Single Fiber Pull-out Test Research Articles

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.

Study on Preparation of Triangular Melt-Spinning Poly (Vinyl Alcohol) Fibers and Its Fabric Strengthening and Toughening Epoxy.

Fiber-reinforced epoxy materials have the advantages of light weight, high strength and designability, which are widely used in high-technology fields. In this paper, triangular poly (vinyl alcohol) (PVA) fibers prepared by melt spinning were used for the first time in reinforcing and toughening epoxy resins. Based on intermolecular complexation and plasticization, the triangular PVA fibers were successfully prepared via melt spinning and hot drawing. The thermal properties, crystallinity, morphology and mechanical properties of the triangular fibers with different draw ratios were characterized by DSC, FTIR, XRD, SEM and tensile testing. The results show that the comprehensive performance of the triangular fibers increased with the increase in the draw ratio. The tensile strength of triangular fibers increased from 0.3 to 4.22 cN/dtex. Then, the triangular PVA fiber and circular PVA fiber-reinforced and toughened epoxy materials were prepared, respectively. The mechanical properties of triangular PVA fiber/epoxy composites were higher than that of circular fiber-reinforced and toughened epoxy materials. Furthermore, the single-fiber pull-out test was used to analyze the interface capability of fibers and epoxy. The pull-out force of the circular fiber was 1.24 N, while that of the triangular fiber was 2.64 N. The specific surface area of the triangular PVA fiber was larger than that of the circular PVA fiber, which better made its contact with epoxy and was not easily pulled out. Experiments prove that triangular PVA fiber is an ideal material for strengthening and toughening epoxy resin.

Role of interfacial adhesion and fiber length on the mechanical performance fiber reinforced thermoplastic elastomers

Thermoplastic elastomers combine inherent benefits such as the processability and recyclability of thermoplastics and the elasticity, toughness, and flexibility of elastomers. In this work, polyester based thermoplastic elastomers have been reinforced with carbon and poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers of similar strength and stiffness. Testing of the respective mechanical properties revealed that the PBO reinforced composites yielded higher tensile strength. Single fiber pullout testing was also performed to evaluate the respective interfacial and mechanical properties where it was found that the apparent interfacial shear strength for carbon fibers was two to three times greater than their PBO counterparts. Further analysis revealed that the fiber length of the PBO composites was much greater than for carbon fiber, resulting in superior strength of the PBO reinforced elastomers.

Polymeric Coatings for AR-Glass Fibers in Cement-Based Matrices: Effect of Nanoclay on the Fiber-Matrix Interaction

Polymeric coatings are widely used to enhance the load bearing capacity and chemical durability of alkali-resistant glass (AR-glass) textile in cement-based composites. The contact zone between coated yarns and concrete matrix plays a major role to enable the stress transfer and has still to be improved for the full exploitation of the mechanical behavior of the composite. As a new approach, this paper studies how the addition of nanoclay particles in the polymer coating formulation can increase the chemical bond between organic coating and inorganic matrix. This includes the description of the water-based coating preparation by dispersing sodium montmorillonites, whereby the resulting coating nanostructure is characterized by X-ray diffraction and energy dispersive X-ray spectroscopy. Single glass fibers were treated by dip-coating. Atomic force microscopy was used to determine the surface roughness, and the effect on the fiber tensile properties was studied. Moreover, the morphological and chemical characteristics of the coatings were compared with the results obtained from single fiber pull-out (SFPO) tests. It was shown that the incorporation of nanoclays leads to increased interfacial shear strength arising from the ability of nanoclay particles to nucleate hydration products in the fiber-matrix contact zone.

Preparation and characterization of ultra-high-strength and ultra-high-ductility cementitious composites incorporating waste clay brick powder

Large quantities of waste clay bricks from construction and demolition (C&D) waste are currently stockpiled in landfills, which is an environmentally, technically, and economically detrimental procedure. In this work, waste clay bricks powder (WCBP) was assessed to determine its feasibility as an alternative to cement for ultra-high-strength and ultra-high ductility cementitious composites (UHS-UHDCCs). The influences of WCBP on the fluidity, hydration, mechanical properties, fiber/matrix interface performances, autogenous shrinkage, and ecological overload capacity of UHS-UHDCC with different replacement levels ranging from 0 to 50% were investigated. The results indicate that WCBPs reduce the fluidity of the UHS-UHDCC matrix. WCBP content of 20% slightly accelerates the hydration process and exhibits the highest compressive and flexural strength in the specimens. Moreover, all UHS-UHDCCs achieve strain-hardening characteristics and exhibit a tensile strain capacity of 3.18–5.47%. UHS-UHDCC with 20% WCBP obtains excellent ultimate tensile stress and tensile strain capability, reaching around 9.34 MPa and 3.18%, respectively. The performance of the interface frictional bond (τ) with different WCBP content was obtained through single-fiber pullout tests, which reasonably explained the macroscopic tensile properties of UHS-UHDCC. The average interface frictional bond (τ) between the fiber and matrix of UHS-UHDCC reaches the maximum 1.72 MPa in the WCBP20 specimen. The average slip hardening coefficient (β) of UHS-UHDCC is distributed between 0.0031 and 0.0075 and tends to be excellent in the WCBP20 specimen. The theoretical fiber bridging stress vs crack displacement relationship of UHS-UHDCC was calculated through the limitation of interface parameters, which had an excellent correlation with the experimental results. Furthermore, the introduction of WCBP remarkably reduces the autogenous shrinkage of UHS-UHDCC. The ecological overload capacity of UHS-UHDCC is decreased by the introduction of WCBP, which provides an environmentally friendly solution for the C&D and cement industries.

Effect of freeze–thaw cycling on mechanical properties of polyethylene fiber and steel fiber reinforced concrete

This paper experimentally studied the influence of freeze–thaw cycles on the mechanical property of hybrid PE and steel fiber reinforced concretes (FRC). Five different FRC mixtures and a reference mixture were prepared for the comparative study. After 250 freeze–thaw cycles, the tension, compression, bending and fracture tests all showed that freeze–thaw cycles have minimal effect on the strength but significant influence on the deformability of FRCs. Analysis on relative dynamic modulus of elasticity and mass loss ratio indicated that the high compactness of matrix maintains the matrix integrity and thus keeps strength stability. Single fiber pull-out tests revealed the variations of peak strength and the corresponding displacement when fiber is pulled out of matrix before and after freeze–thaw treatment. It is demonstrated that freeze–thaw cycles impair the fiber/matrix interfacial bond, which is attributed to be the main cause to the degradation of deformability.

Influence of cyclic loads on the fiber-matrix-interaction of cellulose and glass fibers in polypropylene

The present study is focused on the characterization of cyclic loads on the fiber matrix adhesion of regenerated cellulose fibers (RCF) and glass fibers (GF) in a PP matrix with varying contents of maleic anhydride-grafted polypropylene (MAPP). Glass fibers were used for reference purposes. By means of the cyclic loaded single fiber pull-out test (SFPT) the interfacial shear strength (IFFS) and the critical fiber length (lc) are determined as a function of the applied loads, the number of cycles and the MAPP content. The results of the material-specific reduction of the IFSS were extrapolated to approximately predict the fatigue behavior of the composites.The investigations show the influence of cyclic loads in the single fiber pull-out test on the fiber–matrix adhesion for the first time. On the one hand, the results show that cyclic loading of the embedded single fibers leads to a significant reduction of fiber–matrix adhesion. On the other hand, it becomes clear that the number of cycles has the greatest effect on IFSS and thus on fatigue, while the various investigated setups (test speeds and forces) show the least effect.The transfer of the resulting findings to the integral fatigue properties of composites can contribute to a much more precise property and failure prediction in the future.

Functionalization of basalt fibers with ZnO nanostructures by electroless deposition for improving the interfacial adhesion of basalt fibers/epoxy resin composites

Basalt fibers were functionalized by ZnO electroless deposition for obtaining a nanostructured interphase for enhancing the interfacial strength with an epoxy resin matrix. The structural, morphological and wetting properties of the pristine basalt fabrics and ZnO-coated basalt fabrics were evaluated. The fabrics were uniformly coated with ZnO nanostructures featuring a wurtzite structure and a twin hexagonal prism morphology. The contact angle measurements revealed that ZnO prisms transformed the hydrophilic basalt fabric into a hydrophobic one (~130°). ZnOs were also grown on the basalt fibers as yarns to evaluate their interfacial adhesion by single fiber pull-out tests. The results emphasize significant improvement in the apparent interfacial shear strength (~42%) with limited degradation of the pristine basalt fiber tensile strength (a reduction of ~ 17%). Therefore, ZnO electroless deposition can be regarded as an effective route to improve the mechanical performance of basalt/epoxy composites expanding their potential range of applications as structural materials.

Three-dimensional mesoscale numerical study on the mechanical behaviors of SFRC tunnel lining segments

This study investigated the mechanical properties and failure patterns of the steel fiber reinforced concrete (SFRC) lining segments with different fiber parameters using a three-dimensional mesoscale model. The research highlights a sophisticated mesoscale model of the SFRC tunnel segment with randomly distributed steel fibers and considers the nonlinear mechanical behaviors of steel fiber, concrete and the bond-slip effect. The entire paper is organized as follows: firstly, a generation algorithm to achieve the discrete distribution of steel fibers and the nonlinear constitutive models of steel fiber and concrete were introduced. The bond-slip effect between steel fiber and concrete was converted equivalently. Secondly, the numerical model was further validated by conducting a series of simulations, including the single steel fiber pullout, uniaxial compression, splitting tension and flexural testing. Finally, the mesoscale model was employed to investigate the influences of steel fiber parameters on the mechanical properties and failure patterns of SFRC tunnel segments. It revealed that the typical features of the cracked SFRC segments in the numerical simulation are highly consistent with the experimental results. The randomly distributed steel fibers can well bear and transfer the tensile stress, which effectively improves the crack resistance of the tunnel segment. The load-bearing capacity of SFRC segments linearly increases with the steel fiber content and length while maintains a negative relationship with the steel fiber diameter. Moreover, the addition of steel fiber improves the toughness of the segment more significantly than the maximum load. The proposed model provides an applicable method for further analysis, design and optimization of SFRC segments in real tunneling engineering.

Low temperature plasma polymerization: An effective process to enhance the basalt fibre/matrix interfacial adhesion

The interface of fibre reinforced composite materials is deeply affected by the type of sizing applied to the surface of the fibres. In this study, a polymer film based on pure tetravinylsilane (TVS) or its mixture with two different oxygen amounts was deposited on the surface of unsized basalt fibres by plasma enhanced chemical vapor deposition. At first, the influence of the plasma process was investigated through a single fibre tensile test characterization, which did not show any strength degradation of the modified fibres. Subsequently, single fibre pull-out tests were performed to study interfacial strength with an epoxy matrix. The presence of oxygen within the polymer film mixture allowed to increase the interfacial shear strength (IFSS) by 79% compared to untreated basalt fibres, highlighting an excellent fibre/matrix interface for the micro composite. After the interfacial adhesion tests, scanning electron microscopy confirmed the strong bond between the fibre surface and the deposited silane sizing.

Investigation on pull‐out behavior and interface critical parameters of polymer fibers embedded in concrete and their correlation with particular fiber properties

AbstractA crucial problem in concrete engineering is the corrosion of steel reinforcements. Polymer fibers as alternative reinforcement material can prevent corrosion; however, high adhesion to concrete and good fiber mechanics are necessary for polymers to be considered as an alternative reinforcement. This study tested different thermoplastic polymer materials to evaluate their level of adhesion to concrete. The adhesion properties of different self‐drawn polymer fibers were analyzed by extracting the fibers from concrete using single fiber pull‐out test (SFPT). To determine the adhesion mechanism, different polymer properties were analyzed and correlated to SFPT. Strong evidence was found that the fibers mechanical properties correlate with SFPT. Roughening the fiber surface increases the SFPT results significantly. While highly polar materials can support the adhesion process, a clear correlation could not be found. This study identifies high stiffness and roughness as the crucial properties of polymer fibers used in concrete engineering. If these factors can be engineered into the fiber, polymer fibers can present an alternative to steel in concrete reinforcement.

Molecular grafting of nanoparticles onto sisal fibers - adhesion to cementitious matrices and novel functionalities

Cement materials have good compression strength, but poor tensile resistence. Aiming to improve this property, natural fibers can be applied as reinforcement. However, natural fibers have low adhesion with cement, and surface treatments are needed. In this work, sisal fibers were submitted to a cationization process followed by the grafting of two kinds of nanoparticles, Fe2O3 and MnO2. It is a usual treatment to dye fibers to the fabrics. The fibers microstructural characteristics were evaluated, before and after treatment, by using FTIR, XRD, TGA, DSC, SEM, water absorption, and tensile strength tests. The effect of the nanoparticles on the sisal fiber-cement matrix adhesion was evaluated by single fiber pull-out tests. The results show the cationization process removes part of lignin, increasing the fiber crystallinity, while the grafting of nanoparticles reduces the fiber crystalline degree, which indicates that nanoparticles grafting in the lignin spaces. The nanoparticles grafting acts as a catalyst of cellulose formation during the thermal degradation changing the initial fiber degradation profile. All treatments have caused a reduction in the absorbed water amount, become the fibers more stable dimensionally. They have improved the fiber adhesion on the cementitious matrix, being the sisal fiber grafting with MnO2 the best result.

Effect of graphene oxide on single fiber pullout behavior

The properties of fiber reinforced concrete (FRC) depends strongly on bond between fiber and cement matrix. Any change in matrix and fiber characteristics affects the bond behavior. In this study, the effect of graphene oxide on cement composite strength and bond behavior between fiber and cement matrix was investigated. The graphene oxide solution with concentration of 10 mg/ml was mixed with cement mortar at 0.05% by weight of cement. Four types of fibers viz., hooked end steel, double hooked end steel, polypropylene and glass fibers were tested by embedding in both plain cement mortar (M) and cement graphene mortar (GM). The single fiber pullout test was performed at the rates of 60 and 180 mm/min. Results were collected in form of failure mode, scanning electron microscope (SEM) images, bond-slip response, bond strength and energy absorption. SEM images showed that the failure modes depended on fiber type, matrix type and loading rate. The steel fibers showed almost no damage except for the change of hooked end shape at the fiber end. For polypropylene fiber, the fiber surface scraping was commonly observed while the debonding of coating material was observed for glass fiber. The bond strength was higher in GM mortar than that of M mortar and increased with the increase in loading rate. In the case of fiber type, steel fibers exhibited higher bond and energy absorption than both polypropylene and glass fibers.

Tailoring of polyethylene fiber surface by coating silane coupling agent for strain hardening cementitious composite

Polyethylene (PE) fiber is a common material for preparing ultra-high-strength strain hardening cementitious composites (SHCC). However, the interfacial bonding between fiber and matrix is weak due to the hydrophobic nature of the PE fiber surface and resulting in low PE fiber utilization. In this study, a method is proposed by applying silane coupling agents (SCA) coating to the fiber surface to enhance the interfacial bonding and further improve the mechanical properties of PE fiber reinforced SHCC. The KH172 is proved to be the best one in five selected SCAs for PE fiber surface modification by surface energy analysis. The coating feasibility and hydrophilicity of SCA coated PE fiber are verified by the infrared absorption test and contact angle test, respectively. The effectiveness of this approach is evaluated by means of the single fiber pullout test and uniaxial tensile test. With the SCA coating, the interfacial fractional bond between fiber and matrix is increased by 52%, and the slip hardening response is remarkable. The ultimate tensile stress and tensile strain capacity of PE fiber-reinforced SHCC are increased by 48% and 112%, respectively. Based on the micromechanical analysis, the mechanism of composite property improvement due to interfacial enhancement by fiber surface SCA coating is demonstrated, and one of the greatest advantages of this approach is the saving on the dosage of fiber.

Enhancement of para-aramid fibers by depositing poly-p-paraphenylene terephthalamide oligomer modified multi-walled carbon nanotubes

In this work, a facile and effective approach was developed to enhance para-aramid fiber (AF). Poly-p-paraphenylene terephthalamide oligomer modified amino-functionalized multiwalled carbon nanotubes (PPTA-MWNTs-x) were prepared and deposited on the surface of AF. TEM and AFM analyses proved the successful grafting of PPTA oligomers onto the surface of MWNTs-NH2. The modified MWNTs showed better dispersion stability compared with that of MWNTs-NH2. PPTA-MWNTs-x deposited on the AF surface was characterized by SEM and XRD. SEM images showed a relatively homogeneous distribution of PPTA-MWNTs-x over the aramid fibers. XRD analysis showed the crystallographic index value of AF/PPTA-MWNTs-x increased significantly as compared to that of AF, suggesting a more regular and ordered crystalline structure. The tensile test and single fiber pullout test were conducted. The results confirmed that PPTA-MWNTs-x could greatly enhance the mechanical properties of AF and improve the interfacial properties of EP/AF and PP/AF composites.

Method for Characterizing the Rate-dependent Behavior of Aramid Fibers Coated with Shear Thickening Fluids

Due to their high specific strength, specific stiffness, and toughness, aramid fabrics have been used in various applications, especially those involving bulletproofing. The bulletproof capability and mechanical properties of aramid fabrics can be enhanced at the mesoscale with shear thickening fluid (STF) coatings; however, little is known about the microscale characteristics of STF-coated aramid fabrics. This study aimed to investigate the microscale rate-dependent behavior of STF-coated single aramid fibers inside aramid yarns using a newly proposed testing method, in which a high shear rate is applied to STF-coated yarns and fibers in a single-fiber pull-out test.

Effect of sand grain size and fibre size on macro–micro interfacial bond behaviour of steel fibres and UHPC mortars

In this paper, the macro–micro interfacial bond behaviour of steel fibres embedded in ultra-high-performance concrete (UHPC) matrix was investigated using a single-fibre pull-out test with the parameters of fibre size, maximum sand grain size and fibre type. Test results showed that the maximum sand grain size (1·18 and 2·36 mm) had a slight effect on the pull-out responses while increasing the maximum sand grain size tended to change the microscopic interfacial failure mode according to observations made using scanning electron microscopy (SEM). Increasing the fibre size led to an increase in the maximum pull-out load, average bond strength and pull-out work but a decrease in the equivalent bond strength. The percentage of end hook contribution to the total pull-out energy ranged from 54·0 to 60·3%, indicating a significant mechanical anchorage contribution. A new evaluation index on the fibre–matrix interfacial bond behaviour was firstly proposed using the concept of the contribution of unit volume from the perspective of structural demands. It was found that, according to the proposed index, hooked-end fibres with a smaller diameter would be a better choice for structural applications.

A comparative study of micro-mechanical models for fiber pullout behavior of reinforced high performance concrete

The pseudo-ductile material behavior of fiber reinforced high performance concrete is mainly characterized by the fiber pullout process. Thereby, complex fiber–concrete interactions, i.e. interface debonding, concrete micro cracks, slippage, adhesion and further unknown processes, are commonly investigated in single-fiber pullout tests. The study in this contribution is based on the experimental results of Gebuhr etal., (2019) and compares three different numerical models applied to the fiber pullout test. An accurate and efficient model for fiber pullout behavior forms the basis for the prediction of the overall behavior by means of composite models or multi-scale approaches in subsequent studies.

Decoration of SiO2 and Fe3O4 Nanoparticles onto the Surface of MWCNT-Grafted Glass Fibers: A Simple Approach for the Creation of Binary Nanoparticle Hierarchical and Multifunctional Composite Interphases.

We report on a versatile method for chemically grafting multiwalled carbon nanotubes (MWCNTs) onto the surface of conventional glass fibers (GFs), as well as depositing further silica (SiO2) or superparamagnetic (SPM) magnetite (Fe3O4) nanoparticles (NPs) creating novel hierarchical reinforcements. The CNT-grafted GFs (GF-CNT) were utilized further as the support to decorate nano-sized SiO2 or Fe3O4 via electrostatic interactions, resulting finally into double hierarchy reinforcements. SiO2 NPs were first used as model nano-particulate objects to investigate the interfacial adhesion properties of binary coated GFs (denoted as GF-CNT/SiO2) in epoxy matrix via single fiber pull-out (SFPO) tests. The results indicated that the apparent interfacial shear strength (IFSS or τapp) was significantly increased compared to the GF-CNT. Fe3O4 NPs were assembled also onto CNT-grafted GFs resulting into GF-CNT/Fe3O4. The fibers exhibited a magnetic response upon being exposed to an external magnet. Scanning electron microscopy (SEM) revealed the surface morphologies of the different hierarchical fibers fabricated in this work. The interphase microstructure of GF-CNT and GF-CNT/SiO2 embedded in epoxy was investigated by transmission electron microscopy (TEM). The hybrid and hierarchical GFs are promising multifunctional reinforcements with appr. 85% increase of the IFSS as compared to typical amino-silane modified GFs. It could be envisaged that, among other purposes, GF-CNT/Fe3O4 could be potentially recyclable reinforcements, especially when embedded in thermoplastic polymer matrices.

Numerical simulation of fiber-matrix debonding in single fiber pull-out tests

The present work deals with the numerical crack simulation of fiber-matrix debonding in single fiber pull-out tests. For this purpose, two models are used: a finite element model (FE model) with the cohesive zone approach and a peridynamic model. For calibration a reference experiment is applied. In addition analytical equations are used for reference values. The influence of the model parameters and the material parameters of the cohesive zone model on the force-displacement curve is investigated. Besides the free fiber length, the critical interface strength, the critical energy release rate as well as the initial interface stiffness have a great influence on the force-displacement curve of the pull-out test. From the crack simulation it can be seen that Mode I has an influence on the crack initiation, but further crack growth after initiation is dominated by Mode II. The FE model can be calibrated in a way that the crack initiation point and the maximum force correspond to the reference experiment. The peridynamic model depicts a comparable crack formation process.