Analytical simulation of tensile response of fabric reinforced cement based composites

A model simulating the tensile behavior of fabric–cement composites is presented to relate the properties of the matrix, fabric, interface and the damage parameters to the overall mechanical response of the composites. Crack spacing parameters measured during tensile tests are used to define the damage parameters, and related to the stiffness degradation as a function of the applied strain. This procedure is integrated in composite laminate theory using an incremental approach to model the uniaxial tensile response. Two approaches of linear and nonlinear fabric bridging models are used. The model is capable of using interface parameters for different fabrics, matrix properties, and processing parameters. Simulation results are studied by means of parametric simulation and a validation of a variety of experimental observations which vary the matrix formulation with flyash, changes in pressure after casting, and fabric type.     

正交铺设陶瓷基复合材料单轴拉伸行为

采用细观力学方法对正交铺设陶瓷基复合材料单轴拉伸应力-应变行为进行了研究。采用剪滞模型分析了复合材料出现损伤时的细观应力场。采用断裂力学方法、 临界基体应变能准则、 应变能释放率准则及Curtin统计模型4种单一失效模型确定了90°铺层横向裂纹间距、 0°铺层基体裂纹间距、 纤维/基体界面脱粘长度和纤维失效体积分数。将剪滞模型与4种单一损伤模型结合, 对各损伤阶段应力-应变曲线进行了模拟, 建立了复合材料强韧性预测模型。与室温下正交铺设陶瓷基复合材料单轴拉伸应力-应变曲线进行了对比, 各个损伤阶段的应力-应变、 失效强度及应变与试验数据吻合较好。分析了90°铺层横向断裂能、 0°铺层纤维/基体界面剪应力、 界面脱粘能、 纤维Weibull模量对复合材料损伤及拉伸应力-应变曲线的影响。      

Fiber-reinforced cement based composites under tensile impact

In many civil engineering structures, adequate resistance of the material to suddenly applied loads is essential. This article describes the improvements in the impact resistance of cement matrices when reinforced with high volume fractions of carbon, steel, and polypropylene micro-fibers. Tensile briquettes were fractured under a rapidly applied load using a newly designed instrumental impact machine. Strength and fracture energy values were measured. When compared with static test results, considerable sensitivity to stress rate was noted. The composites were found to be stronger and tougher under impact and the improvements were more pronounced at higher fiber volume fractions.     

Shrinkage of steel fibre reinforced cement composites

The paper presents results of an experimental investigation on the influence of steel fibres on the free shrinkage of cement-based matrices. Shrinkage tests were carried out on cement paste, mortar and two types of concrete mixes for a period of up to 520 days. Melt extract, crimped and hooked steel fibres were used for reinforcement at volume fractions ranging between 1 and 3%. The results indicate that fibres restrain the shrinkage of the various cement matrices to a significant extent, resulting in reductions of up to 40%. Crimped fibres are the most efficient in providing shrinkage restraint. The paper also presents a theoretical expression and an empirical expression which can be used to predict shrinkage strains of steel fibre reinforced cement matrices. The analysis requires a knowledge of the values of coefficient of friction, μ, at the fibre-matrix interface, which are also derived in this paper. The μ values for steel fibres in normal concrete, mortar and cement paste range between 0.07 and 0.12.     

单向纤维增强陶瓷基复合材料单轴拉伸行为

采用细观力学方法对单向纤维增强陶瓷基复合材料的单轴拉伸应力-应变行为进行了研究。采用Budiansky-Hutchinson-Evans（BHE）剪滞模型分析了复合材料出现损伤时的细观应力场,结合临界基体应变能准则、应变能释放率准则以及Curtin统计模型三种单一失效模型分别描述陶瓷基复合材料基体开裂、界面脱粘以及纤维失效三种损伤机制,确定了基体裂纹间隔、界面脱粘长度和纤维失效体积分数。将剪滞模型与3种单一失效模型相结合,对各个损伤阶段的应力-应变曲线进行模拟,建立了准确的复合材料强韧性预测模型,并讨论了界面参数和纤维韦布尔模量对复合材料损伤以及应力-应变曲线的影响。与室温下陶瓷基复合材料单轴拉伸试验数据进行了对比,各个损伤阶段的应力-应变、失效强度及应变与试验数据吻合较好。     

Properties of cement composites reinforced with Kevlar fibres

The organic polyamide fibre, Kevlar, is promising as an efficient reinforcement for cementitious matrices. For cement boards, in which chopped fibres are distributed randomly in two dimensions, typical mechanical properties obtained with 1.9 vol% fibre addition are as follows: ultimate tensile strength (UTS) 16 MN m^–2; MOR 44 MIN m^–2; impact strength 17 kJ m^–2. The composite material can be produced by autoclaving if desired and at ambient temperatures they are expected to be durable in most environments. The relatively low decomposition point of Kevlar (as opposed to glass fibres or steel) is a disadvantage for its use in building components which may come into contact with high temperatures, as in a fire. It should be noted that a solvent which is used in the manufacture of the fibre and remains in the fibre in minute quantities has been found to produce cancer in rats. There is no evidence of it causing cancer in humans but the significance of this in terms of a possible health risk, if any, will need to be assessed by the appropriate medical authorities in relation to any applications.     

氧化石墨烯增强水泥复合材料的断裂性能

将氧化石墨烯(GO)加入水泥砂浆中,以提高其抗裂性和韧性.通过三点弯曲梁法测试了GO增强水泥砂浆试件的断裂性能,采用双K断裂模型分析了GO掺量对改性水泥砂浆断裂参数的影响.结果表明:GO提高了水泥砂浆试件的起裂韧度,当GO掺量(与胶凝材料质量比)为0.01％～0.07％时,较对照组分别提高了13.4％、25.4％、24...     

The stress-strain behaviour of glass-fibre reinforced cement composites

Equations describing the stress-strain behaviour of continuous, aligned, brittle-fibre-brittlematrix composites have been modified to take account of the construction of practical choppedstrand glass-fibre reinforced cement composites. The effects on the composite strain to failure of a number of factors have been considered and some detailed comparisons made between theoretical predictions and experimental results.     

Aging mechanisms in cellulose fiber reinforced cement composites

This paper examines the effects of laboratory scale accelerated aging exposures on the changes in physical and mechanical properties of commercially produced cellulose fiber reinforced cement composites. Two different accelerated aging methods were used to simulate the possible aging mechanisms for which the material may experience under service conditions, both methods being compared to material naturally weathered for 5 yr in roofing. The first aging method consisted of different cycles of water immersion, carbonation, and heating exposures whereas in the second method, cycles of water immersion, heating and freeze-thaw exposures were used. The porosity, water absorption, permeability of nitrogen and compressive shear strength of the composites were examined before and after aging exposures. The surface morphologies of the composites fractured in compression shear tests were examined using scanning electron microscope. Experimental results showed that the compressive shear strength of the accelerated aged composites were related to the microstructures within the composites. Both natural weathering and accelerated aging in CO₂ environment reduced the porosity, water absorption, and nitrogen permeability in the cement matrix, and enhanced the durability of the cellulose fiber-cement composites. The aging test based on artificial carbonation was more effective in simulating natural aging performance of the composites, while the freeze-thaw cycling method failed to induce significant aging effects on the composites even after 21 cycles.     

Transverse tensile behaviour of fibre reinforced titanium metal matrix composites

Four Ti MMCs have been tested in transverse tension, at ambient temperature and 600 °C. Generally, mechanical properties are reduced compared to monolithic Ti alloys. Transverse Young's modulus is, however, higher than in monolithic alloys, as a result of constraint of the matrix by the fibres.MMC proportional limits are associated with the onset of interfacial failure. Fibre coating cracking and longitudinal fibre splitting may also contribute to MMC yield and the associated acoustic emission peak. The fibre/matrix interface in IMI 834/SM1140+ appears to be weaker than in the other MMCs, resulting in a lower proportional limit and less acoustic emission. Final failure of the MMCs is generally via ductile shearing of matrix ligaments. The exception to this is IMI 834/SM1140+ in which the matrix fails in a brittle manner. This causes poor transverse tensile strength and failure strain in this MMC.A model to predict the MMC proportional limit, previously proposed by Jansson et al., has been modified to take account of the tensile strength of the fibre/matrix interface. The model previously used by Jansson et al. to predict the transverse tensile strength is acceptably accurate provided that the area fraction of matrix appearing on fracture surfaces is accurately determined.     

Rate-dependent tensile behavior of high performance fiber reinforced cementitious composites

High Performance Fiber Reinforced Cementitious Composites (HPFRCC) show strain hardening behavior accompanied with multiple micro-cracks under static tension. The high ductility and load carrying capacity resulting from their strain hardening behavior is expected to increase the resisting capacity of structures subjected to extreme loading situations, e.g., earthquake, impact or blast. However, the promise of HPFRCCs for dynamic loading applications stems from their observed good response under static loading. In fact, very little research has been conducted to investigate if their good static response translates into improved dynamic response and damage tolerance. This experimental study investigates the tensile behavior of HPFRCC using High strength steel fibers (High strength hooked fiber and twisted fiber) under various strain rates ranging from static to seismic rates. The test results indicate that the tensile behavior of HPFRCC using twisted fiber shows rate sensitivity while that using hooked fiber shows no rate sensitivity. The results also show that rate sensitivity in twisted fibers is dependent upon both fiber volume fraction and matrix strength, which influences the interface bond properties.     

Compression creep behaviour of steel fibre reinforced cement composites

The paper presents the results of an experimental investigation to determine the influence of steel fibre reinforcement on the creep of cement matrices under compression. Creep tests were carried out at a number of applied stress-strength ratios ranging between 0.3 to 0.9. Melt extract and hooked steel fibres were used at volume fractions ranging between 0 and 3% by volume of a mix. Three types of cement matrices were used namely cement paste, mortar and two mix proportions of concrete. The results indicate that steel fibres restrain the creep of cement matrices at all stress-strength ratios, the restraint being greater at lower stresses and at higher fibre contents. Steel fibres are effective in restraining only the flow component of creep of cement matrices, the delayed elastic component being unaffected. The reduction in creep of cement pastes, due to fibre reinforcement, is much greater than that for mortar or concrete matrices. Mathematical expressions are given for the creep of steel fibre reinforced cement matrices.

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Résumé Cet article présente les résultats d’une étude expérimentale en vue de déterminer l’influence du renforcement de fibres d’acier sur le fluage de matrices de ciment en compression. On a exécuté les essais de fluage à un certain nombre d’intervalles correspondant à des rapports contrainte/déformation allant de 0,3 à 0,9. On a utilisé des fibres d’acier dans des proportions se situant entre 0 et 3% par volume de mélange, et on s’est servi de trois types de matrices de ciment: pate de ciment, mortier et béton selon deux dosages. Les résultats montrent que les fibres d’acier empêchent le fluage des matrices de ciment dans tous les rapports de contrainte/déformation, l’effet étant plus important pour les contraintes faibles et les teneurs en fibres plus élevées. Les fibres d’acier n’agissent qu’en s’opposant à la déformation plastique des matrices de ciment sans que l’élasticité différée en soit affectée. La diminution du fluage des pates de ciment due au renforcement des fibres est beaucoup plus importante que pour le mortier ou les matrices de béton. On donne des formules mathématiques pour le fluage des matrices de ciment renforcé de fibres d’acier.

     

碳纤维水泥基材料电阻的非线性研究

研究了碳纤维水泥基材料(CFRC) 非线性电阻的伏安特性,并着重讨论了不同的碳纤维掺量和温度对CFRC材料电压-电阻关系的影响.结果表明:在较小外加电压下(＜2V)CFRC材料的电压-电阻曲线会出现明显的平台区,随着电压的进一步增大,其电阻逐渐降低呈现非线性特性.相同纤维掺量的水泥基材料随温度的升高其电压-电阻曲线下降斜率基本保持不变,但初始电阻值下降;而在温度保持不变时,随碳纤维掺量的增加,电阻随电压下降的趋势逐渐减缓.     

Crack growth resistance of hybrid fiber reinforced cement composites

Crack propagation in cement-based matrices carrying hybrid fiber reinforcement was studied using contoured double cantilever beam (CDCB) specimens. Influence of fiber type and combination was quantified using crack growth resistance curves. It was demonstrated that a hybrid combination of steel and polypropylene fibers enhances the resistance to both nucleation and growth of cracks, and that such fundamental fracture tests are very useful in developing high performance hybrid fiber composites. The influence of number of variables which would otherwise have remained obscured in normal tests for engineering properties become apparent in the fracture tests. The paper emphasizes the desired durability characteristics of these composites and discusses their current and future applications.     

Uniaxial and biaxial flexural fatigue of glass reinforced inter-penetrated network polymer composites

Conclusions Flexural fatigue of uniaxially and biaxially stressed IPN/glass mat composites was investigated using four point bend (4PB) and concentrically loaded (CL) specimen geometries. Regions of nearly constant bending moment between the inner spans of a 4PB beam and within the inner annulus of a CL circular plate yield quasi-uniform uniaxial and biaxial stress, respectively, on the tensile faces. The specimen dimensions were optimized for both loading geometries to give: (1) reduced specimen deflection through maximizing the ratio of the induced tensile stresses to the applied load, (2) minimized contact stresses by maximizing the induced stress with respect to the unit contact load, and (3) a large material volume exposed to the maximum cyclic stress (i.e., statistical fracture initiation).A power model was used to analyze the fatigue data for the 4PB and CL specimens. Both IPN composite materials studied fatigued more rapidly under the more severe loading conditions imposed by the CL specimen geometry.Fractography revealed that debond fracture was the dominant damage process for both geometries. The initial debond cracks were uniformly distributed throughout the stressed regions, confirming the presence of nearly uniform tensile stress. Damage localization followed after further cycling and was characterized by a locally high debond fracture density, fiber fracture, and always occurred where several glass strands crossed near the specimen surface. Final specimen failure resulted from the preferential growth of dominant cracks through the specimen thickness.