Characterising the time-dependant behaviour on the single fibre level of SHCC: Part 2: The rate effects on fibre pull-out tests

This paper is the second part of a two paper series about the time-dependant behaviour of Strain Hardening Cement-based Composite (SHCC) on the single fibre level. Having dealt with mechanisms of creep in SHCC in the first part, this paper reports single fibre pull-out tests that were done to investigate the effect of the pull-out rate on the mechanical response of the interface between the fibre and the matrix. It was found that not only the pull-out resistance increased with an increase of the pull-out rate but the probability of fibre rupture during pull-out as well. Another important finding was that the interfacial shear resistance and slip-hardening coefficient are not only dependant on the pull-out rate, but also the embedment length.     

Improved mechanical performance: Shear behaviour of strain-hardening cement-based composites (SHCC)

The retardation of moisture and gas ingress associated with important degradation mechanisms in cement-based composites in general and reinforced concrete or prestressed concrete in particular is an ongoing research focus internationally. A dense outer layer is generally accepted to significantly enhance durability of structural concrete. However, cracking leads to enhanced ingress, unless the cracks are restricted to small widths. Strain-hardening cement-based composites (SHCC) make use of fibres to bridge cracks, whereby they are controlled to small widths over a large tensile deformation range. In this paper, SHCC shear behaviour is studied, verifying that the cracks which arise in pure shear are also controlled to small widths in these materials. The design of an Iosipescu shear test setup and specific SHCC geometry is reported, as well as the results of a test series. A computational model for SHCC, based on finite element theory and continuum damage mechanics, is elaborated and shown to capture the shear behaviour of SHCC.     

Modelling method based on idealised fibre bundles

Abstract

In the fibrous structures such as textiles and composites there are fibre assemblies exhibiting statistical bundle like behaviour. This paper presents a modelling method and software FibreSpace, based on a system of structuralised statistical fibre bundles, so called fibre bundle cells. These fibre bundle cells introduced before represent different idealised and typified fibre properties such as fibre shape, state of deformation, gripping as a connection with the vicinity, and the characteristic of force-transmitting and damage. With the help of the weighted parallel connection of the fibre bundle cells the mechanical behaviour and the damage process of real fibrous systems can be modelled as well as some structural properties or the strength data of single fibres can be determined by a fibre bundle cells model identified on the basis of measurements. The applicability of the fibre bundle cells method and modelling program developed is demonstrated by modelling the load and damage process of real textile structures and unidirectional composites during tensile or flexural test.     

Creep analysis of fibre reinforced adhesives in single lap joints—Experimental study

In this paper, the effect of reinforcing the adhesive on the creep behaviour of single lap joints was studied experimentally. The reinforcement was in the form of fibre and three types of fibres namely aramid, carbon and glass were used. The test was performed at a temperature above the glass transition temperature of the adhesive. The effect of fibre orientation was also investigated. The failure time and initial strain for all the specimens were evaluated and compared to the un-reinforced adhesive joint. According to the results, adding fibres in the bondline considerably affects both the initial strain and the failure time and these effects are dependent on the fibre type and orientations. The fracture surfaces of the specimens were also studied to investigate the failure mechanisms of the reinforced adhesive in creep. The fibre breakage was observed along with adhesive and cohesive failures.     

Total deformation of loaded drying concrete

Usually the total time-dependent deformation of a loaded drying concrete specimen is subdivided into two components, these two being creep and shrinkage. But it turns out that the sum of both, pure creep and pure shrinkage, is always less than the deformation under load and simultaneous drying. Until now the question remains whether there does exist a special mechanism of drying creep or load induced shrinkage. Creep under sealed conditions can be analytically expressed by means of rate theory. It is shown here that tensile stresses in the drying outer shell usually overcome tensile strength of concrete. Thus crack formation takes place and the internal stress is redistributed. Theoretical predictions are compared with experimental results. It may be concluded that creep and shrinkage of a loaded drying specimen cannot be separated. The total deformation is a consequence of the superposition of internal and external state of stress.     

Uniaxial tensile and creep behaviour of an alumina fibre-reinforced ceramic matrix composite: I. Experimental study

The uniaxial tensile and creep behaviour of an alumina fibre-reinforced silicon carbide composite is studied. The damage mechanisms during tensile loading are identified on the basis of the elastic response and in-situ morphological analysis. Tensile tests show that the composite presents a pseudoductile behaviour due to matrix microcracking and fibre-matrix debonding. Temperature induces changes in the tensile behaviour because of variations in load transfer conditions and in the axial residual stress borne by the fibres and the matrix. The creep curves at 1100°C under vacuum present an extended tertiary part, especially at low creep stress. The unloading-reloading loops periodically performed during creep show a progressive decrease in longitudinal stiffness. Progressive interface debonding during creep is invoked to explain: (i) the strain rate increase during tertiary creep, (ii) the decrease of the elastic modulus and (iii) the large fibre pull-out observed on the creep fracture surface. The different creep rupture modes at low and high stresses are related to the capability of the remaining intact fibres to support the overload failure of the first fibres.     

Micromechanical model of time-dependent damage and deformation behavior for an orthogonal 3D-woven SiC/SiC composite at elevated temperature in vacuum

This paper presents a micromechanical model to predict the time-dependent damage and deformation behavior of an orthogonal 3-D woven SiC fiber/BN interface/SiC matrix composite under constant tensile loading at elevated temperature in vacuum. In-situ observation under monotonic tensile loading at room temperature, load–unload tensile testing at 1200 °C in argon, and constant load tensile testing at 1200 °C in vacuum were conducted to investigate the effects of microscopic damage on deformation behavior. The experimentally obtained results led to production of a time-dependent nonlinear stress–strain response model for the orthogonal 3-D woven SiC/SiC. It was established using the linear viscoelastic model, micro-damage propagation model, and a shear-lag model. The predicted creep deformation was found to agree well with the experimentally obtained results.     

Tensile creep of a structural epoxy adhesive: Experimental and analytical characterization

Epoxy adhesives are nowadays being extensively used in Civil Engineering applications, mostly in the scope of the rehabilitation of reinforced concrete (RC) structures. In this context, epoxy adhesives are used to provide adequate stress transference from fibre reinforced polymers (FRP) to the surrounding concrete substrate. Most recently, the possibility of using prestressed FRPs bonded with these epoxy adhesives is also being explored in order to maximize the potentialities of this strengthening approach. In this context, the understanding of the long term behaviour of the involved materials becomes essential. Even when non-prestressed FRPs are used a certain amount of stress is permanently applied on the adhesive interface during the serviceability conditions of the strengthened structure, and the creep of the adhesive may cause a continuous variation in the deformational response of the element. In this context, this paper presents a study aiming to experimentally characterize the tensile creep behaviour of an epoxy-based adhesive currently used in the strengthening of concrete structures with carbon FRP (CFRP) systems. To analytically describe the tensile creep behaviour, the modified Burgers model was fitted to the experimental creep curves, and the obtained results revealed that this model is capable of predicting with very good accuracy the long term behaviour of this material up to a sustained stress level of 60% of the adhesive׳s tensile strength.     

Relation between fibre distribution and post-cracking behaviour in steel fibre reinforced self-compacting concrete panels

In this research, the influence of the fibre distribution and orientation on the post-cracking behaviour of steel fibre reinforced self-compacting concrete (SFRSCC) panels was studied. To perform this evaluation, SFRSCC panels were cast from their centre point. For each SFRSCC panel, cylindrical specimens were extracted and notched either parallel or perpendicular to the concrete flow direction, in order to evaluate the influence of fibre dispersion and orientation on the tensile performance. The post-cracking behaviour was assessed by both splitting tensile tests and uniaxial tensile tests. To assess the fibre density and orientation through the panels, an image analysis technique was employed across cut planes on each tested specimen. It is found that the splitting tensile test overestimates the post-cracking parameters. Specimens with notched plane parallel to the concrete flow direction show considerable higher post-cracking strength than specimens with notched plane perpendicular to the flow direction.     

Damage-Enhanced Creep in a Siliconized Silicon Carbide: Mechanics of Deformation

Continuum mechanics methods were employed to analyze creep deformation of a grade of siliconized silicon carbide at elevated temperatures. Three loading modes (tension, compression, and bending) are considered in this analysis. In tension, deformation is accompanied by cavitation at stresses in excess of a temperature-dependent threshold level, resulting in bilinear power-law creep. In compression, greater applied stresses are required to achieve the same rate of strain, and although bilinear creep behavior is also observed, a single power-law creep equation was assumed to simplify the mathematical analysis of the flexure problem. Asymmetrical creep in siliconized silicon carbide leads to a number of unique features in flexural creep. At steady state, a threshold bending moment exists below which no damage occurs. The neutral axis shifts from the geometric center toward the compressive side of the specimen by an amount that depends on the level of applied stress. Cavitation zone shapes, which are predicted to develop in a four-point bend specimen as a function of load, are found to be in qualitative agreement with those obtained experimentally. For transient creep under bending, the time-dependent neutral axes for stress and strain do not coincide, although they do converge toward a single axis at steady state. Quantitative predictions are given for relaxation of tensile stresses at the outer fiber, reverse loading in the midplane region, and the growth of the damage zone toward the compressive side of the flexural specimen. This load redistribution leads to a prolonged transient stage as compared to its counterpart in uniaxial creep.     

Creep performance of CNT reinforced glass fiber/epoxy composites: Roles of temperature and stress

Carbon nanotubes (CNTs) have been emerged as a potential nanofiller to reinforce polymeric materials to improve their mechanical properties, like strength and modulus. However, time-dependent deformation of such materials under a constant load and elevated temperature is a matter of concern for long-term durability of these materials. The present article primarily demonstrates the effects of creep temperature and stress on the reinforcement efficiency of CNT in a glass fiber/epoxy (GE) composite. Two types of materials were investigated in this study—GE which was used as a control material, as well as CNT embedded GE composite. To elucidate the impact of CNT on the long-term durability of GE composite, creep tests have been performed at different temperatures (50, 80, and 110 °C) under bending loading. As applied stress has also significant contribution toward the elevated creep deformation of materials, creep tests have also been carried out under different stresses (5, 10, and 40 MPa). The strength of the CNT-GE composite exhibited 8.7 and 18.3% higher than that of control GE composite under tensile and bending load, respectively. Results suggest CNT reinforcement to be beneficial for low temperature applications, both in terms of creep strain and strain rate. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47674.     

Microstudy on creep of concrete at early age under biaxial compression

An interesting phenomena of crack restoration and increasing strength of concrete under biaxial compression creep were described in this paper. A small loading apparatus was prepared and a long work distance optical microscope with variable focus was used for studying the cracks. It was found from the micrographs that these cracks diminished under biaxial compression creep. There were increases in the strength of the creep specimens under the sustained biaxial compression load compared with the free companion ones. Multiaxial compression caused by the early temperature rise inside the mass concrete may strengthen the concrete and reduce the tensile cracks during and after temperature drop.     

Preliminary investigations of the dimensional stability of super-critically carbonated glass fibre reinforced cement

The dimensional changes occurring during the super-critical carbonation of glass fibre reinforced cement (GRC) and its subsequent environmental exposure have been investigated. Sheet samples, with embedded stainless steel pins 200 mm apart, were fabricated and then subjected to super-critical carbonation. Test coupons were subsequently exposed to a range of environments including continuous immersion in water, cyclic wetting/drying and outdoor exposure. The super-critical carbonation process resulted in a slight expansion of GRC. This is contrary to the behaviour observed during natural carbonation where irreversible shrinkage normally occurs. Exposure to the various environments mentioned above showed that super-critically carbonated samples had much greater resistance to swelling and shrinkage than uncarbonated specimens. These observations are particularly significant in relation to the practical application of GRC in environments of fluctuating moisture content.     

Comparative evaluation of steel mesh, steel fibre and high-performance polypropylene fibre reinforced shotcrete in panel test

In this study, experimental investigations were performed on steel mesh (SM), steel fibre (SF) and high-performance polypropylene fibre (HPPF) reinforced shotcrete (HPPFRS) panels to evaluate performance characteristics such as toughness, flexural ductility, energy absorption and load capacity. The panel tests, in accordance with European specification for sprayed concrete (EFNARC), were made on 18 prismatic specimens having the same mix designs and were cured for 28 days but reinforced with various fibres. In addition, the rebound characteristics of these mixes were determined to compare the actual in situ fibre contents.Test results show that all reinforcements, including HPPFs that are low-modulus fibres, greatly improved the flexural ductility, toughness, and load-carrying capacity of the brittle matrix. It was seen that there was a positive synergy effect between steel and polypropylene fibre in hybrid fibre usage from a performance point of view. According to results, it can be concluded that a hybrid polypropylene-SF can be used alternatively instead of SM and monosteel fibre as a reinforcement in shotcrete applications to get better efficiency in mechanical properties of composite.     

Uniaxial tensile creep behaviour of ultra high molecular weight linear polyethylene

The present paper examines the room temperature constant-load uniaxial tensile creep response of two ultra high molecular weight linear polyethylene (UHMW LPE) materials and compares it with that of a normal molecular weight linear polyethylene (NMW LPE). It was found that at all stress levels examined, the magnitude of creep deformation is significantly higher in UHMW LPE than in NMW LPE. Possible reasons for this behaviour are explored. Potential techniques for improving the tensile creep behaviour (i.e. decreasing the creep deformation) of LPE are discussed.     

Direct assessment of tensile stress-crack opening behavior of Strain Hardening Cementitious Composites (SHCC)

The process of designing Strain Hardening Cementitious Composites (SHCC) is driven by the need to achieve certain performance parameters in tension. These are typically the pseudo-strain hardening behavior and the ability to develop multiple cracks. The assessment of the tensile load-deformation behavior of these materials is therefore of great importance and is frequently carried out by characterizing the material tensile stress–strain behavior. In this paper an alternative approach to evaluate the tensile performance of SHCC is investigated. The behavior of the material in tension is studied at the level of a single crack. The derived tensile stress-crack opening behavior is utilized to analyze and compare the influence of various composite parameters on the resulting tensile behavior. The deformations occurring during tensile loading are furthermore examined using a digital image-based deformation analysis technique to gain detailed insight into the crack formation, propagation and opening phases.     

砂的颗粒级配对SHCC抗拉性能的影响

本文研究了颗粒级配不同的同批次砂对应变硬化水泥基复合材料(SHCC)抗拉性能的影响。在前期试验基础上,通过流动扩展度试验得到了具有最佳流动性的SHCC;通过对哑铃型试件进行单轴拉伸试验,获得采用不同颗粒级配砂配制的SHCC的应力-应变曲线。结果表明,采用最大粒径为0.3mm的级配砂比最大粒径为0.6mm和1.18mm级配砂所配制出的SHCC能够更好地实现应变硬化和多点开裂,且极限拉应变可达5.8%左右。本文所得结论可为SHCC的科研和工程应用提供理论依据。     

Tensile,compressive and flexural basic creep of concrete at different stress levels

Concrete is brittle and highly sensitive to cracking, which is detrimental to the sustainability of its applications. Although it is well known that cracks occur mainly in tension, research on the mechanical behavior of concrete is usually limited to compression and investigations of creep behavior, a major concern for concrete structures, are no exception in this respect. This paper is intended to help remedy the situation. First, the new experimental set-ups developed to achieve tensile and bending creep are presented. The precautions taken to obtain relevant experimentation are also described. Results for specimens subjected to sustained stresses of 30, 40 and 50% of the tensile or compressive strength are then presented. The final discussion compares basic creep under the different types of loading for the three stress levels.     

Rheological modeling of the tensile creep behavior of paper

Paper is a networked structure of randomly bonded fibers. These fibers are composed of naturally occurring polymeric materials (cellulose, hemicelluloses, and lignin). Polymeric materials such as these exhibit viscoelastic deformation, and as a result, creep under an applied stress. A rheological model has been developed to predict the tensile creep behavior of paper under a uni‐axial stress. Specifically, the focus of this model was to predict creep strain using only stress, time, and efficiency factor (effectiveness of bonding). This rheological model offers insight into creep behavior (drawing from molecular creep mechanisms) and separates total strain from creep into initial elastic, primary creep, and secondary creep components. Interfiber bonding is taken into account through the use of an efficiency factor which represents how effectively bonding is distributing load throughout the fiber network of the paper. As a result, this model makes it possible to predict the creep behavior of paper over a range of bonding levels, induced by mechanical changes in bonded area or chemical modification of specific bond strength, using creep data from paper at any single level of bonding. This utility is retained as long as the fibers and the orientation of the fibers are not changed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007