Uniaxial Tension Behavior Research Articles

ECC와 고장력 철근으로 보강된 철근콘크리트 보의 휨 실험

ECC는 모르타르 매트릭스에 약 2% 이내 체적비의 합성 섬유를 보강하여 1축 인장 하에서 변형률 경화 거동과 다수의 미세균열을 발현하도록 제조한 높은 연성의 시멘트 복합 재료이다. 최근 ECC를 적용하는 연구가 구조물 보수보강 공법, 현장 타설 및 프리캐스트화 공법 등 다양한 형태로 진행되고 있으며, 특히 연성이 요구되는 구조 부재에 적용하고 하는 시도가 끊임없이 이루어지고 있다. 이 연구에서는 성능 저하된 철근콘크리트(RC) 보의 구조 성능 및 균열 제어 성능을 개선하기 위하여 ECC와 고장력 철근을 함께 활용하는 보강 공법을 다루고 있다. 이를 위하여 RC 보실험체를 제작·실험하여 구조 성능과 파괴 형태 등을 연구하였다. 실험 결과, ECC와 고장력 철근으로 보강된 RC보는 우수한 균열 제어 성능과 더불어 휨 내하력 및 휨 강성이 크게 향상되는 것으로 평가되었다. 이 연구 결과는 ECC와 고장력 철근을 활용한 보강 공법의 설계 및 현장 적용을 위한 기초 자료로 활용될 것이다.

Uniaxial Tension Behavior of Reinforced Concrete Members Strengthened with Carbon Fiber Sheets

A two-dimensional (2D) nonlinear numerical analysis code by using the rigid body spring method (RBSM) was developed by the writers at Hokkaido University to simulate the behavior of reinforced concrete (RC) members strengthened with fiber-reinforced polymer (FRP) sheets. The code supports the nonlinear constitutive laws for the different materials and nonlinear bond stress-slip relationships for steel-concrete and FRP sheet-concrete interfaces. This study uses the aforementioned code to examine the uniaxial tension behavior of RC members strengthened with carbon fiber sheets (CFS). Experimental results are compared with relevant numerical outputs to validate the model and confirm its ability to simulate the experimental observations. This study also assesses the influence of the amount of CFS strengthening on the tension-stiffening behavior of strengthened members. Finally, this research also suggests new analytical expressions for the average stress-strain relationships of concrete and steel in tension in the presence of stiffening contributions from internal steel reinforcement bars and externally bonded CFS reinforcement.

Thickness effects in polycrystalline thin films: Surface constraint versus interior constraint

The uniaxial tension behavior of polycrystalline thin films, in which all grain boundaries (GBs) are penetrable by dislocations, is investigated by two-dimensional discrete dislocation dynamics (DDD) method with a penetrable dislocation-GB interaction model. In order to study thickness effect on the tensile strength of thin films with and without surface treatment, three types of thin films are comparatively considered, including the thin films without surface treatment, with surface passivation layers (SPLs) of nanometer thickness and with surface grain refinement zones (SGRZs) consisting of nano-sized grains. Our results show that thickness effects and their underlying dislocation mechanisms are quite distinct among different types of thin films. The thicker thin films without surface treatment are stronger than the thinner ones; however, opposite thickness effects are captured in the thin films with SPLs or SGRZs. Moreover, the underlying dislocation mechanisms of the same thickness effects of thin films with SPLs and SGRZs are different. In the thin films with SPLs, the thickness effect is caused by the sharp increase of dislocation density near the film-passivation interface, while it is mainly due to the sharp decrease of dislocation density within the refined surface grains of the thin films with SGRZs. No matter in what type of thin films, thickness effect gradually disappears when the number of grains in the thickness direction is large enough. Our analysis reveals that general mechanism of those thickness effects lies in the competition between the exterior surface-constraint and interior GB-constraint on gliding dislocations.

섬유 보강 시멘트계 복합재료의 단면 이미지에서 섬유 검출을 위한 섬유 형광 특성 분석

섬유 보강 시멘트계 복합재료의 성능 예측 및 평가에 있어서 중요한 영향을 미치는 섬유 분포 특성을 정량적으로 평가하기 위해서는 복합재료의 단면 이미지에서 섬유를 정확히 검출하여야 한다. 이 논문은 형광원리를 이용하여 합성섬유를 매트릭스로부터 개별적으로 검출하기 위한 기초 연구로써 섬유 보강 시멘트계 복합재료 중의 하나인 ECC에 일반적으로 사용되는 PVA 섬유, PET 섬유, PE 섬유, 그리고 PP 섬유의 형광특성을 분석하였다. 또한 하이브리드 된 경우 합성섬유들 간의 상대 형광 광도값의 차이를 비교하여 종류별로 구분하기 위한 최적의 파장대를 선정하였으며, 그 파장대에서 두 분류대상을 구별할 수 있는 최적의 경계값을 통계적 방법을 이용하여 산출하였다. 또한 형광 현미경을 이용하여 각 섬유를 촬영하여 상대적인 밝기값을 비교하였다. It is important to detect fibers in the sectional image of fiber reinforced cementitious composites (FRCC), since the fiber distribution is a crucial factor to predict or evaluate the mechanical performance of FRCC. In this paper, we investigated the fluorescence characteristics of Polyvinyl Alcohol (PVA) fibers, Polyethylene Terephthalate (PET) fibers, Polyethylene (PE) fibers, and Polypropylene (PP) fibers used in Engineered Cementitious Composites (ECC), which is a special kind of FRCC that incorporates synthetic fibers and exhibits extremely ductile behavior in uniaxial tension, to detect each fiber according to its type. Furthermore, optimum excitation and emission wavelengths were proposed on the basis of maximum difference of Relative Fluorescence Intensity (RFI) between two types of fibers used in the hybrid ECC. Optimum threshold values to discriminate two types of fibers using statistical tools were also proposed. Finally, images of four types of fibers obtained using a fluorescence microscope are compared.

The Development of Superplastic Magnesium Alloy Sheet

While magnesium alloys are routinely used in engineering applications in the form of net shape castings, applications for sheet product have been limited due to the poor cold formability of magnesium combined with the perceived expense of sheet. The issues associated with poor cold formability could largely be overcome if magnesium alloys were to be superplastically formed. Superplasticity in magnesium is well established with research papers on the subject dating back to the late 1960s. In recent years, interest in this area has grown to the point where a number of companies have successfully superplastically formed prototype automotive panels from magnesium alloy sheet. Concurrent to this the scientific community have demonstrated superplasticity in a wide range of magnesium alloys using processing techniques ranging from the exotic (severe plastic deformation) to the mundane (traditional warm rolling). Work by the current authors has shown, rather surprisingly, that superplasticity can be achieved in magnesium alloys in the as-cast condition. This has led to some initial exploratory work involving twin roll casting. The concept being that affordable superplastic magnesium sheet could be produced via twin roll casting with only limited rolling reduction to final gauge. This paper describes the superplastic behaviour (in uniaxial tension) and microstructure of sheet processed from strip cast AZ31 and AZ91. The experimental material has included strip cast AZ91 subjected to large shear strains immediately prior to casting. The material was tested in the as-cast condition and after warm rolling to a number of gauges. Industrially useful superplastic capability was demonstrated in the strip cast alloys. Furthermore, good superplastic capability was also demonstrated in sheet subsequently rolled from the cast metal and rolling strain did not significantly influence the ductilities obtained. The mechanism for achieving superplasticity in as-cast magnesium alloys will be considered and the contrasting deformation characteristics of AZ31 and AZ91 will be discussed in terms of m value analysis and microstructural characterisation.

Effect of amount, size, and spatial distribution of intermetallics on incipient localization during plastic deformation

Virtual microstructures having a systematic variation of amount, mean size, standard deviation of size, and spatial arrangement of intermetallics have been synthesized, and their deformation behavior in uniaxial tension has been evaluated using finite element analysis. Four spatial arrangements of intermetallics have been considered in this work, namely: random, clustered, and two-ordered structures. Various mathematical quantities have been developed to quantify the severity of deformation including plastic work density distribution (PWDD), percentile work-density volume criterion (PWC), and percentile stress volume criterion (PSC). This approach eliminates the need for an external trigger in FEA to achieve localization. The method developed has led to a better understanding of the effect of different microstructural attributes on the process of deformation. This has resulted in guidelines for optimizing the microstructure to minimize material damage and thereby maximize ductility.

Simulation on Uniaxial-Tension Behavior of NiTi Shape Memory Alloy Films

Based on experimental results reported in the reference, Liang-Rogers’ constitutive model for SMA is used to simulate the stress-strain curves of NiTi shape memory alloy films under uniaxial tension with isothermal conditions. The effects of film compositions and temperature on the tensile behavior of NiTi shape memory alloy films are discussed. By comparing the simulation results with the experimental results, it is found that the simulation curves agree basically with the experimental curves except that the phase-transformation regions are wider in the simulation curves. This demonstrates that the Liang-Rogers’ model can be used to predict the thermomechanical behavior of shape memory alloy films roughly. This study provides some theoretical foundation for the quantitative description and prediction of the actuation mechanism when shape memory alloy films are used as micro-actuators.

Uniaxial tension and compression behavior of AZ31 magnesium alloy rolled and extruded materials

Mg alloy as a practical use metal is superior in specific strength and specific stiffness, and also exhibits eminent performance in heat discharging, electromagnetic wave shielding and vibration and shock absorption. Therefore, it is used for the components of cars, aircrafts, precision machines etc. and its applications are broadened to various fields. Mg alloy AZ31 rolled sheet and extruded sheet were considered in this study. It was the experimental observation and investigation of the deformation behavior about the materials manufactured different methods by uniaxial tensile test and compression test under the temperature range including high temperature range. It was also aimed at revealing and investigating the differences of material properties derived from different manufactured methods. A Siebel-type compression test was performed, which can compensate the deviation from a simple compression condition caused by the friction between interfacial surfaces of tools and materials, in order to perform the test based on ideal simple compression condition. The properties of material received by both tests were compared and important findings were obtained for Mg alloy AZ31 sheet.

Temperature and loading rate effects on tensile properties of kenaf bast fiber bundles and composites

This paper presents extensive experiments and micromechanics-based modeling to evaluate systematically the tensile properties of kenaf bast fibers bundle (KBFB) and kenaf bast fiber-reinforced epoxy strands. Uniaxial tension behaviors of KBFBs and KBFB-reinforced epoxy strands were evaluated statistically using large sample sets. The elastic modulus, tensile strength, as well as failure strains of KBFBs, displayed large scatter statistically ranging from 10% to 30%. The loading rate-dependency was evaluated at three strain rates ranging from approximately 10 −4 ∼ 10 −2/s. The tensile strength increases gradually as the loading rate increases, while the tensile modulus almost remains the same as the loading rate increases until the loading rate reaches 10 −2/s, at which a much higher modulus was presented. The high temperatures (170–180 °C), possibly subjected during fiber processing and composite fabrication, do not impose significant effects on the tensile properties of KBFBs if the duration is less than 1-h. The tensile properties of KBFB were not affected by the conditioning at 130 °C for 24-h, which mimics the severe service temperature of automotive front-end components. KBFB-epoxy composite strands were further evaluated at various loading rates. A micromechanics-based Mori–Tanaka model was implemented to predict the anisotropic elastic moduli of KBFB and KBFB-epoxy composite strands based on the microstructural compositions.

Prediction of fabrics mechanical behaviour in uni-axial tension starting from their technical parameters

On the basis of a phenomenological study, which consists in determining the response of a fabric sample to the traction and the relaxation, we proposed a rheological model with 15 coefficients. This model of mechanical behaviour in uni-axial tension takes into account the elastic and the viscous components. To characterize the coefficients of this model, simple tension and relaxation tests were performed for 29 fabrics in the warp and the weft directions. An artificial neuronal network trained with a retro-propagation algorithm performs functional mapping between these mechanical coefficients and the technical parameters of fabric, which allowed us to predict the mechanical behaviour in tension and relaxation of fabrics starting only from their technical parameters.

Dynamic behaviour and constitutive model of concrete at different strain rates

Dynamic tensile and compressive tests are carried out to investigate the effect of strain rates on the dynamic tensile and compressive behaviour of concrete. Results of tests indicate that the tensile and compressive strengths of concrete increase with the loading rate. The initial tangential modulus and the critical strain of concrete in tension are independent of strain rate but those in compression slightly increase with the strain rate. Poisson's ratio of concrete in both tension and compression is not obviously dependent of loading rate. The static Drucker–Prager model is modified to the consistency viscoplastic Drucker–Prager model by considering the effect of strain rate. The simulation results using the proposed model show that the present model can simulate the uniaxial dynamic tension and compression behaviour of concrete well. Finally, the dynamic responses of a concrete beam under impact loading are analysed to investigate the influence of strain rates on dynamic responses of concrete structures.

The manufacture of superplastic Magnesium alloy sheet

AbstractProbably because of their propensity to dynamically recrystallise, superplastic behaviour can be obtained from magnesium alloys considerably more easily than from comparable aluminium alloys. In some cases even as cast magnesium alloys can exhibit reasonable superplasticity and there appears no need for the special alloying additions or complex thermal mechanical treatments required by aluminium alloys such as AA2004 or AA7475. The paper describes the superplastic behaviour (in uniaxial tension) and microstructure of sheet processed from strip cast AZ31 and AZ91. The material was tested in the as‐cast condition and after warm rolling to a number of gauges. Industrially useful superplastic capability was demonstrated in strip cast AZ31 and AZ91 in the as cast condition. Furthermore good superplastic capability was also demonstrated in sheet rolled from the cast metal and the ductilities obtained were not significantly influenced by rolling strain. Twin roll strip casting represents a feasible and simple route for the production of superplastic material either for use in the as cast condition or after rolling to the required gauge.

고로슬래그 미분말이 혼입된 자기충전 및 숏크리트용 ECC의 개발을 위한 시멘트풀 레올로지 제어

An engineered cementitious composite produced with slag particles (Slag-ECC) had been developed based on micromechanical principle. Base grain ingredients were properly selected, and then the mixture proportion was optimized to be capable of achieving robust tensile ductility in the hardened state. The rheological design is performed in the present study by opti- mizing the amount of admixtures suitable for self-consolidating casting and shotcreting process in the fresh state. A special focus is placed on the rheological control which is directly applicable to the construction in field, using prepackaged product with all pulverized ingredients. To control the rheological properties of the composite, which possesses different fluid properties to facilitate two types of processing (i.e., self-consolidating and shotcreting processing), the viscosity change of the cement paste suspensions over time was initially investigated, and then the proper dosage of the admixtures in the cement paste was selected. The two types of mixture proportion were then optimized by self-consolidating & shotcreting tests. A series of self-consolidating and shotcreting tests demonstrated excellent self-consolidation property and sprayability of the Slag-ECC. The rheological properties altered through this approach were revealed to be effective in obtaining Slag-ECC hardened properties, represented by pseudo strain-hard- ening behavior in uniaxial tension, allowing the readily achievement of the desired function of the fresh Slag-ECC. These ductile composites with self-consolidating and shotcreting processing can be broadly utilized for a variety of applications, e.g., in strength- ening seismic resistant structures with congested reinforcements, or in repairing deteriorated infrastructures by shotcreting process.

Constitutive Modeling of the Stress-Stretch Behavior of Two-Dimensional Triangulated Macromolecular Networks Containing Folded Domains

The mechanical behavior of the red blood cell membrane is governed by the lipid bilayer which resists changes in surface area and the underlying spectrin network which resists changes in shape. The constituent spectrin chains of the network consist of a series of domains along the chain, which exhibit noncovalent interactions. Upon sufficient extension of a chain, each folded domain undergoes mechanically-induced unfolding after reaching a chain force between 10 and 35pN. Individual spectrin chains within the network experience their first unfolding event at different levels of macroscopic strain depending on the macroscopic loading conditions and the orientation of each constituent chain with respect to the macroscopic loading. A microstructurally-informed continuum level constitutive model is developed which tracks individual chain deformation behavior as well as the overall macroscopic network stress-strain behavior. Using the introduced continuum approach and statistical mechanics based models of the chain force-extension behavior together with a transition state model of domain unfolding; a constitutive model for the membrane stress-stretch behavior is constructed. Uniaxial tension and simple shear behaviors of the membrane are simulated incorporating the unfolding of the individual chains. A Taylor averaging approach is used as a first approximation to account for the irregularities in the spectrin network which result in a near plateau-like force behavior with increasing stretch.

Diverse Application of ECC Designed with Ground Granulated Blast Furnace Slag

In the recent design of high ductile engineered cementitious composites (ECC), optimizing both processing and mechanical properties for specific applications is critical. This study employs a method to develop useful ECC produced with slag particles (slag-ECC) in the field, which possesses different fluid properties to facilitate diverse types of processing (i.e., self-consolidating or spray processing). Control of rheological modulation was regarded as a key factor to allow the performance of the desired processing while retaining the ductile material properties. To control the rheological properties of the composite, the basic slag-ECC composition was initially obtained, determined based on micromechanics and steady-state cracking theory. The stability and consequent viscosity of the suspensions were then mediated by optimizing the dosage of the chemical and mineral admixtures. The rheological properties altered through this approach were revealed to be effective in obtaining ECC-hardened properties, represented by pseudo strain-hardening behavior in uniaxial tension, allowing the readily achievement of the desired function of the fresh ECC.

Analytical Model for Tensile Strain Hardening and Multiple Cracking Behavior of Hybrid Fiber-Engineered Cementitious Composites

An analytical model for the design of strain-hardening and multiple-cracking behavior of engineered cementitious composites (ECC) containing hybrid fibers is proposed. The model predicts first crack strength and ultimate bridging strength of hybrid fiber ECC. The model also predicts the minimum (critical) volume fraction of fibers required to exhibit strain-hardening and multiple-cracking behavior in uniaxial tension. The model is verified with the experimental results of hybrid fiber ECC specimens. A parametric study is also performed, using this model, to evaluate the effects of fiber length, diameter, and interfacial bond strength on the first crack strength, the ultimate bridging strength and the critical volume fraction of fibers. It is shown that the critical volume fraction of fibers in hybrid fiber composites can be optimized by proper selection of fiber length, diameter, and interfacial bond strength. Low modulus fibers are found to have a more pronounced effect on the strain-hardening and multiple-cracking behaviors of hybrid fiber composites compared to high modulus fibers. The hybrid fiber concept is found to offer additional freedom in the design variables compared to composite containing one type of fiber.

Tensile and fiber dispersion performance of ECC (engineered cementitious composites) produced with ground granulated blast furnace slag

An engineered cementitious composite (ECC) produced with ground granulated blast furnace slag was developed for the purpose of achieving moderately high composite strength while maintaining high ductility, represented by strain-hardening behavior in uniaxial tension. In the material development, single fiber pullout tests and matrix fracture tests were performed, followed by micromechanical analyses to properly select the range of mixture proportion. Subsequent direct tensile tests were employed to assess the strain-hardening behavior of the composite, which exhibited high ductility and strength with the addition of slag. High ductility is most likely due to enhanced workability and fiber dispersion performance which is attributed to the oxidized grain surface of slag, as verified by fiber dispersion tests. These results suggest that, within the limited slag dosage employed in the present study, the contribution of slag to fiber dispersion outweighs the side-effect of decreased potential for saturated multiple cracking, including a slight increase in matrix fracture toughness and fiber/matrix bond strength.

Durability of a multiscale fibre reinforced cement composite in aggressive environment under service load

The LCPC has developed and patented a new ultra high performance fibre reinforced cement composite (UHPFRCC); the introduction of three steel fibre sizes leads to a multiscale fibre reinforced cement composite (MSFRCC) with multi-cracking and hardening behaviour in uniaxial tension ( f t more than 20 MPa). An innovative test of durability is presented. Pre-cracked thin slabs are damaged by fatigue under loading corresponding to service load (30 MPa in bending test) then maintained under bending at the same level. A part of these slabs undergoes 30 weekly wetting–drying cycles in a chloride solution (NaCl 5%, 20 °C). A reloading to failure is led. One notes an absence of corrosion in the micro-cracked area even for cover lower than 300 μm and a strength increase for pre-damaged slabs by fatigue. Under service load and in the presence of chloride water, a quasi-total recovery of initial stiffness is possible; it is accompanied by an increase of the pseudo-elastic behaviour. The development of a better matrix/micro-fibres synergy and a diffuse micro-cracking explains this result.

고로슬래그미분말이 혼입된 ECC(Engineered Cementitious Composite)의 개발

This paper presents both experimental and analytical studies for the development of an ECC(Engineered Cementitious Composites) using ground granulated blast furnace slag(slag). This material has been focused on achieving moderately high composite strength while maintaining high ductility, represented by strain-hardening behavior in uniaxial tension. In the material development, micromechanics was adopted to properly select optimized range of the composition based on steady-state cracking theory and experimental studies on matrix, and interfacial properties. A single fiber pullout test and a wedge splitting test were employed to measure the bond properties of the fiber in a matrix and the fracture toughness of mortar matrix. The addition of the slag resulted in slight increases in the frictional bond strength and the fracture toughness. Subsequent direct tensile tests demonstrate that the fiber reinforced mortar exhibited high ductile uniaxial tension behavior with a maximum strain capacity of 3.6%. Both ductility and tensile strength( MPa) of the composite produced with slag were measured to be significantly higher than those of the composite without slag. The slag particles contribute to improving matrix strength and fiber dispersion, which is incorporated with enhanced workability attributed to the oxidized grain surface. This result suggests that, within the limited slag dosage employed in the present study, the contribution of slag particles to the workability overwhelms the side-effect of decreased potential of saturated multiple cracking.

Equivalent and separable strain-energy functions in compressible, finite elasticity

A material non-uniqueness is identified for isotropic, homogeneous, hyperelastic, compressible materials. This non-uniqueness is a special case of the hyperelastic null Lagrangian. Equivalence classes of strain-energy functions (SEFs) are then obtained. This equivalence can be used to extend in a natural way some of the common SEF used in incompressible elasticity to compressible elasticity. The familiar Valanis–Landel separable form of the SEF for incompressible materials is extended in this way. Some necessary restrictions are imposed on this new form and its behaviour in uniaxial tension is discussed. Previous problems in using the Valanis–Landel form of the SEF in compressible elasticity have been overcome.