基于全试验设计方法的含裂纹铝合金板复合材料修补参数优化

建立含穿透裂纹铝合金板复合材料单面胶接修补板条的三维有限元模型,基于位移外推法对裂纹尖端的应力强度因子(SIF)进行求解。使用全试验设计的方法对不同修补参数下修补板条的单向拉伸试验进行仿真模拟,利用二次方程描述并研究了补片长度、补片厚度及胶层弹性模量共同作用时对SIF的影响,确定了以SIF为评价指标时对修补效果影响最大的修补参数,优化了修补设计,并应用优化修补参数进行单向静拉伸试验。结果表明,当三类修补参数共同作用时,补片长度对修补效果影响最大;使用优化修补参数单面修补试验件的破坏强度比未修补板的提高了12.1%,恢复到完好板的90.5%。     

含孔复合材料层合板孔边的应力集中

针对含孔有限宽复合材料层合板的应力集中问题,提出一种计算孔边应力分布及应力集中因子的方法：先利用经典层板理论,将复合材料层合板化归为各向异性板;再将各向异性板等效转换为一偏轴拉伸的单向纤维层板;最后利用含孔偏轴单向板的孔边应力计算公式来分析一般铺层层合板孔边应力集中情况。根据所推导的计算公式,分析讨论了板宽/孔径比、铺层比例、铺层方式、材料性能参数等因素对孔边应力集中的影响。     

复合材料补片加固含椭圆孔钢板刚度分析

针对复合材料补片加固损伤钢板的受力特点, 建立三维弹性力学模型, 对复合材料补片双面加固轴向受拉平板进行了应力和变形分析。研究了粘贴界面的剪应力分布规律, 钢板和补片内轴向拉力的分布规律, 分析了加固后平板相对加固前的相对刚度。研究了含椭圆孔钢板经复合材料补片加固后相对加固前以及相对完好钢板的刚度。结果表明, 复合材料胶接修补含孔钢板的刚度随着补片厚度的增加而增加, 且增强的趋势逐渐趋于缓和。当补片层数大于5层后, 单纯依靠增加补片的厚度并不能有效提高修复效果。解析法与有限元法的计算结果及试验结果吻合得比较好。      

层压板双面挖补修理的拉伸性能研究及参数分析

对含穿透型损伤层压板双面挖补胶接修补件的拉伸性能及主要影响参数进行了试验研究。结果表明: 双面挖补后的失效拉伸强度恢复率能够达到80%; 当挖补斜度1∶40、 覆盖层取3层、 且靠近母板的铺层方向与母板的最外层的铺层方向保持一致时, 强度恢复率较大; 使用双面挖补后, 母板厚度、 母板铺层方式对强度恢复率的影响不大。同时应用有限元软件ABAQUS, 采用三维实体单层设置建模方式对双面挖补层压板的拉伸性能进行了数值模拟, 模拟计算结果与试验结果基本相符。      

湿热环境下复合材料含孔层合板静力拉伸性能及工程估算模型

湿热环境下的复合材料结构件力学性能预测对其工程应用具有重要意义。文中针对复合材料层合板静力拉伸性能和强度预测问题,开展6种湿热环境下复合材料含孔层合板的静力拉伸试验,分析其结构失效响应及损伤表征。基于应力场强法建立湿热环境下复合材料含孔层合板工程估算模型,与有限元渐进损伤模型和试验结果进行对比,分析了湿热环境对含孔层合板力学性能和拉伸失效的影响。结果表明,工程估算模型预测结果与有限元及试验结果误差范围较小,可用于预测温度和吸湿率对含孔层合板拉伸失效强度的影响;相比于室温干态,75℃吸湿饱和态下试件拉伸失效强度下降了6.1%;25℃干态和75℃吸湿饱和态下含孔层合板0°铺层出现最为严重的纤维拉伸失效,90°铺层出现最为严重的基体拉伸失效,纤维拉伸失效和基体拉伸失效为层合板主要破坏模式;通过扫描电镜对75℃吸湿饱和态下层合板厚度方向微观形貌进行分析,发现试件0°方向纤维与树脂的脱粘程度加重且出现明显的裂痕,90°方向纤维分布较为齐整,但黏附的树脂较少。     

拉伸载荷下贴补复合材料层合板的渐进损伤分析

基于连续介质损伤力学和粘聚区模型建立了贴补复合材料层合板的渐进损伤分析模型,计算了拉伸载荷下修补结构的极限强度。数值仿真结果和实验结果吻合较好,验证了该模型的有效性。基于建立的模型研究了贴补复合材料层合板的损伤演化过程,并讨论了补片参数对修补结构拉伸性能的影响。研究结果表明:补片参数对贴补复合材料层合板的破坏模式与损伤演化过程有显著影响;不同破坏模式下,补片参数的改变对修补结构极限强度的影响效果不同。研究结果可为复合材料层合板的贴补设计提供部分理论参考。     

单面贴补修理后层合板的拉伸性能

对含孔损伤复合材料层合板单面贴补后进行拉伸试验研究。测量了层合板的应变分布、修理后层合板中心点的离面位移及拉伸强度等, 考察了补片的厚度、大小等因素对修理效果的影响。结果显示, 增加补片的厚度和直径能够提高母板的承拉能力, 但是增加补片的厚度会导致层合板离面位移增大。对无侧边支持的单面贴补层合板进行计算分析时, 必须考虑偏心载荷引起的弯矩的影响。在此基础上, 采用分层损伤判据建立了三维有限元模型, 对单面贴补层合板的破坏机理和拉伸强度进行了计算和分析。结果表明, 修理后层合板的拉伸破坏是由补片或母板内与胶接面相邻的层间分层引起的; 计算结果与试验结果一致。      

复合材料层合板胶接贴补修理渐进损伤分析

建立了复合材料层合板胶接贴补修理构型渐进损伤分析的三维有限元模型, 其中层合板和胶层分别采用正交各向异性损伤和各向同性损伤的连续介质损伤力学模型, 整个分析过程中同时考虑层合板和胶层的损伤形成和扩展以及它们之间的相互影响, 单向压缩载荷作用下的层合板贴补修理构型的试验数据验证了该模型的有效性, 采用该模型分析了不同的贴补修理参数对修补强度的影响。 结果表明: 当层合板补片较薄时, 补片损伤是导致修补结构失效的主要原因; 当补片较厚时, 胶层失效是导致修补结构失效的主要原因, 此时补片厚度增加并不能显著增大修补结构的极限强度。在复合材料贴补修理时需要对补片和胶层进行详细优化设计。      

复合材料层板开孔拉伸损伤分析

针对纤维增强复合材料层板开孔拉伸, 将复合材料层板的失效分为层内失效和层间失效, 建立了复合材料层板开孔拉伸损伤分析模型。该模型基于逐渐损伤分析, 对不同复合材料开孔层板进行了失效预测, 并与文献试验结果进行了对比, 破坏强度和失效模式均与文献试验结果非常吻合。结果表明本文中所建立的层板开孔拉伸损伤分析模型能够模拟含孔层合板拉伸过程中的损伤起始、 损伤扩展和最终破坏模式, 并最终预测含孔层合板拉伸失效模式和破坏强度。      

复合材料胶接修补件力学性能的实验研究与数值模拟

进行复合材料修补的铝合金板的静强度实验,测定载荷-位移曲线,分析破坏机理,并讨论了胶层材料性能、复合材料补片性能与厚度等因素对修补件静强度的影响;建立了修补件的三维有限元模型,模拟修补件的载荷-位移曲线和应力分布,验证了模型的有效性;根据应力分布计算结果和失效准则,预测初始损伤及裂纹产生的位置,并估算破坏强度,预测结果...     

On stress concentrations for isotropic/orthotropic plates and cylinders with a circular hole

Scale factors (SFs) are widely used in engineering applications to describe the stress concentration factor (SCF) of a finite width isotropic plate with a circular hole and under uniaxial loading. In this paper, these SFs were also found to be valid in an isotropic plate with biaxial loading and an isotropic cylinder with uniaxial loading or internal pressure, if a suitable hole to structure dimension ratio was chosen. The study was further expanded to consider orthotropic plates and cylinders with a center hole and under uniaxial loading. The applicable range of the SFs was given based on the orthotropic material parameters. The influence of the structural dimension on the SCF was also studied. An empirical calculation method for the stress concentrations for isotropic/orthotropic plates and cylinders with a circular hole was proposed and the results agreed well with the FEM simulations. This research work may provide structure engineers a simple and efficient way to estimate the hole effect on plate structures or pressure vessels made of isotropic or orthotropic materials.     

三维碳/碳复合材料板件拉伸强度预报

针对三维碳/碳复合材料板件开孔敏感性问题进行了试验与有限元分析。建立了三维碳/碳复合材料代表体积单元(RVE)和完好板件、开孔板件的有限元模型,给出了宏观应力到细观应力的转换方法,并基于此方法,实现了完好板件、开孔板件拉伸破坏过程的数值模拟。进行了三维碳/碳复合材料完好板件和开孔板件的单向拉伸试验,模拟计算结果与试验结果吻合良好。分析了不同宽径比(WTDR)对开孔板件拉伸强度的影响,其中当宽径比达到6时,开孔板拉伸极限强度相比完好板下降11.5%,可以认为如果宽径比不低于6,开孔对板件拉伸极限强度影响较小。该结论可以为碳/碳复合材料开孔板件设计提供指导。     

Numerical experiments on the determination of stress concentration factors

A brief discussion of relevant applications of the finite element method is presented, and two cases of relatively high stress concentration are considered. For a slot with semi–circular ends in a plate subjected to uniaxial stress, the stress concentration factors obtained numerically, by use of the finite element method, are compared with corresponding experimental results. The case of a corrugated hole in a plate under 1:1 biaxial stress is also examined, and stress concentration factors determined using the finite element approach are compared with exact values obtained by application of Muskhelishvili's method.     

钛合金平板电子束焊接残余应力的小孔法测量

采用小孔法对TC11平板真空电子束焊接接头的残余应力进行了测量,结果发现,其残余应力为以纵向残余应力为主的单向拉伸应力状态,横向残余应力数值较小,同时还将测量结果与有限元结果进行对比分析,证明了有限元模型的合理性和可靠性。     

Tensile behaviour of patch-repaired CFRP laminates

The present study examines the tensile behaviour of composite structures repaired by bonding external patches. Various patches of different stacking sequences placed on both sides of the parent plate were considered. Damage development and the failure process of the repaired plates were analyzed and a parent plate fracture model has been proposed. Optimised patch repairs were calculated by finite element modelling. It was found that high stress concentration along the longitudinal edges of circular patches and/or at the transverse edges of the hole leads to early damage initiation in the parent plate. However, the position of damage initiation and the process of damage progression depend particularly on the properties of repair patches. In order to optimise patch repairs, finite element modelling was used and it was founded possible to attain over 90% of the strength of an unnotched specimen. The optimised patch design can be characterised by an optimal strength ratio R^*, which should be minimized when selecting repair parameters.     

开孔碳纤维层合板层间应力分析

针对具有典型铺设角 的开孔碳纤维层合板, 采用三维有限元数值模拟方法, 分析了在单向拉伸载荷作用下孔边附近的层间应力, 讨论了界面层参数对层间应力的影响, 详细给出了典型铺设角之间层间应力的分布规律和最大层间应力产生的位置。结果表明: 对于相同铺设角的界面层, 沿厚度方向的位置影响层间应力的大小, 但不影响分布趋势; 而铺层顺序(如 或 )对层间应力的大小和分布趋势影响则较小。最大层间正应力产生于 的界面层, 位于与拉伸方向成90°的位置, 是外加拉伸应力的51%; 最大层间剪应力产生于 的界面层, 最大层间环向剪应力位于与拉伸方向成74°的位置, 是外加拉伸应力的64%; 而最大层间径向剪应力位于与拉伸方向成66°的位置, 是外加拉伸应力的25%。      

DBEM and FEM analysis on non-linear multiple crack propagation in an aeronautic doubler-skin assembly

The performance of a riveted patch repair, applied on a cracked panel, is simulated by using both a commercially available boundary element code (BEASY) and a finite element code (ANSYS). A two-dimensional stress analysis on a single-sided repaired configuration is performed by both methodologies; consequently, the occurrence of out-of-plane bending and its effect on the through-thickness stress intensity factor (SIF) variation is neglected. The connection between the two layers (patch and panel) is realised by 32 rivets, with through-cracks initiated on the most loaded holes. Special elements are used to model the crack: discontinuous elements in the dual boundary element method (DBEM) approach or quarter point elements in the finite element method (FEM) approach. Different loading configurations are considered depending on the presence of a biaxial or uniaxial remote load and the non-linear hole/rivet contact is simulated by gap elements. The most stressed skin holes are highlighted, and the effect of a through crack from such holes is analysed in terms of SIFs and stress redistribution. The accuracy in SIFs assessment by DBEM and FEM and the respective computational and pre-processing efforts are determined. Such a two-dimensional analysis allows us a straightforward pre-processing phase, and very short run times are needed. A peculiar arrangement of the pin configuration in the DBEM analysis allows us to take into account the real in-plane plate stiffness and the transversal pin stiffness, even in a 2D analysis (this is straightforward by using FEM).     

Thermal Response and Stability Characteristics of Bistable Composite Laminates by Considering Temperature Dependent Material Properties and Resin Layers

In this study, the stability characteristics and thermal response of a bistable composite plate with different asymmetric composition were considered. The non-linear finite element method (FEM) was utilized to determine the response of the laminate. Attention was focused on the temperature dependency of laminate mechanical properties, especially on the thermal expansion coefficients of the composite graphite-epoxy plate. Also the effect of including the resin layers on the stability characteristics of the laminate was investigated. The effect of the temperature on the laminate cured configurations in the range of 25°C to 180°C and ?60°C to 40°C was examined. The results indicate that the coefficient of thermal expansions has a major effect on the cured shapes. Next, optical microscopy was used to characterize the laminate composition and for the first time the effect of including the resin layers on the actuation loads that causes snapping behavior between two stable shapes was studied. The results obtained from the finite element simulations were compared with experimental results and a good correlation was obtained. Finally, the stability characteristics of a tapered composite panel were investigated for using in a sample winglet as a candidate application of bistable structures.     

Reduction of the stress concentration factor in a homogeneous panel with hole by using a functionally graded layer

This work aims at understanding the effect of a radially heterogeneous layer around the hole in a homogeneous plate on the stress concentration factor. The problem concerns a single hole in a plate under different far-field in-plane loading conditions. By assuming a radial power law variation of Young’s modulus and constant value for Poisson’s ratio, the governing differential equations for plane stress conditions, and general in-plane loading conditions are studied. The elastic solutions are obtained in closed form and, in order to describe localized interface damage between the ring and the plate, two different interface conditions (perfectly bonded and frictionless contact) are studied. The formulae for the stress concentration factors are explicitly given for uniaxial, biaxial and shear in-plane loading conditions and comparisons with interface hoop stress values are performed. The solutions are investigated to understand the role played by the geometric and graded constitutive parameters. The results are validated with numerical finite element simulations in which some simplified hypotheses assumed in the analytical model, are relaxed to explore the range of validity of the elastic solution presented. In this way the results obtained are useful in tailoring the parameters for specific applications.