THREE-DIMENSIONAL ELLIPTIC CRACK UNDER IMPACT LOADING

The dynamic stress intensity factor of a three-dimensionalelliptic crack under impact loading is determined with the finiteelement method. The computation results can take into account theinfluence of time and the ratio of the wave speeds on the stressintensity factor. The present method is suitable not only forthree-dimensional dynamic crack, but also for three-dimensionaldynamic contact.

Discrete Element Modeling of Asphalt Concrete Cracking Using a User-def ined Three-dimensional Micromechanical Approach

We established a user-defined micromechanical model using discrete element method （DEM） to investigate the cracking behavior of asphalt concrete （AC）. Using the ＂Fish＂ language provided in the particle flow code in 3-Demensions （PFC3D）, the air voids and mastics in asphalt concrete were realistically built as two distinct phases. With the irregular shape of individual aggregate particles modeled using a clump of spheres of different sizes, the three-dimensional （3D） discrete element model was able to account for aggregate gradation and fraction. Laboratory uniaxial complex modulus test and indirect tensile strength test were performed to obtain input material parameters for the numerical simulation. A set of the indirect tensile test were simulated to study the cracking behavior of AC at two levels of temperature, i e, -10 ℃ and 15 ℃. The predicted results of the numerical simulation were compared with laboratory experimental measurements. Results show that the 3D DEM model is able to predict accurately the fracture pattern of different asphalt mixtures. Based on the DEM model, the effects of air void content and aggregate volumetric fraction on the cracking behavior of asphalt concrete were evaluated.

Interactions of penny-shaped cracks in three-dimensional solids

The interaction of arbitrarily distributed penny-shaped cracks in three-dimensional solids is analyzed in this paper. Using oblate spheroidal coordinates and displacement functions, an analytic method is devel- oped in which the opening and the sliding displacements on each crack surface are taken as the basic unknown functions. The basic unknown functions can be expanded in series of Legendre polynomials with unknown coefficients. Based on superposition technique, a set of governing equations for the unknown coefficients are formulated from the traction free conditions on each crack surface. The boundary collocation procedure and the average method for crack-surface tractions are used for solving the governing equations. The solution can be obtained for quite closely located cracks. Numerical examples are given for several crack problems. By comparing the present results with other existing results, one can conclude that the present method provides a direct and efficient approach to deal with three-dimensional solids containing multiple cracks.

A modified maximum tangential tensile stress criterion for three-dimensional crack propagation

The three-dimensional （3D） crack propagation is a hot issue in rock mechanics. To properly simulate 3D crack propagation, a modified maximum tangential tensile stress criterion is proposed. In this modified criterion, it is supposed that cracks propagate only at crack front in the principal normal plane. The tangential tensile stress at crack front in the principal normal plane in local coordinates is employed to determine crack propagation, which is calculated through coordinate transformation from global to local coordinates. New cracks will propagate when the maximum tangential tensile stress at crack front in the principal normal plane reaches the tensile strength of rock-like materials. Compared with the previous crack propagation criteria, the modified crack propagation criterion is helpful in calculating 3D crack stress intensity factor, and can overcome the limitations of propagation step determined by individual experiences in previous studies. Finally, the 3D crack propagation process is traced by element-free Galerkin method. The numerical results agree well with the experimental ones for a frozen resin sample with prefabricated 3D cracks.

CONVOLUTION OF THE IMPACT THREE-DIMENSIONAL ELASTO-DYNAMICS AND DYNAMIC STRESS INTENSITY FACTOR FOR AN ELLIPTIC CRACK

This paper presents a formulation for three-dimensional elasto-dynamics with an elliptic crack based on the Laplace and Fourier transforms and the convolution theorem. The dynamic stress intensity factor for the crack is determined by solving a Fredholm integral equation of the first kind. The results of this paper are very close to those given by the two-dimensional dual integral equation method.

A STUDY ON STRESS INTENSITY FACTORS AND SINGULAR STRESS FIELDS OF THREE-DIMENSIONAL INTERFACE CRACK

By using the finite-part integral concepts and limit technique,the hypersingular inte- grodifferential equations ofthree-dimensional(3D)planar interface crack were obtained; then thedominant-part analysis of 2D hypersingular integral was further usedto investigate the stress fields near the crack front theoretically,and the accurate formulae were obtained for the singular stressfields and the complex stress intensity factors.

INTERFACIAL CRACK ANALYSIS IN THREE-DIMENSIONAL TRANSVERSELY ISOTROPIC BI-MATERIALS BY BOUNDARY INTEGRAL EQUATION METHOD

The integral-differential equations for three-dimensional planar interfacial cracks of arbitrary shape in transversely isotropic bimaterials were derived by virtue of the Somigliana identity and the fundamental solutions, in which the displacement discontinuities across the crack faces are the unknowns to be determined. The interface is parallel to both the planes of isotropy. The singular behaviors of displacement and stress near the crack border were analyzed and the stress singularity indexes were obtained by integral equation method. The stress intensity factors were expressed in terms of the displacement discontinuities. In the non-oscillatory case, the hyper-singular boundary integral-differential equations were reduced to hyper-singular boundary integral equations similar to those of homogeneously isotropic materials.

Three dimensional K-Tz stress fields around the embedded center elliptical crack front in elastic plates

Through detailed three-dimensional （3D） finite element （FE） calculations, the out-of-plane constraints Tz along embedded center-elliptical cracks in mode I elastic plates are studied. The distributions of Tz are obtained near the crack front with aspect ratios （a/c） of 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0. Tz decreases from an approximate value of Poisson ratio v at the crack tip to zero with increasing normalized radial distances （r/a） in the normal plane of the crack front line, and increases gradually when the elliptical parameter angle φ changes from 0° to 90°at the same r/a. With a/c rising to 1.0, Tz is getting nearly independent of φ and is only related to r/a. Based on the present FE calculations for Tz, empirical formulas for Tz are obtained to describe the 3D distribution of Tz for embedded center-elliptical cracks using the least squares method in the range of 0.2 ≤ a/c ≤ 1.0. These Tz results together with the corresponding stress intensity factor K are well suitable for the analysis of the 3D embedded centerelliptical crack from field, and a two-parameter K-Tz principle is proposed.

Prediction of Three Dimensional Crack Path Brittle Fracture in Weldment under Dynamic Load

Three dimensional dynamic stress intensity factors are analyzed for a curved crack with a second order perturbation method. The method is extended to obtain an approximate representation of a three dimensional dynamic stress intensity factors at the tip of a curved crack. Due to three dimensional curved crack growth the dynamic energy release rate can be calculated by using the Irwin＇s formula. A three dimensional curved crack in materials with inhomogeneous fracture toughness are considered. Paths of a brittle three dimensional curved crack propagating along a welded joint are predicted via the present method, where the effects of dynamic applied stresses, residual stresses, and material deterioration due to welding are taken into considerations.

Closed form solution of stress intensity factors for cracks emanating from surface semi-spherical cavity in finite body with energy release rate method

In this paper, a new semi-analytical and semi-engineering method of the closed form solution of stress intensity factors （SIFs） of cracks emanating from a surface semi-spherical cavity in a finite body is derived using the energy release rate theory. A mode of crack opening displacements of a normal slice is established, and the normal slice relevant functions are introduced. The proposed method is both effective and accurate for the problem of three-dimensional cracks emanating from a surface cavity. A series of useful results of SIFs are obtained.

EXPERIMENTAL RESEARCH ON STRESS INTENSITY FACTOR K_Ⅲ FOR 3-D CRACK BY CAUSTICS

An experimental technique for determining the anti plane stress intensity factor K Ⅲ of a three dimensional crack, which is very difficult to obtain by other experimental methods, has been presented by using reflected caustics in combination with the stress freezing and stress releasing technique. The results of this experimental method coincided favorably with the theoretical analysis results of Tweed and Rooke.

STRESS INTENSITY FACTORS FOR CORNER CRACKS AT A HOLE UNDER REMOTE TENSION

The three-dimensional weight function method recently developed by the authors is used to determine stress intensity factors for two symmetric quarter-elliptical corner cracks at a hole in a wide finite-thickness plate subjected to remote tensile loading. The geometry parameters considered are r / t = 0.5, 1, 2; a / c= 0.2, 0.5, 1, 2; a / t = 0.2, 0.5 within c/r= 2. The results are compared, where possible, with other solutions available in the literature. Generally good agreement is observed. The effect of an approximation of the two-dimensional unflawed stress distribution on the accuracy of stress intensity factors by the weight function method is discussed.

An extended J-integral for evaluating fluid-driven cracks in hydraulic fracturing

J-integral has served as a powerful tool in characterizing crack tip status. The main feature, i.e. path- independence, makes it one of the foremost fracture parameters. In order to remain the path- independence for fluid-driven cracks, J-integral is revised. In this paper, we present an extended J-in- tegral explicitly for fluid-driven cracks, e.g. hydraulically induced fractures in petroleum reservoirs, for three-dimensional （3D） problems. Particularly, point-wise 3D extended J-integral is proposed to char- acterize the state of a point along crack front. Besides, applications of the extended J-integral to porous media and thermally induced stress conditions are explored. Numerical results show that the extended J- integral is indeed path-independent, and they are in good agreement with those of equivalent domain integral under linear elastic and elastoplastic conditions. In addition, two distance-independent circular integrals in the K-dominance zone are established, which can be used to calculate the stress intensity factor （SIF）.

THE 3-D STRESS INTENSITY FACTOR FOR A HALF PLANE CRACK IN A TRANSVERSELY ISOTROPIC SOLID DUE TO THE MOTION OF LOADS ON THE CRACK FACES

Three-dimensional analysis of a half plane crack in a transversely isotropic solid is performed. The crack is subjected to a pair of normal point loads moving in a direction perpendicular to the crack edge on its faces. Transform methods are used to reduce the boundary value problem to a single integral equation that can be solved by the Wiener-Hopf technique. The Cagniard-de Hoop method is employed to invert the transforms. An exact expression is derived for the mode I stress intensity factor as a function of time and position along the crack edge. Some features of the solution are discussed through numerical results.

喷射混凝土围岩复合体受压力学特性及本构模型

为探究喷射混凝土围岩复合体受压力学特性,结合三维数字图像相关技术,针对4种不同喷射混凝土围岩复合体试件开展了单轴受压试验,探究了纤维和岩石种类对复合体破坏模式和受压性能的影响.基于Weibull统计损伤理论,构建了适用于复合体的受压应力应变本构模型.试验结果表明,不同围岩基复合体破坏模式存在显著差异,但微裂缝均先从喷射混凝土中发展.破坏时,围岩部分表现单一裂纹模式,而喷射混凝土部分呈现多裂纹扩展状态.4种复合体峰值强度均高于单一喷射混凝土破坏强度,且砂岩基复合体的峰值强度均高于花岗岩基复合体,平均高出5.55%.同种围岩条件下,纤维增强喷射混凝土复合体强度会存在轻微降低,较普通喷射混凝土复合体平均下降1.95%.此外,所建本构模型计算结果与试验数据吻合较好.

动载下含表面裂纹岩石破坏的近场动力学模拟

为了研究动态加载下岩石表面裂纹的扩展,基于前期试验结果使用一种考虑压缩和拉伸强度比的弹塑性常规态基近场动力学模型进行数值试验。模拟动态加载下含不同角度和深度三维表面裂纹岩石的破坏过程。模拟结果表明,使用的近场动力学模型可以很好地模拟岩石三维裂纹的扩展和试样的破坏过程。通过近场动力学模拟可以得到与试验结果一致的强度变化规律。试样表面形成的裂纹深入试样内部一定深度,该深度与表面裂纹深度有关。试样内部产生的壳裂纹会与表面形成的裂纹连接形成复杂的三维裂纹。

基于数字图像相关技术的膨胀土三维裂隙扩展特性研究

为探究膨胀土三维裂隙动态扩展特性及机制,搭建恒温恒湿环境箱,通过双目相机实时采集4、8、16mm厚度下矩形和圆形膨胀土裂隙图像,基于三维数字图像相关技术重建裂隙三维形态,量化裂隙扩展机制和发育特性。结果表明:三维裂隙动态拓展特征方面,土体开裂时存在时空各异的收缩中心,收缩中心的数量随着试样厚度的增加而减少,收缩中心数量越多,裂隙网络形态越复杂;收缩中心运动形式呈纵向沉降,沉降量与裂隙土块含水率相关;裂隙土块周围颗粒运动矢量呈碗状朝收缩中心运动。各收缩中心空间分布存在差异,土体收缩时产生剪切应力,部分裂隙发育方向发生偏转,导致裂隙网络衔接呈多样性。膨胀土开裂过程中往往伴随着裂隙土块的翘曲,裂隙土块翘曲形成及运动机制如下:裂隙土块发育形成后,土块整体呈现“外干内湿”湿度差,随之产生的剖面基质吸力差引起了垂直方向收缩变形梯度,进而产生驱动裂隙土块上凹的曲卷力矩并朝着收缩中心聚拢,促使土块翘曲形成。该试验和分析结果可以为膨胀土裂隙发育规律认知、分析和预测方法提供参考。

基于Q235的水下ACFM可视化检测系统设计及研究

针对复杂海水环境中Q235结构物夹板裂纹,基于交流电磁场检测(ACFM)技术开展了水下可视化检测系统的设计及研究。基于COMSOL仿真分析选取了探头外壳材料。搭建硬件电路及设计缺陷识别及重构的软件系统。基于Q235材料样板上不同长度及不同深度的裂纹进行水下检测。结果表明:Q235材料的不同长度裂纹检测中长度误差和深度检测误差分别为8.53%和15.11%;不同深度裂纹检测中长度误差和深度检测误差分别为8.17%和13.9%。

涡轮叶片裂纹方位角的三维叶尖间隙动态响应特性研究

针对叶尖定时故障诊断方法在检测叶片裂纹时仅关注一维信息、提取故障特征有限的不足,利用叶尖间隙具有三维特征的特点,提出了航空发动机涡轮叶-盘-轴耦合系统的三维叶尖间隙(3D-BTC)有限元模型。以该模型为基础,分析了航空发动机涡轮叶片裂纹方位角对三维叶尖间隙的影响规律。首先,建立了航空发动机涡轮叶-盘-轴耦合系统有限元模型;其次,在叶片尾缘处引入贯穿裂纹,通过瞬态动力学分析计算了在不同横滚角、俯仰角裂纹作用下叶片的变形;最后,采用三维叶尖间隙计算方法获取了叶片三维叶尖间隙时域波形及频谱,研究了航空发动机涡轮叶片裂纹方位角的三维叶尖间隙动态响应特性。计算结果表明:裂纹的出现会降低叶片刚度,从而影响叶片的三维叶尖间隙;叶片三维叶尖间隙随裂纹横滚角的增大而减小,随裂纹俯仰角的增大先减小后增大;随着裂纹两类方位角变化,不同测点位置的叶尖径向间隙基本谐波分量峰值变化率的差值最大为6.1%,周向滑移角和轴向偏转角基本谐波分量峰值变化率的差值小于0.12%,可见综合三维叶尖间隙能够更准确地反映裂纹信息。

引江济淮船闸闸首施工期开裂风险与防裂分析

与混凝土坝相比,船闸闸首、泵站类薄壁大体积混凝土结构体型相对较小,但结构型式与受力复杂、约束明显且采用泵送高性能混凝土浇筑,早期混凝土放热量较常态混凝土大且发热快,施工期比大坝更易产生温度裂缝,其防裂设计成为施工质量控制的难点之一。结合在建的引江济淮枞阳船闸上闸首工程,采用三维有限元法仿真分析手段,模拟真实环境、材料、结构、温控措施的影响,分析结构施工期温度场、应力场以及开裂风险,并提出针对性的防裂措施。研究表明,输水廊道、墩墙、空箱及启闭机房周边大体积混凝土等功能性部位为高风险开裂部位,实现控温浇筑、通水冷却以及表面保温的多措并举且差异化控制,能够大大降低其开裂风险,可为引江济淮工程薄壁结构防裂设计提供参考。