Showing papers on "Stress–strain curve published in 2004"

Mechanical Behavior of Materials

Abstract: 1. Stress and strain 2. Elasticity 3. Mechanical tensile testing 4. Strain hardening of metals 5. Plasticity 6. Strain-rate and temperature dependence of flow stress 7. Slip 8. Dislocation geometry and energy 9. Dislocation mechanics 10. Mechanical twinning 11. Hardening mechanisms 12. Discontinuous and inhomogeneous deformation 13. Ductility and fracture 14. Fracture mechanics 15. Viscoelasticity 16. Creep and stress rupture 17. Fatigue 18. Residual stresses 19. Ceramics 20. Polymers 21. Composites 22. Mechanical working Appendix I. Miller indices Appendix II. Stereographic projection.

Uniaxial True Stress-Strain after Necking

Abstract: A weighted-average method for determining uniaxial, true tensile stress vs. strain relation after necking is presented for strip shaped samples. The method requires identification of a lower and an upper bound for the true stress-strain function after necking and expresses the true stress-strain relation as the weighted average of these two bounds. The weight factor is determined iteratively by a finite element model until best agreement between calculated and experimental loadextension curves is achieved. The method was applied to various alloys.

Stress-strain curves for high strength concrete at elevated temperatures

Abstract: The effects of high temperature on the strength and stress-strain relationship of high strength concrete ~HSC! were investi- gated. Stress-strain curve tests were conducted at various temperatures ~20, 100, 200, 400, 600, and 800°C) for four types of HSC. The variables considered in the experimental study included concrete strength, type of aggregate, and the addition of steel fibers. Results from stress-strain curve tests show that plain HSC exhibits brittle properties below 600°C, and ductility above 600°C. HSC with steel fibers exhibits ductility for temperatures over 400°C. The compressive strength of HSC decreases by about a quarter of its room temperature strength within the range of 100- 400°C. The strength further decreases with the increase of temperature and reaches about a quarter of its initial strength at 800°C. The strain at peak loading increases with temperature, from 0.003 at room temperature to 0.02 at 800°C. Further, the increase in strains for carbonate aggregate HSC is larger than that for siliceous aggregate HSC.

Bio-composites produced from plant microfiber bundles with a nanometer unit web-like network

Abstract: Using plant microfiber bundles with a nanometer unit web-like network, a moulded product with a bending strength of 250 MPa was obtained without the use of binders. High interactive forces seem to be developed between pulp fibers owing to their nanometer unit web-like network. In other words, the area of possible contact points per fiber are increased, so that more hydrogen bonds might be formed or van der Waals forces increased. When 2% oxidized tapioca starch, by weight, was added, the yield strain doubled and the bending strength reached 310 MPa. The starch mixed moulded product had a similar stress strain curve to that for magnesium alloy, and three to four times higher Young's modulus and bending strength values than polycarbonate and GFRP (chopped). The mouldings have a combination of environmentally friendly and high strength properties.

A Model Treating Tensile Deformation of Semicrystalline Polymers: Quasi-Static Stress−Strain Relationship and Viscous Stress Determined for a Sample of Polyethylene

Abstract: Tensile deformation of semicrystalline polymers follows a common scheme with changes in the mechanism at critical strains. Choosing a poly(ethylene-co-12% vinyl acetate) (PEVA12) as an example, we measured true stress−strain relationships at constant strain rates, determined the elastic and plastic part of imposed strain in step-cycle experiments, and followed the stress relaxation at fixed strains. On the basis of the general observations, a model was constructed and then used for a description of the properties of PEVA12. The model treats the stress as arising from three contributions: quasi-static stresses originating from the stretched network of entangled chains in the fluid regions and from the force-transmitting skeleton of crystallites, plus the viscous forces described by Eyring's equation. Adjustment of the measured data to the model provides a decomposition of the stress in the three parts. With increasing strain the dominance shifts from the crystal- to network-transmitted stress, while the v...

Stress-strain characteristics in Ni–Ga–Fe ferromagnetic shape memory alloys

Abstract: Tensile and compressive stress–strain characteristics for Ni–Ga–Fe ferromagnetic shape memory alloys at several temperatures were investigated by mechanical test and a critical stress versus temperature diagram was obtained. The crystal structure of the martensite phase obtained by tensile-stress-induced martensitic transformation was estimated from the degree of the transformation strain. Stress-induced martensite transformed from the parent phase with an L21 structure showed a 14M structure by tensile stress and by further applying stress, the 14M structure martensitically was transformed into an L10 structure. Moreover, it was found in the compressive test that variant rearrangement occurred by very low compressive stress less than 3 MPa, which is similar to the phenomenon seen in Ni–Mn–Ga alloys.

Low strength concrete members externally confined with FRP sheets

Abstract: In this paper axial loading tests on low strength concrete members, which were confined with various thickness of carbon fiber reinforced polymer (CFRP) composite sheets are described. Totally 46 specimens with circular, square and rectangular cross-sections with unconfined concrete compressive strengths between 6 and 10 MPa were included in the test program. During the tests, a photogrammetrical deformation measurement technique was also used, as well as conventional measurement techniques. The contribution of external confinement with CFRP composite sheets to the compressive behavior of the specimens with low strength concrete is evaluated quantitatively, in terms of strength, longitudinal and lateral deformability and energy dissipation. The effects of width/depth ratios and the corner radius of the specimens with rectangular cross-section on the axial behavior were also examined. It was seen that the effectiveness of the external confinement with CFRP composite sheets is much more pronounced, when the unconfined concrete compressive strength is relatively lower. It was also found that the available analytical expressions proposed for normal or high strength concrete confined by CFRP sheets could not predict the strength and deformability of CFRP confined low strength concrete accurately. New expressions are proposed for the compressive strength and the ultimate axial strain of CFRP confined low strength concrete.

Transition of deformation and fracture behaviors in nanostructured face-centered-cubic metals

Abstract: Tensile stress–strain curves demonstrate that single-phase nanocrystalline face-centered-cubic (fcc) metals are intrinsically ductile and their failure begins with necking. However, the area reductions and the fracture behaviors were found to be dependent on the grain size. When plastic deformation is governed by dislocation activity, the nanocrystalline samples behave similar to the conventional coarse-grained materials. As the grain size is reduced to the regime where grain boundary sliding dominates, the material shows very high strain-hardening rate and the tensile samples fail by microcracking with no noticeable reduction in area.

Modeling Cyclic Deformation of HSLA Steels Using Crystal Plasticity

Abstract: High strength low alloy (HSLA) steels, used in a wide variety of applications as structural components are subjected to cyclic loading during their service lives. Understanding the cyclic deformation behavior of HSLA steels is of importance, since it affects the fatigue life of components. This paper combines experiments with finite element based simulations to develop a crystal plasticity model for prediction of the cyclic deformation behavior of HSLA-50 steels. The experiments involve orientation imaging microscopy (OIM) for microstructural characterization and mechanical testing under uniaxial and stress-strain controlled cyclic loading. The computational models incorporate crystallographic orientation distributions from the OIM data. The crystal plasticity model for bcc materials uses a thermally activated energy theory for plastic flow, sell and latent hardening, kinematic hardening, as well as yield point phenomena. Material parameters are calibrated from experiments using a genetic algorithm based minimization process. The computational model is validated with experiments on stress and strain controlled cyclic loading. The effect of grain orientation distributions and overall loading conditions on the evolution of microstructural stresses and strains are investigated.

Stress-Strain Model of Concrete Damaged by Freezing and Thawing Cycles

Abstract: This study investigates the dependence of the mechanical behavior of concrete, such as strength, stiffness, and deformation capacity on the damage caused by freezing and thawing cycles (FTC). A stress-strain model for concrete damaged by freezing and thawing prior to the application of mechanical loading was proposed based on plasticity and fracture of concrete elements. The FTC fracture parameter was introduced to explain the degradation in initial stiffness of concrete resulting from freezing and thawing damage. Based on experimental data, the FTC fracture parameter was empirically formulated as a function of plastic tensile strain caused by freezing and thawing with the assumption that the plastic strain was caused by the combined effects of FTC and mechanical loading damage. The stress-strain relationships obtained by the proposed model were compared with the experimental data.

Finite Element Micromechanics for Stiffness and Strength of Wavy Fiber Composites

Abstract: In this paper, a micromechanical model of a composite lamina material with fiber waviness is described. Results are presented and discussed with regard to stiffness and strength predictions for composite lamina. A micromechanical model of a unit cell from periodically distributed unidirectional waved cylindrical fibers embedded within matrix is proposed to withdraw the different material stiffness parameters. Finite element analysis of the periodic unit cell characterizing the structural stiffness of the composite material is carried out to determine the average stress and strain components. The composite stress-strain relations are then employed to determine the stiffness parameters. Numerical results for a typical composite constituted of polymer matrix and carbon fibers in the form of periodically hexagonal packing and initially sinusoidal waviness are presented for different amplitude to wavelength ratios and a range of fiber volume fractions. The results reveal the presence of local periodic-antisymm...

Effects of Pore Morphology and Bone Ingrowth on Mechanical Properties of Microporous Titanium as an Orthopaedic Implant Material

Abstract: Successful bone formation which leads to functional osseointegration is determined by the local mechanical environment around bone-interfacing implants. In this work, a novel porous titanium material is developed and tested and then impact of porosity on mechanical properties as a function of bone ingrowth is studied numerically. A superplastic foaming technique is used to produce CP-Ti material with rounded, interconnected pores of 50% porosity; the pore size and morphology is particularly suitable for bone ingrowth. In order to understand the structure-property relations for this new material, a numerical simulation is performed to study the effect of the porous microstructure and bone ingrowth on the mechanical properties. Using ABAQUS, we create two-dimensional representative microstructures for fully porous samples, as well as samples with partial and full bone ingrowth. We then use the finite element method to predict the macroscopic mechanical properties of the foam, e.g., overall Young's modulus and yield stress, as well as the local stress and strain pattern of both the titanium foam and bone inclusions. The strain-stress curve, stress concentrations and stress shielding caused by the bone-implant modulus mismatch are examined for different microstructures in both elastic and plastic region. The results are compared with experimental data from the porous titanium samples. Based on the finite element predictions, bone ingrowth is predicted to dramatically reduce stress concentrations around the pores. It is shown that the morphology of the implants will influence both macroscopic properties (such as modulus) and localized behavior (such as stress concentrations). Therefore, these studies provide a methodology for the optimal design of porous titanium as an implant material.

Uniqueness of reverse analysis from conical indentation tests

Abstract: The curvature of the loading curve, the initial slope of the unloading curve, and the ratio of the residual depth to maximum indentation depth are three main quantitiesthat can be established from an indentation load-displacement curve. A relationship among these three quantities was analytically derived. This relationship is valid for elasto-plastic material with power law strain hardening and indented by conical indenters of any geometry. The validity of this relationship is numerically verified through large strain, large deformation finite element analyses. The existence of an intrinsic relationship among the three quantities implies that only two independent quantities can be obtained from the load-displacement curve of a single conical indenter. The reverse analysis of a single load-displacement curve will yield non-unique combinations of elasto-plastic material properties due to the availability of only two independent quantities to solve for the three unknown material properties.

Stress-strain relationships for spruce wood : influence of strain rate, moisture content and loading direction

Abstract: The influence of strain rate, moisture content and loading direction on the stress-strain relationships for spruce wood has been investigated. The strain rates were approximately 8×10−3 s−1, 17s−1 and 1000 s−1, and the states of moisture content were those corresponding to oven dry, fiber saturated and fully saturated. Compressive loads were applied along the principal directions of the stem of the tree, i.e., radially, tangentially and axially. The low and medium strain-rate tests were performed with the aid of a servohydraulic testing machine, while the high strain-rate tests were carried out using the split Hopkinson pressure bar (SHPB) technique. Magnesium or steel bars were used in the different SHPB tests in order to reduce impedance mismatch for the different directions of the wood specimens. The strain rate was found to have large influence on the behavior of the wood, especially under the condition of full saturation, where water transport in the deforming specimen is of major importance.