Study of Mode I Fracture of Graphene Sheets Using Atomistic Based Finite Element Modeling and Virtual Crack Closure Technique

In this letter a finite element based atomistic model is proposed. This model was employed to study the mode I fracture characteristics of graphene monolayers. The proposed model was found to be less numerically intensive with fewer degrees of freedom in comparison to other numerical techniques, such as Monte Carlo and molecular dynamics modeling. An approach based on the virtual crack closure technique was employed to develop the atomistic formulation for estimating strain energy release rates for graphene sheets under opening mode.     

Finite Element Analysis of the Competition Between Crack Deflection and Penetration of Fiber-Reinforced Composites Using Virtual Crack Closure Technique

Crack deflection along the fiber/matrix interface for fiber-reinforced composites is an important condition upon which the toughening mechanisms depend. Sound control for the interface debonding of composites contributes to improving the fracture toughness of composites. Combined with the virtual crack closure technique, a finite element model of composites is proposed to predict the competition between the matrix crack deflection along the interface and the matrix crack penetration into the fibers under the thermomechanical coupling fields. For C/C composites, the effects of the geometry size, fiber volume fraction, fiber coating materials and thermal mismatch on the energy release rate and the crack deflection mechanisms are studied. Results show the fiber coating increases the ability to deflect at large thermal mismatch and small crack sizes, and the TaC coating shows larger effect than the SiC coating. The research provides fundamental method for promoting the toughening design of C/C composites.     

Finite Element Analysis of Delamination Behaviors of Composite Laminates under Hygrothermal Environment Using Virtual Crack Closure Technique

Journal of Failure Analysis and Prevention - This research aims to study the delamination behaviors of T700/8911 composite laminates under hygrothermal environment. For two mixed-mode I/II...     

Effect of Specimen Geometry on Stress Intensity Factors of Inclined Crack by Finite Element Method

A rectangular plate with inclined cracks of different crack lengths at different crack inclination angles under biaxial loading condition are being analyzed in mixed mode condition using finite element method (FEM) for the determination of stress intensity factors (SIFs). With increases of the width of the plate and the crack length ratio, SIF increases up to 45° of crack inclination angle and then decreases; maximum value is obtained at 45° of crack inclination angle. With the increasing value of size factor, the value of SIF starts decreasing. The accuracy of the results of the proposed method is validated by comparing with the previously obtained results by theoretical and experimental methods. The FEM results give significant result for the two-dimensional mixed mode loading conditions.     

某型特种弹尾翼片的有限元分析

利用具有非线性分析能力的Marc2000/Mentat2000有限元分析系统,对尾翼片进行有限元分析,动态模拟了尾翼片的装配和贮存过程的应力应变状态;根据分析结果,提出了尾翼片与固定裙装配部位应进行结构上的改进.     

Fatigue Fracture of a Cast Inconel 718 by Compression Loading and Its Verification by Finite Element Analysis (FEA)

Two out of three struts of a drilling tool’s 6.75″ Flow Diverter failed during operation. This failure resulted in invasion of mud into the electronic module of the tool. During drilling, high stick slip was observed. Measurement While Drilling (MWD) signal was good and Logging While Drilling (LWD) data was used to pick the oil/water contact point. At the end of the run, cracks were observed on the radii of the struts. Failure investigations were performed to identify the cause of the cracks. Material characterization, microstructural examinations, and fractography by SEM technique revealed that fatigue under compression forces was the cause of the cracking of the struts. Finite elemental analysis (FEA) was also used to determine the magnitude and the area of stress concentration. Failure mechanism has been explained. A few issues related to integrity of the material have also been raised.     

Effect of Closure of a Crack Caused by the Roughness of Its Surfaces on Fatigue Fracture under the Conditions of Transverse Shear

We develop a model aimed at the prediction of propagation of mode II fatigue cracks with regard for the interaction of their lips caused by the roughness of the fracture surfaces. In this model, the relationship between normal and shear contact stresses is described by the Amonton's law of friction and the plastic yield of the material in the prefracture zone is taken into account with the help of the model of thin plastic strips. The method of singular integral equations is used to solve the corresponding boundary-value problem for a plate with cracks propagating from two semiinfinite collinear notches. The distribution of contact stresses is determined and the stress intensity factors and displacements of the crack lips are evaluated. The proposed example is used to analyze the basic specific features of the influence of contact of the crack lips on fatigue fracture under the action of shear loads. The obtained results are confirmed by the experimental data on the propagation of mode II fatigue cracks in specimens of HY-130 steel.     

Influence of Boundary Conditions on the Finite Element Creep Stress and Strain Analysis of a Double Shear Specimen for Isotropic Creep Conditions

The present study investigates the influence of boundary conditions on creep finite element stress and strain results for a double shear creep specimen which was developed by Mayr et al. [1]. The results of the study show that a preliminary analysis [2], which was performed using the finite element code ABAQUS using RIGID SURFACE as a boundary condition for loading is conservative, because it predicts that a homogeneous state of shear stress is only maintained up to shear strains of the order of 10%. Using a more realistic boundary condition for loading (ABAQUS option: CONTACT PAIR) it can be shown that the homogeneous state of shear stress in the specimen is maintained throughout creep up to much higher shear strains than originally estimated. Further anisotropic finite element creep stress analysis of our double shear creep specimen will therefore be based on this more realistic loading condition.     

铝合金圆板上碳化硅涂层热应力有限元分析

假设涂层和基体界面处于理想结合状态下,且不考虑涂层中缺陷的影响,采用有限元软件(ANSYS8.0)分析了5～30μm厚碳化硅涂层中的热变形和热应力.结果表明,在平面法线方向(z方向)上,涂层/基体系统在热应力作用下发生热屈曲,圆心处z方向热变形为0.05mm,而在边缘处2方向热变形为-0.08mm;热变形呈现轴对称的特点,其危险区域在上下表面的圆心部位,该处的热变形最大,也最容易造成该处涂层胀裂失效;对于不同直径的圆板,发生热屈曲时均存在一个类似的z方向零位移环,并且该z方向零位移环的位置与圆盘半径有关,而与涂层厚度无关;计算得出5～30μm厚碳化硅涂层中的热应力约为2.45～11.00GPa,该值远高于1mm厚4043铝合金基体中产生的热应力(24.68MPa);圆板热屈曲后拱起高度和热应力均随涂层厚度的增加而增加.     

SAP84有限元软件在复杂高层剪力墙结构动静力分析问题上的应用

本文对SAP84有限元软件系统在解算复杂高层剪力墙结构问题上,作了若干处理。对结构模型化问题,应用软件系统的子结构功能及求解过程等作了阐述。在国产长城0520A微机上实施了25层复杂高层剪力墙结构的动静力分析,并取得了满意的计算结果。     

A Numerical Comparison of Finite Difference and Finite Element Methods for a Stochastic Differential Equation with Polynomial Chaos

A numerical comparison of finite difference (FD) and finite element (FE) methods for a stochastic ordinary differential equation is made. The stochastic ordinary differential equation is turned into a set of ordinary differential equations by applying polynomial chaos, and the FD and FE methods are then implemented. The resulting numerical solutions are all non-negative. When orthogonal polynomials are used for either continuous or discrete processes, numerical experiments also show that the FE method is more accurate and efficient than the FD method.     

Superconvergence of Finite Element Methods for Optimal Control Problems Governed by Parabolic Equations with Time-Dependent Coefficients

In this article, a fully discrete finite element approximation is investigated for constrained parabolic optimal control problems with time-dependent coefficients. The spatial discretisation invokes finite elements, and the time discretisation a nonstandard backward Euler method. On introducing some appropriate intermediate variables and noting properties of the $L^2$ projection and the elliptic projection, we derive the superconvergence for the control, the state and the adjoint state. Finally, we discuss some numerical experiments that illustrate our theoretical results.     

修正结构有限元模型的一种方法

本文讨论了一个寻找结构有限元模型的误差源和对它进行修正的方法.为了减少待修正模型的未知参数数目,将质量、刚度矩阵描述为设计变量的函数.在此基础上,先用少量测得的静位移数据修正刚变矩阵;然后用少数几阶实验的固有频率、固有振型数据修正质量矩阵.为了避免测量旋转自由度的静位移和固有振型分量的困难,还讨论了聚缩模型的修正问题.最后,文中附有算例及结果.     

全金属反射光学系统结构的有限元分析

用有限元分析方法对全金属反射光学系统在各种受力状态下的变形、应力及固有频率做了全面的分析，并给出了结构多次优化后的分析结果。     

Error Estimation of Nanoindentation Mechanical Properties Near a Dissimilar Interface via Finite Element Analysis and Analytical Solution Methods

Nanoindentation methods are well suited for probing the mechanical properties of a heterogeneous surface, since the probe size and contact volumes are small and localized. However, the nanoindentation method may introduce errors in the computed mechanical properties when indenting near the interface between two materials having significantly different mechanical properties. Here we examine the case where a soft material is loaded in close proximity to an interface of higher modulus, such as the case when indenting bone near a metallic implant. Results are derived from both an approximate analytical quarter-space solution and a finite element model, and used to estimate the error in indentation-determined elastic modulus as a function of the distance from the apex of contact to the dissimilar interface, for both Berkovich and spherical indenter geometries. Sample data reveal the potential errors in mechanical property determination that can occur when indenting near an interface having higher stiffness, or when characterizing strongly heterogeneous materials. The results suggest that caution should be used when interpreting results in the near-interfacial region.     

Measurement of Fracture Parameters for a Mixed-Mode Crack Driven by Stress Waves using Image Correlation Technique and High-Speed Digital Photography

Abstract:  Measurement of fracture parameters for a rapidly growing crack in syntactic foam sheets using image correlation technique and high-speed photography is presented. The performance of a rotating mirror-type multi-channel high-speed digital camera to measure transient deformations is assessed by conducting benchmark tests on image intensity variability, rigid translation and rigid rotation. Edge-cracked foam samples are subjected to eccentric impact loading relative to the initial crack plane to produce mixed-mode loading conditions in a three-point bend configuration. High-speed photography is used to record decorated random speckles in the vicinity of the crack tip at a rate of 200 000 frames per second. Two sets of images are recorded, the first set before impact and the second after impact. Using image correlation methodology, crack-tip displacement field histories and dominant strains from the time of impact up to complete fracture are mapped. Over-deterministic least-squares analyses of crack-tip radial and tangential displacements are used to obtain mixed-mode fracture parameters. The measurements are compared with complementary finite element results. The fracture parameters determined from radial displacements seem more robust even when fewer number of higher order terms in the crack-tip asymptotic expansion are used.     

On the robustness of finite element procedures based on Virtual Crack Closure Technique and fail release approach for delamination growth phenomena. Definition and assessment of a novel methodology

Numerical procedures based on the combined use of the Virtual Crack Closure Technique and of a fail release approach have been widely used to simulate delamination growth phenomena of composite material structures. This paper starts explaining why this kind of methodologies might not be robust due to mesh and load step size dependency and introduces a novel approach able to cope with the problems identified. Finally the effectiveness and robustness of the proposed procedure, implemented into a commercial finite element software by means of user subroutines, are assessed by comparing the obtained numerical results for a delamination growth phenomenon against literature experimental data on a stiffened panel with a circular embedded delamination under compressive load.