【作者】 张亚芳；

【导师】 唐春安；

【作者基本信息】 东北大学 ， 采矿工程， 2006， 博士

【摘要】 复合材料破坏过程的研究近几十年来一直是力学界、材料界和工程界的研究难点和研究热点。脆性基复合材料的破裂过程与破坏机理的研究对于许多研究学者来讲都是一个富有挑战性的课题,此课题的研究对揭示脆性基复合材料的破裂机理和增韧机制以及对脆性材料结构的强韧优化设计都具有重要的理论和实际意义。由于脆性材料的抗拉能力大大低于其抗压能力,当材料达到其抗拉强度时失效往往在瞬间发生,造成灾难性的后果,所以对该类材料的强韧化设计是当前复合材料研究的一大热点。由于复合材料破坏过程中的多层次性、相关联性以及各种因素的强耦合效应,对于结构中裂纹的扩展有很大的影响,所以在此领域内尚缺少有效的理论分析研究方法,目前的理论分析工作大量集中于应力状态分析和材料的破坏判据上,对破裂过程的理论分析并不多见。虽然已进行了大量有关脆性基复合材料破裂的物理实验,但是对材料的破裂机制仍然缺乏认识,而且由于其组分材料力学性能的随机性和分散性特征,导致了复合材料损伤和破坏过程的复杂性与随机性,试图通过物理实验的方法来研究所有材料相组合的力学性能和破裂行为既不现实也不可能。众所周知,用数值方法来对复合材料的损伤和破坏进行分析研究是一个非常重要的途径,然而,传统的数值方法例如有限元法遇到破坏过程问题时就显示出了它的局限性。通常来讲,传统的数值方法可以得到满意的初始应力／初始应变场和最后的应力状态,但却不能很好地处理结构的破裂过程,即当复合材料产生破坏时,基于传统宏观意义上的数值方法不能很好地处理其中的破坏单元,也就不能很好地反映破坏过程。而复合材料的破坏机理是与各相材料介质在细观上的破坏过程,即从材料劣化到裂纹萌生、扩展及贯通密切相关的,所以需要利用细观力学的知识和方法来解决这类问题。如今,随着计算环境的改善和工程实际的客观要求,数值模拟分析正转向对结构破坏的全过程进行模拟研究,运用高性能的计算机和大规模科学计算方法对材料结构进行系统的分析研究是当前计算力学的一大特色。本文采用建立在细观损伤力学基础上的RFPA数值分析系统,可以用来模拟复合材料结构的整个破坏过程,比如,裂纹萌生、扩展直至最后的失稳破坏。在模拟过程中,用Weibull随机分布函数来描述脆性基复合材料的细观非均匀性,并通过Monte-Carlo方法实现单元力学属性的随机赋值,建立细观单元的带有残余强度的弹性损伤演化本构模型。复合材料的准脆性或韧性破坏行为的非线性可以用此方法来模拟分析,同时,可以用连续介质力学模型来解决应变软化现象和一些非连续力学问题。从细观层次上对复合材料结构的破坏过程进行分析研究可以帮助理解材料的破裂机制和破坏原理,这些从物理实验和理论分析都是难于得到的。本文对颗粒增强脆性基复合材料在单轴拉伸作用下的破坏全过程进行了分析研究,着重研究了颗粒力学性质、颗粒尺寸对结构的增韧效果,并且对柔性颗粒的增韧效果以及颗粒的不同几何分布所产生的不同破裂机制进行了分析研究。研究了界面的力学性质对复合材料的力学特性和破裂机理的重要作用。为研究界面的影响,在细观尺度上将基体、颗粒及界面都看作是非均匀性的介质,在单轴拉伸荷载作用下分别对具有不同界面强度的刚性颗粒增强复合材料和柔性颗粒增强复合材料的破坏过程进行了分析研究。同时研究了含弱界面的长纤维增强脆性基复合材料在单轴拉伸荷载作用下的破坏过程和破坏机理。可以发现用本文的增韧方法来增强脆性基体结构不但可以提高复合材料的韧性,也会影响复合材料结构的强度和刚度。成功再现了破坏过程中的一些重要破坏现象,例如颗粒的穿晶破坏和沿晶破坏,纤维增强复合材料的界面脱粘／滑移,裂纹偏析／纤维桥联和拔出现象,以及与之相应的声发射现象,并对此做了细观方面的分析和讨论。此外,本文研究了层状复合材料的破坏过程及增韧机制。层状复合材料的增韧机制和传统上通过消除缺陷来提高韧性的方法不同,它是一种能量耗散机制,其结构设计将使材料强度对微缺陷的存在不那么敏感,成为一种耐缺陷材料。本文模拟了层状复合材料的裂纹扩展过程,与实验结果十分吻合。在此基础上,分别讨论了软层的强度、软层的弹性模量以及层状复合材料硬软层的厚度比对复合材料强度和增韧性能的影响。最后,本文还进行了三维数值模拟分析,通过高性能的并行计算,运用RFPA3D数值模拟分析程序模拟和研究了三维条件下层状复合材料和破裂过程和破裂机理,更形象真实地反映了复合材料的破坏过程以及破裂机理。更多还原

【Abstract】 Composite material failure process has been a hotspot in these decades in mechanics investigations, materials and engineering investigations due to the difficulties and the significance related to it. The investigation on failure process and fracture mechanisms on brittle composite matrix is a great challenge to many researchers. However, such research will reveal the mechanism of fracture developing and toughness strengthening, and therefore, it is important and helpful to optimization design of brittle composite materials.Brittle composite materials are weak in their tensile resistance, the failure often occurs in a short time which will lead to a tragic consequence. How to improve their toughness is, therefore, a hot topic in recent composite material studies. However, due to cross coupling properties of various factors, which have great effect on the fracture developing of composite materials, there is lack of powerful means with analytical method to conduct theoretical research in this field. Currently, analytical works concentrate mostly on analyses of stress states and study of fractural criteria. The knowledge to fractural mechanism is still not adequate. Furthermore, the complicated and random behavior in fracture developing test, which can be generally owed to the randomicity and dispersancy of the composite material prosperities, limit the application of laboratory test in such research.It has been well known now that numerical study is one of the important approaches to analyze material failure process. It is almost now our final hope to provide a means to study the failure process of brittle composite in a full scope. However, although finite element method is a useful and effective tool among various numerical methods, the traditional one has its limit when failure problem is encountered. Normally, the traditional numerical methods can be applied to get a satisfactory initial stress, initial strain fields or the final stress state. But it cannot work well with collapsed cells, which means when failure occurs in the composite. New techniques and new methods should be adopted to meet the needs of mesomechanics. Now, with the improvement of computing environment and the practical demands in composite engineering, the failure process analysis is turning its steps to simulate the complete process of the whole structure. The appearance of high performance computing and simulation approaches aiming at large scale structures systematically is one of the main features of current computing mechanics.In the thesis, RFPA, a numerical code developed based on meso-damage mechanics, is adopted to simulate the whole failure process, i.e, crack initialization, coalescence, propagation and final rupture under uniaxial tensile load. In the simulation, heterogeneity in composite is modeled by using stochastic functions, such as Weibull, even and normal distribution function, and Monte-Carlo approach. A relatively simple constitutive law, strain-softening model, is applied to each elementary cell. The cell in the calculation will keep its remaining strength after it collapses.With these techniques, non-linear deformability of a quasi-brittle or ductile failure behavior of the composite can be simulated with RFPA. Furthermore, phenomena of strain-softening and some discontinuum mechanics problems can also be revealed by using a continuum mechanics model. The fracturing evolution in mesoscopic fragments, can help to understand mechanism of failure and principles of composite, which cannot be acquired by laboratory experiments and theoretical analysis.Fundamental theory of this research, numerical modeling method and verification of the modeling have been introduced in Chapters1-3.In Chapter4, whole failure process of brittle composite matrix reinforced by a single particle or particles under uniaxial tensile load has been studied. The properties of particles, the effect of particle size, the different failure mechanisms for particles with different flexibility, and the effect of particles alignment are consequently investigated.In Chapter5, the study revealed that mechanical properties of interface have significant effect on the composite and its final fracture mechanism. To understand such an effect, the failure process of particle reinforced brittle composite with interfaces in different strengths has been studied under uniaxial tensile load, while inhomogeneity in mesoscale is applied to the matrix, particles and interfaces, respectively.In the same Chapter, the failure process and mechanism of a brittle composite matrix, which is reinforced with long fibers with weak interface, is also studied. The fiber reinforcement with weak interface could bring out not only higher strength and stiffness, but also higher toughness of the composite. Some important phenomena, such as the interface debonding and sliding, crack deflexion, fiber bridging and pulling out, and etc. have been successfully reproduced and revealed. Finally, the accompanied result of acoustic emission is also presented.Failure mechanism and toughness enhancement of laminated composite material are studied in Chapter6. Such mechanism is totally different to traditional flaw-elimination method. It is actually a system of energy dissipation. Structurally, it is insensitive to the existing of micro flaws and could be considered as flaw-toleration. The simulation of fractural developing process for laminated material, carried out in this study, matches well with the experimental result. Consequently, the effect on the strength and toughness of the composite, made by the strength and Young’s Modulus of soft layers, thickness ratio of hard and soft layers, is investigated.Finally, A code RFPA3D, is be used to reveal the fractural process and mechanism of laminated composite in a real world scope. The parallel computing technique has been involved in to make the calculation in an acceptable time. The result of this three-dimensional simulation is presented in Chapter7.更多还原