塑性应变疲劳裂纹扩展研究进展

介绍了两类循环Ｊ积分——Ｄｏｗｌｉｎｇ“实用”ΔＪ定义与Ｔａｎａｋａ等的线积分ΔＪｐａｔｈ定义的研究现状，分析了其用于描述塑性应变疲劳裂纹扩展速率时存在的问题．

裂纹扩展寿命模型的对比研究

目前，已提出了许多种随机载荷谱裂纹扩展模型，但还很少有人对这些模型进行对比研究，并对工程应用提出建议。本文使用战斗机综合任务谱，对标准试件，用不同的裂纹扩展模型进行实际测算，并与试验值比较。通过研究得出了一些有益的结论。

半平面多边缘裂纹反平面问题的奇异积分方程

利用复变函数和奇异积分方程方法,求解弹性范围内半平面多边缘裂纹的反平面问题．提出了满足半平面边界自由的由分布位错密度表示的单边缘裂纹的基本解,此基本解由主要部分和辅助部分组成．将半平面多边缘裂纹问题看作是许多单边缘裂纹问题的叠加,建立了一组Cauchy型奇异积分方程．然后,利用半开型积分法则求解该奇异积分方程,得到了裂纹端处的应力强度因子．最后,给出了几个数值算例．

反平面弹性中边缘内分叉裂纹的奇异积分方程解法

利用复变函数和奇异积分方程方法,求解反平面弹性中半平面边缘内分叉裂纹问题。提出了满足半平面边界自由的由分布位错密度表示的半平面中单裂纹的基本解,此基本解由主要部分和辅助部分组成。将半平面边缘内分叉裂纹问题看作是许多单裂纹问题的叠加,建立了以分布位错密度为未知函数的Cauchy型奇异积分方程组。然后,利用半开型积分法则求解奇异积分方程,得到了裂纹端处的应力强度因子。文中给出两个数值算例的计算结果。

混凝土结构断裂损伤的概率分析

混凝土结构的断裂破坏是混凝土中裂缝损伤累积的结果。由于材料性能和荷载等因素的随机性,裂缝的扩展过程也表现出随机的特征。本文应用马尔柯可夫随机过程理论实现了对这一过程的概率分析,并给出了状态转移概率矩阵的确定方法。最后以简单算例说明了文中方法的具体应用。

钢筋混凝土结构裂纹扩展数值模拟

通过FRANC 2D软件模拟裂纹扩展,得出了应力和应力强度因子.对应力的影响因素研究发现:相对保护层厚度对钢筋周围混凝土的最大拉应力影响较大.通过应力和应力强度因子可以确定裂纹起始点,从而进行钢筋混凝土裂纹扩展模拟,以对实际结构中裂纹的扩展提供参考.

Anti-Plane Elasticity Problem and Mode Ⅲ Crack Problem of Cubic Quasicrystal

The fracture theory of cubic quasicrystal was developed. The exact analytic solution of a Mode Ⅲ Griffith crack in the material was obtained by using the Fourier transform and dual integral equations theory, and so the displacement and stress fields, the stress intensity factor and strain energy release rate were determined. The results show that the stress intensity factor is independent of material constants, and the strain energy release rate is dependent on all material constants. These provide important information for studying the deformation and fracture of the new solid material.

STUDY ON CRACKED PLATES WITH BONDED PATCHES UNDER IN PLANE BIAXIAL LOADINGS

Based on the finite element method, a numerical investigation into the bonded repair efficiency of cracked plates under in plane biaxial loadings is presented. The main considerations are: reduction in stress intensity factor (SIF) at the crack tip, the maximum tensile stress in the composite patch and the maximum shear stress in the adhesive bond between the patch and the plate. Without the patch, a tensile or compressive stress parallel to the crack has no effect on the SIF at the crack tip. While with a composite patch, there exists coupling effect between the normal stress parallel to the crack and the SIF, and the coupling effect depends significantly on ply orientation of the patch and the biaxial stress ratio of the plate.

Fracture properties of epoxy asphalt mixture based on extended finite element method

Crack is found to be a major distress that affects the performance of the epoxy asphalt pavement.An extended finite element method was proposed for investigating the fracture properties of the epoxy asphalt mixture.Firstly,the single-edge notched beam test was used to analyze the temperature effect and calculate the material parameters.Then,the mechanical responses were studied using numerical analysis.It is concluded that 5℃ can be selected as the critical temperature that affects the fracture properties,and numerical simulations indicate that crack propagation is found to significantly affect the stress state of the epoxy asphalt mixture.The maximum principal stress at the crack surface exhibits different trends at various temperatures.Numerical solution of stress intensity factor can well meet the theoretical solution,especially when the temperature is lower than 5℃.

Interaction of a circular cavity and a beeline crack in right-angle plane impacted by SH-wave

The problem of scattering of SH-wave by a circular cavity and an arbitrary beeline crack in right-angle plane was investigated using the methods of Green's function,complex variables and muti-polar coordinates.Firstly,we constructed a suitable Green's function,which is an essential solution to the displacement field for the elastic right-angle plane possessing a circular cavity while bearing out-of-plane harmonic line source load at arbitrary point.Secondly,based on the method of crack-division,integration for solution was established,then expressions of displacement and stress were obtained while crack and circular cavities were both in existence.Finally,the dynamic stress concentration factor around the circular cavity and the dynamic stress intensity factor at crack tip were discussed to the cases of different parameters in numerical examples.Calculation results show that the crack produces adverse engineering influence on both of the dynamic stress concentration factor and the dynamic stress intensity factor.

Finite element analysis of road structure containing top-down crack within asphalt concrete layer

In this paper, a four-layered road structure containing a top-down crack is investigated by performing finite element analyses in ABAQUS. In this study, in addition to the vertical load of a vehicle wheel, the horizontal load as well as its position with respect to the crack is also considered in the analyses, and the crack tip parameters including stress intensity factors(SIFs) and T-stress are then calculated. Moreover, influence of elastic modulus and thickness of the pavement layers on the crack tip parameters is studied. Results show that the horizontal and vertical loads along with their position with respect to the crack, elastic modulus and thickness of the road layers influence the crack tip parameters(KⅠ, KⅡ and T-stress) significantly. It was also found that for the cases that the vehicle wheel is positioned near the crack plane, only the shear deformation mode is observed at the crack tip;while, for the vehicle wheel positions far from the crack, only the opening mode is observed, and between these positions, both the opening and shear deformation modes(i.e., mixed mode Ⅰ/Ⅱ) are observed at the crack tip.

Bond interface crack propagation of fresh foundation concrete and rock under blasting load

According to concrete age, the dynamic stress intensity factors of bond interface crack of concrete-rock was calculated. Result shows that the propagation of concreteinterface crack is mainly caused by tensile stress and shear stress for stress wave reflection. With the growth of concrete age, interface crack fracture toughness increases, and itscapacity of resisting blasting load strengthens. Therefore, blasting vibration should bestrictly controlled for fresh concrete.

Dynamic Crack Propagation Analysis Using Scaled Boundary Finite Element Method

The prediction of dynamic crack propagation in brittle materials is still an important issue in many engineering fields. The remeshing technique based on scaled boundary finite element method(SBFEM) is extended to predict the dynamic crack propagation in brittle materials. The structure is firstly divided into a number of superelements, only the boundaries of which need to be discretized with line elements. In the SBFEM formulation, the stiffness and mass matrices of the super-elements can be coupled seamlessly with standard finite elements, thus the advantages of versatility and flexibility of the FEM are well maintained. The transient response of the structure can be calculated directly in the time domain using a standard time-integration scheme. Then the dynamic stress intensity factor(DSIF) during crack propagation can be solved analytically due to the semi-analytical nature of SBFEM. Only the fine mesh discretization for the crack-tip super-element is needed to ensure the required accuracy for the determination of stress intensity factor(SIF). According to the predicted crack-tip position, a simple remeshing algorithm with the minimum mesh changes is suggested to simulate the dynamic crack propagation. Numerical examples indicate that the proposed method can be effectively used to deal with the dynamic crack propagation in a finite sized rectangular plate including a central crack. Comparison is made with the results available in the literature, which shows good agreement between each other.

Stress-drop effect on brittleness evaluation of rock materials

Uniaxial or triaxial compression test of cylindrical rock specimens using rock mechanics testing machine is a basic experimental method to study the strength and deformation characteristics of rock and the development process of rock fracture. Extensive literature review has been conducted on this issue;meanwhile, experimental and numerical studies have been conducted on the stress-drop effect on the brittleness of rock materials. A plastic flow factor of λ is proposed to describe the stress-drop effect. Evaluation methods of the factor λ corresponding to the four yield criteria of rock mass are proposed. Those four yield criteria are Tresca criterion, von-Mises criterion, Mohr-Coulomb criterion and Drucker-Prager criterion. For simplicity purposes, an engineering approximation approach has been proposed to evaluate the stress-drop with a non-zero strain increment. Numerical simulation results validated the effectiveness of the plastic flow factors λ as well as the engineering approximation approach. Based on the results in this study, finite element code can be programmed for brittle materials with stress-drop, which has the potential to be readily incorporated in finite element codes.

Wing crack propagation model under high hydraulic pressure in compressive-shear stress state

A new wing crack model subjected to hydraulic pressure and far-field stresses was proposed considering the effect of hydraulic pressure in wing crack and the connected part of the main crack on the stress intensity factor at the wing crack tip. With the equivalent crack length Ieq of the wing crack introduced, the stress intensity factor Kl at the wing crack tip was as- sumed to the sum of two terms： on one hand a component K1^（1） for a single isolated straight wing crack of length 21, and subjected to hydraulic pressure in the wing crack and far-field stresses; on the other hand a component K1（2） due to the effective shear stress induced by the presence of the equivalent main crack. The lateral tensile stress and hydraulic high pressure are the key factors that induce crack propagation unsteadily. The new wing crack theoretical model proposed can supply references for the study on hydraulic fracture in fractured masses, hydraulic fracturing in rock masses.

Defect assessment of welded specimen considering weld induced residual stresses using SINTAP procedure and FEA

The defect assessment in butt-welded joint of ASTM A36 steel plates and 7075-T7351 aluminum alloy plates containing transverse through thickness crack was analyzed using SINTAP procedure and FEA incorporating weld induced residual stresses. Weld induced longitudinal residual stress profile can be obtained through SINTAP procedure, FEA or experimental analysis. This residual stress profile can be fitted with the trapezoidal residual stress profile available in SINTAP. For three different cases, crack length and residual stress intensity factor （SIF） are calculated and its comparison with the results obtained through FEA is plotted with respect to crack length. The stress intensity factor for mechanical loading is also plotted in the same graph. Using this graphical plot, the total SIF, including residual stress and mechanical loading, can be calculated for any particular crack size. The total SIF can be compared with the fracture toughness of the material for damage tolerance analysis. Also a failure assessment diagram is drawn for welded 7075-T7351 aluminum alloy plates with different crack sizes for as-welded （only residual stress） and mechanical loading along with the existing weld induced residual stresses to show the safety level for a particular crack size and mechanical loading.

Analysis of insidious fault activation and water inrush from the mining floor

Based on the stress field distribution rule of the mining floor under abutment pressure, we have established a simplified mechanical model, which contains multiple factors relating to activation and evolution of insidious water-conductive faults. The influence of normal and shear stresses on fault activation and effective shear stress distribution in the fault plane was acquired under mining conditions.Using fracture mechanics theory to calculate the stress intensity factor of an insidious fault front, we have derived the criterion for main fault activation. Results indicate that during the whole working face advance, transpressions are exerted on fault planes twice successively in opposite directions. In most cases, the second transpression is more likely to lead to fault activation. Activation is influenced by many factors, predominant among which are: burial depth of the insidious fault, friction angle of the fault plane, face advance direction and pore water pressure. Steep fault planes are more easily activated to induce a sustained water inrush in the face.

Simulation of hydraulic fracture utilizing numerical manifold method

A 2nd order numerical manifold method(NMM) based method is developed to simulate the hydraulic fractures propagating process in rock or concrete. The proposed method uses a weak coupling technique to analyze the fluid phase and solid phase. To study the seepage behavior of the fluid phase, all the fractures in solid are identified by a block cutting algorithm and form a flow network. Then the hydraulic heads at crack ends are solved. To study the deformation and destruction of solid phase, the 2-order NMM and sub-region boundary element method are combined to solve the stress-strain field. Crack growth is controlled by the well-accepted criterion, including the tension criterion or Mohr-Coulomb criterion for the initialization of cracks and the maximum circumferential stress theory for crack propagation. Once the crack growth occurs, the seepage and deformation analysis will be resolved in the next simulation step. Such weak coupling analysis will continue until the structure becomes stable or is destructed. Five examples are used to verify the new method. The results demonstrate that the method can solve the SIFs at crack tip and fluid flow in crack network precisely, and the method is effective in simulating the hydraulic facture problem. Besides, the NMM shows great convenience and is of high accuracy in simulating the crack growth problem.

From monoscale to multiscale modeling of fatigue crack growth:Stress and energy density factor

The formalism of the earlier fatigue crack growth models is retained to account for multiscaling of the fatigue process that involves the creation of macrocracks from the accumulation of micro damage.The effects of at least two scales,say micro to macro,must be accounted for.The same data can thus be reinterpreted by the invariancy of the transitional stress intensity factors such that the microcracking and macrocracking data would lie on a straight line.The threshold associated with the sigmoid curve disappears.Scale segmentation is shown to be a necessity for addressing multiscale energy dissipative processes such as fatigue and creep.Path independency and energy release rate are monoscale criteria that can lead to unphysical results,violating the first principles.Application of monoscale failure or fracture criteria to nanomaterials is taking toll at the expense of manufacturing super strength and light materials and structural components.This brief view is offered in the spirit of much needed additional research for the reinforcement of materials by creating nanoscale interfaces with sustainable time in service.The step by step consideraton at the different scales may offer a better understanding of the test data and their limitations with reference to space and time.

Transient near tip fields in crack dynamics

Transient effects of stress-strain fields in the vicinity of a stationary crack tip under high rate loads are discussed.Exact analytical solutions to near tip stresses are compared to fields prescribed by leading terms(one or several) of Williams asymptotic expansion.Influence of load application mode,time(or,which is the same,distance from a crack tip) and Poisson's ratio on this discrepancy is extensively examined.Some effects connected with crack tip propagation speed are also discussed.Significant inconsistencies between real(or received in numerical solutions of state equations-e.g.finite element computations) crack tip fields and stress intensity factor(SIF) singular field observed by numerous researchers are explained.The scope of problems where SIF field can be used for correct prediction of dynamic stress-strain fields in the crack tip region is established.Possibility to correctly approximate fields that are not SIF dominated,accounting additional terms of Williams expansion,is studied.