Fiber-matrix Bond Research Articles

Durability of textile reinforced concrete made with AR glass fibre: effect of the matrix composition

This paper presents the results of recent experimentation performed to study time-dependent changes in the mechanical performance of textile reinforced concrete (TRC) made with AR glass fibre and to specify the decisive mechanisms influencing the durability of this composite material. The effect of the matrix composition was investigated by varying hydration kinetics and alkalinity of the binder mix. At first, tensile tests on (accelerated) aged specimens made of TRC were performed. The results showed a pronounced decrease in the tensile strength and strain capacity for TRC whose matrix was most alkaline (Portland cement was used exclusively as binder in this composition). The performance of TRC made with modified, alkali reduced matrix composition was to a great extent unaffected by exposure to accelerated ageing. In order to investigate the mechanisms leading to such different behaviours, changes in the mechanical performance of the fibre–matrix bond were studied using double-sided pullout test specimens with under-critical fibre reinforcement after they had undergone accelerated ageing. Furthermore, the appearance of the microstructure in the interface between fibre and matrix was described by images obtained from SEM-investigations. Measured reductions in the toughness of the composite materials could be attributed mainly to the visual observed disadvantageous new formation of solid phases in the fibre–matrix interface, while the deterioration of the AR glass fibre seemed to play only a secondary role. It could be shown that the morphology of the formed solid hydration phases depends to a large extent on the matrix composition.

Application of nanoindentation testing to study of the interfacial transition zone in steel fiber reinforced mortar

The characteristics of the profiles of elastic modulus and hardness of the steel fiber–matrix and fiber–matrix–aggregate interfacial zones in steel fiber reinforced mortars have been investigated by using nanoindentation and Scanning Electron Microscopy (SEM), where two sets of parameters, i.e. water/binder ratio and content of silica fume were considered. Different interfacial bond conditions in the interfacial transition zones (ITZ) are discussed. For sample without silica fume, efficient interfacial bonds across the steel fiber–matrix and fiber–matrix–aggregate interfaces are shown in low water/binder ratio mortar; while in high water/binder ratio mortar, due to the discontinuous bleeding voids underneath the fiber, the fiber–matrix bond is not very good. On the other hand, for sample with silica fume, the addition of 10% silica fume leads to no distinct presence of weak ITZ in the steel fiber–matrix interface; but the effect of the silica fume on the steel fiber–matrix–aggregate interfacial zone is not obvious due to voids in the vicinity of steel fiber.

Effect of Fiber Length on Hysteretic Heating of Discontinuous Fiber-Reinforced Polypropylene

Fatigue application of a polymeric material is limited due to its hysteretic heating behavior. In this work, effort has been made to understand the effect of fiber reinforcement and length of the reinforcing fibers on hysteretic heating behavior and its mechanisms. Unreinforced, 20% short glass fiber-reinforced and 20% long glass fiber-reinforced polypropylene materials have been injection-molded and subjected to a finite number of fatigue cycles. The load required for constant deflection and the surface temperature of materials during testing was measured and correlated with the hysteretic heating mechanism. During material deformation, the presence of the reinforced fibers and fiber-matrix interface in the reinforced material contributed more internal friction, and resulted in higher heat generation than unreinforced material. Higher fiber density and the inferior fiber-matrix bonds existing in the short fiber-reinforced material generated higher heat than that of long fiber-reinforced material during testing.

Experimental investigations on the durability of fibre–matrix interfaces in textile-reinforced concrete

Abstract Changes in the strength and toughness of textile-reinforced concrete with increasing age are determined essentially by the durability of the armouring fibres, the matrix itself, and the bond between the matrix and the fibres. Based on new experimental results, important mechanisms influencing the durability of the fibre–matrix bond are treated in this article and discussed as to their significance. The investigations were conducted on multi-filament yarns of AR-glass which were imbedded in matrices of varying alkalinity and hydration kinetics. The loading capacity of the fibre–matrix bonds was determined in direct tension tests on under-reinforced specimens after they had undergone accelerated aging. Further, the condition of the microstructure between fibre and matrix was ascertained with both image analysis and analytical procedures. It was concluded that measured reductions in the toughness of the composite material could be attributed to the diminishing protective effect of organic polymer sizing on the surface of the filaments as well as to the disadvantageous new formation of solid hydration phases (mainly Portlandite) in the fibre–matrix interface.

Ultrasonic Energy Transfer and Stress Concentrations in a Single-Fiber Composite with Absorbing Interface Layer

Elastic wave scattering from coated and layered cylindrical scatterers encased in an elastic matrix is reviewed. Interaction of time harmonic monochromatic longitudinal and shear ultrasonic waves with a single fiber coated by homogeneous dissipative (polymeric) materials and embedded in an unbounded elastic matrix is studied using the classical method of eigenfunction expansion. The novel features of Havriliak—Negami model is employed to include the effect of hysteresis-type of compressional and shear waves' absorption in the interphase material by introduction of proper frequency-dependent complex moduli. The pertinent integrals for the time-averaged total fiber energy induced by the elastic waves refracted along the fiber are derived and numerically evaluated. The numerical results reveal the important effects of the absorptive fiber—matrix bond as well as interphase thickness on fiber/matrix energy transfer and dynamic stress concentrations. The calculated results may be interpreted in the context of phenomena observed in acousto-ultrasonic (AU) experiments on fiber reinforced composite materials by correlating certain parameters in the frequency spectrum of the detected waveform to mechanical characteristics of the dissipative interface layer. Limiting cases are considered and good agreements with well-known solutions are established.

Steel-fibre-reinforcement and hydration coupled effects on concrete tensile behaviour

This paper presents the effect of hydration development on the tensile behaviour of steel-fibre-reinforced concrete. Tensile tests were performed on plain and fibre-reinforced concretes at 2, 7 and 28 days in order to determine the response of the composites, in particular to establish the post-peak behaviour and the evolution of the residual post-peak strength with hydration development and the associated improvement in the fibre–matrix bond. From the laws governing the tensile behaviour of plain concrete on the one hand and the residual strength capacity due to fibre reinforcement on the other, parameters fitting an analytical model of the uniaxial tensile response of fibre-reinforced concrete were determined. The proposed model takes into account the tensile damage to the concrete and the development of hydration. An original aspect of the model is that it also integrates the damage to the fibre–matrix bond.

Temperature dependences of the strengths of a carbon fiber and a three-dimensional reinforced carbon-carbon composite

At a fixed tension rate, the ultimate tensile strength of a carbon fiber decreases nonlinearly with increasing temperature T. This nonlinearity is caused by a change in the statistics of atomic vibrations from quantum (at T 2250 K) statistics. To take into account the quantum statistics, quantum function Fq is introduced into Zhurkov’s equation instead of temperature; the value of this function is calculated from the temperature dependence of the specific heat of carbon. This equation gives the values of the fracture activation energy (≈16 eV) and parameter γ (≈0.15 nm3). The strength of the three-dimensional reinforced carbon-carbon composite decreases up to ≈1800 K and increases as the temperature grows further. The decrease in the strength is explained by an increase in the rate of fiber and matrix fracture with increasing temperature, and the increase in the strength is explained by a decrease in the strength of the fiber-matrix adhesion bonds at high temperatures. As a result of this decrease, fibers begin to move with respect to each other under load, and the stresses applied to them level off. Although the fiber strength continues to decrease with increasing temperature, this effect increases the composite strength.

Mechanical reinforcement of concrete with bi-component fibers

For some applications the reinforcement of concrete with fibers is an economical alternative to conventional steel bar reinforcement. Steel fibers have been the first choice for many years because of their high tensile strength and high elastic modulus. Low modulus fibers, such as polyolefin based fibers generally are thought to be less suitable for this purpose. However, it is shown that polyolefin fibers with sufficient tensile strength can, applying a novel bi-component approach, successfully enhance the mechanical properties of concrete. The effect of the introduction of nanoparticles into the fiber polymers and of a fiber surface structuring on the fiber pull-out characteristics and the fiber–matrix bond strength is presented. The performance of bi-component fiber reinforced concrete is studied in 4-point bending and square slab tests. Ductile post-peak behavior of such fiber reinforced concrete is achieved, making this new fiber technology interesting for applications in pre-cast elements, industrial floors and earth quake protecting systems.

Mechanical behaviour of self-compacting concrete with hybrid fibre reinforcement

In this study, fibre-reinforced self-compacting concretes were developed for precast building components, incorporating either adherent metal fibres or polymeric synthetic slipping fibres or a combination of both. To achieve the warranted workability, compressive and splitting tensile strengths, compositions were determined by preliminary tests on self-compacting materials with various proportions of metal fibres. Bending tests in controlled deflection confirmed the positive contribution of fibres in the mechanical behaviour of self-compacting concrete. The comparison between vibrated and self-compacting concretes of similar mechanical characteristics indicated a possible better fibre-matrix bond in the case of self-compacting types. The results also showed that the properties of the hybrid fibre-reinforced self-compacting concrete could be inferred from the properties of the individual single-fibre reinforcements and their respective proportions through simple mix-rules.

Microfiber and Macrofiber Hybrid Fiber-Reinforced Concrete

To realize the potential of micro- and macrofiber blends in a concrete matrix, a concrete containing polyvinyl alcohol (PVA) or steel microfibers and steel macrofibers was designed using a mixture proportioning method that provides good workability in concretes containing microfibers. This procedure is based on an optimum paste volume fraction determined from the relationship between flow and paste content. The mechanical performance, water permeability of the cracked material, and shrinkage crack resistance of cast concrete were evaluated. In the hybrid concrete, the microfibers delayed the development of macrocracks and so the composite demonstrated greater strength and crack resistance than a similar matrix reinforced with macrofibers only. This influence was less pronounced than was observed with a mortar matrix in a previous study. This is explained by differences in the failure mechanism of the fibers: A stronger fiber-matrix bond resulting from a lower water-to-binder ratio caused the microfibers to break instead of pull out. The macrofibers were also more likely to break in the hybrid fiber-reinforced concrete than in the same matrix containing macrofibers alone—since the microfibers reinforced the matrix, increasing the macrofiber pullout resistance.

Impact response of the fiber-matrix bond in concrete

Fiber reinforced concrete (FRC) is a heterogeneous material comprising of distinct components: fiber, matrix, and fiber-matrix interface. Many applications require an enhanced resistance to impact loading from FRC. While designing for impact, the fiber-matrix interaction is by far the most critical. In the study reported here, three polymeric fibers and one steel fiber were investigated to examine the influence of fiber material, geometry, orientation, and the matrix strength under static and impact loading. Results show bond stiffening under impact loading. This was evident through higher peak loads, lower corresponding values of crack opening and a transition in the failure mode of some polymeric fibers from pull-out to fracture. Higher matrix strength resulted in a stiffer bond for all fiber types. Inclined loading produced better results for steel fibers. Under high crack opening rates, the bond stiffening in deformed polymeric fibers brought it closer in performance to the steel fiber.Key words: bond–COD response, impact loading, steel fiber, polymeric fiber, pull-out test, rate effects.

Coconut fibre reinforced polyethylene composites: effect of natural waxy surface layer of the fibre on fibre/matrix interfacial bonding and strength of composites

Coconut fibre has been used as reinforcement in low-density polyethylene. The effect of natural waxy surface layer of the fibre on fibre/matrix interfacial bonding and composite properties has been studied by single fibre pullout test and evaluating the tensile properties of oriented discontinuous fibre composites. The waxy layer provided good fibre–matrix bond such that removal of the layer resulted in drastic decrease of the fibre pullout stress, increase of the critical fibre length and corresponding decrease in tensile strength and modulus of the composites. The waxy layer of polymeric nature also exhibited a stronger effect on interfacial bonding than by grafted layer of a C 15 long-chain alkyl molecule onto the wax-free fibre. The morphological features of the fibre along with its surface compatibility with the matrix favours oriented flow of relatively long fibres along with the molten matrix during extrusion.

Micromechanical modeling of viscoelastic composites with compliant fiber–matrix bonding

The micromechanical method of cells is used to calculate the average time-dependent constitutive properties of the homogenized substitute continuum from the viscoelastic material properties and volume fractions of the individual phases as well as from the interphase behavior of polymeric fiber composites. Two approaches are investigated to establish and solve the micromechanical model equations of the rate-dependent material phases. The first one is based on the L aplace-transformation of the time-dependent material functions and the application of the correspondence principle of linear viscoelasticity to the governing equations of the micromechanical model. In the second approach a numerical time integration scheme is developed to compute the convolution integrals. For the study of the influence of compliant fiber–matrix bonds alternative models are proposed: A representative volume element with three constituents accounts for the properties of the flexible interphase between the fibers and the matrix. The three-phase model is then compared to a two-phase model with a time-dependent flexible interface to simulate the compliant fiber–matrix bond. Various examples show the performance of both models and the numerical algorithms presented.

Improvement in fracture toughness of an epoxy/copper composite through the use of various end shaped fibers

Improvement in composite fracture toughness can be achieved by using short ductile fibers with shaped ends which utilizes the plastic work potential of the fiber volume through anchoring of the fiber end into the matrix. Single fiber pullout tests performed on copper wire demonstrates that a 2D flat-end-impacted fiber geometry that is easier to produce improves the fracture toughness increment at least as well as a 3D “axisymmetric” end-impacted fiber. Calculations based on pull out tests and a model for predicting the fiber contribution to the fracture toughness increment “Δ G” show a 46% higher Δ G for the flat-end-impacted fiber compared to a straight fiber at 0° orientation. Results further indicate that for a given fiber geometry there is an optimum end volume; above or below this volume results in a lower Δ G. Due to the small fiber end volume, the packing density of the fibers will not be significantly affected by the end shaping of the fiber. Annealing and subsequent oxide removal of the end-impacted fiber is not necessary because there is no significant improvement in the Δ G. However, a moderate 3.5–15% decrease in the debonding force is seen in the annealed fibers, depending on the embedment depth. This indicates that annealing weakens the fiber–matrix bond. Furthermore, results indicate that a fiber that has multiple mechanical interlocks in the matrix is not as effective in fracture toughening as single anchoring of a shaped fiber end. However, the load displacement curve for the light rippled fiber shows a unique “wavy” behavior related to the geometry of the fiber end which may be useful in other applications. Round fiber ends produced with an acetylene torch had a 70% lower Δ G compared to a straight fiber because of the large fiber end volume and the poor microstructure resulting from the high torch temperature.

Effects of Particle-Dispersion on the Strength of an Alumina Fiber-Reinforced Aluminum Alloy Matrix Composite

Aluminum alloy matrix hybrid composites, in which alumina particles were dispersed among continuous alumina fibers, were fabricated by squeeze casting, and the influence of temperature and the effects of the particle-dispersion on the strength of the composites were then investigated. The particle-dispersion among the fibers minimized preform contraction and fiber-to-fiber contact due to the melt infiltration during the squeeze casting. The tensile strength, 0.2% proof stress and elastic modulus of the composites in the longitudinal direction increased with increasing the fiber volume fraction, retaining nearly the same values up to 623 K. These values of the hybrid composite were larger than those of the particle-free composite at every temperature. This is because the fibers were distributed uniformly owing to the particles that prevented the fiber-to-fiber contact, leading to the reduction of stress concentration at the points of direct fiber contact, and stress transmission between the fiber and the matrix becomes easy. At every temperature, the transverse tensile strength and proof stress of the particle-free composite was lower than that of the unreinforced alloy, because the fracture was initiated at the fiber-to-fiber contact point and the cracks propagated mainly along the fiber-matrix interface. In contrast, the strength of the hybrid composite was close to that of the unreinforced alloy even at high-temperature, because the cracks propagated mainly throughout the matrix owing to the uniform distribution of the fibers and the strong fiber-matrix bond.

EFFECT OF IMPERFECT INTERFACE ON THERMAL STRESSES-ASSISTED MATRIX CRACKING IN FIBER COMPOSITES

In this article we study the effects of imperfect bonding on stress intensity factors (SIFs) calculated at a radial matrix crack in a fiber (inclusion) composite subjected to thermal loading. We use analytic continuation to extend existing series methods to obtain series representations of deformation and stress fields in both the inclusion and the surrounding matrix in the presence of the crack. The interaction between the crack and the inclusion is demonstrated numerically for different elastic materials, geometries, and varying degrees of bonding (represented by imperfect interface parameters) at the interface. The results show that, generally, SIF decreases monotonically with the interface imperfection at both nearby and distant crack tips. The sliding and nonslipping interfaces have a relatively small effect on the reduction of thermal stresses-induced SIF as compared to a general imperfect interface. Therefore, the pronounced effect of the imperfect fiber-matrix bond on the thermal stresses-assisted SIF at the crack tips should not be ignored.

Three‐dimensionally braided carbon fiber–epoxy composites, a new type of materials for osteosynthesis devices. II. Influence of fiber surface treatment

AbstractInterfacial adhesion between carbon fiber and epoxy resin plays an important role in determining performance of carbon–epoxy composites. The objective of this research is to determine the effect of fiber surface treatment (oxidization in air) on the mechanical properties (flexural strength and modulus, shear and impact strengths) of three‐dimensionally (3D) braided carbon‐fiber‐reinforced epoxy (C3D/EP) composites. Carbon fibers were air‐treated under various conditions to improve fiber–matrix adhesion. It is found that excessive oxidation will cause formation of micropits. These micropits are preferably formed in crevices of fiber surfaces. The micropits formed on fiber surfaces produce strengthened fiber–matrix bond, but cause great loss of fiber strength and is probably harmful to the overall performance of the corresponding composites. A trade‐off between the fiber–matrix bond and fiber strength loss should be considered. The effectiveness of fiber surface treatment on performance improvement of the C3D/EP composites was compared with that of the unidirectional carbon fiber–epoxy composites. In addition, the effects of fiber volume fraction (Vf) and braiding angle on relative performance improvements were determined. Results reveal obvious effects of Vf and braiding angle. A mechanism was proposed to explain the experimental phenomena. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1040–1046, 2002

Surface modification of textile fibers for improvement of adhesion to polymeric matrices: a review

Surface treatments have long been utilized to modify the chemical and physical structures of the surface layers of textile fibers, thus improving the properties of fibers in many applications. This review discusses the feasibility and characteristics of different methods of surface modification of polymeric textile fibers, focusing on tailoring fiber-matrix bond strength in fiber-reinforced composite materials. The influence of various treatments on the chemical and mechanical properties of different fibers is discussed. Some very recent developments in surface modification of textile fibers are highlighted.

Cold-temperature and simultaneous aqueous environment related degradation of carbon/vinylester composites

Abstract The effects of non-cryogenic, cold temperature conditions especially as related to freeze–thaw cycling both in the presence of aqueous solutions, and just ambient humidity, on carbon fiber composites is not well understood. Based on processing characteristics and lower cost, vinylester resin systems are increasingly being used in civil infrastructure and in off-shore applications. The use of these systems with carbon fibers causes some concern due to the current lack of appropriate finish on the fibers. In addition the use of ambient cure processes such as wet layup and resin infusion raise concerns of durability since these resin systems typically do not achieve full conversion and are thus susceptible to moisture induced degradation. This paper reports on results of short-term exposure of thin ambient cured carbon/vinylester specimens, processed following wet layup procedures similar to those used in rehabilitation of structural components, to freeze and freeze–thaw cycling. It is shown that freeze–thaw can cause significant reduction in mechanical properties and in glass-transition temperature with immersion in salt water having a larger effect on fiber-matrix bond deterioration and matrix cracking than other exposures.

An Analysis of Microdroplet Test: Effects of Specimen Geometry, Matrix Type, Fibre Treatment and Water Ageing

The microdroplet test has been used to characterize the interfacial bond in fibrous composite materials. The specimen consists of a length of fibre with a drop of resin cured on it and pulled out while the drop is being supported by a platinum disc with a hole. A properly tested specimen fails at the droplet's tip-fibre interface, revealing the ultimate interfacial shear strength. The microdroplet test is carried out under optimal testing conditions, established from our previous work, in order to obtain fully debonded specimens for the interfacial shear stress analysis of the fibre-matrix bond. Three thermosetting resins are applied on silane coated and acetone washed (silane removed) glass fibres in order to investigate the influence of the silane coupling agent on the fibre-matrix bond. Also, these systems were aged in water at room temperature for periods of 3, 6 and 10 weeks and the influence of water degradation on the fibre-matrix bond is investigated for both groups of glass fibres (silane coated and acetone washed). The group of glass fibres washed in acetone produced weaker fibre-matrix bond and accelerated water degradation of the interfacial bond. It is found that the influences of two dominant geometric parameters (conus angle and droplet length) on the test results are reciprocally opposite, generating scatter in the experimental results. It is shown in this work that the microdroplet test can be used as a reliable tool for measurement of the fibre-matrix adhesion.