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.     

Damage Mechanisms in Injection Molded Unreinforced, Glass and Carbon Reinforced Nylon 66 Spur Gears

Application of short fiber reinforced thermoplastic materials are limited to functionally less important components due to heterogeneous characteristics of material and incomplete understanding of failure mechanisms involved. Reinforcement not only affects mechanical and electrical properties but also the failure mechanisms. Gears used in power/motion transmission may fail in many ways. This paper discusses the various types of failures exhibited by unreinforced and fiber reinforced Nylon 66 gears. Injection molded unreinforced, glass reinforced and carbon reinforced Nylon 66 spur gears were tested in a power absorption type gear test rig. Failed gears were observed under optical and scanning electron microscope to understand the damage mechanism. Different types of failures such as gear tooth wear, cracking at the tooth surface, tooth root cracking and severe tooth shape deformation were observed. Material compositions and applied torque decides the type of failure mechanism. Low interfacial strength between fibers and matrix in the reinforced gears causes fiber pullout. Reinforced gears exhibited longer life compared with the unreinforced gears due to superior mechanical strength and thermal resistance.     

A theoretical solution for the pullout properties of a single FRP rod embedded in a bond type anchorage

ABSTRACT

This paper proposes a theoretical solution for predicting the pullout properties of a single fiber-reinforced polymer (FRP) rod embedded in a bond type anchorage based on a trilinear bond–slip model. The radial variation of the shear stress and reaction of the steel sleeve are considered in the solution. Pullout procedure with elastic, elastic-softening, elastic-softening-debonding, pure softening, softening-debonding, and debonding stages, as well as the corresponding critical stages, are analyzed. In this theoretical solution, the maximum pullout load, shear stress along the rod–grout interface, axial tensile stress of the FRP rod, and load–slip relationship are derived with explicit formulations. Effective bond length of bond type anchorage is also discussed. The solution is validated against experimental results available in literature. The theoretical solution reveals that the anchorage may attain its maximum pullout load in the elastic-softening, pure softening, or elastic-softening-debonding stage. Moreover, the effects of embedded length, ultimate shear stress, and residual shear stress on maximum pullout load closely related with the stage in which the anchorage attains its maximum pullout load. However, the effect of radius of FRP rod on the maximum pullout load increases with the embedded length, no matter in which stage the anchorage attains its maximum pullout load.     

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.     

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.     

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.     

力、热载荷作用下纤维在复合材料中的界面应力传递行为

采用三维光弹性实验应力分析和有限元计算两种方法,在拉拔载荷和热残余应力联合作用下,对单丝拔出树脂基复合材料三维冻结切片界面剪应力进行了研究。实验结果和计算表明,在单纤维与基体界面的埋入端及埋入末端附近出现界面残余剪应力的极值;力、热载荷作用下纤维界面剪应力呈抛物线分布,单丝埋入端附近是应力的主要传递区域,最先达到危险应力,出现界面脱胶破坏,然后剪应力沿纤维埋入长度由纤维埋入端附近向埋入末端逐渐传递;界面热残余应力对界面剪应力的影响是使纤维埋入末端应力集中程度降低,使界面剪应力最大值增大。      

异型钢纤维拉拔界面模型

主要提出了一种新型的异型纤维拉拔界面模型,完成了整个拉拔过程中拉拔荷载与拉拔位移的对应计算。同传统的异型纤维拉拔计算模型相比,本模型不仅考虑到了拉拔过程中由于纤维的塑性变形而且考虑到了界面压力变化导致的界面摩擦变化所引起的荷载阻抗,此外,模型中还量化模拟了基体材料的剥落损伤行为及其对载荷释放的影响。最后从计算模拟结果与试验结果的对比来看,本模型具有较好的适应性。     

Predicting the pullout response of inclined straight steel fibers

Fiber pullout tests have been used for decades to characterize and optimize bond strength on fiber reinforced concretes. However most of the investigations focus on the behavior of fibers aligned with load direction whose pullout mechanisms are not representative of the ones existing in real applications, where random orientation of fibers is likely to occur. In this paper a new predictive model for the pullout response of steel fibers embedded in cement matrices with any inclination respect to loading direction is provided. Comparisons with experimental results highlight the capacity of the model on describing appropriately the entire load–crack width behavior. The procedure differentiates itself from previous works by introducing clear and comprehensive concepts within a straightforward approach.     

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.     

CFRP筋粘结型锚固系统数值模拟及失效分析

针对碳纤维增强复合材料(CFRP)筋粘结型锚固系统,采用数值分析研究了不同锚具内壁与锚固胶体间的摩擦系数、不同锥形倾角、锚具前端有无直筒过渡段及有无端堵约束对筋材应力状态的影响,找出了筋材在靠近锚具端部发生破坏的原因,提出了相关强度分析方法及锚固改进方法,并用试验结果验证了方法的有效性。分析结果表明:筋材在摩擦系数为0.7时受到的径向挤压应力峰值只有摩擦系数为0.3时受到的径向挤压应力峰值的45.01%;随着锥形倾角增大,CFRP筋在锚具锥段所受的径向挤压应力峰值逐渐减小,所受剪应力峰值逐渐增大;在锥段末端增设直筒过渡段可以缓解筋材所受到的径向挤压应力与剪应力峰值,缓解系数分别为76.78%与52.90%;端堵约束的存在使锚具端部应力集中现象明显,导致筋材在锚具端部发生破坏。     

Properties of high-strength steel fiber-reinforced concrete beams in bending

This paper presents research results of ten high-strength reinforced concrete beams and steel fiber-reinforced high strength concrete beams, with steel fiber content of 1% by volume. The enlarged ends of mild carbon steel fibers with three different dimensions were selected. This research shows that the flexural rigidity before yield stage and the displacement at 80% ultimate load in the descending curve are improved, and crack number and length at comparable loads is reduced after the addition of steel fibers. The descending part of the load-displacement curve of the concrete beams without steel fibers is much steeper than that with steel fibers, which shows that the addition of steel fibers makes the high strength concrete beams more ductile.     

Static and fatigue mechanical characterizations of variable diameter fibers reinforced bone cement

Fibers can be used to improve the mechanical properties of bone cement for the long-term stability of hip prostheses. However, debonding of the fibers from the matrix due to the poor fiber/matrix interface is a major failure mechanism for such fiber reinforced bone cements. In this study, a novel fiber (variable diameter fibers or VDFs) technology for reinforced bone cement was studied to overcome the interface problem of short-fiber composites. These fibers change their diameters along their length to improve the fiber/matrix interfacial bond by the mechanical interlock between the VDFs and the matrix. A novel composite made from novel ceramic VDFs incorporated in PMMA matrix was developed. Both static and fatigue tests were carried out on the composites. Conventional straight fiber (CSF) reinforced bone cement was also tested for comparison purposes. Results demonstrated that both the stiffness and the fatigue life of VDF reinforced bone cement are significantly improved (P < 0.05) compared with the unreinforced bone cement. VDF contents of 10% by volume increased the fatigue life over unreinforced bone cement by up to 100-fold. Also, the fatigue life and modulus of toughness of VDF reinforced cement were significantly greater than those of CSF reinforced cement (P < 0.05 and P < 0.001, respectively). Scanning electron microscopy (SEM) micrographs revealed that VDFs can bridge the matrix cracks effectively and pullout of VDFs results in much more extensive matrix damage than pullout of CSFs increasing the resistance to fatigue. Therefore, VDF reinforced cement was significantly tougher, having a greater energy dissipation capacity than CSF reinforced cement. VDFs added to bone cement could potentially avoid implant loosening due to the mantle fracture of bone cement and delay the need for revision surgery.     

Geometrical and mechanical aspects of fabric bonding and pullout in cement composites

Fabric reinforced cement composites are a new class of cementitious materials with enhanced tensile strength and ductility. The reinforcing mechanisms of 2-D fabric structures in cement matrix are studied using a fabric pullout model based on nonlinear finite difference method. Three main aspects of the composite are evaluated: nonlinear bond slip characteristic at interface; slack in longitudinal warp yarns, and mechanical anchorage provided by cross yarn junctions. Parametric studies of these key parameters indicate that an increase in the interfacial bond strength directly increases the pullout strength. Grid structures offering mechanical anchorage at cross yarn junctions can substantially increase the pullout resistance. Presence of slack in the yarn geometry causes an apparently weaker and more compliant pullout response. The model was calibrated using a variety of test data on the experimental pullout response of AR-Glass specimens, manufactured by different techniques to investigate the relative force contribution from bond at interface and from cross yarn junctions of alkaline resistant glass fabric reinforced cement composites.     

Fabrication and Testing of Carbon Fiber Reinforced Truss Core Sandwich Panels

Truss core sandwich panels reinforced by carbon fibers were assembled with bonded laminate facesheets and carbon fiber reinforced truss cores.The top and bottom facesheets were interconnected with truss cores.Both ends of the truss cores were embedded into four layers of top and bottom facesheets.The mechanical properties of truss core sandwich panels were then investigated under out-of-plane and in-plane compression loadings to reveal the failure mechanisms of sandwich panels.Experimental results indicated...     

Investigation of the damage evolution of metal inserts embedded in carbon fiber reinforced plastic by means of computed tomography and acoustic emission

Carbon fiber reinforced plastics are promising materials for lightweight structures, for instance in automotive applications due to their outstanding specific mechanical properties. However, the load transfer in structural carbon fiber reinforced plastics parts via a detachable connection poses a challenge for the composite. Conventionally, the parts have to be drilled for this purpose whereby the fiber continuity is interrupted and hence the associated local stress accumulation decreases the load bearing capacity of the composite. This can be prevented by using embedded metal elements, so‐called inserts, for joining parts in structures. The damage behavior under tensile loading of inserts turned out to be extremely complex and is based on different mechanisms. In order to understand the damage evolution under tensile loading detailed knowledge about the deformation of the insert, crack growth in the laminate and debonding between metal insert and carbon fiber reinforced plastics is necessary. This paper aims for an investigation of the in situ failure behavior during tensile loading of composite sheets equipped with two different types of inserts by means of acoustic emission and computed tomography analysis. An inductive strain gauge was additionally installed underneath the laminate when performing the tensile tests monitored by acoustic emission analysis.     

Experimental analysis on bond between PBO-FRCM strengthening materials and concrete

The effectiveness of externally bonded strengthening for reinforced concrete (RC) elements strongly depends on the bond between the strengthening material and the concrete and on the mechanical properties of the concrete cover. In this paper the bond between fiber reinforced cementitious matrix (FRCM) materials made out of a poliparafenilenbenzobisoxazole (PBO) net embedded in a cement based matrix and the concrete is experimentally analyzed. Experimental results of double shear tests involving different bond lengths and fibers cross sections are presented. The results allow to estimate the effective anchorage length and evidence that the debonding occurs at the fibers/matrix interface after a considerable fibers/matrix slip. They also confirms the effectiveness of the FRCM materials as external reinforcements for concrete. The obtained experimental results can be used to calibrate a local bond-slip relation to be used in the design of the external reinforcement.     

Transverse plastic deformation of metal-matrix with randomly arranged continuous fibers

Within the framework of continuum plasticity theory, a numerical analysis in this investigation is made of the role of microstructures of fibrous composites against transverse plastic flow by means of the finite element method (FEM). In this way, the effective mechanical properties can be related quantitatively to the micro structures of composites reinforced by randomly arranged fibers. The effects of different cross-sectional geometry, such as the fiber shape (circular, square and lozenge), size, and random fiber distribution on the transverse elastic and plastic deformation of the metal-matrix composites with specific randomly distributed, aligned continuous fibers, are examined. Numerical results show that the overall transverse plastic flow of the composites is rather sensitive to the fiber geometric parameters while the elastic properties exhibit a much lower sensitivity to the fiber distribution. The interference of fibers with flow paths is seen from stress contours analysis to play an important role in the transverse strengthening due to the constraint imposed by the reinforcements. The calculations of the alterations in matrix field quantities in response to controlled changes in the random fiber distribution give valuable insights into the effects of fiber clustering on the transverse tensile properties.     

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.     

Determination of mechanical strength properties of hemp fibers using near-infrared fourier transform Raman microspectroscopy

Fourier transform near-infrared (FT-NIR) Raman microspectroscopy was adopted for analyzing the micro mechanical tensile deformation behavior of cellulosic plant fibers. Mechanical strength parameters such as tensile strength, failure strain, and Young's modulus of diversified hemp fibers were determined within the range of single fiber cells and fiber filaments. The analysis of fiber deformation at the molecular level was followed by the response of a characteristic Raman signal of fiber cellulose that is sensitive to the tensile load applied. The frequency shift of the Raman signal at 1095 cm(-1) to lower wavenumbers was observed when the fibers were subjected to tensile strain. Microstructural investigations using electron microscopy under environmental conditions supported the discussion of mechanical properties of hemp fibers in relation to several fiber variabilities. Generally, mechanical strength properties of diversified hemp fibers were discussed at the molecular, microstructural, and macroscale level. It was observed that mechanical strength properties of the fibers can be controlled in a broad range by appropriate mercerization parameters such as alkali concentration, fiber shrinkage, and tensile stress applied to the fibers during the alkaline treatments.