Tensile Stress-strain Diagram Research Articles

Mechanical characterization of lightweight engineered geopolymer composites exposed to elevated temperatures

In this study, the effect of different factors, such as PVA fibers (2% by total volume) and precursor type (slag, fly ash, or a combination of both), on the behavior of green lightweight engineered geopolymer composites (LEGC) and lightweight engineered cementitious composites (LECC) after exposure to temperatures up to 800 °C for 1 h is investigated. Expanded glass granules were used as lightweight aggregate instead of silica sand to reduce the spalling tendency and density of the composite. The flowability, density, color change, mass loss, spalling resistance, residual mechanical properties (compressive strength, stress-strain diagram, tensile stress-strain diagram, load-deflection response, failure mode), and microstructural analysis (by scanning electron microscopy) were investigated before and after exposure to thermal deterioration. The findings pointed out that the dry density, compressive strength, fiber bridging stress, strain capacity, maximum load, and maximum deflection of the developed mixtures before exposure to fire deterioration were in the range of 1703–1883 kg/m3, 16.66–64.11 MPa, 2.66–4.97 MPa, 2.40–3.33%, 1573–4824 N, and 2.92–5.53 mm respectively. It's worth mentioning that the substitution of 50% slag in the lightweight EGC mixture demonstrated the optimal tensile strain capacity and deformation capacity and further enhanced both ultimate tensile strength and flexural strength of fly ash-based EGC (FA-EGC) mixtures. After heat exposure, both LEGC and LECC composites demonstrated strain hardening behavior and deflection hardening behavior up to 300 °C of heat treatment, while after exposure to a temperature of 300 °C and above, both deflection hardening behavior and strain hardening behavior are dramatically damaged. This is attributable to the melting of the PVA fibers. Also, the microstructural analysis showed that incorporating fly ash into lightweight EGC mixtures can effectively reduce the melting point of PVA fibers and further improve the fire resistance of EGC mixtures.

Modeling the Nonlinear Deformation of Highly Porous Cellular Plastics Filled with Clay Nanoplatelets.

Rigid low-density plastic foams subjected to mechanical loads typically exhibit a nonlinear deformation stage preceding failure. At moderate strains, when the geometrical nonlinearity is negligible, such foam response is predominantly caused by the nonlinearity of deformation of their principal structural elements—foam struts. Orientational averaging of stresses in foam struts enables estimation of the stresses taken up by foams at a given applied strain. Based on a structural model of highly porous anisotropic cellular plastics filled with clay nanoplatelets and the orientational averaging, a method for calculating their nonlinear deformation is derived in terms of structural parameters of the porous material, the mechanical properties of the monolithic polymer, and filler particles and their spatial orientation. The method is applied to predicting the tensile stress-strain diagrams of organoclay-filled low-density rigid polyurethane foams, and reasonable agreement with experimental data is demonstrated.

Analysis of high strength composite structure developed for low-carbon-low-manganese steel sheet by laser surface treatment

In the present work, a high-strength composite steel structure developed by imparting laser surface hardening treatment on a low-carbon low-Manganese automotive steel sheet has been comprehensively analysed. The layered composite steel structure has been successfully developed by re-engineering the surface of the steel using diode laser hardening treatment up to a depth of 250–300 µm through its thickness. Hardness at the treated surface improved by 150% to that of its base due to formation of a mixture of hard phases constituting martensite and bainite along with retained ferrite. Indeed, coupling EBSD technique with Weibull distribution of various phase fractions determined by image quality helped analyse microstructure effectively. The tensile property of the layered composite steel sheet was found to yield significant improvements in both Yield Strength (YS) (40–44%) and Ultimate Tensile Strength (UTS) (19–21%) due to sandwich effect of composite layer constituting hardened layer and soft base accomplished by a strengthening mechanism associated with rule of mixtures concept. In fact, Young’s modulus of the composite steel sheet, determined from slope of tensile stress–strain diagram was found to be convergent with ultrasonic test result. Additionally, the crystallographic texturing effects of the hardened layer and untreated base measured using standard XRD technique re-confirmed their influence on Young’s Modulus and r-bar values obtained.

Експериментальний аналіз механічних характеристик деталей ракет-носіїв, виготовлених за допомогою FDM адитивних технологій

Additive manufacturing is very promising for aerospace engineering and aircraft construction. Using these technologies, light structures with preset strength properties can be made. For lack of tables of the mechanical properties of materials made by additive technologies, any calculation must be accompanied by the experimental determination of their mechanical properties. This paper presents an experimental approach to the determination of the mechanical characteristics of parts printed by FDM technologies. Parts manufactured from polymers by FDM technologies are shown to be orthotropic. Therefore, their elastic properties are described by nine constants: three Young’s moduli, three shear moduli, and three Poisson ratios. A cube is printed for the experimental determination of these constants. Six specimens are cut out from the cube. Three specimens are cut parallel to the cube edges, and the other three are cut at an angle of 45° to them. Each such specimen is manufactured in five pieces. This makes it possible to average the tensile stress–strain diagrams obtained for all the components of the stress tensor. The mechanical properties of the material are determined from these diagrams. The three Young’s moduli and the three Poisson ratios are determined from the three specimen types parallel to the cube edges. The three shear moduli are determined from the specimens cut at an angle of 45° to the cube edges. To determine these constants, tensile stress–strain diagrams are obtained experimentally. A technology is presented for manufacturing specimens on a Stratasys FORTUS 900 MC 3D printer. The mechanical properties of two polymer materials (ULTEM 9085 and PLA) are determined and compared. PLA has higher Young’s moduli and shear moduli and lower Poisson ratios than ULTEM 9085.

СТРУКТУРНО-ФЕНОМЕНОЛОГИЧЕСКИЕ МОДЕЛИ МЕТАЛЛОВ ДЛЯ ВАГОНОСТРОЕНИЯ

The paper provides examples of negative changes of mechanical properties of metals, including 20L cast steel, that are used in manufacturing of bearing elements of bogies for freight cars, which are manifested with growth of cyclic operating time. In order to study relationship of mechanical properties the authors have carried out a mathematical description of processes occurring at uniaxial quasi-static stretching in representative volume of fragile and viscous steel for car manufacturing with various cyclic operating time. Well known models of structural steel are supplemented by the Mironov - Yakushev model that considers a softening stage in separate microstructural seeds and that is useful for studying fragile and viscous steel. The authors have studied damage of steel by a method of preventive unloading on a stage of poet-failure deformation with a construction of full tensile stress-strain diagrams at stretching of studied volume of metal and have compared with results of tests of full-scale samples. Results of modeling show a decrease of all mechanical properties of metals after operating time and an interaction with the tensile stress-strain diagram.

Computational Model for the Evaluation of the Service Life Of Fiber-Reinforced Concrete Structures Under Long-Term Static Loading

We propose a computational model for the evaluation of the service life of fiber-reinforced concrete structural elements subjected to long-term tension. The model is based on the first law of thermodynamics and a proposed idealized tensile stress-strain diagram of fibrous concrete. This model is used for the evaluation of the service life of a fiber-reinforced concrete plate containing a circular hole under the conditions of long-term bilateral tension.

Hydrogen Effect on Strength and Plasticity of Steels After Ion Nitriding in Hydrogen and Hydrogen-Free Environments

The authors present the results of experimental investigations of strength and plasticity characteristics, as well as fracture nature, of the specimens of different steels under tension after ion nitriding in hydrogen (nitrogen with hydrogen) and hydrogen-free (nitrogen with argon) environments. The obtained characteristics are compared with those before nitriding. It is established that during ion nitriding in the hydrogen-free environment, tensile strength of the specimens has increased by 4–11%, whereas relative elongation and contraction have decreased by 1.1–3 times based on the nitriding mode and steel type. The authors have built the tensile stress-strain diagram for the specimens made of steel St3, 40Kh, and 12Kh18N10T without chemical and thermal treatment, ion nitriding in the hydrogen and hydrogen-free environments. The influence of hydrogen concentration in the saturation environment on the strength and plasticity characteristics has been defined. Under concentrations higher than 10 vol.% there is embrittlement with the decrease of strength characteristics (to 15%) and a significant decrease of plasticity (to 40%) for low-alloyed steels. For high-alloyed steels, the negative influence of hydrogen on the strength characteristics does not increase by 3%, while the influence on the plasticity characteristics – by 8%. The authors have investigated the initiation and propagation of microcracks under external loading on the steel with nitride layers.

Mechanical behavior of concrete prisms reinforced with steel and GFRP bar systems

Being immune to corrosion, and having a tensile strength up to three times higher than structural steel, glass fiber reinforced polymer (GFRP) bars are suitable for reinforcing concrete structures exposed to aggressive environmental conditions. However, a relatively low elasticity modulus of GFRP bars (in respect to the steel) favors the occurrence of relatively large deformability of cracked reinforced concrete. Lack of ductility and degradation of properties under high temperature can be also identified as debilities of GFRP bars over steel ones. Combining GFRP and steel bars can be a suitable solution to overcoming these concerns. Nevertheless, the application of such hybrid reinforcement systems requires reliable material models. The influence of the relative area of GFRP and steel bars on the tensile capacity of cracked concrete (generally known as tension-stiffening effect), was never investigated from the experimental point of view, mainly crossing results from different tools on the assessment of the cracking process. This paper experimentally investigates deformations and cracking behavior of concrete prisms reinforced with steel bars and GFRP bars in different combinations. The test results of 11 elements are reported. A tensile stress-strain diagram is conceptually proposed for modelling the tension-stiffening effect in elements with such hybrid combination of the reinforcement. The cracking process in terms of crack width and crack spacing is analyzed considering the hybrid reinforcement particularities and a preliminary approach is proposed for the prediction of the crack width for this type of reinforced concrete elements.

Tensile modeling of steel fiber reinforced concrete

The use of steel fiber reinforced concrete (SFRC) as a structural material has shown an ever-increasing growth. Steel fibers have a significant effect on the mechanical properties of reinforced concrete, especially on its tensile behavior. For SFRC modeling, it is necessary to determine the stress–strain equations. Due to the complexity of strain–strain tests and the lack of access to these results before modeling and design, determining these equations without testing is a modeling requirement. So far, limited studies have been conducted on estimating the SFRC tensile stress–strain equation based on other SFRC characteristics. In this study, the results of 41 experimental specimens collected from the literature are used to present an equation to estimate the SFRC tensile stress–strain diagrams using the genetic programming (GP) method. The proposed equation had acceptable accuracy compared to the experimental results and other equations presented in the literature.

Influence of laser surface hardened layer on mechanical properties of re-engineered low carbon steel sheet

In the present work, a C-Mn low carbon automotive steel sheet has been used for laser surface hardening treatment. The steel sheet has been successfully surface hardened using high power diode laser up to certain depth (250–300µm) to make layered composite structure through its thickness. The microstructure of the treated layered steel sheet have been evaluated by different characterization techniques such as optical microscopy, scanning electron microscopy, X-ray diffraction. The hardness at the treated surface improved significantly (133–145% increase as compared to base hardness) due to formation of harder phases, which are identified as a mixture of martensite, bainite and prior-ferrite using EBSD technique. The tensile property of the layered steel sheet was also evaluated and found to yield significant improvements in both Yield Strength (YS) (25–40%) and Ultimate Tensile Strength (UTS) (20%) due to the sandwich effect of a composite layer constituting hardened layer and base accomplished by a strengthening mechanism associated with rule of mixtures concept. Correspondingly, the Young's modulus of the layered steel sheet determined from the slope of tensile stress-strain diagram was also found to be consistent with ultrasonic test results. Additionally, the crystallographic texturing effects of the hardened layer and untreated base measured using standard XRD technique re-confirmed their effect on Young's Modulus and r-bar values obtained.

Analysis of deformation induced martensite in AISI 316L stainless steel

Abstract Metastable austenite stainless steel AISI 316L is sensitive to cold deformation, where transformation from austenite to martensite occurred. The bending deformation as the formation process leads to tensile and compression troughout the thickness of the billet. Tensile testing of the specimen causes differences in the true stress-strain along the contraction neck prior to fracture as well. The aim of the paper is to find correlation between microhardness as brief inspection parameters and extension of martensitic transformation. The total equivalent plastic strain extend diagram obtained by numerical simulation of bending was compared with tensile true stress-strain diagram. Results show very good correlation between hardness, true strain and martesite content. Therefore, one can conclude that by hardness measurement, it is possible to measure the level of equivalent plastic strain until ultimate tensile stress as a linear correlation between hardness, true strain and martesite content.

Microstructural evolution and mechanical behaviour of surface hardened low carbon hot rolled steel

Surface hardening of low carbon hot rolled C–Mn steel has been successfully performed by high power diode laser with an achievable case depth of about 300μm. The laser treated samples have been characterised using optical microscope, scanning electron microscope, transmission electron microscope, and X-ray diffraction techniques. Higher hardness level is achieved in laser surface hardened zone (≈300HV) than in the base alloy region (≈140HV). The variation in hardness as a function of distance across the laser tracks is observed during multi-track laser hardening. Laser hardened steel sheets show enhanced mechanical strength (YS: 383–443MPa, UTS: 476–506MPa) with the lowering of percentage total elongation (23–28%) compared to the base alloy (YS: 351MPa, UTS: 450MPa and total elongation is 32%). Strain hardening exponent (‘n’) has been evaluated from true tensile stress–strain diagram and it shows a similar nature for both base alloy and laser treated steels. The microstructure in the base alloy region consists of a mixture of ferrite and pearlite, whereas predominantly lath martensite is present in the laser hardened surface layer. The improvement of mechanical strength is discussed in terms of the formation of this hardened layer on the surface.

Prediction Model of Deflections in PET Fiber Reinforced Concrete Beams

ABSTRACTThe increasing use of polymeric reinforcements in concrete structures requires either the development of a new design theory or the adaptation of current designs considering the engineering properties of this type of materials. In this work a method for calculating the deflections of reinforced concrete elements is proposed, which can be used in predicting the flexural behavior of longitudinally reinforced concrete with PET strips in amounts up to 1%. The model theory assumes that concrete has a tensile load capacity different to zero, characterized by a uniaxial tensile stress-strain diagram. A series of tests were conducted to corroborate the validity of the suggested method, showing that the theory also correctly predicts the creep deformation post-cracking. The deflection results of reinforced concrete with recycled PET strips are presented. The tests are carried out by a simple beam with center-point loading, using three different amounts of reinforcement and comparing the experimental results with the theoretical results of the proposed model.

Structure of Bulat

Results of a chemical analysis of the metal of three antique blades are given, which, based on their chemical composition and the presence of patterns on their surface, can be classified as bulat arms. Alloys that contain 1.5–3.46% carbon have been melted, from which plates with a bulat pattern have been fabricated. The micro- and macrostructure of seven alloys containing 2.4–3.48% C have been studied in detail. The microhard-ness of the structural constituents of these alloys has been determined; the tensile stress-strain diagram of the alloy with 3.02% C has been constructed. The results obtained are compared with P.P. Anosov’s data on bulats.

Modelling of high-temperature inelastic behaviour of the austenitic steel AISI type 316 using a continuum damage mechanics approach

A conventional material behaviour model can be extended to taking into account varying thermo-mechanical loading conditions in wide stress range. The motivation for developing this model is given by the well documented failure case study of high-temperature components at unit 1 of the Eddystone fossil power plant (Pennsylvania, USA), which have operated for 130,520 h in creep–fatigue interaction conditions. The developed model basis is a creep constitutive law in the form of hyperbolic sine stress response function originally proposed by Nadai (1938). The constitutive law is extended to assume the damage process by the introduction of scalar damage parameter and appropriate evolution equation according to Kachanov–Rabotnov concept. The research task is the introduction into the constitutive model of a few additional material state variables, able to reflect hardening and recovery effects under cyclic loading conditions. The first variable is represented by the relatively fast saturating back-stress [Formula: see text] describing kinematic hardening. The second variable is represented by the relatively slow saturating parameter [Formula: see text] describing isotropic hardening. Evolution equations for [Formula: see text] and [Formula: see text] are formulated in a modified form originally proposed by Chaboche and based on the Frederick–Armstrong concept. The uniaxial modelling results are compared with cyclic stress–strain diagrams and alternative experimental data in the form of creep curves, tensile stress–strain diagrams, relaxation curves, etc., for the austenitic steel AISI type 316 at 600 °C in a wide stress range.

Determination of the fracture energy of a composite by the method of digital speckle correlation

We have developed a computational-and-experimental procedure for evaluating the fracture energy of composite materials, which lies in the determination of surface displacements in a neighborhood of the tip of a stress concentrator under static tension by the method of digital speckle correlation. Using the known displacement distribution, we calculate strains in the prefracture zone as well as the opening and shear of crack lips for given loading force and measuring base. The static tensile stress-strain diagram, together with the established specimen strain, enables us to find stresses in the prefracture zone. Using these data, we find the fracture energy of the composite. We also present some results of investigations for a three-layer composite material based on an epoxy-phenol plastic reinforced chaotically with disperse particles.

Assessment of the parameters influencing the fiber characteristics of electrospun poly(ethyl methacrylate) membranes

This work evaluates the influence of electrospinning process parameters on the mean diameter and standard deviation of fiber diameters in electrospun poly(ethyl methacrylate) (PEMA). Processing conditions were selected using Taguchi’s statistical method. Oriented and unoriented electrospun mats with good mechanical properties were produced and demonstrated with tensile stress–strain diagrams. Differential scanning calorimetry (DSC) experiments showed that the polymer chains were forced into non-equilibrium conformations due to electrospinning. Enthalpic recovery during a heating scan is shown by an endothermic peak in the initial DSC heating scan that disappears in subsequent heating scans. An increase in the glass transition temperature with respect to PEMA films shows that the polymer is not equilibrated by heating above glass transition. Cell attachment was tested with MC3T3-E1 pre-osteoblastic cells cultured for short time periods on the electrospun mats. It is shown that the cells present less extended morphology with more diffuse perimetral focal adhesions than cells cultured on flat substrates. A tendency of cells to align in the direction of the substrate fibers in oriented electrospun membranes was also found.

The effect of small additions of a liquid-crystalline polymer on the mechanical properties of polyethylene

Results of an investigation into the mechanical properties of polyethylene with small, no more than 3 wt.%, additions of a thermotropic liquid-crystalline polymer (LCP) are reported. Experimental data on the influence of LCP additions on the tensile stress–strain diagram, elastic modulus, long-term (up to a 1000-h) creep, and creep recovery after unloading of the specimens are obtained. The elastic properties of the materials are analyzed taking into account the effect of self-reinforcement owing to the formation of extended LCP inclusions. The results obtained are compared with experimental data. Analytical descriptions of the creep and creep recovery of the materials investigated are also presented..

Effect of tension of a composite propellant on its burning rate

Existing concepts of the effect of tensile strains on the burning rate of propellants are analyzed. It is demonstrated that the basic mechanism of increasing burning rate of composite propellants under tension is spalling of the binder from oxidizer particles, formation of an additional burning surface, and changes in the combustion-zone structure. To describe this effect, a rheological model of a composite solid propellant is developed, which takes into account separation of the binder from disperse particles of the fillers (oxidizers, coolants, metals, etc.). A criterion is found, which describes the difference between the propellant behavior under tension with spalling of the binder from the particles and the tension of the same material without the emergence of internal defects. A method of experimental determination of the number of defects arising in the propellant under tension, based on analyzing the tensile stress-strain diagram of the material, is proposed. A mathematical model of composite propellant combustion is developed, which takes into account separation of the binder from the oxidizer particles and formation of an additional burning surface. A correlation between the change in the burning rate of the propellant under tension and parameters of the propellant tensile diagram is found. A method for predicting the change in the burning rate of the propellant under tension on the basis of the propellant tensile diagram shape is developed.

Effect of the strain rate and test temperature on the mechanical properties of a Ti-Ni-Fe shape memory alloy

The test temperature (from −196 to +50°C) and the strain rate (from 10−4 to 103 s−1) are found to affect the character of deformation of a shape memory alloy TN1K based on titanium nickelide and alloyed with iron. The shapes of the tensile and compressive curves are shown to depend on the position of the test temperature with respect to the characteristic phase-transition temperatures. The mechanical properties are extremal in the temperature ranges corresponding to the R phase region. As the strain rate increases in the quasi-static range, the strength characteristics of the material increase and the plastic characteristics decrease. As the strain rate increases in the quasi-static range, the yield strength changes analogously; in this case, a yield drop appears in the compressive and tensile stress-strain diagrams. The data obtained are used to optimize the technology of the thermomechanical joints of pipelines and construction elements.