短纤维增强金属基复合材料基体中的应力分布及其变形特征

基体行为是影响随机分布短纤维增强金属基复合材料力学性能的一个重要因素。本文作者将在短纤维复合材料单纤维三维模型的基础上, 借助于弹塑性有限元分析方法, 研究在加载过程中, 不同纤维位向和界面结合状态下基体中的应力分布情况及基体的变形特征。研究表明, 该类复合材料中基体的应力分布情况和变形特征将受到纤维位向改变和界面结合强弱的显著影响。      

界面对短纤维增强金属基复合材料力学行为的影响

界面是复合材料中一个非常重要的因素, 本文将在实验分析的基础上建立合理的理论分析模型, 借助于轴对称和三维有限元分析方法, 对界面性能的变化对短纤维增强金属基复合材料力学行为的影响作较为系统和深入的研究, 其中包括界面性能对应力传递机制、弹性模量、应力-应变曲线以及断裂机理的影响。研究表明, 界面性能的好坏显著影响基体与纤维间的应力传递, 从而对复合材料的弹性模量、应力-应变行为和断裂机理产生较大的影响, 界面控制是复合材料设计中不可忽视的重要环节。      

氧化铝短纤维增强铝基复合材料的蠕变破坏行为

研究了挤压铸造Al₂O₃短纤维增强铝基复合材料在350℃恒应力条件下的蠕变行为。蠕变试验过程中采用中断实验的方法对复合材料的显微组织进行观察,发现复合材料在蠕变过程中纤维发生断裂,弱界面发生破坏以及基体合金在应力作用下发生变形。根据复合材料在蠕变三个阶段中显微组织的变化情况,对其宏观蠕变行为进行了分析,认为位错在复合材料中滑移和攀移控制整个蠕变过程,并提出了短纤维增强金属基复合材料的蠕变断裂机理,合理地解释了复合材料的蠕变过程。      

聚丙烯纤维网混凝土桥面铺装的性能研究

纤维增强混凝土简称纤维混凝土,是在素混凝土基体中掺入均匀分散的短纤维而组成的一种复合材料。其主要工作机理是利用均匀分散的短纤维来改善混凝土的脆性。在受力过程中,发挥纤维抗拉强度高、混凝土抗压强度高的各自优势,从而显著提高纤维混凝土材料的各项技术性能,使其具有优良的抗拉、抗弯、抗疲劳、抗冲击性能以及耐一磨耗、韧性高等特点。本文研究了聚丙烯纤维对桥面铺装混凝土的改性作用与机理。     

氧化铝短纤维增强铝合金复合材料界面的研究

本文在浸渗法制备复合材料的基础上,研究了氧化铝短纤维增强铝合金复合材料的界面.研究结果表明,在复合材料中,纤维与基体之间存在明显的界面层,合金元素通过适当的化学反应可改善浸润状况,有利于纤维与基体的结合.试样断口分析还表明,复合材料的界面结合状况良好,可传递足够的载荷到纤维上,发挥其增强作用.     

钢纤维聚合物混凝土抗压本构关系

钢纤维聚合物混凝土是由聚合物混凝土基体和钢纤维共同组成的纤维增强复合材料，它的力学行为不仅依赖于聚合物混凝土基体的行为，而且与钢纤维的掺量、分散特征以及钢纤维的几何尺寸有关，本文将基于损伤力学原理研究在不同纤维掺量下的钢纤维聚合物混凝土的全程压力-应变曲线及其本构模型。     

基体特性对δ-Al2O3/Al合金复合材料力学行为的影响

基体行为是短纤维增强金属基复合材料中一个重要的因素.本文将通过实验分析方法和在一定的理论分析模型上,借助于弹塑性有限元分析方法,对基体特性的变化对δ-Al2O3/Al合金复合材料力学行为的影响作较为详细的研究,其中包括基体性能对应力传递、抗拉强度以及断裂机理的影响.研究表明,基体性能的变化显著影响基体与纤维间的应力传递,从而对复合材料的抗拉强度和断裂机理产生较大的影响.     

界面脱粘对正交铺设SiC/CAS复合材料基体开裂的影响

采用细观力学方法研究了正交铺设SiC/CAS复合材料在单轴拉伸载荷作用下界面脱粘对基体开裂的影响。采用断裂力学界面脱粘准则确定了0°铺层纤维/基体界面脱粘长度, 结合能量平衡法得到了主裂纹且纤维/基体界面发生脱粘(即模式3)和次裂纹且纤维/基体界面发生脱粘(即模式5)的临界开裂应力, 讨论了纤维/基体界面剪应力、 界面脱粘能对基体开裂应力的影响。结果表明, 模式3和模式5的基体开裂应力随纤维/基体界面剪应力、 界面脱粘能的增加而增加。将这一结果与Chiang考虑界面脱粘对单向纤维增强陶瓷基复合材料初始基体开裂影响的试验研究结果进行对比表明, 该变化趋势与单向SiC增强玻璃陶瓷基复合材料的试验研究结果一致。     

界面相性态对纤维增强复合材料内应力传递的影响

本文用有限元法研究了具有基体裂纹的纤维增强复合材料内的应力传递问题。假设纤维与基体的界面为非理想的，文中运用“弹簧层”模型首先分析了在不同的组分弹性模量比、纤维体积含量与边界约束条件下，界面相性态对复合材料的应力传宾影响，然后进一步考察了在几种典型的损伤模式下界面附近的应力分布情况。     

界面对硅酸铝短纤维增强AZ91D镁基复合材料蠕变性能的影响

界面对复合材料蠕变性能的影响很大。在试验分析的基础上建立了硅酸铝短纤维增强AZ91D镁基复合材料理论分析模型,利用三维有限元分析方法,系统研究了界面特性、界面上应力应变分布和短纤维位向变化对硅酸铝短纤维增强AZ91D镁基复合材料蠕变性能的影响。研究表明：界面特性,如厚度、模量,均对纤维最大轴应力和稳态蠕变速率有影响,当界面厚度增加,纤维最大轴应力减小而稳态蠕变速率增大;当界面模量增大,纤维最大轴应力增大而稳态蠕变速率减小,但当界面模量高于基体模量时,纤维最大轴应力和稳态蠕变速率均保持不变;纤维位向也影响轴应力分布和稳态蠕变速率,纤维在其末端界面上存在较大的应力和应变,此处容易产生微裂纹而使材料抗蠕变能力下降;界面对硅酸铝短纤维增强AZ91D镁基复合材料的蠕变曲线和蠕变断裂机制也有影响,其影响程度还与纤维位向有关。     

Effects of interfacial bonding on sliding phenomena during compressive loading of an embedded fibre

The effects of interfacial bonding on the sliding phenomena at the fibre/matrix interface are considered for fibre-reinforced ceramic composites when an axial compressive stress is applied at the exposed end of an embedded fibre. Sliding occurs at the interface when the interfacial shear strength is exceeded. The interfacial shear stress, the stress transfer from the fibre to the matrix, the length of the sliding zone, and the fibre displacement are analysed in the present study. The results show that at a fixed load, the sliding length and the fibre displacement decrease with increase in the interfacial shear strength. Effects of interfacial bonding on the applied stress-fibre displacement relationship during compressive loading and subsequent unloading are also revealed.     

Some further considerations of the theory of fibre debonding and pull-out from an elastic matrix. Part 2: Non-constant interfacial frictional shear stress

Solutions for fibre debonding and pull-out from an elastic matrix have previously been obtained under the condition that the interfacial frictional shear stress is independent of the bond length. However, this condition is not strictly valid. In this paper, an analysis of fibre debonding and pull-out is carried out by considering the dependence of the interfacial frictional shear stress on bond length. The maximum fibre pull-out stress necessary to cause complete debonding and eventual pull-out is determined to be dependent on the fibre-to-matrix elastic modulus ratio, the interfacial shear strength, the interfacial frictional coefficient, the fibre volume fraction and the embedded fibre length. Excellent agreement between the present theoretical analyses and existing experimental results is obtained.     

A study of mechanisms of stress transfer in continuous- and discontinuous-fibre model composites by laser Raman spectroscopy

The micromechanics of stress transfer in single-fibre/epoxy-resin model composites has been investigated. Two specimen geometries are examined incorporating both continuous and discontinuous fibres in epoxy resin blocks. In both cases, the point-by-point strain in the fibre is measured from the fibre Raman spectrum and its strain dependence. The continuous-fibre model composites (CFMC) are subjected to incremental tensile loading and the fibre fragmentation process is continuously monitored. The short-fibre model composites (SFMC) are loaded incrementally to levels of stress of sufficient magnitude to cause interfacial failure and the fibre strain profiles are obtained at each level of applied stress.

In addition to fibre strain measurements, the interfacial shear stress distribution is derived at each increment of applied stress by means of a balance-of-forces argument. The effects of fibre surface treatment and fibre modulus on the strain transfer profile and the distribution of the interfacial shear stress along the fibre are examined. The importance of parameters such as fibre/matrix debonding and interphase yielding in the vicinity of fibre breaks or fibre ends is discussed.     

短纤维增强混凝土应力传递剪滞理论的改进

根据混凝土的非线弹性性质、纤维端部应力和由于温度变化产生的温度应力对剪应力及纤维正应力的影响,对短纤维增强混凝土应力传递的剪滞理论进行改进。结合碳纤维增强混凝土分析了混凝土应变、纤维端部正应力系数和温度差对应力传递的影响。认为在混凝土弹性阶段剪应力和纤维正应力受应变影响较小,塑性阶段应变对应力传递影响较大;纤维端部应力系数对剪应力和纤维正应力表现为线性影响;剪应力与温度变化呈同方向变化,但纤维正应力不受温度影响。     

Monitoring the micromechanics of reinforcement in carbon fibre/epoxy resin systems

The technique of laser Raman spectroscopy (LRS) was employed to obtain the interfacial shear stress (ISS) distribution along a short high-modulus carbon fibre embedded in epoxy resin at different levels of applied stress. Up to 0.6% applied strain, the ISS reached a maximum at the bonded fibre ends and decayed to zero at the middle of the fibre. At higher applied strains, the maximum value of the ISS distribution shifted away from the fibre ends towards the middle of the fibre. At the point of fibre fracture, fibre/matrix debonding was found to initiate at the fibre breaks. Further increase of applied strain resulted also in debonding initiation at the fibre ends. Current analytical stress-transfer models are reviewed in the light of the experimental data.     

Predicting the tensile strength of natural fibre reinforced thermoplastics

The tensile strength of short natural fibre reinforced thermoplastics (NFRT) was modeled using a modified rule of mixtures (ROM) strength equation. A clustering parameter, requiring the maximum composite fibre volume fraction, forms the basis of the modification. The clustering parameter highlights that as fibre loading increases, the available fibre stress transfer area is decreased. Consequently, at high volume fractions this decrease in stress transfer area increases the brittleness of the short fibre composite and decreases the tensile strength of the material. A key parameter, the interfacial shear strength, was determined by fitting the micromechanical strength model to tensile strength data at low fibre loading (10 wt%) where there is minimal fibre clustering.To test the modified ROM strength model, compression molded specimens of high-density polyethylene (HDPE) reinforced with hemp fibres, hardwood fibres, rice hulls, and E-glass fibres were created with fibre mass fractions of 10–60 wt%. The modified ROM strength model was found to adequately predict the tensile strength of the various composite specimens.     

THE EFFECT OF INTERFACIAL PROPERTIES ON THE FLOW STRENGTH OF DISCONTINUOUS REINFORCED METAL-MATRIX COMPOSITES

Abstract

The effect of interfacial properties on the strength of discontinuous reinforced metal-matrix composites is systematically studied by theoretical modelling. The calculations were carried out within the framework of continuum plasticity theory using cell models and the finite element method. A wide range of inclusion aspect ratios, volume fractions, and interfacial strengths were investigated for perfectly plastic and hardening matrices. Interfaces were modelled either as strongly bonded, or as shearable but strong normal to the inclusions, or as debonding at the reinforcement ends but strong on the sides. Additionally, the effects of reinforcement arrangement and extensive damage to continuous fibre composites were addressed. Debonding at the ends of the inclusions was found to have the most deleterious effect on the strength of the composite. When debonding does not occur but interface sliding takes place freely, an amount of strengthening is seen which is a function of the inclusion volume fraction but is primarily independent of the inclusion aspect ratio. For extensively damaged continuous fibre composites, a weak interface yields a steady-state composite flow strength slightly higher than the volume fraction of the matrix times the yield strength of the matrix. This increases linearly with the interfacial shear strength up to the level for strongly bonded composites and can be estimated from the intact fibre aspect ratio, the matrix yield stress, the volume fraction, and the interfacial strength     

Modification of residual thermal stress in a metal-matrix composite with the use of a tailored interfacial region

A three-component cylinder model of a unidirectional composite is used to evaluate the importance of an interfacial region in the build-up of residual thermal stress. The components consist of the fibre, an interfacial region and the surrounding matrix. Four mismatch of thermal expansion terms are obtained from this analysis. Each is examined for its contribution to the residual stress state by comparing the coefficients of each mismatch term. The residual stress problem is seen as being influenced by more than the mismatch between the fibre and the matrix. When anisotropic fibre expansion exists in the specific metal-matrix composite, the interfacial region has a significant contribution to residual stress build-up. Further, an interfacial region with an elastic modulus of the order of the elastic modulus of the fibre has a larger effect on the residual stress than when it is of the order of that of the matrix. For an aluminium-matrix composite this large contribution is deleterious and a compliant interfacial region is preferred due to the present considerations.     

The strength of the fibre-polymer interface in short glass fibre-reinforced polypropylene

A method for the determination of the interfacial bond strength in well-aligned short glass fibre-reinforced polypropylene samples is discussed. The method takes into account the variation of the interfacial shear stress during the deformation process; consequently, it yields very consistent results at all values of the composite strain. The influence of the fibre orientation with respect to the load axis is appropriately considered using macro-mechanical analyses for stiffness and strength of the composite. The method is compared with two other methods reported in the literature.     

Interfacial shear stress distribution in model composites: the effect of fibre modulus

The micromechanics of reinforcement have been investigated for a continuous intermediate-modulus (IM) carbon fibre embedded in an epoxy resin (MY-750). The embedded single-fibre (fragmentation) geometry was employed as the loading configuration. A laser Raman spectroscopic method was used to obtain the fibre strain distribution along the embedded fibre fragments, at various levels of applied strain. The interfacial shear stress distribution along the fibre was derived through a balance of forces analysis.A number of parameters, such as the maximum interfacial shear stress at each level of applied strain and the fibre debonded length, were evaluated. The maximum interfacial shear stress of the IM fibre system was found to increase by 80%, compared with the high-modulus fibre system examined previously, while the distance from the fibre end where the interfacial shear stress maximizes was significantly shorter. The debonded length was found to increase only marginally up to an applied strain of 1.8%, followed by a dramatic rate of increase between 1.8% and 2.5% of applied strain.