Simulation of single fiber pullout response with account of fiber morphology

A finite element model was developed at the single fiber length scale to predict the quasi-static pullout response of individual fibers from cementitious composites. The model accounts for energy dissipation through granular flow of the interfacial transition zone (ITZ) and matrix, plastic work in the fiber, and frictional dissipation at the fiber–ITZ interface. The considered fiber morphology was a triangular cross section that had been uniformly twisted along the fiber length. The model was calibrated to published experimental data for fiber pitches of 12.7 and 38.1 mm/revolution pulled from cement mortar with a 44-MPa unconfined compressive strength. The model was used to investigate slip-hardening behavior, tunneling of the cement mortar, in situ pullout behavior of helically twisted fibers at a crack plane, and provide an alternate explanation for the pullout response of twisted fibers from a 84-MPa unconfined compressive strength matrix containing silica fume. Calculations show that twisted fibers can provide up to 5 times the peak pullout force and 10 times the total work compared with straight fibers and infer work-hardening behavior during fiber pullout. The findings indicate that the tailoring of fiber morphology and control of constituent properties are important avenues for achieving significant improvements in the performance of fiber-reinforced cementitious composites.     

Effect of shrinkage reducing agent on pullout resistance of high-strength steel fibers embedded in ultra-high-performance concrete

The interfacial bond strength of long high-strength steel fibers embedded in ultra-high-performance concrete (UHPC) reinforced with short steel microfibers was investigated by conducting single-fiber pullout tests. In particular, the influence of the addition of a shrinkage-reducing to a UHPC matrix on the pullout resistance of high-strength steel fibers was investigated. The addition of a shrinkage-reducing agent produced a noticeable reduction in the fiber pullout resistance owing to the lower matrix shrinkage, although the reduction of pullout resistance differed according to the type of fiber. Long smooth and twisted steel fibers were highly sensitive to the addition of the shrinkage-reducing agent whereas hooked fibers were not. Among the various high-strength steel fibers tested, twisted steel macrofibers showed the highest interfacial bond resistance, although twisted fibers embedded in UHPC showed slip softening pullout behavior rather than the typical slip hardening behavior observed in mortar.     

Effect of loading rates on pullout behavior of high strength steel fibers embedded in ultra-high performance concrete

In this paper single fiber pull-out performance of high strength steel fibers embedded in ultra-high performance concrete (UHPC) is investigated. The research emphasis is placed on the experimental performance at various pullout rates to better understand the dynamic tensile behavior of ultra-high performance fiber reinforced concrete (UHP-FRC). Based on the knowledge that crack formation is strain rate sensitive, it is hypothesized that the formation of micro-splitting cracks and the damage of cement-based matrix in the fiber tunnel are mainly attributing to the rate sensitivity. Hereby, different pull-out mechanisms of straight and mechanically bonded fibers will be examined more closely. The experimental investigation considers four types of high strength steel fibers as follows: straight smooth brass-coated with a diameter of 0.2 mm and 0.38 mm, half end hooked with a diameter of 0.38 mm and twisted fibers with an equivalent diameter of 0.3 mm. Four different pull out loading rates were applied ranging from 0.025 mm/s to 25 mm/s. The loading rate effects on maximum fiber tensile stress, use of material, pullout energy, equivalent bond strength, and average bond strength are characterized and analyzed. The test results indicate that half-hooked fibers exhibit highest loading rate sensitivity of all fibers used in this research, which might be attributed to potential matrix split cracking. Furthermore, the effect of fiber embedment angles on the loading rate sensitivity of fiber pullout behavior is investigated. Three fiber embedment angles, 0°, 20°, and 45°, are considered. The results reveal that there is a correlation between fiber embedment angle and loading rate sensitivity of fiber pullout behavior.     

A pullout model for inclined carbon nanotube

The mechanical properties of carbon nanotubes (CNTs) reinforced composites would mainly depend on the pullout behavior of carbon nanotubes which are randomly distributed in matrix. In this paper, an analytical pullout model is developed for an inclined CNT embedded into matrix to study the mechanisms for improving mechanical properties of inclined CNTs reinforced composites. In this model, by employing the assumptions of constant compression stress as well as the punch strength of matrix and a perfect plastic matrix near exit point, the maximum pullout load can be predicted analytically and the entire pullout process can be characterized. Moreover, by extending the definition for inclination angle this model can be fit to more complicated loading situations. Due to all the derivations are based on assumption of continuum mechanics, this model can be used for various inclined fibers besides CNTs.     

Bio-inspired tapered fibers for composites with superior toughness

The toughness of fiber-reinforced composites largely relies on crack bridging. More specifically, intact fibers left behind the tip of a propagating crack are progressively pulled out of the matrix, dissipating energy which translates into toughness. While short fibers are traditionally straight, recent work has showed that they can be shaped to increase the pullout strength, but not necessarily the energy to pullout. In this work we have modeled, fabricated and tested short fibers with tapered ends inspired from a high-performance natural material: nacre from mollusc shells. The main idea was to duplicate a key mechanism where a slight waviness of the inclusion can generate strain hardening and energy dissipation when the inclusion is pulled out. We have incorporated a similar feature to short fibers, in the form of tapered ends with well defined opening angles. We performed pullout tests on tapered steel fibers in epoxy matrices, which showed that the pullout of tapered fiber dissipates up to 27 times more energy than straight fibers. The experimental results also indicated the existence of an optimum taper angle to maximize work of pullout while preventing the brittle fracture of the matrix. An analytical model was developed to capture the pullout mechanism and the interaction between fiber and matrix. The analytical model can guide the design of tapered fibers by providing predictions on the influence of different parameters.     

Tow pullout behavior of polymer-coated Kevlar fabric

Tow pullout response of polymer-coated woven Kevlar fabric is investigated experimentally. Various material microstructures are created by applying heat and pressure to polymer-coated fabric to change polymer surface morphology. The pullout behavior is studied by conducting tow pullout tests using a pullout fixture. Single tow is pulled from the fabric loaded with tension in the direction transverse to that of pull. The tests are performed at various loading rates to investigate rate sensitivity of pullout load and energy. Load versus displacement data is obtained, from which the peak loads and pullout energies are computed. The results are compared with baseline neat fabric as well as baseline fabric with polymer coating. The results of the investigation show that for the Kevlar fabric under consideration, the neat fabric shows no loading rate dependence. On the other hand, the polymer-coated fabric shows loading rate dependence. Moreover, the heat-compressing of the fabric seems to have significant influence on tow pullout behavior. The results of pullout tests show that by simply compressing the fabric in the presence of heat may influence the interactions at tow cross-over regions to results in higher pullout energy.     

Mechanical and statistical analyses of fracture of whisker-reinforced ceramic-matrix composites

This paper analyzes the fracture toughness of short-fiber reinforced ceramic-matrix composites (CMC). The effects of crack deflection and fiber pullout on matrix cracking are examined using a combination of mechanical and statistical models. First, the stress intensity factors of a deflected crack subjected to closure stress due to fiber pullout are analyzed based upon the mechanical model. Distributed dislocation method is used for the elastic analysis. Since the deflected crack is subjected to biaxial loading, a mixed mode fracture criterion in linear elastic fracture mechanics is applied to calculate the fracture toughness. Secondly, the number of pullout fibers on the fracture surface is treated as a random variable, and the statistical distribution of these fibers has been determined. The pullout force acting on a deflected crack is also obtained as a random variable by assuming a simple mechanism of fiber pullout. The probability of failure of CMC can thus be estimated from the strength characteristics of the fiber and matrix as well as the interface between these two.     

Investigation of fiber configurations of chafer cuticle by SEM, mechanical modeling and test of pullout forces

Insect cuticle as a natural biocomposite includes many favored microstructures which have been refined over centuries and endow the cuticle eminent mechanical properties. This paper first studies the microstructures of chafer cuticle through SEM observations. Several peculiar fiber configurations and fiber-ply arrangements such as branched fiber, acanth-fiber and helicoid plies are observed. These microstructures are useful for man-made fiber-reinforced composites to improve their mechanical properties. Then, a special configuration of the branched fiber found in chafer cuticle is in details analyzed through a mechanical model and experimental verification. The pullout force of fibers as an index is firstly studied through parameter study. The factors, which can improve the pullout forces, are identified. Finally, the maximal pullout force of the branched fiber is experimentally tested and compared with that of plain straight fiber. It is proved that the maximal pullout force of branched fibers is obviously greater than that of the plain straight fibers.     

Bridging and toughening of short fibers in SMC and parametric optimum

A simple analytical approach was proposed to simulate the bridging and toughening of randomly oriented short fibers in brittle matrix. Based on a magnitude order analysis, the energy dissipation before fiber pullout was neglected. The coupling of matrix spalling and fiber pullout was simulated by an approximate bridging model to determine the energy dissipation in the whole pull-out process of randomly oriented short fibers in brittle matrix. The proposed approach was applied to analyze the bridging/toughening of short fibers in SMC plates with mode I crack and to discuss the effects and optimum of parameters. The modelling results show that the main parameters for toughening are fiber length and interface friction stress and there is an optimal combination, i.e., their product nearly equals a constant (about 15–17 MPa-mm).     

水泥基材料中倾斜端钩型钢纤维拔出力学性能计算模型

端钩型钢纤维是结构工程中应用最广泛的钢纤维品类之一,单根钢纤维拔出力学性能对于确定钢纤维混凝土的受拉本构及受拉韧性具有重要意义。为了得到能够有效预测倾斜端钩型钢纤维拔出荷载-端部位移曲线的理论模型,首先将倾斜端钩型钢纤维拔出过程分为完全黏结、脱黏和拔出滑移阶段三种受力状态,考虑不同拔出阶段及基体孔道损伤,建立了钢纤维黏结应力与纤维端部位移之间的关系,同时考虑钢纤维塑性变形、附加摩擦力及纤维拔出角度导致的基体剥落和挤压摩擦效应,建立了一种可以预测倾斜端钩型钢纤维拔出全过程的理论计算模型,在此基础上提出形式简单的简化模型,选取已有试验数据对提出的计算模型进行验证,结果表明:本文提出的两种模型均能够有效预测端钩型钢纤维拔出全过程,具有较高的计算精度且变异系数小,为进一步分析钢纤维对水泥基材料受拉性能的增强作用提供了理论依据。      

Crack/fiber interaction and crack growth resistance behavior in microfiber reinforced mortar specimens

Performance enhancement due to microfibers is well known. However, fracture processes that lead to strain hardening behavior in microfiber reinforced composites are not well understood. Crack growth resistance behavior of mortar reinforced with steel microfibers and polypropylene microfibers was investigated in-situ during load application. The polypropylene fibers were inter-ground in the cement mill to enhance the fiber/matrix interfacial frictional stress. A more homogeneous fiber distribution was observed in the inter-ground polypropylene composites compared to the steel microfiber reinforced composites. In steel microfiber reinforced composites the dominant toughening mechanisms were multiple microcracking and successive debonding along the fiber/matrix interface. Fiber pullout, the dominant mechanism in conventional macrofiber reinforced composites was rarely observed. In-situ observation of crack/fiber interaction in the inter-ground polymer fibers also revealed multiple microcracking along the length of the fibers followed by fiber pullout.     

Assessment of flexural toughness and impact resistance of bundle-type polyamide fiber-reinforced concrete

This study compares the fiber/matrix bonding strength and flexural properties of bundle-type polyamide fibers to those of hooked-end steel fibers. Their fracture behavior upon impact with a high-velocity projectile is also assessed in terms of penetration depth, crater diameter and rear-side scabbing. The results obtained demonstrate that the bundle-type polyamide fibers undergo fracture without fiber pullout because of the increased interfiber gap and specific surface area for bonding, but exhibit poorer flexural fracture behavior with a lower flexural strength and fracture energy when compared to hooked-end steel fibers. Yet despite this, concrete reinforced with bundle-type fibers is shown to more effectively suppress scabbing during high-velocity impact, which is attributed to a more efficacious dispersion of shock stress due to the increased number of individual fibers.     

硅烷偶联剂对玻璃织物/水泥复合材料界面行为的影响

本文通过织物抽拔试验和抽拔试验后残留在水泥基体中的纤维表面形貌扫描电镜照片分析,研究了在玻璃纤维织物表面涂覆硅烷偶联剂或涂蜡,对它增强的水泥基复合材料界面行为的影响.实验结果表明:在玻璃纤维织物表面涂覆硅烷偶联剂有利于它与水泥基体间的界面粘结,改善它们间的界面行为,在玻璃纤维织物表面涂蜡则不利于它与水泥基体间的界面粘结.     

Tailored interyarn friction in aramid fabrics through morphology control of surface grown ZnO nanowires

Para-aramid fibers, such as Kevlar, have been the most common material used for ballistic applications by providing a variety of energy absorption mechanisms to dissipate a projectile’s momentum. One important mechanism is interyarn friction, which can be controlled through surface treatment of the fibers. It was recently shown that the growth of ZnO nanowires on the surface of the fibers could increase the interyarn friction, producing 10.85 times higher peak load and 22.70 times higher energy absorption compared to neat fabrics. Here, it is demonstrated that variation of the morphology of the nanowires produces a drastic change in the pullout behavior with low aspect ratio nanowires producing the largest increase in pullout force. While weight of the modified fabrics increased only 20% compared to the bare ones, tensile test results show that the growth of ZnO nanowires enhances both tensile strength and elastic modulus of the fabrics by 13% and 10%, respectively. A comprehensive post-test micrograph analysis is performed to reveal possible enhancement mechanisms of modified aramid fabrics with different nanowire morphologies.     

Processing and tensile characterization of composites composed of carbon nanotube-grown carbon fibers

The growth of carbon nanotubes (CNTs) on carbon fibers was conducted via chemical vapor deposition. A solution approach has been used to distribute nickel particles on the fiber, and the carbon source was a methane gas. The resulting CNTs are about 10 μm in length and 50 nm in outer diameter. After CNT growth, a fiber bundle was impregnated with an epoxy resin to form a unidirectional composite. Tensile tests were carried out, and the induced fracture surface was examined by microscopes. Three types of CNT fracture during fiber pullout are discussed. The results show that fracture in the CNT/fiber joint is the major mode. Pullout of CNTs was also observed. While pullout of fibers leaves micro-scale holes, pullout of CNTs leaves nano-scale holes. The multi-scale fracture behavior generates new parameters for material design and processing. Some concepts regarding the microstructural design for this special composite are discussed.     

Fiber-end deformation effects in enlarged-end, fiber-reinforced composites

Composite materials reinforced by fibers with enlarged ends are known to have significantly better strength and toughness than those reinforced by flat-end fibers. The objective of this study is to develop an analytical model to determine the importance of deformation of the enlarged end on the reinforcement performance of ellipsoidal enlarged-end fibers. The resisting pullout load of the fiber is composed of a component due to interfacial bond at the fiber/matrix interface and a component due to mechanical anchorage at the embedded enlarged end of the fiber. The component due to mechanical anchorage at the enlarged end is due to both mechanical interlock and deformation of the enlarged end. In the past, little has been done to account for the deformation of the enlarged end. To account for this component of the mechanical anchorage resistance at the embedded enlarged end, a spring component is introduced to connect the embedded fiber with the enlarged ellipsoid. Analytical solutions were derived to predict the effects of the rigid enlarged end shape on the pullout load and stress distribution. These solutions were then compared to finite element solutions. It is shown that the shape of enlarged end has a significant influence on the stress distribution of the short fiber. Specially, the model demonstrates that the enlarged ends deform significantly for some shapes and are not effective for long fibers.     

Pullout resistance of straight NiTi shape memory alloy fibers in cement mortar after cold drawing and heat treatment

In our study, we found cold drawing to be an effective method for enhancing the pullout resistance of NiTi shape memory alloy (SMA) fibers in concrete. The pullout resistance was observed to be dependent on the contact pressure and friction coefficient at the interface between the fibers and the mortar matrix. The drawing process increased the stiffness and yield stress of the fibers and consequently increased the contact pressure at the interface between the fibers and the mortar matrix. Moreover, heat treatment of the fibers after cold drawing was found to noticeably recover the fiber diameter, thereby significantly enhancing the pullout resistance. The enhancement of the interfacial bond strength by heat treatment verified the crack-closing capabilities of SMA-fiber-reinforced cement composites.     

Bond between deformed reinforcement and normal and high-strength concrete with and without fibers

This paper describes an experimental study that consisted of pullout tests of deformed reinforcing bars in NSC and HSC specimens, with and without hooked-end steel fibers. Two types of test setups were applied, direct and flexural tests, and three bar diameters were tested (8, 12 and 20 mm). The experimental setups were based on standard RILEM pullout (direct) and beam tests, with several modifications. The experimental program included study of the effects of concrete strength and inclusion of steel fibers on the bond strength, as well as the influence of bar geometry and concrete cover. Discussion of the results shows coupling of these effects and proposes an empirical expression that represents this coupling. The results from the current study are also compared with the design bond strengths specified in American and European standards as well as a known model.     

A theoretical model on piezoelectric fibre pullout with electric input

A theoretical model of a single piezoelectric fibre pullout from an elastic matrix was developed to study the effect of an input electric field. The stress distributions in the fibre under both mechanical and electric loads are obtained. The relationships between pullout force, induced electric potential and deformation are evaluated by computer simulation. The effects of electric input, piezoelectric parameters and fibre volume fraction on the load- displacement curve for fibre pullout are discussed. Numerical results obtained in this study indicate that the pullout force can be adjusted by changing the value or the direction of the applied electric field. Also, the results show that piezoelectric parameters and fibre volume fraction play important roles on the pullout force in the piezoelectric fibre.     

Interface engineering of fiber-reinforced all-oxide composites fabricated by the sol–gel method with fugitive pyrolytic carbon coatings

This paper describes the interface engineering of three–dimensional (3D) Nextel™440 fiber-reinforced aluminosilicate composites fabricated by the sol–gel method with fugitive pyrolytic carbon (PyC) coatings. The coating thickness on the fiber strength, interfacial characteristics and there corresponding effects on mechanical properties of the composites were investigated. The study shows that the fiber strength was influenced by the coating thickness and optimized with the thickness of 0.15 μm. The composites with uncoated fibers showed brittle fracture behavior without fiber pullout because of strong interactions between the fiber and the matrix. However, higher strengths and pseudo-ductile fracture behaviors were obtained in the composites with PyC interphases, where different deflections and branches of propagating cracks and fiber pullout patterns were observed. Moreover, induced fugitive PyC interface conditions have great effects on the density, microstructure and mechanical properties of the resultant composites.