建立了TC4钛合金材料的疲劳裂纹萌生寿命的预测模型,并通过试验验证了此模型在预测TC4钛合金材料疲劳裂纹萌生寿命时的可行性.基于裂纹萌生的细观位错模型,采用Tanaka-Mura的开裂寿命公式,考虑了表面粗糙度,提出了分析TC4钛合金材料疲劳裂纹萌生的有限元模型,并通过实验验证仿真模型的有效性.结果 表明:模型裂纹萌生形式在不同载荷水平下存在差异,高应力水平状态下,模型除了主裂纹的萌生扩展还伴有大量独立微裂纹,裂纹密度大,而在低应力水平状态下,模型存在少量独立微裂纹,裂纹密度小.

Sandwich structure T-joints are increasingly broadly applied in aviation and aerospace industries due to the need for lightweight design. This paper deals with the lightweight optimization of a typical adhesively bonded Nomex honeycomb-core sandwich T-joint in side bending load, considering the strength constraints. The optimization problem, with discrete and continuous design variables, is a compound optimization problem involving size optimization for the whole structure and stacking sequence optimization for multiple variable-thickness composite laminates. A self-adjusted parametric modeling with user-defined suppression process is proposed. An integrated combination of progressive damage model methodology, self-adjusted parametric modeling with user-defined suppression process and multi-island genetic algorithm is applied for the optimization problem. The optimization result showed 30.75% weight reduction compared to the original T-joint configuration. On the basis of history data, we investigate the correlations between design variables and concerned constraint variables.

This paper deals with the optimization of multi-laminate structures by Multi-Island Genetic Algorithm (MIGA) coupled with CAE solver. The optimization problem is a compound problem which relates to size optimization for object structure and stacking sequence optimization for variable-thickness composite laminates. Taking a typical adhesive bonded sandwich T-joint under a reference pull-off load as an instance object and establishing strength conditions on the basis of progressive damage analysis, optimum design is carried out with the total weight of joint as the target function. Progressive damage model (PDM) methodology and cohesive zone model (CZM) methodology are employed to develop an exact finite element model of the object structure. Classified failure criteria are chosen to investigate the capability of the joint in bearing the applied load. The optimization procedure on the typical adhesive bonded sandwich T-joint showed 34.24% weight reduction compared to the initial laminated structure. On the basis of history data, the study further brings out the influence of design variables on some main constraint variables.

The variational asymptotic homogenization (VAM) theory is extended to access freely to commercial finite element (FE) software to deal with periodic plate structures. In this work, the finite element format for periodic plate structures based on the variational asymptotic homogenization is developed, ensuring the commercial finite element software can be utilized to obtain the effective plate stiffness. A standard numerical framework and an integration algorithm are proposed for unifying the dimensional reduction analysis and the homogenization analysis in a formalized manner. As for model validation, the periodic plates composed of unit cells with three-dimension (3D) heterogeneous geometry are simulated by various elements and modeling techniques using the commercial FE software rather than programming in-house code. Compared to the results provided in the existing literature, the proposed approach shows excellent performance in terms of computational efficiency and time without compromising the VAM accuracy. It is preferable to enhance the application of the variational asymptotic homogenization theory for the more sophisticated heterogeneous plate structures.

飞机活动面缝隙采用封严结构可以有效提升隐身性能.本文提出了一种橡胶夹层结构的封严板结构,对该封严板使用的橡胶材料进行了单轴、等双轴与平面拉伸试验.对常用的Mooney-Rivlin、二次多项式、Neo-Hookean、Yeoh和三次Ogden本构模型进行拟合分析,确定了二次多项式和三次Ogden形式的高弹性材料模型,在此基础上建立了封严板结构有限元模型并进行装配分析和气动载荷作用分析.最后通过试验验证了所选用的橡胶材料高弹性本构模型与橡胶夹层封严板结构仿真分析方法的正确性.

本文提出通过丝网印刷技术,将石墨烯导电碳油墨以及银浆油墨印刷在复合材料结构表面形成智能传感层,对复合材料结构中的损伤进行监测和识别.采用准静态压痕的方式在复合材料结构中引入多个损伤,通过在印刷传感层的边界电极依次注入微小的电流,测量获得损伤前后传感层边界电压变化,重建损伤引起的电导率变化图像,从而提供有关损伤的信息.试验结果表明,该印刷传感层性能良好,重建的电导率变化图像能够较好地反映损伤的数量、位置和近似尺寸,为复合材料结构中损伤的定量监测和识别提供了一种有前景的应用技术.

Using the fracture mechanics and finite element analysis theory,the effect of section types of the ribs and multiple-site damage on the crack turning behavior and fracture characteristics of the integral stiffened panel is investigated.First the crack growth model of the integral stiffened panel was established,and the reliability of the established model was proved in comparison with the test results of the crack growth rates.Based on the above model,the influence of section types of the ribs on the crack fracture parameters and crack turning was discussed.Finally the multiple cracks problem was investigated.The study showed that the effect of the sectional types of the ribs on the fracture parameters was not significant and the crack turning was more likely to occur when the crack was growing far away from the ribs.In the case of multiple cracks,cracks will not turn,but grow straightly and connect to form a large crack.Moreover,the strong interference of adjacent crack tips can lead to the severe increase of stress intensity factor and acceleration of the crack growth.Furthermore,the crack propagation life will be seriously shortened.Therefore,effective repairing methods are needed to improve the residual strength and service life of the structure.

To make sense of the crack growth rate distribution in graded material,a standard three-point bending fatigue test was performed on two groups of cracked graded titanium alloys TC11-TC4 and TA15-TA2.Test results show that the same components in different parts of graded structure shear an identical propagation property and the Paris formulas of the four kinds of 3D printing titanium alloy have been determined.The variation distribution of e-lastic modulus influences the stress intensity factor and inhibits the propagation of crack in the lower modulus side. During influence thickness of transition layer,propagation rate is between that of the two components and varies continuously,proving that gradient is able to eliminate the interface effect in the connection of dissimilar materials. A mixing law based on volume rate has been proposed to describe the distribution of crack propagation rate.In the test under constant periodic load,there was a remarkable difference in the propagation life of specimens only propa-gated along different directions.It means damage tolerance in graded structures can be improved with a reasonable arrangement of the gradient.The variation distribution of modulus and fatigue property both influence the propaga-tion lives.

Based on the classical composite laminate theory ,a finite composite plate weakened by multiple elliptical holes is treated as an anisotropic plate .Using the Faber series method in complex theory combined with the least squares boundary collocation techniques on the finite boundaries ,the bending problem of a finite composite plate weakened by elliptical rigid inclusions is studied by means of the complex variable method .As a result ,concise and high accuracy solutions are presented for the stress distribution around the rigid inclusions .Finally ,numerical examples are presented to discuss the effects of some parameters on the stress concentration around the rigid inclusions .The results showed that the present method is not only very efficient for analysis of the stress distribution of finite laminates with multiple elliptical rigid inclusions ,but also is highly accurate and needs short computer time .

为了模拟功能梯度材料(FGM)在工程应用中可能会出现的断裂问题并计算相应的开裂载荷,通过编写用户自定义UEL子程序将梯度扩展单元嵌入到ABAQUS软件中模拟功能梯度材料的物理场,并编写交互能量积分后处理子程序计算裂纹尖端的混合模式应力强度因子(SIF),采用最大周向应力准则编写子程序计算裂纹的偏转角,并模拟了裂纹扩展路径,计算了裂纹的起裂载荷.讨论了材料梯度参数对裂纹扩展路径以及起裂载荷的影响规律.通过与均匀材料的对比,验证了功能梯度材料断裂性能的优越性.研究表明:外载平行于梯度方向时,垂直梯度方向的初始裂纹朝着等效弹性模量小的方向扩展,且偏转角在梯度指数线性时出现峰值,并随着组分弹性模量比的增加而变大;当外载和初始裂纹均平行于梯度方向时,材料等效弹性模量和断裂韧性的增加或者梯度指数的减小都导致起裂载荷变大.

In this paper, the residual strength of metal gradient materials with I cracks is observed through the test and the numerical simulation. A three-point bending test of TC4-TC11 metal gradient material with notches is completed. The model of the residual strength is built to have a research on metal gradient material with cracks, based on the extended finite element method. The stress intensity factor is calculated by the use of the extended finite element method of interactive energy integral. Comparing the experi-mental results with the calculated results, the model is verified to be valid. The influence of the material parameters on the residual strength is discussed, based on the finite element model to get some useful conclusions.

Graded extended finite element was proposed for fracture characteristic analysis in graded composites whose varying properties were predicted by micromechanics method.The spatially varying stiffness matrix was calculated through the linear interpolation of displacement field and a continuous gradient finite element model was established.The superiority of graded extended finite element method(XFEM) was verified through comparing the results of graded element,homogeneous element and relevant literatures.Furthermore,the influence of material properties on stress intensity factor(SIF) was discussed in detail.The results of graded XFEM can converge to the exact solutions quickly as increasing mesh density,whereas the calculation error of homogeneous XFEM cannot vanish as increasing mesh density,and it rises with the increase of crack length and property gradient.The increase of property gradient and thickness ratio of coating and substrate enlarge the SIF in coating graded material.The increase of crack length and the decrease of thickness ratio of connecting layer and substrate lead to the raise of SIF in connecting graded material.

To further improve the level of mechanical analysis and design of graded composites,a graded extended finite element method(XFEM) is proposed for fracture characteristic analysis in two-directional graded composites whose varying properties along gradient directions are predicted by a micromechanics method.The spatially varying stiffness matrices of 4-node graded extended finite elements are calculated by linear interpolation of displacement fields and a continuous gradient finite element model is established.The stress intensity factors(SIFs) of crack-tip are finally calculated by the interaction energy integral method.The superiority of graded XFEM is verified through comparison with relevant literature.Furthermore,the influence of material parameters on SIFs in two-directional graded structures is discussed in detail.The calculation accuracy of SIFs can be obviously improved by graded XFEM and the results converge to accurate solutions quickly as mesh density increases.The SIFs in two-directional graded structures can be markedly affected by constituent distribution and property gradients.In two-directional graded structures with multiple interior cracks,the SIFs are enlarged by the interaction between cracks and are larger at positions with higher elastic modulus.

According to the macrostructure and microstructure of the composite laminates,a 3D finite element model for analyzing high velocity impact damage of composite laminates was established based on ABAQUS software platform.The cohesive elements were involved between two layers to simulate delaminations of the composite laminates,and a 3D visco-elastic constitutive model coupled with Hashin failure criteria was used to predict the in-plane failure of composite laminates.By adopting this model,the ballistic performance and damage characteristics of composites laminates under high velocity impact were studied.The numerical results agree well with the experimental results.Parametric studies were conducted to study the effects of the strength parameters on the damage of composite laminates under high velocity impact.Some valuable conclusions were obtained.

Based on a progressive damage method, the mechanical behaviors of stitched composite laminates containing initial delamination were investigated. A finite element （FE） model was established to predict the residual compressive strength through ABAQUS. Laminate buckling and delamination propagation were considered during the analytical process. The failures of laminates , interlaminar and stitching were simulated by UMAT subroutine embedded truss elements were used to provide the bridge-forces of stitching. Hashin criterion and stiffness reduction method were introduced to predict fiber failure and matrix failure. The progressive damage process was investigated and the damage mechanisms were analyzed in detail. Effects of different stitching parameters on residual compressive strength were discussed. The predicted failure modes and strength have excellent agreement with experimental results. It shows that stitching increases laminate buckling load-capacity, restrains delamination propagation and increases the residual compressive strength.

Using the complex potential method in the bending theory of elasticity for an anisotropic body,the stress distribution in an infinite plate containing multiple elliptical holes is proposed with the help of Faber series expansion and conformal mapping.The effects of the relative distance between holes,total number of holes,the ellipticity of holes and the material system of the plate are studied in detail.Some useful conclusions are drawn.Results indicate that the present method has many advantages such as high accuracy,good convergence and great convenience.

A strength analysis model was presented to study the progressive damage of integral stiffened composite panels subjected to compressive loading by using the nonlinear finite element method.In the model,the debonding failure of the adhesive between the skin and stiffener was considered by adding cohesive elements between the shell elements.Quads failure criteria and Hashin＇s failure criteria were adopted to identify the occurring of damage events of the cohesive elements and the composite panels,respectively.Based on ABAQUS,a material degradation rule containing continuum damage status variables was presented.The process of damage initiation,propagation and catastrophic failure of the integral stiffened composite panels was simulated in detail by the proposed model,and the initial geometric imperfection was taken into account.Axial stiffness ratio of stiffener and skin was defined and conducted to study the effects to the structure on the carrying capacity and failure modes.The results indicate that： the model can predict the damage process of integral stiffened panel effectively;under the condition the ply design is reasonable,increasing the stiffness ratio can to some extent improve the unit area bearing capacity of the cross section of stiffened composite panels.

Based on the assumption of hexagonal cross-section of yarns, a parameterized representative volume element （RVE） of 2D 1 × 1 biaxial braided composites was established with the consideration of the interaction and the cross-sectional deformation of the yarns. Coupled with the periodical boundary condition, the structural RVE was adopted to predict the elastic properties of the braided composites by the finite element method. The influences of braid angle and fiber volume fraction on the effective elastic constants were studied, and the overall stress distribution nephogram of the RVE under typical loads were obtained and analyzed. The results indicate tha the spatial configuration and the mutual squeezing of the yarns are validly reflected in the RVE, and the structural RVE with different structural parameters can be quickly regenerated, and the prediction results coincide with the experimental data well. The finite element model provides reasonable stress filed, which establishes a good foundation for structure optimization and failure analysis of the 2D braided composites.

A nonlinear finite element model with interface phase was presented to simulate the damage and failure of 3D five-directional braided composites under unidirectional tension.Tsai-Wu and Mises criterions were considered for initial damage prediction of yarns and matrix.Quads failure criteria was adopted to identify the onset of debonding of the interface.An anisotropic damage model was used to analyze the damage evolution of yarns those damage modes were characterized by Murakami's damage tensor.In addition,a isotropic damage model was used to matrix and interface.The whole process of damage initiation,propagation and catastrophic failure of five-directional braided composites with typical braided angle were simulated in detail.The damage mechanisms were revealed in the simulation process and the strength of the braided composites was predicted from the calculated stress-strain curve.The numerical results show good consistent with experiment results,which verifies that the proposed simulation method is very useful for the evaluation of damage mechanisms of 3D five-directional braided composites.

Using the complex potential method in anisotropic elasticity,the stress distribution in a finite plate containing an elliptical hole is proposed with the help of Faber series expansion.The effects of the related parameters on stresses are studied in detail.Some helpful conclusions are drawn.