Pull-out Process Research Articles

Pull-Out of Pristine and Functionalized Carbon Nanotubes from Cement: A Molecular Modelling Study

Carbon nanotubes (CNTs) are widely used as reinforcements in cement-based composites. The improvement in the mechanical properties of the resulting materials depends on the characteristics of the interface formed between CNTs and the cement matrix. The experimental characterization of the interfacial properties of these composites is still limited and hard to achieve with currently available technologies. In this work, molecular dynamics and molecular mechanics pull-out simulations of pristine and functionalized CNTs, taken from a tobermorite crystal, were carried out to study interfacial shear strength (ISS) from an atomic perspective. ISS was calculated from the potential energy of the systems. The effects of the CNT diameter and the degree of functionalization on the pull-out process were analyzed according to the ISS and non-bonded energy results. The influence of H-bonding and electrostatic interactions between the CNT and the matrix were also studied. The results show that ISS decreases with increasing CNT radius for pristine CNTs and depends upon the number of H-bonds for functionalized CNTs. ISS values are positively correlated to Enon-bonded energy, which is related to the number of carboxyl groups on the CNT surface. A high degree of functionalization increases both the number of H-bonds and the number of Ca2+-O interactions between the CNT and the tobermorite surface. This results in a stronger interfacial interaction and, therefore, an elevated ISS value.

Characterizing and predicting the relationship between translaminar fracture toughness and pull-out length distributions under distinct temperatures.

The translaminar fracture toughness reflects the damage tolerance of a fibre-reinforced composite under longitudinal tension, which often governs the final failure of structures. One of the main energy-dissipation mechanisms that contributes to the translaminar toughness of composites is the fibre pull-out process. The present study aims to quantify and model the statistical distribution of fibre pull-out lengths formed on the translaminar fracture surface of composites, for the first time in the literature; this is done under different temperatures, so that the relationship between pull-out length distributions, micromechanical properties and the translaminar fracture toughness can be established. The fracture surfaces of cross-ply compact tension specimens tested under three different temperatures have been scanned through X-ray computed tomography to quantify the extent of fibre pull-out on the fracture surfaces; the distribution of pull-out lengths showed alarger average and larger variability with an increase in temperature, which also lead to an increase in translaminar fracture toughness. A similar trend has been captured by the proposed analytical model, which predicts the pull-out length distribution based on the analysis of quasi-fractal idealizations of the fracture surface, yielding an overall accuracy of more than 85%. This article is part of the theme issue 'Ageing and durability of composite materials'.

Kevlar 49 yarn pull-out test and numerical analysis

The yarn pull-out tests are of great importance to study the frictional energy dissipation mechanism between yarns and the overall deformation behavior of fabrics. In this study, the stress transfer mechanism of the yarn pull-out process of Kevlar 49 woven fabric is investigated by the digital image correlation (DIC) marker method, considering both the warp waves and transverse preload variables, and have analyzed the shear deformation behavior of textile by the change history of fabric shear angle. The results show that the peak load of the yarn pull-out process increases nonlinearly with the increase of the transverse preload force and the number of warp waves, the maximum pull-out displacement of the yarn is sensitive to the preload force only and is largely independent of warp waves. Finally, the finite element (FE) calculation of the single yarn pull-out process was carried out by using the mixed element method with the restart analysis technique, and the numerical simulation results achieved a good match with the peak load of the experimental data.

Mixed mode fracture of geometrically similar FRUHPC notched beams with the localizing gradient damage model

A localizing gradient damage model is proposed in this paper to capture the mechanisms underlying the fracture of fiber reinforced ultra-high performance concrete (FRUHPC): (i) an initially expanding damage process zone due to the formation of numerous fine cracks, each widening up slowly due to the fiber bridging effect; (ii) a gradual formation of a localized macroscopic crack, when at least one of the fine cracks has opened up sufficiently to trigger a fiber pull-out process. Restricting to tensile-shear dominated failures in monotonic loadings, the simplest form of damage model is adopted, where an isotropic damage variable is driven by a strain-like variable. The performance of the proposed localizing gradient damage model is first demonstrated by comparing the damage evolution process against the corresponding intensity of acoustic emission (AE) events obtained experimentally, for a notched FRUHPC beam in three-point bending. It was shown that the model is able to reproduce the slightly widening fracture process zone (FPZ) during strain hardening, as well as the eventual localized damage process zone during strain softening. Specifically, during strain hardening, the proposed localizing gradient damage model predicts an energy dissipation bandwidth that compares well with the AE energy maps, together with a consistent propagation of crack front. With a correct description of the underlying damage mechanisms, the predictive capability of the proposed model is next demonstrated through the mixed mode fracture of a series of geometrically similar FRUHPC notched beams. Since the specimens are of the same casting batch as those in the three-point bending test, the numerical material parameters calibrated earlier are adopted directly for the mixed mode fracture tests. Without any further undue calibrations, the localizing gradient damage model is shown to predict well the structure responses and size effect of the series of geometrically similar FRUHPC notched beams in mixed mode fracture.

RETRACTED: Evaluation of pseudo-strain hardening behavior of hybrid fiber reinforced ultra-high performance concrete containing coarse aggregates by using micromechanical principles

In this study, pseudo-strain hardening (PSH) behavior of hybrid fiber reinforced ultra-high performance concrete containing coarse aggregates (UHPC-CA) and the synergistic effect of hybrid fibers were investigated. First, three types of fiber hybrid samples, namely, 2% short steel fiber (SSF2), 1.5% short steel fiber + 0.5% long steel fiber (SSF1.5 + LSF0.5) and 1.5% short steel fiber + 0.5% PE fiber (SSF1.5 + PE0.5), were designed and tested for their tensile properties. The results showed that the tensile strength and peak tensile strain of SSF1.5 + LSF0.5 were 19.9% and 183.3% higher than the corresponding values of SSF2, respectively. The tensile strength of SSF1.5 + PE0.5 was 3.6% lower than that of SSF2, but its ductility was 4.5 times higher than that of SSF2. Next, three-point bending tests on notched beams and single-crack tensile tests were conducted to evaluate the PSH behavior of hybrid fiber reinforced UHPC-CA by using micromechanical principles. The results showed that all groups met the strength and energy criteria with PSH index values of 5.86, 11.28, and 14.66 for SSF2, SSF1.5 + LSF0.5, and SSF1.5 + PE0.5, respectively. The higher the PSH index of the specimens, the greater the tensile ductility, which is consistent with the tensile test results. These results suggest that it is reasonable to evaluate the PSH behavior of hybrid fiber reinforced UHPC-CA by using micromechanical principles. Third, mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) analyses were performed to clarify the potential micromechanical properties of the hybrid fibers in improving the tensile properties of UHPC-CA. The MIP results showed that the total porosity was closely related to the initial cracking strength of the matrix; The smaller the total porosity, the higher the initial cracking strength of the matrix. The SEM results showed the characteristics of the PE fibers and the matrix. The bond strength between the PE fibers and the matrix was found to be high, leading to various damage patterns during the pull-out process of the PE fibers. Moreover, the combination of PE fibers and steel fibers showed a good synergistic effect, which explains the reason for the significant improvement in the ductility of the specimens when 1.5% steel fibers and 0.5% PE fibers were mixed.

The effect of cumulative damage on the ballistic performance of plain weaves

This paper investigates the effect of cumulative damage on the energy absorption capability of plain-woven fabric systems. The cumulative damage was generated by creating pre-damaged holes, aiming to simulate the post-impacted sample targets. Ballistic impact tests were performed to characterize the ballistic protection of the pre-damaged and undamaged systems, and FE simulation was used to study the underlying mechanisms of energy dissipation. The results showed that the pre-damaged systems appear to be less protective than the undamaged system in most cases. The ballistic performance of the fabrics is more sensitive to the pre-damaged holes that are located on the primary yarns than that on the secondary yarns. When the primary yarns are broken, the holes are detrimental to the evolution of the transverse deflection but are beneficial in increasing the amount of energy dissipated by frictional sliding during the yarn pull-out process. Angle-plying the constituent layers of multi-ply panels is found to be useful in minimizing property degradation. It was also determined that the angle-plied systems absorb less energy than the aligned system regardless of the existence of the pre-damaged holes.

Interfacial bond characteristics of polypropylene fiber in steel/polypropylene blended fiber reinforced cementitious composite

This paper investigates for the first time the interfacial bond characteristics of polypropylene fiber in blended hybrid steel-polypropylene fiber reinforced concrete, which contributes to improving the polypropylene fiber utilization efficiency and then promoting the overall performance of the composite. Single-sided fiber pullout tests were performed on 21 groups of macro polypropylene fiber pullout specimens. The complete fiber pullout load-slip responses were captured, and the fiber interfacial properties were analyzed for different hybrid fiber dosage, fiber embedded length and matrix length. Results show that partial fiber rupture with a non-zero residual pullout load is the dominant failure mode. Increasingly severe abrasion at the PF surface is observed during the pullout process, resulting in a strain hardening behavior after a bent-over point is reached in a typical pullout load-slip curve. An appropriate polypropylene fiber addition in the concrete matrix can effectively improve the maximum pullout load, residual pullout load, the pullout energy, and the contribution of polypropylene fibers is more critical than that of steel fibers. Besides, the reinforcement of polypropylene fibers on the toughness and cracks resistance is proven to be more pronounced in plain concrete than that in high-strength concrete. Finally, theoretical analyses were performed on the polypropylene fiber direct pullout problem and the frictional bond stress vs. slippage relation is obtained with hybrid fiber content taken into account. The research provides a base for an optimized hybrid fiber reinforced concrete with high fiber efficiency and less cement consumption.

Load transfer mechanism at the calcium silicate hydrate/carbon nanotubes interface changed by carbon nanotubes surface modification investigated from atomic simulation

The surface modification of carbon nanotubes (CNT) can further improve the ductility of the cement/CNT composites. Atomic simulations were employed to decode the alteration of the interfacial load transfer mechanism by PVA addition, non-covalent bond modification of CNTs. The results of the pull-out simulations show that PVA significantly enhances the load transfer at the organic–inorganic interface. Interfacial load transfer depends on effective interfacial interactions, which in turn depend on interfacial chemical bonding. The interfacial region is more complexly reorganized by PVA, accompanied by the renewal of interfacial bonding. The addition of PVA could add new ion pairs (OCNT-Ca-OPVA-Ca-OCSH and HCNT-OPVA-Ca-OCSH) and lots of hydrogen bonds without detriment to the original linkage (OCNT-Ca-OCSH), in turn forming a strong network of chemical bonds intertwined with direct and indirect connections. Due to the intertwined network, the load transfer at the interface is partitioned and transferred in multiple paths, contributing to an increased interfacial strength. During the pull-out process, the load distribution between CNT, PVA and CSH evolves constantly, which is beneficial to damage retardation of the cement/CNT composites.

Structure insight into the interfacial effect of fiber-low density polyethylene composites by the combination of experiment and finite element simulation

Good interfacial adhesion is one of the key factors to ensure the excellent performance of composite materials. However, the interfacial effect of fiber-polymer composites remain incompletely understood. Herein, a self-designed mold was used to prepare UHMWPE (Ultra-high molecular weight polyethylene)-fiber/LDPE (Low density polyethylene)-matrix (LU) and PET (polyethylene terephthalate)-fiber/LDPE-matrix composite materials (LP). The interfacial bonding effect was explored by the interfacial shear strength (IFSS), and the interfacial structure and failure mechanism during the fiber pull-out process were analyzed by the combination of experiment and finite element simulation. The results show that the IFSS of LU (3.59 MPa) is 3 times greater than that of LP (1.18 MPa), giving rise to a higher tensile strength. Moreover, Raman mapping and DSC results suggest that UHMWPE-fiber act as nucleation sites to improve the orientation and crystallization of LDPE molecular chains, generating an obvious interfacial transition zone between UHMWPE-fiber and LDPE, while no obvious interfacial transition zone is found between PET-fiber and LDPE. These findings will be expected to give some guidelines for the design and manufacturing high-performance fiber-polymer composites.

Modeling of filament level plain woven Kevlar 49 fabric for accurate prediction of yarn pull-out behavior

This paper proposes two numerical and one analytical yarn pull-out models to investigate pull-out behavior and its energy transfer mechanism. First, the microgeometry of a two-dimensional woven fabric is generated with 112 fibers per yarn mesh implementing the digital element approach. In this approach, one yarn consists of a bundle of fibers and one fiber is made of a chain of rod elements. As such, the realistic yarn surface and yarn cross-section shapes are derived from fiber alignment. Meanwhile, a yarn-level fabric model with constant yarn cross-sectional shape is built on commercial textile modeling software TexGen. Then, both numerical and analytical yarn pull-out models are proposed to investigate the fabric performance. Results show the pull-out force and transferred energy of the digital element approach model is higher than the other two models and closer to the experimental data. This indicates that the realistic yarn surface introduces more contact points and the period of the pull-out force is determined by material properties rather than detailed fabric geometric configuration. Second, a detailed parametric study is carried out with a broad range of friction coefficient and tensile preload. The simulation results find out, when the tensile pre-load is between 50 N and 300 N, the transverse force drops immediately after the pull-out process begins. When the tensile pre-load reaches 400 N, the transverse force increases instead. This phenomenon indicates that the transverse force is sensitive to tensile pre-load and can be used as an indicator to detect residual stress in fabric.

Modelling of bond behavior of deformed bar embedded in concrete after heating to high temperatures: A mesoscale study

A three-dimensional (3D) mesoscale model was established in the present work with considering the heterogeneity of concrete materials and the surface characteristics of deformed steel bars. In the simulation, the interaction between deformed rebar and concrete is taken into account, including mechanical interlocking and frictional resistance. The sequentially coupled thermal-stress analysis procedure was employed to model the process of pull-out of deformed steel bar after cooling from high temperature. Comparison with the results of the pull-out test illustrates the correctness of the mesoscale simulation results. Then, the influence of aggregate types and elevated temperatures on the bond properties were discussed. Considering the simulation results, a calculation formula for the bond stress-slip curve after cooling from high temperature was proposed based on the recommendations in the existing codes for the cases of room temperature. The results show that the reduction of peak bond strength is directly bound up with the heat transfer and mechanical performance of the aggregates. When the deformed rebar is pulled out under the displacement load, the development process of concrete cracks with different aggregates is obviously different. Comparing the results obtained by the calculation formula proposed in the present work with the numerical simulation results, one can notice the good consistency between them, which means the effectiveness of the proposed formula.

Steel fibers from waste tires as reinforcements in concrete: Analysis of bond mechanism

This paper mainly studies the bond performance between a single recycled steel fiber and concrete, to attain the overall bond strength of recycled steel fibers reinforced concrete. By measuring the load displacement curve of the recycled steel fibers under pullout, we ascertain the maximum bonding stresses of the recycled steel fibers with different depths, angles and shapes while undergoing interfacial pull-out. At the same time, through observations and calculations, the failure phenomena of three stages and the changing trend of bond stresses of recycled steel fibers along the anchorage section are obtained. It is evidenced that the recycled steel fibers reinforced concrete has good interfacial bonding performance. Numerical simulation results of fiber pull-out process via the finite elements provide conditions for the analysis of bond strength of recycled steel fibers reinforced concrete cubes under different circumstances.

Numerical modelling of steel fibers pull-out from cementitious matrixes

The improvements of cementitious matrixes due to fibers addition are directly related to fibers’ pull-out mechanism and hence the proper simulation of this phenomenon is essential for the numerical simulation of fiber reinforced cementitious composites behavior.A steel fiber pull-out model is proposed in this paper. The model takes into account the matrix properties, the fibers shape, properties and orientation and uses parameters that can be obtained from pull-out tests of straight fibers. The paper includes calibration with experimental results, application examples and comparison with other experimental results that show the ability of the model to reproduce steel fibers pull-out behavior from different types of concrete matrix. The variation of the model parameters with concrete type and strength and fiber type and geometry is also included. It is shown that the model is able to describe not only fibers pull-out process but also matrix and fibers failure for different fiber geometries and orientations.

Bond enhancement for BFRP bar in concrete by using a resin-filled FRP tube anchorage

A novel additional anchorage system made out of epoxy resin-filled FRP tubes was proposed for FRP bars in concrete. A total of 33 specimens were carried out via the direct pull-out test to investigate the effectiveness of the developed anchorage in enhancing the bond between BFRP bars and concrete. The investigated variables consisted of the bond length of FRP bar, anchorage length, and the anchorage spacing. Due to the improved bond transfer mechanism for BFRP bar in concrete by using the proposed anchorage, the bond indexes, including the initial bond stiffness, ultimate bond strength, and the residual bond strength, were found to have the maximum improvements of 165.1%, 39.7%, and 162.9%, respectively. In addition, two current bond stress-slip constitutive models for unanchored FRP bars were modified to simulate the pull-out process of the anchored BFRP bars with good accuracy.

Ultra-High-Performance Concrete Crack Propagation Based on Fiber Random Distribution Model

The tensile strength, crack behavior, and strain-hardening properties of ultra-high-performance concrete (UHPC) are mainly affected by the steel fibers distributed in the matrix. In this study, a mesoscopic model of UHPC was established using a numerical simulation method to study the effects of steel fibers on the crack propagation and tensile properties of UHPC. First, the exponential cohesive model was used to simulate the pull-out of a fiber embedded in a UHPC matrix. Then, the distribution and variation of the interfacial shear stress during the fiber pull-out process were obtained, and the UHPC fiber-pull-out load–displacement curve was obtained. Using the Monte Carlo method, a meso-scale finite element model of UHPC was established by modeling randomly distributed steel fibers in the UHPC matrix. After verification, the model was used to study the effects of fiber characteristics, interface strength, and matrix strength on the crack propagation path and tensile properties of UHPC. The results showed that the exponential cohesive constitutive model with a softening coefficient of –1 can effectively characterize the mechanical behavior of the interface between the steel fibers and matrix in UHPC. Affected by the random distribution of fibers, the main propagation mode of cracks in UHPC was that the cracks bypassed the fiber-dense area and extended to the fiber-sparse area, and the crack propagation path was mainly affected by the fiber distribution. The distribution of fibers significantly affected the tensile strength and peak strain of UHPC. When the fiber inclination angle was in the range of 15°–30°, the comprehensive tensile properties of UHPC were the best. With increasing fiber volume fraction and fiber length, UHPC gradually began to exhibit multi-cracking and strain hardening (when the fiber volume fraction was more than 2.5% or fiber length was more than 15.5 mm). The interface strength and matrix strength had little effect on the crack propagation path, but had significant effects on the tensile strength and toughness of the UHPC. The higher the strength of the UHPC matrix, the more fully the anti-cracking effect of the steel fiber that can be achieved.

Mechanical properties and microstructure of spark plasma sintered Al2O3-SiCw-Si3N4 composite ceramic tool materials

In this study, Al2O3-SiCw-Si3N4-based ceramic tool materials were fabricated by spark plasma sintering technique. The impact of sintering temperature on mechanical properties and local microstructure was thoroughly investigated. Moreover, detailed comparison was performed between spark plasma sintering and early hot-pressing sintering methods. Evolution law and mechanism of high-temperature mechanical properties were also investigated. Composites with higher density and enhanced mechanical properties were obtained by spark plasma sintering at temperature of 1500 °C, which was 100 °C lower than hot-pressing temperature. The cross-interlocking structure of generated β-Sialon particles and silicon carbide (SiC) whiskers could effectively improve mechanical properties of material configuration. Moreover, Vickers hardness and fracture toughness of ceramic composites decreased by elevating temperature (20–1000 °C); however, following percentages at 1000 °C were still maintained with respect to values at room temperature, i.e., 67 and 51%. This effect is attributed to existence of Sialon phase, healing of partial cracks, as well as manifestation of whisker pullout process. As a result, decreased attenuation rate under high-temperature conditions of respective mechanical properties was observed. Our approach provides useful insights into potential fabrication of novel material configurations with relatively low thermal budget and enhanced mechanical performance.

Pullout resistance of geogrid and steel reinforcement embedded in lightweight cellular concrete backfill

Lightweight Cellular Concrete (LCC) has been increasingly used as backfill material for retaining walls, ground improvement, and pavements due to its low self-weight, quick installation, and high compressive strength as compared with soils. This paper presents a series of pullout tests performed in the laboratory to investigate the pullout resistance of geogrid (extensible reinforcement) and steel strip (inextensible reinforcement) embedded in LCC. Pullout displacements and pullout forces were monitored using displacement transducers (DT) and a load cell during the pullout process. This study investigated the effects of age, normal stress, fly ash, the presence of a cold joint, and re-pullout on the pullout resistance and calculated the pullout resistance factors F* for geogrid and steel strip embedded in LCC. Test results show that for the geogrid embedded in LCC, the maximum pullout force increased as the normal stress increased. For the steel strip embedded in LCC, the maximum pullout force was independent of the normal stress and increased as the age and the cement to fly ash ratio increased. Test results also show that the presence of a cold joint did not reduce the pullout resistance, while the re-pullout test had lower pullout resistance compared to the original pullout test for the same specimen. The pullout resistance factors F* for steel strips were greater than those for geogrids and these factors decreased as the normal stress increased.

Influence of Corrosion on Anchoring Bond Behavior of Jointed Rock Mass

In order to investigate the effect of corrosion on bond behavior of anchored jointed rock masses, based on the migration law of pitting corrosion obtained by laboratory tests and the concept of strength reduction, the anchoring pull-out test of jointed rock mass was simulated by using Discrete Element Method (DEM). The results show that for uncorroded rock specimens, due to the existence of joints, there are significant differences for granular displacement and bond failure on both sides of the rock bolt during pull-out process, and phenomenon of eccentric tension also appears; for corroded rock specimens, the reduction rate of bond strength and slip ratio of single and double joint specimens decrease linearly with the increase of pitting corrosion rate. The decreasing rate gets fast with the increase of joints, but it is not a multiple relationship. Compared with the test with uncorroded specimens, when the pitting corrosion rate is 15%, the maximum pull-out load of the single-joint specimen with joint angel of 45° is reduced by 1 7.8%, and the slip value is reduced by 14.6%; the maximum pull-out load of the double-joint specimen is reduced by 31.9%, and the slip value is reduced by 24.9%. The numerical test results are in good agreement with the laboratory test results. The research results can provide reference for the selection and optimization of the design parameters of anchoring support structure of jointed rock mass under the erosion environment in the future.

Study on shear behavior and damage constitutive model of tendon-grout interface

Ground anchors are responsible for maintaining the stability and integrity as well as controlling the deformation, displacement, and crack development in the surrounding ground. Tendon-grout (T-G) interface, which is the weak part of anchorage structures, affects the anchorage performance of the ground anchor during load-transfer processes. To investigate the shear mechanical properties of the tendon-grout interface, a series of pull-out tests were conducted on anchorage element under constant radial stiffness boundary conditions. Moreover, the equivalent calculation for the constant radial stiffness boundary condition was analyzed. The mechanical performance of cement grouts cured for different periods was investigated, including the compressive strength and shear strength. Additionally, the failure modes of tendon-grout systems and effects of different tendon quantity, spacing and offset, were assessed. The failure evolution mechanism of the interface during the pull-out process was clarified. In addition, the microscopic structural characteristics of the tendon-grout interface were investigated. Two interfacial shear stress-displacement relationships were developed based on the interface failure modes, i.e., stress attenuation type and ideal elasto-plastic type. According to the test results, the peak strength of the interface increased with the tendon spacing, while the residual strength decreased. The effect of the tendon offset on the shear stiffness and strength of the interface is significant. A damage constitutive model was proposed to model the shear behavior of the tendon-grout interface. The proposed model verified by comparing with the test results and further adopted in pull-out test simulation. The conclusions will provide a better understanding on the shear behaviors of tendon-grout interface, and facilitate the development of a general evaluation method for ground anchor systems.

Microscopic Bearing Behavior of Horizontally Loaded Vertical Plate Anchors in Sandy Soil

Vertical plate anchors are an effective technique to enhance the stability of various structures, such as retaining walls and sheet piles. More research has been devoted to their bearing capacity and macroscopically affecting parameters, while less research can be found on their microscopic bearing behavior. In this paper, the microscopic bearing behavior of vertical plate anchors subjected to a horizontal pullout load in sandy soil was investigated with the particle flow code (PFC) based on the model test results. Results show that the larger‐sized anchor plates withstand greater soil pressure and affect a broader range of soil during the pullout process. The soil not behind the anchor plate is pressed, and the pressure in front of the anchor plate increases with increasing size. The soil close to the plate anchor suffers larger pressure while the soil far away from the plate anchor is less affected, and the soil is redistributed to a more stable state during the pullout process of the plate anchor. The particles with a long axis distributed in the horizontal direction are the most stable, while those with a long axis distributed in the vertical direction are the most unstable.