Single Fiber Pull-out Test Research Articles

An experimental and numerical study on the age depended bond-slip behavior between nano-silica modified carbon fibers and cementitious matrices

Electrophoretic deposition of nano-silica on the surfaces of carbon fibers (CF) is a novel approach for modifying the interfacial bond properties between the fibers and cementitious matrices. To understand its pullout behavior with increasing age, single fiber pullout tests and scanning electron microscopy were performed at various curing times for pristine and modified fibers. In this article, a new approach for calculating the debonding energy is introduced and two theoretical models – a stress-controlled and an energy-controlled – were employed to obtain the critical interface failure criterion. The results showed that interfacial adhesion at fiber-matrix interface was increasingly improved with age compared to the untreated fiber, due to the formation of C–S–H gel in the vicinity of the fiber surface. Meanwhile, the untreated fibers exhibited only a slightly increasing trend of interfacial bonding with longer curing time. A finite element model was applied to simulate the fiber pullout failure, which incorporated a discrete cohesive concept into the smeared phase-field method in a Representative Crack Element (RCE) framework. Using a phenomenologically defined cohesive traction-separation law, the numerical simulations successfully captured the characteristics of the experimental pullout curves. According to proper parametric calibrations, good agreement was obtained between the numerical results and the respective experimental evaluations.

Experimental and Numerical Investigation of Single Fiber Pull-Out Tests of Steel Macro-Fiber and Glass Micro-Fiber in a Cementitious Matrix

Abstract This paper presents a modeling-based relationship between fiber diameter and frictional bond strength for steel macro-fiber (0.8-mm diameter) and glass micro-fiber (5–24-μm diameter) embedded in a cementitious matrix. A combined approach of experimental stress-displacement results obtained through single fiber pull-out tests and finite element modeling (FEM)–based stress-displacement results yielded the friction coefficient for the two types of fibers used. Further analysis relating to the pressure exerted on the fiber caused by the surrounding concrete reveals that a smaller fiber diameter would improve the bond strength. The improvements in frictional bond strength due to reduced diameter are based on FEM analysis. Validation of the experimental results is done through numerical approaches. The results showed that in the case of the same diameter, the pressure of the surrounding concrete decreases throughout the fiber sliding. The results also showed that the smaller diameter of fiber improves the properties of fiber-reinforced concrete interfaces.

Behaviour of Steel Fibre Reinforced Concrete Pavements on A Single Fibre Level

Concrete is a popular construction material used all around the globe. It is strong in compression and weak in tension. To mitigate this tensile weakness, various reinforcement methods have been used over the years, of these, fibre reinforced concrete (FRC) has gained popularity. Fibres not only improve the tensile strength of the concrete matrix, but also control crack propagation through crack bridging action. The goal of this study is to investigate the long-term behaviour of FRC pavements through conduction of static and fatigue meso-scale level tests using different steel fibres. To achieve this goal, the influence of different fibre geometries and embedment angles on the pull-out behaviour of fibres in FRC was investigated first. This was carried out by examining 60mm Dramix 3D steel and 5D steel fibres at single fibre level. The embedment depth was kept constant at half the fibre length, and the fibre embedment angles were tested at 0°, 15° and 30°. The average maximum pull-out load, Amax, was determined first through static tests then fatigue tests were carried out using 85% of the Amax value on pre-damaged samples. These tests indicated that an increase in fibre embedment angle and number of hooks leads to additional anchorage of the fibre. Fibre pull-out was found to be the dominant fibre failure mechanism for the static single fibre pull-out tests (SFPTs).

Mechanical properties of high ductile alkali-activated fiber reinforced composites incorporating red mud under different curing conditions

Red mud (RM) is an industrial by-product created during the production of alumina/aluminium that has engendered severe environmental concerns arising from its improper disposal. To mitigate the negative environmental impact of RM waste and produce a sustainable material having high mechanical performance (particularly ductility) for a variety of applications, this study developed an alkali-activated fiber reinforced composite (AAFRC) using RM, ground granulated blast-furnace slag (or slag), and silica fume as the precursors. The effects of two different curing conditions (ambient-curing and heat-curing) on the mechanical properties and microscopic characteristics of the material were systematically investigated. The heat-cured RM-derived AAFRC exhibited excellent tensile strain capacity (or ductility) of up to 5.6%, which was significantly higher than that of its counterpart that was cured under ambient condition. Despite the reduction in strength caused by heat-curing, AAFRC retained adequate tensile and compressive strengths of 4.0 and 75 MPa, respectively. The results of three-point bending, single crack tensile, and single fiber pullout tests provided reasonable explanations for the tensile behaviors of the studied material. Scanning electron microscopy revealed that a less compacted matrix was formed in the RM composite due to heat-curing. The combined results from energy dispersive spectroscopy and nanoindentation tests indicated that adding red mud did not alter the types of reaction products for the material, which were similar in composition to conventional alkali-activated slag while also including C-A-S-H and N-A-S-H gels in addition to a type of layered double hydroxide. These findings can serve as theoretical guidelines for the future design and application of eco-friendly, high-ductile cementless composites prepared from RM waste.

Micromechanical Tests on Natural Fibre Composites with Enzymatically Enhanced Fibre–Matrix Adhesion

Natural fibre–reinforced composites are more sustainable than other composites with respect to the raw materials. Their properties are attractive due to high specific properties, and especially so wherever high damping is valued. As the interphase between fibre and matrix is the region of highest stresses, a strong bond between fibre and matrix is essential for any composites’ properties. The present study compares two methods of determining the interfacial shear stress in natural fibre–reinforced composites: the single fibre fragmentation test and the single fibre pullout test. The studied composites are flax fibre reinforced epoxy. For a variety of fibre–matrix interaction, the fibres are treated with a laccase enzyme and dopamine, which is known to improve the fibre–matrix shear strength. In the observed samples, single fibre fragmentation test data, i.e. of fracture mode and fragment length, scatter when compared to pullout data. In single fibre pullout tests, the local interfacial shear strength showed a 30% increase in the laccase-treated samples, compared to the control samples. The method also permitted an evaluation of the frictional stress occurring after surface failure.

Mesoscopic modelling of UHPCC material under dynamic tensile loadings

This paper presents a new 3D mesoscopic model of ultra-high performance cement-based composite (UHPCC) to investigate its dynamic tensile behavior. In this model, the UHPCC is regarded as a two-phase material composed of cementitious matrix and randomly distributed fibers. The model is established using the commercial software LS-DYNA and involves generating the randomly distributed fiber elements with considerations of diameter, length, orientation and volume fraction, and then fully constraining them with the matrix. In particular, to capture the slipping effect between fibers and matrix that has a strong influence on the dynamic tensile behavior, the fibers are modelled by a fictitious material represented by the load-slip relation. The strain-rate effect of slipping force neglected in most of previous studies is considered by calibrating constitutive parameters of the fictitious material under different strain-rates based on the single fiber pullout tests. Finally, the 3D mesoscopic model is validated against three sets of tension-dominated experiments covered a wide range of loading intensity. Numerical predictions demonstrate that strain-rate effect of slipping force must be considered, and the neglect of it may lead to a great underestimation of the dynamic tensile strength of UHPCC material and would unavoidably underestimate the blast resistance of UHPCC components.

Enhanced interfacial shear strength in ultra-high molecular weight polyethylene epoxy composites through a zinc oxide nanowire interphase

The mechanical properties of fiber-reinforced composites are largely dictated by fiber-matrix interactions, where a well-adhered interface is desired. Ultra-high molecular weight polyethylene (UHMWPE) fibers suffer from poor interfacial properties due to their smooth and inert surface. Therefore, enhancing the interfacial properties of such composites will lead to an increased use in structural applications where high modulus, tenacity, low density, high impact and abrasion resistance properties are needed. In this work, an interfacial modification process consisting of an oxygen plasma surface functionalization followed by the grafting of zinc oxide nanowires (ZnO NWs) onto the UHMWPE fiber surface is investigated to improve the fiber-matrix interaction in UHMWPE composites. The UHMWPE fiber surface is subjected to varying oxygen plasma treatment durations such that the surface oxygen content can be increased to improve ZnO NW adhesion. Through single fiber pullout testing, the interfacial shear strength (IFSS) of oxygen plasma functionalized and ZnO NW coated UHMWPE fibers was shown a maximum increase of 135% with a 30 s plasma treatment prior to ZnO NWs. This increase in IFSS is attributed to an increased surface area interaction and mechanical interlocking between the fiber and matrix. The results detailed in this work demonstrate a benign, simple, and effective way to significantly improve the interfacial properties of UHMWPE fiber reinforced composites.

Multi-level study on UHPFRC incorporating ECat

The suitability of a recently developed ultra-high performance fibre reinforced cementitious composite (UHPFRC) incorporating Spent Equilibrium Catalyst, ECat, for structural applications is investigated through a systematic multi-level investigation across micro, meso and composite levels. Scanning electron microscopy, isothermal calorimetry, thermogravimetric analysis, and mercury intrusion porosimetry tests were performed to evaluate the microstructure of the composite. At the meso-level, the mechanical properties of fibre to matrix ITZ were characterised by single fibre pullout tests on fibres embedded with various fibre orientation angles. At the composite level, specimens with 3% fibre content and different fibre orientation profiles were prepared to determine uniaxial tensile behaviour. The relation between the tensile fracture parameters and fibre structure parameter was assessed. In each level, the results are compared to a conventional ternary UHPFRC mixture and point towards the suitability of the newly developed mixture for structural applications.

Experimental investigation on sing fiber pullout behaviour on steel fiber-matrix of reactive powder concrete (RPC)

A new single fiber pullout testing method was applied to characterize the interfacial bond performance, which includes the pullout load-slip relationship, pullout energy, bond strength, and fiber utilization ratio, of the steel fiber-matrix in reactive powder concrete (RPC). The bond properties and bond strengthening mechanism were investigated from the perspective of certain parameters, including the fiber type, fiber embedment angle, silica fume content, magnetic nanoparticles content, and curing time. The microstructure of the steel fiber and longitudinal morphology of the tunnel after the pullout test were determined by scanning electron microscope (SEM). In terms of the results, the bond strength of the end-hooked steel fiber is much stronger than that of the straight steel fiber. With the increasing of embedment angle, the ultimate pullout load and pullout energy of straight steel fibers increase, and the interfacial bond strength increases. It is found that the incorporation of silica fume can effectively enhance the matrix strength. In addition, RPC containing 20%∼30% silica fume shows significant improvement in pullout behavior. The appropriate curing age contributes greatly to the hydration reaction and the matrix strength of RPC. Steaming curing for 2 d can effectively improve the interfacial bond performance. A content of 1% magnetic nanoparticles enhances the pullout behavior of straight steel fibers with no brass coating, because the maximum pullout load and pullout energy increase by 33.6% and 79.0%, respectively. The microstructural observations confirm the conclusions regarding the bond mechanism drawn from the pullout tests. In conclusion, the performance of the matrix, the characteristics of the fiber-matrix interface area and the friction of the steel fiber in the tunnel can influence the bond performance of the fiber-matrix interface of RPC.

Mechanical and fiber/matrix interfacial behavior of ultra-high-strength and high-ductility cementitious composites incorporating waste glass powder

Most of the waste glass is landfilled, which is not conducive to the sustainability of the environment. However, waste glass powder (WGP) with sufficient fineness and high alkali reactivity exhibits excellent pozzolanic behavior, promoting its potential application in the concrete industry. This work evaluated the feasibility of replacing cement with WGP ranging from 0 to 30% to produce Ultra-High-Strength and High-Ductility Cementitious Composites (UHS-HDCC). The influences of WGP on the fresh flowability, mechanical properties, and fiber/matrix interfacial behavior of UHS-HDCC were investigated. The introduction of WGP enhanced the flowability of fresh UHS-HDCC mixtures. UHS-HDCC incorporating 10% WGP achieved incredible compressive strength, ultimate four-point bending stress, and ultimate uniaxial tensile stress, reaching around 158.4 MPa, 43.22 MPa, and 12.47 MPa, respectively. Moreover, all UHS-HDCCs exhibited strain-hardening features and multiple cracking behaviors. The maximum mid-span deflection and ultimate tensile strain capacity of UHS-HDCCs with WGP were distributed in 9.84 mm–13.83 mm and 5.88%–7.47%, respectively. Compared with WGP0, WGP10 and WGP20, the crack distribution in the WGP30 was more saturated and the number of narrow cracks was the largest. The results of the single-fiber pullout test indicated that the interface frictional bonding stress of WGP10 was reached 2.22 MPa, which represented a high ultimate four-point bending stress and ultimate uniaxial tensile stress. Furthermore, the average slip hardening coefficient of UHS-HDCCs ranged from 0.0032 to 0.0041. Based on the micromechanical model, the fiber ruptured fraction of WGP10 in the development of crack opening was calculated to reach 31.5%, which explained the decrease in maximum mid-span deflection and tensile strain capacity. Therefore, the experimental research will provide preliminary data for the scientific and efficient use of waste glass in engineering construction.

LaPO4 coating on alumina-based fiber: Strength retention of fiber and improvement of interfacial performances

The characteristics of the interface are the key factors that determine the mechanical properties and fracture behavior of fiber-reinforced ceramic matrix composites. Design and preparation of coatings which can preserve fiber strength and maintain appropriate interfacial bonding strength are of great challenges. LaPO4 coating is a promising weak interface coating for oxide fiber reinforced oxide ceramic matrix composites. Through this coating, the toughening mechanism of the composite such as fiber pulling out and fiber debonding is triggered. The LaPO4 coating was deposited on the surface of alumina-based fibers by a solution precursor heterogeneous precipitation method. The effects of different precursors and different deposition temperatures on fiber strength were studied, and the mechanism of the strength degradation of the coated fiber was analyzed. It was found that the fibers coated with phytic acid precursor and deposited at 90 °C had the highest tensile strength compared to other coated fibers. The retention of strength is attributed to its loosely stacked coating. Besides, a single fiber pullout test was carried out to evaluate the effect of the coating on the interface of the composites. The results show that the composites coated by depositing citric acid precursor and phytic acid precursor at 90 °C can reduce the interfacial bonding strength by 32.5% and 46.7%, respectively compared to uncoated composites. This study has potential application value in the preparation of ceramic matrix composites used in oxidation and high temperature environments.

Experimental Investigation and Modelling of the Layered Concrete with Different Concentration of Short Fibers in the Layers

The use of steel fiber reinforced concrete (SFRC) in structures with high physical-mechanical characteristics allows engineers to reduce the weight and costs of the structures, to simplify the technology of their production, to reduce or completely eliminate the manual labor needed for reinforcement, at the same time increasing reliability and durability. Commonly accepted technology is exploiting randomly distributed in the concrete volume fibers with random each fiber orientation. In structural members subjected to bending, major loads are bearing fibers located close to outer member surfaces. The majority of fibers are slightly loaded. The aim of the present research is to create an SFRC construction with non-homogeneously distributed fibers. We prepared layered SFRC prismatic specimens. Each layer had different amount of short fibers. Specimens were tested by four point bending till the rupture. Material fracture process was modelled based on the single fiber pull-out test results. Modelling results were compared with the experimental curves for beams. Predictions generated by the model were validated by 4PBT of 100 × 100 × 400 mm prisms. Investigation had shown higher load-bearing capacity of layered concrete plates comparing with plate having homogeneously distributed the same amount of fibers. This mechanism is strongly dependent on fiber concentration. A high amount of fibers is leading to new failure mechanisms—pull-out of FRC blocks and decrease of load-bearing capacity. Fracture surface analysis was realized for broken prisms with the goal to analyze fracture process and to improve accuracy of the elaborated model. The general conclusion with regard to modelling results is that the agreement with experimental data is good, numeric modelling results successfully align with the experimental data. Modelling has indicated the existence of additional failure processes besides simple fiber pull-out, which could be expected when fiber concentration exceeds the critical value.

Influence of sodium bicarbonate treatment on the free vibration characteristics of Phoenix sp. fiber loaded polyester composites

Natural fiber-based polymeric composites have become increasingly popular in machine tool and automobile applications in recent years. While, using the composite in such applications, it may fail due to excessive vibrations caused by dynamic loads. Therefore, it is necessary to understand the free vibration behavior of composites before employing it to such applications. The purpose of this work is to compare the free vibration characteristics of untreated and sodium bicarbonate treated Phoenix sp. fiber reinforced polyester composites fabricated through compression molding technique with different fiber content (0, 10, 20, 30 and 40 wt.%) and constant fiber length (20 mm). The chemical treatment was carried out for different time periods (24, 120 and 240 h) with 10 wt.% concentration of sodium bicarbonate solution. The results revealed that the natural frequency of treated fiber composites was greater than the untreated fiber composites which maybe ascribed to the increased stiffness of the fibers and interfacial bonding after the treatment. In addition to this single fiber pull-out test was also performed to assess the interfacial strength among the composite constituents. Especially, the composites loaded with 120 h treated fiber showed better results and it can be suggested for various engineering applications.

Enhancing the hydrophobic PP fiber/cement matrix interface by coating nano-AlOOH to the fiber surface in a facile method

A variety of fiber surface modification methods, based on oxidation or coating, have been developed to improve the weak hydrophobic polymer fiber/cement matrix interface, which is critical for the mechanical properties of fiber-reinforced cement-based materials. However, these methods require special equipment for oxidizing the fiber, consumes costly raw materials for synthesizing the coating layer, or produces massive waste of organic solvent. This study improves the interface between the hydrophobic polypropylene (PP) fiber and the cement matrix, by synthesizing and coating AlOOH with a facile method without any of the drawbacks above. Material characterizations (SEM-EDS, FTIR, XPS, and XRD) of the treated fiber confirms nano-plates of γ-AlOOH are successfully coated onto the fiber surface. The water contact angle test and the characterization of the precipitates on the fiber after conditioning in mimic cement pore solutions (saturated Ca(OH)2 w/or w/o sulfates) are conducted. The results indicate that AlOOH coating enhances wettability and chemical reactivity of the fiber, respectively. In the presence of sulfate, AFm and AFt (ettringite) are found in the precipitates, showing the effectiveness of engaging the sulfates in cement of AlOOH coating. Single-fiber pull-out test shows that the coating enhanced the frictional bond and the energy absorption at the interface by 113% and 80% respectively, which is among the most enhanced in comparison with previously reported fiber treatments. The significant enhancement can be attributed to the more densified and more tortuous fiber/matrix interface.

Experimental Study on the Mechanical Properties of Cotton Straw Fiber-Reinforced Soil Interface under Dry–Wet Cycles

Fiber-reinforced soil technology has been employed in engineering. However, reinforced soil often undergoes dry–wet (D–W) changes because of high temperatures and rain in summer; consequently, its strength deteriorates. Its mechanical properties are mainly affected by the force between a fiber interface and a soil matrix. In this study, the effect of D– W cycles on the interfacial mechanical properties of cotton straw fiber-reinforced soil was investigated. Fiber-reinforced soil with different normal stresses was subjected to single fiber pullout tests on the 0th, 1st, 3rd, 6th, and 9th D–W cycles. Results showed that the trend of the changes in the pullout force–displacement curve was consistent; in particular, the trend initially showed a linear increase. Then, it peaked, decreased, and finally stabilized. The pullout force had a linear relationship with normal stress and increased linearly as normal stress increased. The relationship between interfacial shear strength and the number of D–W cycles exponentially decreased, and the most significant decrease occurred in the first to three D–W cycles.

Increasing the Fatigue Resistance of Strain-Hardening Cement-Based Composites (SHCC) by Experimental-Virtual Multi-Scale Material Design.

Strain-hardening cement-based composites are a promising class of materials for a wide variety of applications due to their considerable tensile strength and pronounced ductility caused by the development of multiple fine cracks. Nevertheless, the safe use of such composites requires sound knowledge of their mechanical behaviour under different types of loading, particularly under fatigue loading, while considering distinct influences like initial crack width and fibre orientation. To deepen this knowledge, single-fibre pull-out tests on PVA-fibres from a cementitious matrix were carried out to gain information about the micro-mechanical and degradation processes of the fibre. It could be shown that the fibres tend to rupture instead of being pulled out under quasi-static loading. When changing the loading regime to alternating loading, this failure mechanism shifts to pull-out. By varying the experimental parameters such as initial crack width, inclination angle or compressive-force level a clear influence on the fibre’s crack bridging capacity could be observed associated with effects on the degradation processes. Based on the data obtained, a micro-mechanical numerical model was developed to support the assumptions and observations from single-fibre pull-out tests and to enable predictions of the performance of the material on the microscale under cyclic loading.

Mechanical properties of high ductile alkali-activated fiber reinforced composites with different curing ages

Combining the high ductility of engineered cementitious composites and sustainable advantages of alkali activated materials, high ductile alkali-activated fiber reinforced composite (AAFRC) is receiving a lot of attentions. In this paper, ground granulated blast-furnace slag (or slag)-based, polyethylene fiber reinforced AAFRCs were prepared. The macro- and meso-scale mechanical properties of the mixes with different curing ages varying from 1d to 120d were experimental investigated. The results show that both the compressive strength and the tensile strength increase first and then stabilize with the ages increasing, nonetheless reaching the maximum values of 100.7 MPa at 28d and 5.99 MPa at 56d, respectively. The tensile strain capacities maintain a higher level of 5.74–6.14% after 7d. At meso-scale, the parameters derived from three-point bending, single crack tensile and single fiber pullout tests can be well correlated with the tensile properties of the studied mixes at different ages. The analysis of FT-IR reveals that the amorphous silicate and aluminosilicate gels are the main polymerization reaction products; and the crosslinking degree of the gels has a tendency to be higher after 7d.

Experimental, Analytical and Numerical Studies of Interfacial Bonding Properties between Silane-Coated Steel Fibres and Mortar

A systematic investigation of the effects of silane coatings on steel fibre–mortar interfacial bond properties was conducted, combining pullout tests, analytical solutions, and meso-scale FE simulations. Nine silane coatings were tested, and their effects were evaluated by 30 single fibre pullout tests. They were found to increase the peak force and energy consumption up to 5.75 times and 2.48 times, respectively. Closed-form analytical solutions for pullout load, displacement, and interfacial stress distribution during the whole pullout process were derived based on a tri-linear bond-slip model, whose parameters were calibrated against the pullout tests. Finally, the calibrated bond-slip models were used to simulate the pullout tests and complex failure of multi-fibre specimens in mesoscale finite element models. Such an approach of combining pullout tests, analytical solutions, and mesoscale modelling provides a reliable way to investigate the effects of fibre–mortar interfacial properties on the mechanical behaviour of steel fibre reinforced concrete members in terms of structural strength, stiffness, ductility, and failure mechanisms.

Microbond multiple fiber pull-out test to evaluate interface properties of UHMWPE/LDPE self-reinforced polymer composites

Interfacial properties of composite materials play an important role in overall efficiency and reliability of these materials in structural applications. The objective of this study is to develop a multiple fiber microbond pull-out test to determine the interfacial properties of self-reinforced polymer composites (SRPC) and compare it with single fiber multiple fiber pull-out tests. SRPC possess better interfacial adhesion due to their similarity in chemical structure. The system used in this study is LDPE sheet reinforced with plain weave ultra-high molecular weight polyethylene (LDPE/ UHMWPE). The optimal operating temperature was estimated with DSC and TGA analysis. The micromechanical and meso-mechanical approaches were compared to validate the results. A fractographic study was performed to correlate lamina and meso-mechanical properties found in this study. It was observed that the multiple fiber pullout test explained in this study is on par with or better than the other conventional methods to evaluate interface properties.

Role of sizing agent on the microstructure morphology and mechanical properties of mineral-impregnated carbon-fiber (MCF) reinforcement made with geopolymers

This report deals with the influence of different sizing agents on carbon fiber (CF) heavy tows towards their impregnation with a geopolymer (GP) suspension and resulting properties of mineral-impregnated carbon-fiber composites (MCF) produced in an automated, continuous process. Three different commercial CFs were investigated after treatment with either thermoplastic, epoxy, or vinyl-ester sizing agents and then compared to unsized CFs. During impregnating, diverse yarn-spreading behavioral modes and degrees of wetness were observed, indicating different degrees of impregnation, both in quality and quantity. All sizing agents decreased the hydrophobic nature of unsized CFs significantly. Supported by microscopic investigation, water contact angle measurement, and thermal gravimetric and mercury intrusion porosity, the epoxy sizing showed the best fiber-matrix distribution over the cross-section and a dense microstructure. Moreover, in single-fiber pullout tests each sizing brought about a significant increase in maximum pullout force, indicating enhanced bond between fiber and matrix. This is attributed to more intense interaction in the interfacial region, fully corroborating the topological characteristics obtained in microscopic analysis. With higher interfacial bond strength, the thermoplastic sizing enabled the highest tensile properties of MCF despite slightly less impregnation quality in comparison with epoxy-sized samples.