【作者】 刘军；

【导师】 黄宝宗；

【作者基本信息】 东北大学 ， 结构工程， 2006， 博士

【摘要】 纤维增强复合材料具有比强度高、比刚度大和材料性能可设计等一系列优点,因此复合材料,特别是纤维增强树脂基层合复合材料,在航空、航天等高科技领域得到越来越广泛的应用。大量的试验和观察发现纤维增强复合材料层合板中基体微裂纹是最早发生的一类损伤,应力集中区的高应力、高应变更容易引发基体微裂纹损伤,而微裂纹损伤的出现,会引起层板刚度下降,从而影响层合板的性能。另外,分层也是复合材料层合板破坏的主要形式,它导致结构承载能力明显降低,成为层合板使用中的主要隐患,因此,研究分层裂尖场的特征和提高层合板的抗分层能力具有重要的意义和应用价值。本文的主要工作分为两方面,首先研究了微裂纹损伤及其演化对分层尖端场、能量释放率和失效行为的影响。其次用解析法研究了短纤维层间增韧—这一新的增韧方法的增韧机理和主要参数的影响,得到了一些有意义的结论。基于连续介质损伤力学,本文采用多标量损伤变量来描述复合材料基体微裂纹损伤,将单层板的实验结果推广到复杂应力状态,认为损伤的演化仅依赖于材料主方向的应变,从而得到全量形式的损伤演化方程。损伤模型中考虑了准脆性材料的应变软化特征,分别采用了四种软化参数,来分析应变软化的影响。对于含贯穿分层裂纹的层合板,为了研究基体微裂纹损伤对分层裂尖场的影响,将复合材料层板简化为平面应变问题,利用虚功原理推导了全量形式的平衡方程和增量迭代方程,考虑几何非线性和损伤引起的物理非线性,编制了FORTRAN程序,用于分析损伤及损伤参数对分层尖端场奇异性的影响、层合板发生局部屈曲后裂尖能量释放率的变化以及极限损伤区的扩展。研究表明微裂纹损伤使分层尖端应力的奇异性指数降低；损伤和应变软化使裂尖的能量释放率不再随载荷单调增加,而出现波动,已经难以反映分层特征；极限损伤区域的扩展受软化参数的影响,扩展方向受铺层方式的影响。为了研究微裂纹损伤对复合材料层合板强度的影响,本文利用基于修正的一阶剪切理论建立的一种拟协调复合材料层合板单元,考虑单层板破坏前由微裂纹损伤引起的刚度下降,利用Hoffman准则对含孔层合板在单向拉伸载荷下和方板在均布载荷作用下的破坏过程进行了数值分析,并与不考虑损伤演化的分析结果进行了比较。结果表明损伤演化影响复合材料层合板的破坏过程,单层板破坏前的损伤演化使应力向未损伤单元转移,使应力集中得到缓解,单层破坏延迟。对于正交铺设的层合板可使极限破坏载荷明显提高。常见的各种层间增韧方法如：改善基体增韧、层间加入韧性胶膜、层间加入韧性颗粒,缝纫技术等,虽然都能有效改善层间韧性,但有的方法工艺复杂,有的方法引起面内性能的降低。相比之下,一种新的增韧方式—层间短纤维增韧具有工艺简单,增韧效果明显,而且对面内性能影响小等特点,具有很大的发展前景。本文采用解析法对层间短纤维增韧的机理进行了研究,建立了适于层间浅埋纤维桥联分析的计算模型。分析了Ⅰ、Ⅱ型裂纹扩展过程中桥联纤维的脱粘和拔出过程,讨论了纤维弹性模量、桥联纤维密度和纤维基体界面参数(如：基体与短纤维界面强度、脱粘后界面摩擦力等)对分层韧性的影响,对增韧效果作出了预测。结果表明：桥联纤维从基体中的拔出过程消耗较多的能量,是层间韧性增加的主要因素；高弹性模量纤维的增韧效果优于低模量纤维；界面强度对增韧效果影响不大；脱粘后中等摩擦力界面的增韧效果最佳。另外,层间短纤维中有一部分位于界面层的中部,不参与桥联,但这部分纤维在其它桥联纤维的拔出过程中起到了干扰作用,而这种干扰作用是不能忽视的。更多还原

【Abstract】 Due to high strength, low weight and good durability, fiber reinforced composite material has been widely used in the field of aviation and space. Experiments show that the matrix micro-cracks in fiber reinforced laminates are the damage that occure first and it ieasily caused by the high stress and high strain in the stress concentration region.The matrix micro-crack damage results in the decrease of rigidity and other property of laminate. In addition, delamination is another main damage and hidden trouble of composite laminate which can lead to decrease of structural carrying capacity. So the studies on delamination crack tip stress field and interlaminar toughening are very important. This paper focus on two aspects:firstly investigation of the the influence of micro-crack damage and evolution on delamination tip field, energy release ratio and laminate failure behavior; secondly analytical research for the interlaminar chopped fiber toughening mechanism and the main parameters.Based on the continuum damage theory and existing experiment results, a multi-scalar damage model and damage evolutiont equations are presented. In the model, the damage depends on the strain in main direction of material, the strain softening characteristics of quasi-brittle materials are considered and four kinds of softening parameters are adapted to analyze the influence of strain softening.In order to study influence of the micro-crack on crack tip field, laminates with through-crack are considered under plane strain assumption. Using the principle of virtual work, the equations of equilibrium and their increment iteration form are deduced. Considering the geometry nonlinearity and physical nonlinearity caused by damage, a FORTRAN program is made to analyze the influence of damage on crack tip field, energy release ratio and expand of ultimate damage region. The computational results show that the damage of micro-crack can make the stress singularity decreased, damage and strain softening makes the crack tip energy release ratio no longer increase monotonously with the load, therefore it can not be used to describe the characteristic of delamimation growth.The expanding of ultimate damage region is influenced by softening parameters and fiber directions.To determine the influence of micro-crack damage on the laminate failure, a quasi-forming laminated plate element based on first order shear theory is used in FE analysis. Considering the rigidity decrease caused by micro-crack and using the Hoffman criterion, the numerical analysis of destroy course of laminates with a hole under tension and square laminated plates under uniform pressure are carried on. Compare of the numerical results with and without micro-crack damage indicates that damage evolution before destroy can chang stress transmission in laminates, release the stress concentration, delay the destroy of laminates, especially ones with0/90plies, and therefore ultimate carrying capability may be improved obviously.The commonly used interlaminar toughening methods are:improving matrix tougheness, interleaving, adding plastic particle to interlayer and stitiching etc. Although these methods can improve interlaminar tougheness, they are complex in technique and make the decrease of in-plane performances. But a new approach to toughening from interlaminar chopped-fibers may overcome the shortcomings. In this paper, a modeling for bridging of chopped fibers lying almost within the delamination plane is carried out by a analytical method. In the model, the spalling of matrix and pull-out of bridging fibers are considered, the effects of some parameters on tougheness increment are discussed including fiber elastic modulus, bridging fiber density and property of fiber matrix interface. Results show that the energy dissipation mainly comes from pull-out of bridging fibers, the high modulus Kevlar fibers and moderate interface friction force are more effective for improving delamination toughness, but the influence of interface strength is small. In addition, a part of chopped fibers in the middle of interlayer is not in bridging but they disturb pull-out of bridging fiber and they are not ignored.更多还原