层板分层后屈曲分析

本文给出了改进二维分层Mindlin模型^[1]过渡区中的内力内力矩、应变、位移协调关系，以及有限元模型，进而分析了含有圆形和椭圆形分层的复合材料层合板后屈曲行为，表明这一模型的可靠性和实用性。     

含分层损伤复合材料层合板的屈曲特性研究

本文利用有限元方法,对压缩载荷作用下含各种形状脱层损伤的复合层合板的屈曲模态和载荷进行了研究。对不同的脱层损伤尺寸,层合板分别有整体,混合和局部三种屈曲模态,穿透脱层和埋藏脱层的层合板其临界载荷的变化规律是相同的。     

板锥网壳结构复合材料层合板的屈曲分析

板锥网壳结构是一种受力性能合理,技术经济效益良好的新型空间结构形式.本文结合复合材料力学理论和复合材料结构力学(板壳理论),采用组合结构有限元对复合材料层合板自身的局部稳定性和主要影响因素进行全面和深入的研究,研究复合材料板锥网壳结构的受力性能,为复合材料板锥网壳结构设计和层合板铺层设计提供理论上的依据,得出了可应用于工程实践的重要结论.     

含不同类型分层缺陷层合板的屈曲失效性能

对含分层缺陷的层合板材料在压缩载荷下进行分层屈曲试验,研究中央分层及边缘分层两种缺陷下的屈曲失效模式,同时结合厚度、分层深度、位置等因素,对两种分层类型进行综合对比分析。结合有限元方法,运用界面元研究了层间的分层缺陷的损伤行为,有限元模拟得到了与实验吻合度较好的数值结果,并进一步研究了分层尺寸及试样尺寸效应的影响。     

含预置分层复合材料层合板低速冲击损伤分析

以T300/QY8911碳纤维复合材料层合板为研究对象,利用三维Hashin失效准则对层内损伤进行预测,引入Cohesive界面单元模拟层间分层破坏,使用Camanho刚度退化准则确定材料参数退化方案,并编写了用户材料子程序Vumat,利用ABAQUS软件对预置不同分层损伤复合材料层合板低速冲击过程进行了数值仿真模拟。在冲击载荷作用下研究层合板的动力学响应,得到不同预置分层对层合板总分层损伤面积的影响,同时分析了冲击过程中冲击点位移随时间变化的规律。结果表明：预置分层位置离层合板中间层越远,层合板抵抗冲击承载能力越弱。预置分层的分层数目越多,层合板的刚度越低,层合板抵抗冲击承载能力越弱。在预置分层的两侧,层间分层损伤面积呈塔状分布。含预置分层层合板受到冲击时,预置分层对分层损伤的扩展具有一定的抑制作用。     

埋置元件复合材料层合板的分层损伤

通过对含与不含薄膜埋置元件复合材料层合板的拉伸和层间韧性的力学性能表征,探讨影响埋置元件结构完整性的一些关键因素。利用ABAQUS有限元软件,基于断裂力学用以研究裂纹扩展问题的虚拟裂纹闭合技术(VCCT),建立了在拉伸荷载作用下层合板分层起裂和扩展过程的数值模拟方法。在模拟分析中,考虑了裂纹萌生位置对分层扩展规律及结构强度的影响,并与实验结果比较,验证了本方法的合理性和有效性。     

含分层损伤复合材料组合层合梁的屈曲性态

本文基于Ｍｉｎｄｌｉｎ假定推导了考虑剪切的组合层合梁的有限元列式,并在此基础上计算出含分层损伤的各种层合梁在受轴向压力情况下的屈曲模式。从中得出梁的屈曲性态与铺设角,梁的几何形状,分层位置及分层大小等因素的关系,所得结论具有普遍意义。     

含孔复合材料层合板的结构参数对其屈曲性能的影响

利用ABAQUS软件,建立了含孔复合材料层合板的有限元模型,在层合板四边简支,承受轴向压力载荷作用下,分析讨论了孔径/板宽比、铺层比例、铺层角度、材料性能等结构参数对层合板的线性屈曲的影响。结果表明,孔径/板宽比d/w越大,屈曲载荷越小,当d/w≤0.1时,可认为是小孔洞情况,可近似按无限大层合板处理;90°铺层(与载荷受力方向一致)比例越大,或0°铺层(与载荷受力方向垂直)比例越小,屈曲载荷越大,增加±45°铺层比例,有利于降低层合板各向异性程度;对于斜交对称铺层的层合板,铺层角度对其屈曲载荷影响很大,最大值与最小值相差13.05N;材料主方向结构性能参数差别较大时,屈曲载荷也相差较大,材料的结构性能参数对屈曲承载力具有明显的影响。     

纤维曲线铺放的变刚度复合材料层合板的屈曲

本文介绍了一种纤维曲线铺放的变刚度复合材料层合板的概念,利用这种新的铺放方法所引起的复合材料层合板面内应力的重新分布,提高了复合材料层合板的屈曲载荷.并且利用ABAQUsTM软件对这种纤维曲线铺放的复合材料层合板进行了建模计算,验证了其面内受力情况下,屈曲载荷显著提高,幅度达14%左右.     

复合材料层合板分层疲劳性能研究进展

介绍了近年来复合材料层合板分层疲劳模型、数值模拟、以及I型Ⅱ型和混合型分层疲劳性能试验的研究进展,并对复合材料层合板分层疲劳性能进一步的研究进行了展望。     

Delamination Cracking in a Laminated Ceramic-Matrix Composite

Delamination crack propagation has been investigated in a laminated fiber-reinforced ceramic-matrix composite. The crack growth initiation resistance has been shown to be dominated by the critical strain energy release rate for the matrix. However, the resistance increases with crack extension because of bridging effects associated with intact fibers and, in some cases, intact segments of matrix. The delamination cracks also assume a steady-state trajectory within a 0° layer close to the 0°/90° interface.     

Steady-State Mechanics of Delamination Cracking in Laminated Ceramic-Matrix Composites

A fracture mechanics delamination cracking model has been developed for brittle-matrix composite laminates. The near-tip mechanics is discussed in the context of material orthotropy and composite material inhomogeneities. A fracture mechanics framework based on the near-tip energy release rate and the associated phase angle Ψ has been adopted. In the case of steady-state delamination cracking in a prenotched cross-ply symmetric laminated beam, analytical expressions for the steady-state energy release rate, _ss, have been obtained for the combined applied loading of an axial force and a bending moment. Parameter studies assessing the effects on _ss of load coupling, crack location, and lamination morphology which includes the total number of layers, layer thickness, and material properties are presented. Thus, composite homogenization criteria with respect to the total number of layers placed along the beam height can be obtained for a wide range of material selection. The associated phase angle Ψ at the delamination crack tip is discussed in the context of existing solutions. The analysis has been developed based on a theory for structural laminates. The delamination model can be used in conjunction with experimental data obtained from model geometries to extract the mixed-mode transverse composite fracture toughness. Thus, conditions for stable delamination crack growth can be established and design criteria based on toughness for composite laminates and composite fasteners can be obtained.     

Delamination propagation analyses of spar wingskin joints made with curved laminated FRP composite panels

Initiation and propagation of inter-laminar delamination in adhesive bonded spar wingskin joint (SWJ) made with laminated fibre-reinforced plastic (FRP) composite curved panels have been studied employing three-dimensional finite element analyses. In-plane and out-of-plane normal and shear stress distributions are seen to be highly three-dimensional in nature. Tsai-Wu coupled stress failure criteria have been employed to identify critical locations of onset of delamination-induced damage. This occurs underneath the toe-end of the spar overlap and at the inter-laminar surface between the first and second plies of the curved FRP wingskin panel. Significant edge effects on the joint strength have been observed due to the curvature geometry of the composite wingskin panels. Non-linear finite element analyses have been carried out for study of delamination propagation using contact and multi point constraint (MPC) elements. The use of contact elements prevents inter-penetration of delaminated surfaces. Whereas, sequential release of MPC elements facilitates computation of opening, sliding and cross-sliding modes of delamination-induced strain energy release rates (SERR) by using virtual crack closure technique. Variation in delamination lengths significantly effects the variation of peel and inter-laminar shear stresses and different modes of SERRs. Variations on the two delamination fronts are seen to be quite different indicating dis-similar propagation rates. The Mode I SERR (G_I) predominantly governs the delamination propagation in the SWJ.     

Influence of Residual Stresses on the Wear Behavior of Alumina/Alumina–Zirconia Laminated Composites

Symmetric structures of laminated ceramic composites were produced by superimposing alternating layers of Al₂O₃ and Al₂O₃/ZrO₂ composite prepared by tape casting. The composites were designed to have an alumina surface layer on either side. This configuration caused residual compressive stresses to be induced on the surface due to the different thermal expansion coefficients of the various layers, leading to an increase in the apparent surface toughness. The amount of residual stress was determined using the indentation technique. The tribological behavior of these laminated structures was evaluated using the pin-on-disk method for different loads and sliding speeds. Comparison with the results obtained from stress-free alumina showed that, within the range of these experimental conditions, the improvement in surface toughness leads to a reduced friction coefficient and increased wear resistance of the composites. Possible wear mechanisms are proposed.     

Cathodic Delamination of an Epoxy/Polyamide Coating from Steel

X-ray photoelectron spectroscopy (XPS) has been used to determine the mechanism responsible for debonding of an epoxy/polyamide coating from steel during cathodic delamination in 3.5% aqueous NaCl solutions. Coating failure always occurred near the interface between the coating and the oxide. The nitrogen content of the free surface of the prepared coatings was about 10%. However, the nitrogen content of the free surface dropped to only 5% after exposure to 1 N NaOH for four weeks and that of the coating failure surface after cathodic delamination was only about 2%, implying that the failure involved degradation of the polyamide curing agent by hydroxide ions formed at the steel surface by reduction of oxygen. That conclusion was supported by results obtained from curve fitting of C(1s) and O(1s) spectra. The intensity of components in the C(1s) spectra due to C—N and C≡O bonds in amide functional groups decreased significantly after coatings were exposed to NaOH or subjected to cathodic delamination. Small amounts of organic materials characteristic of the coating were observed on the substrate failure surface, perhaps indicating that the failure was cohesive within the coating but very close to the interface or that some products from degradation of the curing agent precipitated on the substrate. Use of silane coupling agents to retard cathodic delamination was also investigated. Coupling agents were added directly to the coating or applied to the substrate as a primer before application of the coating. Significant reduction in the rate of cathodic delamination was seen only when the silane coupling agent was applied to the substrate and cured at elevated temperatures before the epoxy/polyamide coating was applied.     

三聚氰胺甲醛／聚乙烯复合层压板生产工艺

介绍了以纸基三聚氰胺甲醛（ＭＦ）塑料为面层、废旧聚乙烯（ＰＥ）塑料为芯层,用层压工艺制造复合板材的工艺过程,并重点论述了阻燃型板材的配方设计及工艺控制。     

Cathodic Delamination of an Epoxy/Polyamide Coating from Steel

X-ray photoelectron spectroscopy (XPS) has been used to determine the mechanism responsible for debonding of an epoxy/polyamide coating from steel during cathodic delamination in 3.5% aqueous NaCl solutions. Coating failure always occurred near the interface between the coating and the oxide. The nitrogen content of the free surface of the prepared coatings was about 10%. However, the nitrogen content of the free surface dropped to only 5% after exposure to 1 N NaOH for four weeks and that of the coating failure surface after cathodic delamination was only about 2%, implying that the failure involved degradation of the polyamide curing agent by hydroxide ions formed at the steel surface by reduction of oxygen. That conclusion was supported by results obtained from curve fitting of C(1s) and O(1s) spectra. The intensity of components in the C(1s) spectra due to C—N and C≡O bonds in amide functional groups decreased significantly after coatings were exposed to NaOH or subjected to cathodic delamination. Small amounts of organic materials characteristic of the coating were observed on the substrate failure surface, perhaps indicating that the failure was cohesive within the coating but very close to the interface or that some products from degradation of the curing agent precipitated on the substrate. Use of silane coupling agents to retard cathodic delamination was also investigated. Coupling agents were added directly to the coating or applied to the substrate as a primer before application of the coating. Significant reduction in the rate of cathodic delamination was seen only when the silane coupling agent was applied to the substrate and cured at elevated temperatures before the epoxy/polyamide coating was applied.     

Thermodynamics of a Stable Blister Delamination at Elevated Temperature

A new blister test using thermal expansion of an internal working gas trapped at a dissimilar interface between a thin polymer coating and a rigid adherend is developed to measure the adhesive strength at elevated temperature. The blister dimensions are measured by a thermomechanical analyser (TMA) and an optical microscope as a function of temperature. The thermodynamics is presented based on both linear elastic fracture mechanics and the ideal gas law.     

Mixed-Mode Cathodic Delamination of Rubber From Steel

A series of scarf specimens with a range of scarf angles was used to examine the effect of fracture mode-mix on the cathodic delamination of rubber from steel in an aggressive (1N NaOH) environment. Fracture parameters were extracted from finite element analyses that accounted for the nonlinearly elastic response and the finite deformations in the rubber layer. Delamination growth rates were found to be independent of energy release rate levels that were chosen for the study but were strongly dependent on crack opening angle, the parameter that was used to represent the local fracture mode-mix.