Characterising the time-dependant behaviour on the single fibre level of SHCC: Part 2: The rate effects on fibre pull-out tests

This paper is the second part of a two paper series about the time-dependant behaviour of Strain Hardening Cement-based Composite (SHCC) on the single fibre level. Having dealt with mechanisms of creep in SHCC in the first part, this paper reports single fibre pull-out tests that were done to investigate the effect of the pull-out rate on the mechanical response of the interface between the fibre and the matrix. It was found that not only the pull-out resistance increased with an increase of the pull-out rate but the probability of fibre rupture during pull-out as well. Another important finding was that the interfacial shear resistance and slip-hardening coefficient are not only dependant on the pull-out rate, but also the embedment length.     

Improved mechanical performance: Shear behaviour of strain-hardening cement-based composites (SHCC)

The retardation of moisture and gas ingress associated with important degradation mechanisms in cement-based composites in general and reinforced concrete or prestressed concrete in particular is an ongoing research focus internationally. A dense outer layer is generally accepted to significantly enhance durability of structural concrete. However, cracking leads to enhanced ingress, unless the cracks are restricted to small widths. Strain-hardening cement-based composites (SHCC) make use of fibres to bridge cracks, whereby they are controlled to small widths over a large tensile deformation range. In this paper, SHCC shear behaviour is studied, verifying that the cracks which arise in pure shear are also controlled to small widths in these materials. The design of an Iosipescu shear test setup and specific SHCC geometry is reported, as well as the results of a test series. A computational model for SHCC, based on finite element theory and continuum damage mechanics, is elaborated and shown to capture the shear behaviour of SHCC.     

Positive synergy between steel-fibres and rubber aggregates: Effect on the resistance of cement-based mortars to shrinkage cracking

Cement-based materials suffer from their low tensile strength and their poor straining capacity: they are sensitive to cracking, particularly shrinkage cracking. Enhancing the cracking resistance of cementitious materials is the challenge of a broad ongoing research programme. In this regard, the aim of the present work was the design of a cement composite exhibiting a high straining capacity before macrocracking localisation. It was assumed that incorporation of aggregates with low elastic modulus could be a solution. Actually rubber aggregates obtained from shredded non-reusable tyres were used, conferring an environmental interest on the study.After a previous contribution focusing on the basic mechanical properties of rubberised mortar, the purpose of this paper is to present the influence of rubber aggregates on the load-deflection relationship of mortar in flexure. The synergy between rubber aggregate substitution and metal-fibre reinforcement was also investigated. Despite the low strength and high shrinkage length change of rubberised mortars, ring-tests showed that the composite materials exhibited an enhanced resistance to shrinkage cracking. In this regard, a positive synergy effect between rubber aggregates and steel-fibres was evidenced: shrinkage cracking was delayed and when it occurred, multiple cracking with thinner crack openings was observed.     

Preliminary investigations of the dimensional stability of super-critically carbonated glass fibre reinforced cement

The dimensional changes occurring during the super-critical carbonation of glass fibre reinforced cement (GRC) and its subsequent environmental exposure have been investigated. Sheet samples, with embedded stainless steel pins 200 mm apart, were fabricated and then subjected to super-critical carbonation. Test coupons were subsequently exposed to a range of environments including continuous immersion in water, cyclic wetting/drying and outdoor exposure. The super-critical carbonation process resulted in a slight expansion of GRC. This is contrary to the behaviour observed during natural carbonation where irreversible shrinkage normally occurs. Exposure to the various environments mentioned above showed that super-critically carbonated samples had much greater resistance to swelling and shrinkage than uncarbonated specimens. These observations are particularly significant in relation to the practical application of GRC in environments of fluctuating moisture content.     

Mechanical and durable behaviour of alkaline cement mortars reinforced with polypropylene fibres

The development of new binders, alternative to traditional cements and concretes obtained by the alkaline activation of different industrial by-products (blast furnace slags and/or fly ashes), is an ongoing study and research topic of the scientific community.

The mechanical and durable behaviour of alkaline cement mortars reinforced with polypropylene fibres has been the object of the present investigation. Three different alkaline matrices were used: (a) granulated blast furnace slag activated with waterglass (Na₂SiO₃+NaOH) with a concentration of 4% Na₂O by mass of slag and cured at room temperature, (b) aluminosilicate fly ash activated with 8M NaOH and cured at 85 °C during the first 24 h and (c) 50% fly ash+50% slag activated with 8M NaOH solution at room temperature. In the mechanical tests (flexural and compressive strengths), two different dosages of fibres were used: 0.5% and 1% by mortar volume. Shrinkage tests according to ASTM C 806-87 standard with (1%) and without fibres were also carried out. The durability tests carried out were freeze/thaw and wet/dry cycles. In these tests, the dosage of fibre was 0.5% by mortar volume. The results obtained show that the nature of the matrix is the most important factor to strength development, more than fibre presence and content amount.     

A comparative investigation of interfacial adhesion behaviour of polyamide based self-reinforced polymer composites by single fibre and multiple fibre pull-out tests

Abstract

Interfacial adhesion of composite materials plays an important role in their mechanical properties and performance. In the present investigation, analysis of the interfacial properties of self-reinforced polyamide composites by using microbond multiple fibre pull-out test is emphasised. Microbond specimens prepared through thermal processing are tested for their interfacial properties by multiple fibre bundle pull-out tests and compared with that of traditional single fibre pull-out test specimens. Multiple fibre pull-out addresses the volume fraction as well as eliminates the possibility of fibre breakage before matrix shear. Higher scatter in the data in the samples is addressed in the present studies. FTIR and Fractographic studies are carried out for deep understanding of the post pull-out interfacial adhesion.     

The effect of molecular orientation on the physical ageing of amorphous polymers-dilatometric and mechanical creep behaviour

The objective of this study was to determine whether molecular orientation has an effect on the rate of physical ageing in amorphous glassy polymers. To achieve this, samples of atactic polystyrene and bisphenol-A polycarbonate were uniaxially hot-drawn to various stretch ratios and then quenched into the glassy state to freeze in orientation. Physical ageing rates were then measured as a function of orientation with dilatometry and tensile creep measurements. The volume relaxation rate, β, was approximately 50% higher for the stretched samples and did not vary with orientation over the range of stretch ratios tested. This was true despite the fact that the initial free volume was actually decreasing with increasing elongation. In contrast, creep measurements showed a slight decrease in the horizontal shift rate, μ_h, upon stretching (i.e. a decreased ageing rate). Possible explanations for these unusual trends in terms of a stretch-induced activated state are discussed.     

The single fiber pull-out test analysis: influence of a compliant coating on the stresses at bonded interfaces

The mechanical properties at the fiber/matrix interface play a significant role in controlling the fracture resistance of fiber-reinforced composites. By coating the fiber with sizing and coupling agents, these interfacial properties can be modified. The aim of the present analysis was to examine the effects of the coating thickness and modulus on the stresses at the bonded interfaces between the fiber, coating, and matrix. Using the fiber pull-out test as the analytical model, the stresses are first obtained by minimizing the total complementary energy in the coated fiber/matrix composite. The analytical results show that the interfacial shear stress between the fiber and the coating is higher than that between the matrix and the coating. Also, a thin and compliant coating reduces substantially the peak interfacial shear stress but not the interfacial radial stress due to Poisson's effect on the fiber. Furthermore, the shear stress transfer from the fiber to the matrix across the coating layer is found to be more uniform. The implications of these findings are discussed.     

On the characterization of tensile creep resistance of polyamide 66 nanocomposites. Part II: Modeling and prediction of long-term performance

The Part I of this study [Yang JL, Zhang Z, Schlarb AK, Friedrich K. Polymer 2006;47:2791-801] provided systematic experiments and general discussions on the creep resistance of polyamide 66 nanocomposites. To promote these works, here we present some modeling and prediction attempts in order to further understand the phenomena and mechanisms. Both a viscoelastic creep model named Burgers (or four-element model) and an empirical method called Findley power law are applied. The simulating results from both models agree quite well with the experimental data. An additional effort is conducted to understand the structure-property relationship based on the parameter analysis of the Burgers model, since the variations in the simulating parameters illustrate the influence of nanofillers on the creep performance of the bulk matrix. Moreover, the Eyring stress-activated process is taken into account by considering the activation volume. Furthermore, in order to predict the long-term behavior based on the short-term experimental data, both the Burgers and Findley models as well as the time-temperature superposition principle (TTSP) were employed. The predicting capability of these modeling approaches is compared and the Findley power law is preferred to be adopted. Master curves with extended time scale are constructed by applying TTSP to horizontally shift the short-time experimental data. The predicting results confirm the enhanced creep resistance of nanofillers even at extended long time scale.     

A new method for study of the fiber-matrix interface in composites: single fiber pull-out from a microcomposite

—A new method, single fiber pull-out from a microcomposite (SFPOM), was developed to study the fiber/matrix interface in composites. By pulling a fiber out of a seven-fiber microcomposite, the SFPOM test provides the real feeling of a fiber pulled out of an environment similar to that in a real composite. Interfacial shear strength decreased as the fiber volume fraction increased in the fiber-matrix system tested in the experiment. Three factors were suggested to be responsible for the phenomenon: (1) poor bonding between fibers when close to each other; (2) shear stress concentration in the matrix between neighboring fibers; and (3) possible change in matrix properties, thus altering the failure mechanism from interfacial debonding to a mixture of interfacial debonding and matrix fracture.     

Derivation of a crack opening deflection relationship for fibre reinforced concrete panels using a stochastic model: Application for predicting the flexural behaviour of round panels using stress crack opening diagrams

This study is aimed at proposing a simple analytical model to investigate the post-cracking behaviour of FRC panels, using an arbitrary tension softening, stress crack opening diagram, as the input. A new relationship that links the crack opening to the panel deflection is proposed. Due to the stochastic nature of material properties, the random fibre distribution, and other uncertainties that are involved in concrete mix, this relationship is developed from the analysis of beams having the same thickness using the Monte Carlo simulation (MCS) technique. The softening diagrams obtained from direct tensile tests are used as the input for the calculation, in a deterministic way, of the mean load displacement response of round panels. A good agreement is found between the model predictions and the experimental results.     

Effect of powder purification by heat-treatment on the creep behaviour of dense boron carbide monoliths

High temperature compressive creep tests have been performed at 1650−1750 °C under applied stresses of 50−150 MPa on sintered boron carbide samples exhibiting high relative density and a mean grain size of 0.5 μm. The creep behaviour of two types of materials, sintered by spark plasma sintering from both raw and heat-treated powders, are characterized. For both materials, the identification of creep parameters (i.e. apparent activation energy and stress exponent values) coupled with TEM structural observations suggest a power law creep regime controlled by dislocation glide, which is limited by the presence of twins. However, the TP material exhibits lower stationary strain rates. This improved creep resistance seems to be directly correlated to the stoichiometry modification of the carbide induced by the powder pre-heat treatment, i.e. increase of structural carbon content and slight decrease of oxygen amount.     

An integrated approach for modelling the tensile behaviour of steel fibre reinforced self-compacting concrete

The present work resumes the experimental and numerical research carried out for the development of a numerical tool able of simulating the tensile behaviour of steel fibre reinforced self-compacting concrete (SFRSCC). SFRSCC is assumed as a two phase material, where the nonlinear material behaviour of SCC matrix is modelled by a 3D smeared crack model, and steel fibres are assumed as embedded short cables distributed within the SCC matrix according to a Monte Carlo method. The internal forces in the steel fibres are obtained from the stress–slip laws derived from the executed fibre pullout tests. The performance of this numerical strategy was appraised by simulating the tensile tests carried out. The numerical simulations showed a good agreement with the experimental results.     

Analysis of a pull-out test with real specimen geometry. Part II: the effect of meniscus

This paper continues our study on the platelet model of the pull-out specimen, in which the matrix droplet shape is approximated by a set of thin parallel disks with the diameters varying along the embedded fiber. Using this model, the fiber tensile stress and the interfacial shear stress profiles were calculated for real-shaped matrix droplets, including menisci (wetting cones) on the fibers, taking into account residual thermal stresses and interfacial friction. Then, these profiles were used to numerically simulate the processes of crack initiation and propagation in the pull-out test and to obtain theoretical force-displacement curves for specimens with different embedded lengths and wetting cone angles. Our simulations showed that the interfacial crack in real-shaped droplets initiated at very small (practically zero) force applied to the fiber, in contrast to the popular ‘equivalent cylinder’ approximation. As a result, the equivalent cylinder approach underestimated the interfacial shear strength (IFSS) value determined from the pull-out test and at the same time overestimated the interfacial frictional stress; the smaller was the wetting cone angle, the greater the difference. We also investigated the effects of the embedded fiber length and interfacial frictional stress in debonded areas on the calculated IFSS. The simulated force–displacement curves for the real-shaped droplets showed better agreement with experimental curves than those plotted using the equivalent cylinder approach.     

Effect of short fibers on residual permeability and mechanical properties of hybrid fibre reinforced high strength concrete after heat exposition

Mechanical and permeability performance of fibre reinforced high strength concrete after heat exposition were evaluated in the experimental study. Cylindrical concrete specimens were exposed to heat with the rate of 10 °C/min of up to 400 °C. In order to study the effect of short fibres on residual performance of heated high strength concrete, polypropylene and steel fibres had been added into the concrete mix. The melting and vaporization of its fibre constituents were found to be responsible for the significant reduction in residual properties of polypropylene fibre reinforced high strength concrete. In terms of non-destructive measurement, UPV test was proposed as a promising initial inspection method for fire damaged concrete structure. Furthermore, the effect of hybrid fibre on the residual properties of heated fibre reinforced high strength concrete was also presented.     

Analysis of a pull-out test with real specimen geometry. Part I: matrix droplet in the shape of a spherical segment

We compared two models of the pull-out specimen – the ‘equivalent cylinder’ and the platelet models in which the matrix droplet is represented as a set of thin parallel disks with the diameters varying along the embedded fiber to approximate the real droplet shape. Analytical expressions for the profiles of the fiber tensile stress and the interfacial shear stress have been derived for the matrix droplet in the shape of a spherical segment, including the effects of residual thermal stresses and interfacial friction. Using these expressions, we analyzed the process of crack initiation and propagation in the platelet model and investigated the effect of the specimen shape on the force–displacement curves. The interfacial stress near the loaded fiber end in the platelet model is higher than in the equivalent cylinder model, which gives rise to earlier crack initiation and smoother shape of the force–displacement curve. As a result, the calculated interfacial shear strength values may be underestimated by 10–20%, if the equivalent cylinder is used instead of the real droplet shape. A method of correction to the equivalent cylinder model in order to avoid this underestimation is proposed.     

Experimental and theoretical investigation on the postcracking inelastic behavior of synthetic fiber reinforced concrete beams

A realistic method of analysis for the postcracking behavior of newly developed structural synthetic fiber reinforced concrete beams is proposed. In order to predict the postcracking behavior, pullout behavior of single fiber is identified by tests and employed in the model in addition to the realistic stress-strain behavior of concrete in compression and tension. A probabilistic approach is used to calculate the effective number of fibers across the crack faces and to calculate the probability of nonpullout failure of fibers. The proposed theory is compared with test data and shows good agreement. The proposed theory can be efficiently used to predict the load-deflection behavior, moment-curvature relation, load-crack mouth opening displacement (CMOD) relation of synthetic fiber reinforced concrete beams.     

Effect of high-energy radiation on the uniaxial tensile creep behaviour of ultra-high molecular weight linear polyethylene

The tensile creep (and other tensile) properties of ultra-high molecular weight polyethylene (UHMW PE) have been determined before and after electron beam irradiation and compared with similar results on normal molecular weight high-density polyethylene (NMW PE). In both polymers, irradiation increases the tensile modulus and the yield stress whilst reducing creep. The major effects occur over the first 20 MRad irradiation dose, though creep strain continues to diminish with dose in UHMW PE up to 64 MRad. Most of the effects can be attributed to crosslinking in the amorphous phase, though the rise in yield stress seems to require crosslinking in the crystalline phase, and the initial rise in modulus in UHMW PE seems to reflect a rise in crystallinity. Comparison with other polymers shows that the creep behaviour of UHMW PE remains relatively poor, even after irradiation. The improvements obtained may, however, be significant in applications where creep resistance is of secondary importance compared with, say, impact and wear resistance, in which UHMW PE excels.     

Part 1: A study on the properties of synthetic hydroxyledgrewite

In this work, for the first time chemical and physical properties of hydroxyledgrewite synthesized under hydrothermal conditions were examined. Hydroxyledgrewite was synthesized in the primary mixture consisted of CaO and amorphous SiO₂·nH₂O, when the molar ratio of CaO/SiO₂ was equal to 2.25. The synthesis has been carried out in unstirred suspensions under saturated steam pressure at 200°C temperature for 48 hours. It was proved that synthetic hydroxyledgrewite is stable till 675°C and at higher temperature recrystallized to γ‐C₂S, ‐C₂S, while upon subsequent cooling transformed into β‐C₂S. In addition to this, it was also determined that the density and specific heat capacity at 25°C are equal to 2.668 ± 5 g/cm³ and 0.928 J/(g·K), respectively. Synthetic hydroxyledgrewite showed disordered aggregates of plate‐like particles, while calculated S_BET value is equal to 13.961 m²/g. According gas adsorption results, it was obtained that hydroxyledgrewite is a mesoporous material. Also, it was obtained that after 72 hours of activated hydroxyledgrewite hydration, the amount of released heat was equal to 74.34 J/g. The product of synthesis and calcination were characterized by XRD, STA, DSC, SEM, TEM, FT‐IR analyses, and BET method.     

Influence of chemical and physical properties of the last generation of silicon carbide fibres on the mechanical behaviour of SiC/SiC composite

SiC/SiC composites reinforced with near stoichiometric SiC ceramic fibres (Hi-Nicalon S and SA3 Tyranno fibres) are attractive materials to be used in nuclear environment. Netherless, their mechanical properties must be improved and controlled. For example, SA3 Tyranno fibres (TSA3) -reinforced composites exhibit a brittle behaviour whereas composites reinforced by Hi-Nicalon S (HNS) fibres exhibit a conventional damage tolerant response. This difference is related to the nature of the fibre/matrix (F/M) coupling. The aim of this work was to identify the SiC fibres characteristics influencing the F/M coupling and consequently the mechanical properties of the composites. The experimental results point out that the TSA3 fibres exhibit a granular and rough surface leading to an increase of the residual stress and the interfacial shear stress in the SiC/SiC composites. Beside the roughness, the experimental results also point out that the surface chemistry of the SiC fibres significantly influence the F/M bonding.