Tensile Stress-strain Response Research Articles

Influence of pyrolytic waste tire residue on the residual performance of high strength concrete exposed to elevated temperatures

An environmental friendly fire-resisting high strength concrete (HSC) was developed by incorporating waste tire residue (WTR) char as an additive in concrete. The residual mechanical properties such as compressive strength, split tensile strength, stress-strain response, elastic modulus, and mass loss of controlled and modified formulations were analyzed at elevated temperatures of 23, 100, 200, 400, 600, and 800 °C. The modified blends of waste tire residue char high strength concrete (WTR-HSC) were promising to retain the mechanical attributes at elevated temperatures along with the reduced potential of spalling. Micro-forensics besides visual assessment such as scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and thermal conductivity reveal that comburent WTR char produces adequate nanopores which relieve the developed pore pressure in elevated conditions compared with control HSC. Experimental data has been used to develop mathematical relations for the mechanical properties of WTR-HSC as a function of temperature using a statistical tool in Minitab software.

Nonlinear Inverse Analysis for Predicting the Tensile Properties of Strain-Softening and Strain-Hardening UHPFRC.

The tensile stress–strain response is considered to be the most important and fundamental mechanical property of ultra-high-performance fiber-reinforced concrete (UHPFRC). Nevertheless, it is still a challenging matter for researchers to determine the tensile properties of UHPFRC. As a simpler alternative to the direct tensile test, bending tests are widely performed to characterize the tensile behavior of UHPFRC, but require further consideration and a sophisticated inverse analysis procedure. In order to efficiently predict the tensile properties of UHPFRC, a nonlinear inverse method based on notched three-point bending tests (3PBT) was proposed in this paper. A total of fifteen UHPFRC beams were fabricated and tested to evaluate the sensitivity of the predicted tensile behavior to variations in fiber volume fraction. A segmented stress–strain model was used, which is capable of describing the various tensile properties of UHPFRC, including strain softening and strain hardening. A more approximate formulation was adopted to simulate the load–deflection response of UHPFRC beam specimens. The closed-form analytical solutions were validated by tensile test results and existing methods in literature. Finally, parametric studies were also conducted to investigate the robustness of the proposed method. The load–deflection responses obtained from notched 3PBT could be easily converted into tensile properties with this inverse method.

Work hardening in metastable high entropy alloys: a modified five-parameter model

Excellent work hardening in transformation-induced plasticity (TRIP)-enabled metastable high entropy alloys (HEAs) owe to persistent austenite (γ) to martensite (ε) phase transformation; non-basal slip activity and deformation twinning in the transformed martensitic phase are additional deformation mechanisms that contribute to work hardening in selected TRIP HEAs. Mechanical response of TRIP HEAs under uniaxial tension is characterized by an intermediate stage distinguished by a gradual increase in work hardening rate preceded and succeeded by stages of rapid drop in the work hardening rate. A five-parameter empirical model that replicates the nature of the work hardening rate curve in TRIP HEAs has been developed. The ease of parametric identification of the model from the global stress-strain response simplifies implementation of the model over physically based models. As the propensity of stress induced transformation in TRIP HEAs is related to the stacking fault energy (SFE) of the parent austenite phase, an attempt is made to correlate the model parameters with SFE. Based on the trends indicated in the correlation of model parameters with SFE as well as the initial microstructure, a method is proposed to predict the tensile stress-strain response of TRIP HEAs.

Temperature and pressure effect on tensile behavior of ice-Ih under low strain rate: A molecular dynamics study

The mechanical behavior of ice is complex, especially with the effect of temperature and pressure, which may significantly affect the safety in various engineering, where slow deformation of ice may bring more hidden, sudden, and severe damage. The mechanical behavior of ice-Ih under a low strain rate with the effect of temperature and confining pressure is still unclear, especially at the nanoscale. In this work, the tensile behavior of ice-Ih is investigated using Molecular Dynamics simulation method. The tensile test of ice-Ih was all performed along the y-direction ([010] crystal orientation) with a constant strain rate of 1 × 108 s−1. The effect of temperature (73 to 270 K with 1 atm), as well as the confining pressure (1 atm to 200 MPa, combined with 100 K, 150 K, 200 K, and 250 K) on the tensile behavior of ice-Ih, was studied, respectively. The tensile stress–strain response under different temperatures was obtained, and the linear relationship of temperature and Young’s modulus was consistent with other previous studies. The tensile strength of ice-Ih decreased linearly with increasing temperature under low strain rate, which is quite different from the cases under high strain rate. The solid-solid phase transition was observed at 73 ∼ 140 K, and the solid-liquid phase transition at 150 ∼ 270 K, where more energy is required in completely solid-liquid phase transition at 250 ∼ 270 K than solid-solid phase transition at 73 ∼ 140 K. The evolution of total potential energy increment and structural failure with strain significantly depended on the phase transition of ice-Ih. Moreover, the ice-Ih is earlier and more likely to be melted at higher confining pressure.

The effect of tension stiffening in moment-curvature responses of prestressed concrete members

Although tensile resistance of concrete is often neglected in calculating the nominal flexural capacity of a concrete member, it needs to be taken into account to accurately trace the deformation response of concrete members as it affects pre- and post-cracking behavior. To adequately address tensile resistance contributed by concrete in moment–curvature analysis, it is necessary to exploit a proper concrete tensile stress–strain model. While constitutive stress–strain models of reinforced concrete in tension have been studied by many researchers and well documented, those of prestressed concrete have gained relatively less attention. There are also conflicting results on how prestressing force affects concrete tension stiffening. This study fills the gap in existing studies and suggests an appropriate and simple model capturing the concrete tensile stress–strain relationship of prestressed concrete in a moment–curvature analysis. Based on an inverse-engineering approach using experimental test data and nonlinear analysis results, the effect of concrete tension stiffening varied by the level of prestressing force is investigated. The results show that concrete tensile properties hardly affect the force–deformation response of beams with high-level prestressing. Additionally, several parameters related to concrete tension stiffening are examined in the parameter study. By comparing predicted moment–curvature responses with test data, ideal values for considered parameters are obtained. Based on the parameter study, simple equations are proposed that are expressed in concrete compressive strength to simulate the concrete tensile stress–strain responses of partially prestressed members. The proposed equations are demonstrated using several prestressed beam tests found in previous studies.

Highly ductile fiber reinforced geopolymers under tensile impact

Eco-friendly Ductile Geopolymer Composites (EDGCs) are a class of fiber reinforced geopolymers with strain capacity exceeding 3% in pure tension. In this study, EDGCs were produced using a hybridized binder carrying Class F fly ash, Class C fly ash and slag activated by an alkaline solution. The alkaline activator comprised of sodium silicate and 12 M sodium hydroxide solution in the ratio 2.5:1. Workability, setting time, compressive strength, and tensile stress-strain response under quasi-static (strain rate of 10−4 s−1) and dynamic loading (strain rate of 0.51 ± 0.2 s−1) were investigated. Heat curing weakened the EDGCs due to early age drying shrinkage, reducing their compressive and tensile strength. All EDGCs irrespective of curing type or source material depicted a ductile strain-hardening behavior under quasi-static loading. Under dynamic loading, some ambient-cured EDGCs depicted a brittle failure, and some ductile. All heat-cured EDGCs showed a ductile behavior under dynamic loading. The peak tensile stress under dynamic loading was significantly higher than that under quasia-static loading irrespective of matrix type or curing regime. The dynamic increase factors (DIF) ranged between 1.18 and 1.86 for ambient-cured specimens and between 1.68 and 2.2 for heat-cured specimens. These DIFs were higher than the models predicted. Under dynamic loading, the strain capacity of the EDGCs was also smaller than under quasi-static loading, and the dynamic strain factors observed were 0.08–0.34 for ambient-cured, and 0.12 to 0.57 for heat-cured EDGCs. Overall, the hybrid EDGCs demonstrated better tensile properties under both quasi-static and dynamic loading curing.

An efficient algorithm of dislocation-precipitate interactions for single crystal nickel-based superalloys within discrete dislocation dynamics and its application

In this paper, the discrete dislocation dynamics (DDD) framework for single crystal nickel-based superalloys (SCNBSs) modeling is extended to simulate the superdislocation pairs shearing numerous precipitates more efficiently. An adaptive dislocation segment meshing scheme by specially treating the dislocation segments deposited on the γ/γ′ interfaces is also used to decrease the computational expense. In addition, the MPI parallel algorithm is also realized to increase the computational speed. Through this DDD framework, the size-related plastic response of SCNBSs microcrystal containing collections of precipitates is systematically investigated. Two types of SCNBSs microcrystal samples, one with intact precipitates and the other with partial precipitates truncated by free surfaces, are established for different sample sizes. The influence of the sample size, two types of boundary, and the coherency stress induced by lattice mismatch between the two phases are discussed. The results show that the influence of sample size on the yield strength and the dispersity of stress–strain curves are relatively weak when more than four precipitates across the cross section. And the effect of sample size on deformation mode and the dislocation density is still evident for all the considered sample sizes. For two types (intact and truncated precipitates) of SCNBSs microcrystal samples, the remarkable difference in their mechanical responses and dislocation evolution appears when there is only one precipitate across the cross section. In addition, the misfit stress can significantly change the dislocation distribution in different channels. However, it has less influence on the tensile stress–strain response for the considered tensile loading condition. Our results indicate that to properly characterize the global mechanical behavior of bulk SCNBSs by micro-test, the microcrystal sample should present more than sixteen whole precipitates across the cross section.

Predicting the complete tensile properties of additively manufactured Ti-6Al-4V by integrating three-dimensional microstructure statistics with a crystal plasticity model

A multiscale finite element model integrating microstructure statistics with an enhanced three-dimensional (3D) crystal plasticity model including damage has been developed to predict the complete tensile stress-strain response of Ti-6Al-4V manufactured by selective laser melting. A statistically equivalent representative volume element (SERVE) was constructed from an orthogonal set of electron backscatter diffraction (EBSD) images capturing key statistical information such as the spatial distribution of the grain size and the crystallographic and morphological orientation of the grains. Working from this 3D SERVE as a statistical model of the real microstructure, the crystal plasticity finite element (CP-FE) method was then used to predict the complete tensile stress-strain response of the material, including damage post necking. For the first time, crack-band theory (CBT) was incorporated into the CP-FF model to accurately predict ductility and to minimize mesh size sensitivity, which is a common issue in existing models. The effects of the 3D SERVE size on the stress-strain response was investigated through a rigorous sensitivity analysis. The model was calibrated using one sample and validated against a second sample with a different microstructure and properties. The new multiscale model provides a basis for a comprehensive Integrated Computational Materials Engineering (ICME) tool to enable the rational design of new high-performance materials.

Effects of temperature and strain rate on the tension-compression asymmetric plastic deformation behavior of ME20M magnesium alloy: Experiments and modeling

The tension-compression asymmetry of a rare-earth-containing alloy sheet, ME20M, is experimentally investigated along the rolling direction over a wide strain-rate range from 0.001 to 2500 s−1 and at temperatures from 213 to 488 K. The asymmetry of yield stress in tension and compression decreases with increasing temperature and decreasing strain rate. The strain-rate sensitivity of the tension-compression asymmetry during the plastic deformation stage increases with increasing temperature. It is observed that the compressive strain-hardening rate is much higher than the tensile one due to more {101-2} tension twins occurring in the compressive plastic deformation. A viscoplastic self-consistent (VPSC) model is employed to simulate the tensile and compressive stress-strain responses, deformation texture, and deformation mechanism evolution over wide ranges of strain rates and temperatures. Results indicate that the VPSC model is capable of capturing the macroscopic plastic deformation characteristics of ME20M alloy. Tensile plastic deformation is dominated by slip at strain rates up to 103 s−1, and basal slip and tension twinning are the main deformation modes in compressive plastic deformation. Effects of strain rate and temperature on the main plastic deformation mechanism of ME20M alloy are insignificant within the tested rate and temperature ranges.

Experimental and numerical investigation of progressive damage and failure behavior for 2.5D woven alumina fiber/silica matrix composites under a complex in-plane stress state

The focus of this work is to characterize the progressive damage and failure behavior of 2.5D woven alumina fiber reinforced silica ceramic matrix thin composite specimen under a planar tension-shear stress state. The tensile and shear stress–strain diagrams in the principal material coordinate system were obtained using the off-axis tension test with the digital image correlation technique. A mesoscale finite element model of the composite material was developed within the framework of continuum damage mechanics. The thickness effect on the macroscopic mechanical response was considered by removing the periodic boundary conditions along the thickness direction. The macroscopic stress–strain response given by the numerical model was validated by the experimental results. The interaction of the planar biaxial tension and shear stresses was taken into account in the model, quantified using a shear contribution coefficient in the modified 3D Hashin failure criterion. The results from the damage evolution analysis indicate that the linear tensile stress–strain response with a brittle fracture characteristic for the on-axis tension specimen is governed by the axial fiber bundle. The nonlinear tensile stress–strain response with the significantly reduced ultimate stress under biaxial tension and shear stress state results from multiple damages with evolution in both axial and transverse fiber bundles. Both the effective tensile modulus and the ultimate tensile strength increase with increasing the thickness of the specimen due to the increase of the fiber volume fraction. The strengthening induced by the size effect is less significant for the specimen under biaxial tension and shear stress state than that for the on-axis tension specimen owing to the involvement of shear response dominated by the matrix of the material. The results of this study provide new insight into the failure of 2.5D woven ceramic-based composite material, which contributes to the optimal design of the reusable thermal protection system structure.

Tensile Testing of Carbon FRP (CFRP) and Glass FRP (GFRP) Bars: An Experimental Study

Abstract The available ASTM recommends grout-filled steel tube anchors (STAs) for the tensile testing of 6.4- to 32-mm-diameter glass fiber-reinforced polymer (GFRP) bars and a 9.5-mm-diameter carbon FRP (CFRP) bar. The available ASTM provides minimum STA dimensions for tensile testing of the FRP bars. However, because of the large variations in the FRP bars, the exact STA dimensions for successfully testing the FRP bars in tension vary significantly and depend on the type and diameter of the FRP bar. In this experimental study, tensile testing of 9- and 15-mm-diameter CFRP bars and a 15.9-mm-diameter GFRP bar was successfully conducted using cement grout–filled STAs. For the 9-mm-diameter CFRP bar, a STA of 31.8-mm outer diameter (OD), 17.6-mm inner diameter (ID), and 460-mm anchorage length (La) was used. For the 15-mm-diameter CFRP bar, a STA of 46.3-mm OD, 27.1-mm ID, and 600 mm La was used. For the 15.9-mm-diameter GFRP bar, a STA of 46.3-mm OD, 27.1-mm ID, and 460-mm La was used. This study investigated the effect of anchorage length of STAs, number of sand coat layers, and grout age on the tensile stress–strain response of the FRP bars. The experimental results showed that increasing the anchorage length of STAs and the number of layers of sand coating improved the frictional resistance between the FRP bars and the surrounding cement grout, which resulted in rupturing of the FRP bars. The experimental results showed that FRP bars with grout-filled STAs tested at 3 days of casting failed by the rupturing of FRP bars, whereas FRP bars with grout filled STA tested at 28 days of casting slipped out from STAs during the tensile test.

Thermomechanical Behavior of Bone-Shaped SWCNT/Polyethylene Nanocomposites via Molecular Dynamics.

In the present study, the thermomechanical effects of adding a newly proposed nanoparticle within a polymer matrix such as polyethylene are being investigated. The nanoparticle is formed by a typical single-walled carbon nanotube (SWCNT) and two equivalent giant carbon fullerenes that are attached with the nanotube edges through covalent bonds. In this way, a bone-shaped nanofiber is developed that may offer enhanced thermomechanical characteristics when used as a polymer filler, due to each unique shape and chemical nature. The investigation is based on molecular dynamics simulations of the tensile stress–strain response of polymer nanocomposites under a variety of temperatures. The thermomechanical behavior of the bone-shaped nanofiber-reinforced polyethylene is compared with that of an equivalent nanocomposite filled with ordinary capped single-walled carbon nanotubes, in order to reach some coherent fundamental conclusions. The study focuses on the evaluation of some basic, temperature-dependent properties of the nanocomposite reinforced with these innovative bone-shaped allotropes of carbon.

A physically-based structure-property model for additively manufactured Ti-6Al-4V

A physically-based, mixed-phase structure-property model is presented for microstructure-sensitivity of tensile stress-strain response, including yield stress, ultimate tensile strength, uniform elongation and flow stress (strain hardening), for additively manufactured Ti-6Al-4V. The interdependent effects of solutes, grain size, phase volume fraction and dislocation density are explicitly included. Solid-state phase transformation and dislocation density evolution are incorporated to simulate the effects of martensite dissolution and α-β transformation at high temperature. Predictions are validated by comparison with measured tensile test data for (i) effects of additive manufacturing process conditions (such as build orientation and sample size) on tensile properties, based on the microstructure attributes inherited from the process, and (ii) the effect of temperature on tensile stress-strain response across a broad range of temperatures. The model is thus applicable for rapid process-structure-property prediction, in conjunction with AM process modelling, to capture the effects of key manufacturing variables and for process optimization.

Low-cycle fatigue performance of high-strength steel reinforcing bars considering the effect of inelastic buckling

A comprehensive experimental study was carried out to examine the low-cycle fatigue performance of ASTM A1035 Grade 690 rebars under cyclic-strain reversals with total strain amplitudes ranging from 1% to 4%. 12.7- and 15.8-mm diameter unmachined rebars were tested. To evaluate the effect of inelastic buckling on the low-cycle fatigue performance, bar unsupported length was varied between 6db and 15db in 3dbincrements, where db is the bar diameter. Rebar buckling was not completely prevented even in specimens with bar unsupported length of 6db, hence cyclic stress-strain responses were unsymmetrical for all the specimens. However, decreasing the strain amplitude and bar unsupported length generally increased the fatigue life and total energy dissipation of ASTM A1035 Grade 690 reinforcing bars. Existing strain and energy-based fatigue-life models’ constants were calibrated using the generated experimental fatigue data. The proposed models are applicable to ASTM A1035 Grade 690 rebars subjected to cyclic-strain reversals with total strain amplitude ranging from 1% to 4%. The results revealed that utilizing the previous fatigue-life models with constants calibrated using data obtained from tests on other types and grades of steel with nearly identical monotonic tensile stress-strain responses compared to that of ASTM A1035 Grade 690 steel would lead to inaccurate fatigue life predictions. The effect of inelastic buckling was incorporated into the proposed models by correlating their constants with a buckling parameter. Based on the proposed models’ predictions, ASTM A1035 Grade 690 reinforcing bars were found to have the potential to be used in reinforced concrete columns designed for a maximum target ductility demand of 2 provided that the center-to-center spacing between the transverse reinforcements is limited to 6db.

Constitutive modelling and mechanical properties of cementitious composites incorporating recycled vinyl banner plastics

This paper describes an experimental study, which has been lacking to date, into the mechanical properties of cementitious composites incorporating granules and fibres from recycled Reinforced PVC (RPVC) banners. A detailed account of over 140 tests on cylindrical, cubic and prismatic samples tested in compression and flexure, with up to 20% replacement of mineral aggregates, is given. Based on the test results, the uniaxial properties of selected recycled materials are examined in conjunction with a detailed characterisation of the RPVC granule size and geometry. Experimental measurements using digital image correlation techniques enable a detailed interpretation of the full constitutive response in terms of compression stress-strain behaviour and flexural stress-crack opening curves, as well as key mechanical parameters such as strength, elastic modulus and fracture energy. It is shown that the mechanical properties decrease proportionally with the amount of RPVC. For each 10% increment of volumetric replacement of mineral aggregates, the compressive strength is halved whilst the flexural strength is reduced by about 30% compared to their conventional counterparts. The reduction in strength is counterbalanced by an improved ductility represented by a favourable post-peak response in compression and an enhanced flexural softening and post-cracking performance. Smaller particles, with a relatively long acicular or triangular geometry, exhibited better behaviour as these acted as fibres with improved bond properties in comparison with intermediate and large size granules. The test results and observations enable the definition of a series of expressions to determine the mechanical properties of cementitious materials incorporating RPVC and other waste plastics. These expressions are then used as a basis for an analytical model for assessing the compressive and tensile stress-strain response of such materials. Validations carried out against the tests undertaken in this study, as well as from previous investigations, indicate that the proposed expressions and the developed constitutive model offer reliable representations for practical application.

Mechanical characterization of 304L-VAR stainless steel in tension with a full coverage of low, intermediate, and high strain rates

A 304L-VAR stainless steel is mechanically characterized in tension over a full range of strain rates from low, intermediate, to high using a variety of apparatuses. While low- and high-strain-rate tests are conducted with a conventional Instron and a Kolsky tension bar, the tensile tests at intermediate strain rates are conducted with a fast MTS and a Drop-Hopkinson bar. The fast MTS used in this study is able to obtain reliable tensile response at the strain rates up to 150 s−1, whereas the lower limit for the Drop-Hopkinson bar is 100 s−1. Combining the fast MTS and the Drop-Hopkinson bar closes the gap within the intermediate strain rate regime. Using these four apparatuses, the tensile stress-strain curves of the 304L-VAR stainless steel are obtained at strain rates on each order of magnitude ranging from 0.0001 to 2580 s−1. All tensile stress-strain curves exhibit linear elasticity followed by significant work hardening prior to necking. After necking occurrs, the specimen load decreases, and the deformation becomes highly localized until fracture. The tensile stress-strain response of the 304L-VAR stainless steel exhibits strain rate dependence. The flow stress increases with increasing strain rate and is described with a power law. The strain-rate sensitivity is also strain-dependent, possibly due to thermosoftening caused by adiabatic heating at high strain rates. The 304L-VAR stainless steel shows significant ductility. The true strains at the onset of necking and at failure are determined. The results show that the true strains at both onset of necking and failure decrease with increasing strain rate. The true failure strains are approximately 200% at low strain rates but are significantly lower (~100%) at high strain rates. The transition of true failure strain occurs within the intermediate strain rate range between 10−2 and 102 s−1. A Boltzmann description is used to present the effect of nominal strain rate on true failure strain.

Micro-scale computational modeling of graphene-based nanocomposites – Influence of filler-matrix interface failure

The effect of failure at the filler-matrix interface on the tensile stress-strain response of graphene-based polymer nanocomposites is studied using micro-scale finite element (FE) modeling. Interfacial failure can occur in the form of debonding or slip. Based on previous studies (Jiang et al., 2014; Anagnostopoulos et al., 2015; Li et al., 2017), interfacial slip, which causes friction-like interaction between the filler and matrix, is identified as the major failure mechanism that affects the nanocomposite response. By establishing single-filler models in ABAQUS, it is demonstrated that interfacial slip affects the stress-strain response of the nanocomposite in two ways. Firstly, it limits the load that can be transferred to the filler. Secondly, it causes a shear displacement discontinuity across the interface, and this generates additional strain over and above those of the matrix and filler; this reduces the level of stress enhancement in the nanocomposite that the incorporation of fillers contribute, and under certain conditions, even weaken the nanocomposite. Models with fully aligned and randomly oriented fillers are also established to examine the influence of parameters such as filler orientation, interfacial slip strength, and filler aspect ratio, on the tensile response of the nanocomposite. The model developed is employed to describe the tensile response of an actual polyvinyl alcohol (PVA)- graphene oxide (GO) nanocomposite, and good correlation with experiments is observed.

Characterizing and predicting the tensile mechanical behavior and failure mechanisms of notched FMLs—Combined with DIC and numerical techniques

This paper mainly extensively investigates the effects of layer direction, notch shape and notch size on the tensile mechanical behavior and failure mechanisms of notched fiber metal laminates (FMLs) by numerical and experimental methods. In virtue of Digital image correlation (DIC) technique, tensile tests are implemented to measure tensile stress-strain responses, displacement distributions and localized strain fields of different notched FMLs. Subsequently, the progressive damage analysis considering thermal residual stress is conducted to characterize failure modes and progressive damage evolution of notched FMLs, which contains the intra-laminar damage in composite laminates and the interfacial delamination between aluminum sheets/composite laminates simultaneously. Results demonstrate that the ultimate notch strength is more sensitive to layer direction and notch size in comparison with notch shape. FMLs with smaller notches exhibit more prominent directional dependence than those with larger notches. Additionally, regarding to fracture morphologies, notched FMLs present typical brittle fracture for on-axis case, while they manifest as initial transverse fracture following tilted cracking along fiber directions for off-axis case. Consequently, with the increase of layer direction, the final failure mode of notched FMLs results from fiber-driven to aluminum-driven, and the critical damage mechanism gradually transfers from tension dominated to tension-shear dominated.

Tensegrity Modelling and the High Toughness of Spider Dragline Silk.

This work establishes a tensegrity model of spider dragline silk. Tensegrity systems are ubiquitous in nature, being able to capture the mechanics of biological shapes through simple and effective modes of deformation via extension and contraction. Guided by quantitative microstructural characterization via air plasma etching and low voltage scanning electron microscopy, we report that this model is able to capture experimentally observed phenomena such as the Poisson effect, tensile stress-strain response, and fibre toughness. This is achieved by accounting for spider silks’ hierarchical organization into microfibrils with radially variable properties. Each fibril is described as a chain of polypeptide tensegrity units formed by crystalline granules operating under compression, which are connected to each other by amorphous links acting under tension. Our results demonstrate, for the first time, that a radial variability in the ductility of tensegrity chains is responsible for high fibre toughness, a defining and desirable feature of spider silk. Based on this model, a discussion about the use of graded tensegrity structures for the optimal design of next-generation biomimetic fibres is presented.

Direct Tension Test for Characterization of Tensile Behavior of Ultra-High Performance Concrete

Abstract Ultra-high performance concrete (UHPC) is characterized by its superior strength, ductility, durability, and particularly its unique post-cracking performance in tension. Dog-bone–shaped specimens are widely used for determination of the tensile behavior of UHPC, but there is no standard test method or specimen design for the characterization of tensile behavior. In this study, an evolving strategy on designing a direct tension test (DTT) specimen is first conducted using numerical finite element analysis. Seven series of DTT specimens made of UHPC and with well-designed dimensions to avoid local stress concentration are then tested experimentally. Results indicate that the post-cracking localization within the gauge measurement region is guaranteed, and the DTT specimen is capable of fully capturing tensile stress–strain responses of UHPC. An idealized constitutive model with three linear phases is proposed to fit the experimental data and thus characterize the linear elastic, strain-hardening, and strain-softening behavior of UHPC in tension. Four tensile material parameters extracted from the experimental stress–strain curves are implemented in the idealized constitutive model from which the multi-phase responses of UHPC in tension are reconstructed. It is found that most tensile material parameters extracted from experimental stress–strain curves, including tensile strength, modulus of elasticity, and dissipated energy, increase with the increased volume fraction of steel fibers, curing age, and displacement loading rate, while the strain capacity at the first cracking remains nearly constant. The DTT specimen developed can be used effectively to characterize the tensile behavior of ductile fiber-reinforced cementitious materials.