钛基复合材料中的微区应力分布

使用三纤维/基体有限元模型研究了纤维失效和基体屈服后钛基复合材料内微区应力分布,结果表明:钛基复合材料内纤维失效端面的轴向应力降为0,承载能力降低,相邻基体和未失效纤维的承载能力升高;随着纤维体积分数的增大,失效后应力和失效前应力的比值增大;当中心纤维断裂时,纤维体积分数高的复合材料立即失效,且失效形式为共面失效;对于纤维体积分数低的复合材料,基体屈服对纤维与基体之间的载荷传递有重要的影响.     

Effect of a soft interlayer with changing properties on the stress distribution around inclusions and yielding of composites

A model is presented that takes into account the presence of a soft interlayer with changing properties around an inclusion embedded into an infinite matrix. Property change in the interphase is expressed by a power law function of the form of f(r)=J(R/r)^p. The equilibrium equation is satisfied by a spherical and a dipole like solution. The continuity of stresses and displacements at all interfaces are used as boundary condition. Calculations are carried out for a matrix/elastomer/filler system to predict the effect of the soft interlayer on stress distribution. According to the model the deformation of this interlayer is very large compared to that of the matrix. Very soft interlayers lead almost exclusively to compressive deformations in the entire deformed volume. Radial stress decreases considerably and the stress maximum shifts to the surface of the inclusion. Although in a somewhat lesser extent than radial stress, shear stress also decreases in the presence of the soft interlayer and stress maxima become less localized. In such a case yielding is initiated at the equator of the inclusion. Particles covered with a very soft interphase behave like cavities. A soft interphase is advantageous because it changes stress distribution and decreases stress concentration, but particles loose their reinforcing effect already at a relatively thin layer thickness.     

Deformation and fracture properties of damage tolerant in-situ titanium matrix composites

This paper discusses the tensile response and fracture toughness of in-situ titanium alloy metal matrices discontinuously-reinforced with whiskers of titanium boride which were success-fully produced by ingot metallurgy techniques. Additions of elemental boron resulted in a near uniform dispersion of the rod-like titanium boride (TiB) reinforcements in the alloy matrix. Such composites have engendered considerable scientific and technological interest due to their attractive combinations of improved mechanical properties and low manufacturing cost. The improved elastic moduli of the composites are explained using shear lag and rule-of-mixtures theories. The increased strengths of the in-situ composites are rationalized by considering the combined effects of deformation restrains imposed by the stiff whiskers and strengthening contributions arising from the substructure that evolves from the presence of additional dislocations.     

Tensile fracture behavior of notched fiber reinforced titanium metal matrix composite

The static fracture behavior of a titanium based metal matrix composite (MMC) with a central hole or a straight notch was investigated. The MMC used was SCS-6/Ti-β21-S with a quasi-isotropic lay-up. Different sizes of hole or notch were used which provided cut-out size to specimen width ratios from 0·1 to 0·4. Two test temperatures were used: ambient and 650°C. At both temperatures, the tested MMC showed a mild hole size effect or notch sensitivity. The failure mechanisms involved the debonding of fibers followed by failure of fibers, and then by failure of the matrix.     

Controlling fracture toughness of matrix for ductile fiber reinforced cementitious composites

An experimental study was carried out to find material parameters for making fiber reinforced cementitious composites (FRCC) more ductile. One of the dominant factors to control the ductility might be hidden in fracture property of matrix as well as the interface property between fiber and matrix. Therefore this study varied air content and water-binder ratio as the parameters to change the fracture property of matrix and experimentally examined their influence on the ductility of FRCC by three-point bend test with notched beams. As a result, it is concluded that fracture toughness of the matrix could be one of key parameters to control the ductility of FRCC. In case of a polyethylene fiber used in this study, the optimum value of the fracture toughness (critical strain energy release rate): G_IC of the matrix was obtained to be 7.5-8.0 N/m.     

Microstructural efficiency and fracture toughness of short fiber/thermoplastic matrix composites

This paper outlines the fracture behavior of composites with thermoplastic matrices of different fracture toughness K_cm (increasing in the order PPS → PET (I) → PET (II) → ETFE → PC). In particular, the way in which the fracture toughness of these composite systems, K_cc, is affected by the volume fraction, orientation and distribution of short glass fibers across the plaque thickness (fiber length ≈ 200 μm, fiber diameter ≈ 10 μm), and by the quality of their interfacial bonding to the matrix is discussed. SEM studies were carried out to define the microstructural details and the dominant mechanisms of energy adsorption during breakdown of the composites.In general, an increase in composite toughness can be expected with increasing extent of reinforcement if the matrix is in a brittle condition (here also verified by K_c-tests at lower temperatures) and if the fibers are well bonded and mostly oriented perpendicular to the crack front. An opposite tendency may occur for matrices which behave in a highly ductile manner even in the presence of fibers. The probability of this behavior is favored in poorly bonded systems. The results are discussed in terms of a ‘microstructural efficiency factor’ M, which mainly considers the relative contributions of fiber and matrix related mechanisms to energy dissipation during breakdown of a composite (‘energy absorption ratio’ n) as well as the reinforcement content and its arrangement in the matrix (‘reinforcing effectiveness parameter’ R).     

基体合金对连续M40石墨纤维/Al复合材料纤维损伤及断裂机制的影响

选用M40石墨纤维为增强体,采用真空气压浸渗法制备了纤维体积分数为40%,基体合金分别为ZL102、ZL114A、ZL205A及ZL301合金的连续M40/Al复合材料,并用NaOH溶液萃取出M40纤维,研究了基体合金对连续M40/Al复合材料纤维损伤和断裂机制的影响。结果表明:不同的基体合金对M40纤维造成的损伤差异较大,从M40/ZL301复合材料中萃取的纤维拉伸强度最高,其拉伸强度为1 686 MPa,约为纤维原丝拉伸强度的38.3%;而从M40/ZL102复合材料中萃取的纤维拉伸强度最低,其拉伸强度仅为687 MPa,且纤维表面粗糙程度不一。不同M40/Al复合材料的断裂机制存在明显差别,M40/ZL102和M40/ZL114A复合材料断裂时无纤维拔出及界面脱粘,裂纹横向穿过纤维导致复合材料在低应力下失效;M40/ZL205A复合材料则表现为少量纤维拔出,界面轻微脱粘;同时,M40/ZL301复合材料表现为大量纤维拔出,裂纹沿界面纵向扩展,界面脱粘明显,纤维充分发挥其承载作用,复合材料的拉伸强度最高,达到了670.2 MPa。      

Effect of thickness on fracture criterion in general yielding fracture mechanics

In this research work, the effect of thickness on fracture criterion is studied for extra deep drawn (EDD) steel sheets. Experimental results are generated on fracture toughness of EDD steel sheets using compact tension specimens and a ‘maximum load’ as a fracture criterion. Critical crack tip opening displacement (CTOD) is found with the help of three methods: plastic hinge model (PHM), crack flank opening angle (CFOA) and finite element model (FEM). The fracture toughness is found to increase with increase in thickness of specimens. The fracture behaviour exhibited characteristics of general yielding fracture mechanics.     

Effects of nonuniformity of fiber distribution on thermally-induced residual stresses and cracking in ceramic matrix composites

A three-dimensional finite element analysis is conducted to estimate stresses induced by thermal cooldown in unidirectionally fiber-reinforced ceramic matrix composites. Various configurations of nonuniform fiber distributions are considered. Both cases of thermal expansion mismatch between isotropic, linearly thermoelastic fibers and matrix are studied. Significant effects of nonuniformity of fiber distributions on the local stress states are found. The initiation of various possible cracking modes is discussed in the light of these results.     

Interfacial debonding in laminated titanium matrix composites

The results of an experimental program in which multiaxial loads were applied to [0₄] and [±45]_s silicon carbide/titanium (SiC/Ti) tubes are reviewed showing that stress coupling, matrix viscoplasticity (including room temperature creep) and fiber/matrix interfacial damage all contribute to nonlinear response and permanent strains in titanium matrix composites (TMC). A micromechanical model that explicitly considers the aforementioned phenomena is presented herein. The model assumes a periodic microstructure and uses finite elements to analyze a representative volume element. The composite is assumed to be in a state of generalized plane strain making it possible to discretize only a generic transverse plane while still being able to apply three-dimensional loading through appropriate boundary conditions. The response of laminated composites is predicted by incorporating the micromechanical results into nonlinear lamination theory. Predictions are presented to show the influence of the model parameters on the effective composite response of unidirectional [0₄] and angle-ply [±45]_s TMC laminates.     

Effect of friction between fiber and matrix on the fracture toughness of the composite interface

A method of evaluating the interfacial fracture toughness using a single-fibre composite test is proposed. In contrast with the existing techniques, the method takes into account the phenomenon of friction between the fibre and matrix in the debonding zone. A general mathematical solution of the problem and modelling of the friction phenomenon are presented. Finite-element analysis using a contact statement is utilized for numerical evaluation of the stress–strain state. The influence of the coefficient of friction and interfacial debonding length is analysed in detail. It is shown that the friction reduces the calculated value of the elastic strain energy release rate for a given debonding length, relative to that obtained when friction is neglected. The magnitude of the difference depends on the coefficient of friction, the elastic properties of the fibre and matrix, and the characteristics of the debonding mechanism. Experimental data on debonding in a series of glass-epoxy single-fibre composites are analysed using the proposed numerical technique to obtain the effects of fibre surface treatments and fibre strain-to-break on the interfacial fracture toughness. © Kluwer Academic Publishers     

Effect of matrix modification on durability of glass fiber reinforced cement composites

The primary concern for glass fiber reinforced concrete (GFRC) is the long-term durability of glass fibers in alkaline environment of cement. The objectives of the present work were to study the effectiveness of dimension-stabilizing admixture (NSR) and blast furnace slag with and without acrylic polymer on the mechanical properties of GFRC panels determined by flexural and tensile tests with aging. Control GFRC specimens without NSR and slag were also examined for comparison. Also, the influence of humidity condition of the specimens during testing was studied. Dry specimens (at standard room condition) and saturated specimens were tested in flexure and tension. Specimens after 28-day normal curing were submerged in hot water at 50°C for up to 84 days and then tested. The results indicated that the NSR admixture with slag provided significant improvement in aging behavior of the GFRC composite. The improvement was seen in both, strength and toughness. The greatest improvement was for NSR and slag with acrylic polymer tested at saturated condition.

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Résumé La résistance au vieillissement des fibres de verres en milieu basique, domaine de pH caractéristique du ciment, constitue le paramètre primordial pour l'utilisation de ciments armés à la fibre de verre (GFRC). Les objectifs de cette étude visent à analyser l'efficacité de produits auxiliaires de stabilité dimensionnelle (NSR) et du laitier de haut fourneau avec et sans polyacrylique sur les propriétés mécaniques de panneaux en GFRC. Pour cela on réalise des tests de flexion et de traction sur des échantillons de panneaux préalablement vieillis. Ces résultats sont comparés avec des échantillons de référence sans NSR ni laitier. L'influence du taux d'humidité des échantillons sur leurs propriétés mécaniques fait aussi l'objet de cette étude. Trois échantillons avec un taux d'humidité standard ainsi que des échantillons saturés en humidité sont soumis à des tests en flexion et en traction. Après une phase de prise du ciment portée 28 jours, les échantillons sont plongés dans un bain d'eau chaude à 50°C pendant 84 jours et puis testés. Les résultats indiquent que l'addition de NSR et de laitier conduisent à une amélioration significative du comportement au vieillissement des composites GFRC. Ceci est valable aussi bien pour le comportement à la flexion qu'à la traction. Le meilleur résultat est obtenu pour un apport constitué d'un mélange de NSR, de laitier et de polyacrylique sur des échantillons saturés en humidité.

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Editorial note Dr. Alva Peled is a RILEM Senior Member. She participates in RILEM TC TRC ‘Textile reinforced concrete’. Prof. Dr. Surendra P. Shah is a member of the Bureau of RILEM as well as the Editor-in-Chief of Materials and Structures. ACBM Center (Northwestern University) is a RILEM Titular Member.     

Effect of fiber coating on the longitudinal damping capacity of fiber-reinforced metal matrix composites

A novel model for calculating the damping capacity of continuous fiber-reinforced metal matrix composites (FMMCs) is proposed based on the viewpoint of energy loss. Finite element method (FEM) has been employed to investigate the effect of fiber coating on the longitudinal damping capacity of a composite by varying the thickness and the material properties of the coating. The results show that the damping of a composite containing the elastic coating increases with a decrease in the elastic modulus of the coating, while for the case of plastic coating, the weak coating or the high elastic modulus coating may help in improving the overall damping of composite.     

Effect of composite layering on the fracture toughness of brittle matrix/particulate composites

Glass matrix/Ni particulate composites, ranging from 0 to 25% particulate phase, were prepared both as single-volume-fraction composites and as multi-volume-fraction layered composites. Fracture toughness (K_c) measurements were made on all composites using the applied moment double cantilever beam technique. The measured toughness values for the layered composites were found to be equivalent to those of the single-volume-fraction composites. The fracture toughness measured for the layered composites was found to be dependent on the volume of composite phase tested and ultimately on the number of crack-particle interactions which occurred. R-curve like behaviour was observed in the layered composites.     

Effect of strain rate on the fracture of ceramic fibre reinforced glass matrix composites

The effect of strain rate on the fracture behaviour of two ceramic fibre reinforced glass matrix composites was studied. Increasing the strain rate was found to enhance catastrophic failure in both of these composites. This was attributed to the crack deflection and changes in the fibre pullout length as a function of strain rate. Enhanced strain rates were found to decrease the strength, static toughness and fracture energy of the composites. This effect was more pronounced in the case of the coated fibre composites as compared to the uncoated fibre composites. This is because of fibre/matrix isolation, obtained as a result of the coating.     

Transverse failure of carbon fiber composites: Analytical sensitivity to the distribution of fiber/matrix interface properties

An analytic differentiation method is presented to calculate the sensitivity of the transverse failure response of carbon fiber composite laminates to the distribution parameters of the fiber/matrix interface properties. The method starts with the evaluation of the sensitivities of the transverse failure response with respect to the interface properties of each fiber, ie, the cohesive failure strength and the critical displacement jump. These individual sensitivities are then used to calculate the sensitivities with respect to the mean and standard deviation of the interface properties. The derived sensitivities are implemented in a nonlinear interface-enriched generalized finite element method solver specially developed for this application. The interface-enriched generalized finite element method solver combines a cohesive modeling of the fiber/matrix interface failure with finite element meshes that do not conform to the composite microstructure. The approach is first demonstrated on a model material involving a one-dimensional domain containing N cohesive interfaces described by randomly selected cohesive failure properties. The method is then applied to the more complex problem of a composite laminate involving a large number of fibers.     

High temperature fracture behavior of tungsten fiber reinforced copper matrix composites under dynamic compression

W_f/Cu₈₂Al₁₀Fe₄Ni₄ composite was fabricated by flow casting method. Dynamic compression tests with strain rate of 1600 s⁻¹ at 20 °C, 200 °C, 400 °C and 600 °C were finished by means of Split Hopkinson Pressure Bar (SHPB). The results showed that the composites possessed obvious high temperature softening behaviors. The damages of W_f/Cu₈₂Al₁₀Fe₄Ni₄ composites all occurred within the tungsten fibers when compressed at 20 °C, 200 °C and 400 °C, indicating that the interface strength of the composites was high. While the damages of the composites occurred either in the tungsten fibers or in the matrix at 600 °C, in addition, the melt of matrix alloy also occurred. Microstructure of the composites after dynamic compressing at 600 °C was analyzed by transmission electron microscope (TEM), observation revealed that there were a lot of high-density dislocations, stacking faults and twins existing in the matrix. It was also found that the precipitated phase in the matrix played the role of the second phase strengthening.