Characterization of the orientation profile of steel fiber reinforced concrete

The orientation of fibers has a pronounced influence on the tensile behavior of steel fiber reinforced concrete (SFRC) and, consequently, this aspect should be considered in modeling the material constitutive law. Previous works have shown that the tensile strength of SFRC is directly related to the average orientation of the fibers. However, few studies have investigated the correlation between the variation of distribution of fiber orientation and material strength. This paper introduces a new concept of orientation profile in order to characterize fiber orientation through an unambiguous method. From the investigation on experimental data it could be observed that fiber orientation follows a Gaussian law and that the distribution and average values of single fiber orientations are correlated with each other. Conclusions from this paper are particularly relevant for the development of micromechanical models for SFRC.     

Investigation of microstructure and flexural behavior of steel-fiber reinforced concrete

This paper presents microstructure and flexural behavior of steel-fiber reinforced concrete produced with different steel fibers volume fraction and aspect ratio. Prismatic concrete specimens of 100 × 100 × 350 mm were prepared with and without steel fiber. Two different steel fiber types (both is hooked-end) were used by ratio of 0% (control), 0.2, 0.4, 0.6 and 0.8% by volume. Specimens were de-molded after 24 h and cured in water until 7, 28, 56, 180 and 360 days. On the prisms, flexural strength has been defined for every age. The crack widths have also been measured after maximum bearing loads. Microstructure of SFRC was studied by scanning electron microscopy and optical microscopy for 180 aged specimens. The results showed that the polarized microcopy images may be used for observing the bond characteristic of SFRC as alternatively to SEM. A good bond was observed between steel fiber and concrete matrix interface zone by using polarizing microscopy, too. Flexural strength of SFRC increased with the concrete age and fiber volume fraction. Besides, the first crack development significantly decreased by increasing of fiber volume fraction in the all concrete ages.     

Influence of fiber chemical coating on the acoustic emission behavior of steel fiber reinforced concrete

The current study focuses on the effect of chemical coating on the acoustic emission (AE) characteristics monitored during the fracture process in steel fiber reinforced concrete (SFRC). Different shapes of chemically treated and un-treated steel fibers are used to create specimens which are subjected to four point bending up to failure. Sensitive AE indices demonstrate that the coating gives distinct characteristics to the interface bonding between the fiber and the concrete matrix, which are evident mainly during the pull-out stage, after the moment of macroscopic crack formation. Specifically, AE average frequency and RA value, which defines the rising angle of the waveforms indicate that coating results in extensive matrix cracking in addition to the friction between fiber and concrete which characterizes the uncoated fibers. AE analysis can be used for interpretation of the fracturing stage and characterization of the fracture mode. It is shown that the surface conditioning of the fibers leaves a clear fingerprint on the AE signals, shedding light into the processes that occur during failure in SFRC.     

Flexural fatigue behavior of steel fibre reinforced concrete before and after cracking

This paper presents the results of a study of steel-fiber-reinforced concrete (SFRC) in flexural fatigue. An experimental technique was developed to determine the moment at which cracking is initiated, thus allowing a quantification of the survival life beyond cracking. Basically, the experimental program consisted of 8 series of flexural fatigue tests (under third-point loading) performed at three different levels of stress (70%, 75% and 85% of first-crack strength). Six SFRC mixtures (at a fiber dosage of 40 kg/m³) were prepared and tested. The variables were the water/cement ratio (0.45 and 0.35), and the fiber geometry (hooked, anchored, and crimped fibers). Two similar plain concretes (w/c=0.45 and 0.35) were used as reference mixtures. The fatigue response of the SFRC mixtures was found to be quite variable, both before and after cracking. The survival life appeared to be significant, especially at the lower level of stress investigated, but the overall variability prevented the identification of specific trends concerning the influence of the water/cement ratio and the type of fibers. The variability of the number of fibers found in the bottom half of the specimens at the critical section could not explain the variability of the survival life. It was concluded that the orientation of the fibers also had an influence in this respect, and that a fiber content higher than that utilized, or the use of larger test specimens, was probably required to limit this variability.     

Modeling steel fiber reinforced concrete: numerical immersed boundary approach and a phenomenological mesomodel for concrete‐fiber interaction

Steel fiber reinforced concrete (SFRC) allows overcoming brittleness and weakness under tension, the main drawbacks of plain concrete. The influence of the fibers on the behavior of SFRC depends on their shape, length, slenderness, and also on their orientation and distribution into the plain concrete. The goal of this paper is to develop an ad hoc numerical strategy to account for the contribution of the fibers in the simulation of the mechanical response of SFRC. In the model presented, the individual fibers immersed in the concrete bulk are accounted for in their actual location and orientation. The selected approach is based on the ideas introduced in the immersed boundary (IB) methods. These methods were developed to account for 1D (or 2D) solids immersed in 2D (or 3D) fluids. Here, the concrete bulk is playing the role of the fluid and the cloud of steel fibers is acting as the immerse boundary (that is, a 1D structure in a 2D or 3D continuous). Thus, the philosophy of the IB methodology is used to couple the behavior of the two systems, the concrete bulk and fiber cloud, precluding the need of matching finite element meshes. Note that, considering the different size scales and the intricate geometry of the fiber cloud, the conformal matching of the meshes would be a restriction resulting in a practically unaffordable mesh. In the proposed approach, the meshes of the concrete bulk and fiber cloud are independent, and the models are coupled imposing displacement compatibility and equilibrium of the two systems. In the applications presented here, the concrete bulk is modeled using a standard nonlinear damage model. The constitutive model for the fibers is designed to account for the complex interaction between fibers and concrete. The fiber models are based on the previous investigations describing the concrete‐fiber interaction and its dependence on the factors identified to be relevant: shape of the fiber (straight or hooked) and angle between the fiber and crack plane. Copyright © 2011 John Wiley & Sons, Ltd.     

Fatigue behaviour of steel fiber reinforced concrete

The results of an experimental investigation on the fatigue characteristics and residual strength of steel fiber reinforced concrete (SFRC) are reported. The testing program included flexural specimens as well as split-cylinders and cubes reinforced with two fiber types at a low volume content. One of the fibers was of the deformed slit-sheet type available at aspect ratios of 45 and 60. It is shown that SFRC has a better fatigue response than plain concrete and that the deformed slit-sheet fiber has an effect almost identical to hooked-end fiber of similar dimensions. There is no increase in residual strength measured by split-tension when specimens are subjected to fatigue stress above the endurance limit. Fatigue characteristics of SFRC from this testing program as well as previous works can be interpreted as a function of the fiber factor (i.e. a parameter accounting for volume fraction, aspect ratio and fiber type) to provide design charts. More experimental work is needed to provide an acceptable database for fatigue design of SFRC.     

An experimental study on the tensile strength of steel fiber reinforced concrete

This paper deals with steel fiber reinforced concrete mechanical static behaviour and with its classification with respect to fibers content and mix-design variations. A number of experimental tests were conducted to investigate uniaxial compressive strength and tensile strength. Different mixtures were prepared varying both mix-design and fiber length. Fibers content in volume was of 1% and 2%. Mechanical characterization was performed by means of uniaxial compression tests with the aim of deriving the ultimate compressive strength of fiber concrete. Four-point bending tests on notched specimens were carried out to derive the first crack strength and the ductility indexes. The tensile strength of steel fiber reinforced concrete (SFRC) was obtained both from an experimental procedure and by using an analytical modelling. The experimental tests showed the different behaviour of SFRC with respect of the different fiber content and length. Based on the experimental results, an analytical model, reported in literature and used for the theoretical determination of direct tensile strength, was applied with the aim of making a comparison with experimental results. The comparison showed good overall agreement.     

局部高密度钢纤维混凝土弯曲疲劳损伤演变规律

在混凝土弯曲构件底部用高密度钢纤维局部增强称为局部高密度钢纤维混凝土(PHPFRC)与同样纤维掺量的传统钢纤维混凝土(SFRC)比较它可以用相近的价格获得高得多的刚度﹑承载力和抗疲劳断裂性能为了能够预测PHPFRC在循环荷载作用下的疲劳寿命需要确定其疲劳损伤演变规律本文通过试验发现局部高密度钢纤维混凝土的弯曲疲劳损伤表现出韧性材料所具有的性质基本上接近韧性损伤这与素混凝土及传统钢纤维混凝土有本质的不同根据其疲劳特性的实验结果探讨了PHPFRC弯曲疲劳损伤阈值结合损伤理论建立了适合于纤维体积掺量为1.2%的局部高密度钢纤维混凝土试件的弯曲疲劳损伤演变方程     

Numerical Study on Mechanical Properties of Steel Fiber Reinforced Concrete by Statistical Second-order Two-scale Method

The present study aims to evaluate the mechanical properties of steel fiber reinforced concrete (SFRC) by the statistical second-order two-scale (SSOTS) method. At first, the representation for microstructure of SFRC is described by a concept of statistical screen. According to the microstructure representation, the SSOTS method is displayed in a concise way. This method is on the basis of asymptotic expansion homogenization and Monte Carlo method, and can calculate the local strain and stress field through the two-order displacement solution. As the classical homogenization method, the expression of homogenized elastic modulus is derived analytically. Then combined with the appropriate strength criterion and correspondence principle, the homogenized strength and viscoelastic properties of SFRC are obtained respectively. The validity of the SSOTS method is confirmed by the comparison between numerical results and the available experiment data. Results show that the SSOTS method is effective to evaluate the elastic, strength and viscoelastic properties of SFRC. In addition, the influence of distribution of steel fibers on the macroscopic mechanical properties of SFRC is discussed.     

Optimal fiber distribution for tensile properties of injection molded composite

The survival rate of a composite is the residual fiber length divided by the initial fiber length, and it decreases with the initial fiber length and fiber volume content (V_f) during injection molding processes. The degree of damage is higher for carbon fiber than for glass fiber, and the survival rate increases with a hyperbolic tangent relationship as the nozzle diameter increases. Higher survival rate corresponds to a stronger material. Five different lengths of fiber with 29 different size fibers were selected based on the distribution and shape of residual fiber in experimental works. These were examined to study the effects of fiber distribution on the tensile properties of a short-fiber reinforced composite (SFRC). Compared with the experimental results, the modulus predicted using the Halpin-Tsai relation shows reasonable agreement with the prediction obtained using the residual fiber length instead of the initial fiber length. It was found that the tensile modulus and strength generally differ by a factor of up to 3.2, depending on the fiber distribution patterns with V_f = 30%, and the trend is more significant as the fiber aspect ratio increases. The interactions between the fiber and matrix and the staggered-type distribution are the most important factors in the reinforcement of the SFRC. With the same combination of short fiber length, an optimized fiber distribution pattern is suggested.     

单向受拉状态下的钢纤维混凝土本构模型

在混凝土中添加随机分布的钢纤维能有效提高混凝土力学性能。为了更好地考虑纤维对单向受拉状态下钢纤维混凝土（SFRC）的增强作用，提出一个钢纤维混凝土的弥散开裂本构模型。在弹性阶段，纤维混凝土被视为简单复合材料，基于两相复合材料理论，对SFRC的弹性刚度矩阵进行修正；在受拉开裂后，混凝土的塑性变形量被视为纤维与混凝土界面脱粘过程中滑移量，利用粘结滑移模型计算纤维在混凝土开裂面上的桥接作用。该文通过有限元软件ABAQUS中子程序二次开发接口Umat，进行Fortran编程，在ABAQUS中实现该本构模型。通过数值模拟结果与受拉实验数据进行对比，验证了该本构模型的准确性。通过数值模拟分析，进一步探究钢纤维混凝土相关参数对抗拉性能的影响，为钢纤维混凝土在实际的工程中的应用提供建议。     

STOKES FLOW THROUGH A SYSTEM OF PARALLEL INFINITE CYLINDERS WITH AXES ORIENTED AT AN ANGLE TO THE DIRECTION Of MEAN FLOW

Abstract

Theoretical models based on Stokes flow of air through a fibrous filter predict a significantly higher pressure drop than experimentally measured values. This discrepancy persists even when the interaction of the flow between) neighboring fibers is accounted for. Various authors have attributed this discrepancy to the inhomogeneity of the fiber distribution within the filter and to the possibility that some fibers are partially orientation in the directon of mean flow. It has been shown that fiber density inhomogeneity does indeed contribute to this discrepancy

In this paper, the effect on the flow and subsequent pressure drop when the fibers are oriented at an angle to the directon of mean flow is studied. The solution of the three dimensional equation for creeping, incompressible flow in a doubly periodic, infinite lattice of infinite circular cylinders when there is a constant mean flow whose direction makes an acute angle with the axes of the cylinders is given. If the volume fraction of fibers is small, the periodic boundary conditions can be replaced by requiring zero vorticity at the outer boundary of an imagined cylindrical cell of fluid surrounding one of the cylinders. The resulting parallel and transverse problems have known solutions and give an approximate solution to the flow through the periodic lattice. The resulting drag is used to compute the dimensionless pressure drop across a filter for several values of the volume fraction of fiber and is compared to the experimentally determined formula of Davies. It is shown that the average drag over a uniform distribution of fiber orientations yields a pressure drop which is significantly closer to the experimental values of Davies than that resulting from strictly transverse flow.     

Shear behavior model for steel fiber-reinforced concrete members without transverse reinforcements

Due to the complex shear mechanism of steel fiber-reinforced concrete (SFRC) members, there is lack of comprehensive shear behavior models for SFRC members. The shear behavior model, based on a smeared crack model, requires the tensile stress–strain constitutive equation of SFRC membrane subjected to biaxial stresses. After SFRC panel tests under biaxial stresses were recently conducted, it has been possible to create a more complete smeared crack model for estimating the shear behavior of SFRC members. It is, however, very difficult to conduct such experiments for different types of steel fibers, various amount of steel fibers, different ranges of concrete strengths, etc. Thus, in this study, steel fibers are modeled as average direct tensile contribution elements in a modified smeared crack truss model, considering directionality and distribution of fibers. In this way, only simple bond tests are required to reflect the effects of different characteristics of SFRC. In addition, the shear contribution of steel fibers can be obtained considering the bond failure of steel fibers. The proposed model was compared to the test results of 8 SFRC panels and 80 SFRC beams, and the shear behavior of the SFRC members was well estimated.     

Statistical continuum theory for the effective conductivity of fiber filled polymer composites: Effect of orientation distribution and aspect ratio

Effective conductivity of polymer composites, filled with conducting fibers such as carbon nanotubes, is studied using statistical continuum theory. The fiber orientation distribution in the matrix plays a very important role on their effective properties. To take into account their orientation, shape and distribution, two-point and three-point probability distribution functions are used. The effect of fibers orientation is illustrated by comparing the effective conductivity of microstructures with oriented and non-oriented fibers. The randomly oriented fibers result in an isotropic effective conductivity. The increased fiber orientation distribution can lead to higher anisotropy in conductivity. The effect of fiber’s aspect ratio on the effective conductivity is studied by comparing microstructures with varying degrees of fiber orientation distribution. Results show that the increase in anisotropy leads to higher conductivity in the maximum fiber orientation distribution direction and lower conductivity in the transverse direction. These results are in agreement with various models from the literature that show the increase of the aspect ratio of fibers improves the electrical and thermal conductivity.     

钢纤维混凝土在低周反复荷载下力学性能的研究

对钢纤维混凝土棱柱体试件做了低周轴心压缩循环加、卸载试验,试验采用恒位移控制,通过试验和参数研究,提出了钢纤维混凝土反复加、卸载应力~应变曲线方程及计算变形的公式,这些数学表达式简便且可根据材料特性进行调整,具有较广泛的适应性。     

短纤维增强橡胶密封复合材料界面疲劳损伤行为

依据广义自洽方法,建立了包含芳纶纤维、界面相、橡胶基体和等效介质的代表性体积单元(RVE)模型。采用自定义材料子程序对内聚力疲劳累积损伤模型进行编译,分别在基体/界面相的界面和纤维/界面相的界面设置内聚力单元,研究界面相性能参数对纤维增强橡胶密封复合材料(SFRC)界面疲劳损伤行为的影响。探讨了界面相厚度和模量的确定方法,获得了不同界面相厚度和模量下SFRC界面脱粘起始位置以及脱粘起始疲劳次数。结果表明,较低的界面相模量能够抑制界面脱粘的产生;随着界面相厚度的增加,界面脱粘的起始疲劳次数增加,SFRC抗疲劳损伤能力得到提高。     

钢纤维混凝土多轴损伤比强度准则EI北大核心CSCD

以损伤比强度理论为基础,建立了钢纤维混凝土真三轴损伤比强度准则,并根据钢纤维混凝土试验资料,推荐了钢纤维混凝土损伤比变量表达式中的6个经验参数。利用钢纤维混凝土在单轴、双轴和三轴受力状态下的应力-应变曲线试验结果验证了损伤比取值合理性,对比了单轴受拉、单轴受压和双轴等压等典型受力状态下钢纤维混凝土和普通混凝土损伤比变量取值的差异。通过与国内外共104组钢纤维体积率为0.5%~2.5%的钢纤维混凝土三轴强度试验资料的比较,表明六经验参数钢纤维混凝土损伤比强度准则的三维破坏包络面接近已有认识;通过与国内外强度准则比较,表明损伤比强度准则与钢纤维混凝土三轴试验数据有较高的吻合度。对于围压三轴受力状态,提出简化的钢纤维混凝土常规三轴强度准则,并与已有常规三轴强度准则进行比较分析。此外,对于材料处于二轴受力,推荐了简化的损伤比二轴强度准则中的经验参数取值。     

GMT流动成型纤维取向研究

GMT材料流动成型后玻纤在平面内发生取向，导致模压件呈各向异性，本研究从成型后制品上取样烧尽树脂，由扫描仪获取纤维数值图像，用Photoshop软件将图像反相，增强，再利用MATLAB软件确定纤维取向分布，研究表明，GMT单向流动成型时纤维沿流动方向取向，随流动距离增大，取向趋向更为明显，而均匀双向拉伸流动纤维取向程度较小，与片材相比，材料力学性能沿取向方向增大，但垂直取向方向材料性能变差。     

Surface modification of fiber reinforced polymer composites and their attachment to bone simulating material

The purpose of this study was to investigate the effect of fiber orientation of a fiber-reinforced composite (FRC) made of poly-methyl-methacrylate (PMMA) and E-glass to the surface fabrication process by solvent dissolution. Intention of the dissolution process was to expose the fibers and create a macroporous surface onto the FRC to enhance bone bonding of the material. The effect of dissolution and fiber direction to the bone bonding capability of the FRC material was also tested. Three groups of FRC specimens (n = 18/group) were made of PMMA and E-glass fiber reinforcement: (a) group with continuous fibers parallel to the surface of the specimen, (b) continuous fibers oriented perpendicularly to the surface, (c) randomly oriented short (discontinuous) fibers. Fourth specimen group (n = 18) made of plain PMMA served as controls. The specimens were subjected to a solvent treatment by tetrahydrofuran (THF) of either 5, 15 or 30 min of time (n = 6/time point), and the advancement of the dissolution (front) was measured. The solvent treatment also exposed the fibers and created a surface roughness on to the specimens. The solvent treated specimens were embedded into plaster of Paris to simulate bone bonding by mechanical locking and a pull-out test was undertaken to determine the strength of the attachment. All the FRC specimens dissolved as function of time, as the control group showed no marked dissolution during the study period. The specimens with fibers along the direction of long axis of specimen began to dissolve significantly faster than specimens in other groups, but the test specimens with randomly oriented short fibers showed the greatest depth of dissolution after 30 min. The pull-out test showed that the PMMA specimens with fibers were retained better by the plaster of Paris than specimens without fibers. However, direction of the fibers considerably influenced the force of attachment. The fiber reinforcement increases significantly the dissolution speed, and the orientation of the glass fibers has great effect on the dissolving depth of the polymer matrix of the composite, and thus on the exposure of fibers. The glass fibers exposed by the solvent treatment enhanced effectively the attachment of the specimen to the bone modeling material.     

Investigation on Thermal Conductivity of Steel Fiber Reinforced Concrete Using Mesoscale Modeling

A mesoscale model was developed to investigate the effect of steel fiber on the thermal conductivity of steel fiber-reinforced concrete (SFRC). Delaunay triangulation was employed to generate the unstructured mesh for SFRC materials. The model was validated using the existing experimental data. Then, it was used to study how model thickness affected simulation outcomes of thermal conductivity of models with different fiber lengths, by which an appropriate thickness was determined for the later analyses. The validated and optimized model was applied to the study of relationships between thermal conductivity and factors such as fiber content, fiber aspect ratio and different parts of an SFRC block by conducting steady-state heat analyses with the finite element analysis software ANSYS. The simulation results reveal that adding steel fiber increases thermal conductivity considerably, while fiber aspect ratio only has an insignificant effect. Besides, the presence of steel fibers has an obvious impact on the distribution of temperature and heat flux vector of the SFRC blocks.