Stress-strain Results Research Articles

Local and global tensile deformation behavior of AA7075 sheet material at 673oK and different strain rates

The constitutive behavior of AA7075 aluminum sheet like other metallic sheet materials is highly temperature and strain rate dependent at elevated temperatures. The uniaxial tensile test is commonly employed to characterize the temperature and strain rate dependent deformation behavior. However, the traditional uniaxial tensile testing and data analysis procedure treats the gauge length of sample as a uniformly-deforming (or homogeneous) region. This may not be accurate due to early localized deformation that is initiated in the gauge region at elevated temperatures. Treating the post-necking hardening behavior of uniaxial tensile samples has been an important task to expand the effective range of true stress-true strain curves at elevated temperatures for metal and alloys. To explore this issue, uniaxial tensile tests on AA7075-F sheets (with as-fabricated temper) were conducted at 673o K and four different test speeds. The test procedure also utilized an on-line 2-D digital image correlation (DIC) system method to obtain full-field strain map from the deforming gauge region of the test specimen. Local stress-strain curves at different points along the gauge length were then calculated based on the recorded local in-plane principal stress and strain values from macroscopic force data and current cross-section of the specimen at the chosen points. In this manner, the DIC strain analysis not only enabled the determination of stress-strain response but also the determination of strain rates at specific chosen points along the gauge length. The local stress-strain curves revealed significant deviation at an earlier stage of deformation from the global (or macroscopic) stress-strain response based on the entire gauge region. To estimate the constitutive behavior at different strain rates, a correction method to traditional stress-strain curves was applied to obtain a family of stress-strain curves at different constant strain rates based on the local stress-strain curves. Additionally, the material deformation response in the stress, strain and strain rate space in the 3D Cartesian coordinate system was also constructed for tests conducted at different stretching speeds. Further, a verification test was conducted to see if the above methodology could provide an accurate estimation of local deformation behavior of AA7075 sheet. Furthermore, the above experimental method was also coupled with the Gurson-Tvergarrd-Needleman (GTN) ductile void damage model and incorporated into ABAQUS-Explicit general purpose finite element (FE) analysis code via a VUMAT material subroutine. Good agreement was obtained for both the local and global stress-strain curves between the FE test simulation and experimental stress-strain results. The proposed experimental methodology and the resulting stress-strain-strain rate behavior is believed to be applicable in describing the strain rate-dependent constitutive behavior of AA7075-F sheet metals at 673o K.

Optimization of quadcopter frame using generative design and comparison with DJI F450 drone frame

This research accentuates to explore designing the drone frame using Generative design tools. A quadcopter is designed using Autodesk generative design embedded in Fusion 360. The simulation results such as static stress-strain, modal frequency and displacement results of additive manufactured quadcopter are compared with a DJI flame wheel F450 drone frame. The generative designed frame has minimum displacement compared to traditional designed drone frame. It is observed that generative designing technique along with additive manufactured frames yields better frames with improved resistance to fracture and minimum displacement compared to traditional designed DJI flame wheel F450 drone frame.

Influence of Cu volume fraction on the flow behavior of Fe based composites

This paper studied the flow behavior of two-phase Fe-Cu polycrystal using an elasto-viscoplastic material model based on micromechanics approach and finite-element simulations. In particular, three different iron (Fe) based composites are studied i.e., Fe-67 wt%Cu, Fe-50 wt%Cu and Fe-33 wt%Cu. To obtain the initial crystal orientations of grains, a set of 1000 discrete random orientations are considered. The mechanical behaviour of the aforementioned polycrystalline was investigated by finite-element compression tests and micromechanical formulation compression tests. The results (finite-element and micromechanical formulation) showed that the flow behavior for these three kinds of Fe-Cu polycrystalline is in good agreement with the experimental stress–strain results.

A Study to Define Critical Current Limit for Covetic Al Formation

This paper presents the production steps of Al covetic material, as well as a margin for the critical current in its production process along with stress-strain mechanical measurement results. As the need for energy is increasing every day, it is getting important not only its production but also its transmission in power grids. Covetic metals, one alternate to superconductors, show high electrical conductivity property for that purpose. During covetic production, a metal matrix is first melted and then mixed with Carbon Nanotubes (CNTs) before a high DC current is applied to the molten material. The role of this DC current is important in covetic material production and still under investigation. In the course of study, covetic samples have been produced for four different current values, namely 100A, 200A, 300A and 400A, applied for 10 minutes, and one sample without a DC current, all having CNT-Al mixing ratio of 1% by weight. Another sample was also produced without CNT doping for comparison purposes. Obtained results have showed that covetic material formation takes place for DC current values greater than or equal to 200A based on conductivity comparison of the samples. It is also observed that covetic materials show increased flexibility.

Analysis and statistical interpretation of biaxial material constants derived according to EN 17117-1:2018

Utilising fabrics as an engineering material allows for the creation of lightweight structures, but poses some challenges when it comes to deriving material properties for use in numerical models due to the complex composite-like nature of these coated fabrics.Until recently, no international standard regarding testing these materials and deriving their material properties existed. With the publication of EN 17117-1:2018, guidelines to biaxially test fabrics and derive material constants are now available.To evaluate the methodology presented in EN 17117-1:2018 an analysis of the test results was carried out. A statistical interpretation allowed for the evaluation of the data variability while verifying the feasibility of a stochastic approach describing the derived material constants. This latter could be a valuable addition in e.g.reliability analyses.Results obtained from ten tests utilising the methodology suggested in EN 17117-1:2018 are discussed and evaluated. Next, a method to verify and evaluate the statistical properties of the derived constants is proposed.The obtained stress–strain results and material constants indicate a good consistency between the conducted tests. The process of applying statistical distributions showed to be relatively straightforward and yields compatible distributions describing the data. However, some limitations, mostly related to the sample size, still exist.

High-Temperature Mechanical Behaviors of SiO2-Based Ceramic Core for Directional Solidification of Turbine Blades.

The high-temperature mechanical behaviors of SiO2-based ceramic cores for the directional solidification of turbine hollow blades were investigated. Isothermal uniaxial compression tests of ceramic core samples were conducted on a Gleeble-1500D mechanical simulator with an innovative auxiliary thermal system. The stress–strain results and macro- and micro- structures of SiO2-based ceramic cores were investigated experimentally. The microstructures were characterized by the scanning electron microscope (SEM). Based on the experimental data, a nonlinear constitutive model for high temperature compressive damage was established. The statistical results of Weibull moduli show that the stability of hot deformation increases with the increase of temperature. The fracture type of the SiO2-based core samples is brittle fracture, but when the temperature exceeds 1400 °C, the mechanical behavior exhibits thermo-viscoelastic and viscoplastic property. Under high-temperature (>1400 °C) and stress conditions, the strength of the ceramic core is weakened owing to the viscous slip of SiO2, which is initially melted at the temperature of 1400 °C. The comparison results between the predictions of nonlinear model and experimental values indicate that the model is applicable.

Effect of fiber reinforcement on the mechanical behavior of bio-cemented sand

To explore the possibility of providing additional reinforcement to bio-cemented sand through fiber inclusions, a series of sand-fiber mixtures were prepared by mixing sand with three types of fibers (polypropylene fiber, basalt fiber and carbon fiber) at different fiber contents (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.8% by volume of sand). Each mixture was subjected to two cycles of controlled microbial-induced calcite precipitation (MICP) treatments. Unconfined compression tests and calcium carbonate content tests were performed to characterize their mechanical properties and calcite amounts, respectively. Stress-strain results indicate that fiber inclusions change the bio-cemented soil from brittle to ductile failure modes. Basalt fibers with the highest tensile strength were most effective to increase the unconfined compressive strength (UCS) of the mixture. Due to higher elongation at breakage, polypropylene fiber is superior to the other two fiber types for energy absorption during the softening phase, resulting in higher residual strength. Scanning electron microscopy (SEM) photographs reveal the spatial distribution of calcite precipitations at the inter-particle contacts, on sand grain surfaces, and on the fiber surfaces, which primarily contributes to the roughening of the surface texture of particles and fibers and increases the internal friction and inter-particle bonding.

Fabrication of foam-like oil sorbent from polylactic acid and Calotropis gigantea fiber for effective oil absorption

A variety of oil sorbents with good hydrophobicity and oil-absorption capacity have been reported. However, their practical application is severely hindered by complex preparation process, high application cost and non-biodegradability. Here, Calotropis gigantea fiber (CGF) from plant fibers and polylactic acid (PLA) from renewable resources were utilized to construct a biodegradable foam-like oil sorbent (marked as PCF), by using a tandem freezing-solvent replacement method. CGF has large hollow structure for oil absorption, PLA can provide the oil sorbent with physical rigidity, and additional freezing process is expected to produce the oil sorbent with higher porosity for accommodating more oils. SEM images indicated the stable porous structure of PCF, and stress-strain results gave the support for good mechanical property, as the data showing that PCF remained intact structure at 70% deformation (pressure up to 185 kPa). PCF had the oil-absorption capacity ranging from 22.8 to 48.3 g/g, and could selectively absorb the oils in water for its water contact angle >130°. With added power, PCF also had the ability to transfer the oils from water. After ten “absorption-desorption” cycles, the absorption percentage of PCF reached still 81.3% for soybean oil and up to 88.1% for kerosene, indicating its excellent reusability. Compared with commercial oil sorbent, the treatment cost using PCF could be reduced by more than 80% from a preliminary evaluation, demonstrating that the as-developed PCF showed great commercial viability for treating oily wastewaters to provide a cleaner water environment for humans.

Analysis and modelling of tensile and torsional behaviour at different strain rates of Ti6Al4V alloy additive manufactured by electron beam melting (EBM)

Additive manufactured Ti6Al4V alloy has become a widely used metallic material in many structural applications including aerospace components and medical implants, where its balance of mechanical performance and flexibility in assuming complex shapes is very appreciated. As a consequence, extensive efforts have been directed towards its mechanical characterization and extended data collections are reported in the literature. However, in the basic characterization, some prominent aspects related to “true” stress-strain behaviour and strain rate effects appear to have been substantially neglected. In this work the tensile behaviour of a Ti6Al4V alloy produced by Electron Beam Melting is investigated at static and dynamic rates together with the torsion response at static rates, in order to evaluate the effects of the strain rate and the Lode stress-state parameter on the elastoplastic response and the fracture initiation. The experimental procedure adopted here relied on enhanced true stress-true strain measurements based on optical measurements of the necking cross section of the specimen, continuously acquired by video recordings, as an alternative to the traditional elongation-based approach to the true stress-true strain evaluation. In the case of static torsion tests, the Nádai method has been used, delivering the equivalent stress-strain curves directly from the torque-rotation experimental data. Moreover, the real stress amplification due to high strain rates has been obtained subtracting the thermal softening effect from the flow curves of the dynamic tests. In addition to the mechanical characterization, micrographic analyses allowed to relate the microstructural properties to the additive process conditions and the fracture morphology to the various test conditions imposed to different specimens. Finally the stress-strain results from experiments are compared with literature data for Ti6Al4V alloy, manufactured by the same or different production processes, evidencing how not only manufacturing technique and process conditions, but also the approach used in the characterization affects the results of material behaviour assessment.

Numerical simulation of mechanical resistance of wet and dry powder cakes

This paper presents a multi-scale modelling approach to describe the mechanical behaviour of caked powders as an aid to properly design and analyse the results of laboratory caking tests to assess the propensity of powders to cake. This approach is based on the integration of particle-particle interactions due to liquid or solid bridges over the total volume of a cake using Discrete Element Method (DEM). The overall objective of the study was to simulate the behaviour of agglomerated (caked) samples subjected to a mechanical, compressive or tractive, stress. First, simulations were performed on an assembly of particles subjected to capillary forces. The capillary forces being amenable to a theoretical description, the tensile strength of wet agglomerates was calculated theoretically and used as a reference to validate the numerical results. Simulations were based on the combination of conventional DEM method and the phenomenological model of capillary condensation. The stress-strain curves were then simulated and compared to the theoretical values predicted by Rumpf's model for wet agglomerates. After validation, numerical simulations were used to study the mechanical behaviour of wet or dry agglomerates subjected to compression. In order to take into account the cohesive effect of solid bridges at the level of elementary contacts in discrete modelling, an original Euler-Bernoulli type beam model between particles in contact was established. The simulated stress-strain results was in very good agreement with experimental data.

Quantitative Representation of Mechanical Behavior of the Surface Hardening Layer in Shot-Peened Nickel-Based Superalloy.

Surface hardening treatment can usually introduce severe grain distortion with a large gradient in the surface layer. It results in mechanical properties being difficult to accurately determine through macroscopic tests due to the non-uniformity of the shot-peened material. In this study, the mechanical behavior of uniformly pre-deformed nickel-based superalloy IN718 was investigated with monotonic tensile tests and instrumented indentation tests. For the shot-peened material, the hardness distribution of the surface hardening layer after shot peening was identified through the instrumented indentation method. According to the stress–strain results of pre-deformed materials, Ramberg–Osgood model parameters could be presented with plastic deformation. Assuming the power-law relationship between hardness and plastic deformation, the plastic deformation distribution along the depth of the surface hardening layer was clarified. Based on the results, a method to identify the stress–strain relationships of hardened material at different depths was established. Finally, the finite-element simulations of the instrumented indentation test considered residual stress and strain hardening were built to verify the method presented herein. The results show that the solution to evaluate the mechanical properties of hardening layer materials in the microscopic zone is feasible, which can provide the foundation for the failure analysis of shot-peened materials.

Deformation mechanisms and evolution of mechanical properties in damaged advanced ceramics

We investigated the uniaxial compressive behavior of damaged and intact alumina using quantitative X-ray computed tomography (XCT) analysis coupled with digital image correlation (DIC) for mechanical characterization. Internal three-dimensional crack characteristics such as crack surface area and orientation were quantified using XCT to assess the level of damage. From the quasi-static and dynamic stress–strain results, the primary effects of crack damage are to reduce the initial stiffness and rate of lateral expansion in damaged alumina. With increasing axial strain, crack closure was found to lead to a recovery of elastic properties, in some cases to intact levels, in the damaged specimens. Localized deformation mechanisms related to the crack structure, including lateral crack closure, axial crack opening and closing, and inclined crack sliding, were visualized in-situ and connected to XCT reconstructions. High-speed imaging also revealed a mixed fracture mode for damaged alumina that included axial splitting and failure along pre-existing cracks.

Model for description of nonlinear unloading-reloading stress-strain response with special reference to plastic-strain dependent chord modulus

To describe the nonlinear stress-strain response in the unloading-reloading process, a model was proposed for the variation of the elastic modulus during the process. This model is directly associated with the widely used equation for the plastic-strain dependent chord modulus, and no additional material parameters are needed for modeling. In this model, nonlinearly changing elastic modulus is explicitly determined from the current stress state, instead of using an accumulated-strain measure. Therefore, it is robust against the numerical noise (e.g., stress oscillation) in computation. Based on the same elastic modulus model, two types of calculation models were constructed, namely, a micro-plasticity model and nonlinear-elasticity model. From the numerical simulations of the non-proportional unloading-reloading processes, it was found that the stress-strain responses calculated by the two models were very similar to each other. The model was validated by comparing the calculated stress-strain results with the corresponding experimental observations on a DP980 advanced high-strength steel sheet (AHSS). The influence of the nonlinear unloading behavior on the springback prediction was discussed by performing two cases of springback simulations (a uniform bending-unbending and a hat-shaped draw-bending) on two types of AHSSs (dual-phase DP980 and precipitation strengthen 780 R), by using several types of elastic modulus models. The best choice for springback simulation is to use the nonlinear elastic model to describe the realistic stress-strain responses in the yield surface, along with an appropriate kinematic hardening model of plasticity to express smooth elastic-plastic transition behavior. In comparison with the springback calculation by the nonlinear model, a certain amount of errors were found when using the plastic-strain dependent linear models, however, they were not very large. Thus the second best option in springback simulation would be to use the chord modulus model because of its simple modeling and high computation efficiency.

Macro scale material characterisation in support of meso scale modelling of masonry under uniaxial in-plane loading

The amount of detailed experimental data on the mechanical properties of brick masonry in the literature is limited regarding the orthotropic strength and fracture energy. A combined experimental/numerical methodology is proposed for the derivation of the macro scale properties of masonry. The experimental aspect deals with the mechanical characterisation of the individual materials, small masonry samples and masonry wallettes, including the relation of couplet to wallette strength. The numerical aspect is the calculation of the macroscopic properties of the masonry through discrete cracking calculations in two orthogonal directions. The numerical analysis results are compared with the experimental stress-strain results and Digital Image Correlation analysis. The Young's modulus and compressive fracture energy for the masonry composite are derived. The results are analysed in view of the resulting anisotropy of masonry and the obtained failure modes.

The embankment-subsoil subjected to multidirectional seismic loading simultaneously-prediction of strain mechanism

The limitation of research in using numerical simulation is related to simulate boundary condition and application of seismic load in the multidirectional to the model. The nonlinear loading is applied in one direction which is horizontal direction was presented in several kinds of literature. In the simulated, a model is subjected to the seismic loading needs to apply near-fault ground motions on multidirectional simultaneously for the prediction of the straining mechanism. In this study, by using nonlinear finite element software, the strain and stress were analyzed when the near-fault ground motion was applied to the embankment-subsoil model in a single direction and multidirectional. To control the quality of the numerical simulation the statistical analysis was applied. The maximum stress-strain results of the several stages of numerical simulation pulled out for statistical analysis and prediction of the strain-stress density mechanism. According to the statistical analysis, the results reveal the apply multidirectional seismic loading to the embankment-subsoil model has better results quality. The outcome of this study enhances the quality of numerical simulation and prediction of the embankment-subsoil model seismic response. The method is presented in this study can apply to several engineering problems in which they are subjected to the nonlinear and linear dynamic loads in multidirectional simultaneous.

Macro scale material characterisation in support of meso scale modelling of masonry under uniaxial in-plane loading

The amount of detailed experimental data on the mechanical properties of brick masonry in the literature is limited regarding the orthotropic strength and fracture energy. A combined experimental/numerical methodology is proposed for the derivation of the macro scale properties of masonry. The experimental aspect deals with the mechanical characterisation of the individual materials, small masonry samples and masonry wallettes, including the relation of couplet to wallette strength. The numerical aspect is the calculation of the macroscopic properties of the masonry through discrete cracking calculations in two orthogonal directions. The numerical analysis results are compared with the experimental stress-strain results and Digital Image Correlation analysis. The Young's modulus and compressive fracture energy for the masonry composite are derived. The results are analysed in view of the resulting anisotropy of masonry and the obtained failure modes.

Microstructure prediction in low and high strain deformation of Al6063 using artificial neural network and finite element simulation

The final microstructure of materials under interactive effects of critical elements of deformation: strain, strain-rate, and temperature, often follows complex trajectories. Capturing the existing process-structure linkages is fundamental for controlling product outcomes, yet it calls for establishing the constitutive relationships that describe material behavior. In this paper, a backpropagation Artificial Neural Network (ANN) is proposed for microstructure prediction for a wide range of strain, strain-rate, and temperature conditions. Microstructural changes in Al6063 are experimentally examined using (i) quasi-static universal testing apparatus, Drop Weight Impact Tester (DWIT), and Split Hopkinson Pressure Bar (SHPB) tests for low strain regimes at elevated temperatures, and (ii) Plane Strain Machining (PSM) for high strain, high strain-rate, and the accompanied temperature rise conditions. Two ANNs are established to predict microstructure responses, grain and subgrain sizes, in low and high strain regimes, respectively. Additionally, the stress-strain results obtained from the low strain regime are used to calculate the Johnson–Cook (J-C) material model constants, which are then incorporated in the finite element (FE) simulation along with the developed ANN algorithm, to predict microstructure response for different cutting conditions. The performance of the ANNs and the FE simulations was evaluated using statistical indices. The comparative assessment of the models’ outcomes indicates close agreements with the experimental results in both low- and high-level deformations. The accurate predictions from PSM conditions can potentially be applicable for microstructural prediction of the machined surface.

Strain Measurement on Quasi-Isotropic HTS Strand at 77 K by Fiber Bragg Grating and Extensometer Sensor

This article measures the strain response of quasi-isotropic strand at 77 K when subjected to uniaxial tensile loads by using extensometer sensor and simultaneously verifies the possibility of utilizing Fiber Bragg grating (FBG) sensor for the same measurements. Through matching the measured data from the two sensors, strain sensitivity of FBG sensor is calibrated and fitted to be 1.21 pm/ μ ϵ. In elastic deformation region, the stress–strain curve of the strand captured by FBG sensor coincides well with that from the extensometer sensor, but the range it could reach is only 0.31% strain ( ϵ tens ∼0.31%) due to the limitation of the elongation. Through fitting the stress–strain curve obtained by the extensometer, elastic modulus 115.5 GPa and yield stress 471 MPa of the strand are, respectively, determined. Then, depending on the measured stress–strain results and the equation on critical current as a function of tensile strain, critical stress of the strand is estimated to be 464 MPa. Furthermore, a three-dimensional finite element method model of the strand established by Comsol software is employed to understand the testing results.

Hot compression deformation behavior of Mg-5Zn-3.5Sn-1Mn-0.5Ca-0.5Cu alloy

The hot deformation behavior of Mg-5Zn-3.5Sn-1Mn-0.5Ca-0.5Cu alloy containing coarse second phase particles is studied. The flow stress increases with decreasing temperature and increasing strain rate. The stress-strain result is fitted to the hyperbolic sine equation to obtain the constitutive equation of the alloy. The processing map is constructed and it shows that low temperature and high strain rate lead to both low efficiency of power dissipation and instability and two domains with high efficiency of power dissipation out of the instability area is suitable for hot processing of the alloy: 580 K–670 K, 0.001 s−1–0.01 s−1 and 630 K–670 K, 0.01 s−1–1 s−1, the latter one is recommended for hot deformation for the higher strain rate. It was revealed that the effect of coarse particles on the restoration mechanism is dependent on the deformation condition. Under low Zener-Hollomon value, particles hinder discontinuous dynamic recrystallization through grain boundary pinning making dynamic recovery and continuous dynamic recrystallization the main restoration mechanism. Under high Zener-Hollomon value, the misorientation gradient near particles increases and the efficiency of particle stimulated nucleation increases. The enhanced particle stimulated nucleation accompanied by more active twinning and twin related recrystallization make the fraction of dynamic recrystallized grain to be higher under high Zener-Hollomon value.

Structural Modelling of Automotive Engine Mounting from Kenaf Fibre Reinforced Natural Rubber/Thermoplastic Polyurethane “Green” Composites

This paper presents an investigation of deformation analysis using ANSYS of automotive engine rubber mount made from kenaf fiber reinforced natural rubber (NR)/thermoplastic polyurethane (TPU) composites. The modelling of mounting was conducted using CATIA software. To determine the of the Poisson’s ratio of this new material composites is important, which was required data to be filled into the ANSYS software. The Poisson’s ratio was calculated based on the previous data experiment from the stress-strain results. The analysis was focussing on the deformation and stress analysis effected from the pressure that was applied in the modelling and in the ANSYS simulation software which is 250 psi. The deformation and stress simulation results were then identified and discussed and the results were compared to the Natural Rubber material of the same design simulation.