Showing papers on "Stress–strain curve published in 2006"

FRP-confined concrete under axial cyclic compression

Abstract: One important application of fiber reinforced polymer (FRP) composites in construction is as FRP jackets to confine concrete in the seismic retrofit of reinforced concrete (RC) structures, as FRP confinement can enhance both the compressive strength and ultimate strain of concrete. For the safe and economic design of FRP jackets, the stress–strain behavior of FRP-confined concrete under cyclic compression needs to be properly understood and modeled. This paper presents the results of an experimental study on the behavior of FRP-confined concrete under cyclic compression. Test results obtained from CFRP-wrapped concrete cylinders are presented and examined, which allows a number of significant conclusions to be drawn, including the existence of an envelope curve and the cumulative effect of loading cycles. The results are also compared with two existing stress–strain models for FRP-confined concrete, one for monotonic loading and another one for cyclic loading. The monotonic stress–strain model of Lam and Teng is shown to be able to provide accurate predictions of the envelope curve, but the only existing cyclic stress–strain model is shown to require improvement.

Effect of strain rate on stress-strain behavior of alloy 309 and 304L austenitic stainless steel

Abstract: The effect of strain rate on stress-strain behavior of austenitic stainless steel 309 and 304L was investigated. Tensile tests were conducted at room temperature at strain rates ranging from 1.25×10−4s−1 to 400 s−1. The evolution of volume fraction martensite that formed during plastic deformation was measured with X-ray diffraction and characterized with light microscopy. Alloy 304L was found to transform readily with strain, with martensite nucleating on slip bands and at slip band intersections. Alloy 309 did not exhibit strain-induced transformation. Variations in ductility and strength with strain rate are explained in terms of the competition between hardening, from the martensitic transformation and a positive strain rate sensitivity, and softening due to deformational heating. Existing models used to predict the increase in volume fraction martensite with strain were examined and modified to fit the experimental data of this study as well as recent data for alloys 304 and 301LN obtained from the literature.

Phase-Transformation Fronts Evolution for Stress- and Strain-Controlled Tension Tests in TiNi Shape Memory Alloy

Abstract: Nucleation and development of phase transformation fronts in TiNi shape memory alloy subjected to the stress- and strain-controlled tension tests were investigated. A thermovision camera was applied to register the distribution of infrared radiation emitted by the specimen and to find its temperature variations. During the loading, narrow bands of considerably higher temperature corresponding to the martensitic phase, starting from the central part of the specimen and developing towards the specimen grips, under both approaches, were registered. The inclined bands of heterogeneous temperature distribution were observed also during the unloading process of the SMA, while the reverse transformation accompanied by temperature decrease took place. Thermomechanical aspects of martensitic and reverse transformations for various strain rates were analyzed under both stress- and strain-controlled tests.

Tribological contact analysis of a rigid ball sliding on a hard coated surface. Part I: Modelling stresses and strains

Abstract: The stress and fracture conditions of a coated surface, that are the origin to wear, were analysed by three-dimensional finite element method (FEM) modelling on microlevel, by stress and strain computer simulations and by experimental studies with a scratch tester The studied tribological contact was a 02 mm radius diamond ball sliding with increasing load on a thin, 2 Am thick titanium nitride (TiN) coating on a flat high speed steel substrate The ball was modelled as rigid, the coating linearly elastic and the steel substrate elastic–plastic taking into account strain hardening effects The stresses and strains generated in the surface during sliding are the result of four different mechanisms: the pulling and pushing by the friction force; the geometrical indent, groove, and torus shaped deformations of the flat surface; the bulk plasticity concentration and curvature minimum effects; and the residual stresses in the coating In a sliding contact the first crack is initiated at the top of the coating from bending and pulling actions and it grows down through the coating In the modelled scratch tester system a complex stress field is formed at the surface including remaining residual stresses in the coating behind the sliding contact The stress fields are very different in a scratched uncoated steel sample Some residual tensile stresses are formed in the groove behind the tip but they are very much lower than for the TiN coated case A displacement controlled FEM model was found to better represent the real situation and correspond to experimental results than a force controlled model D 2005 Elsevier BV All rights reserved

Stress–Strain Behavior of Large-Scale Circular Columns Confined with FRP Composites

Abstract: For a reliable and efficient design of fiber reinforced polymer (FRP) wrapping reinforcement, an accurate stress–strain prediction of reinforced concrete (RC) columns confined with FRP composites is essential. The stress–strain characteristics of FRP-confined concrete have been extensively studied, testing small-scale cylindrical specimens. In this paper, the stress–strain response of axially loaded large-scale columns confined with FRP wrapping reinforcement is investigated. The effective circumferential FRP failure strain and the effect of increasing confining action on the stress–strain behavior of RC columns confined with FRP are examined. Different models for the prediction of the stress–strain behavior of FRP-confined concrete have been reviewed. The existing stress–strain models are applied to predict the stress–strain response of the experimental results presented in this study. A comparison between the experimental results and those predicted by the existing models are presented. Few of the available models that were derived based on small-size cylinders were found to accurately predict the stress–strain response of large-scale columns.

On the determination of spherical nanoindentation stress-strain curves

Abstract: Instrumented nanoindentation experiments, especially with sharp tips, are a well-established technique to measure the hardness and moduli values of a wide range of materials. However, and despite the fact that they can accurately delineate the onset of the elasto-plastic transition of solids, spherical nanoindentation experiments are less common. In this article we propose a technique in which we combine (i) the results of continuous stiffness measurements with spherical indenters–with radii of 1 μm and/or 13.5 μm, (ii) Hertzian theory, and (iii) Berkovich nanoindentations, to convert load/depth of indentation curves to their corresponding indentation stress–strain curves. We applied the technique to fused silica, aluminum, iron and single crystals of sapphire and ZnO. In all cases, the resulting indentation stress–strain curves obtained clearly showed the details of the elastic-to-plastic transition (i.e., the onset of yield, and, as important, the steady state hardness values that were comparable with the Vickers microhardness values obtained on the same surfaces). Furthermore, when both the 1 μm and 13.5 μm indenters were used on the same material, for the most part, the indentation stress–strain curves traced one trajectory. The method is versatile and can be used over a large range of moduli and hardness values.

Influence of initial microstructure on the hot working flow stress of Mg-3Al-1Zn

Abstract: The hot working flow stress is examined for wrought and as-cast magnesium alloy AZ31, deformed in both compression and torsion. It is found that the hot deformation behaviour is sensitive to the deformation conditions, initial microstructure and the deformation mode. The peak stresses in compression are higher for as-cast material at low strain rates and high temperatures, whereas they are higher for fine-grained wrought material at high strain rates and low temperatures. Also, the stress–strain curves in torsion exhibit considerably higher strains to the peak flow stress than in compression, particularly at lower temperatures. Most noticeably in the compression of the wrought material, the shape of the stress–strain curves change considerably as the temperature is reduced and the strain rate is increased. These key features of the deformation behaviour are explained in terms of initial grain size, texture, twinning and dynamic recrystallization.

Analytical Stress—Strain Relationship for Concrete Confined by Steel Stirrups and/or FRP Jackets

Abstract: This paper presents a plain strain analytical model, based on the elasticity theory, to determine the confining pressures of transverse reinforcements on the concrete core of a reinforced concrete member. The analytical evaluation of the confining pressures was first carried out on reinforced sections with square and circular stirrups, and subsequently on reinforcement configurations of greater complexity with square and rectangular stirrups and supplementary cross ties. Finally, the model has been used to evaluate the confining pressures applied by external wrapping in any material [fiber-reinforced polymer (FRP), S-glass, steel, etc.] and to design better combinations of techniques and confinement materials. In order to obtain the stress–strain curves due to passive confinement, an analogy between square and circular sections has been introduced. In this way, any active confinement model derived by triaxial tests on cylindrical specimens can be used. The model has been validated by comparing its predict...

Stress-strain model for fiber-reinforced polymer jacketed concrete columns

Abstract: Fiber-reinforced polymer (FRP) jackets are often used to confine and reinforce concrete columns. This article reports on a study of the stress-strain behavior of FRP-confined concrete columns that focused on rectangular column sections. The authors developed a new design-oriented model of the stress-strain response of FRP confined columns. Their test variables included the volumetric ratio of the FRP jackets, the aspect ratio of the column section, and the area of longitudinal and lateral steel reinforcement. Results showed that jacketing rectangular column sections with FRP sheets increases their axial strength and ductility. FRP jackets can be used to prevent premature failure of the concrete cover and buckling of the steel bars, leading to substantially improved performance. The authors include a discussion of the main parameters that control the stress and strain characteristics of FRP-confined rectangular column sections. They propose a general design model of the stress-strain response of FRP-confined concrete. The authors conclude that the results predicted by this model showed very good agreement with other test data of FRP-confined circular and rectangular columns reported in the literature.

Multi-scale modeling, stress and failure analyses of 3-D woven composites

Abstract: The very complex, multi-level hierarchical construction of textile composites and their structural components commonly manifests via significant property variation even at the macro-level. The concept of a “meso-volume” (introduced by this author in early 1990s) is consistently applied in this work to 3-D stress/strain and failure analyses of 3-D woven composites at several levels of structural hierarchy. The meso-volume is defined as homogeneous, anisotropic block of composite material with effective elastic properties determined through volumetrically averaged 3-D stress and strain fields computed at a lower (“finer”) level of structural hierarchy and application of generalized Hooke’s law to the averaged fields. The meso-volume can represent a relatively large, homogenized section of a composite structural component, a lamina in laminated composite structure, a homogenized assembly of several textile composite unit cells, a single homogenized unit cell, a resin-impregnated yarn, a single carbon fiber, even a carbon nanotube assembly. When composed together, distinct meso-volumes constitute a 3-D Mosaic model at the respective hierarchy level. A multi-scale methodology presented in this paper first illustrates 3-D stress/strain analysis of the Mosaic unidirectional composite, computation of its effective elastic properties and their further use in 3-D stress/strain analysis of the Mosaic model of 3-D woven composite Unit Cell. The obtained 3-D stress/strain fields are then volumetrically averaged within the Unit Cell, and its effective elastic properties are computed. The predicted effective elastic properties of 3-D woven composite are compared with experimental data and show very good agreement. Further, those effective elastic properties are used in 3-D simulations of three-point bending tests of 3-D woven composite; theoretical predictions for central deflection show excellent agreement with experimental data. Finally, a 3-D progressive failure analysis of generic 3-D Mosaic structure is developed using ultimate strain criterion and illustrated on the 3-D woven composite Unit Cell. The predicted strength values are compared to experimental results. The presented comparisons of theoretical and experimental results validate the adequacy and accuracy of the developed material models, mathematical algorithms, and computational tools.

Unloading and Reloading Stress–Strain Model for Confined Concrete

Abstract: Current engineering practice evaluates the seismic performance of bridges based on nonlinear dynamic response analyses. Although various models exist for envelope curves of concrete confined by transverse reinforcement, only a few simple models represent the hysteretic behavior of the confined concrete; therefore, development of an accurate stress–strain model of unloading and reloading paths for confined concrete is urgently needed. In this paper, an unloading and reloading stress–strain model for concrete confined by transverse reinforcement is presented, which is based on the results from a series of compressive loading tests of reinforced concrete column specimens. The model focuses on the effect of repeated unloading/reloading cycles and partial loading excursions, and provides equations that take into account the number of unloading/reloading cycles to predict essential parameters, such as plastic strain and unloaded stress after repeated loading cycles. The proposed model provides good agreement with the test results.

The mechanical properties of freestanding electroplated Cu thin films

Abstract: The plane-strain bulge test is used to investigate the mechanical behavior of freestanding electroplated Cu thin films as a function of film thickness and microstructure. The stiffness of the films increases slightly with decreasing film thickness because of changes in the crystallographic texture and the elastic anisotropy of Cu. Experimental stiffness values agree well with values derived from single-crystal elastic constants and the appropriate orientation distribution functions. No modulus deficit is observed. The yield stress of the films varies with film thickness and heat treatment as a result of changes in the grain size of the films. The yield stress follows typical Hall-Petch behavior if twins are counted as distinct grains, indicating that twin boundaries are effective barriers to dislocation motion. The Hall-Petch coefficient is in good agreement with values reported for bulk Cu. Film thickness and crystallographic texture have a negligible effect on the yield stress of the films.

Effects of Strain Rate on Tensile Behavior of Reactive Powder Concrete

Abstract: Reactive Powder Concrete (RPC) reinforced with short steel fibers is characterized by ultra-high strength and high fracture toughness Because of its excellent properties, RPC may be suitable as an advanced material for reinforced concrete structures subjected to impact loading Thus, the objective of this study was to find out the effects of strain rates on tensile behaviors of RPC specimens subjected to rapid loading The influence of the loading rates on failure modes, tensile stress-elongation curves and tensile stress-crack opening curves was investigated Furthermore, based on the test results, a rate-dependent bridging law expressing the relation between tensile stress and crack opening was proposed

Determination of tensile properties by instrumented indentation technique: Representative stress and strain approach

Abstract: Tensile properties can be evaluated by defining representative stress and strain with the parameters obtained from instrumented indentation tests using a spherical indenter. The accuracy of this approach depends strongly on how the contact depth is analyzed and how the representative stress and strain are defined. The primary factors influencing the determination of contact depth, pile-up/sink-in and elastic deflection, were quantified by analyzing indentation morphology by finite element simulation; then plastic pile-up/sink-in behavior was formulated in terms of the strain-hardening exponent and the ratio of indentation depth to indenter radius. For the representative strain, the definition by tangent function was determined to be more appropriate for assessing tensile properties based on derived behaviors of the strain-hardening exponent. This approach was experimentally verified by comparing tensile properties of 10 metallic materials from uniaxial tensile tests and instrumented indentation tests.

Modeling Viscoelastic and Viscoplastic Behavior of High Density Polyethylene (HDPE)

Abstract: The viscoelastic and viscoplastic behaviors of high density polyethylene (HDPE) under uniaxial monotonic and cyclic loading are modeled using the modified viscoplasticity theory based on overstress (VBO). The viscoelastic modeling capabilities of the modified VBO are investigated by simulating the behavior of semicrystalline HDPE under uniaxial compression tests at different strain rates. In addition, the effects of the modification (introducing the variable "C" into an elastic strain rate equation) on VBO that has been made to construct the change in the elastic stiffness while loading and unloading are investigated. During first loading and unloading, the modification in the elastic strain rate equation improves the unloading behavior. To investigate how the variable "C" that is introduced in the elastic strain rate equation evolves during reloading, the cyclic behavior of HDPE is modeled. For a complete viscoelastic and viscoplastic behavior, the relaxation and creep behaviors of HDPE are simulated as well in addition to stress and strain rate dependency. The influences of the strain (stress) levels where the relaxation (creep) experiments are performed are investigated. The simulation results are compared with the experimental data obtained by Zhang and Moore (1997, Polym. Eng. Sci., 37, pp. 404-413). A good match between experimental and simulation results are observed.

Thermo-Mechanical Reliability of 3D-integrated Microstructures in Stacked Silicon

Abstract: This paper investigates the thermo-mechanical reliability of inter-chip-vias (ICV) for 3D chip stacking after processing and under external thermal loads relevant for the envisaged field of application (mobile, automotive) by Finite Element simulation. First the materials are characterised by nano-indentation to determine elasto-plastic data. Finite Element simulations are used to reproduce these data and to extract local material properties like E-modulus and yield stress. Accumulated plastic strain is used as failure indicator under periodic thermal loading of an ICV. Geometrical, material and process-related parameters are varied to obtain first design guidelines for this new technology. The locations of stress and strain accumulation are given.