Investigating the influence of delamination on the stiffness of composite pipes under compressive transverse loading using cohesive zone method

The effect of delamination on the stiffness reduction of composite pipes is studied in this research. The stiffness test of filament wound composite pipes is simulated using cohesive zone method. The modeling is accomplished to study the effect of the geometrical parameters including delamination size and its position with respect to loading direction on stiffness of the composite pipes. At first, finite element results for stiffness test of a perfect pipe without delamination are validated with the experimental results according to ASTM D2412. It is seen that the finite element results agree well with experimental results. Then the finite element model is developed for composite pips with delaminated areas with different primary shapes. Thus, the effect of the size of delaminated region on longitudinal and tangential directions and also its orientation with respect to loading direction on delamination propagation and stiffness reduction of the pipes is assessed.     

Beam and shell element model for advanced analysis of steel structural members

Plastic zone method of advanced analysis, which uses shell elements to model the entire structure, is the most accurate method available to predict the ultimate strength and behavior of steel frames. The disadvantage of such full shell plastic zone models is that it is computationally expensive and hence its use is limited to small structures. Beam elements in commercial finite element packages can model residual stress and capture spread of plasticity, but cannot model local buckling of plates that the member is made up of, which leads to unloading and failure in steel frames. A hybrid model using shell elements only in the regions vulnerable to elastic or inelastic local buckling and beam elements in other locations could overcome this limitation of full beam element model. The issues in using this hybrid model are, knowing a priori the location and length of the shell element region and connecting the beam and shell regions without any artificial stress concentrations or incompatible displacements. In this study, in addition to addressing these issues, the hybrid model is systematically evaluated by studying its performance in structural elements. It is seen that the hybrid model strength predictions has an average error of only 0.91% but requires on an average 83% less computational time when compared to the full shell plastic zone models.     

Beam and shell element model for advanced analysis of steel structural members

Plastic zone method of advanced analysis, which uses shell elements to model the entire structure, is the most accurate method available to predict the ultimate strength and behavior of steel frames. The disadvantage of such full shell plastic zone models is that it is computationally expensive and hence its use is limited to small structures. Beam elements in commercial finite element packages can model residual stress and capture spread of plasticity, but cannot model local buckling of plates that the member is made up of, which leads to unloading and failure in steel frames. A hybrid model using shell elements only in the regions vulnerable to elastic or inelastic local buckling and beam elements in other locations could overcome this limitation of full beam element model. The issues in using this hybrid model are, knowing a priori the location and length of the shell element region and connecting the beam and shell regions without any artificial stress concentrations or incompatible displacements. In this study, in addition to addressing these issues, the hybrid model is systematically evaluated by studying its performance in structural elements. It is seen that the hybrid model strength predictions has an average error of only 0.91% but requires on an average 83% less computational time when compared to the full shell plastic zone models.     

组合肋壳非线性分析

组合肋壳是一种新型组合空间结构形式,基于非线性有限元理论,利用空间梁单元和曲边壳单元,考虑结构非线性影响,推导了T.L.坐标系下结构单元刚度矩阵,采用增量迭代法和弧长法对整个受力过程进行全过程跟踪,得到组合肋壳的极限承载力,并对结构承载力影响因素进行了系统分析,最后与带肋钢筋混凝土薄壳以及光滑混凝土壳体进行了比较.分析结果表明,随着矢跨比和截面尺寸的增大、边界条件接近刚性,组合肋壳的极限承载力增大;与其它两类壳体相比,由于组合肋中钢肋的存在,使得组合肋壳整体刚度增大,承载能力提高,结构的薄膜内力分布更为合理.     

Static buckling analysis of thin cylindrical shell with centrally located dent under uniform lateral pressure

One of the common failure modes of thin cylindrical shell subjected to external pressure is buckling. The buckling pressure of these shell structures are dominantly affected by the geometrical imperfections present in the cylindrical shell which are very difficult to alleviate during manufacturing process. Dent is one of the common geometrical imperfections present in thin shell structures which may be formed due to mechanical damage caused by accidental loading or impact. In this work, influence of various dent parameters (dent length, dent width, dent depth and angle of orientation of the dent) on the critical buckling pressure of thin cylindrical shells with a centrally located dent is studied using non-linear static finite-element analysis of ANSYS under external pressure with simply supported boundary conditions at the top and bottom edges of the thin cylindrical shell.     

Hybrid Progressive Damage Prediction Model for Loaded Marine Sandwich Composite Structures Subjected to a Fire

The risk of fire, and of fire-related structural degradation, represents an ever present challenge to the safe design of marine sandwich composite structures. Current state-of-the-art fire analysis and damage assessment tool has ignored the coexistence of discrete (delamination) and continuum damage in a sandwich composite structure after a fire exposure. To capture the synergistic interaction between the discrete and continuum damage and their compounding effects to the final failure, a fire damage assessment tool for a marine composite sandwich structure was developed by integrating the 1D fire model of a composite sandwich panel with a hybrid progressive damage prediction module within the LS-DYNA3D computational framework. The 1D fire model for a combustible skin and decomposable core was employed to characterize the temperature and mass dependent heat conduction, energy consumption resulting from the decomposition, and the energy transfer associated with vaporous migration. For a given thermal and mechanical damage of a sandwich composite system, a multi-scale solution framework was formulated to determine the material response and failure at the structural level from the damage progression at its constituent, ply, and laminated plate level. The delamination failure along an interface was characterized by a cohesive element approach with a user-defined cohesive law while the diffuse damage resulting from the fiber/tow/matrix failure in the sandwich composite was characterized using a continuum damage model. A loaded sandwich beam with and without a fire was considered to demonstrate the effects of fire damage on the ultimate strength of the sandwich structure.     

清水混凝土双曲面薄壳模板施工

以银川车站改造工程为例,着重对现浇清水混凝土双曲薄壳板的模板制作安装、拆除及技术保证措施等方面进行了具体分析,经实践取得了良好效果,为今后同类模板施工提供了一定经验。     

土结接触面黏聚区域模型及渐进累积破坏分析

首先探讨了土结接触面的含义、接触面相对位移的组成以及接触面的破坏形式,然后提出以黏聚区域模型描述接触面滑动和拉裂破坏变形、邻近非接触面单元描述接触面基本变形的分析思路,建立了基于黏聚区域模型的无厚度接触面单元,有效避免了接触面厚度确定的困难。在强化有限单元法（ FEM⁺⁺ ）的框架内完成了接触面模型的数值实现后,以直剪试验的模拟揭示了土与结构接触面的渐进累积破坏过程,说明了接触面模型与平均剪应力–剪切位移试验曲线的差异,并对模型本构参数的选取进行了阐述。     

网状扁壳与带肋扁壳组合结构的拟三层壳分析法

本文对网状扁壳与带肋扁壳共同工作的组合结构(可简称组合网状扁壳),采用连续化的拟三层壳的计算模型,按弹性小挠度薄壳理论进行分析计算,推导建立了混合法的基本方程式。由于这种构造上的拟三层壳在一般情况下不存在中面,因而壳体的薄膜内力、弯矩与薄膜应变,弯曲应变是耦合的,存在一个耦合矩阵,使得基本方程式比单层光面的符氏扁壳方程要复杂得多。对于周边简支的组合网状扁壳可求得基本方程式的解析解。文中对三向、四向组合网状扁壳进行了详细讨论,并指出了在特定条件下,可退化为一个当量的各向同性单层扁壳。对于一般网状扁壳的拟壳分析法及带肋扁壳的拟壳分析法分别属于本文的两种特殊情况。文中附有计算例题。     

A non-asymptotic model for the stability analysis of thin biperiodic cylindrical shells

The object of considerations is a thin linear-elastic cylindrical shell with arbitrary cross-section having a periodic structure in both directions tangent to the shell midsurface. The aim of this paper is to propose a new averaged non-asymptotic model of dynamical stability problems for such shells. As a tool of modeling we shall apply the tolerance averaging technique, given for periodic structures in Woźniak and Wierzbicki [Averaging techniques in thermomechanics of composite solids, Wydawnictwo Politechniki Częstochowskiej, 2000]. In contrast with models obtained by means of asymptotic homogenization methods, the proposed one makes it possible to describe the effect of the period lengths on the global shell stability, called a length-scale effect.     

Finite element based coupled mode initial post-buckling analysis of a composite cylindrical shell

This paper presents a finite element formulation of Koiter's initial post-buckling theory using a multi-mode approach. Initial post-buckling theory provides direct information about the imperfection sensitivity of a structure under compressive loading, and is also the basis of a nonlinear reduced order model. The objective of the present work is to illustrate the capability of the implementation for buckling analysis of shell structures including modal interaction. A coupled mode initial post-buckling analysis for a composite cylindrical shell under axial compression, including the effect of a nonlinear pre-buckling state, has been carried out using a small number of representative modes. For small imperfection amplitudes the limit-point buckling loads obtained with the reduced order model compare reasonably well with full model nonlinear analysis, illustrating that a fast prediction of the coupled mode response of imperfect shells is possible using the approach proposed.     

宝石群单层折面网壳构形研究和简化杆系计算模型

深圳大运会体育馆、体育场的主体结构是由三角形网格组成,其外形如宝石群,为单层折面空间网壳结构.通过构形研究和机动分析,论证了宝石群单层折面网壳是一种满足几何不变性基本要求的静定空间桁架结构.通过研究圆形平面的宝石群单层折面网壳,提出了简化杆系计算模型及其计算方法,可精确求得网壳结构的内力和位移.在基本结构体系基础上,增...     

Free vibration analysis of a toroidal shell

The free vibration of a toroidal shell (curved pipe) is studied using DMV (Donnell-Mushtari-Vlasov) thin shell theory, Fourier's series and Galerkin's method. Some behaviors of the vibration of cylindrical shell, as a special case of toroidal shell, are discussed. Also, a 90-degree-bend curved pipe is studied numerically. Results obtained by the present method are compared to those obtained by finite element and other methods. DMV linear thin shell theory is employed for simplicity. However, the method applies to any other shell theories without difficulties.     

带裂缝服役混凝土结构力学性能的多尺度模拟方法

为了研究既有裂缝及其扩展对服役混凝土结构力学性能的影响,建立了以随机骨料和黏结单元为基础的细观结构黏聚区模型,并进一步建立了带裂缝混凝土梁的一致多尺度等效模型。在开裂区域引入符合牵引分离准则的黏结单元来考虑裂缝的随机扩展,通过随机骨料投放来模拟骨料、砂浆等混凝土的细观结构组成。采用已有试验结果对细观结构黏聚区模型进行了验证,该方法能够模拟混凝土裂缝的随机扩展及其应力-应变全过程曲线。然后,对不同裂缝深度的混凝土试件进行单轴拉伸试验的数值模拟,采用一致多尺度模型对不同裂缝深度的混凝土梁进行模拟,并对其计算效率和精度进行了比较。结果表明：对于初设裂缝深度小于0.3h(h为混凝土试件高度),试件尺寸对混凝土抗拉强度的影响很小,而当初设裂缝深度大于0.3h时,试件尺寸对混凝土抗拉强度的影响明显;随着初设裂缝深度的增加,混凝土的抗拉强度和弹性模量以及混凝土梁的抗弯刚度、受弯承载力逐渐减小;在不同区域采用不同控制尺度建立的一致多尺度细观结构等效模型能够模拟混凝土梁的裂缝及其随机扩展,并且能在不显著影响其计算效率的情况下保证其计算精度。     

Elastic modulus reduction method for limit analysis of thin plate and shell structures

The paper proposes an efficient numerical method, namely, the elastic modulus reduction method (EMRM) to calculate lower-bound limit loads of thin plate and shell structures. To determine the elastic modulus adjustment parameter q in a rational way, the precisions and convergent properties of four different strategies of elastic modulus adjustment (SEMAs) combined with the EMRM are investigated. Furthermore, to take into consideration the plastic resistances as well as the internal forces in the processes of elastic modulus adjustment, the element bearing ratio (EBR) of thin plate/shell element is defined as the governing parameter based on the generalised yield criterions. The flexibility and accuracy of the proposed EMRM combined with both the strain energy equilibrium principle (SEEP) and the fixed strain method (FSM) are demonstrated in the numerical example.     

大跨度钢-混凝土组合薄壳屋盖模型试验研究

本文对一种新型的结构形式——钢-混凝土组合薄壳进行缩尺破坏试验,得出该结构在多点集中荷载作用下的非线性特征以及承载性能,对裂缝的出现与开展、单元连接螺栓处失效以及底板屈曲行为作了详细的观察分析.试验结果表明,由于螺栓节点的存在,导致壳体内部刚度的不连续,从而减弱了组合壳的整体性能.本文实验结果为后续对该结构的非线性有限元分析提供了可靠依据.     

应用于单层网壳结构动力响应精细化模拟的杆件计算模型

在动力荷载作用下单层网壳结构杆件受力和结构承载力处于动态变化中,因此要得到准确的结构实时动力响应,必须建立能够模拟杆件实时受力性能的杆件计算模型.根据单层网壳结构杆件的受力特点,总结出受压杆件的两种失稳类型.引入国际标准化组织推荐的空间受力圆钢管失稳判别准则,采用3节点塑性铰单元和Marshall模型分别模拟受压杆件失...     

叠合梁屈曲的分析与数值研究

根据Euler-Bernouli梁与古典叠合梁理论，提出了一个分析模型，用于获得叠合梁的屈曲荷载，同时，用有限元方法验证了该模型。数值模拟采用8节点壳单元和接触对单元。对铺层方向、排列次序、铺层在宽度方向上的位置、铺层长度以及总层数等参数的影响进行了分析。研究表明，应用R2分析模型给出了比R1更精确的结果；同时发现，重要的表达方式必然包含于反对称次级迭合单元的弯曲刚度阵中；此外，叠合层的长度也影响精确性和分析预测之间的差异。但分析结果和模型值吻合很好。     

An elastic–plastic analysis of large deflection of thin shell structure using a delta-sequence function

The analysis of thin shell under large deflection has complex problem associated with the geometrical and material nonlinearities, in which the solutions for stress and deformation are desired to obtain the same level accuracy. One of ordinary and powerful method for this subject is a finite element method that has generally inconvenience to be necessarily extensive calculation due to large number of freedom.This paper is concerned with the elastic–plastic analysis of thin shell structures by the hybrid method using a functional for the principle of modified complementary energy. For elastic–plastic materials, the numerical calculation could be well executed by the way that the stress distribution across the panel thickness is expressed as continuous function using the delta-sequence function. This approach introduces a considerable way for the reduction of computing volume. The proposed method was applied to discuss the resisting mechanism of thin shell structure under large deflection.