A computer simulation on tensile strength of surface-Damaged fibers

The strength of surface-damaged fibers was studied by means of a computer simulation experiment based on the Monte-Carlo method using a simple model which assumes that the surface flaws can be regarded as mode I notches on fiber surfaces, the strength of undamaged fibers obeys the Weibull distribution function, and the largest flaw determines the strength of damaged fibers. Normal and exponential distribution functions were taken as the flaw size distribution function. By employing the present simulation method, the effects of average flaw size, coefficient of variation of flaw size, density of flaws, and gage length on average strength and its coefficient of variation were studied. It was found that the surface-damaged fibers can retain their full strength only when the average flaw size is small, the coefficient of variation of flaw size is small, density of flaw size is low, and gage length is short. Otherwise the average strength of damaged fibers was reduced seriously. It was emphasized that the scatter of size of flaws and density of flaws strongly affect the strength of fibers as well as flaw size and gage length.     

A computer simulation of strength of metal matrix

The deformation and fracture behavior of metal matrix composites with a reaction layer at the fiber-matrix interface was studied by means of a computer simulation experiment, using a two-dimensional model, and the results of the simulation experiment were compared with the predictions based on the single fiber model, which has been proposed to describe the reduction of strength of composites due to a reaction layer. In the simulation experiment, the composite was regarded as an assembly of single fiber elements, in which, for each element, the reaction layer introduces a notch on the fiber surface when it is broken, which reduces the strength of the fiber if the thickness of the layer is thinner than a critical value, as has been studied by using the single fiber model. The strength of composites was reduced with increasing thickness of the reaction layer and the fracture mode became catastrophic. The strength values obtained by the simulation were equal to those based on the single fiber model only when the fracture of the fiber was caused by the extension of the notch having been introduced by premature fracture of the reaction layer. In other cases, the strength values of the simulation were lower than those predicted by the single fiber model, although the single fiber model gave approximate values.     

Transverse strength of metal matrix composites reinforced with strongly bonded continuous fibers in regular arrangements

The composite limit flow stress for transverse loading of metal matrix composites reinforced with a regular array of uniform continuous fibers is calculated using the finite element method. The effects of volume fraction and matrix work hardening are investigated for fibers of circular cross section distributed in both sqyare and hexagonal arrangements. The hexagonal arrangement is seen to behave isotropically with respect to the limit stress, whereas the square arrangement of fibers results in a composite which is much stronger when loaded in the direction of nearest neighbors and weak when loaded at 45° to this direction. The interference of fibers with flow planes is seen to play an important role in the strengthening mechanism. The influence of matrix hardening as a strengthening mechanism in these composites increases with volume fraction due to increasing fiber interaction. The results for a power law hardening matrix are also applicable to the steady state creep for these composites. The influence of volume fraction on failure parameters in these composites is addressed. Large increases in the maximum values of hydrostatic tension, equivalent plastic stain, and tensile stress normal to the fiber-matrix interface are seen to accompany large increases in composite strength.     

An ultimate tensile strength dependence on processing for consolidated metal matrix composites

The ultimate tensile strength (UTS) of metal and intermetallic matrix unidirectional composites can be significantly lower than expected from the rule of mixtures prediction. One possible explanation is that the fibers in the as-processed state are in a residual state of stress and in some cases are broken because of the inhomogeneous nature of the densification during manufacture. Three main results emerge from the effort to include the effect of this processing damage on the composite UTS. First is the development of a simple but accurate analytical version of Curtin's model for predicting the stress-strain response and UTS of this class of composites. Second is the generalization of Curtin's model to include both process induced fiber bending and fracture. Third is that the reduction in strength is a sensitive function of the consolidation conditions; thus a link is established between the quality of the composite and the conditions of its manufacture.     

The strength of metal matrix composites

There is intensive interest in metal matrix composites (MMCs) for automotive components, and the first production applications in Japan use discontinuous fibers as the reinforcements. These fibers are randomly oriented, resulting in an MMC with isotropic properties. However, there are conflicting reports on the tensile strengths attainable. In some cases, the strength increases with increasing volume fraction(V _f) of fibers, while in other cases, there is little or no benefit. A simple method is proposed to calculate the strength of this type of MMC. It is shown that the fibers oriented perpendicular to the stress direction play a key role, and the strength depends upon the strength of the interfacial bond. Upper and lower limits of the composite strength are calculated. If the bond strength is larger than the matrix strength, the composite strength has a maximum value which increases withV _f. If the bond strength is weaker than the matrix, the composite strength has a minimum value which is either weakly dependent or even independent ofV _f. These calculations are in good agreement with examples taken from the literature of aluminum composites reinforced with either A1₂O₃, graphite, or SiC. The strength of the matrix alloy is shown to be a very important parameter: weak alloys are easily strengthened, while in certain cases, strong alloys may be weakened.     

Creep behavior of fiber reinforced metal matrix composites

A theoretical model of the creep behavior of metal matrix composites having strong fiber-matrix interfaces is described in terms of creep parameters of the matrix and fibers. The available experimental data, obtained from the unidirectionally solidified aluminum-nickel eutectic containing 10 vol pct Al₃Ni fibers, are in good agreement with the theoretical model. The creep activation energy of the composite is described in terms of the creep activation energy of fibers and the matrix. The experimentally de-termined data of (Co, Cr)-(Co, Cr)₇C₃ and Al-Al₃Ni eutectics are in agreement with those values as predicted.     

Creep behavior of fiber reinforced metal matrix composites

A theoretical model of the creep behavior of metal matrix composites having strong fiber-matrix interfaces is described in terms of creep parameters of the matrix and fibers. The available experimental data, obtained from the unidirectionally solidified aluminum-nickel eutectic containing 10 vol pct Al₃Ni fibers, are in good agreement with the theoretical model. The creep activation energy of the composite is described in terms of the creep activation energy of fibers and the matrix. The experimentally de-termined data of (Co, Cr)-(Co, Cr)₇C₃ and Al-Al₃Ni eutectics are in agreement with those values as predicted. Formerly a Visiting Scholar, Materials Department, University of California, Los Angeles.     

The transverse tensile properties of boron fiber reinforced aluminum matrix composites

The transverse tensile properties of boron fiber reinforced aluminum have been determined as a function of fabrication parameters, matrix alloy and fiber types, fiber content, specimen geometry, and thermal environment. Matrix alloys investigated include 2024, 6061, 5052, 5056, 2219, 1100, and Al-7 pct Si. The fibers investigated include 4.0 mil boron, 4.2 mil BORSIC, R.F. boron, 5.6 mil boron, 5.7 mil BORSIC, and 4.0 mil silicon carbide. It was shown that the composite transverse tensile performance is a function of all of these variables and that transverse strengths of up to 45,000 psi can be achieved by the choice of the proper combination of matrix, fiber type and fabrication procedures.     

Aspects of fracture in particulate reinforced metal matrix composites

The tensile deformation and fracture behaviour of the aluminium alloy 6061 reinforced with SiC has been investigated. In the T4 temper plastic deformation occurs throughout the gauge length and the extent of SiC particle cracking increases with increasing strain. In the T6 temper strain becomes localised and particle cracking is more concentrated close to the fracture. The elastic modulus decreases with increasing particle damage and this allows a damage parameter to be identified. The fraction of SiC particles which fracture is less than 5%, and over most of the strain range the damage controlling the tensile ductility can be recovered, indicating that other factors, in addition to particle cracking are important in influencing tensile ductility. It is suggested that macroscopic fracture is initiated by the SiC particle clusters that are present in these composites as a result of the processing. The matrix within the clusters is subjected to high levels of triaxial stress due to elastic misfit and the constraints exerted on the matrix by the surrounding particles. Final fracture is then produced by crack propagation through the matrix between the clusters.     

Tensile and fatigue behavior of Al-based metal matrix composites reinforced with continuous carbon or alumina fibers: Part I. Quasi-unidirectional composites

The thermomechanical (dilatometric, tensile, and fatigue) behavior of Al-based metal matrix composites (MMCs) is investigated. These composites are reinforced by quasi-unidirectional (quasi-UD) woven fabric preforms with 90 pct of continuous fibers in the longitudinal direction and 10 pct in the transverse direction. The two composite systems investigated feature a highly ductile matrix (AU2: Al-2Cu wt pct) with a strongly bonded fiber-matrix interface (N610 alumina fibers) and an alloyed, high-strength matrix (A357: Al-7Si-0.6Mg wt pct) with a weak fiber-matrix interface (K139 carbon fibers). Microstructural investigation of the tested specimens has permitted identification of the specific characteristics of these composites: undulation of the longitudinal bundles, presence of the straight transverse bundles, interply shearing, and role of brittle phases. Moreover, simple semiquantitative models (e.g., interply shearing) have enabled explanation of the specific mechanical behavior of these quasi-UD composites, which exhibit high tensile and fatigue strengths, as compared with the corresponding pure UD composites. Knowledge of the specific characteristics and mechanical behavior of these quasi-UD composites will facilitate the further investigation of the (0, ±45, 90 deg) quasi-UD laminates (Part II). At a more theoretical viewpoint, the specific geometry and behavior of these quasi-UD composites allows exacerbation of fatigue mechanisms, even more intense than in “model” composites.     

Temperature-dependence mechanism of tensile strength of Si-Ti-C-0 Fiber-Aluminum matrix composites

The mechanism for the temperature dependence of the tensile strength of unidirectional hybrid type Si-Ti-C-O (Tyranno) fiber-reinforced aluminum matrix composite, in which SiC-particles are dispersed in the matrix, is discussed, focusing on the temperature dependencies of the stress concentration arising from broken fibers and critical length and their influences on the composite strength, by means of a shear-lag analysis and a Monte Carlo simulation. The main results are summarized as follows. The softening of the matrix at high temperatures raises the composite strength from the point of decrease in stress concentration, but on the other hand, it also reduces strength from the point of increase in critical length, which reduces the stress-carrying capacity of broken fibers over a long distance. The reason why the measured strength of composite decreased with increasing temperature could be attributed to the predominacy of the latter effect over the former one. The results of the simulation indicated that the hybridization of the composites improved room-temperature and high-temperature strengths through the strengthening of the matrix.     

On the strength of ductile particle reinforced brittle matrix composites

This study examines theoretically the strength characteristics of ductile particle reinforced brittle materials in which the strength enhancement is derived from the crack bridging process. The crack bridging is characterized by rectilinear and linear softening bridging laws that relate the crack surface traction to the crack opening displacement. The composite strength is expressed in terms of two non-dimensional parameters that combine the effects of the flaw size, elastic modulus, matrix toughness, and the bridging-law parameters. It is shown that such composites can be substantially more flaw tolerant than the monolithic matrices owing to a narrowing of the strength distribution. The role of interface debond length is also examined. It is shown that there exists optimal debond lengths of which the composite strength is maximized. In contrast, the steady state toughness increases monotonically with debond length. The implications of these results on the design of composite microstructures are briefly described.     

碳纤维表面状态及其对复合材料剪切性能的影响

采用X射线衍射仪和扫描电镜观察了碳纤维表面的微观结构以及复合材料的截面形貌,分析了进口碳纤维和国产碳纤维的表面状态差异,以及此差异对碳纤维复合材料进行层间剪切强度(ILSS)的影响.结果表明进口碳纤维表面粗糙度更大,沟槽深度和宽度均大于国产碳纤维.在树脂基复合材料中,进口碳纤维与树脂基体结合更为紧密,固化后形成制件的孔隙率更低,其室温和高温层间剪切强度都高于相应国产碳纤维体系.因此,尽管表面处理会对碳纤维表面造成一定的影响,但处理后得到的高粗糙度表面是纤维在复合材料中形成较强界面的根本原因.     

化学镀碳纤维增强羟基磷灰石复合材料的性能

通过化学镀方法,在碳纤维表面分别镀上Ni和Cu+Ni镀层,以这种表面改性碳纤维与羟基磷灰石陶瓷复合,制备表面改性碳纤维增韧增强羟基磷灰石复合材料,研究各种碳纤维的含量对复合材料的抗弯强度、断裂韧度、尺寸变化率和孔隙率的影响。结果表明,表面改性碳纤维可以显著提高材料的性能,尤其是铜镍复合镀碳纤维的效果更好,其断裂韧度可达基体断裂韧度的2.5倍,抗弯强度可达基体抗弯强度的3.4倍,增韧增强后的复合材料的尺寸和孔隙率变化不大。     

非连续相混杂增强金属基复合材料的研究进展

论述了非连续增强金属基复合材料的研究概况,简要介绍了非连续相混杂增强金属基复合材料常用的几种制备方法,包括搅拌熔铸法、压力铸造法、无压浸渗法、喷射沉积法、粉末冶金法、原位反应法等,同时对常见的3种增强体的混杂类型:颗粒+颗粒、短纤维+晶须(短纤维)、颗粒+晶须(短纤维)增强金属基复合材料的性能和国内外研究现状进行了综述,指出了非连续相混杂增强金属基复合材料存在的问题,并对其今后的发展进行了展望。     

The deformation of particle reinforced metal matrix composites during temperature cycling

Superplasticity during temperature cycling of particle reinforced metal matrix composites has been studied over a range of reinforcement sizes and volume fractions. Above a critical volume and thermal cycle amplitude, the mean strain per cycle is proportional to stress and approximately proportional to cycle amplitude. For a given thermal cycle the constant of proportionality with respect to stress increases with reinforcement fraction to a maximum at around 30%; it then decreases with further increase in reinforcement. Transmission electron microscopy revealed no characteristics dislocation substructure; even after 90% strain the material was indistinguishable from its undeformed state. The experimental results confirm an internal plastic flow model for the phenomenon rather than an enhanced creep. A model of the process derived from the Lévy-Von Mises equations predicts both the effect of thermal cycle amplitude the MMC microstructure on the enhanced creep rate.