Characterising the time-dependant behaviour on the single fibre level of SHCC: Part 1: Mechanism of fibre pull-out creep

SHCC (Strain Hardening Cement-based Composite) has been designed and optimised to overcome the main weaknesses of ordinary concrete, which is its brittleness. SHCC shows a high tensile ductility and can resist the full load at a tensile strain of more than 4%. An in depth investigation into the time-dependant behaviour is still lacking for SHCC. This paper is the first part of a two paper series about the time-dependant behaviour on the single fibre level. In this paper, the tensile creep behaviour of SHCC is studied to distinguish mechanisms of creep. Tensile creep and shrinkage test results are reported for dumbbell type SHCC specimens. The specimens are pre-cracked to simulate in-service conditions, with subsequent sustained load at various levels, here chosen as 30%, 50%, 70% and 80% of the ultimate resistance. To distinguish the sources of significant creep deformation under these sustained loads, single fibre pull-out tests are performed under sustained load. It is shown that the time-dependent fibre pull-out is a significant source of time-dependent deformation, along with the formation of new cracks in SHCC under sustained load.     

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.     

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.     

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.     

Wood fiber surface treatment level effects on selected mechanical properties of wood fiber-cement composites

The objective of this study was to determine the effects of sodium (N) silicate, potassium (K) silicate, and silane (Si) treatment levels on newspaper and unbleached kraft fibers for enhancing selected mechanical properties of wood fiber-cement composites compared to untreated wood fiber-cement composites. Both wood fiber types were treated with selected aqueous solution strengths, air dried, and mixed with water and cement. The bending and compression properties of the specimens were determined after 28 days of hydration. Results of this study indicated that the aqueous chemical treatments of the wood fibers enhanced some of the mechanical properties of wood fiber-cement composites compared to the untreated wood fiber-cement composites. The enhancement depended on chemical treatment and wood fiber type. All three chemical treatments of newspaper fiber enhanced the normalized toughness values compared to the untreated newspaper fiber-cement composites. In addition, higher treatment levels using N silicate with newspaper fiber improved the compressive strength and bending modulus of the composites compared to the untreated newspaper fiber-cement composites. Kraft fiber treated with all three chemicals enhanced the compressive strength, bending modulus and bending strength compared to the untreated kraft fiber-cement composites. However, only silane-treated kraft fiber improved the normalized toughness values compared to the untreated kraft fiber-cement composites. The results of the study indicated that certain chemical treatments react better with different wood fiber types resulting in selected mechanical property enhancements.     

Effect of hygrothermal conditioning on the energy release rate and failure mechanism of metal only adherend-glass fibre prepreg co-cured single lap joints

Abstract

Fibre-reinforced composite materials are extensively used in repair and rehabilitation of oil and gas metal infrastructures which are largely exposed to water and hydrocarbon. An important aspect to this is applying adequate surface preparation to the metal to ensure a durable bond between the composite and metal substrate. In this paper, mild steel surface was prepared using grit blasting and single lap joint (SLJ) test specimens were manufactured and tested to investigate the adhesion in terms of total energy release rate (G_T) of the interface between mild steel adherend and glass fibre prepreg. An out-of-water usable epoxy resin primer was incorporated to join mild steel adherend with glass fibre prepreg by curing at a temperature of 55 °C for 48 h. Upon durability testing of the SLJ specimens using hygrothermal conditioning at a temperature of 55 °C for 1000 h, the experimental G_T values were seen to reduce significantly. Comparatively lower amount of cohesive failure and increased amount of swelling or delamination of the adhesive was observed for conditioned SLJ specimens when compared to controlled SLJ specimens. Furthermore, the experimental G_T values were found to correlate well with an analytical adhesive interface model.     

Indirect estimation of fiber/polymer bond strength and interfacial friction from maximum load values recorded in the microbond and pull-out tests. Part II: Critical energy release rate

A new approach to experimental data treatment in the pull-out and microbond tests has been developed. It uses the relationship between the maximum force recorded in these tests and the embedded length ('scale factor') to separately determine adhesional interfacial parameters (critical energy release rate, local bond strength) and interfacial friction in debonded regions. The new method does not require the measurement of the debond force, which corresponds to interfacial crack initiation, and is, therefore, much more convenient and simpler than 'direct' techniques involving continuous monitoring of crack growth. Using the equation for the current crack length as a function of the load applied to the fiber, based on a fracture mechanics analysis of interfacial debonding, we modeled the pull-out and microbond experiments and obtained the maximum force versus the embedded length. By varying the critical energy release rate and interfacial frictional stress to fit experimental plots, both interfacial parameters were determined for several fiber-polymer pairs. Effects of specimen geometry, residual thermal stresses, and interfacial friction on the measured values are discussed. The results are compared with those obtained with our similar stress-based approach. The energy criterion works when the embedded length is not very short, and in this range of embedded length it is better than the stress criterion. Both criteria can be complementarily used for interface characterization.     

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.     

The effects of amine additives and flow rate on the performance of mixed-bed ion exchange at ultralow concentrations

Experimental data were obtained to evaluate the effects of amine additives for pH control of solution and the volumetric flow rate of feed solution on the performance of mixed-bed ion exchange for the removal of ionic impurities in solution. The experiments were performed under various temperatures and cation resin ratios by using a continuous column system with NaCl solution. The breakthrough curves of ions, plotted as the ratio of the effluent to influent concentration versus run time or treated solution volume, give detailed results about the effects of the existence of the pH controller, such as ammonia and morpholine, and the variable flow rate on the fate of each ion in the units. The experimental results show that the morpholine breakthrough occurs earlier than the ammonia breakthrough and that the effect of ammonia on both sodium and chloride exchange rates is more significant than that of morpholine. The addition of ammonia in solution results in the decrease of cation resin capacity for the sodium removal much more than the addition of morpholine. The step changes in the flow rate affect significantly the shapes of sodium and chloride breakthrough curves. The effluent concentrations of sodium and chloride change according to the flow rate. However, the effect increases with decreasing operation capacity of cation resin, while it becomes serious around the breakthrough time of chloride and negligible after the time.     

The effect of the Zn/Sn ratio on the formation of single phase kesterite Cu2ZnSnS4 solar cell material

The effect of the Zn/Sn ratio in the solution on the properties of Cu₂ZnSnS₄ films prepared by sol-gel method has been investigated. As the Zn/Sn ratio in the solution increases to a certain value, a pure single phase kesterite CZTS is obtained and confirmed by XRD, XPS and Raman. Through controlling the Zn/Sn ratio in the solution, secondary phases such as SnO₂ can be avoided and an optimal condition for single phase kesterite CZTS can be achieved. Surface SEM images of the CZTS films are investigated and the optical band gap of the optimized CZTS film is found to be 1.23 eV.     

The effects of alkali treatment on the mechanical and morphological properties of Pennisetum purpureum/glass-reinforced epoxy hybrid composites

The effects of alkali treatment on the mechanical properties, void contents and morphological of Pennisetum purpureum/glass-reinforced epoxy hybrid composites were studied. The composites were produced using the vacuum infusion method. Pennisetum purpureum fibres were treated with 5 and 10% dilute sodium hydroxide (NaOH) solution for 6?h. The epoxy resin was mixed with either the treated or untreated Pennisetum purpureum/glass fibres to a ratio of 70:30 (by volume). Tensile and flexural tests were performed on the composites in accordance with ASTM D638 and ASTM D790, respectively. The hybrid composites that contained 5% NaOH-treated Pennisetum purpureum fibres exhibited the greatest tensile and flexural strengths with lower void contents. Field-emission scanning electron microscopy fractography supported the findings showing lesser voids and fibre pull-outs suggesting good interfacial bonding between the matrix and reinforcement. The reduced of the void contents is suspected due to the reduced hemicellulose content within the treated Pennisetum purpureum fibre.     

Raman spectroscopy study of HM carbon fibres: effect of plasma treatment on the interfacial properties of single fibre/epoxy composites

The effect of an oxygen plasma treatment upon the structural and morphological properties of high-modulus carbon fibres has been studied by means of several characterisation techniques. Scanning electron microscopy showed that there were only minor changes of the morphology of the fibres following treatment. X-ray diffraction traces revealed that there were differences in structural parameters between the untreated fibres but no further modifications in the crystalline structure were detected after the plasma oxidation. Raman spectroscopy was used to follow the changes on the fibre surface structure following treatment. The peak positions and widths of the four main Raman bands (D, G, D′ and G′) were determined, with no significant changes observed after the surface treatment. A relationship between the width of the G band and the crystal parameter d₀₀₂ was found, with the magnitudes of both decreasing as the fibre modulus increased. A reference order parameter I_D/(I_D+I_G) ratio was calculated from the intensities of D and G bands. The treated fibres exhibited a more highly disordered surface structure that the untreated ones, as revealed by the increase of I_D/(I_D+I_G) after the plasma oxidation.     

Polypropylene cellulose‐based composites: The effect of bagasse reinforcement and polybutadiene isocyanate treatment on the mechanical properties

Using bagasse fiber as the reinforcing filler and polypropylene as the thermoplastic matrix polymer, a reinforced composite was prepared, and its mechanical properties examined as a function of the amount of compatibilizing agents used. In the sample preparation, four levels of fiber loading (10, 20, 30, and 40 wt %), three levels of polybutadiene isocyanate (PBNCO) content (0, 2, and 4 wt %) and two levels of maleated polypropylenes (MAPP) content (0 and 3 wt %) as compatibilizing agents were used. The tensile properties of the composites improved as the fiber loading and the compatibilizing agents increased, but the impact strength was significantly decreased. The mechanical study revealed that the positive effect of compatibilizing agents on interfacial bonding. The composites treated with PBNCO showed superior tensile and impact properties than those without treatment. The findings indicated that bagasse as agro‐waste material is a valuable renewable natural resource for composite production and could be utilized as a substitute for wood in composite industries. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Recycled poly(ethylene terephthalate) and its short glass fibres composites: effects of hygrothermal aging on the thermo-mechanical behaviour

This paper reports on the effects of hygrothermal aging at 70 °C in water and at 80% relative humidity, on the thermo-mechanical properties, molar mass and microstructure of recycled poly(ethylene terephthalate) (rPET) and its short glass fibres composites.For all the investigated materials, the elastic mechanical properties (tensile and storage moduli) determined at low strain levels resulted practically unaffected by hygrothermal aging under the selected conditions. On the other hand, a marked reduction of the tensile strength and apparent fracture toughness has been observed for rPET matrix and its composites during hygrothermal aging, more markedly for materials immersed in water than for those aged at 80% RH. Both properties resulted to be related on the molar mass of the rPET matrix, that decreased during hygrothermal aging as a consequence of the hydrolysis process.The materials glass transition, evaluated as the temperature of the loss factor peak, increased during hygrothermal aging due to the progressively restricted mobility of the amorphous phase caused by a concurrent crystallinity increase. This crystallization process (chemicrystallization) is favoured by temperature, by the plasticizing effect of water and by the reduction of molar mass.Consistently with the mechanical measurements, the morphology of fracture surfaces exposed to hygrothermal aging in water revealed a reduction of plastic deformation of the rPET matrix and a weakening of the fibre-matrix interface for rPET composites.     

The effects of tungsten doping on the mechanical properties of Bi2O2Se

In this study, we systemically investigate the effects of tungsten doping on the structural, electronic, and mechanical properties of Bi₂O₂Se by using the first-principles method. It is found that tungsten doping significantly influences the electronic structure and mechanical properties of Bi₂O₂Se. The electrons are distributed on the Fermi level, and the doped Bi₂O₂Se exhibits metallic-like character. Meanwhile, tungsten doping improves the ductility and toughness of Bi₂O₂Se and reduces its lattice thermal conductivity. This study demonstrates that tungsten doping is an effective method to engineer the physical properties of Bi₂O₂Se.     

The influence of microstructure on the static and dynamic strength of transparent Magnesium Aluminate Spinel (MgAl2O4)

The dynamic flexural strength of transparent ceramics is an important property for demanding applications. Therefore, this work reports a systematic study of the grain size influence on the dynamic and static flexural strength of Magnesium Aluminate Spinel (MgAl₂O₄), utilizing a modified split Hopkinson (Kolsky) apparatus. An ultra-high-speed camera (8?×?10⁶ frames/sec), was extensively used and provided precious insight of the dynamic crack initiation and propagation. It was found that fine microstructure Spinel exhibits slightly higher static flexural strength than coarser microstructure. However, in the dynamic regime, no significant difference was found. Moreover, the fracture timing method, i.e. fracture gauge or high-speed camera, influences significantly the dynamic strength results. While the fracture gauges indicate complete fracture, the high-speed camera captures the onset of the fracture as a true material property. Consequently, Spinel’s dynamic flexural strength is not rate sensitive, based on high-speed imaging.     

Influence of learning effects on the results of the cap‐sorting test Roth 28‐hue (E) desaturated: Influence of learning effects on colour‐arrangement tests

The results obtained in colour vision tests can be influenced by many factors. It is possible that a learning effect disguises the fact that an acquired colour vision disturbance has progressively either deteriorated or been successfully treated. Therefore, the primary object of this study was to examine whether a possible learning effect occurred if screening by the colour vision test Roth 28‐hue (E) desaturated was repeated several times, and if this learning effect was age dependent. Sixty‐five ocularly and generally healthy subjects participated in the study and were divided into two age groups: group A: 20–39 years, n = 35; group B: 40–59 years, n = 30. Besides their ophthalmological status (visual acuity, refraction, intraocular pressure, cup/disk ratio, central fundus), the cap‐sorting test Roth 28‐hue (E) desaturated was performed under standardized test conditions. The measurements were repeated after 5 ± 1.72 days (T1), 15 ± 3.53 days (T2), 32 ± 6.97 days (T3), and 189 ± 16.85 days (T4). The ophthalmological parameters of all subjects were inconspicuous. The individual evaluation of the error scores in the cap‐sorting test Roth 28‐hue (E) desaturated showed large‐scale variations. For both age groups there was no statistically significant difference between the right and left eye at any time. The mean values of the younger group remained relatively constant after the first measurement. This age group showed a quick, clearly visible learning effect that persisted over the whole test period. With regard to the older age group the average values deteriorated, remained solid for one month increasing again after 6 months. The results showed an age‐related learning effect. Therefore, it is important to repeat the colour vision test within 5 days for the age group 20–39 years. This second test result will then serve as a stable basis for further comparative examinations within a period of 6 months. The subjects of the age group 40–59 years ought to repeat a first colour vision test after 5 and again after 15 days. The result of the second repetition will then offer stable basic values for subsequent tests. © 2006 Wiley Periodicals, Inc. Col Res Appl, 32, 16–21, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20282     

Polypropylene–rubber blends: 1. The effect of the matrix properties on the impact behaviour

The effect of matrix properties, i.e. crystallinity and molecular weight, on the impact behaviour of polypropylene–EPDM blends was studied. The blends were made on a twin-screw extruder. The impact strength was determined as a function of temperature, using a notched Izod impact test. The matrix crystallinity was varied by varying the matrix isotacticity, and ranged from 33 to 50 wt%.

With increasing temperature the polymers show a sharp brittle–ductile transition. This brittle–ductile transition temperature (T_bd) shifts to higher temperatures with increasing crystallinity of the polypropylene. However, the balance of properties and the modulus–T_bd relationship were better with blends made with higher crystalline PP.

The matrix molecular weight was decreased by treating a high molecular weight PP–EPDM (85/15 vol%) master blend with peroxide. In this way blends were obtained with a high MFI and a small rubber particle size. The matrix MFI of the blends thus obtained ranged from 2 to 30 dg min⁻¹. With decreasing matrix molecular weight the T_bd increased. The peroxide treated blends exhibited a considerably lower T_bd than comparable blends made in the standard way with a similarly small particle size. Peroxide treatment of a master blend is an effective method of preparing blends with a high MFI, small particle size and good ductility.