Effect of specimen size on flexural compressive strength of reinforced concrete members

It is important to consider the effect of member size when estimating the ultimate strength of a concrete flexural member, because the strength always decreases with an increase of member size except for well-reinforced members. Research conducted previously in this area include axial compressive strength size effect on cylindrical specimens and flexural compressive strength size effect on C-shaped specimens, notched cylindrical specimens, and axially loaded double cantilever beam (DCB) specimens. Since the most widely used flexural member type is reinforced concrete (RC) beams, it is logical to extend the study of flexural compressive strength size effect to flexural loaded RC beam members. Previously, several researchers have reported from their studies that flexural compressive strength size effect does not exist. However, the analyses show that the specimens used for the study had limited size variation and the neutral axis depth variations were too similar to show distinct size effect. Therefore, this study enforced distinct neutral axis depth variations for all of the tested specimens.In this study, the size effect of a RC beam was experimentally investigated. For this purpose, a series of beam specimens subjected to four-point loading was tested. RC beams with three different effective depths were tested to investigate the size effect. The shear-span to depth ratio and the thickness of the specimens were kept constant to eliminate the out-of-plane size effect.The test results are curve fitted using Levenberg–Marquardt’s Least Square Method (LSM) to obtain parameters for Modified Size Effect Law (MSEL) by Kim et al. The analysis results show that the flexural compression strength and ultimate strain decrease as the specimen size increases. Comparisons with existing research results considering the depth of neutral axis were also performed. They also show that the current strength criteria-based design practice should be reviewed to include member size effect.     

Critical fibre length and tensile strength for carbon fibre-epoxy composites

The tensile strength of epoxy resin reinforced with a random-planar orientation of short carbon fibres decreases with increasing temperature. This decrease may be estimated by the strain rate and temperature dependence of both the yield shear strength at the fibre-matrix interphase and the critical fibre length obtained by taking the distribution of fibre strength into consideration. The experimental value at room temperature is smaller than the calculated value. It is inferred that this result is attributed to the stress concentration caused by ineffective fibres produced during preparation which were shorter than the critical fibre length.     

The effect of transverse compressive stresses on tensile failure of glass fibre-epoxy

Four point bending tests were carried out on 32-ply glass fibre-epoxy specimens with 0 ° plies on the surface. Different combinations of 0 ° and ± 45 ° interior plies allowed the transverse stresses at the surface to be varied. All specimens failed by fibre tension, and the strength was relatively insensitive to transverse compressive strengths. The maximum stress failure criterion was found to provide a reasonable fit to the experimental data, and is recommended for design purposes.     

Effects of geometry and specimen size on out-of-plane tensile strength of aligned CFRP determined by direct tensile method

The out-of-plane tensile strength of CFRP laminate determined by the direct tensile method varies with specimen geometry and size. This effect was first experimentally observed using aligned CFRP. To explain the geometry and size effects from a mechanical point of view, an analytical model combining Weibull statistics, including the concept of effective volume, and a fracture criterion under multi-axial loading was constructed on the basis of stress distributions calculated using the finite element method. The predicted out-of-plane tensile strength of aligned CFRP was found to be consistent with experimental results. Thus, the present model is useful for reducing experimentally determined out-of-plane tensile strength under complex stress distributions to that under a uniaxial and uniform stress distribution.     

Transition in flexural microbuckling mechanisms in unidirectional glass fibre-reinforced thermoplastics

A transition in the mechanism of flexural failure previously observed in low matrix modulus unidirectional glass fibre composites is semi-quantitatively explained by considering the criterion for each of the failure modes. The failure strength for cooperative fibre microbuckling is controlled by the shear modulus of the composite which is linearly related to the Young's modulus of the matrix, while the failure strength for delamination splitting microbuckling is controlled by the composite shear strength which is not as strongly dependent on the Young's modulus of the matrix. Because the critical failure stresses have different dependencies on the matrix modulus, a transition from cooperative fibre microbuckling to delamination splitting microbuckling occurs as the matrix modulus increases. Due to the stress gradient in the beam, the compressive failure behaviour in bending is not the same as in uniform compression. When the failure mode is cooperative fibre microbuckling, the bending strength is higher than expected, especially in the thin beams. In bending, the delamination splitting microbuckling mode does not lead to abrupt splitting of the entire beam, but rather occurs by gradual accumulation of surface damage.     

The longitudinal tensile strength of unidirectional fibrous composites

When a fibre-plastic composite in which the fibres are brittle, continuous, and unidirectional is subjected to longitudinal tension under essentially static loading conditions, there exists a range of possible composite strengths. This paper presents a model which may be used to predict that range of possible composite strengths. An important feature of the model is that it considers both static and dynamic stress concentration effects on intact fibres which result from a fibre failure. A computer simulation technique is used to generate a set of generalized scatter limits for the average fibre stress at composite failure from the model. The generalized scatter limits may be used to predict the range of strengths for a composite material. The model results are used to predict the ranges of strength for composite materials prepared from three types of carbon fibre and these are compared with experimental results.     

The effect of hydrostatic pressure on transverse strength of glass and carbon fibre-epoxy composites

An attempt was made to measure indirectly the transverse tensile strengths of uniaxially aligned fibre pultrusions by the diametral compression of disc-shaped samples using concave loading anvils. Two types of composite were investigated, containing 60%V _f of either type AS carbon fibres (CFRP) or S glass fibres (GRP), both in an epoxy resin matrix. Testing was carried out at atmospheric and under superposed hydrostatic pressures, –H, extending to 300 M Pa. The resultant principal stresses at the disc centre were ₁ = _A +H; ₂ =H; ₃ = –3_A +H, where _A = 2P/dt for a disc of diameter,d, and thickness,t, subjected to a loadP. Deviations from linearity in the load-deflection response were detected throughout the pressure range at ã70% and ã90% of the failure load for CFRP and GRP, respectively, and these were associated with resin yielding. The pressure dependence of _A, approximately –0.1H, was consistent with a two-parameter yielding criterion predicting hypothetical yield stresses in simple tension and compression of ã81 and –109 MPa, respectively, for both matrix materials. Irrespective of pressureeventual fractures took place along the loading diameter, but in the CFRP specimens tested under pressure initial cracks at the disc centres were at ã45 ° to the loading axis, i.e. on the plane of maximum shear stress. Fractographic observations were consistent with transverse failure taking place by fibre-matrix decohesion in GRP and by resin fracture in CFRP. Other than the atmospheric datum point for CFRP, the pressure dependence of _A for failure, _F, was also approximately –0.1H. Of the various stress, strain and strain energy criteria for failure examined, only critical tensile strain was found consistent with this pressure dependence.     

Effect of interfacial adhesion and statistical fiber strength on tensile strength of unidirectional glass fiber/epoxy composites. Part. II: comparison with prediction

An analytic model by Curtin and Takeda (Curtin WA, Takeda N. Tensile strength of fiber-reinforced composites: I. Model and effects of local fiber geometry. Journal of Composite Materials 1998;32(22):2042–59; Curtin WA, Takeda N. Tensile strength of fiber-reinforced composites: II. Application to polymer matrix composites. Journal of Composite Materials 1998;32(22):2060–81) is used to predict the ultimate tensile strength (UTS) of unidirectional (UD) glass fiber/epoxy composites with different interfacial adhesion and statistical fiber strength. Data for the fiber strength σ_c at the critical fiber length δ_c for five kinds of treated fibers are used for predicting UTS, which is obtained from both single fiber composite (SFC) and single fiber tension (SFT) tests. σ_c from SFC is attained using the Curtin theory on the fragmentation of SFC, while that from SFT is determined using a linear extrapolation of SFT data. Under good interface adhesion, the predicted UTS values based on the SFC data show the best agreement with measured ones at various fiber volume fractions, but a higher predicted value is obtained if the interface failure is matrix-controlled. For poor interfacial adhesion, the predicted UTS values are rather high compared to experimental ones due to the ineffective stress transfer. The predicted values based on the SFT data are much higher than the measured value for good interfacial adhesion.     

Hygrothermal ageing and fracture of glass fibre-epoxy composites

The effects of hygrothermal ageing upon the failure characteristics and work of fracture of glass fibres in epoxy are described. A collection of data based on the direct observation and measurement of fibre debond length and fibre pull-out length after fracture is displayed in empirical failure diagrams. Similarly, a vast number of experimental measurements of work of fracture is displayed in a three-dimensional diagram where the axes are work of fracture, humidity and ageing time. This information is combined with models of fracture in the construction of a fracture map which is used to interpret hygrothermal ageing phenomena.     

Interlaminar tensile strength (ILTS) measurement of woven glass/polyester laminates using four-point curved beam specimen

Based on a review of the current methods for the measurement of interlaminar tensile strength (ILTS), a novel specimen of a four-point curved beam is evaluated for this purpose. Detailed finite element analysis is carried out to investigate the appropriateness of the data interpretation formula. A simple design method to choose the appropriate parameters in order to guarantee the delamination failure is provided. This specimen is used to measure the ILTS of woven glass/polyester laminates. The results are found to be comparable with other methods. The most attractive advantage of this test method is its simplicity and that it requires no special techniques in specimen preparation and test set-up.     

Tension and flexural strength of silicon carbide fibre-reinforced glass ceramics

The use of silicon carbide-type fibres to reinforce lithium aluminosilicate glass ceramics results in composites with exceptional levels of strength and toughness. It is demonstrated that composite strength and stress-strain behaviour depend onin situ fibre strength, matrix composition, test technique and atmosphere of test. Both linear and non-linear tensile stress-strain curves are obtained with ultimate strengths at 22° C approaching 700 MPa and failure strains of 1%. Flexure tests performed at up to 1000° C in air are compared with data obtained in argon to demonstrate a significant dependence of strength and failure mode on test atmosphere. Finally, glass ceramic matrix composite performance is compared with a silicon carbide fibre-reinforced epoxy system to demonstrate the importance of matrix failure strain on strength and stress-strain behaviour.     

Effect of interfacial adhesion and statistical fiber strength on tensile strength of unidirectional glass fiber/epoxy composites. Part I: experiment results

The effects of fiber surface treatment on ultimate tensile strength (UTS) of unidirectional (UD) epoxy resin matrix composites are examined experimentally. The interfacial shear strength (IFSS) and statistical fiber strength are significantly altered by five different kinds of surface treatments, which are: (a) unsized and untreated; (b) γ-glycidoxypropyltrimethoxysilane (γ-GPS); (c) γ-methacryloxypropyltrimethoxysilane (γ-MPS); (d) mixture of γ-aminoxypropyltrimethoxysilane (γ-APS), film former (urethane) and lubricant (paraffin); and (e) urethane-sized. The maximum UTS is obtained for the relatively strong interfacial adhesion (glass/γ-MPS/epoxy) but not for the strongest interfacial adhesion (glass/γ-GPS/epoxy). The governing micro-damage mode around a broken fiber and the interface region is matrix cracking for γ-GPS treated fibers, and a combination of interfacial debonding and matrix cracking for γ-MPS treated fibers. The micro-damage mode related to the interfacial adhesion strongly affects the fracture process, and thus the UTS of UD composites. The results also indicate that the interfacial adhesion can be optimized for effective utilization of fiber strength for fiber composites. A parameter called “efficiency ratio” of fiber strength in UD composites is proposed to examine and distinguish different effects of IFSS and fiber strength on the UTS of UD composites. The experimental results show that improved UTS of UD composites due to surface treatments mainly result from the increase in fiber strength but not from the modified interface.     

Determination of tensile strength of glass fiber straps

Glass fibers straps have been used for strengthening of masonry and concrete structures in the last decade. Recently, their use has become greater. This paper describes the research of measuring tensile strength of dry glass fiber straps as well as straps that were made of glass fibers and epoxy coating. The effect of strap widths, the effect of loading speeds and the effect of epoxy coating placed on fiber straps, on tensile strength of straps have been analyzed. Differences of tension strengths of fiber straps with and without epoxy coating are shown.     

Short time tensile behavior of unidirectional glass and carbon fiber reinforced polymer laminates with curved shapes

A newly developed confining system for rectangular columns required a wrapping, which sustained a large amount of parasitic bending due to the curved shape of the laminates at the cross-section corners. To investigate the effect of parasitic bending on unidirectional carbon (CFRP) and glass fiber (GFRP) reinforced polymer laminates, 4 ply coupons were laminated with a semi-elastic hybrid resin and cured in a curved shape. During the tensile tests, the curved coupons stretched and failed after further loading. Due to the parasitic bending, occurring during stretching, the tensile resistance was reduced by 48% for the CFRP and by only 18% for the GFRP coupons. Tests with high-strength, high-modulus epoxy resin laminated GFRP coupons were drawn upon and compared with the semi-elastic hybrid resin laminated GFRP coupons. There is a beneficial effect on the tensile resistance with the use of semi-elastic hybrid resin.     

Strain-rate dependence of the tensile strength of glass fibers

It is well known that the strength of glass fibers increases with increasing strain rate. Consequently, impact strength of glass fiber is competitive with that of carbon fiber. This strengthening phenomenon is well recognized for bulk glass. Strain-rate dependence of the strength for bulk glass was described by considering slow crack growth in glass. The analytical model that considered the slow crack growth of glass is proposed to predict the strength of glass fibers. The proposed model considered the stress corrosion limit and a constant crack velocity region. Calculations showed almost same results with the previous model, however, some differences were confirmed. To discuss the validity of the analysis, tensile tests of E-glass fiber bundles were conducted at various strain rates. It was observed that the fracture behaviors differ with the strain rates. Experimental results showed that the strength of E-glass fibers increased with increasing strain rate. Furthermore, we confirmed that the analytical results were in good agreement with the experimental results. The strain-rate dependence of the strength of glass fibers was successfully predicted by considering the slow crack growth in glass.     

A general relation between tensile strength and specimen geometry for concrete-like materials

The tensile strength of concrete-like materials varies wiries when different test procedures or even different shapes and sizes of specimens are employed. Correlations have previously been established but restricted to particular test conditions. The approach here presented, based on the “weakest link of the chain” concepts, offers the possibility of correlating the results of the different tensile tests of concrete-like materials. For that purpose, the amount of material under a tensile stress greater than 0.95 of the maximum tensile stress in the specimen must be evaluated. Relating the maximum tensile stress at the moment of failure with the above mentioned volume (Highly Stressed Volume), a decreasing function is obtained. The function fitted can be used to obtain a tensile strength value of the material, free from the influence of the characteristics of tests and specimens. In this experimental work, the function is established by testing seven different mortar mixes, subjected to nine different test conditions. Centered-point and third-point loading flexural tests and splitting tests were carried out on specimens of different sizes. Direct tension on a briquertte was also applied. The test results showed a decreasing linear regression between the logarithm of the maximum tensile stress at the moment of failure and the logarithm of the highly stressed volume. The slope of that line remained fairly constant for the seven mortar mixes tested.     

Fracture toughness and mechanical properties of glass fibre-epoxy composites

The present study investigated fracture properties and various mechanical properties of a set of unidirectional glass fibre-epoxy resin composites. This set was comprised of samples with volume fraction of fibres in the range 0.29 to 0.75. An identical set of composites was water-boil treated for 7 days, and the effect of this treatment on the above properties was examined. The work of fracture (γ _F) and the fracture surface energy of initiation (γ ₁) results were compared with existing theoretical models for the prediction of fracture toughness. It was discovered that theγ _F results agreed with the pull-out model [6], suggesting that this was the major contribution to the fracture energy of the complete process. Theγ ₁ values corresponded generally with the surfaces formation model [9], proposing that the creation of new fibre, matrix and fibre-matrix surfaces controls the stage of fracture initiation.     

Influences of interfacial bonding strength and scatter of fibre strength on tensile behaviour of unidirectional metal matrix composites

The influences of interfacial bonding strength and scatter of strength of fibres on tensile behaviour of unidirectional metal matrix composites, whose matrix has low yield stress in comparison to the strength of fibres, were studied using the Monte-Carlo simulation technique using two-dimensional model composites. The following results were found. The strength of composites increases with increasing bonding strength, especially when the bonding strength exceeds the shear yield stress of the matrix and then remains nearly constant. The strength of composites is very sensitive to bonding strength when the scatter of fibre strength is large, but not when it is small. The fracture mode varies from non-cumulative to cumulative with increasing scatter of fibre strength for both cases of weak and strong interfacial bondings. The fracture surface becomes irregular when bonding strength becomes low and scatter of fibre strength becomes large. The applicability of the Rosen and Zweben models and the rule of mixtures to predict the strength of composites was examined.