Compressive strength of unidirectional fiber composites with matrix non-linearity

A study has been made of the effect of fiber misalignment and non-linear behavior of the matrix on fiber microbuckling and the compressive strength of a unidirectional fiber composite. The initial fiber misalignment constituted the combined axial and shear stress state in the matrix, and the state of stress just prior to the buckling was considered to be the initial state of stress in bifurcation analysis. The expression for the critical microbuckling stress was found to be the same as that for the elastic shear-mode microbuckling stress except that the matrix elastic shear modulus was replaced by the matrix elastic-plastic shear modulus. Incremental theory of plasticity and deformation theory of plasticity were used to model the matrix non-linearity. The analysis results showed reasonable correlation with available experimental data for AS4/3501-6 and AS4/PEEK graphite composites with 2° to 4° range of initial fiber misalignment.     

Failure mechanisms in thick composites under compressive loading

Failure mechanisms were studied in a unidirectional carbon/epoxy composite under uniform and linearly varying longitudinal compression. The first failure mechanism is shear yielding or shear failure in the matrix precipitated by initial fiber misalignment. It was shown how an initial fiber misalignment of 1.5° can produce the measured compressive strength of 1725 MPa (250 ksi). Matrix failure is followed by fiber buckling and fracture, resulting in the formation of a kink band. The kink band orientation is constant in the range of β = 20–30°, whereas the kink angle a varies from a small initial value to a maximum value of 2β. Kink band widths varied between 4 and 20 fiber diameters. Kink bands can occur on different planes which can rotate along the band. Kink band multiplication or broadening with increasing stress was observed at points where the maximum kink angle was reached.     

碳纤维增强铜基复合材料制备方法研究进展

综述了碳纤维增强铜基复合材料的主要制备方法及其发展现状,重点讨论了粉末冶金法、热压扩散烧结法、熔渗法、PVD法、CVD法及电镀法等常用制备工艺的原理及特性,分析了不同制备方法的优缺点及适用领域,提出了现有方法中存在的问题,并展望了碳纤维增强铜基复合材料的发展趋势及在输变电领域的应用前景。     

连续纤维增强PPESK树脂基复合材料的界面性能

用SEM观察了复合材料的微观断面结构,用横向拉伸强度和层间剪切强度表征玻璃纤维(GF)、T700碳纤维(CF)、芳纶纤维(F-12)增强PPESK树脂基复合材料的界面性能,研究了界面性能对三种复合材料耐湿热性能的影响.结果表明,T700/PPESK和F-12/PPESK复合材料的界面粘接性能均优于GF/PPESK复合体系.三种纤维复合材料的破坏机理不同:玻璃纤维发生纤维与树脂的界面脱粘破坏,碳纤维复合材料在破坏时,树脂与纤维并没有完全脱粘,破坏发生在树脂内;而芳纶纤维复合材料的破坏总伴随着纤维本身横向的撕裂破坏.三种复合材料体系均具有较低的吸湿率和良好的耐湿热性能,T700/PPESK复合材料在湿热条件下的性能保持率最高.     

A study of continuous filament reinforced aluminum matrix composites

Some principal results of the research work on metal matrix composites at Beijing Institute of Aeronautical Materials, concerning CVD-produced continuous B and SiC filaments reinforced aluminum and its alloys, are summarized. The processing, fiber degradation, interface, mechanical properties and fracture behavior of the composites are discussed.Abbreviations CVD chemical vapor deposition - MMCs metal matrix composites - ROM rule of mixtures     

Epoxy-carbon nanotubes as matrix in glass fiber reinforced laminated composites

A manufacturing process using vacuum-assisted resin infusion molding was used to produce glass fiber-reinforced laminates with the epoxy matrix and with the 0.5 weight % NH₂ functionalized-MWCNT based epoxy matrix. Images obtained from the TEM and SEM indicates that MWCNTs are well dispersed into the epoxy matrix. Microstructures observations of the composites from SEM show the better interaction between CNTs and epoxy. The mechanical and thermo-mechanical behavior of the glass fiber-epoxy system and glass fiber-CNT/epoxy system was characterized through flexural test and Dynamic Mechanical Analysis (DMA).     

The effect of hydrostatic pressure and loading rate on compressive failure of fiber-reinforced ceramic-matrix composites

The influence of hydrostatic confinement on compressive strength and corresponding failure mechanisms is explored for SiC-reinforced glass-ceramics tested at different strain rates. Two composite architectures (0° and 0°/90°) are studied, and their behavior is compared with that of monolithic glass-ceramic tested under similar conditions. Composite confined pressure results are interpreted in terms of fiber buckling under quasi-static conditions and fiber kinking at high pressures, and compared with monolithic (non-composite) microfracture coalescence at low pressures and shear band formation under more intense confinement. In particular, dilatational fracture within the matrix dominates composite failure at low pressures, while high pressures cause a transition to shear-dominated mechanisms based on fiber kinking.     

Computational modeling of damage evolution in unidirectional fiber reinforced ceramic matrix composites

A finite element model for investigating damage evolution in brittle matrix composites was developed. This modeling is based on an axisymmetric unit cell composed of a fiber and its surrounding matrix. The unit cell was discretized into linearly elastic elements for the fiber and the matrix and cohesive elements which allow cracking in the matrix, fiber-matrix interface, and fiber. The cohesive elements failed according to critical stress and critical energy release rate criteria (in shear and/or in tension). The tension and shear aspects of failure were uncoupled. In order to obtain converged solutions for the axisymmetric composite unit cell problem, inertia and viscous damping were added to the formulation, and the resulting dynamic problem was solved implicitly using the Newmark Method. Parametric studies of the interface toughness and strength and the matrix toughness were performed. Details of the propagation of matrix cracks and the initiation of debonds were also observed.     

不同热塑性树脂基体对单向碳纤维复合材料吸湿行为的影响

采用溶液浸渍和热压成型分别制备了以聚醚砜和聚醚砜/聚苯硫醚为基体的单向连续碳纤维增强复合材料,研究了不同成型温度下复合材料的层间剪切强度和湿热环境下对水分的吸湿过程。结果表明,添加了聚苯硫醚的复合材料在较佳的加工工艺窗口下可以保持原有的层间剪切强度,同时大幅度地增强复合材料的抗水分侵蚀能力。这是由于聚苯硫醚在热压成型过程中,以聚醚砜为晶核在碳纤维表面结晶,形成较为致密的结构。同时红外分析表明,混合树脂极性较小。二者共同作用,可以提高复合材料在使用过程中的安全性和结构的完整性。性能较好的复合材料吸湿过程符合Fick第二定律。     

Cracking mechanisms in durable sisal fiber reinforced cement composites

Fiber reinforced cement composite laminates with long sisal fibers were manufactured using a cast hand lay up technique. A matrix with partial cement replacement by metakaolin and calcined waste crushed clay brick was used in order to improve the durability aspects. Mechanical response was measured under tension and bending tests while crack formation was investigated using a high resolution image capturing procedure. Crack spacing was measured using image analysis and correlated with the applied strain under both the tensile and bending response. Various stages of loading corresponding to initiation, propagation, distribution, opening, and localization of a crack system in the specimen are discussed. The effect of flexural cracking on the location of neutral axis during the bending tests was measured using strain-gages.     

Transverse elastic moduli of unidirectional fiber composites with fiber/matrix interfacial debonding

An elastic contact model is developed to predict the transverse Young's modulus, Poisson's ratio and shear modulus of unidirectional fiber composites with interfacial debonding. The elastic deformation formulae of the fiber under contact pressure are derived by the use of elasticity theory. These results are then used in the formulation of an analytic boundaryelement method for solving the interfacial debonding problem. The two extreme cases of perfect bonding and the fiber-like void are also studied. On the basis of this theory, the upper and lower bounds of the transverse moduli for unidirectional fiber composites with imperfect fiber/matrix interfaces are provided. Numerical calculations of parametric studies are conducted for four composites, and some basic characteristics of the transverse elastic moduli of unidirectional fiber composites with interfacial debonding are presented.     

Prediction of the failure properties of short fiber reinforced composites with metal and polymer matrix

The statistical strength of short fiber reinforced composites such as metal matrix composites (MMCs) and polymer matrix composites (PMCs) with different fiber volume fractions is investigated in this work using combined cell models (CCM) and Weibull statistical approach. Statistical combined cell models (SCCM) have been developed, which were originally developed for fiber fracture in composites. This allows to calculate separately the two types of unit cells, i.e. unit cells with unbroken fibers, and with broken ones. The global mechanical behavior of metal or PMCs reinforced with randomly oriented short fibers (e.g., an Al/15 vol.% Al₂O₃ aluminium matrix composite and PMC with 20 or 30 vol.% glass-fibers) is calculated based on the rule of mixture. In all cases, predictions of the behavior by the SCCM are compared with experiments and they show good agreement.     

The mechanics of matrix cracking in fiber reinforced ceramic composites containing a viscous interface

A detailed fracture mechanics analysis of matrix cracking in a fiber reinforced ceramic composite is presented for the case where the fiber—matrix interface exhibits viscous flow as can be the case when ceramic composites containing amorphous interfacial layers are subjected to loads at elevated temperatures. The analysis considers the case where matrix cracks are fully bridged by fibers, and the role of the viscous interface is to introduce a time dependence into the stress-intensity formulations. Such time-dependence arises because the bridging fibers are able to pull out of the matrix by viscous interfacial flow, with the result that the crack opening, as well as the actual (or shielded) matrix crack-tip stress-intensity factor, increase with time under the action of a constant externally applied load to the composite. The differential equation governing the mechanics of the fiber pull-out is derived. This is then applied to obtain expressions for the time-dependence of the crack opening and the effective crack-tip stress-intensity factor in terms of material and microstructural factors. These expressions predict that the matrix crack will exhibit stable crack growth, with the crack growth rate being essentially crack length (and time) independent and a function only of the applied stress and of material and microstructural factors. It is also shown that the composite lifetime is independent of the sizes of pre-existing cracks and is dependent only on a critical microstructure dependent flaw size, applied stress and microstructural factors.     

玻纤增强复合材料抗爆震性能研究

采用炸药柱近似模拟平面波发生器 ,利用传感器对 3种玻纤复合材料的抗爆震性能进行了研究。试验结果表明 ,玻纤复合材料对爆炸冲击波衰减作用的大小与基体材料的阻尼特性有关。基体材料的阻尼性能越好 ,复合材料对冲击波的衰减作用也就越好。     

碳纤维增强水泥基复合材料屏蔽性能的研究

现代电子技术的迅速发展不可避免地带来了电磁辐射所造成的环境污染.选择合适的屏蔽材料,利用其反射、吸收、多层反射等功能是消除电磁污染的有效手段.碳纤维除具有高弹性、高模量、低密度、耐腐蚀等优良性能被当作许多复合材料的增强体外,还具有良好的导电性,将其加入到水泥基体中制成碳纤维增强水泥基复合材料(CFRC),不仅使水泥自身力学性能得到改善,而且可作为防止电磁辐射或核辐射的优良屏蔽体.     

NiTi纤维表面处理及其树脂基复合材料的界面特性

NiTi纤维表面呈惰性,表面能低,NiTi纤维与树脂间界面粘结性差.为了提高界面性能,采用硅烷偶联剂、表面涂层和低温冷等离子体技术等方法对NiTi纤维表面进行处理,改善纤维表面的浸润性,达到纤维与树脂界面良好的粘结.对复合材料进行界面剪切强度的测定,并利用扫描电镜观察拔脱纤维表面形貌的变化.研究表明:NiTi纤维经不同方法处理后,纤维的浸润性和界面的粘结强度均有不同程度的改变,其中冷等离子体处理的纤维再经硅烷处理,其复合材料IFSS提高2.94倍,ILSS提高1.45倍,且纤维与树脂粘合较好.     

Surface damage of unidirectional carbon fiber reinforced epoxy composites under reciprocating sliding in ambient air

For many technical applications friction and wear are critical issues. Reinforced polymer-matrix composites are widely used under vibrating contact condition in various automotive and aerospace applications as well as in structural engineering. In this paper, the friction and wear of bulk epoxy and unidirectional carbon fiber reinforced epoxy composite have been investigated under reciprocating sliding against either alumina or stainless steel balls in ambient air. The effect of sliding direction with respect to the long and unidirectional carbon fibers has been studied. We demonstrate that the carbon fiber reinforcement greatly improves the tribological properties of the thermoset epoxy: it reduces the coefficient of friction and the debris formation. It was found that on sliding in the anti-parallel direction a more significant degradation takes place than in the parallel direction. The coefficient of friction measured on bulk epoxy sliding against either stainless steel or alumina is around 0.65, whereas the coefficient of friction measured on epoxy reinforced with carbon fibers is significantly lower, namely down to 0.11. It was found that sliding with a stainless steel ball in a direction parallel to the fiber orientation results in a lower coefficient of friction than sliding in anti-parallel direction. The reduced coefficient of friction is largely influenced by the carbon fiber reinforcement due to the auto-protecting film formed as a paste in the contact area and along the wear track edges. The relationship between friction and degradation of the composite material including surface wear and debris formation are discussed based on an in-depth analysis of the worn surfaces by optical and scanning electron microscopy, micro-Raman spectroscopy, and white light interferometry.     

Micromechanics based analysis of randomly distributed fiber reinforced composites using simplified unit cell model

The geometry of the simplified unit cell (SUC) model [Aghdam MM, Smith DJ, Pavier MJ. Finite element micromechanical modeling of yield and collapse behavior of metal matrix composites. J Mech Phys Solids 48 (2000) 499–528] is extended to study effects of random fiber arrangement on the mechanical and thermal characterizations of unidirectional composites. The representative volume element (RVE) considered in the model consists of an r × c unit cells in which fibers are surrounded randomly by matrix cells. The presented model is general and can be used to predict the behavior of a fibrous composite subjected to thermal and mechanical, normal and shear, loading. The model also is capable of analyzing various combinations of these loading conditions such as off-axis test of unidirectional coupons. Both random and repeating fiber arrays can be considered in the model. Results for the overall thermal and elastic properties of a SiC/Ti metal matrix composite (MMC) show good agreement with both the finite element and other analytical models with repeating fiber arrays. Results of transverse properties also revealed that hexagonal array assumption for fiber arrangement is more realistic than square array assumption.     

Compressive response of glass–fiber reinforced polymeric composites to increasing compressive strain rates

The utilization of composite materials instead of traditional materials in structural high-speed applications has induced the need for a proper knowledge of dynamic behavior as well as static behavior of them. The material and structural response vary significantly under dynamic loading as compared to static loading conditions. In order to investigate the dynamic responses of composite materials under dynamic loading at various strain rates, special testing machines are needed. Most of the researches in this field are focused on applying real loading and gripping boundary conditions on the testing specimens.The present study is carried out in order to characterize the compressive properties of unidirectional glass–fiber reinforced polymeric composites using a servo-hydraulic testing apparatus at varying strain rates, ranging from 0.001 to 100 s⁻¹. For performing practical tests, a jig and a fixture are designed and manufactured, which could insure the alignment of axial loads on the specimens. During of tests, the performance of the test jig is evaluated. It is found that the designed jig and the fixture perform very well during the test process. The results of the dynamic tests are compared with the results of the static tests carried out on specimens with identical geometry. Based on the experimental results obtained from the tests, empirical functions for the mechanical properties are proposed in terms of strain rates. The results of the study indicate that strain rate has a significant effect on the material response. It is found that the compressive strength and modulus both increased with increasing the strain rate. Also, the results show that the compressive strain to failure is generally insensitive to strain rate.     

The dispersion of SWCNTs treated by dispersing agents in glass fiber reinforced polymer composites

It is an obstacle issue for carbon nanotubes (CNTs) particularly for single-wall carbon nanotubes (SWCNTs) with nano-level dispersion in fiber reinforced polymer matrix composites. In this paper, the dispersing agents such as Volan and BYK-9076 were employed to treat SWCNTs to improve their dispersion in the glass fiber/epoxy (GF/EP) composites. The dispersing results of SWCNTs in composites were observed by scanning electron microscopy (SEM). Then the glass transition temperature (T_g) of these kinds of composites with treated and untreated SWCNTs were obtained by dynamic mechanical thermal analysis (DMTA). Moreover, the flexural tests were performed on these composites. Based on the experiment results, the dispersion of SWCNTs was improved and the flexural property of SWCNTs/GF/EP composite was enhanced too.