Uni-axial tension test for steel fibre reinforced concrete

RILEM Technical CommitteesRILEM TC 162-TDF: Test and Design Methods for Steel Fibre Reinforced Concrete

Uni-axial tension test for steel fibre reinforced concrete Recommendations     

Fibre fragment distribution in a single-fibre composite tension test

Single fibre fragmentation tests are performed for brittle fibres with Weibull strength distribution and different surface treatments. The fragmentation process is modelled and closed-form expressions for break spacing distribution are obtained. The model accounts for the effect of finite fibre length on the initial fragmentation as well as for break interaction on the advanced fragmentation stage. It is assumed that the exclusion zone due to fibre–matrix interface failure and stress recovery in the fibre is linearly dependent on the applied load. This assumption is validated experimentally. The derived theoretical average fragment length dependence on applied load is used to determine the fibre strength distribution parameters and the effective interfacial shear stress for carbon/epoxy single fibre composites with different fibre surface treatment and for glass/vinylester single fibre composite. Fragment length distribution is predicted for several load levels. Predictions are in good agreement with experimental data.     

Carbon fibre reinforced glass matrix composite tension and flexure properties

Carbon fibre reinforced borosilicate glass matrix composites have been fabricated to determine their mechanical properties in tension and flexure. Composite tensile stress-strain properties, including elastic modulus, proportional limit and ultimate tensile strength, have been measured as a function of fibre content. Composite tensile properties were also obtained at temperatures of up to 625° C through the testing of 0/90 cross-plied specimens. Composite short-beam shear strength was found to depend on specimen orientation and also on the composition of the glass matrix. This compositional dependence was associated with an independent measurement of the fibre-matrix interfacial shear strength and was related to the degree of fibre-matrix reaction taking place during composite fabrication.     

Statistical prediction of fatigue failure of fibre reinforced composite materials

A statistical approach is proposed to evaluate the residual strength and life of unidirectional and angle-ply composite laminates subjected to in-plane tensile cyclic stresses. The method is based on the extension of previous static failure criteria describing independently the fibre failure and matrix failure modes, combined with the statistical nature of fatigue failure of fibre-reinforced composites. The static and fatigue strengths of composite laminates at any off-axis angle are evaluated using the fatigue failure functions for the three principal failure modes, which are determined from the fatigue behaviour of unidirectional composites subjected to longitudinal and transverse tension as well as in-plane shear stresses. The evaluations of the fatigue strength of unidirectional E-glass/epoxy laminates under off-axis fatigue loading and angle-ply S-glass/epoxy laminates under in-plane fatigue loading show good agreement between theoretical predictions and experimental results.     

Compressive failure of finite size unidirectional composite laminates with a region of fibre waviness

The compressive behaviour of finite unidirectional composites with a region of misaligned reinforcement is investigated via finite element analyses. Models with and without fibre bending stiffness are compared, confirming that compressive strength is accurately predicted without modelling fibre bending stiffness for real composite components which typically have waviness defects of several millimetres wavelength. Various defect parameters are investigated. Results confirm the well-known sensitivity of compressive strength to misalignment angle, and also show that compressive strength falls rapidly with the proportion of laminate width covered by the wavy region. A simple empirical equation is proposed to model the effect of a single patch of waviness in finite specimens. Other parameters such as length and position of the wavy region are found to have a smaller effect on compressive strength. The modelling approach is finally adapted to model distributed waviness and thus determine the compressive strength of composites with realistic waviness defects.     

The micromechanisms of fracture of alumina and a ceramic-based fibre composite: Modelling the failure processes

The development of structural ceramics and ceramic composites often relies on assumptions about their cracking and fracture characteristics under load. It is most important to understand the nature of the cracking processes and the interaction between neighbouring cracks. In other words, a clear picture of the dynamic micromechanisms of cracking and the accumulation of damage is essential to the development of sound physical models to explain measurements of the strength and toughness of these materials.     

Experimental investigation of bridging law for single stitch fibre using Interlaminar tension test

In this study, a novel Interlaminar tension test (ITT) method was performed to experimentally investigate the bridging and fracture process of a single stitch fibre used to improve the delamination strength of composite laminates. Kevlar-29, of various thread thicknesses (44, 66, 88 and 132 tex), was used as the through-thickness stitch fibre in the ITT experiments. Key empirical force and displacement parameters, which governed the stitch fibre bridging law, were characterised and identified. Relationships of such parameters with thread thicknesses were determined. Fibre fracture load and fibre fracture energy are found to increase with increasing thread thickness. Frictional pull-out force greatly depends on the type of stitch fracture modes, which can be grouped into three categories. This paper aims to provide better physical understanding of the mechanics and mechanisms of stitch fibre fracture. By correlating critical stitch fracture parameters with stitch fibre thicknesses, the results expect to provide useful reference, which is essential and important for accurate stitch computational modeling and strength prediction of composites using stitching as the interlaminar reinforcement technique.     

Effect of interfacial debonding on the failure behavior in a fiber-reinforced composite subjected to transverse tension

By using computational micromechanics, macroscopic stress–strain curves for a glass/epoxy lamina subjected to transverse tension were determined in this paper. To compute stress for given strain, a finite element model of a three-phased unit cell with the hexagonal symmetry was employed. Mixed mode debonding conditions between reinforcement and matrix were modeled by a bilinear cohesive law. A stress transfer between matrix and fiber was simulated by a inhomogeneous interphase. A detailed analysis of a debonding growth was also presented. Parametrical studies showed that a choice of values for cohesive parameters as well as debonding locations do have an influence on the macroscopic response of material. An ability of the proposed model to simulate the softening behavior of the material under transverse tension and to predict final failure was demonstrated.     

The failure of fibre composites and adhesively bonded fibre composites under high rates of test

The failure of fibre composites and adhesively bonded fibre composites under high rates of test, up to rates of about 15 m s^–1 were studied in detail. The present paper. Part I of the series, considers the experimental aspects of the mode I fracture of the fibre composite materials and joints. Part II will analyse the dynamic effects which are invariably associated with high-rate tests, and will show how these effects influence the observed behaviour of the test specimens. Part III will report the results from mode II and mixed-mode I/II tests on the fibre composite materials.     

The failure of fibre composites and adhesively bonded fibre composites under high rates of test

The dynamic effects which are commonly encountered during high-rate DCB tests with fibre composite and adhesively bonded fibre composite arms have been studied in detail. This paper, Part II of the series, follows Part I, which described the experimental aspects of the high-rate testing. Part III will report the results from mode II and mixed-mode I/II tests on the fibre-composite materials.Nomenclature a crack length - a ₀ initial crack length - a crack speed - ä crack acceleration - c longitudinal wave speed - h thickness of single arm of test specimen - p crack length perturbation (i.e. the measured value of the crack length minus the value predic ted by steady-state theory) - p crack velocity perturbation - crack acceleration perturbation - t time - t ₀ time taken for crack to initiate during the mode I test - u ₀ load-line vertical displacement of single arm of test specimen (/2 in Part I) - u(x) vertical displacement of specimen at distance x from the load-line - u(x) vertical displacement rate of specimen at distance x from the load-line - x distance along the test specimen from the load-line - A constant relating the steady state crack length to root time - B width of specimen - C compliance of the specimen (u ₀/P) - E ₁₁ axial modulus of the fibre-composite beam - G mode I energy release rate - G _Ic mode I critical energy release rate or fracture toughness - G ₁ half the value of G _Ic during steady-state propagation (i.e. calculated for half the beam as shown in Fig. 1) - G ₂ half the value of G _Ic at crack initiation - P end load applied to specimen - U _ext external work done - U _s strain energy - U _k kinetic energy - V velocity of a single arm of test specimen (i.e. half the measured test velocity) - dynamic term, governed by the ratio of the energy to initiate versus that to propagate a crack - _I mode I crack shear deflection and root rotation correction term - crack length correction term, evaluated by the negative intercept on the a versus t ^1/2 plot - dynamic term controlling the form of the computed perturbations - Poisson's ratio for the fibre-composite beams density of the fibre-composite beams - time, normalized by the initiation time, t ₀ and thus equivalent to (t/t ₀) - values of at which crack arrest occurs. n = 1,2,3... - ratio of distance along beam to crack length (x/a)     

Discrete failure mode detection in a woven SiC cloth reinforced glass composite

Single layer silicon carbide cloth reinforced glass composites were fabricated and subjected to three-point bending in order to develop better models of failure mechanisms. Acoustic emission (AE) monitoring was also performed during the bend tests to help isolate these mechanisms. During the basic flexural tests, discrete failure modes, which were often not visible from specimen surfaces, displayed their existence through characteristic load-deflection curve unloading regions and abrupt changes in acoustic activity. Microscopic three-point bend tests were then performed to elaborate on the results of the conventional bend tests. Observations made during the microscopic bend tests provided a one-to-one correlation with load-deflection curve anomalies and acoustic emission activity. As a result of the different mechanical, optical and acoustical techniques used, discrete failure mechanisms for the cloth reinforced ceramic matrix composite (CMC) were conclusively established.     

A new theory to obtain Weibull fibre strength parameters from a single-fibre composite test

A theory is developed to obtain the Weibull scale and shape parameters for in situ fibre strength utilizing the data obtained from a single-fibre-composite (SFC) test. It is well known that during the SFC test, the fibre fractures several times along its length at successive weak points that are randomly located. The SFC technique, although most commonly used for measuring the fibre/matrix interfacial shear strength, is an excellent way to determine the fibre flaw spacings and the in situ fibre failure stress at every fracture location. In the present technique, the SFC specimen is partitioned into a relatively large number of small sections of equal length such that each section will have either one or no fibre fracture point. Because all the sections may not include the fibre fracture points because of their random nature, the test data are regarded as “censored data”. In other words, the theory is constructed for the estimation of Weibull parameters that takes into account the censored nature of the data. The Weibull parameters predicted using the present theory are in the same range as those obtained from the single fibre tension tests. For several reasons the values obtained from the SFC test tend to be slightly higher than those obtained from the simple tension tests. However, the in situ fibre strength values obtained using SFC technique may be more realistic and thus may be more useful in modelling composite strength.     

On fracture path stability in the compact tension test

A simple criterion based on the engineers' theory of bending is suggested for the determination of the stability of fracture path in the compact tension test.

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Zusammenfassung Zur Bestimmung der Stabilität des Fortpflanzungspfades einer Rißes beim Zugversuch an kompakten Proben, wird ein einfacher, von der Ingenieurtheorie fur Biegung abgeleiteter Kennwerk vorgeschlagen.

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Résumé Un critére simple, basé sur l'équation d'équarissagé, est propose pour la détermination de la stabilité du parcours que suit une rupture au cours d'un essai de traction sur éprouvette compacte.

     

Experimental evaluation of residual stresses in single fibre composites by means of the fragmentation test

The residual stresses in both thermosetting and thermoplastic single-fibre composites have been experimentally evaluated by means of an original technique based on the continuous monitoring of the fragmentation test performed at various temperatures. The difference between the strain at the break of a single fibre in air and one embedded in a polymeric matrix has been measured as a function of temperature. By considering the compressive fibre modulus this strain difference has been converted into fibre compressive stresses related to the matrix thermal shrinkage after curing of the samples. In fact, as the test temperature increased, the thermal compressive stresses decreased until a zero value was obtained, corresponding to a so called stress free temperature, equal to the curing temperature for amorphous thermosetting matrix composites or equal to the matrix melting temperature for semicrystalline-thermoplastic matrix composites. The experimental results have been compared with data obtained from a theoretical model and a good agreement was found especially if the temperature dependence of the matrix Young's modulus and matrix thermal expansion coefficient are accounted for in the computation.     

A micrographic study of bending failure in five thermoplastic-carbon fibre composite laminates

The local deformation and failure sequences of five thermoplastic matrix composites were microscopically observed while bending the samples in a small fixture attached to a microscope stage. The thermoplastics are polycarbonate, polysulphone, polyphenylenesulphide, polyethersulphone and polyetheretherketone. The composites made from these plastics contain a variety of carbon fibres, though all with similar properties, and have fibre volume fractions ranging from 32 to 66%. Comparison is made to an epoxy matrix composite, 5208-T-300. Laminates tested are (0/90)_2S, with outer ply fibres parallel to the beam axis. Four-point bending is used at a typical span-to-thickness ratio of 39:1. A shallow notch is put in the samples at mid-span to avoid failure under the loading pins. It was found that all the thermoplastic composites failed by abrupt longitudinal compression buckling of the outer ply. Very little precursory damage was observed. Micrographs reveal typical fibre kinking associated with longitudinal compression failure. Curved fracture surfaces on the fibres suggest they failed in bending rather than direct compression. Delamination was suppressed in the thermoplastic composites, and the delamination that did occur was found to be the result of compression buckling, rather than vice versa. Microbuckling also caused other subsequent damage such as ply splitting, transverse ply shear failure, fibre tensile failure, and transverse ply cracking.