Showing papers on "Stress–strain curve published in 2015"

Polymer engineering science and viscoelasticity : an introduction

Abstract: Introduction.- Stress and Strain Analysis and Measurement.- Characteristics, Applications and Properties of Polymers.- Polymerization and Classification.- Differential Constitutive Equations.- Hereditary Integral Representations of Stress and Strain.- Time and Temperature Behavior of Polymers.- Elementary Viscoelastic Stress Analysis for Bars and Beams.- Viscoelastic Stress Analysis in Two and Three Dimensions.- Nonlinear Viscoelasticity.- Rate and Time-Dependent Failure: Mechanics and Predictive Models.

Stress-Strain Behavior of Sands Cemented by Microbially Induced Calcite Precipitation

Abstract: Microbial induced calcite precipitation (MICP) is a novel biomediated ground improvement method that can be used to increase the shear strength and stiffness of soil. The evolution of the shear strength and stiffness of sand subjected to undrained and drained shearing is evaluated using triaxial tests. MICP treated sands with cementation levels ranging from young, uncemented sand to a highly cemented sandstonelike condition are subjected to undrained shear. A transition from strain hardening to strain softening behavior and a corresponding transition of global to localized failure as cementation is increased is observed. Moderately cemented specimens are subjected to various stress paths, which result in a change to the shear strength and volumetric behavior. Shear wave velocity is used to nondestructively monitor the change in small-strain stiffness during shearing, which provides an indication of cementation degradation as a function of strain level. Because shear wave velocity is influenced by ...

Spherical nanoindentation stress–strain curves

Abstract: Although indentation experiments have long been used to measure the hardness and Young's modulus, the utility of this technique in analyzing the complete elastic–plastic response of materials under contact loading has only been realized in the past few years – mostly due to recent advances in testing equipment and analysis protocols. This paper provides a timely review of the recent progress made in this respect in extracting meaningful indentation stress–strain curves from the raw datasets measured in instrumented spherical nanoindentation experiments. These indentation stress–strain curves have produced highly reliable estimates of the indentation modulus and the indentation yield strength in the sample, as well as certain aspects of their post-yield behavior, and have been critically validated through numerical simulations using finite element models as well as direct in situ scanning electron microscopy (SEM) measurements on micro-pillars. Much of this recent progress was made possible through the introduction of a new measure of indentation strain and the development of new protocols to locate the effective zero-point of initial contact between the indenter and the sample in the measured datasets. This has led to an important key advance in this field where it is now possible to reliably identify and analyze the initial loading segment in the indentation experiments. Major advances have also been made in correlating the local mechanical response measured in nanoindentation with the local measurements of structure at the indentation site using complementary techniques. For example, it has been shown that the combined use of Orientation Imaging Microscopy (OIM, using Electron BackScattered Diffraction (EBSD)) and nanoindentation on polycrystalline metallic samples can yield important information on the orientation dependence of indentation yield stress, which can in turn be used to estimate percentage increase in the local slip resistance in deformed samples. The same methods have been used successfully to probe the intrinsic role of grain boundaries in the overall mechanical deformation of the sample. More recently, these protocols have been extended to characterize local mechanical property changes in the damaged layers in ion-irradiated metals. Similarly, the combined use of Raman spectroscopy and nanoindentation on samples of mouse bone has revealed tissue-level correlations between the mineral content at the indentation site and the associated local mechanical properties. The new protocols have also provided several new insights into the buckling response in dense carbon nanotube (CNT) brushes. These and other recent successful applications of nanoindentation are expected to provide the critically needed information for the maturation of physics-based multiscale models for the mechanical behavior of most advanced materials. In this paper, we review these latest developments and identify the future challenges that lie ahead.

Mechanical Behavior of Fiber Reinforced Engineered Cementitious Composites in Uniaxial Compression

Abstract: Polyvinyl alcohol (PVA) fiber reinforced engineered cementitious composite (ECC) is a class of high performance cementitious composites with pseudo strain-hardening behavior and excellent crack control when subjected to uniaxial tension. However, the compressive behavior of ECC has not been well characterized in the literature. In this paper, uniaxial compression tests were carried out on ECC with five different mix proportions and compressive strength ranging from 35 MPa to 60 MPa. Complete stress-strain curves were obtained. Based on the test results, the compressive parameters, such as the elastic modulus, engineering strain at the peak stress, the Poisson's ratio and the toughness index, were studied. A new constitutive model was proposed to express the pre- and post-peak mechanical behavior of ECC under uniaxial compression. The proposed model showed a good agreement with the experimental curves. The model proposed should be a valuable reference for the nonlinear analysis of ECC material in the part of structures under uniaxial compression.

A Comparative Evaluation of Stress–Strain and Acoustic Emission Methods for Quantitative Damage Assessments of Brittle Rock

Abstract: The purpose of this study is to identify the crack initiation and damage stress thresholds of granite from the Korea atomic energy research institute’s Underground Research Tunnel (KURT). From this, a quantitative damage evolution was inferred using various methods, including the crack volumetric strain, b value, the damage parameter from the moment tensor, and the acoustic emission (AE) energy. Uniaxial compression tests were conducted, during which both the stress–strain and AE activity were recorded simultaneously. The crack initiation threshold was found at a stress level of 0.42–0.53 σc, and the crack damage threshold was identified at 0.62–0.84 σc. The normalized integrity of KURT granite was inferred at each stress level from the damage parameter by assuming that the damage is accumulated beyond the crack initiation stress threshold. The maximum deviation between the crack volumetric strain and the AE method was 16.0 %, which was noted at a stress level of 0.84 σc. The damage parameters of KURT granite derived from a mechanically measured stress–strain relationship (crack volumetric strain) were successfully related and compared to those derived from physically detected acoustic emission waves. From a comprehensive comparison of damage identification and quantification methods, it was finally suggested that damage estimations using the AE energy method are preferred from the perspectives of practical field applicability and the reliability of the obtained damage values.

Strain rate and anisotropy effects on the tensile failure characteristics of human skin.

Abstract: The anisotropic failure characteristics of human skin are relatively unknown at strain rates typical in impact biomechanics. This study reports the results of an experimental protocol to quantify the effect of dynamic strain rates and the effect of sample orientation with respect to the Langer lines. Uniaxial tensile tests were carried out at three strain rates (0.06 s−1, 53 s−1, and 167 s−1) on 33 test samples excised from the back of a fresh cadaver. The mean ultimate tensile stress , mean elastic modulus and mean strain energy increased with increasing strain rates. While the stretch ratio at ultimate tensile stress was not affected by the strain rate, it was influenced by the orientation of the samples (parallel and perpendicular to the Langer lines. The orientation of the sample also had a strong influence on the ultimate tensile stress, with a mean value of 28.0±5.7 MPa for parallel samples, and 15.6±5.2 MPa for perpendicular samples, and on the elastic modulus, with corresponding mean values of 160.8 MPa±53.2 MPa and 70.6 MPa±59.5 MPa. The study also pointed out the difficulties in controlling the effective applied strain rate in dynamic characterization of soft tissue and the resulting abnormal stress–strain relationships. Finally, data collected in this study can be used to develop constitutive models where high loading rates are of primary interest.

Modelling of concrete behaviour in uniaxial compression and tension with DEM

Abstract: The paper focuses on the DEM modelling of the behaviour of plain concrete during uniaxial compression and uniaxial tension using the discrete element method. The model takes into account the concrete heterogeneity at the meso-scale level. The effects of concrete density, size of aggregate grains and specimen size on the stress–strain curve, volume changes and fracture process are studied. In addition, the evolution of contact forces, grain rotations, displacement fluctuations and strain localization during deformation is investigated. The elastic, kinetic, plastic and numerical dissipated energy is calculated and analysed at a different stress–strain stage. Concrete is described as a 1-phase or 3-phase material. The macroscopic 2D and 3D results are compared with the corresponding experiments. A satisfactory agreement between experiments and calculations is achieved.

General Stress-Strain Model for Steel- and FRP-Confined Concrete

Abstract: A new and general stress-strain model for concrete confined by steel or fiber reinforced polymer (FRP) is developed in this work. One additional variable and one constant are added to the well-known Popovics model to control the type and the shape of the stress-strain curve. The proposed model has one simple, continuous, and explicit expression and can exhibit either hardening or softening types of responses. This general model provides a unified platform for modeling stress-strain of concrete confined by different materials, such as steel or FRP, and help to overcome inconsistency or complexity. The parameters of the stress-strain model are determined by analytical study and data regression using a large and up-to-date test database. The proposed stress-strain model is validated with experimental results and compared with existing models; it shows good performance and superior flexibility and versatility of the model.

Unified Stress-Strain Model for FRP and Actively Confined Normal-Strength and High-Strength Concrete

Abstract: Accurate modeling of the complete stress-strain relationship of confined and unconfined concrete is of vital importance in predicting the overall flexural behavior of reinforced concrete structures. The analysis-oriented models, which utilize the dilation characteristics of confined concretes for stress-strain relationship prediction, are well recognized for their versatility in such modeling applications. These models assume that at a given lateral strain, the axial compressive stress and strain of fiber-reinforced polymer (FRP)–confined concrete are the same as those of the same concrete when it is actively confined under a confining pressure equal to that supplied by the FRP jacket. However, this assumption has recently been demonstrated experimentally to be inaccurate for high-strength concrete (HSC). It was shown that at a given axial strain, lateral strains of actively confined and FRP-confined concretes of the same concrete strength correspond when they are subjected to the same lateral con...

Experiments and FE modeling of stress-strain state in ReBCO tape under tensile, torsional and transverse load

Abstract: For high current superconductors in high magnet fields with currents in the order of 50 kA, single ReBCO coated conductors must be assembled in a cable. The geometry of such a cable is mostly such that combined torsion, axial and transverse loading states are anticipated in the tapes and tape joints. The resulting strain distribution, caused by different thermal contraction and electromagnetic forces, will affect the critical current of the tapes. Tape performance when subjected to torsion, tensile and transverse loading is the key to understanding limitations for the composite cable performance. The individual tape material components can be deformed, not only elastically but also plastically under these loads. A set of experimental setups, as well as a convenient and accurate method of stress–strain state modeling based on the finite element method have been developed. Systematic measurements on single ReBCO tapes are carried out combining axial tension and torsion as well as transverse loading. Then the behavior of a single tape subjected to the various applied loads is simulated in the model. This paper presents the results of experimental tests and detailed FE modeling of the 3D stress–strain state in a single ReBCO tape under different loads, taking into account the temperature dependence and the elastic-plastic properties of the tape materials, starting from the initial tape processing conditions during its manufacture up to magnet operating conditions. Furthermore a comparison of the simulations with experiments is presented with special attention for the critical force, the threshold where the tape performance becomes irreversibly degraded. We verified the influence of tape surface profile non-uniformity and copper stabilizer thickness on the critical force. The FE models appear to describe the tape experiments adequately and can thus be used as a solid basis for optimization of various cabling concepts.

Effect of Water on the Deformation and Failure of Rock in Uniaxial Tension

Abstract: To design and construct underground structures, it is essential to understand the mechanical properties of rock in not only compression but also tension. It is well known that water is one of the important factors affecting the deformation and failure of rock. In this study, laboratory tests and numerical simulations were conducted to understand the effect of water on rock properties in uniaxial tension. In the experiments, a testing machine previously used for uniaxial tension tests in dry conditions was modified for tests in wet conditions. Using this machine, complete stress–strain curves from the pre- to postpeak regions of water-saturated specimens in uniaxial tension were obtained. The results for granite, tuff, and two types of andesite showed that the stress–strain curves in wet conditions have a lower initial slope and lower strength than those in dry conditions, and they are strongly nonlinear in the prepeak region. Comparing the changes in the results for uniaxial tension versus compression due to water, it was found that the reduction rate of uniaxial tensile strength was greater than that of uniaxial compressive strength, while the ratio between the reduction rates was almost constant for various rocks. In numerical simulations, the stress–strain curves in the prepeak region under dry and wet conditions could be reproduced by crack extension models under uniaxial tensile stress. Numerical analyses indicated that the nonlinearity of the stress–strain curves is probably due to the longer crack extension in wet compared with dry conditions.

Effect of rock strength on failure mode and mechanical behavior of composite samples

Abstract: Many dynamic events in coal mine are caused by the instability of coal–rock body. In order to study the influence of rock strength on this type of instability, uniaxial compression experiments of rock–coal–rock composite samples with different rock strengths are carried out, and the effect and mechanism of rock strength on the mechanical behavior and fracture mode of the composite samples are analyzed. The results show that major failure modes of the composite samples are conjugate X-shaped shearing fracture and splitting fracture. The angle between the shear fracture surface and the end face increases with rock strength. The splitting fracture in the coal body expands to the rock when the rock strength is low. The strength properties of the composite samples mainly depend on the coal strength instead of the rock strength. With the rock strength increasing, the peak strain of the composite samples decrease, and the differences from the coal strain and strain rate to rock strain and strain rate become greater. These failure modes and characteristics of deformation are shown to be determined by the difference between the elastic modulus of rock and coal constituting the composite samples.

Mechanical response of TiAl6V4 lattice structures manufactured by selective laser melting in quasistatic and dynamic compression tests

Abstract: This paper focusses on the investigation of the mechanical properties of lattice structures manufactured by selective laser melting using contour-hatch scan strategy. The motivation for this research is the systematic investigation of the elastic and plastic deformation of TiAl6V4 at different strain rates. To investigate the influence of the strain rate on the mechanical response (e.g., energy absorption) of TiAl6V4 structures, compression tests on TiAl6V4-lattice structures with different strain rates are carried out to determine the mechanical response from the resulting stress-strain curves. Results are compared to the mechanical response of stainless steel lattice structures (316L). It is shown that heat-treated TiAl6V4 specimens have a larger breaking strain and a lower drop of stress after failure initiation. Main finding is that TiAl6V4 lattice structures show brittle behavior and low energy absorption capabilities compared to the ductile behaving 316L lattice structures. For larger strain rates, ultimate tensile strength of TiAl6V4 structures is more than 20% higher compared to lower strain rates due to cold work hardening.

Analysis-Oriented Stress-Strain Model for Concrete under Combined FRP-Steel Confinement

Abstract: Extensive research has been conducted on fiber-reinforced polymer (FRP)-confined plain and RC columns, leading to a large number of stress–strain models. Most of these models have been developed for FRP-confined plain concrete and are thus applicable only to concrete in FRP-confined RC columns with a negligible amount of transverse steel reinforcement. The few models that have been developed for concrete under the combined confinement of FRP and transverse steel reinforcement are either inaccurate or too complex for direct use in design. This paper presents an accurate design-oriented stress–strain model for concrete under combined FRP-steel confinement in FRP-confined circular RC columns. The proposed model is formulated on the basis of extensive numerical results generated using an analysis-oriented stress–strain model recently proposed by the authors and properly captures the key characteristics of FRP-steel-confined concrete as revealed by existing test results. The model strikes a good balanc...