干预裂纹前缘形状,解决了裂纹前缘端点跨越模型角点时的网格畸变问题;在裂纹扩展法则中使用了AFGROW裂纹闭合模型,并引入多裂纹协调扩展算法,避免了逐一应力循环计算,缩短了计算时间;试件失效判据使用净截面屈服和裂纹前缘塑性区连通准则,试件满足任一判据即判定为失效;最后在ANSYS软件的基础上进行二次开发,设计了适应多种模型的全寿命自动分析程序,并使用VB语言编写了用户界面,分别对单孔和一排五孔两种平板试件从锪窝孔边起裂的角裂纹扩展进行了数值模拟,并与试验结果进行了对比。结果表明:使用AFGROW裂纹闭合模型对寿命的预测精度明显高于Paris模型,其寿命预测误差在5%以内;对位于模型表面的裂纹尺寸的模拟与试验结果吻合较好,并很好地模拟了五孔模型在发生裂纹连通之后加速扩展的情况。     

空心车轴不同深度表面裂纹应力强度因子研究

分析了空心车轴的旋转弯曲载荷的特点,建立了空心轴表面周向半椭圆裂纹的模型,给出了半椭圆裂纹的构形参数定义,即形状比、深度比和裂纹前缘相对位置。采用四分之一20节点等参退化奇异单元,通过有限元计算,模拟裂纹前沿的应力奇异性。在此基础上,计算了裂纹前缘表面点和中心点的应力强度因子随着裂纹扩展深度和旋转角度的变化。计算结果表明,对于给定的裂纹构形,在车轴的一个载荷循环中,裂纹前缘同一相对位置的应力强度因子是不断变化的,不同位置的应力强度因子在达到最大值的角度也是不同的,这就导致了裂纹前缘表面点在一些角度下的扩展是不对称的。这些结果为进一步研究空心轴表面裂纹的扩展路径和寿命提供了参考。     

电磁热效应裂纹止裂研究的进展

利用电磁场的热效应对带有裂纹的载流导体进行裂纹止裂,是达到延长其工作寿命、提高安全性和可靠性的一种行之有效的方法．理论计算和试验观察都证明了：载流导体中裂纹尖端处的热集中效应,能够在附近的一个很小范围内熔化形成微小的焊口,裂纹前缘处的曲率半径瞬间增加了２～３个数量级,显著地减少了应力集中,阻止了干线裂纹源的形成,有效地遏制了裂纹的扩展。本文阐述了这一物理过程的研究现状、进展情况以及未来应用前景．     

焊趾表面裂纹的形态发展曲线与疲劳寿命预测

以作者建立的焊地椭圆表面裂纹应力强度因子数据库以及复杂应力场中焊践半随圆表面裂纹前缘应力强度因子分布计算的基本模式法为基础上,给出了复杂应力场中焊践表面表纹在疲劳扩展过程中形态变化规律及寿命的工程分析方法。     

楔型向错偶极子与含界面裂纹圆形夹杂的弹性干涉

运用共形映照、解析延拓以及柯西型积分运算等复变函数方法研究无限大平面中楔型向错偶极子与界面裂纹、圆形夹杂的弹性干涉问题.求出含一条裂纹应力场的封闭形式解后,推导了裂纹尖端应力强度因子和作用在向错偶极子中心的向错力的表达式,研究了偶极子位置、方向、偶臂长度及材料性质对应力强度因子的影响,并分析了裂纹及夹杂对逐渐靠近的向错偶极子排斥或吸引作用的规律.     

双轴压缩载荷下裂纹动态传播的实验研究

本文研究了双轴压缩载荷下裂纹的起裂和非共面传播问题，用动态光弹性方法成功地记录下裂纹的动态扩展过程。获得了双轴压缩载荷下扩展裂纹的传播速度，临界开裂载荷以及开裂角第一系重要断裂参数。研究表明，在双轴压缩载荷下不闭合裂纹前缘应力场是一个混合型的奇异场，裂纹的扩展过程为稳态过程并且表现出显著的阶段性特点。     

金属材料疲劳损伤的宏细观理论

工程结构的疲劳损伤发展过程经历了由初始缺陷的形成、裂纹的稳态扩展直到最后失效的不同发展阶段,通常疲劳损伤的演化可以概括为以下几个阶段:(1) 亚结构和微观结构的变化引起永久损伤的形成,产生微观裂纹;(2) 微观缺陷的长大会合形成主裂纹;(3) 主裂纹稳态扩展;(4) 结构失稳或完全失效.首先论述了疲劳裂纹扩展的物理机制,并从细观和宏观两个方面总结了处理疲劳裂纹问题的最新研究成果,对位错力学在处理短裂纹扩展问题中的应用,以及无位错区(DFZ)在疲劳裂纹扩展中的作用进行了较详细讨论.      

Interface crack between a compressible elastomer and a rigid substrate with finite slippage

We study the deformation of a crack between a soft elastomer and a rigid substrate with finite interfacial slippage. It is assumed that slippage occurs when the interfacial shear traction exceeds a threshold. This leads to a slip zone ahead of the crack tip where the shear traction is assumed to be equal to the constant threshold. We perform asymptotic analysis and determine closed-form solutions describing the near-tip crack opening displacement and the corresponding stress distributions. These solutions are consistent with numerical results based on finite element analysis. Our results reveal that slippage can significantly affect the deformation and stress fields near the tip of the interface crack. Specifically, depending on the direction of slippage, the crack opening profile may appear more blunted or sharpened than the parabola arising from for the case of zero interfacial shear traction or free slippage. The detailed crack opening profile is determined by the constant shear traction in the slip zone. More importantly, we find that the normal stress perpendicular to the interface can increase or decrease when slippage occurs, depending on the direction of slippage and the shear traction in the slip zone.     

A contact force model in the dynamic analysis of contactable cracks

In this paper a finite element method in the time domain is proposed to analyze the dynamic response of contactable cracks. By this method, the contact normal stress caused by impact or contact of the crack faces can be directly obtained and then the motion of the model containing the contactable cracks can be calculated, and the processes of contact, slip and separation of the crack faces can also be correctly determined and simulated. The method proposed can be used to study the scattering problems of an elastic wave caused by the crack and the dynamic extension problems of the crack arising from impact or cyclic loading. Numerical examples indicate that the method is efficient and accurate enough. It is also shown that the effect of dynamic contact of crack faces on the motion of itself and the area near the crack is significant by comparing the results with the contact considered and neglected.     

Dwell fatigue crack nucleation model based on crystal plasticity finite element simulations of polycrystalline titanium alloys

In this paper a crystal plasticity-based crack nucleation model is developed for polycrystalline microstructures undergoing cyclic dwell loading. The fatigue crack nucleation model is developed for dual-phase titanium alloys admitting room temperature creep phenomenon. It is a non-local model that accounts for the cumulative effect of slip on multiple slip systems, and involves evolving mixed-mode stresses in the grain along with dislocation pileups in contiguous grains. Rate dependent, highly anisotropic behavior causes significant localized stress concentration that increases with loading cycles. The crystal plasticity finite element (CPFE) model uses rate and size-dependent anisotropic elasto-crystal plasticity constitutive model to account for these effects. Stress rise in the hard grain is a consequence of time-dependent load shedding in adjacent soft grains, and is the main cause of crack nucleation in the polycrystalline titanium microstructure. CPFE simulation results are post-processed to provide inputs to the crack nucleation model. The nucleation model is calibrated and satisfactorily validated using data available from acoustic microscopy experiments for monitoring crack evolution in dwell fatigue experiments.     

Kinematics of layered reinforced-concrete planar beam finite elements with embedded transversal cracking

In this work crack formation and development is addressed and implemented in a planar layered reinforced-concrete beam element. The crack initiation and growth is described using the strength criterion in conjunction with exact kinematics of the interlayer connection. In this way a novel embedded-discontinuity beam finite element is derived in which the tensile stresses in concrete at the crack position reaching the tensile strength will trigger a crack to open. Since the element is multi-layered, in this way the crack is allowed to propagate through the depth of the beam. The cracked layer(s) will involve discontinuity in the cross-sectional rotation equal to the crack-profile angle, as well as a discontinuity in the position vector of the layer’s reference line. A bond–slip relationship is superimposed onto this model in a kinematically consistent manner with reinforcement being treated as an additional layer of zero thickness with its own material parameters and a constitutive law implemented in the multi-layered beam element.     

铁中Ⅰ型裂纹裂尖场与裂尖形变机制分析

应用有限元及连续介质力学计算了平面应交条件下体心立方铁中不同取向裂纹的裂尖应力场和弹性应变能密度分布,从宏微观相结合角度分析了裂尖场分布与裂尖微结构演化的相互关联.指出裂尖塑性变形的具体形式与裂尖滑移面上分切应力的大小密切相关,并从能量角度解释了裂尖相变产生的原因,最后结合相关分子动力学研究成果探讨了裂尖奇异性区域.     

螺栓连接件微动疲劳特性分析

工程中钢结构构件经常采用螺栓连接,被连接部位多处于复杂受力状态,单轴疲劳理论已无法满足该形式下构件的寿命评估需求.针对现有理论不足,本文建立了螺栓连接件有限元模型,研究了不同工况下被连接件微动疲劳裂纹萌生位置,并基于临界平面的多轴疲劳理论,比较了四种常用模型的适用性以及预测了连接件的疲劳寿命.结果 表明:(1)裂纹萌生...     

纳米晶体铝在单轴拉伸下的位错堆积对微裂纹扩展的影响

摘要：针对纳米晶体材料,研究了单轴拉伸载荷作用下纳米晶体铝中的裂纹与裂纹尖端发射的位错所形成的滑移面之间的相互作用。通过分布位错法,将裂纹和滑移面等效为均匀分布的连续位错,获得了裂纹面上应力场。并引入裂纹尖端的无位错区,研究了裂纹尖端无位错区对微裂纹的萌生和主裂扩展之间的影响。结果表明,不考虑裂纹尖端无位错区时,裂纹长度较短,会先在晶界处形成微裂纹,主裂纹较长时,主裂纹会直接穿晶扩展。滑移面与裂纹尖端夹角较大时,会增加裂纹尖端发射的位错个数,从而抑制主裂纹的扩展。考虑裂纹尖端无位错区时,无位错区先于晶界处出现微裂纹,通过主裂纹与微裂纹之间位错的相互发射,导致裂纹与尖端处微裂纹汇合,有效加速了主裂纹的扩展。     

Growth of a penny-shaped crack with a nonsmall fracture process zone in a composite

The paper studies the stress rupture behavior of a reinforced viscoelastic composite through which a penny-shaped mode I crack propagates under a constant load. The composite has hexagonal symmetry and consists of elastic isotropic fibers and viscoelastic isotropic matrix. The material is modeled as a transversely isotropic homogeneous viscoelastic medium with effective characteristics. The crack is in the isotropy plane. The ring-shaped fracture process zone at the crack front is modeled by a modified Dugdale zone with time-dependent stresses. The viscoelastic properties of the matrix are characterized using a resolvent integral operator. Use is made of Volterra's principle, the method of operator continued fractions, and the theory of precritical crack growth in viscoelastic bodies. The problem is reduced to nonlinear integral equations. Numerical results are obtained for certain components of the composite, constant volume fractions, and different fracture strengths Translated from Prikladnaya Mekhanika, Vol. 44, No. 8, pp. 45–51, August 2008.     

同震断层三维裂纹扩展波动时域超奇异积分法

应用波动时域超奇异积分法将P波、S波和磁电热弹多场耦合作用下同震断层任意形状三维裂纹扩展问题转化为求解以广义位移间断率为未知函数的超奇异积分方程组问题;定义了广义应力强度因子,得到裂纹前沿广义奇异应力增量解析表达式;应用波动时域有限部积分概念及体积力法,为超奇异积分方程组建立了数值求解方法,编制了FORTRAN程序,以三维矩形裂纹扩展问题为例,通过典型算例,研究了广义应力强度因子随裂纹位置变化规律;分析了同震断层裂纹扩展中力、磁、电场辐射规律.      

Note on arc crack experiencing slip

Under certain loading conditions a crack surface may undergo partial sliding (slip). Such slip may be triggered by non-uniformity of frictional characteristics along the crack surface, variability of applied stresses or curvilinearity of a crack path. In the present work we study the influence of a curvilinear shape of a crack on slip evolution. The analysis is carried out for the case of a two-dimensional circular arc crack. Initiation and propagation of a slip zone is investigated based on the criterion that the shear stress intensity factor vanishes at endpoints of the slip zone. Two case scenarios are studied: first, when slip is attributed to the non-uniform distribution of a coefficient of friction and, second, when slip is initiated by the far field compressive loads. The curvilinear effects are estimated by comparing the obtained solutions with the ones for a straight crack. Analytical expressions for the stress intensity factors (SIFs) derived in this work may also present certain interest of their own.