Experimental investigation and modelling of the temperature effects on the tensile behavior of textile reinforced refractory concretes

The work in hand presents results of an experimental and numerical research on the post-heating residual strength of a basalt textile refractory composite submitted to tensile loading. The tensile tests were performed after a preheating process at temperatures ranging from 25 to 1000 °C. The mechanical performance and cracking mechanisms were discussed and compared to that obtained at room temperature. Image analysis by means of digital image correlation method was used to obtain the evolution of crack width which was subsequently correlated with the stress response for all target temperatures. Scanning electron microscopy was used to investigate the damage processes in the fiber–matrix interfaces after exposure to high temperatures. A finite difference model was used to simulate the tension stiffening behavior of TRC (Textile Reinforced Concrete) systems predicting their crack spacing and stress vs. strain responses. The obtained results indicated that due to the coating decomposition the reliability of basalt TRC can only be guaranteed from room temperature to 150 °C.     

短切纤维及预应力对玄武岩织物增强水泥基复合材料拉伸力学性能的影响

通过静态拉伸试验研究不同体积掺量的短切碳纤维、钢纤维、耐碱玻璃纤维及预应力对5层玄武岩织物增强水泥基复合材料(BTRC)拉伸性能的影响。试验结果表明:短切碳纤维、玻璃纤维可以提高基体和BTRC的开裂强度,且开裂强度随着碳纤维掺量的增加而增加;预应力使基体产生预压力,明显提高其开裂强度。短切纤维及预应力都显著提高BTRC的峰值荷载和韧性,但峰值应变基本不变;峰值荷载和韧性随着钢纤维掺量的增加而增加,体积分数为1.5vol%掺量时达到最大值;随着碳纤维掺量增加,峰值荷载和韧性先增加后减小,体积分数为1.0vol%掺量时最大。施加预应力且掺入短切碳纤维或钢纤维时,短切纤维增强的基体可以更好地承受张拉力释放后纤维束径向变形引起的环向应力,进一步提高了织物与基体界面的挤压作用力及摩擦力,从而增强效果最明显,峰值荷载分别增加50.4%和58.9%,韧性分别增加84.7%和79.5%。BTRC材料掺入短切玻璃纤维、钢纤维及施加预应力均可以增加其受力后的裂缝条数,减小裂缝间距和裂缝宽度。     

An experimental analysis of fracture mechanisms of short glass fibre reinforced polyamide 6,6 (SGFR-PA66)

In the present work, toughness of unfilled polyamide 6,6 (PA66) and short glass fibre reinforced polyamide 6,6 (SGFR-PA66) was investigated. Digital image correlation (DIC) was used with a single camera for in-plane displacement field measurement and then strain computation. The results allowed to extract the resistance curve for the PA66 and critical stress intensity factors, K_Ic, for the SGFR-PA66 with three glass fibre contents (15%, 30% and 50% (wt)) and under room temperature (20 °C). The tests were carried out on single edge notched tension (SENT) specimens. The DIC technique allowed to precise the spatial distribution of the local strains in a defined region including the crack tip at different steps of the loading. Scanning electron microscopy observations illustrated different damage mechanisms occurring in the studied composites: matrix crack, fibre–matrix interface failure and fibres pull out.     

Experimental investigation of surface crack initiation, propagation and tension stiffening in self-compacting steel–fibre-reinforced concrete

To investigate crack initiation and propagation in reinforced, self-compacting, steel?Cfibre-reinforced concrete (SCSFRC) members, tie elements were tested in tension. Strain and surface crack formation were monitored with an optical strain measurement system based on digital image correlation. In addition, to capture the softening behaviour (???Cw) of the material, uni-axial tension testing was performed on SCSFRC cylinders. The results show that, with the optical strain measurement system, it was possible to detect different cracking modes and to follow the crack growth. It was especially of interest to recognize that high fibre amounts tend to change a sudden opening of a crack (as in non-fibrous concrete) into a more stable procedure. It was found that, for a given crack width, the SCSFRC specimens exhibited a noticeably higher tension stiffening than the specimens without fibres. Moreover, at a given load, the crack widths decreased by as much as 65% for the SCSFRC specimens with a nominal fibre content of 1%. For the uni-axial tension tests the results showed that with higher fibre content, for this type of fibre and concrete, both the peak stress and the residual tensile stress were increased. Additionally, it was noted for both specimen types that the scatter in fibre distribution decreased with increasing fibre content.     

Modeling of tension stiffening in reinforced cement composites: Part II. Simulations versus experimental results

This is the second part of a two-part paper involving a numerical model for simulations of tensile behaviour of reinforced cement-based composites. The model simulates the tensile stress strain response of a brittle matrix composite, tension stiffening effect of cracked matrix, and crack spacing evolution in tension members. The paper presents the simulations of four independent experimental results obtained from literature: steel reinforced concrete, concrete reinforced with steel and glass fiber reinforced plastic (GFRP), alkali resistant (AR) glass textile reinforced concrete and AR glass fabric reinforced cement pastes. The first and third experiments had complete input information for the simulations, and the predicted responses compare quite well to the experimental results. The second and last experiments did not have complete input data but, the properties can be estimated from other sources or by means of back calculations. The predicted responses reasonably agreed with the experimental results.     

Modeling of tension stiffening in reinforced cement composites: Part I. Theoretical modeling

This is the second part of a two-part paper involving a numerical model for simulations of tensile behaviour of reinforced cement-based composites. The model simulates the tensile stress strain response of a brittle matrix composite, tension stiffening effect of cracked matrix, and crack spacing evolution in tension members. The paper presents the simulations of four independent experimental results obtained from literature: steel reinforced concrete, concrete reinforced with steel and glass fiber reinforced plastic (GFRP), alkali resistant (AR) glass textile reinforced concrete and AR glass fabric reinforced cement pastes. The first and third experiments had complete input information for the simulations, and the predicted responses compare quite well to the experimental results. The second and last experiments did not have complete input data but, the properties can be estimated from other sources or by means of back calculations. The predicted responses reasonably agreed with the experimental results.     

Experimental and numerical studies in model composites Part I: Experimental results

Results on strength, apparent toughness, fatigue crack growth and fiber debonding on specially made composite materials are reported. The compact tension composite specimen used consisted of an epoxy matrix and layers of long aligned glass or kevlar fibers that were equally spaced. The experimental data on crack initiation strength showed that for a range of fiber spacing , the composite's strength _A , scaled with the fiber spacing in the form of % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaciaacaqabeaadaqaaqaaaOqaaiabeo8aZnaaBa% aaleaaieGacaWFbbaabeaakmaakaaabaGaeq4UdWgaleqaaOGaeyyp% a0JaeqOUdSgaaa!3EB5!\[\sigma _A \sqrt \lambda = \kappa \]. The apparent toughness of the composite specimens increased with a decrease in fiber spacing. Two sets of fatigue crack propagation experiments were performed. The first one was on specimens with the same fiber spacing and under different applied loads. The second set was on specimens with different fiber diameter and the same loading conditions. While crack arrest was observed in the first set, crack arrest was seen in the second set for the relatively large diameter fibers and specimen fracture for the relatively thin fibers. A method, based on fracture mechanics principles and crack opening displacements, for evaluating bridging tractions is outlined. Using this method, simulations for the bridging tractions and stress intensity factor were carried out using a linear crack opening profile. The total stress intensity factor was found to decrease with crack length. The debonding in the bridging zone, on specimens with different fiber spacing, was evaluated using a one dimensional debonding analysis. The model was calibrated with the debonding on the first fiber and consequently used to describe debonding on the bridging zone of specimens with different fiber spacing. In spite of the assumptions adopted in the present studies, the model seems to describe debonding well.     

Cracking mechanisms in durable sisal fiber reinforced cement composites

Fiber reinforced cement composite laminates with long sisal fibers were manufactured using a cast hand lay up technique. A matrix with partial cement replacement by metakaolin and calcined waste crushed clay brick was used in order to improve the durability aspects. Mechanical response was measured under tension and bending tests while crack formation was investigated using a high resolution image capturing procedure. Crack spacing was measured using image analysis and correlated with the applied strain under both the tensile and bending response. Various stages of loading corresponding to initiation, propagation, distribution, opening, and localization of a crack system in the specimen are discussed. The effect of flexural cracking on the location of neutral axis during the bending tests was measured using strain-gages.     

Fatigue crack growth with fiber failure in metal-matrix composites

Crack growth during the fatigue of fiber-reinforced metal-matrix composites can be predicted analytically by determining the reduction in the crack tip stress intensity range resulting from fiber bridging. Various canonical functions exist that relate the crack tip stress intensity range to bridged crack geometries and loading for both infinite and finite width specimens; however, comprehensive crack growth predictions incorporating fiber failure require knowledge of the maximum fiber stress in the bridged zone for all notch sizes and crack lengths. Previous modeling efforts have been extended to predict complete growth curves with fiber failure for specimens of finite width. Functions for maximum fiber stresses in the bridged zone are presented here for a center crack in tension and edge cracks in tension and bending. The rapid increase in crack growth when fibers fail emphasizes the importance of determining the loads and notch sizes that mark the beginning of fiber failure. Critical loads for given notch sizes and fiber strengths are easily determined for finite width specimens using the functions presented in this work.     

Tension of reinforced concrete prisms. Bond properties of reinforcement bars embedded in concrete tie elements. Summary of a RILEM round-robin investigation arranged by TC 147-FMB ‘Fracture Mechanics to Anchorage and Bond’

Results are presented from a RILEM Round Robin Investigation. It deals with tension stiffening of reinforcement bars embedded in concrete tie elements. Seven groups of researchers have performed some 50 tests and analyses. Parameters discussed are: cover thickness, crack spacing, bar size, tension stiffening of naked reinforcing bars, and influence of concrete strain softening.     

Influence of short dispersed and short integral glass fibres on the mechanical behaviour of textile-reinforced concrete

This article addresses the influence of the addition of short dispersed and short integral fibres made of alkali-resistant (AR) glass on the fracture behaviour of textile-reinforced concrete (TRC) subject to tensile loading. A series of uniaxial, deformation-controlled tension tests was performed to study the strength, deformation, and fracture behaviour of thin, narrow plates made of TRC, both with and without the addition of short fibres. Additionally, uniaxial tension tests on specimens reinforced with only short fibres were performed to figure out the difference in behaviour in the absence of textile reinforcement. Furthermore, multifilament-yarn and single-fibre pullout tests were carried out to gain a better understanding of bonding properties and crack-bridging behaviour. While pronounced enhancement of first-crack stress was achieved due to the addition of short dispersed fibres (the value increased by a factor of 2), a significant improvement in tensile strength was recorded for TRC specimens with the addition of integral glass fibres; the value increased by approximately 30 %. Moreover, TRC specimens reinforced with short dispersed glass fibres showed formation of more and finer cracks in comparison to the specimens with integral fibres. It was also found that short integral fibres can improve the bond between multifilament-yarns and the surrounding matrix by means of “special” cross-links. In TRC with short dispersed fibres this phenomenon was less pronounced. The investigations were accompanied by microscopical investigations which provided additional basis for an in-depth discussion of the decisive working mechanisms of hybrid reinforcement.     

Effect of embedded optical fiber sensors on transverse crack spacing of smart composite structures

In this study the effect of the presence of embedded optical fiber sensors on the transverse cracking of cross-ply laminates was investigated. The transverse crack spacing of cross-ply laminates with embedded optical fiber sensors was predicted using modified shear-lag analysis considering the presence of optical fibers and compared with experimental results. The effect of the orientation and quantity of optical fibers was evaluated and the effect of the coating of optical fiber was also investigated. Specimens were made with transparent glass/epoxy prepreg because the transverse crack and other damages such as delamination, splitting and bleeding of laser can be examined directly and visually. It has been found that the transverse crack spacing was not affected significantly by the embedding of optical fibers at low volume fraction of optical fibers. However, the cracks of specimens with embedded optical fibers which were initiated at a slightly lower stress level showed smaller spacing at the same stress level than those of specimens without embedded optical fibers. The theoretical crack spacing evaluated from the shear lag analysis showed good agreements with experimental results.     

Influence of concrete strength on crack development in SFRC members

Tension stiffening is still a matter of discussion into the scientific community; the study of this phenomenon is even more relevant in structural members where the total reinforcement consists of a proper combination of traditional rebars and steel fibers. In fact, fiber reinforced concrete is now a worldwide-used material characterized by an enhanced behavior at ultimate limit states as well as at serviceability limit states, thanks to its ability in providing a better crack control.This paper aims at investigating tension stiffening by discussing pure-tension tests on reinforced concrete prisms having different sizes, reinforcement ratios, amount of steel fibers and concrete strength. The latter two parameters are deeply studied in order to determine the influence of fibers on crack patterns as well as the significant effect of the concrete strength; both parameters determine narrower cracks characterized by a smaller crack width.     

Characterisation of crack-tip fields in biaxial fatigue based on high-magnification image correlation and electro-spray technique

This work presents a novel methodology for characterising fatigue cracks under biaxial conditions on a low carbon steel. It allows both short crack and early propagation stages to be studied in tubular specimens. Short crack growth is studied with a long-distance microscope acquiring images of the bare metal surface. Results showed oscillations in crack growth rate due to microstructure. Early propagation stage is studied with high magnification Digital Image Correlation (DIC) technique for measuring displacement and strain crack-tip fields. By applying micro-speckle pattern on the metal surface it is possible to achieve high magnification for DIC technique. Ultra-fine black and white speckles were created by electro-spray technique. The validity of this novel technique is demonstrated by direct comparison with extensometer measurements, under combined tension–compression and torsion conditions. It was also possible to estimate satisfactorily the mixed-mode stress intensity factor.     

超高分子量聚乙烯架桥纤维与裂缝的交互微观力学

使用拉曼光谱研究了架桥纤维与裂缝微观力学,以超高分子量聚乙烯(UHMWPE)纤维为例,将纤维搭桥试样进行微拉伸试验,着重分析架桥纤维的止裂作用和架桥纤维/环氧树脂界面的应力分布,并对不同位置架桥试样的裂缝扩展速度和应力分布进行分析,并进一步运用剪切滞后模型,对架桥纤维在不同拉伸载荷下的应力分布进行了拟合分析,结果表明:架桥纤维能够分散部分外载应力,对于裂纹扩展具有显著的止裂作用。在低于UHMWPE纤维最大应变拉伸时,发现在裂缝中心位置处架桥纤维所承受的应力最大,其应力不超过2GPa,而基体树脂的应力可达到12GPa,架桥纤维/基体界面的应力传递达不到100%。以UHMWPE为架桥的应力传递模型呈"正抛物线"型,应力分布存在于粘结区、脱粘区和架桥区。     

Statistical study of cracking in reinforced and prestressed concrete members

Abstract

Crack width and crack spacing in concrete flexural members are affected by two parameters, namely, the bond stress between the steel and concrete, and the effective concrete area in tension. This paper presents a statistical study applying dimensional analysis and regression analysis to existing test data to evaluate these two parameters as well as the crack spacing and crack width. It is found that the crack spacing is strongly dependent on the diameter of reinforcing bars and the reinforcement ratio. It is also found that a reduction of steel stress should be considered in calculating the crack width. Based on the results of this study, equations are developed for the calculation of crack width in reinforced and prestressed flexural members.     

Effect of overload on crack closure in thick and thin specimens via digital image correlation

The displacement field of compact tension (CT) specimens have been mapped by digital image correlation (DIC) local to growing fatigue cracks to study overload effects for plane stress and plane strain. We have extracted crack opening displacement (ΔCOD) and stress intensity (K) determined by a Muskhelishvili fit to the crack tip displacement field to infer the closure load. In both cases a classical knee was observed upon unloading consistent with closure which disappeared during the accelerated growth following OL, before increasing during retardation. In both cases following OL the crack growth rate is perturbed for a distance similar to the plastic zone.     

Distributed cracking and stiffness degradation in fabric-cement composites

Formation of distributed cracking and the associated degradation in the stiffness of fabric-cement composites under tensile loading were studied. Composites made from low modulus woven polyethylene fabric and bonded Alkali Resistant (AR) glass mesh were manufactured by means of pultrusion technique. The influence of fabric type, matrix modification and curing as well as the pressure applied after pultrusion were studied using tensile stress strain response. Three distinct measures of damage including quantitative crack spacing by image analysis, stiffness degradation, and microstructural observation by optical and scanning electron microscopy are evaluated. The evolution of crack spacing as a function of applied strain was correlated with the tensile response as well as with the stiffness degradation for various composites. Also, the microstructure of the different composites was characterized and correlated with their mechanical properties using optical and scanning electron microscopy. It was observed that the mechanical properties as well as crack spacing and composite stiffness were significantly affected by the matrix formulation, curing procedure, and the intensity of the pressure applied after the pultrusion process. The best tensile performance was achieved for glass fabric composites with a high content of fly ash.     

Microscale characterization of granular deformation near a crack tip

This paper presents a study of microscale plastic deformation at the crack tip and the effect of microstructure feature on the local deformation of aluminum specimen during fracture test. Three-point bending test of aluminum specimen was conducted inside a scanning electron microscopy (SEM) imaging system. The crack tip deformation was measured in situ utilizing SEM imaging capabilities and the digital image correlation (DIC) full-field deformation measurement technique. The microstructure feature at the crack tip was examined to understand its effect on the local deformation fields. Microscale pattern that was suitable for the DIC technique was generated on the specimen surface using sputter coating through a copper mesh before the fracture test. A series of SEM images of the specimen surface were acquired using in situ backscattered electronic imaging (BEI) mode during the test. The DIC technique was then applied to these SEM images to calculate the full-field deformation around the crack tip. The grain orientation map at the same location was obtained from electron backscattered diffraction (EBSD), which was superimposed on a DIC strain map to study the relationship between the microstructure feature and the evolution of plastic deformation at the crack tip. This approach enables to track the initiation and evolution of plastic deformation in grains adjacent to the crack tip. Furthermore, bifurcation of the crack due to intragranular and intergranular crack growth was observed. There was also localization of strain along a grain boundary ahead of and parallel to the crack after the maximum load was reached, which was a characteristic of Dugdale–Barenblatt strip-yield zone. Thus, it appears that there is a mixture of effects in the fracture process zone at the crack tip where the weaker aspects of the grain boundary controls the growth of the crack and the more ductile aspects of the grains themselves dissipate the energy and the corresponding strain level available for these processes through plastic work.     

Tension stiffening effect and bonding characteristics of chemically prestressed concrete under tension

The tension stiffening effect of chemically prestressed concrete (CPC) under uniaxial tension was experimentally investigated and compared with those of reinforced concrete (RC). A special specimen profile was designed to avoid the effect from end parts. The tension stiffening of both RC and CPC were compared with the current tension stiffening model. The crack pattern was observed after loading. The results show that the CPC has superior tension stiffening than RC and the conventional model for RC substantially underestimates the tension stiffening of CPC. In addition, the number of cracks in CPC is less than in RC at the same load. Further investigation on bonding characteristics of CPC under tension was subsequently conducted. Effects of cross section’s size and amount of expansive additive were also investigated. The strain distribution of rebar was measured from strain gages attached with 20 mm interval. Local bond, slip and average bond stress were then calculated. The results show that bond of CPC near loading end is higher than that of RC, although the average bond is almost same. The results of this study can partly explain some of unique tensile properties of CPC which can be related to its cracking resistance.