Pull-out Mechanism Research Articles

Evaluation of pull out behavior of plain and deformed steel bars in tire shred –sand mixture using large scale model

Interaction with soil strengthening is a critical problem in the construction of reinforced soil structures. It is therefore, important to examine the pullout mechanism and determine the relationship between soil and reinforcement. Most of the pullout experiments that have been performed in geotextile to date. This research was conducted to evaluate the pullout resistance of plain and deformed steel bar embedded in a mixture of tire shred sand and sand alone. Large-scale test model was designed and fabricated. Large-scale test model measurements were 1.2 m × 0.9 m × 1.5 m. Moreover, the research system was made of steel; it researched the impact of tire shreds and the diameter of deformed bars. It was inferred from the results, that pullout resistance enhanced with the introduction of tire shreds (2.267 KN/m) in the sand only (1.682 KN/m), and pullout resistance increased with an increase in normal load (1.7 to 2.0 KN/m). With deformed bars (1.032 KN/m), the pullout resistance of deformed bars was greater than for plain bars (0.868 KN/m).

Pullout tests on the connection to an existing foundation of a steel warehouse rebuilt after the 2012 Emilia (Italy) earthquakes

The tests described in this paper were aimed at evaluating the tensile capacity of the anchorages connecting an automated pallet warehouse with an existing RC foundation. The warehouse is a new steel structure erected in the place of a previous warehouse collapsed due to the Emilia earthquake, but whose foundation remained undamaged. The investigated fastening consists of 10 post-installed, bonded threaded rods with diameter (d) and embedment depth (hef) of 20 and 500 mm, respectively. Neither anchor arrangement nor embedment depth (hef > 20d) was covered by current standards for fastening design. To reproduce the in-situ actual conditions of the fastening, an unconfined test configuration was used. The maximum loads achieved were more than 3 times greater than the seismic demand for the fastening. The tests highlighted the crucial role played by the reinforcing steel which was present in the foundation. Concrete-related failure mechanisms, such as the combined pullout and concrete cone failure mechanism typical of bonded anchors, were not activated. The observed crack patterns rather suggest the onset of a flexural failure mechanism of the concrete slab. This feature is confirmed by analytical calculations showing that, at the maximum loads achieved in the tests, the top reinforcement was likely to be yielded. In six preliminary unconfined tension tests on single anchors, steel rod failure was achieved, associated with limited cracking of the concrete surface in proximity of the anchor.

Creep Mechanisms in Precracked Polypropylene and Steel Fiber–Reinforced Concrete

Recently, fiber-reinforced concrete (FRC) creep behavior has become a much addressed topic, and the main gap found in the literature is whether cracked FRC is stable in the serviceability limit state. Therefore, this work aims to investigate the creep behavior of steel and polypropylene (PP) FRC by analyzing the growth of the crack opening displacement over time in prismatic specimens subjected to sustained bending. Sustained load tests were also performed on FRC constituents and on the fiber matrix interface in order to evaluate individually their long-term response, and to understand their contribution in bending. Based on a model proposed, the rotation in a plastic hinge located in the crack area could be calculated considering the three mechanisms evaluated—fiber pullout, concrete compression, and shrinkage. It could be concluded that the fiber pullout mechanism plays a primary role in creep deformation in precracked elements subjected to sustained bending loads.

Textile-cement bond enhancement: Sprinkle some hydrophilic powder

Textile reinforced concrete (TRC) has recently gained significant attention in the field of construction. The textile is made of multifilament yarns that exhibit telescopic pull-out behavior reducing its reinforcing efficiency. The remedy lies in epoxy coating, which facilitates full utilization of all filaments within the yarn during loading. Nevertheless, the obtained interfacial bonding between the hydrophilic cementitious matrix and hydrophobic epoxy coating is rather weak. In this study, several coating strategies were employed to enhance the interface bonding by modifying the textile surface with graphene oxide or cement powder. The effect of these coatings on the pull-out behavior of a single multifilament yarn from a cementitious matrix was systematically investigated and compared to that of plain or neat epoxy-coated yarns. The pull-out mechanism was found to involve adhesion followed by friction. The best adhesion between the epoxy-coated yarn and the cementitious matrix was obtained for cement powder sprinkled over freshly coated epoxy yarn, yielding an enhancement in chemical debonding energy of 472% compared to a plain yarn. The highest slippage energy (indicating friction) was obtained for a coated epoxy surface decorated with a small amount of cement or graphene oxide particles, which provided improved mechanical anchoring.

Fractographic analysis of scarf repaired carbon/epoxy laminates submitted to tensile strength

This study presents the main fractographic aspects and failure modes involved on the fracture of scarf repaired laminates submitted to tensile test, using scanning electron microscopy (SEM) technique. A detailed analysis of the fractographic aspects was carried out correlating them to the failure modes, the local crack propagation, and the mechanical performance of the scarf joint. SEM analyses showed cohesive failure of the adhesive film associated with a strong mechanical anchoring between the adhesive and the laminate, as well as a good patch ply consolidation on the adhesive interface, showing the joining mechanism efficiency. Fiber pull-out mechanism associated with the translaminar fracture, combined with intralaminar fracture were the predominant failure modes observed in the parent laminate and in the patch repaired region, evidencing the complexity on crack growing in woven fabric composite materials. Fractographic analyses provide evidences that justify the good mechanical behavior of the repaired laminates that is attributed to high interfacial shear strength of the scarf joint and overall good quality of the repair manufacturing process.

Mechanical Properties of the Carbon Nanotube Modified Epoxy-Carbon Fiber Unidirectional Prepreg Laminates.

The effect of plasma treatment of the multi-walled carbon nanotube (MWCNT) surface on the fracture toughness of an aerospace grade epoxy resin and its unidirectional (UD) carbon fiber prepreg laminates has attracted scientific interest. A prepreg route eliminates the possible risk of carbon nanotube filtration by unidirectional carbon fibers. X-ray photoelectron spectroscopy results suggested that oxygen atom concentration at the nanotube surface was increased from 0.9% to 3.7% after plasma modification of the carbon nanotubes. A low number (up to 0.5 wt.%) of MWCNTs was added to epoxy resin and their carbon fiber prepreg laminates. Transmission electron micrographs revealed that the plasma treatment resulted in a better dispersion and distribution of MWCNTs in the epoxy resin. Plasma-treated MWCNTs resulted in a more pronounced resistance to the crack propagation of epoxy resin. During the production of the reference and nanotube-modified prepregs, a comparable prepreg quality was achieved. Neat nanotubes agglomerated strongly in the resin-rich regions of laminates lowering the interlaminar fracture toughness under mode I and mode II loading. However, plasma-treated nanotubes were found mostly as single particles in the resin-rich regions of laminates promoting higher energy dissipation during crack propagation via a CNT pull-out mechanism.

Effect of Multiwalled Carbon Nanotubes on Improvement of Fracture Toughness of Spark-Plasma-Sintered Yttria-Stabilized Zirconia Nanocomposites

Highly dense yttria-stabilized zirconia (YSZ) nano-ceramics reinforced with TC-CVD-synthesized multiwall carbon nanotubes (MWCNTs) were fabricated using spark plasma sintering at a temperature of 1350°C, the heating rate of 100 °C/min and pressure of 50MPa with a dwell time of 10 minutes. The identical parameters were utilized for fabricating composites with a varying weight ratio of YSZ and MWNCTs. The samples were characterized for their phase transformation, microstructure and elemental composition using x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The physical and mechanical properties such as density, porosity, hardness, fracture toughness and wear were also investigated. The increase in the MWCNTs concentration has resulted in the deterioration of the hardness due to CNT agglomerations. The wear resistance of the composites revealed MWNCTs enhanced wear resistance of YSZ nanocomposite by undergoing MWNCTs pull-out and crack branching mechanisms. Further indentation method and single-beam V-notch beam (SEVNB) methods were utilized to study the effect of MWCNTs on the fracture toughness of the nanocomposites. The fracture toughness of YC1 (6.58 ± 0.3 MPa m1/2) was 21% higher than the YSZ (5.21 ± 0.2 MPa m1/2) due to the toughening mechanisms attributable to crack deflection, branching and bridging of MWCNTs.

Microscale Cohesive-Friction-Based Finite Element Model for the Crack Opening Mechanism of Hooked-End Steel Fiber-Reinforced Concrete.

The entire mechanical properties of steel fiber-reinforced concrete (SFRC) are significantly dependent on the fiber–matrix interactions. In the current study, a finite element (FE) model was developed to simulate the pullout response of hooked-end SFRC employing cohesive–frictional interactions. Plain stress elements were adapted in the model to exemplify the fiber process constituents, taking into consideration the material nonlinearity of the hooked-end fiber. Additionally, a surface-to-surface contact model was used to simulate the fiber’s behavior in the pullout mechanism. The model was calibrated against experimental observations, and a modification factor model was proposed to account for the 3D phenomenalistic behavior of the pullout behavior. Realistic predictions were obtained by using this factor to predict the entire pullout-slip curves and independent results for the peak pullout load. The numerical results indicated that the increased fiber diameter would alter the mode of crack opening from fiber–matrix damage to that combined with matrix spalling, which can neutralize the sensitivity of the entire pullout response of hooked-end steel fiber to embedment depth. Additionally, the fiber–matrix bond was enhanced by increasing the fiber’s surface area, sensibly leading to a higher pullout peak load and toughness. The developed FE model was also proficient in predicting microstructural stress distribution and deformations during the crack opening of SFRC. This model could be extended to fully model a loaded SFRC composite material by the inclusion of various randomly oriented dosages of fibers in the concrete block.

Pullout behavior of lap splice connections between double-steel-plate composite walls and RC raft foundation in nuclear engineering

Considering the flexibility of construction and convenience of transportation, the lap splice connection using dowel or lapped bars between double-steel-plate composite (DSC) walls and reinforced concrete (RC) raft foundation is becoming increasingly popular in nuclear engineering. Because the anchorage of dowel or lapped bars in RC members has already been stipulated by many codes, this study mainly focus on the portion in DSC wall of this connection. Eight 1/2 scaled DSC wall – RC foundation connection specimens were tested with monotonic tensile loading to study the pullout mechanism of this connection and provide solid basis for design. The investigated variables are the locations of lapped bars and tie configuration including the spacing and diameter of tie bars. A new tie form using internal steel webs instead of tie bars is also proposed. The test results are analyzed and discussed in terms of crack pattern, load-displacement relations, strains of key components including steel faceplates, tie bars and lapped bars. Based on the experimental findings and previous research, five possible failure modes are concluded and the steel faceplates yielding is regarded as the design target. To avoid the other failure modes, design recommendations on tie bars, lapped bars and shear studs are given. Particularly, a new formula is proposed to design the tie bars to avoid splitting failure based on the nonlinear strain distribution along the length of connection. In addition, the internal steel webs are recommended as a promising alternative forms of tie bars, which has shown better mechanical performance as well as more convenient construction process.

Pull-out and Critical Embedment Length of Grouted Rebar Rock Bolts-Mechanisms When Approaching and Reaching the Ultimate Load

Rock bolts are one of the main measures used to reinforce unstable blocks in a rock mass. The embedment length of fully grouted bolts in the stable and competent rock stratum behind the unstable rock blocks is an important parameter in determining overall bolt length. It is required that the bolt section in the stable stratum must be longer than the critical embedment length to ensure the bolt will not slip when loaded. Several series of pull tests were carried out on fully grouted rebar bolts to evaluate the pull-out mechanics of the bolts. Bolt specimens with different embedment lengths and water/cement ratios were installed in either a concrete block of one cubic meter or in steel cylinders. Load displacement was recorded during testing. For some of the bolts loaded beyond the yield load, permanent plastic steel deformation was also recorded. Based on the test results, three types of failure mechanisms were identified, corresponding to three loading conditions: (1) pull-out below the yield strength of the bolt steel; (2) pull-out between the yield and ultimate loads, that is, during strain hardening of the steel; and (3) steel failure at the ultimate load. For failure mechanisms 2 and 3, it was found that the critical embedment length of the bolt included three components: an elastic deformation length, a plastic deformation length and a completely debonded length due to the formation of a failure cone at the borehole collar.

Mechanical Properties of Flax Tape-Reinforced Thermoset Composites.

Three thermoset resin systems—bio-epoxy, epoxy, and polyester-with 30 v% flax fiber reinforcement have been studied to identify the optimal fiber–resin combination in a typical composite structure. Tensile, interface and interlaminar shear strength together with flexural and impact damage tolerance were compared in this study. The results revealed that mechanical and interfacial properties were not significantly affected by the different resin systems. Microscopy studies reveal that epoxy laminates predominantly fail by fibre linear breakage, polyester laminates by fiber pull-out, and bio-epoxy laminates by a combination of the two. The higher failure strains and pull-out mechanism may explain the better impact damage tolerance of polyester composites. Flow experiments were also conducted, revealing faster impregnation and lower void content with polyester resin, followed by bio-epoxy, due to their lower viscosities. Overall, bio-epoxy resin demonstrates comparable performance to epoxy and polyester resins for use in (semi-)structural bio-composites.

Effects of HfC/PyC core-shell structure nanowires on the microstructure and mechanical properties of Hf1-xZrxC coating

To elevate the mechanical and anti-ablation properties of Hf1-xZrxC coating on C/C composites, HfC/PyC core-shell structure nanowires (HfCnw/PyC) with different PyC layer thickness were synthesized by two steps of CVD. Influences of HfCnw/PyC on the microstructure and mechanical properties of Hf1-xZrxC coating were researched. Toughening mechanism of HfCnw/PyC was also investigated. PyC layer exhibited a lamellar structure and combined well with HfCnw. After incorporating HfCnw/PyC, Hf1-xZrxC coating structure converted from columnar crystal to isometric crystal. HfCnw improved H, E, Kc and bonding strength of Hf1-xZrxC coating, which is ascribed to the nanowire pullout, debonding, bridging and crack deflection mechanism. With the PyC layer thickness increasing, H and E of the coating reduced, Kc and bonding strength of the coating increased. Because of the moderate bonding strength between HfCnw/PyC and coating matrix, lamellar structure of PyC layer and higher Kc of PyC, toughening effectiveness of the core-shell structures gradually enhanced with the PyC layer thickness increasing.

Effect of carbon nanotubes grown temperature on the fracture behavior of carbon fiber reinforced magnesium matrix composites: Interlaminar shear strength and tensile strength

The design of an interfacial structure is particularly important for load transfer in composites. In this paper, different amounts of carbon nanotubes (CNTs) were grafted onto the carbon fiber (CF) surface by adjusting grown temperature using injection chemical vapor deposition (ICVD). The prepared CF preform grafted with CNTs (CNTs-CF) were used to reinforce magnesium alloy by squeeze casting process. The microstructures were analyzed by means of optical microscope (OM) and scanning electron microscope (SEM), and the interlaminar shear strength (ILSS) and tensile strength of the composites were determined by double-notch shear test and tensile test. The results indicated that moderate ILSS was more conducive to improving the tensile properties of carbon fiber reinforced magnesium matrix (Cf/Mg) composites. Compared with Cf/Mg, the tensile strength of composite with CNTs increased by about 80%. For Cf/Mg composites grafted with CNTs, CNTs had the effects of delaying crack propagation and increasing energy consumption by the pull-out and bridging mechanism, which were the main reasons for improving the strength. The analysis of shear fracture surface showed that the crack propagation path can be optimized by adjusting the amounts of grafted CNTs. The presence of CNTs affects the stress distribution and consequently the crack initiation as well as the crack propagation.

Role of CeAl11O18 in reinforcing Al2O3/Ti composites by adding CeO2

AbstractThis work discusses the reinforcement effect of elongated CeAl11O18 phase on multifunctional Al2O3/Ti composites by adding CeO2 to inhibit interfacial reaction and strengthen interface for inducing optimized performances. For this purpose, Al2O3/Ti composite with different contents of CeO2 was fabricated and the microstructure, mechanical and electrical properties were studied. Results indicated that after CeO2 was added, elongated CeAl11O18 phase was formed within these composites. Owing to inhibited interfacial reaction between Al2O3 and Ti, Ti content was increased and compositions of composites were calculated using Rietveld method based on X‐ray diffraction patterns. Attributed to the strengthening and toughening effects of CeAl11O18 phase, 2 mol% CeO2 added composite showed the highest flexural strength and fracture toughness of 576 MPa and 5.15 MPa·m1/2, which increased by 21% and 20% compared to Al2O3/Ti composite without CeO2 addition. In this case, crack bridging by both Ti and CeAl11O18 particles was the major toughening mechanism and the additional fracture toughness caused by CeAl11O18 toughening effect reached a maximum. The role (crack bridging or particle pull‐out mechanism) of CeAl11O18 in toughening Al2O3/Ti composites depended on the aspect ratio of these elongated particles, which was directly related to CeO2 content. Because of the inhibition of interfacial reaction and the increase in Ti content, excellent electrical resistivity of composites after CeO2 addition was maintained in spite of the formation of insulated CeAl11O18 phase. All samples showed relatively low electrical resistivity of ~10−3 Ωcm.

Room and high-temperature mechanical properties of ZrB2-based composite alloyed with Ti and refractory metal Nb

ZrB2 and ZrB2-based composites have extensive applications; however, their application is hindered by their low fracture toughness at room temperature. A ZrB2/(NbTi) composite with 60 wt% ZrB2 was densified to 97.1% through hot-pressing sintering at 1600 °C under 30 MPa. The phase, microstructure, and room and high-temperature (800–1200 °C) mechanical properties of the composite were investigated. The composite mainly consisted of a (Ti,Nb,Zr) solid solution (ss), Nb-rich (Ti,Nb)B, Ti-rich (Nb,Ti)B, and ZrB2. The room-temperature fracture toughness of the composite was up to 6.9 MPa·m1/2 due to crack bridging, crack defection, and pull-out toughening mechanisms. The high temperature deformability of ZrB2/(NbTi) improved as the test temperature increased from 800 °C to 1200 °C due to the plastic rheological behaviour of the ductile (Ti,Nb,Zr)ss phase and the localised temperature rise. The room and high-temperature compressive deformation processes and failure mechanisms were discussed in detail.

Pullout Behavior of Recycled Waste Fishing Net Fibers Embedded in Cement Mortar.

In this study, recycled waste fishing net (WFN) short fibers were proposed to be used as short fiber reinforcements. The pullout resistance of WFN short fibers embedded in cement mortar was investigated by conducting fiber pullout tests. Three types of WFN short fibers and two types of commercial polypropylene (CP) fibers were investigated. To quantitatively compare the pullout resistance of WFN short fibers and CP fibers, pullout parameters, including peak pullout load (peak bond strength), peak fiber stress, slip at peak load, and pullout energy (equivalent bond strength) of the pullout specimens, were analyzed. In addition, the analysis of fiber images, captured by using a stereoscopic digital microscope, before and after pullout tests, elucidated the different mechanisms of fiber pullout corresponding to the type of fibers. The bundled structures of the WFN fibers generated mechanical interaction between fiber and matrix during fiber pullout; consequently, they produced higher bond resistance and more damage on the surface of fibers after the pullout. Therefore, the bundled WFN fibers showed comparable pullout resistance with CP fibers.

An MD study of the polymer–polymer adhesion via connector chains: some aspects of the competition between bulk dissipation in the interphase and pull-out of interface-connecting molecules

Atomistic simulation approach was used to analyze the separation behavior of polymer–polymer interfaces. This was achieved using coarse-grained molecular dynamics (MD) simulations of some connector chains embedded in two adjacent polymer entangled melts. To separate the contribution of polymer molecules pull-out from the micro-bulk dissipation in the viscoplastic interphase, surrounding the surface of separation, two kinds of simulations were considered: (a), simulations of the pull-out mechanism, where the extraction of the connectors out of the melts takes place through a complex mechanism of forced reptation only; (b), simulations of the full debonding process where the micro-bulk dissipation contributes to the overall crack opening and can skew the description of the interface separation by pure connector molecules pull-out. From the MD simulation results, some relevant parameters (e.g., a cohesive strength or an adhesion energy) of the separation behavior of the interface being modelized were extracted. In addition, one was able to separate quantitatively connector pull-out from micro-bulk dissipation (in the surrounding bulk region of the interface). This means that we were able to emphasize: (i) how the straining divides into interface opening and bulk deformation and (ii), how the macroscopic opening rate diffuses throughout the melts and the interface opening rate, since the mechanical behavior of polymers is strain-rate dependent. A comparison between two kinds of MD simulations, where a system is stretched along one direction, is shortly discussed as a necessary prelude to future investigations.

Chromium carbide micro-whiskers: Preparation and strengthening effects in extreme conditions with experiments and molecular dynamics simulations

The rod-like Cr3C2 micro-whiskers were successfully synthesized from chromic oxide and blank carbon as reaction sources and halide salts and nickel as catalysts by using a carbothermal reduction method assisted with high energy milling and liquid phase catalysis. The optimal preparing condition was to calcine the mixture of Cr2O3:C:NaCl:KCl:Ni ​= ​1:8:0.4:0.4:0.3 (mole ratio) at 1300 ​°C for 2h without other post-treatments. The growth of Cr3C2 micro-whiskers was based on S-L-S mechanism that the brief reaction process of Cr2O3+C→Cr7C3→Cr3C2 occurred in molten catalyst liquids. Besides, the deficiency of carbon led to the generation of Cr7C3 particles, and its excess could induce the formation of coral clusters. The obtained Cr3C2 whiskers were not oxidized in air atmosphere at temperature lower than 1000 ​°C, and a thin Cr2O3 coating which prevented further oxidation gradually formed on the outside of whiskers from 1000 to 1200 ​°C. In addition, the bonding strength of heat resistant adhesive was increased by 16–28% from 500 to 1500 ​°C after adding Cr3C2 whiskers, which was mainly attributed to the pull-out mechanism, whereas the added carbon fibers were completely oxidized and left many sags in adhesive structure, thus severely decreasing the strength. Molecular dynamics simulations with the tersoff forcefield were carried out to facilitate fundamental understanding of the micro-whiskers growing in preferred directions. Combined with theoretical and experimental studies, this work significantly expands the design space and motivates future efforts in exploring the emerging applications of the mechanical properties of Cr3C2 whiskers in severe occasions.

Enhanced toughness and strength of boron carbide ceramics with reduced graphene oxide fabricated by hot pressing

Boron carbide (B4C) hybrids with different contents of graphene oxide (GO) were prepared by a heterogeneous co-precipitation method using cetyltrimethyl ammonium bromide (CTAB) as the cationic surfactant. The as-obtained mixtures were further hot-pressed at 1950 °C for 60 min under 30 MPa, by which B4C–reduced GO (rGO) composites were fabricated. It was found that the addition of only 0.5 wt% rGO could alter the predominance of trans-granular fracture in monolithic B4C ceramic material to mixed trans-granular and inter-granular modes in B4C–rGO composites. The flexural strength and fracture toughness of the B4C–2 wt% rGO were increased by 31% (from 350 to 455 MPa) and 83% (from 3.20 to 5.85 MPa·m1/2), respectively, compared with those of pure B4C. The improved mechanical properties are attributed to the mechanisms of pull-out and bridging of rGO and crack deflection, as evidenced by microstructural observations. The energy dissipation in the present B4C–rGO composites was further verified using two micromechanical models.

Mapping the residual strain of carbon nanotubes in DWCNT/epoxy nanocomposites after tensile load using Raman microscopy

The toughness mechanisms of epoxy-based nanocomposites reinforced by double-walled carbon nanotubes (DWCNT) are investigated using Raman microscopy mapping. Maps of the residual strain of the carbon nanotubes were generated for the fracture surfaces obtained after tensile loading. By imaging through different microscope objectives, Raman mapping allowed to determine the distribution of the nanotube residual strain, indicating tension or compression, in different sampling volumes near the fracture surface. The G′ band with its high sensitivity to nanotube diameter variations, and not overlapping with the characteristic epoxy Raman lines, was selected for the mapping. Results provide a quantitative analysis of the nanotube bridging and pull-out mechanisms identified by scanning electron microscopy (SEM) inspection of the fracture surfaces.