【作者】 孙丽莉；

【导师】 孙胜；

【作者基本信息】 山东大学 ， 材料加工工程， 2009， 博士

【摘要】 纤维增强聚合物基复合材料因其良好的力学性能在航空航天、造船、汽车、建筑、体育器材及医疗器械等方面的应用日益广泛。复合材料多变量、可设计的特征成就了其许多优良的特殊性能,但其多尺度、多层次的缺陷也导致了其强度预测及破坏过程分析的复杂性。基于宏观强度准则将复合材料看作单一材料的研究,无法分析其组分、内部缺陷及界面的脱粘和滑移对复合材料整体力学性能的影响,也无法从机理上对复合材料的破坏过程进行研究,更不能提供充分的材料优化设计和服役安全性判据。开展复合材料细观层次及细观尺度的缺陷形成机理分析、服役条件下细观缺陷的演化规律及调控机制研究,并在此基础上进行复合材料强度预测,具有非常重要的科学意义和工程价值。单纤维复合材料模型具备复合材料的最基本的组成单元:纤维、基体和界面,是实际复合材料的最基本模型,被广泛用于研究复合材料的细观力学行为,相应的理论模型—剪切滞后模型也日益发展。目前国内外相关的研究工作主要集中于单纤维复合材料模型中已断纤维对周围环境应力场的扰动,以及在准静载情况下纤维、基体及两者之间界面的性能对复合材料力学性能与破坏过程的影响,但基于多纤维复合材料细观力学实验模型的相关研究工作开展较少。在国家重点基础研究发展计划（973计划）项目的资助下,本文选用玻璃纤维/环氧树脂复合材料为研究体系,以单/多纤维复合材料模型为研究对象,采用实验与数值模拟相结合的手段,研究了各组分对复合材料破坏过程的影响,并且考察了应变速率对复合材料力学性能及破坏行为的影响,将细观缺陷演化与宏观破坏行为联系起来,总结了复合材料破坏模式的重要影响因素。本文作者参与研发了所用的部分实验设备,如纤维定位仪、显微拉伸机等,其中纤维定位仪的定位精度高、制样方便,显微拉伸机配有带偏光系统的显微摄像头,不仅能实时监测复合材料的细观破坏过程,还能实时捕获并记录拉伸试样中的应力集中现象。界面对复合材料的力学性能及破坏行为起着至关重要的作用。基于本文实验中所观测到的,在不同界面强度的复合材料中,纤维断点所引起的裂纹扩展模式不同,本文采用显式动力学有限元模拟软件ANSYS/LS-DYNA与自主开发程序相结合的方法,建立了带有复合材料缺陷的细观力学分析模型。以节点的有效塑性应变作为断裂判据,数值模拟了在不同界面强度下复合材料细观裂纹的扩展过程以及已断纤维对局部应力分布的影响,预测了在不同界面强度下复合材料局部和整体的抗破坏能力,数值模拟结果与实验结果吻合良好。结果表明,随着界面强度由低逐渐增高,已断纤维所引发的裂纹有五种典型的扩展模式;界面强度越高,复合材料的局部抗裂纹扩展能力越强但整体抗破坏能力越低。当界面达到一定强度以后,界面强度对局部抗裂纹扩展能力及整体抗破坏能力不再有影响。在复合材料固化过程中,纤维对液态树脂中某些分子的优先吸附将使纤维周围的界面层形成非化学计量比的固化产物。本文实验研究了基体的力学性质、纤维与环氧树脂基体之间的界面性能（界面破坏模式和界面剪切强度）、基体对纤维的润湿性等物理量与固化剂含量的依赖性关系。结果表明,除初始弹性模量以外,基体的其它力学性质,如屈服强度、屈服应变及断裂强度,都对固化剂含量有依赖性关系,当固化剂与环氧树脂配比为化学计量比时,力学性能指标达到极值。同时,纤维与基体之间的界面性能以及润湿性也达到最佳。研究结果还表明,当基体中具有不同的固化剂含量时,复合材料的界面剪切强度（IFSS）和粘附功（W_a）基本成线性关系。由于基体中的固化剂含量改变,基体的力学性质发生了改变,而基体的力学性质对于纤维与基体之间的界面剪切强度有不可忽视的影响。为了更准确地量化粘附功与界面剪切强度之间的关系,本文在保持基体中固化剂含量不变的前提下,进行了改变纤维的表面性质,进而改变纤维与基体之间的界面性能的研究工作。即在玻璃纤维表面自组装疏水膜,使玻璃纤维表面性质归一化,然后控制表面自组装疏水膜的玻璃纤维在一定浓度的臭氧中的氧化时间,以控制纤维表面的亲水性。实验结果表明,在本文的研究体系中,界面剪切强度与粘附功基本成线性关系,即界面粘结强度可以从力学和热力学两个角度来评价,且评价结果是基本一致的。本文还估算出界面上原子（分子）的平衡距离基本在0.3-0.6 nm范围内波动,且平衡距离与界面剪切强度和粘附功分别成线性关系,即界面粘结强度越高（粘附功或界面剪切强度越大）,纤维与基体之间界面上原子（分子）的平衡距离越小。利用现有的实验条件,本文研究了在低应变速率水平(2E-5 s^-1-2E-2 s^-1)下拉伸应变速率对单/多纤维复合材料及纯环氧树脂基体的力学性质、应力松弛及破坏行为的影响。结果发现,纯环氧树脂基体和纤维复合材料的初始弹性模量对应变速率的依赖性不显著,但极限应力和断裂强度随着应变速率的增加而增大;随着应变速率的增加,纯环氧树脂基体和纤维复合材料的破坏模式都由韧性破坏向脆性破坏转变,但发生转变时的应变速率范围不同;应变速率越高,在试样断裂后,纤维断点周围的残余切应力所引起的双折射现象越弱、消失越快,本文提出了界面相-链式模型对这一现象进行解释;对于多纤维复合材料,随着应变速率的增加,纤维与纤维之间的相互作用有逐渐减弱的趋势。本文在综合研究了三种不同纤维与两种不同基体交叉复合所得的不同的复合材料对应变速率的响应后,得到应变速率对复合材料力学性质影响的一般规律,如初始弹性模量的率不敏感性,极限应力和破坏模式的率敏感性等,还进一步总结出应变速率及各组分相对复合材料破坏模式综合影响的规律。如纤维断裂释放能量的大小（与纤维的断裂强度、弹性模量和断裂应变有关）、断点周围的基体和界面吸收能量的能力（与界面强度、基体屈服强度、屈服应变及弹性模量有关）、断点尺寸（与纤维直径有关）、纤维断点产生的时期（即断点产生在基体或界面的弹性变形阶段还是塑性变形阶段）及外载速率大小（即拉伸应变速率）等对复合材料破坏模式的影响规律。更多还原

【Abstract】 Fiber-reinforced composites have been used increasingly in aviation,aerospace, shipbuilding,automobile,building,sports,medical instrument,and so on due to their good mechanical properties.The multivariable and designable features make composites possess many good and special performances,which result in their complex strength prediction and failure process.Composites have been seen as homogenous materials based on traditional strength criterion,which can not reveal the influences of the components,internal defects,interface debonding and sliding on the mechanical properties of the whole composites.The optimum design of the material can not be conducted without clear failure mechanisms.Thus,it is very important to study the strength prediction,multi-level/-scale damage forming mechanism,damage evaluation in service and the controlling mechanism of composites.There are complete composite components in the single-fiber composite,including fiber,matrix and interface,which is the basic composite model and has been widely used in studying mesoscopic failure behaviors of composites.And the relevant theoretical model-shear-lag model has been improving.However,the main researches were concentrated on stress disturbances of the broken fiber on the surroundings,and influences of fiber,matrix and interface on mechanical properties and failure process of composites in the case of quasi-static loading.The multi-fiber composite investigations were incomplete due to the difficult specimen preparation.As being financed by the National Major Fundamental Research Program of China （973 Project）,meso-mechanical failure performances of glass fiber/epoxy matrix composites were studied using single-/multi-fiber composite models by both numerical and experimental methods.Further more,the strain rate dependence has been investigated.As a result,the relation between mesoscopic failure evolution and macroscopic failure behavior were established,and the important influencing factors on failure mode were concluded.The present author participated in developing some of the used equipments,such as the fiber-positioning apparatus and the tensile tester. Typically,the fiber arrays are controlled much more precisely and the operation is more convenient by conducting the fiber-positioning apparatus.And the tensile tester is equipped with a microscope attached with a polarizer,which brings off real-time monitoring the mesoscopic failure process and further capturing stress concentration images during testing.Interface plays a vital important role in mechanical properties of composites and the failure behavior.Based on the experimental result that there are different crack propagation modes in composites with different interfaces,a meso-mechanical finite element model with a defect was constructed to simulate the crack propagation routes and local stress distributions affected by the broken fiber under different interfacial strengths by using the ANSYS/LS-DYNA soft and the designed program.The failure criterion is a certain effective plastic strain.The local and whole resistance abilities to failure of the composite under different interfacial strengths have been evaluated,and the simulation results agreed well with the experimental results.The results show that, with the interfacial strength varying from weak to strong,there are five kinds of representative crack propagation modes observed in the composite in tension.The stronger the interface is,the better the local resistance to crack propagation,but the worse the resistance to the whole destroyed is.The resistance to crack propagation and the whole destroyed will not be improved any more if the interfacial strength increases to some certain values.During the curing process of composite,some kinds of the molecules in matrix will be adsorbed to fibers preferentially,which results in non-stoichiometric cured epoxy matrix at the interphase around fiber.So,the effect of curing agent ratio to epoxy on mechanical properties of matrix,interface properties between fiber and matrix （interface failure behaviors and the interfacial shear strength）,and the wettability of matrix to fiber were studied experimentally.It was found that,excluding the initial elastic modulus,the mechanical properties of epoxy matrix such as yield strength, yield strain and fracture strength were all dependent on the curing agent ratio to epoxy, which reached their maximum values at the stoichiometric ratio.The interface property and the wettability between fiber and matrix performed the best around the stoichiometric ratio also.Further more,it has been found that the interfacial shear strength is linear to the work of adhesion approximately by varying the curing agent ratio.However,there are variations in the mechanical properties of matrix with varying curing agent ratio which do have effects on the interfacial shear strength as well.In order to quantify the relationship between the interfacial shear strength and the work of adhesion definitely,the investigation of changing fiber surface treatment with the curing agent ratio to epoxy invariable has been carried out.First,self-assembled hydrophobic monolayers were prepared on the glass fiber surface to normalize the glass fiber surface property.The hydrophilicity of the glass fiber surface was controlled by the oxidation time in ozone.According to the comprehensive analysis of the experimental data,it is indicated that the interfacial shear strength does be linear to the work of adhesion in the current researching system.That is to say,the mechanical parameter—the interfacial shear strength and the thermodynamical parameter—the work of adhesion are indeed in close agreement with each other when characterizing the interface cohesive property.Additionally,the equilibrium interatomic/intermolecular distance at the interface has been evaluated to be approximately within 0.3-0.6 nm,which is linear to the interfacial shear strength and the work of adhesion,respectively.Concretely,the stronger the interface is,the smaller the equilibrium interatomic/intermolecular distance at the interface will be.Based on the current experimental conditions,the effect of tensile strain rate on mechanical properties,stress relaxation and failure behaviors of single-/multi-fiber composites and pure epoxy matrix were investigated at low strain rate level(range from 2E-5 s^-1 to 2E-2 s^-1).It was found that,the dependence of the initial elastic modulus of the pure epoxy matrix and fiber composite on strain rate is indistinct, however,the ultimate stress and the fracture strength increase with strain rate increasing.The fracture modes of both pure epoxy matrix and fiber composite change from ductile fracture to brittle fracture with strain rate increasing,but the strain rate of changing fracture mode for them are different.The higher the pre-tensile strain rate is, the weaker the birefringence induced by residual shear stress around fiber break is and the faster it fades,to which the conclusion obtained "the relaxation time decreases with pre-tensile strain rate increase" and the interphase-chain model introduced in the thesis can give a reasonable explanation.By studying the mesoscopic failure modes of multi-fiber composites,it was found that fiber-fiber interactions showed a sign of decrease while increasing tensile strain rate.The influences of strain rate on different composites cross-compounded by three kinds of fibers and two kinds of matrices were investigated systematically in this thesis.The results of general effects of strain rate on mechanical properties were obtained,including the rate independence of the initial elastic modulus,the rate dependence of the ultimate stress and fracture mode.Further more,the combined effects of strain rate and components on fracture mode of composites were summarized.For example,the released fracture energy of the fiber（is related to fracture strength/elastic modulus/fracture strain of the fiber）,the capacity of the surroundings to absorb the released energy（is related to yield strength/yield strain/elastic modulus of the matrix and the interfacial strength）,the dimension of the breakpoint（is related to fiber diameter）,the stage of fiber breakpoint generates（in elastic or plastic stage of the matrix or the interface）,and the external loading rate （tensile strain rate） all have effects on the fracture mode of composite.更多还原