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.     

Mechanical behavior of cement-based materials reinforced with sisal fibers

Fiber-reinforced cement composites were produced in Brazil using blast furnace slag cement reinforced with pulped fibers of sisal originated from agricultural by-products. Thin pads were produced by slurring the raw materials in water, followed by de-watering and pressing stages. Studies of mechanical behavior included observations of stable crack growth behavior under monotonic loading (resistance-curve behavior), followed by scanning electron microscopy (SEM) analysis of the fracture surfaces. Reinforcement with cellulose fibers resulted in improved fracture toughness, even after 9 months in laboratory environment. Microscopic analysis indicated a considerable incidence of crack bridging and fiber pull-out in the composite. The shielding contributions from crack bridging are estimated using a fracture mechanics model, before comparing with the measured resistance-curve behavior.     

碳酸钙晶须对混杂纤维增强高延性水泥基复合材料力学性能的影响

为了探究碳酸钙晶须对钢纤维/PVA混杂纤维增强高延性水泥基复合材料(HyFRHDCC)力学性能的影响,利用2%体积掺量的廉价碳酸钙晶须替代部分纤维,研究了不同纤维掺量HyFRHDCC的压缩性能和拉伸性能,利用扫描电子显微镜观察了HyFRHDCC的微观结构。研究结果表明,引入碳酸钙晶须能够提高HyFRHDCC的初裂拉伸应变和峰前压缩韧性;在1.5%PVA+0.25%钢纤维HyFRHDCC中掺入2%碳酸钙晶须可以改善材料的拉伸性能;当PVA纤维减少至1%时,HyFRHDCC出现了明显的应变软化行为。微观形貌分析发现,碳酸钙晶须能够通过裂纹偏转、晶须拔出以及裂缝桥联等微观机制改善HyFRHDCC的应变硬化行为。     

水灰比对PVA纤维增强水泥基复合材料性能和显微结构的影响

对3种不同水灰比(0.2,0.4,0.65)形成的聚乙烯醇(PVA)纤维增强水泥基材料,通过三点弯曲试验,结合表观裂缝形状和裂缝处PVA纤维形态,研究了水灰比对材料弯曲性能的影响;通过对断裂面处纤维表面、纤维嵌入端和纤维拉断或拔出端的SEM影像分析,从微观层面研究了水灰比对PVA纤维-基体界面显微结构的影响。弯曲试验结果表明:随着水灰比增加,跨中部位裂缝数量明显增加,裂缝处拔出的纤维数量增多而拉断的数量减少,材料的弯曲韧度和开裂强度到弯曲强度的增强幅度提高。界面显微结构表明:随着水灰比增加,基体结构由致密变疏松,界面粘结力减弱,桥接裂缝的PVA纤维状态由瞬间猝断转变为滑动拔出且表面有轻微刮削,纤维对材料增强增韧的效率显著提高。     

聚乙烯醇纤维增强钢渣粉-水泥复合材料基本力学性能及微观结构

通过掺加钢渣粉来制备聚乙烯醇（PVA）纤维增强钢渣粉-水泥基复合材料,从宏微观两个方面研究了这种复合材料的性能。考虑了基体材料的水胶比（0.25和0.35）、不同钢渣粉质量分数（0、30wt%、60wt%、80wt%）,采用抗压强度试验、薄板四点弯曲试验研究了PVA纤维增强钢渣粉-水泥基复合材料的基本力学性能变化规律及其在弯曲荷载作用下的裂缝控制能力,采用扫描电镜观测了破坏后试样的微观结构。结果表明,水胶比和钢渣粉掺量均可明显影响PVA纤维增强钢渣粉-水泥基复合材料的基本力学性能,在低水胶比条件下（水胶比为0.25）,钢渣粉掺量达到80wt%时,试样表现出较高的韧性指数和良好的裂缝控制能力,基本满足工程所需强度要求,水胶比为0.35时钢渣掺量不宜超过60wt%;同时,从节能减排的角度考虑,利用钢渣粉制备PVA纤维增强钢渣粉-水泥基复合材料是可行的。      

单边切口薄板研究聚乙烯醇纤维增强水泥基复合材料断裂韧性

利用单边切口薄板对配制的聚乙烯醇纤维增强水泥基复合材料进行单轴直接拉伸试验研究,得到硬化的荷载-裂缝张开位移全曲线。通过试验观察到缺口尖端处出现呈发散状的多条微小裂纹,部分试件在远离切口处还有多条裂缝出现,并不像混凝土或水泥净浆这类半脆性、脆性材料只有单条裂缝并沿着单条路径开裂,因此,适用于应变软化材料的双K断裂理论以及断裂能理论不能直接用在应变已经发生假硬化的材料中。鉴于上述原因,该文提出起裂断裂韧度和耗散能两个韧性评价指标。     

Influence of fiber and grain bridging on crack profiles in SiC fiber-reinforced alumina-matrix composites

Initial crack extension and crack propagation in sintered SiC fiber reinforced alumina matrix composites were observed in situ in the scanning electron microscope (SEM). In these composites, coupled toughening mechanisms associated with both grain and fiber bridging are operative in the crack wake. Their contribution to toughness were obtained from crack profiles measured in the crack wake at various applied loads and crack lengths. The crack profiles of the fiber reinforced samples reveal much smaller crack opening displacements compared to the monolithic samples. The fibers act as ligaments which bridge and thereby exert closure stresses on the crack surfaces. The profiles revealed pronounced reduction in the crack opening displacement (COD) around the fiber positions. Accordingly, grain bridging in the vicinity of the fibers was still operative up to an applied stress intensity factor of 5.7 MPa m^1/2 which is 30% above the maximum toughness that could be obtained with the monolithic samples for the same crack length.     

Flexural behavior of geopolymer composites reinforced with steel and polypropylene macro fibers

Like ordinary Portland cement concrete, the matrix brittleness in geopolymer composites can be reduced by introducing appropriate fiber reinforcement. Several studies on fiber reinforced geopolymer composites are available, however there is still a gap to understand and optimize their performance. This paper presents the flexural behavior of fly ash-based geopolymer composites reinforced with different types of macro steel and polypropylene fibers with higher aspect ratio. Three types (length-deformed, end-deformed and straight) of steel fibers and another type of length-deformed polypropylene fiber with optimum fiber volume fraction of 0.5% are studied. The effects of different geometries of the fibers, curing regimes (ambient cured and heat cured at 60 °C for 24 h) and concentration of NaOH activator (10 M and 12 M) on the first peak strength, modulus of rupture and toughness of the geopolymer composites are investigated. The quantitative effect of fiber geometry on geopolymer composite performance was also analyzed through a fiber deformation ratio. The compressive strength, splitting tensile strength and flexural toughness are significantly improved with macro fibers reinforcement and heat curing. The results also show that heat curing increases the first peak load of all fiber-reinforced geopolymers composites. End-deformed steel fibers exhibit the most ductile flexural response compared to other steel fibers in both heat and ambient-cured fiber reinforced geopolymer composites.     

Modeling of fiber toughening in cementitious materials using an R-curve approach

This paper presents the theoretical formulation describing the role of fibers in enhancing the fracture toughness of quasi-brittle cement based materials. The formulation is based on the well known R-curve approach which correlates the increase of the apparent fracture toughness of a material with the existence of a pre-critical stable crack growth region.By assuming that the critical crack length in plain matrix is a function of an initial crack length a ₀, a formulation for the R-curves has recently been derived and applied to predict the response of positive and negative geometry specimens of various sizes and materials. This approach is further applied to uniaxial tensile specimens containing various fiber types. Fiber reinforcement is modeled by means of applying closing pressure on crack surfaces resulting in closure of the crack faces and a decrease in the stress intensity factor at the tip of the propagating crack. Incorporation of these two factors in the energy balance equations for crack growth results in increases in both the slope and the plateau value of the R-curve representing matrix response. Enhancement in material response is shown to occur only if precritical crack growth exists, causing fibers to convert the stable cracking process into an increase in load carrying capacity of the material. Fracture response of fiber reinforced composites can be predicted up to the bend-over-point. The theoretical predictions are compared with the experimental results of cement-based composites containing unidirectional, continuous glass, steel or polypropylene fibers.     

Effect of shrinkage reducing admixture on flexural behaviors of fiber reinforced cementitious composites

The use of shrinkage reducing admixture (SRA) at various concentrations was investigated in fiber reinforced cementitious composites. Both mortar and high strength concrete (HSC) matrices were tested. Two types of fibers—steel and polypropylene—were assessed. The effect of SRA was measured on the fundamental properties such as surface tension of the bulk fluids and the contact angle developed between the fibers and the bulk fluids, on the fresh properties such as the air content and the density, and finally on the hardened mechanical properties, specially the flexural behaviors. It was noted that SRA enhances the wettability of fibers and reduces the air content of fiber reinforced cement mortars, while critical SRA concentrations are existing. SRA with critical concentration can significantly improve the flexural toughness and residual strength of steel fiber reinforced cement mortar. In the case of polypropylene fiber, SRA is not as effective in enhancing the flexural behaviors as it is in the case of steel fiber. SRA is generally ineffective in reducing the air content of HSC and the properties of steel fiber reinforced HSC with SRA are inferior to those without SRA.     

钢筋增强超高韧性水泥基复合材料(RUHTCC)梁弯曲性能影响的几何参数分析

具有超高韧性新型随机PVA短纤维增强的水泥基复合材料(UHTCC)代替传统的具有准脆性应力软化特征的混凝土或纤维混凝土材料制作的钢筋(RUHTCC)受弯梁,可提高承载力,改善构件的延性,并具有良好的损伤演变能力,被认为是一种抗震性能较好的新型构件形式。除了配筋率和UHTCC拉压材料性能外,截面几何尺寸是影响其弯曲性能的一个重要因素。基于受弯理论分析和试验验证,采用该理论公式对截面几何尺寸(截面高度、宽度以及面积)的影响规律进行了系列分析。结果发现:对承载力,梁高度比宽度影响明显,而对承载力提高幅度和变形而言,随梁高的增加而减小,梁宽没有影响;对裂缝控制来说,只要梁下边缘的极限拉应变小于UHTCC材料的极限拉应变,截面尺寸的变化几乎不影响裂缝宽度的大小。并进一步针对RUHTCC梁的受弯设计提出了一些设计建议。     

Creep crack growth in fiber reinforced composites with rate-dependent bridging - Role of a viscous fiber matrix interface

A model is developed for creep crack growth in continuous fiber reinforced composites wherein a growing matrix crack is subjected to rate-dependent bridging by unbroken fibers. The rate-dependence in this model arises as a result of the presence of a viscous fiber/matrix interfacial layer. Under load this layer undergoes shear flow causing time-dependent pull-out of bridging fibers from the crack surfaces. The mechanics of time-dependent bridging is combined with a failure criterion based on secondary failure in a crack-tip creep process zone. The dependence of the matrix creep crack growth rates on flaw size and crack wake parameters as well as on composite microstructure is derived. It is shown that the crack wake plays a predominant role in influencing not only the magnitude of creep crack growth rates but also the relationship of growth rates to the crack sizes. A closed form expression is derived for the dependence of crack growth rates on loading, creep and bridging parameters in the regime wherein crack growth rates are independent of crack size.     

Fracture characterization of random short fiber reinforced thermoset resin composites

The fracture toughness characterization of random fiber reinforced polymer composites has been investigated by several research groups in recent years. This paper discusses the methods and results of some of these investigations with regard to the applicability of classical linear elastic fracture mechanics approaches to such materials. In polymers randomly reinforced with short fibers (0.5 mm) the region of inelastic behavior ahead of crack tips is sufficiently small that LEFM toughness tests are valid with standard specimen sizes. However, calculations suggest that during fracture of composites with “long” fibers (24 or 50 mm), inelasticities occur to such an extent that the small scale yielding requirements of LEFM are not satisfied. An alternative approach based on the material tension-softening curve may be more appropriate to characterize fracture toughness in fiber reinforced composites.     

Comparison of fibres for creep strengthening of zinc-aluminium foundry alloys

A comparative evaluation is made of a variety of possible fibrous reinforcements for strengthening zinc-aluminium foundry alloys. The composites are processed by squeeze casting, using preforms of alumina, carbon, stainless steel or low carbon steel fibres. A drastic improvement of the creep strength is achieved with the use of alumina or steel fibres. However, an acceptable level of fracture toughness is maintained only in the composites reinforced with steel fibres. This property results from the low interface adhesion which allows bridging of the crack by the fibres. Low carbon steel fibres do not exhibit more interface reaction than stainless steel fibres. It is concluded that low carbon steel fibres provide a better compromise when taking into account the creep strength, the fracture toughness and the cost of the composite.     

Fracture processes of hybrid fiber-reinforced mortar

Subregion Scanning Computer Vision (SSCV), a digital image based method for measuring surface deformation is used to examine the role of the fibers in the fracture process of mortars reinforced with hybrid blends of microfiber (less than 22 μm in diameter) and macrofiber (500 μm in diameter). Closely-spaced microfibers interact with cracks at the microstructural level and hamper the widening of coalesced microcracks, thus encouraging the growth of multiple cracks. The microfibers improved pre-peak mechanical performance and strength by delaying the formation of a through-specimen macrocrack. Macrofibers were most effective at bridging macrocracks and imparting ductility to the composite due to their geometry and greater length. Compared to mortar reinforced with a single fiber type, an increase in strength and toughness was seen with a blend of steel macrofibers and either steel or PVA microfibers. Finally, based on the crack topography observed, the reduction in water permeability of cracked mortar achieved with hybrid fiber-reinforcement, measured directly in a parallel study, was governed by multiple crack development.     

Synchronous effects of multiscale reinforced and toughened CFRP composites by MWNTs-EP/PSF hybrid nanofibers with preferred orientation

MWNTs-EP/PSF (polysulfone) hybrid nanofibers with preferred orientation were directly electrospun onto carbon fiber/epoxy prepregs and interlaminar synchronously reinforced and toughened CFRP composites were successfully fabricated. With MWNTs-EP loading increasing, the oriented nanofibers were obtained accompanying with enhanced alignment of inner MWNTs-EP. Flexural properties and interlaminar shear strength of composites were improved with increasing MWNTs-EP loadings, whereas fracture toughness attained maximum at 10 wt% MWNTs-EP loading and then decreased. Based on these results, multiscale schematic modeling and mechanism schematic of hybrid nanofibers reinforced and toughened composites were suggested. Due to the preferred orientation of nanofibers, MWNTs-EP was inclined to align vertically to carbon fiber direction along the in-plane of interface layer. The proposed network structures, containing four correlative phases of MWNTs-EP/PSF sphere/carbon fiber/epoxy matrix, contributed to simultaneous improvement of strength and toughness of composites, which was realized by crack pinning, crack deflection, crack bridging and effective load transfer.     

晶须增韧陶瓷基复合材料裂纹扩展行为模型

在考虑晶须桥联和裂纹转两种增韧机理的基础上，建立了晶须增韧陶瓷复合材料裂纹扩展行为的理论模型，利用该型计算了单边切口梁三点弯曲的Ｒ－阻力曲线和载荷－位移曲线。     

Tensile fracture process of Strain Hardening Cementitious Composites by means of three-dimensional meso-scale analysis

There are a wide variety of short fiber reinforced cement composites. Among these materials are Strain Hardening Cementitious Composites (SHCC) that exhibit strain hardening and multiple cracking in tension. Quantitative material design methods considering the properties of matrix, fiber and their interface should be established. In addition, numerical models to simulate the fracture process including crack width and crack distribution for the material are needed.This paper introduces a numerical model for three-dimensional analysis of SHCC fracture, in which the salient features of the material meso-scale (i.e. matrix, fibers and their interface) are discretized. The fibers are randomly arranged within the specimen models. Load test simulations are conducted and compared with experimental results. It is seen that the proposed model can well simulate the tensile failure of Ultra High Performance-Strain Hardening Cementitious Composites (UHP-SHCC) including strain-hardening behavior and crack patterns. The effects of matrix strength, its probability distribution inside the specimen and fiber distribution on the tensile fracture are numerically investigated. Consideration of the probability distributions of material properties, such as matrix strength, appears to be essential for predicting the fracture process of SHCC.     

Tensile strength enhancement in interground fiber cement composites

Interground fiber cement (IFC) is a new process where fibers are ground in with the cement clinker during the dry cement manufacturing process. With IFC considerable strength enhancement can be achieved compared to ordinary cement even at a fiber volume as low as 0.2% due to homogeneous fiber distribution and fiber surface modifications associated with the milling process. The cracking mechanisms associated with the strength enhancement were observed in real time during load application using a custom designed loading device. The homogeneous fiber distribution stabilizes crack growth. Formation of multiple, stable secondary microcracks was observed during the strain hardening regime, enhancing the strain capacity at ultimate strength. Fiber pullout was the dominant toughening mechanism in the strain softening regime. For fibers inclined to the propagating crack, fiber pullout was preceded by secondary microcrack formations along the fiber/matrix interface.     

碳纤维增强油井水泥石的力学性能

油井水泥石在井下易脆裂,造成油井层间封隔失效,进而影响油井开采。为了解决这一问题,需要对水泥石进行降脆增韧。首先,考察了甲基纤维素和羧甲基纤维素对碳纤维的分散效果;然后,研究了碳纤维对油井水泥石抗压强度、抗折强度和劈裂抗拉强度的影响,并模拟井下环境测试了水泥石单轴和三轴应力-应变曲线;最后,使用扫描电子显微镜对碳纤维增强水泥石的微观形貌进行观察,探讨碳纤维对水泥石的增韧机制。结果表明:0.2wt%的羧甲基纤维素溶液可有效分散碳纤维;养护28d后,0.3wt%碳纤维增强水泥石的抗压强度、抗折强度和劈裂抗拉强度较纯水泥石的分别提高8.6%、31.5%和52.4%,三轴直接加载条件下,其弹性模量较纯水泥石的低49.5%;经过分散的碳纤维在水泥石中乱向分布,形成三维网络结构,通过桥联、剥离及拔出耗能作用增强水泥石。研究结果为解决油井水泥石易脆裂的问题提供了理论参考。