New Micromechanics Design Theory for Pseudostrain Hardening Cementitious Composite

The micromechanics design theory has realized random short fiber-reinforced cement composites showing pseudostrain hardening (PSH) behavior with over 5% of strain capacity under tension. Nevertheless, this existing theory currently is limited to specific constituent properties, which does not account for chemical bond and fiber rupture. This article presents a new design theory that eliminates this restriction, achieving fiber rupture type PSH-random short fiber-reinforced cement composites with high-performance hydrophilic fibers like polyvinyl alcohol fibers. Uniaxial tensile tests are conducted employing polyvinyl alcohol fiber composites, the results of which support the validity of the proposed theory. Furthermore, parametric study employing the proposed theory quantitatively evaluates the effects of composite's micromechanics parameters, such as bond strength and fiber strength, on composite performance. This parametric study reveals that continuously increasing the degree of fiber rupture (fiber rupture intensity) enhances the strength performance of composites but not energy performance. However, an optimum rupture intensity exists for maximizing energy performance, which is critical for PSH behavior. The consistency between theoretical predictions and experimental results consequently demonstrates that the proposed theory can be utilized practically as a powerful and comprehensive tool for PSH composite design.     

Interfacial shear strength of cast and directionally solidified NiAl-sapphire fiber composites

The feasibility of fabricating intermetallic NiAl-sapphire fiber composites by casting and zone directional solidification has been examined. The fiber-matrix interfacial shear strengths measured using a fiber push-out technique in both cast and directionally solidified composites are greater than the strengths reported for composites fabricated by powder cloth process using organic binders. Microscopic examination of fibers extracted from cast, directionally solidified (DS), and thermally cycled composites, and the high values of interfacial shear strengths suggest that the fiber-matrix interface does not degrade due to casting and directional solidification. Sapphire fibers do not pin grain boundaries during directional solidification, suggesting that this technique can be used to fabricate sapphire fiber reinforced NiAl composites with single crystal matrices.     

Micromechanical modeling of unidirectional continuous sigma fiber-reinforced Ti-6Al-4V subjected to transverse tensile loading

A micromodeling analysis of unidirectionally reinforced Ti-6-4/SM1140+ composites subjected to transverse tensile loading has been performed using the finite-element method (FEM). The composite is assumed to the infinite and regular, with either hexagonal or rectangular arrays of fibers in an elastic-plastic matrix. Unit cells of these arrays are applied in this modeling analysis. Factors affecting transverse properties of the composites, such as thermal residual stresses caused by cooling from the composite processing temperature, fiber-matrix interface conditions, fiber volume fraction, fiber spacing, fiber packing, and test temperature are discussed. Predictions of stress-strain curves are compared with experimental results. A hexagonal fiber-packing model with a weak fiber-matrix interfacial strength predicts the transverse tensile behavior of the composite Ti-6-4/SM1140+ most accurately.     

Design,analysis and application of an improved push-through test for the measurement of interface properties in composites

An improved fiber push-through test has been designed and used to obtain new information about interfaces in composites consisting of matrices of a Ti alloy and borosilicate glass, both reinforced with SiC fibers. Interpretation of these results is accomplished through an analysis of coupled debonding and push-through, followed by push-back. The sliding stress is found to vary with push-out distance and to be substantially reduced in the vicinity of a fatigue crack in the Ti matrix composite. These effects are attributed to asperity wear, matrix plasticity and fragmentation of the fiber coating around the debonded interface. Reseating effects on push-back have been demonstrated, but have been found to diminish as the relative fiber-matrix displacement increases. Fiber roughness has been identified as an important aspect of interface sliding.     

Crack Growth Resistance of Hybrid Fiber-Reinforced Cement Matrix Composites

The effect of hybrid fiber reinforcement on fracture energy and crack propagation in cement matrix composites is examined. The crack in cement matrix composites is allowed to fracture under mode-I loading with three-point bending beam specimens. The influence of fiber types and their combination is quantified by using the toughness index and fracture energy. A proper hybrid combination of steel fibers and polyvinyl alcohol microfibers enhances the resistance to both the nucleation and growth of the crack. The micromechanical model of hybrid composites by using a fiber bridging law is emphasized, and the numerical model prediction closely matches the behavior obtained from the experiment. The influencing role of the material parameters in the fracture tests (e.g., the fracture toughness index and fracture energy) becomes more apparent than ones used in some conventional strength-based or fiber pullout tests, and these fracture parameters could screen the effect of fiber/microfiber reinforcement in enhancing the crack growth resistance of cementitious composites. This study demonstrates that fundamental fracture tests are effective to characterize and develop high-performance hybrid fiber–reinforced cement matrix composites.     

Influence of fabrication technique on the fiber pushout behavior in a sapphire-reinforced NiAl matrix composite

Directional solidification (DS) of “powder-cloth” (PC) processed sapphire-NiAl composites was carried out to examine the influence of fabrication technique on the fiber-matrix interfacial shear strength, measured using a fiber-pushout technique. The DS process replaced the fine, equiaxed NiAl grain structure of the PC composites with an oriented grain structure comprised of large columnar NiAl grains aligned parallel to the fiber axis, with fibers either completely engulfed within the NiAl grains or anchored at one to three grain boundaries. The load-displacement behavior during the pushout test exhibited an initial “pseudoelastic” response, followed by an “inelastic” response, and finally a “frictional” sliding response. The fiber-matrix interfacial shear strength and the fracture behavior during fiber pushout were investigated using an interrupted pushout test and fractography, as functions of specimen thickness (240 to 730 μm) and fabrication technique. The composites fabricated using the PC and the DS techniques had different matrix and interface structures and appreciably different interfacial shear strengths. In the DS composites, where the fiber-matrix interfaces were identical for all the fibers, the interfacial debond shear stresses were larger for the fibers embedded completely within the NiAl grains and smaller for the fibers anchored at a few grain boundaries. The matrix grain boundaries coincident on sapphire fibers were observed to be the preferred sites for crack formation and propagation. While the frictional sliding stress appeared to be independent of the fabrication technique, the interfacial debond shear stresses were larger for the DS composites compared to the PC composites. The study highlights the potential of the DS technique to grow single-crystal NiAl matrix composites reinforced with sapphire fibers, with fiber-matrix interfacial shear strength appreciably greater than that attainable by the current solid-state fabrication techniques.     

Bridging mechanisms in a Tiβ21s/SCS-6 composite: <Emphasis Type="Italic">In-Situ</Emphasis> scanning electron microscopy study

The physical nature of fiber bridging in a Tiβ21s/SCS-6 composite has been studied using in-situ loading in tension within a field-emission gun scanning electron microscope (SEM). Single-edge-notched specimens were prefatigued under tension-tension to produce mode I bridged cracks and were then monotonically loaded in tension within the field emission gun SEM. Crack opening displacements (CODs) were measured, and the response of the fiber-matrix interface to the applied load was examined. The results confirm that load transfer does occur along debonded fiber-matrix interfaces but also that the efficiency of such load transfer varies with the position of the fiber along the crack path. Based on the information obtained, the physical nature of fiber bridging in the composite is discussed.     

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.     

Finite-Element Model for Failure Study of Two-Dimensional Triaxially Braided Composite

A new three-dimensional finite-element model of two-dimensional, triaxially braided composites is presented in this paper. This mesoscale modeling technique is used to examine and predict the deformation and damage observed in tests of straight-sided specimens. A unit cell-based approach is used to consider the braiding architecture and the mechanical properties of the fiber tows, the matrix, and the fiber tow-matrix interface. A 0°/±60° braiding configuration has been investigated by conducting static finite-element analyses. Failure initiation and progressive degradation has been simulated in the fiber tows by using the Hashin failure criteria and a damage evolution law. The fiber tow-matrix interface was modeled by using a cohesive zone approach to capture any fiber-matrix debonding. By comparing the analytical results with those obtained experimentally, the applicability of the developed model was assessed and the failure process was investigated.     

Failure diagrams for unidirectional

Pertinent failure processes in unidirectional fiber metal-matrix composites (MMCs) have been identified and analyzed. The critical conditions for interface delamination in several composites are compared with the theoretical delamination diagram proposed by He and Hutchinson, in which interface delamination and fiber fracture are delineated on the basis of their relative tough-ness values. It is shown that the delamination diagram does not provide information about the extent of interface cracking or the onset of fiber bridging. An alternative failure diagram that depicts composite fracture processes, such as matrix yielding followed by fiber fracture, inter-face cracking, and fiber bridging, is proposed. Development of the composite failure diagramvia micromechanical modeling of individual mechanisms is presented together with experimental results from the literature. Good correlation between theory and experiment suggests that the composite failure diagram might be used for tailoring composite properties through control of the dominant fracture mechanism.     

Scalings in the statistical failure of brittle matrix composites with discontinuous fibers—I. Analysis and Monte Carlo simulations

This paper addresses the statistical aspects of the failure of unidirectional composites having brittle matrices reinforced with discontinuous brittle fibers. The failure process involves quasi-periodic matrix cracking, frictional sliding of the fibers in fiber break zones and fiber bridging of matrix cracks in a global load-sharing framework. We consider a composite section of “characteristic” length and develop its distribution for strength in terms of certain characteristic stress and length scales. Continuous sections of the fibers follow the usual Weibull distribution for strength. We also introduce random discontinuities along the fiber, originating say from processing damage whose spacins follow a Poisson process where the rate α is the mean number of discontinuities per characteristic length of the fiber. We derive two approximations for the mean and standard deviation of such characteristics composites. A realistic Monte Carlo simulation model is developed to test these analytical results and to study the fiber pull-out properties. The composites turn out to be quite insensitive to initial damage. The mean pull-out length is found to increase with increasing α and becomes independent of the Weibull modulus for large values of α.     

钢铁基复合材料的研究现状及发展前景

钢铁基复合材料由于具有低成本及优异的综合性能,逐渐成为复合材料开发的热点。简要阐述了钢铁基复合材料性能的影响因素,包括钢铁基体、增强体和复合界面三者的性质,归纳了目前钢铁基复合材料的主要制备方法,列举了颗粒增强钢铁基复合材料、钢铁基复合板以及纤维增强钢铁基复合材料的研究现状。最后,探讨了钢铁基复合材料的研究方向和应用前景,指出制备方法协同化、构型复杂化、用途多元化和高通量集成技术将在今后钢铁基复合材料的发展中扮演重要角色。     

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.     

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.     

Fatigue in selectively fiber-reinforced titanium matrix composites

Many applications of the Ti alloy matrix composites (TMCs) reinforced with SiC fibers are expected to use the selective reinforcement concept in order to optimize the processing and increase the cost-effectiveness. In this work, unnotched fatigue behavior of a Ti-6Al-4V matrix selectively reinforced with SCS-6 SiC fibers has been examined. Experiments have been conducted on two different model panels. Results show that the fatigue life of the selectively reinforced composites is far inferior to that of the all-TMC panel. The fatigue life decreases with the decreasing effective fiber volume fraction. Suppression of multiple matrix cracking in the selectively reinforced panels was identified as the reason for their lack of fatigue resistance. Fatigue endurance limit as a function of the clad thickness was calculated using the modified Smith-Watson-Topper (SWT) parameter and the effective fiber volume fraction approach. The regime over which multiple matrix cracking occurs is identified using the bridging fiber fracture criterion. A fatigue failure map for the selectively reinforced TMCs is constructed on the basis of the observed damage mechanisms. Possible applications of such maps are discussed.     

Influence of Cr and W alloying on the fiber-matrix interfacial shear strength in cast and directionally solidified sapphire nial composites

Sapphire-reinforced NiAl matrix composites with chromium or tungsten as alloying additions were synthesized using casting and zone directional solidification (DS) techniques and characterized by a fiber pushout test as well as by microhardness measurements. The sapphire-NiAl(Cr) specimens exhibited an interlayer of Cr rich eutectic at the fiber-matrix interface and a higher interfacial shear strength compared to unalloyed sapphire-NiAl specimens processed under identical conditions. In contrast, the sapphire-NiAl(W) specimens did not show interfacial excess of tungsten rich phases, although the interfacial shear strength was high and comparable to that of sapphire-NiAl(Cr). The postdebond sliding stress was higher in sapphire-NiAl(Cr) than in sapphire-NiAl(W) due to interface enrichment with chromium particles. The matrix microhardness progressively decreased with increasing distance from the interface in both DS NiAl and NiAl(Cr) specimens. The study highlights the potential of casting and DS techniques to improve the toughness and strength of NiAl by designing dual-phase microstructures in NiAl alloys reinforced with sapphire fibers. R. TIWARI, formerly Research Associate, Department of Chemical Engineering, Cleveland State University     

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。     

Pullout Response of a Smooth Fiber with an End Anchorage

The main objective of this study is to develop an analytical model to predict the pullout load versus end slip relationship of a smooth fiber having an end anchorage embedded in a matrix. The resisting pullout load of the fiber is composed of a component due to interfacial bond at the fiber-matrix interface and a component due to mechanical anchorage at the embedded end of the fiber. The concept of a relationship between the bond shear stress and the slip at the fiber-matrix interface is used to obtain the force component due to the interfacial bond. To account for the mechanical anchorage resistance at the embedded end, a spring component at the embedded end of the fiber is used. The constitutive property of the spring is assumed to be nonlinear. Based on these concepts and assumptions, a set of analytical solutions to predict the pullout load vesus end slip relationship is derived and then solved by an iterative procedure. Examples predicting the pullout load versus slip curve of a smooth fiber with and without mechanical anchorage at the embedded end are shown and compared. Different modeling aspects of the fiber end anchorage effect and its influence on the pullout load versus slip response are investigated and discussed, with particular emphasis on the tensile force and the bond shear stress distribution along the fiber-matrix interface.     

Modeling of anisotropic behavior of weakly bonded fiber reinforced MMC's

The anisotropic mechanical behavior of a continuous fiber reinforced Ti alloy matrix composite which possesses a weak fiber matrix interface is modeled numerically. Effects of interface properties and residual stresses incurred during the fabrication are addressed in detail. The computational modeling is guided by comparison with experimental data. The study provides an understanding which will be used to model the multiaxial behavior of weakly bonded composites and to provide a tool for predicting the failure of composite structures.