碳纤维增强树脂复合材料细观蠕变性能

碳纤维增强树脂复合材料以其优异的性能,在各领域得到广泛应用。由于树脂基体具有黏弹性,使其合成的复合材料也表现出黏弹性行为。蠕变是材料黏弹性行为中最典型的一类现象,因此对碳纤维增强树脂复合材料细观蠕变性能的研究具有重要意义。室温下利用纳米压痕技术对碳纤维增强树脂复合材料中的基体、界面及纤维相在不同峰值载荷下的细观蠕变行为进行分析。结果表明:在相同的蠕变时间下,最大载荷为2 mN和10 mN的纤维蠕变位移约为基体蠕变位移的1/3和1/2,界面的蠕变位移介于两者之间;稳态蠕变阶段的蠕变速率小于0.1%;基体、界面、纤维的蠕变应力指数分别为3.6、2.9和2.1。同时根据Kelvin-Voigt模型得到了基体、界面及纤维的第一、第二复数模量、黏度系数及蠕变柔量。      

纤维复合材料粘弹性动态性能的细观力学分析

建立了纤维增强复合材料粘弹性动态性能的细观力学模型.首先建立了树脂基体相各向同性的粘弹性模型,模型参数由纯树脂基体材料的蠕变试验获取,在此基础上分别建立了针对单向和正交铺层复合材料的横观各相同性、正交各相异性的细观粘弹性模型.对单向、正交铺层复合材料进行了静态和动态试验,分别通过试验和上述理论模型得到了其阻尼比、动态储存模量、损耗模量和损耗因子,理论预测与试验结果吻合.     

三维机织复合材料多尺度黏弹性分析

建立了一种三维机织复合材料多尺度的黏弹性分析模型。首先构造了微观尺度纱线束胞元和细观尺度复合材料周期结构胞元两级有限元模型, 由微观尺度胞元分析得到纱线束的弹性常数, 再代入细观尺度胞元计算出复合材料的平均弹性常数。两级胞元模型均施加周期边界条件, 保证了胞元边界上位移和应力满足周期性和连续性。随后分别建立了树脂基体和浸润树脂纱线束的蠕变模型, 用实验标定树脂的蠕变参数, 代入微观尺度胞元进行蠕变计算来修正纱线束蠕变模型的参数。最后将树脂和纱线束的蠕变本构关系应用于细观尺度胞元, 得到材料宏观平均的应力-应变响应, 模拟了三维机织复合材料的蠕变实验曲线。本文模型对于该种复合材料弹性常数和蠕变性能的预测, 均与实验吻合。     

复合材料弹性模量随机计算模型

从纤维在复合材料中排列不规则的事实出发, 运用数值模拟的方法初步研究了纤维排列的随机性, 并将结果初步应用在复合材料模量的细观力学计算当中, 发现纤维随机排列对细观力学研究复合材料的力学性能有较大的影响, 表明如何在复合材料细观力学中恰当地考虑纤维随机排列是一个值得研究的问题。     

聚合物基纤维增强复合材料的蠕变混合律

本文首次对聚合物基纤维增强复合材料的蠕变混合律进行了研究。推导出具有正交双向铺设纤维复合材料的微观力学模型,用此模型对聚合物基纤维增强复合材料的蠕变性能进行了预测,并对其线性蠕变混合律进行了实验验证。实验结果表明,这个理论对复合材料线性蠕变的预测是很精确的。     

残余应力对复合材料弹2塑性变形的影响

从细观力学的角度给出了分析残余应力对一般复合材料塑性性能影响的一种解析方法, 该方法基于应力二阶矩的割线模量法及Ponte Castaneda 和W illis 给出的弹性细观模型。有残余应力时, 所提的细观解析模型能够同时考虑纤维形状, 体积百分比, 纤维取向及纤维的分布对复合材料变形的影响。计算结果表明, 残余应力的存在会引起复合材料拉压变形的不对称, 材料宏观的拉压硬化曲线又与复合材料的细观结构参数密切相关。对单向复合材料, 本文作者对其等效割线热膨胀系数, 拉压应力-应变曲线的有限元分析结果与给出的细观解析模型定量吻合。     

化学气相渗透2D-SiCf/SiC复合材料的蠕变性能及损伤机理

研究了采用化学气相渗透工艺制备2D-SiC_f/SiC复合材料的真空蠕变性能, 蠕变温度为 1200、1300和1400 ℃, 应力水平范围为100~140 MPa。用扫描电子显微镜(SEM)和高分辨透射电子显微镜(TEM)分别观察分析了2D-SiC_f/SiC复合材料的蠕变断口形貌和微观结构。结果表明, 2D-SiC_f/SiC复合材料的主要蠕变损伤模式包括基体开裂、界面脱粘和纤维蠕变。桥接裂纹的纤维发生蠕变并促进了基体裂纹的张开、位移增大, 进一步导致复合材料蠕变断裂, 在复合材料蠕变过程中起决定性作用。2D-SiC_f/SiC复合材料的蠕变性能与SiC纤维微观结构的稳定性密切相关。在1200 ℃/100 MPa时, 纤维晶粒没有长大, 复合材料的蠕变断裂时间大于200 h; 蠕变温度为1400 ℃时, 纤维晶粒明显长大, 2D-SiC_f/SiC复合材料蠕变断裂时间缩短至8.6 h, 稳态蠕变速率增大了三个数量级。     

基于细观力学的纤维沥青混凝土有效松弛模量

为了研究纤维沥青混凝土的本构模型,将其视为以沥青混合料为粘弹性基体,纤维为弹性夹杂的两相复合材料。对基于复合材料细观力学理论建立的有效模量表达式进行了修正,提出了纤维沥青混凝土的割线有效松弛模量。以聚酯纤维沥青混凝土为例进行了有效松弛模量的解析分析和模拟蠕变实验的有限元分析,分析结果与试验数据的比较表明,该文提出的割线有效松弛模量模型对于纤维沥青混凝土粘弹性力学行为具有很好的预测能力。应用该模型对路面弯沉变形进行了有限元分析,结果表明:纤维的加入有效的改善了沥青混凝土路面的粘弹性性能。     

复合材料层合板多尺度交互渐进损伤分析

纤维增强复合材料强度的准确表征是复合材料力学性能研究的核心问题之一。该文以碳纤维增强树脂基复合材料层合板为研究对象,基于宏观-细观多尺度分析方法,根据复合材料的物理失效模式分别给出了基体和纤维的细观失效准则,同时考虑基体失效对复合材料层合板纤维轴向力学性能的影响。提出了新的刚度退化方式,可准确表征复合材料层合板的损伤演化过程,开展了复合材料层合板四点弯模型的多尺度交互渐进损伤分析和试验验证。结果表明:基于多尺度方法的复合材料层合板宏-细观交互渐进损伤分析结果与试验结果吻合较好,新的刚度退化方式可以准确模拟层合板的失效过程。     

纤维增强高分子聚合物基复合材料有效性能的三维数值分析

将细观力学和计算力学方法相结合用以确定复合材料中的局部和平均应力-应变场.对旋转体和非旋转体纤维增强复合材料的有效模量进行了三维有限元数值计算.分析了纤维的排列分布和纤维的几何形状对有效模量的影响.数值结果表明,轴向杨氏模量对细观结构不敏感,而纤维的形状排列方式对横观有效性能影响是显着的.     

Creep of granular materials

This paper examines the creep of brittle granular materials subjected to one-dimensional compression. One-dimensional creep tests were performed on aggregates of brittle pasta and compared with the behaviour of sand at much higher stress levels. It was found that for both materials, creep strain is proportional to the logarithm of time. One possible mechanism for creep is particle crushing. However, it is usually difficult to measure changes in the particle size distribution during creep because the fines produced are so small, and the mass of fines is too small to measure accurately unless creep is permitted for a very long time. However, for pasta, the particle fragments produced are large, and it is found that particle crushing does occur during creep for 24 hours. This is consistent with the proposition that the behaviour of all brittle granular materials is essentially the same. A micro mechanical argument is then summarised which predicts that creep strain should be proportional to log time.     

Effects of inclusion on the creep rate of piezoelectric films

This paper presents an analytical method to study the effect of inclusions in piezoelectric materials on the creep rate. The driven force for the creep rate of piezoelectric materials with inclusion is from diffusional mass transport along the inclusion interface. The results show that the creep rate of piezoelectric materials containing the rigid inclusion with the shape parameter m = 0.8 appears at the minimum, and the effect of inclusion volume fraction on the creep rate of piezoelectric materials with soft inclusion becomes the smaller and smaller, as the stiffness of soft inclusion decreases. Thus, the effect of inclusion on creep characteristics can be improved by controlling the material property, the sizes, shapes, and volume ratio of inclusions.     

金属材料的抗蠕变机理及方法综述

本文分析了传统金属材料抗蠕变性能的不足以及研制抗蠕变性能优良的金属材料的必要性。通过分析影响蠕变的因素,得出蠕变温度和蠕变应力能够影响蠕变行为,而析出相又是影响蠕变行为的关键因素这一结论。综述了固溶强化、析出强化、弥散强化和晶界强化等几种提高材料抗蠕变性能的方法及机理,列举了几种常见的蠕变试验方法。最后展望了抗蠕变金属材料广泛的应用前景。     

Dislocation creep in particle-strengthened systems

A model for climb-controlled creep in two-phase materials is proposed which invokes the presence of a back-stress for providing the necessary mobile dislocations for creep. It is shown that by incorporating this stress in the creep equation it is possible to reduce both the activation energy for creep and the stress exponents to values normally observed in single-phase materials. Creep data on TD nickel and yttriated superalloy when analysed on this basis confirm the applicability of this model.     

Size dependency of self-diffusion and creep behavior of nanostructured metals

Grain boundary and bulk diffusion are two main governing processes of creep deformation in materials. Since the diffusion activation energy for nanostructured materials is lower than that for bulk, the diffusion is enhanced at the nano-scale, and nanosructured materials are expected to creep at lower temperatures and stresses. In this paper, a model has been developed to explain the effect of grain size on diffusion and creep behavior of nanostructures. Diffusion and creep phenomena have been shown to depend significantly upon size. Comparisons have been made with reported experimental results and related theoretical studies.     

Mathematical description of the mechanical behaviour of metallic materials under creep conditions

The metallic materials creep behaviour has been described and a complete model is presented. The basic constitutive equation, as well as the structure parameters, have been derived from a mathematical analysis that represents the dominant physical procedures and mechanisms. The model is very general because it is referred to all stages of creep and describes the creep behaviour of all metallic materials, including those strengthened by a dispersion of second-phase particles. A creep function has been derived from the constitutive equation describing all three stages of creep under constant loading. The function has the minimum possible number of fitting, parameters. The dependence of the fitting parameters on the loading conditions has been described using very simple mathematical relations. Applications and predictions have been carried out in a wide range of metallic materials. Good agreement has been shown by a comparison made also between the creep curves determined experimentally, and those obtained from creep function and determined fitting parameters.     

High-temperature mechanical properties of Li2O-Al2O3-SiO2 (LAS) glass ceramics

The microstructures and creep properties of one experimental and two commercial LAS glass ceramics have been studied. The commercial materials creep at lower temperatures and exhibit more prominently non-linear creep (creep rate decreasing with time). Heat treatment causes grain growth and a redistribution of precipitate and leads to a reduction in creep rates in all materials.     

Recovery creep in materials hardened by a second phase

A recovery creep model, based upon previous theories by McLean and co-workers, has been developed for creep in materials hardened by a second phase. According to the model the increased creep strength in these materials is caused by a decrease in the recovery rate, and this in turn is due to a decrease of the driving force for the recovery process and of the mobility of the climbing dislocations involved in the process. It is shown that the model can account for the very large stress-dependence of the creep-rate often found for alloys hardened by a second phase. Another support for the model is the observation that changes in the creep-rate for materials in different states of precipitation-hardening are entirely due to changes in the recovery rate.     

Proposal of a new concept on creep fracture remnant life for a precracked specimen

Abstract

The creep life time of a smooth specimen can be predicted using existing laws for creep deformation and steady state creep rate. When crack growth behaviour is involved, it is necessary to construct a law of creep crack growth rate to predict creep fracture life. Creep fracture life can be measured by integrating the law of creep crack growth rate. One example is the creep crack growth rate, represented by the parameter Q*. In this study, we investigated the applicability of this prediction method to creep fracture remnant life for a cracked specimen. The Ω criterion is proposed to predict creep fracture remnant life for a smooth specimen for creep ductile materials. In this study, the correlation between Q*_L derived from the paremeters Q* and Ω is investigated. The correlation between Q_L* and Ω provided a unified theoretical prediction law of creep fracture remnant life for high-temperature creep-ductile materials in the range from smooth to precracked specimens.     

Irradiation induced grain boundary flow-a new creep mechanism at the nanoscale

A new mechanism of irradiation enhanced creep is proposed for nanocrystalline materials. It derives from local relaxations within the grain boundaries as they absorb point defects produced by irradiation. The process is studied by inserting point defects into the grain boundaries and following the materials response by molecular dynamics. Calculated creep compliances are found in good agreement with those measured in dilute nanocrystalline Cu-W alloys [Tai, K.; Averback, R. S.; Bellon, P.; Ashkenazy Y. Scr. Mater.2011, 65, 163]. The simulations provide a direct link between irradiation induced creep in nanocrystalline materials with radiation-induced viscous flow in amorphous materials, suggesting that grain boundaries in these materials can be treated as an amorphous phase. We provide a simple analytic model based on this assumption that reproduces the main features of the observed creep rates, a linear dependence on stress, inverse dependence of grain size, a weak dependence on temperature, and a reasonable estimate of the absolute creep rate.