Showing papers on "Stress–strain curve published in 2012"

Estimation of tensile load-bearing capacity of ductile metallic materials weakened by a V-notch: The equivalent material concept

Abstract: Ductile commercial steel was equated from the viewpoint of strain energy density with a virtual brittle material of the same elastic modulus by using a novel concept, called the equivalent material concept (EMC). By determining the ultimate tensile strength of the virtual material and also, by assuming that the values of the plane–strain fracture toughness for real and virtual materials are equal, the tensile load-bearing capacity of several V-notched samples of steel reported in literature was theoretically estimated by using the mean stress (MS) criterion as a well-known brittle fracture theory. It was found that the theoretical results of the MS-EMC criterion for the imaginary brittle specimens are in a very good consistency with the experimental results reported for real steel samples.

Stress–strain model for concrete under cyclic loading

Abstract: A hysteretic stress–strain model is developed for unconfined concrete with the intention of providing efficient modelling for the structural behaviour of concrete in seismic regions. The proposed model is based on the findings of previous experimental and analytical studies. The model for concrete subjected to monotonic and cyclic loading comprises four components in compression and tension – an envelope curve (for monotonic and cyclic loading), an unloading curve, a reloading curve and a transition curve. Formulations for partial unloading and partial reloading curves are also presented. The reliability of the proposed constitutive model is investigated for a reinforced concrete member using a non-linear finite-element analysis program. Comparisons with test results showed that the proposed model provides a good fit to a wide range of experimentally established hysteresis loops.

Laboratory Test on Dynamic Material Properties of Annealed Float Glass

Abstract: In this study, laboratory tests were conducted to investigate the dynamic material properties of annealed float glass, which is widely used in building applications. The influence of strain rate effect on glass strength and Young's modulus is studied. Quasi-static tests were performed first to determine the glass static strength and Young's modulus; then dynamic compressive tests were carried out at the strain rates from 98/s to 376/s using a modified Split Hopkinson Pressure Bar. Tensile tests were performed in the strain rate range of 35/s to 990/s through splitting tensile test (Brazilian test). Test results reveal that the compressive and tensile strengths of annealed glass are very sensitive to strain rate. Dynamic increment on glass compressive strength is found more significant than its tensile strength, a phenomenon different from other brittle materials such as concrete. The Young's modulus is found relatively insensitive to strain rate in the testing range, and is slightly larger in compressive ...

Stress-strain curve of paper revisited

Abstract: We have investigated a relation between micromechanical processes and the stress-strain curve of a dry fiber network during tensile loading. By using a detailed particle-level simulation tool we in ...

Effective mechanical properties of ''fuzzy fiber'' composites

Abstract: In this paper we investigate the mechanical behavior of carbon fiber composites, where the carbon fibers are coated with radially aligned carbon nanotubes. For this purpose we develop a general micromechanics method for fiber composites, where fibers are coated with radially aligned microfibers (“fuzzy fiber” composites). The mechanical effective properties are computed with a special extension of the composite cylinders method. The in-plane shear modulus is determined using an extended version of the Christensen’s generalized self consistent composite cylinders method. The proposed methodology provides stress and strain concentration tensors. The results of the method are compared with numerical approaches based on the asymptotic expansion homogenization method. The combination of composite cylinders method and Mori–Tanaka method allows us to compute effective properties of composites with multiple types of “fuzzy fibers”. Numerical examples of composites made of epoxy resin, carbon fibers and carbon nanotubes are presented and the impact of the carbon nanotubes length and volume fraction in the overall composite properties is studied.

Effects of Pore Position in Depth on Stress/Strain Concentration and Fatigue Crack Initiation

Abstract: The stress field around a pore was analyzed as a function of the pore position in depth in the surface of a linear elastic solid using finite element modeling It was found that the pore depth dominated the stress field around the pore on the surface and that the maximum stress was increased sharply when the pore intercepted with the surface at its top Given the applied nominal stress, the magnitude of the maximum main stress only depended on the relative depth of the pore, while the pore size affected the stress distribution in the surface An elastic-plastic model was also used to account for the yielding effect in the region where stress was over the yield strength The results still indicated a significant maximum stress concentration when the pore was just buried underneath the surface, but with a lowered value than that of the linear elastic model These results were consistent with the experimental observations that fatigue cracks were preferably initiated from pores and particles, which were just intercepted at their top with the sample surface or just buried beneath the surface

Fabric-based Tsai-Wu yield criteria for vertebral trabecular bone in stress and strain space.

Abstract: Osteoporosis related vertebral fractures are an increasing clinical problem in ageing societies. The prediction of vertebral fracture load from QCT-based anatomy-specific finite element simulations could be very useful in the management of patients with osteoporosis, especially with regard to a possible fracture prevention or treatment optimisation. A key property in finite element analysis is the yield surface for the trabecular bone material. This study is aimed at identifying continuum-level yield criteria for vertebral trabecular bone using micro-finite element models subjected to uni-axial, shear, and tri-axial loading. A fabric-dependent, orthotropic Tsai-Wu yield criterion is proposed in both stress and strain spaces. Nonlinear micro-finite element models of cubic vertebral trabecular bone samples with 5.62 mm edge length were generated from μCT-scans. Kinematic boundary conditions were imposed and the specimen was loaded force controlled beyond yield in 17 different load cases (six uni-axial, three shear and eight multi-axial). The proposed yield criteria were fitted to the resulting yield data. Yield strains on-axis were significantly lower (10% in tension and 6% in compression) than in the transverse directions. Average yield strains were 0.7% in tension, 1.1% in compression, 1.0% in shear and ranged from 0.6% to 1.1% under multi-axial loading. In axial direction, maximum yield stress was 2.6 MPa in tension and 4.7 MPa in compression. Lowest shear stress was found in the transverse plane with 1.3 MPa. Multi-axial yield stresses ranged between values for uni-axial tension and compression. Yield stresses depended significantly and substantially on both volume fraction and fabric. Yield strains depended also significantly on both bone volume fraction and fabric, but only weakly on the former. The standard error of the estimate and the concordance correlation coefficient of the yield surface were 5.47% and 0.93 in strain space and 13.58% and 0.96 in stress space. The results of this study are not only consistent with experimental data from the literature but also extend the current knowledge of yield to multi-axial load cases that can hardly be realised in a biomechanical experiment. The presented yield data and criteria will help improving the prediction of vertebral ultimate load using anatomy-specific finite element models.

The tensile behavior of off-axis loaded plant fiber composites: An insight on the nonlinear stress–strain response

Abstract: Composites in load-bearing applications are often exposed to off-axis loads. For plant fiber composites (PFCs) to be seriously and readily considered in structural applications, knowledge and reliable prediction of their response to off-axis loads is critical. This article (i) characterizes the stress–strain response, (ii) investigates the tensile properties, and (iii) analyses the fracture modes, of unidirectional flax-polyester composites subjected to off-axis tensile loading. A key finding of this study is that due to the nonlinear stress–strain response of PFCs, the apparent stiffness of the composite reduces by ∼30% in the strain range of 0.05 to 0.25%. In addition, through cyclic tests on the composites, the elastic strain limit is found to be only ∼0.15%. This has major implications on the strain range to be used for the determination of the composite elastic Young's modulus. Consequently, it is proposed that the tensile modulus for PFCs should be measured in the strain range of 0.025 to 0.100%. Through comparison with experimental data, conventional composite micromechanical models are found to be adequate in quantitatively describing the tensile behavior of off-axis loaded PFCs. The application of such models has also enabled the determination of, otherwise difficult to measure, material properties, such as fiber shear and transverse modulus. Off-axis loaded PFCs fail by three distinct fracture modes in three different off-axis ranges; each fracture mode produces a unique fracture surface.

Experiment and modeling on axial behavior of carbon fiber reinforced polymer confined concrete cylinders with different sizes

Abstract: In most experiments, small-size fiber reinforced polymer–confined concrete cylinders are used to study the axial compressive behavior. Based on the experimental results, many strength models and st...

A constitutive model for polyurethane foam with strain rate sensitivity

Abstract: The present work investigates the strain rate dependent behavior of polyurethane foams and formulates a new constitutive model in order to improve the fit of the experimental data at various strain rates. The model has seven parameters that are decided by quasi-static compression tests at two strain rates. Two models for low and high density polyurethane foams are shown to give stress strain relation at various strain rates. Dynamic compression tests were carried out to give stress strain data at high strain rate and the results are compared with those of the constitutive model.

New model for the indirect determination of the tensile stress–strain curve of concrete by means of the Brazilian test

Abstract: On the basis of the results from an experimental campaign and using simple expressions, a model for the indirect determination of the tensile stress–strain curve of concrete by means of a splitting tensile test (Brazilian test) is proposed. By testing complete specimens as well as specimens cut along the loading plane it was possible to determine the equivalent tensile strength component produced in the cylinder subjected to diametral compression. The model made it possible to reproduce adequately the behavior observed in tests carried out with both cylindrical and cubic specimens of materials such as concrete, mortar and rock. This model, if complemented with a more extensive experimental campaign, would provide an expression for the determination of the tensile stress–strain curve of several concretes or quasi-fragile materials.

Mechanical properties and microscopic deformation mechanism of polycrystalline magnesium under high-strain-rate compressive loadings

Abstract: Polycrystalline magnesium was compressed under different strain rates (0.001, 800, 1000, 2000, and 3600 s−1) to investigate its dynamic mechanical properties, and microstructural characterization was performed to uncover the deformation mechanism. The results show that yield strength is insensitive to strain rate, while ultimate strength, fracture strain, and work hardening rate are highly sensitive to strain rate. Three deformation regimes (I, II, and III) were observed on the quasi-static and dynamic stress–strain curves. These regimes show respectively increasing work hardening rate in the early stage of plastic deformation, constant work hardening rate in the intermediate plastic deformation region, and decreasing work hardening rate in the end region right before fracture. Different deformation mechanisms operate for the quasi-static and dynamic loading conditions. Microscopically, twinning/detwinning is the dominating mechanism for quasi-static testing, while dynamic recrystallization and twinning/detwinning are the dominating mechanisms for dynamic testing. Analytic constitutive models were derived for predicting the dynamic stress–strain relations. The analysis indicated that different factors were in effect for different loading strain rates. The stress–strain relations were primarily affected by strain hardening for quasi-static testing; by strain hardening, strain rate hardening, and thermal softening for dynamic testing with e ˙ ≤ 2000 s − 1 ; and by strain hardening, damping, and thermal softening for dynamic testing with e ˙ > 2000 s − 1 , respectively.

Influence of geometry on mechanical properties of bio-inspired silica-based hierarchical materials.

Abstract: Diatoms, bone, nacre and deep-sea sponges are mineralized natural structures found abundantly in nature. They exhibit mechanical properties on par with advanced engineering materials, yet their fundamental building blocks are brittle and weak. An intriguing characteristic of these structures is their heterogeneous distribution of mechanical properties. Specifically, diatoms exhibit nanoscale porosity in specific geometrical configurations to create regions with distinct stress strain responses, notably based on a single and simple building block, silica. The study reported here, using models derived from first principles based full atomistic studies with the ReaxFF reactive force field, focuses on the mechanics and deformation mechanisms of silica-based nanocomposites inspired by mineralized structures. We examine single edged notched tensile specimens and analyze stress and strain fields under varied sample size in order to gain fundamental insights into the deformation mechanisms of structures with distinct ordered arrangements of soft and stiff phases. We find that hierarchical arrangements of silica nanostructures markedly change the stress and strain transfer in the samples. The combined action of strain transfer in the deformable phase, and stress transfer in the strong phase, acts synergistically to reduce the intensity of stress concentrations around a crack tip, and renders the resulting composites less sensitive to the presence of flaws, for certain geometrical configurations it even leads to stable crack propagation. A systematic study allows us to identify composite structures with superior fracture mechanical properties relative to their constituents, akin to many natural biomineralized materials that turn the weaknesses of building blocks into a strength of the overall system.