短纤维层间增韧的三维有限元分析

层间短纤维强韧化是复合材料层合板层间增韧的有效途径之一。采用三维非线性有限元方法分析了层间短纤维强韧化复合材料层合板的分层扩展问题,探讨其影响因素。以混合模式下通用裂纹扩展的能量准则作为分层扩展判据,通过虚拟裂纹闭合技术计算分层尖端能量释放率。用节点双编号和单元生死技术模拟分层扩展。采用弹簧单元模拟层间短纤维作用,通过改变弹簧刚度修正短纤维桥联力。采用GAP元处理分层区的接触非线性问题。分析了分层区桥联力对层间应力场、位移场以及分层扩展的影响。研究结果表明,层间短纤维有效延缓了分层扩展,增韧效果明显。     

3D复合材料中纤维搭桥对脱层屈曲的影响

本文用能量法求解了绑结层合板和编织复合材料等3D复合材料中有纤维搭桥的圆形脱层屈曲问题。搭桥纤维被视为连续分布于脱层面上、可提供与脱层屈曲挠度成正比的恢复力的线性弹簧，根据脱层夹支边界条件以及脱层只能向一个方向挠曲的约束条件，导出了一个特征长度a0，当脱层半径a超出a0时，脱层屈曲将仅发主在r＜a0的子圆域之内，并不跨越整个脱层区域（r≤a）；当脱层半径a小于a0时，脱层屈曲将跨越整个脱层区域，并随着a的减小，脱层屈曲的临界载荷将逐渐提高。     

玻璃纤维-Al 混杂复合层板的疲劳裂缝扩展行为

本文研究了一种新型结构复合材料玻璃纤维-Al 混杂复合层板(Glass Aluminum Laminates,GLALL)在不同外载荷作用下的疲劳裂缝扩展行为,以及疲劳破坏规律。试验发现,由于未断高强度纤维对裂缝的桥接作用降低了裂缝尖端的有效应力场强度因子,GLALL 的疲劳裂缝扩展速率远低于单一Al 合金。伴随 Al 合金层内疲劳裂缝的扩展,GLALL 疲劳裂缝附近区域会产生脱层破坏,脱层区宽度随外加载荷增大而增大。脱层区宽度越小,纤维对疲劳裂缝的桥接作用越强。     

贝壳珍珠层断裂过程的原位观察及其增韧机制分析

天然生物材料的组织结构特征及其与性能间的关系研究对于材料的仿生设计有重要意义.本文利用扫描电镜原位观察了受拉伸载荷作用下珍珠层中裂纹的萌生及扩展方式,并结合SEM和TEM技术研究了贝壳珍珠层微观组织结构,探讨了裂纹扩展过程中的增韧机制.结果表明,珍珠层相邻片层凹凸镶嵌互补,多边形文石晶体是由纳米级颗粒构成的多晶体.裂纹偏转,有机物桥联,纤维拔出,小孔聚结等多种增韧机制在裂纹扩展过程中协同作用,都源自珍珠层独特的微观结构,并提出片层的球冠型结构是导致珍珠层具有超常韧性的机制之一.     

纤维桥联效应下复合材料层合板的屈曲及分层扩展模拟

纤维增强复合材料层合板由于层间力学性能弱,容易出现分层损伤。分层的扩展往往伴随着纤维桥联效应,纤维桥联能显著增大层合板尤其是多向层合板分层扩展的阻力。考虑纤维桥联效应的三线性内聚力模型能表征分层扩展实验中断裂韧性的“R曲线”特征,比传统的双线性模型能更为准确地描述复合材料的分层扩展行为。本论文基于三线性内聚力模型,对含圆形分层复合材料层合板的轴向压缩进行数值模拟,探讨纤维桥联效应对分层扩展及后屈曲行为的影响规律。研究结果发现,纤维桥联对层合板的屈曲载荷影响较小;混合屈曲模式下,三线性模型预测的上下子板相对法向位移明显低于双线性模型;相同分层深度下,三线性模型预测的层合板后屈曲更早转变为整体屈曲模式。随着分层深度的增加,层合板的屈曲模式由局部屈曲逐步过渡为混合屈曲和整体屈曲;当分层深度较浅时,Ⅰ型分层扩展占主导;随着分层深度的增加,Ⅰ型分层逐渐减弱,而Ⅱ型和Ⅲ型分层扩展则显著增强;当分层接近板中面时,Ⅰ型分层停止扩展,以Ⅱ型及Ⅲ型分层为主。     

PEI纳米纤维层间增韧碳纤维环氧复合材料性能研究

基于聚醚酰亚胺优越的力学性能和纳米纤维膜高比表面积、高孔隙率的特性,利用气泡静电纺丝工艺制备不同厚度的纳米纤维膜改善碳纤维环氧复合材料的层间韧性。结果发现不同膜厚度增韧的双悬臂梁(DCB)试件的I型层间断裂值(GIC)均有所提高,特别是膜厚为0.058±0.007 mm时,层合板的增韧效果最好,比未增韧试件提高了114.55%。通过复合材料层间断裂界面的SEM照片证实了纳米纤维膜在界面处通过桥联约束效应及钉锚作用有效提高了复合材料的层间断裂韧性。     

界面结合状态对玻璃纤维-Al混杂复合层板疲劳行为的影响

通过两种 Al 合金表面处理方法及水煮老化处理,得到了不同界面结合状态的玻璃纤维-Al 混杂复合层板,进行了层板层间剪切强度和疲劳裂缝扩展试验,研究了界面结合强度对层板疲劳行为的影响。试验发现,当界面结合强度较高时,层板的疲劳裂缝扩展速率较低,伴随疲劳裂缝扩展而产生的脱层破坏区尺寸较小,脱层破坏发生在树脂层与增强纤维之间;界面结合强度较低时,层板的疲劳裂缝扩展速率较高,脱层区尺寸较大,脱层破坏发生在 Al 层和树脂层之间。     

横向线动载荷作用下复合材料层合梁的脱层扩展分析

该文基于可动边界变分原理对层合梁脱层扩展进行了分析,分析中考虑脱层间的接触效应,建立了层合梁在横向线动载荷作用下的非线性控制微分方程及相应的定解条件;同时应用Griffith准则,导出了脱层前缘各点处的能量释放率表达式;算例中讨论了脱层长度、脱层深度、几何尺寸及材料性质等因素对脱层扩展的影响。研究表明:脱层梁承受横向拉力作用时,其发生脱层扩展的可能性要比承受横向压力时大得多;脱层越长、动载荷幅值越大,脱层越容易扩展;脱层越深、梁的长高比L/h及材料的E11/E22比值越大,脱层发生扩展的可能性越小。     

纤维增强复合材料界面脱层和基体裂纹的模拟分析

基于Ghosh提出的Voronoi单元有限元方法,构造能同时反映纤维增强复合材料界面脱层和基体裂纹扩展的单元(X-VCFEM单元);应用界面力学理论和断裂力学理论,建立界面脱层、界面裂纹扩展方向和基体裂纹扩展的判断准则;结合网格重划分技术,模拟分析了只含有一个夹杂时界面脱层和基体裂纹扩展的过程,并通过与传统有限元计算结果的比较,验证X-VCFEM单元的可靠性和有效性;同时,模拟分析含任意随机分布夹杂的纤维增强复合材料界面脱层和基体裂纹的产生和扩展过程。结果表明：应用该方法模拟复杂多相复合材料裂纹问题具有计算速度快和精度高的优越性。     

轮胎结构中层间裂纹扩展过程的数值模拟

轮胎是一个十分复杂的橡胶基复合材料结构,在使用过程中常常发生层间脱层破坏。提出了一种新的复合材料结构中层间脱层裂纹扩展模型,并利用Irwin的虚拟裂纹闭合技术和有限元分析技术对轮胎结构带束层中的层间脱层裂纹扩展过程进行了数值模拟。结果表明,提出的模型能够比较真实地揭示复杂复合材料结构层间脱层裂纹扩展复杂性的特点。     

含椭圆分层损伤复合材料层合板分层前缘接触效应研究

采用基于Mindlin 一阶剪切理论的四节点板单元, 分析了分层前缘闭合接触效应对含椭圆分层复合材料层合板前后屈曲行为的影响。通过在分层区域内引入虚杆单元来防止上、下子板发生相互重叠, 并假设接触区内没有摩擦。结果表明, 分层前缘的闭合接触效应受分层几何形状的影响很大, 当椭圆分层子板的长轴与压缩载荷方向一致时, 闭合接触效应对含分层层合板前后屈曲行为最大。      

Finite element analysis of interface delamination and buckling in thin film systems by wedge indentation

A finite element method is used to study the interface delamination and buckling of thin film systems subject to microwedge indentation. In the formulation, the interface adjoining the thin film and substrate is assumed to be the only site where cracking may occur. Both the thin film and the substrate are taken to be ductile materials with finite deformation. A traction-separation law, with two major parameters: interface strength and interface energy, is introduced to simulate the adhesive and failure behaviors of the interface between the film and the substrate. The effects of the interface adhesive properties and the thickness of the thin film on the onset and growth of interface delamination and the film buckling are investigated.     

A finite element model for the analysis of buckling driven delaminations of thin films on rigid substrates

The delamination process of thin films on rigid substrates is investigated. Such systems are typically subject to high residual compression and modest adhesion causing them to buckling driven blisters. In certain cases buckles with the shape of telephone cords are observed. A finite element model for quasi–static delamination growth is developed. Applying a Reissner–Mindlin shell kinematic for the film allows C ⁰− continuous shape functions. The traction vector at the film–substrate interface is obtained from the derivative of a cohesive free energy. Incorporation of loading and unloading conditions is considered for the irreversible process. The equilibrium state is computed iteratively in dependence of the compressive residual stresses. The computed telephone cord delaminations are stable asymmetric configurations whereas the symmetric configurations are unstable.     

Influence of interfacial delamination on channel cracking of elastic thin films

Channeling cracks in brittle thin films have been observed to be a key reliability issue for advanced interconnects and other integrated structures. Most theoretical studies to date have assumed no delamination at the interface, while experiments have observed channel cracks both with and without interfacial delamination. This paper analyzes the effect of interfacial delamination on the fracture condition of brittle thin films on elastic substrates. It is found that, depending on the elastic mismatch and interface toughness, a channel crack may grow with no delamination, with a stable delamination, or with unstable delamination. For a film on a relatively compliant substrate, a critical interface toughness is predicted, which separates stable and unstable delamination. For a film on a relatively stiff substrate, however, a channel crack grows with no delamination when the interface toughness is greater than a critical value, while stable delamination along with the channel crack is possible only in a small range of interface toughness for a specific elastic mismatch. An effective energy release rate for the steady-state growth of a channel crack is defined to account for the influence of interfacial delamination on both the fracture driving force and the resistance, which can be significantly higher than the energy release rate assuming no delamination.     

The stability of delamination growth in compressively loaded composite plates

The stability of growth of internal delaminations in composite plates subjected to compressive loading is investigated. Due to the compressive loading, these structures can undergo buckling of the delaminated layer and subsequently growth of the delamination. The study does not impose any restrictive assumptions regarding the delamination thickness and plate length (as opposed to the usual thin film assumptions). The growth characteristics of the delamination under monotonic compressive loading are obtained on the basis of a combined delamination buckling/postbuckling and fracture mechanics model. The postbuckling solution is derived through a perturbation procedure, which is based on an asymptotic expansion of the load and deformation quantities in terms of the distortion parameter of the delaminated layer, the latter being considered a compressive elastica. The closed form solutions for the energy release rate at the delamination tip versus applied compressive strain during the initial postbuckling phase are used to define the combinations of delamination length and applied strain that lead to unstable growth. This would practically cause either contained jump growth or complete (catastrophic) growth of the delamination. Estimates for the lower and upper bounds of the jump distance (unstable growth) are provided. Moreover, a study of the influence of the mode dependence of interface toughness on the conditions of initiation and extent of delamination growth is performed.     

Delamination buckling of FRP layer in laminated wood beams

An analytical solution for predicting delamination buckling and growth of a thin fiber reinforced-plastic (FRP) layer in laminated wood beams under bending is presented. Based on a strength-of-materials approach, displacement functions for a delaminated beam under four-point bending are derived. Using force and displacement compatibility conditions, an explicit form relating the applied transverse load with the delamination buckling load is established. An explicit form of the strain-energy release rate is presented to study the delamination growth in beams under bending. The analytical solution is evaluated using experimental data for glued-laminated timber (glulam) beams reinforced with a thin fiber-reinforced plastic composite on the compression face. The delamination growth in bending is shown to behave differently to that of the in-plane loading case.     

含任意分层缝纫正交层合板压缩屈曲分析

建立了分析含任意形状分层的缝纫增强复合材料层合板压缩屈曲问题的连续分析模型。该模型允许缝纫层合板含有一个或多个形状不同的分层。分析结果表明,缝纫针脚在分层区域的分布、缝纫密度和缝线的等效刚度系数对分层子板的压缩屈曲应变均有较大影响。      

Influence of substrate compliance on buckling delamination of thin films

A thin film subject to in-plane compressive stress is susceptible to buckling-driven delamination. This paper analyzes a straight-sided delamination buckle with a focus on the effects of substrate compliance, following earlier work by B. Cotterell and Z. Chen. The critical buckling condition, the energy release rate and the mode mix of the interface delamination crack are calculated as a function of the elastic mismatch between the film and substrate. The average energy release rate at the curved end of a tunneling straight-sided blister is also determined. The more compliant the substrate, the easier for the film to buckle and the higher the energy release rates. The effect becomes significant when the modulus of the substrate is appreciably less than that of the film. When the substrate modulus is comparable to that of the film, or higher, the usual assumption is justified to the effect that the film is clamped along its edges. When the substrate is very compliant the energy release rate at the curved front exceeds that along the straight sides.     

Delamination growth in long composite tubes under external pressure

Delamination growth is a phenomenon known to reduce the integrity of laminated composite structural elements and may lead to premature failures. In the present study, state of the art procedures of delamination growth analysis are overviewed. The energy release rate calculation is formulated for composite delaminated tubular cross sections and specialized to a finite element model for delamination buckling and growth analysis of long laminated composite tubes taking into account initial geometric imperfections, large deformations, contact between delamination faces and material degradation. It is, then, used to study the potential of delamination growth in a hybrid composite tube. Parametric studies are conducted to assess the effects of delamination length, location and geometric imperfection on growth.     

含内埋矩形脱层板屈曲分析的传递函数方法

研究了含任意内埋矩形脱层复合材料层合板的屈曲问题。采用一种基于Mindlin一阶剪切理论的条形传递函数方法,将含内埋矩形脱层的复合材料层合板分成含脱层和不含脱层的两种矩形超级单元,然后由各超级单元之间连接结点处的位移连续和力平衡条件得到脱层板屈曲的特征方程,进而得到脱层板的屈曲载荷和屈曲模态。进行参数分析发现,脱层大小、深度、位置以及脱层板的边界条件和复合材料铺层方向对脱层板屈曲载荷的影响较显著。