Embedded Fiber Length Research Articles

Pull-out behaviour of hooked steel fibres

A programme of research is described that has investigated the fracture of steel fibre reinforced sprayed concrete under flexural load, with the aim of developing a stress-profile model to predict flexural behaviour in the form of a load-deflection response. This paper reports the work associated with establishing the pull-out characteristics of hooked end fibres. The effects of matrix strength, fibre embedment length and orientation are described, together with the interaction of these parameters. The relationships established can be used to model the tensile response of a beam at varying crack width and hence form a key part of the stress-profile for predicting residual flexural strength, which is an essential requirement of a much needed design rationale for steel fibre concrete.

Modelling toughening of composites with interleaved chopped fibres

This paper presents a micromechanical model for the toughening of continuous carbon fibre composites with interleaved layers of chopped fibres. Toughening results from a bridging process, involving chopped fibres that are almost parallel to the crack plane. This process can be divided into two stages - in plane cracking or spalling (matrix fracture following by separation of fibre away from the matrix) and fibre pullout. In the early stages of fibre bridging, the matrix surrounding the chopped fibres offers little resistance to fibre bending, so that in plane cracking occurs. In the later stages of fibre bridging, with a reduction in the embedded fibre length and an increase in the tensile stress on the fibres, fibre pullout occurs. The proposed model deals with both in plane cracking and fibre pullout.

Measuring and Modifying Interface Properties of PVA Fibers in ECC Matrix

A single fiber pullout test was used in this study to measure the bond properties of polyvinyl alcohol fibers that are available at various diameters in a mortar matrix. Despite short fiber embedment lengths, the small diameter fibers ruptured during the pullout tests. However, it is shown that even if full fiber pullout is not achieved, it is still possible to determine a chemical debonding energy, Gd, and an initial interfacial frictional bond strength, τ0. Despite high Gd values, the fibers did not rupture during the fiber chemical debonding process, but during fiber pull-out, a strong slip-hardening effect, characterized by the high values of the slip-hardening coefficient, β, induced severe abrasion damage visible under scanning electron microscope on the fiber surface. As a consequence, when the fiber apparent tensile strength was exceeded, fibers ruptured by delamination. Finally, an attempt was made to lower the values of the bond properties to minimize fiber rupture during pullout. This goal was ...

鋼繊維補強コンクリートの一軸引張およびせん断に対する変形挙動のモデル化

The deformational behavior of steel fiber reinforced concrete is important because it affects the crack width which influences the durability of the structure. In this study the deformational behavior of steel fiber reinforced concrete subjected to uniaxial tension and shear was investigated experimentally. In the experiment the effects of the volumetric fiber content, the fiber geometry and the fiber embedment length were investigated. Highstrentgh concrete was used in the specimen. From the experimental results models for the deformational behavior of steel fiber reinforced concrete under uniaxial tension and shear were obtained.

Fiber-stretching test: a new technique for characterizing the fiber–matrix interface using direct observation of crack initiation and propagation

A new micromechanical technique for experimental determination of fiber-matrix interfacial properties is presented. This technique consists in tensile loading of the fiber, with a matrix droplet on it, at both ends, accompanied by continuous direct observation of interfacial crack propagation. In comparison with the well-known microbond test, the new method has two important advantages. First, crack propagation is stable for any embedded fiber length and any relation between adhesion and friction at the interface. Second, compliance of the test equipment does not affect the results, and specimens with long free fiber ends can be successfully tested. A similar result can be reached using the pull-out or microbond test with an 'infinite' (very long) embedded fiber length. An algorithm for separate determination of the interfacial adhesion and friction from experimental relationships between the crack length and applied load is described. The new test was employed to determine the interfacial parameters for composites of glass fibers with polypropylene, polystyrene, and polycarbonate. For each fiber-polymer system investigated, the following parameters were calculated: ultimate interfacial shear strength; critical energy release rate for crack propagation; and adhesional pressure. Our approach to the estimation of the work of adhesion, WA, from micromechanical tests, based on the concept of adhesional pressure, allowed us to calculate the WA values for several thermoplastic matrix-glass fiber pairs and to obtain values consistent with previous estimations made according to other approaches.

Interphase characterization in polymer composites - monitoring of interphasial behavior in dependence on the mode of loading

Single fiber model composites consisting of epoxy resin matrix and differently sized glass fibers were investigated using pull-out tests, scanning electron microscopy (SEM), scanning force microscopy (SFM) and single fiber dynamic load test (SFDL). The inhomogeneous stress distribution along the embedded fiber length could be visualized by monitoring. SEM images showed either cohesive fracture or adhesive failure on pulled-out fibers with different sizings. The crack initiation and propagation were detected randomly and multiply distributed as the inhomogeneous interphase itself and depending strongly on the fiber-matrix model combination. The meniscus region acts as a material inhomogeneity and its appearence depends on the surface free energies of fiber and matrix and on the curing conditions of the resin. SFM in force modulation mode has visualized different interphase thicknesses and gradients of local stiffness. The SFDL test has been shown as a worthful tool for the comprehensive determination of fiber-matrix interaction.

Comparison of the stress transfer in single- and multi-fiber composite pull-out tests

Analyses of the elastic stress transfer taking place across the fiber-matrix interface are presented for single- and multi-fiber composite pull-out tests. The multi-fiber composite is treated as a three-cylinder assemblage consisting of a central fiber, a matrix annulus, and a composite medium. The forms of the fiber axial stress and the interface shear stress distributions along the embedded fiber length are determined for single- and multi-fiber composite pull-out tests and their dependences on the fiber volume fraction, the dimensions of the specimen, the fiber-to-matrix modulus ratio, and the embedded fiber aspect ratio are displayed. The stress transfer for a perfectly bonded interface for the two pull-out tests is compared and the difference is clearly shown. In addition, for the single-fiber composite pull-out test, the present theory is compared with some existing theories.

A technological solution to the testing and data reduction of single fibre fragmentation tests

Abstract The single fibre fragmentation test was first introduced in 1965 for measuring interfacial adhesion in fibre reinforced composites. However, this test is still far from a routine test and no suitable testing standard exists. Consequently, this method is hardly used by research laboratories of the reinforcing fibre manufacturers, composite fabricators and other standardisation organisations. The main reasons for the failure of the fragmentation test to establish as a standard quality control tool are the time consuming manual testing procedures and the poor confidence in the data reduction techniques used for calculating the interfacial adhesion parameter. An integrated machine, software and data reduction approach has been developed for the fragmentation test. The new approach allows the stoppage of the fragmentation test at pre-defined strains and digitisation of the whole embedded fibre length using a microscope attached with a video-camera and Sun work-station. Subsequently, the fragment and associated debond lengths are measured using a Visual Basic program in a semi-automatic process. The problems associated with the huge amount of computer graphics data and compatibility of different graphics formats have been sorted out. This leads to substantial savings in the time required for the fragmentation test. Using this integrated machine and software approach, the single fibre fragmentation test has been carried out on carbon fibre samples with well-defined interfaces. The results were analysed using the conventional Kelly–Tyson model and the cumulative stress transfer function (CSTF) technique, at different applied strains. It has been shown that the CSTF technique can be used for assessing the fibre-matrix adhesion from the single fibre fragmentation test at any applied strain before or after the saturation of the fragmentation process, thus eliminating the need for specially formulated resin for the fragmentation test.

Measurement of interfacial shear strength of carbon fibre/epoxy composites using a single fibre pull-out test

Single fiber pull-out tests were conducted for G34-700/ Araldite-F, T700S/Araldite-F and T800H/Araldite-F model composites using an improved experimental set-up to evaluate the interfacial shear strength (IFSS). The modified method made it possible to reduce the formation of resin meniscus and to accurately define the embedded fibre length, and it also provided a practical approach to conveniently estimate the IFSS for comparison of the fibre-matrix adhesion between different composite systems or different fibre surface conditions. A strong interfacial adhesion and unstable debonding process without detectable friction stages were found for all types of model composites with fibres being chemically treated/sized or sizing being removed. The electrochemical oxidation treatment and sizing on the carbon fibres noticeably increase the IFSS of the composites.

Deformation Micromechanics in Model Carbon Fiber Reinforced Composites Part II: The Microbond Test

Raman spectroscopy is used to study the deformation micromechanics of the microbond test for a carbon fiber epoxy resin system using surface-treated and untreated PAN-based fibers, and the results are compared with those of the conventional microbond test. Fiber strain and interfacial shear stress (ISS) are mapped along the embedded regions of fibers during the test using the Raman technique, and the maximum value of ISS, τmax, is determined. The maximum τmax value can be used to characterize the strength of the fiber matrix interface, and it is higher for specimens with surface-treated fibers. The apparent value of interfacial shear strength, τ a, determined from conventional analysis of the microbond test, is a function of the embedded fiber length. However, the value of τ a extrapolated to zero embedded length, τ i, is comparable to the maximum ISS value, τmax, determined from the Raman analysis. The influence on the results of radial compression and geometric factors, such as droplet shape and size and separation of the knife edges, is also discussed.

Fracture mechanics of single-fibre pull-out test

Pull-out of an elastic fibre from an elastic matrix was investigated. A simple pull-out mechanics has been developed, based on the fracture energy criterion, to describe the debonding process, including friction. Experiments were carried out using polytetrafluoroethylene (PTFE) fibres embedded in a polypropylene (PP) matrix. It was found that growth of an interfacial crack was stable after the initiation of a debond at the loaded fibre end. At first, the debonding force increased linearly with the crack length due to friction in the debonded region. However, the crack accelerated after reaching a critical length, dependent on the embedded fibre length. At this point, the force required to propagate the debond levelled off. Thus, further increase in the debonding force was not necessary to further complete the debonding process. The debonding force was found to be in good agreement with that predicted by the present theory. Techniques for determining the interfacial properties, including adhesive fracture energy, compressive residual stress and coefficient of friction, were considered. In addition, a simple criterion has been derived to predict which fibre end, either embedded end or loaded end, will debond first when the specimen is subjected to an axial load.

Is there any contradiction between the stress and energy failure criteria in micromechanical tests? Part I. Crack initiation: stress-controlled or energy-controlled?

The relationship between energy-based (interfacial toughness, Gic) and stress-based (ultimate shear strength, τult) criteria of interfacial failure in micromechanical tests is investigated within the frames of the Nairn shear-lag approach. To determine which criterion is responsible for interfacial failure in particular fiber-polymer systems, the data obtained in microbond tests for the joints of carbon, glass and aramid fibers with various thermoplastic and thermosetting matrices are analyzed. Experimental relations between the debond force and the embedded fiber length appear to be satisfactorily described by curves plotted according to both theories. It is demonstrated that, in a similar way as a deformed spring can be equally characterized by the reaction force and potential energy, both ultimate shear strength and energy release rate (as well as any combination of those) can be measures of interfacial strength. However, only such function f = f(Gic, τult) can be a real characteristic of the bimaterial interface (rather than of the particular specimen only), which does not depend on the specimen geometry. Theoretical consideration of micromechanical tests has shown that of all geometrical dimensions of a specimen, the fiber diameter, d, has the most prominent effect on the calculated value of the failure criterion. The analysis of data available in literature on microbond experiments with glass fibers having different diameters has revealed that for the glass fiber/epoxy matrix τult is proportional to d-½ and Gic is approximately constant; consequently, interfacial failure can be considered as energy-controlled.

Numerical study of the single fibre push-out test

The boundary element method (BEM) is used to analyze the stress transfer behaviour in the single fibre push-out test. The result shows that the stress distributions are well compared to those obtained from the shear lag analysis for the majority of the embedded fibre length. The BEM analysis clearly depicts the singular behaviour of interface stresses, particularly the radial and shear stress components, at the loaded and supported fibre ends. The shear lag analysis is unable to describe the stress singularity at these free edges. The implications of the biaxial interface stress components are discussed with respect to the interface debond criteria.

Transcrystallization of PTFE fiber/PP composites. II. Effect of transcrystallinity on the interfacial strength

It has been found that transcrystallinity of polypropylene (PP) develops easily on the polytetrafluoroethylene (PTFE) fiber surface in spite of the low surface energy of the fiber. Effect of the transcrystallinity on the interfacial strength has been extensively investigated using a single-fiber pull-out test. By controlling the crystallization temperature, range 25-130°C, the thickness of the transcrystalline layer varied from 0 to 175 μm for thick specimens, ca. 1 mm thick. Measurements of the adhesive fracture energy, the interfacial shear strength and the frictional stress were carried out for specimens with different embedded fiber lengths. Results show that interfacial strength and fracture energy are independent of the transcrystalline thickness. The calculated value of interfacial shear strength is 3.6 MPa, and the fracture energy for debonding is 2.1 J/m 2 . The presence of transcrystallinity does not promote the level of adhesion in PTFE/PP composites. However, the frictional stresses at the debonded fiber/matrix interface increase with transcrystalline thickness. It is attributed to the residual stresses which arise from shrinkage when specimens are cooled from crystallization temperature to room temperature.

Chemical stability, microstructure and mechanical behavior of LaPO 4-containing ceramics

The use of LaPO 4 as a weak interface in composites for high temperature applications was investigated using tape-cast laminates and fiber model systems. Three laminates were fabricated with LaPO 4 as one component and Al 2O 3, Y 3Al 5O 12 or LaAl 11O 18 as the other. The chemical compatibility between the different components of the laminates, as well as the mechanical responses to flexural deformation and the propagation of indentation cracks, were examined. Two fiber model systems (Al 2O 3 fiber/LaPO 4 coating/Al 2O 3 matrix and Y 3Al 5O 12 fiber/LaPO 4 coating/Al 2O 3 matrix) were studied by fiber pushout tests to measure the interfacial shear strengths. The interfacial shear strengths were calculated by the linear and shear-lag approaches for different embedded fiber lengths. The results suggest that Y 3Al 5O 12 fiber-reinforced composites with LaPO 4 coatings have potential as high temperature materials in oxidizing environments.

A MODEL TO PREDICT THE FATIGUE LIFE OF FIBRE‐REINFORCED TITANIUM MATRIX COMPOSITES UNDER CONSTANT AMPLITUDE LOADING

Abstract—A model based on micro‐mechanical concepts has been developed for predicting fatigue crack growth in titanium alloy matrix composites. In terms of the model, the crack system is composed of three zones: the crack, the plastic zone and the fibre. Crack tip plasticity is constrained by the fibres and remains so until certain conditions are met. The condition for crack propagation is that fibre constraint is overcome when the stress at the location of the fibre ahead of the crack tip attains a critical level required for debonding. Crack tip plasticity then increases and the crack is able to propagate round the fibre. The debonding stress is calculated using the shear lag model from values of interfacial shear strength and embedded fibre length published in the literature.If the fibres in the crack wake remain unbroken, friction stresses on the crack flanks are generated, as a result of the matrix sliding along the fibres. The friction stresses (known as the bridging effect) shield the crack tip from the remote stress, reducing the crack growth relative to that of the matrix alone. The bridging stress is calculated by adding together the friction stresses, at each fibre row bridging the crack, which are assumed to be a function of crack opening displacement and sliding distance at each row. The friction stresses at each fibre row will increase as the crack propagates further until a critical level for fibre failure is reached. Fibre failure is modelled through Weibull statistics and published experimental results. Fibre failure will reduce the bridging effect and increase the crack propagation rate.Calculated fatigue lives and crack propagation rates are compared with experimental results for three different materials (32% SCS6/Ti‐15‐3, 32% and 38% SCS6/Ti‐6‐4) subjected to mode I fatigue loading. The good agreement shown by these comparisons demonstrates the applicability of the model to predict the fatigue damage in Ti‐based MMCs.

Effect‐of Processing Varcables on Interfacial Properties of an SiG‐Fiber‐Reinforced Reaction‐Bonded Si3N4 Matrix Composite

Fiber/matrix interfacial debonding and frictional sliding stresses were evaluated by single‐fiber pushout tests on unidirectional continuous silicon‐carbide‐fiber‐reinforced, reaction‐bonded silicon nitride matrix composites. The debonding and maximum pushout loads required to overcome interfacial friction were obtained from load–displacement plots of pushout tests. Interfacial debonding and frictional sliding stresses were evaluated for composites with various fiber contents and fiber surface conditions (coated and uncoated), and after matrix densification by hot isostatic pressing (HIPing). For as‐fabricated composites, both debonding and frictional sliding stresses decreased with increasing fiber content. The HIPed composites, however, exhibited higher interfacial debonding and frictional sliding stresses than those of the as‐fabricated composites. These results were related to variations in axial and transverse residual stresses on fibers in the composites. A shear‐lag model developed for a partially debonded composite, including full residual stress field, was employed to analyze the nonlinear dependence of maximum pushout load on embedded fiber length for as‐fabricated and HIPed composites. Interfacial friction coefficients of 0.1–0.16 fitted the experimental data well. The extremely high debonding stress observed in uncoated fibers is believed to be due to strong chemical bonding between fiber and matrix.

Analysis of Fiber Frictional Sliding in Fiber Bundle Pushout Test

A simple theoretical model is developed to analyze the fiber frictional sliding resistance in a fiber bundle pushout test. The effect of the radial constraint imposed by the neighboring fibers on the stress transfer and frictional pushout stress is included in this analysis. Comparisons of theoretical results of this study and those of two existing fiber pushout models (i.e., single‐fiber pushout and three–cylinder) are also presented. For SiC–RBSN and SiC–glass composites with short embedded fiber lengths less than 1 mm, there is little difference between all these models. However, for larger embedded fiber lengths, the present model gives the highest frictional pushout stress caused by the more realistic radial constraint condition used in the analysis.

Interfacial Debonding and Sliding in Brittle Cementmatrix Composites from Steel Fiber Pullout Tests

AbstractA specially designed single fiber pullout apparatus was used to provide simultaneous results on total fiber displacement versus load in addition to monitoring the fiber displacement at the free end. In this apparatus the fiber was going through the entire specimen, which made it possible to determine the point of complete debonding. To control the embedment length a plastic tube was inserted around the fiber. The described fiber pullout test method coupled with an appropriate analysis provides a quantitative determination of interfacial properties which are relevant to toughening of brittle materials through fiber-reinforcement. The technique was used on a high strength cement-based matrix called the Densified Small Particle system (DSP), and on an ordinary strength matrix. Other parameters investigated included fiber embedment length and fiber volume fraction in the cement matrix. The results indicate that: (1) the dense DSP matrix has significantly improved interfacial properties as compared to the ordinary strength matrix; (2) the major energy of pullout in both systems is due to sliding; and (3) both the debonding energy and sliding energy increase with fiber embedment length. These results are important in the understanding of the role of steel fibers in improving the tensile properties of high performance fiber reinforced composites.

Fiber Twist Test: A New Test Technique to Measure Composite Interface Properties

ABSTRACTThis paper discusses the work accomplished, thus far, in the development of the Fiber Twist Test (FTT) as an alternative and improved technique for testing the interface properties of continuous fiber composite materials. The unique features of the fiber twist test include the absence of Poisson effects during testing. This eliminates the nonlinear variation, with changing embedded fiber lengths, of fracture modes and accompanying debond energy, and of friction along the interface. Since the fiber is not removed from its surrounding matrix during testing, the interface area remains constant. This facilitates a more accurate measurement of friction properties at the interface.