【作者】 王放；

【导师】 张俊乾；

【作者基本信息】 上海大学 ， 固体力学， 2008， 博士

【摘要】 纤维增强金属基复合材料(MMC_f)是以金属或者合金为基体,以不同材料的纤维为增强相的复合材料,其特点在于有一个连续的金属或者合金基体,其它组元相则是均匀地分布在基体中。作为一种非均匀和各向异性材料,它具有传统单一材料不可具备的优越性能,在诸如先进涡轮发动机和超音速飞行器等航空结构中的应用具有令人鼓舞的前景。这些结构要求承受非常复杂的热/机械循环载荷。为了这类MMC_f结构的安全设计和有效使用,一个非常紧迫的任务是要深刻地认识MMC_f的热机械静力与疲劳渐进破坏过程的细观机理以及发展热机械强度和疲劳破坏分析的理论方法。复合材料的破坏过程,从本质上讲是损伤不断累积,材料性质不断退化,应力重新分布的动态过程。本文基于剪切滞后模型和影响函数叠加的方法,考虑纤维和基体同时承受轴向拉伸载荷,纤维和基体之间的应力通过两者之间的剪切层来传递,剪切层以剪应力为主,基体采用理想弹塑性模型。针对MMC_f在裂纹和界面附近可能产生局部循环热塑性变形及其诱导的纤维/基体脱粘(debonding)的特点,提出了纤维断裂伴随局部基体拉伸屈服、局部界面剪切屈服以及界面脱粘的细观破坏力学模型。将每一根纤维断裂及其相伴的基体屈服、界面屈服以及界面脱粘称为一个损伤实体,将多损伤实体相互作用的复杂问题转化为多个单纤维断裂与单基体断裂问题,从而发展了一套包含纤维的统计强度分布、基体的拉伸屈服特性、纤维和基体界面局部循环热塑性以及纤维/基体脱粘的多纤维断裂诱导应力与变形重分配的近似应力分析方法。研究证明:断裂纤维引起的应力扰动具有明显的局部化。因此本文取简化的有限根纤维模型进行了研究,即:将应力场的控制微分方程建立在仅受纤维断裂和基体断裂影响的有限区域之内。通过对在热/机械载荷作用下,金属基复合材料在多重损伤模式下应力响应特征的分析,发现材料的应力分布对机械载荷和温度的变化相当敏感,原因之一在于金属基体的屈服极限随着温度升高而快速地降低。实验结果进一步表明:即使对于处理非弹性这种复杂的破坏问题,这套发展的影响函数叠加方法仍然是可行的,它能够充分地考虑到破坏过程中缺陷之间的相互影响,同时保证简化后的问题经过加权叠加最终是同原问题等价的。复合材料的静力拉伸破坏过程表现为一种渐进的损伤过程:在加载的初始阶段,由于缺陷的存在,部分纤维会首先发生断裂,在纤维断裂附近的基体以及基体和纤维的界面会产生局部热塑性变形,细观应力变形重分配,伴随更多的纤维破坏及局部塑性,大量纤维的失稳破坏亦即复合材料的最终破坏。因此,在所建立的多重损伤实体细观力学模型及其应力分析方法的基础上,本文假设纤维的拉伸强度服从Weibull分布,同时考虑纤维断裂、基体拉伸屈服、界面剪切屈服和基体与界面同时发生塑性变形四种损伤模式,提出了一个能够较好地模拟复合材料在拉伸条件下损伤演化过程的Monte-Carlo二维模型,得到了材料的宏观应力-应变曲线,研究发现复合材料在热机械载荷条件下的最终拉伸强度不仅与复合材料的尺寸大小有关,而且取决于纤维强度分布的统计性质及其应力分布状况。另一方面,也分析了采用Weak-link原理能对不同尺寸复合材料的拉伸强度分布规律进行预测的适应范围,可以认为只有在一定的尺寸范围内,Weak-link原理才是适用的,而且还与纤维强度分布中的形状参数等有关。对复合材料静强度的研究,有助于从细观的破坏机理上理解和认识拉伸破坏过程,为进一步发展基于破坏机理分析的疲劳寿命预测理论奠定了基础。对于高应变、短寿命的复合材料热/机械疲劳,对应的主要细观破坏机理是纤维断裂。由于纤维拉伸强度的随机特性,在第一个循环载荷峰值点,部分纤维发生断裂,导致纤维断裂附近基体和基体/纤维界面产生局部循环热塑性变形,循环塑性引起基体/纤维界面脱粘,细观应力变形重分配。随着循环的增加,更多的纤维断裂及伴随的局部塑性和debonding,直至复合材料断裂破坏。这一区段的疲劳破坏特点是:破坏过程是渐进的、寿命具有随机性且对载荷大小非常敏感。本文从纤维增强金属基复合材料热/机械疲劳渐进破坏过程机理的分析出发,利用所建立的多纤维断裂诱导应力与变形重分配的细观损伤力学模型,考虑金属基体在热/机械载荷条件下循环响应的特征,引入一个循环周期内基体的塑性剪切应变的变化幅值作为判断纤维与基体发生脱粘的重要参数,发展了纤维断裂、局部基体拉伸循环塑性、局部界面剪切循环塑性以及基体/纤维脱粘等细观破坏模式控制的疲劳破坏分析模型,建立了一套能够预测复合材料热/机械疲劳寿命的理论体系和方法。通过采用Monte-Carlo方法较好地模拟了复合材料的热机械疲劳损伤演化过程,揭示了不同热机械载荷水平与疲劳破坏的细观机制之间的内在联系。研究结果反映:在不同热机械载荷水平作用下,材料的细观破坏模式是不一致的,因而使得解释和预测同相(in-phase)热机械疲劳(TMF)的S-N曲线与反相(out-of-phase)TMF的S-N曲线发生交叉成为可能,即高应变TMF加载时,同相TMF寿命比反向TMF寿命低,而在应变幅较低时,同相TMF寿命比反向TMF寿命高。同时,还研究了试件大小、纤维体积分数等细观结构参数对热机械疲劳寿命的影响,将热机械疲劳寿命S-N曲线与基体循环热塑性、体积分数、纤维统计强度以及纤维/基体界面特性定量地联系起来。纤维增强金属基复合材料热/机械变形与破坏的研究一直是国际固体力学和材料科学领域众多学者关注的热点,是一个非常具有挑战性的课题。其特点亦即难点在于如何概括某些对宏观力学行为起敏感作用的细观和微观因素,以及这些因素的演化,从而使得非均匀材料的强化、韧化以及破坏分析立足于科学的认识之上。本论文从纤维增强金属基复合材料热机械静力和疲劳渐进破坏过程机理的分析出发,发展了一种热/机械强度和疲劳破坏分析的理论方法。模型的最大特点之一是,通过多尺度的连续介质力学研究,将材料的宏观力学行为与细观破坏因素定性和定量地联系起来。这种方法为分析多尺度的材料/结构力学行为提供了新的途径。通过本文的研究,希望提升人们对纤维增强金属基复合材料变形与破坏的认识水平,从破坏机理上理解并预测其热/机械强度和疲劳寿命。因而,既具有工程应用价值又具有科学意义。更多还原

【Abstract】 Continuous fiber-reinforced metal matrix composites, shortened by MMC_f, are made up of matrix and reinforcement. The former usually consists of metal or alloy and the latter is composed of many kinds of fibers. More distinctly, it is characterized by a continuous metal or alloy matrix which is equably crammed with other constituent phases. As a heterogeneous and anisotropic material, this composite has an inspiring prospect of wide practical applications, especially in aeronautic and aerospace structures, such as advanced turbine engines and ultrasonic aerocraft, which displays unique superior performance as compared to other conventional materials. In many typical applications, MMC_f are subjected to a cyclic mechanical loading along with a superimposed variation in temperature. This type of complicated loading condition is referred to as cyclic thermomechanical loading. To submit an efficient employment and to take advantage of the entire application potential, it is an urgent task to require a through understanding of the micro-mechanism for the progressive failure under thermo-mechanical static and cyclic loading and to develop methodology to predict their strength and fatigue life.As a matter of fact, the failure process of the composites includes the ceaseless accumulation of damage, gradual degradation of property and stress redistribution. Based upon the shear lag model, we have presented a micromechanically analytical model using an influence function superimposition technique to derive stress profiles for any configuration of breaks in MMC_f under thermo-mechanical loading, by considering the effect of variations in fiber strength, local matrix tensile yield and interface yield (or sliding). Compared with the other models, both the matrix tensile stress and the fiber tensile one have been taken into account. The shear stress is transferred through the interface between fiber and matrix. The local plasticity is modeled by the elastic, perfectly-plastic shear stress-strain relation. In this study, we have restricted our attention to establish a model characterizing progressive failure of MMC_f from the view of micromechanism, which are concerned with the both matrix tensile yield and interface yield and debonding around broken fibers. A broken fiber, accompanying with its yielding matrix and its yielding interface and its debonding interface is called as a damage entity. The case of the interactions among these multi entities is divided into two sub-cases: a single fiber break and a single matrix break, so the solution for the shear yield of interphase or matrix tensile yielding or matrix tensile yield and interface shear yield triggered by multiple fiber breaks can be availably obtained. According to the fact of dramatic localization of stress disturbance, a simplified model is adopted, through taking the governing differential equations controlled inside some limited regions which are affected by broken fibers. The characteristics of stress distribution for multiple damages under thermomechanical loading illustrate that the stress distribution is strongly sensitive to the outside surroundings, such as mechanical load and temperature, especially for in-phase and out-of-phase conditions. It is due to the sharp decline of the yield stress for the matrix with the rising temperature. The experimental observations can further emphasize that this superimposition is not merely simple but completely feasible. For such a non-elastic case, it is reasonable for the solved case to satisfy the origin condition through superimposition, more importantly, including the interactions among those disfigurements.Under static loading, the peculiarity of the tensile damage process appears to be progressive: Since the fiber strength exhibits large variability due to statistic distribution of defect, the first fiber break, usually, takes place at very early loading stage, leading to local thermo plasticity for matrix and interface around the broken fiber. Consequently, the microstress is redistributed due to local stress concentration. More fiber breaks and serious local plastic deformation occur as the applied load increases further, leading to the final failure of the composite at some loading level. In other word, the ultimate failure of the fiber-reinforced composites is largely dominated by accumulation of the large amounts of fiber breakage. Therefore, based upon the micromechanically analytical model under multiple damages, considering that the tensile strength of fibers follows a two-parameter Weibull distribution, a 2-D Monte-Carlo model is developed to simulate failure process for MMC_f under tensile thermo-mechanical loading, which is a combination of fiber fracture, local matrix tensile yield and interface yield throughout the application of applied loading. The macro stress-strain response for the composites shows that the microdamage mechanism is derived from a dominant "critical cluster" of breaks. The results from the several hundred Monte-Carlo simulations indicate that the ultimate tensile strength of the composites not only depends upon the composite length and width, but also is dominated by fiber strength statistics and stress redistribution due to progressive microdamage. Secondly, The mean tensile strength of the composite strongly depends on the magnitude ofβ. As the scatter of the fiber tensile strength increases the tensile strength for composites will dramatically decrease. On the other hand, it is shown that weak link scaling works very well within some limited range, which leads us to a significative conclusion that larger sizes are required to produce weak link scaling with smaller shape parameterβ. Modeling of the thermo-mechanical failure process of the fiber composite materials with ductile matrix helps us a comprehensive understanding of damage behavior and failure mechanism, which can be extended to approaches to fatigue life modeling of ductile matrix composites.In regime 1 for the fatigue life diagram of the maximum applied strain versus the cycles to failure, its corresponding failure micro-mechanism is rested with fiber breakage under high applied strain and low cycles. Just for the statistical particularity of fiber tensile strength, very small fiber break, usually, take places at peak values during the first loading cycle, stimulating local cyclic plasticity deformation for constituent material and interfacial debonding around the fiber break, then the microstress is redistributed in order to release the internal enriched force. Accompanying serious local plastic deformation and debonding, more and more fiber will continually be fractured as the number of load cycles increases further, leading to the fatigue failure of MMC_f. The distinctive property for this regime is the progressive failure and the sensitivity of fatigue life to the amplitude of applied loads. Based on the above characteristics of progressive damage mechanisms, a micromechanical model was then introduced to predict the evolution of fatigue damage, which described the development of a constituent micro-mechanical damage model and reflected the nonlinear cyclic response for the metal matrix, taking into account the behavior of each constituent (i.e. the fibers, the matrix and the interface) of the composite. The alternating plastic shear strain range of matrix is taken as an important index for debonding. Constitutive equations incorporating the effect of damage development and evolution are developed at the constituent level and then used to predict the overall behavior of the material system. The fatigue failure process is modeled by Monte-Carlo method. It is illustrated that the fatigue life of such a composite depends on the evolution of damage, which is a combination of fiber fracture, interfacial debonding, slipping and inelastic matrix deformation during the regular service life. Note that two curves for fatigue life cross over one another, i.e. the fatigue life for in-phase TMF conditions at higher stresses was considerably less than those obtained under the comparable out-of-phase TMF conditions. On the other hand, in the case of low-stress, the fatigue life for in-phase conditions is seen to increase considerably from that obtained under comparable out-of-phase conditions. This is due to the difference of the dominated failure mode for this two TMF conditions. Micro structural parameters are also studied to investigate the dependence of thermo-mechanical fatigue life on these factors. To our delight, the curve of fatigue life of MMC_f under thermo-mechanical loading is quantitatively related to the local thermo-plasticity of matrix properties, the volume fraction, the statistical strength of fiber and the interface characters between fiber and matrix.It is a very challenging task for fiber-reinforced metal matrix composites to be predicted the complex deformation behavior and to be controlled the damage tolerance characteristics under thermo-mechanical loading. To our surprise, it has engendered considerable scientific and technological interest for a long time. Its peculiarity and difficulty is how to gather up those meso and micro damage mechanisms, which can strongly affect the macro-mechanical behaviors, and its evolution, thereby the analysis of mechanical performance for heterogeneous materials can be established upon the scientific understanding. The project aims to develop a theoretic model that can predict thermomechanical strength and fatigue life of fiber reinforced metal-matrix composites and to preferably disclose the inherent relation between such thermo-mechanical propeties and the micromechanical mechanism of damage. The brightest point is that through multi-scale continuum mechanics the macro mechanical behavior can be qualitatively and quantitatively related to micro-factor of deformation and failure. It is shown that this adoptive method serves a new route for analyzing multi-scale mechanical behavior for materials/structures. Therefore, this research is expected to take an important effect on promoting our understanding ability of damage and failure behavior and enhancing our capability for comprehending and predicting the tensile strength and thermomechanical fatigue life for MMC_f. In a word, it is self-evident for the engineering application value and scientific significance.更多还原