Quasi-static Uniaxial Tensile Tests Research Articles

Modified constitutive models for Inconel 718 considering current density and temperature in electrically assisted forming process

Electrically Assisted Forming (EAF) technology has obvious advantages in material forming. To develop an effective constitutive model considering electrical effects, room temperature and electrically assisted quasi-static uniaxial tensile tests were conducted using ultrathin nickel-based superalloy plates with a thickness of 0.25 mm. The research focused on the two most widely recognized effects: the Joule thermal and the electric athermal effects. The mechanism of current action can be divided into two scenarios: one considering the Joule thermal effect only, and the other considering both effects simultaneously. Two basic constitutive models, namely the Modified-Hollomon model and the Johnson-Cook (J-C) model, were selected to be optimized through the classification of two different situations, and four optimized constitutive models were proposed. It was found that the J-C model with simultaneous consideration of the Joule thermal effect and electric athermal effect had the best prediction effect by comparing the results of these four models. Finally, the accuracy of the optimization model was verified by finite element simulation of the electrically assisted stretching optimization model. The results show that the constitutive model can effectively predict the temperature effect caused by the Joule heat effect and the athermal effect of current on the material.

Stereolithography for manufacturing of advanced porous solids

The aim of this paper is focused on benefits of stereolithography (SLA) technology for the fabrication of the lightweight lattice structures with potential for load-bearing function and high absorption of impact energy. SLA is an additive manufacturing technology employing the principle of liquid resins curing moderated by radiation of a wavelength from ultra-violet band where resulting material parameters are tunable by setting of the curing process. The batches of samples manufactured using three different resins were subjected to quasi-static uni-axial tensile and compression tests. Acquired data were processed to derive deformation behaviour expressed as stress-strain diagrams and fundamental material properties. Based on the knowledge obtained from the mechanical tests, the setup of the fabrication parameters, the most suitable resin for manufacturing of the lattice structures and the overall suitability of SLA technology for the fabrication of advanced porous materials, were determined.

Effect of moisture on the delamination properties of fractured PVB-laminated glass: A joint experimental and numerical study

In practical applications of laminated glass, local moisture exchange may occur through panel edges or glass cracks and leads to a significant impact on the post-fracture behavior of laminated glass. In this paper, the effect of moisture on the interfacial delamination properties of PVB-laminated glass is studied. At the experimental level, quasi-static uniaxial tensile tests on pure PVB and through-cracked tensile tests on PVB-laminated glass were conducted at room temperature, considering four initial moisture levels, i.e., 0.2%, 0.4%, 0.6%, and 0.8%, corresponding to relative humidity of 13.6%, 25.6%, 36.1%, and 44.5%, respectively. It is worth noting that for each laminated glass specimen used in the TCT tests, a controlled moisture content was uniformly introduced into the interlayer during the manufacturing process, which is different from traditional aging tests. This allowed for characterization of the moisture-dependent material properties of PVB and interfacial delamination properties of fractured PVB-laminated glass under controlled and uniform moisture conditions. At the numerical level, the moisture-dependent material behavior of the PVB interlayer was first calibrated with 5-parameter Mooney-Rivlin hyperelastic material laws. The moisture-dependent material laws and through-cracked tensile (TCT) test results were then used for an inverse determination of the cohesive strength and interfacial fracture energy at PVB-glass interface through a numerical cohesive zone approach. This study found that the moisture content can significantly affect the interfacial properties of the PVB-laminated glass, with the cohesive strength and interfacial fracture energy decreasing by approximately 70% and 50%, respectively, as the moisture content increases from 0.2% to 0.8%. The changes of the above interfacial parameters can be approximated by linear relationships. In addition, the energy absorption of fractured PVB laminated glass decreases significantly with increasing moisture content due to the degradation of PVB interlayer performance and reduced interfacial adhesion. Therefore, the effect of moisture content should be considered in the practical design of certain laminated glass applications to ensure post-fracture load-bearing capacity.

A dynamic constitutive model for high-density rigid polyurethane foam subjected to impact loading

This study experimentally and numerically investigated the dynamic response of high-density rigid polyurethane foam (RPUF). Firstly, drop-weight tests were conducted under three impact velocities, 2.42 m/s, 3.43 m/s, and 4.85 m/s. Test results showed that the dent depth and absorbed energy rise as impact velocity increases. Then the constitutive model for high-density RPUF based on an ideal elastoplastic constitutive model is proposed based on the measured mechanical properties, including quasi-static uniaxial compression tests, quasi-static uniaxial tensile tests, triaxial confining pressure tests, and split-Hopkinson pressure bar tests. Finally, the numerical simulation of the drop-weight tests was conducted. It was shown that the proposed dynamic constitutive model can accurately describe the dynamic performance of a high-density RPUF subjected to impact loading.

Plastic deformation and ductile fracture of L907A ship steel at increasing strain rate and temperature

A combined theoretical, numerical, and experimental approach is adopted to systematically characterize the plastic deformation and ductile fracture behaviors of L907A low-alloy ship steel widely used in ship construction, with the effects of strain rate and thermal softening duly accounted for. A mixed Swift-Voce hardening model coupled with strain rate and temperature-dependent terms is proposed, while the Hosford-Coulomb model of ductile fracture is modified by introducing temperature-dependent term to account for the effect of thermal softening. To calibrate the parameters appearing in the constitutive models, dynamic uniaxial tensile tests at varying strain rates (0.001 ∼ 5200 s−1) and quasi-static uniaxial tensile tests at varying temperatures (20 ∼ 800 °C) are separately performed, with the coupling between strain rate and temperature effects ignored. The calibrated plasticity and ductile fracture models are numerically implemented with the method of finite elements (FE), and their applicability to practical applications is validated against experimental results of ballistic impacts. Normal and oblique impacts on L907A target plates with conical, hemispherical, and blunt projectiles are considered. Good agreement between FE simulation results of plastic deformation and ductile fracture modes and those measured experimentally is achieved, thus demonstrating the viability of the established plasticity and fracture models of L907A steel for applications in complex ballistic impact scenarios.

Anisotropic tensile behavior of laser powder-bed fusion made Ti5553 parts

Directional heating during metal additive manufacturing promotes the growth of a columnar-grained microstructure along the building direction with a preferred crystal orientation, which results in anisotropy of properties. In this work, the anisotropy of laser powder bed fused Ti-5553 metastable β titanium alloy components are investigated. Samples printed in the normal (vertically printed), and parallel (horizontally printed) orientations to the building direction were subjected to quasi-static uniaxial tensile tests. In comparison, samples printed parallel to the building direction exhibit a significantly higher ultimate strength of 846 ± 6 MPa, whereas the samples printed normal to the building direction reached 780 ± 10 MPa. Fracture initiation and propagation were investigated using interrupted tensile testing. The fracture path of partially fractured specimens reveals the development of slip bands oriented at a 45 ± 5° angle with respect to the loading direction, which promoted fracture by coalescence of aligned pores. Electron backscatter diffraction results indicate that the grain boundaries act as favorable nucleation sites for fracture initiation, particularly when they align perpendicular to the loading direction. Conversely, fracture in samples printed parallel to the building direction was predominantly transgranular, which is expected to be a major contributing factor to the higher strength observed.

Deformation dynamics of a neutron-irradiated aluminum alloy: An in situ synchrotron tomography study

To reveal the effects of long-term neutron irradiation on the mechanical behavior of nuclear materials, in situ quasi-static uniaxial tensile tests are conducted on an LT21 aluminum (Al) alloy from a decommissioned research reactor. Scanning/Transmission electron microscopy (SEM/TEM), and micro CT are first applied to characterize the microstructures of LT21 Al alloys aged naturally and neutron-irradiated for 30 years. In situ synchrotron micro computed tomography (CT) is adopted to characterize the deformation and damage dynamics of this LT21 Al. A new particle tracking analysis technique is proposed to quantify the displacement/strain fields and microstructural evolution (e.g., particle movement and pore growth) in the irradiated sample. Long-term irradiation induces considerable microstructural changes in the LT21 Al alloy, including the precipitation/aggregation of micron-sized Si particles and nanoscale Mg2Si particles. In addition, the size of pores and particles in the irradiated material appear considerably larger than that in the unirradiated material. During tension, the alloy undergoes elastic-plastic deformation followed by shear-dominated necking failure. The porosity and pore size exhibits an overall increase with increasing loading. Formation of new pores occurs in two modes, formation at the particle-matrix interfaces and random formation across the sample. For mode one, pores tend to nucleate at the top and bottom ends of a particle (relative to the loading direction). Formation of new pores and growth of initial and newly-nucleated pores occur simultaneously and contributes approximately equally to damage accumulation in the irradiated LT21 Al alloy before necking occurs. The deformation dynamics deepens our understanding of the aging of nuclear materials.

Construction of hyperelastic model of human periodontal ligament based on collagen fibers distribution

ObjectiveThe human periodontal ligament (PDL) dominated by collagen fibers showed hyperelastic mechanical behavior under orthodontic force. Despite previous researches on the hyperelastic model of PDL, there were certain limitations because of the types of samples and the ignorance of distribution of collagen fibers. Therefore, the aim of this study was to quantify the effect of collagen fibers distribution of human PDL on its hyperelastic behavior. MethodsA total of 6 human PDL samples of root neck, root middle and root apex were cut from human maxillary central incisor and lateral incisor. The spatial angles of collagen fibers were observed by optical microscope, the hyperelastic model was constructed combined with the observation results. The quasi-static uniaxial tensile tests with displacement load 0.05 mm/min were carried out, and the test data were used to identify the parameters of model. ResultsThe mechanical behavior of human PDL conformed to the trend of hyperelastic materials, and greatly depended on the spatial angles of internal collagen fibers. The R2 value statistical fit of the constitutive model to the data is excellent (R2 > 0.98). This model could excellently describe the hyperelastic properties of human PDL. SignificanceIn this study, we quantitatively described the effect of spatial distribution of collagen fibers on the mechanical properties of human PDL. The accuracy of this model was verified by the uniaxial test data, and the relevant model parameters were acquired, which have certain reference value in subsequent researches on hyperelasticity of human PDL and clinical treatment.

Dynamic increase factors of concrete-like materials at very high strain rates

Strain rate effects on the compressive and tensile strengths of a concrete-like material play an important part in the construction of its dynamic constitutive model. So far no data for the same material covering a wide range of strain rates were reported in the literature, not to mention the data for compression at strain rates greater than 103 s−1. This paper presents a combined theoretical and experimental study on the dynamic increase factors of concrete-like materials at very high strain rates. First, a method is proposed to determine dynamic increase factor for a concrete-like material in compression due to strain rate effect only (that is to say to eliminate the inertial (containment) effect) at very high strain rates (>103 s−1) by in-depth analysis of its Hugoniot elastic limit (HEL). Then, various material tests (i.e. quasi-static uniaxial compression and tensile tests, SHPB and SHTB tests, plate impact test and dynamic compression-shear test) were performed on a certain type of cement mortar in a wider range of strain rates (i.e. 10-5 s−1 ∼ 106 s−1). Finally, the test data are compared with the previously proposed semi-empirical equations for dynamic increase factors. Furthermore, the plate impact test results for ultra high performance concrete (UHPC) available in the literature are also analyzed using the proposed method. Dynamic increase factor for tension at strain rate of 103 s−1 is obtained indirectly by the relationship between dynamic uniaxial tensile strength and dynamic shear strength through the shear wave tracking (SWT) technique which is based on dynamic compression-shear test. It is found that dynamic increase factor for concrete-like materials in compression due to strain rate effect only can be regarded as a constant at very high strain rates. It is also found that the previously proposed semi-empirical equations can accurately describe the strain rate sensitive behavior of concrete-like materials in both tension and compression.

Synthesizing of Metallized Acrylic Containing Both Gadolinium and Lead as a Transparent Radiation Shielding Material and Its Physical Properties

In this study, a series of optically transparent metallized acrylics containing Gd and Pb were synthesized by the bulk polymerization of Gd(MAA)3, Pb(MAA)2 and AM according to different polymerization procedures. The variation of their optic transmittance and mechanical performance with Gd contents was investigated. Then, quasi-static uniaxial tensile tests under different strain rates and temperatures were performed to study the influence of strain rate and temperature on the mechanical properties of radiation-shielding metallized acrylic containing both Gd and Pb. The tensile responses of this material distinctly exhibit nonlinear characteristics and strongly depend on both temperature and strain rate. Based on the experimental results, a modified Zhu–Wang–Tang (ZWT) constitutive model, in which the standard elastic component was replaced by the Mooney–Rivlin hyperelastic model, was implemented to characterize the observed both hyperelastic and viscoelastic behaviors. The constitutive parameters were expressed as functions of temperature and determined by experimental data. The model fitting results indicate that the selected constitutive model can accurately describe the nonlinear tensile stress–strain responses of metallized acrylic containing Gd and Pb. Furthermore, the great difference in constitutive parameters implies that the viscoelastic behavior of the as-prepared metallized acrylic affects the response to quasi-static tensile loading the most.

On the use of the Gleeble® test as a heterogeneous test: sensitivity analysis on temperature, strain and strain rate

The development of hot and warm forming processes for the manufacture of complex-shaped alloy components has been increasing. At the same time, the knowledge of the effect of certain variables involved in such processes such as temperature and strain rate is essential for an accurate modelling of the materials and processes. This work provides a sensitivity study on the effect of different testing conditions variables on beta titanium-molybdenum (Ti-Mo) alloys. Data from quasi-static uniaxial tensile tests performed on a dog bone shaped specimen using a Gleeble® machine is post-processed with Aramis digital image correlation (DIC) software and numerical models of the tests are developed using Abaqus® finite element analysis (FEA) software. Results considering different temperature conditions, different applied strain rates and different gauge lengths for the determination of the mechanical properties of the material are discussed.

Nonlinear visco-hyperelastic tensile constitutive model of spray polyurea within wide strain-rate range

Spray polyurea (SPUA) is widely used as a protective coating material for structures subjected to the intensive impulsive loadings, attributed to its prominent large tensile deformation capacity and strain-rate hardening effect. This paper aims to study the tensile properties of SPUA within a wide strain-rate range both experimentally and theoretically. Firstly, by reasonably designing the geometry and dimension of specimens, a series of quasi-static and dynamic uniaxial tensile tests were carried out, and the nonlinear stress-strain curves corresponding to the strain-rates ranging from 10−3 s−1 to 103 s−1 were obtained. It showed that the mechanical properties of SPUA are sensitive to the strain-rate, i.e., the elastic modulus and tensile stress increase with the rising of the strain-rate, while the fracture strain is linearly negatively correlated with the logarithm of strain-rate. Then, a nonlinear visco-hyperelastic (NVHE) constitutive model composed of a 3rd-order polynomial hyperelastic model and seven Prony series terms describing the viscoelastic properties were established, and the corresponding twenty-three model parameters were calibrated through a multi-objective optimization based on genetic algorithm. Furthermore, based on the finite element program ABAQUS/Explicit, the validations of the proposed NVHE model and the parameters determination approach are verified by the present tensile test and existing drop hammer impact test. This work could provide a helpful reference for the retrofitting design of the protective structures.

Identification of strain-rate-dependent hardening model for aluminium alloy sheet in electromagnetic forming

The deformation rate of aluminium alloy sheet reaches up to thousands per second in the electromagnetic forming process, and how to obtain the hardening behaviour of aluminium alloy sheet at such high strain rate becomes an essential issue. The electromagnetic hole-flanging test is proposed to simplify the deformation process. Based on the electromagnetic hole-flanging test, the inverse identification procedures are adopted to calibrate the strain-rate-dependent hardening models of aluminium alloy sheet. For the Johnson-Cook and Cowper-Symonds hardening models, the strain-hardening terms were initially determined by the quasi-static uniaxial tensile tests to simplify the inverse procedures. Then, the strain-rate-dependent terms were identified by comparing the simulated and experimental strains of the electromagnetic hole-flanging test. It was validated to calibrate the Johnson-Cook and Cowper-Symonds models of AA5754 aluminium alloy sheet by combining the electromagnetic hole-flanging test with the quasi-static tensile test.

Cellulose nanofibers and the film-formation dilemma: Drying temperature and tunable optical, mechanical and wetting properties of nanocomposite films composed of waterborne sulfopolyesters

HypothesisWaterborne sulfopolyesters have gained considerable interest as coating materials due to their excellent film-forming and optical properties. Their commercial use has been limited, however, due to their fragile nature. Incorporating cellulose nanofiber (CNF), a sustainable biopolymer, into the polymer matrix is expected to enhance the mechanical integrity of the nanocomposite as these two components synergistically interact. ExperimentsIn this study, we have investigated the suspension and film characteristics of three sulfopolyesters varying in charge density, glass transition temperature and molecular weight, as well as their mixtures with CNF. We have performed steady-shear rheology on mixtures with different CNF loading levels, and resulting films have been subjected to quasistatic uniaxial tensile and water contact-angle tests to elucidate the effects of CNF on mechanical and surface properties. FindingsAddition of CNF to waterborne polyester promotes shear-thinning behavior that remains unaffected by the CNF content. Solid films cast from these suspensions possess enhanced mechanical properties, as well as tailorable surface hydrophilicity, depending on composition and film-drying temperature. Tensile tests reveal that films containing 10 wt% CNF display the greatest mechanical improvements, suggesting the existence of a previously unidentified Goldilocks composition window.

Experimental and numerical study of notched SHS made of different S355 steels

This study compares the mechanical behaviour of square hollow sections (SHS) made of three different types of S355: cold-formed, hot-rolled and offshore steel. A material model and failure criterion for each steel type were calibrated based on quasi-static uniaxial tensile tests. The failure criterion applies a recently proposed through-thickness damage regularisation model with the purpose of accurately describing the load-bearing capacity using shell elements. Experimental three-point bending tests were conducted at both quasi-static and dynamic conditions. Notches were used to trigger failure in the tests. The cold-formed steel exhibited the highest yield stress of the three steel types, while the offshore steel displayed better ductility than the other two. The numerical simulations showed that a shell element model of the SHS incorporating the regularisation scheme was able to describe the material behaviour and predict failure.

Multiscale modeling of coupling mechanisms in electrically assisted deformation of ultrathin sheets: An example on a nickel-based superalloy

Electrically assisted (EA) forming has ubiquitous merits over room-temperature (RT) forming and thermally aided forming for the fabrication of difficult-to-form microscale products. However, it is difficult to predict the material deformation behavior in the EA microforming process owing to the coupling between the electric current and microstructural size effect. To develop a robust constitutive model that considers the interplay between the electric and grain size effects, RT and EA quasi-static uniaxial tensile tests were performed on ultrathin nickel-based superalloy sheets with a thickness of 0.2 mm and grain sizes ranging from 27.2 to 79.4 μm. The experimental results demonstrated that the Joule heating effect and the normalized flow stress reduction were non-monotonically related to the grain size of the superalloy. The grain size effect in the polycrystalline superalloy was suppressed by the enhanced current density. A multiscale constitutive model that considered multiple strengthening mechanisms was proposed to describe the EA deformation behavior of ultrathin superalloy sheets. The multiscale model was proven to have a desirable predictive ability for the EA drawing force of thin-walled superalloy capillaries. Furthermore, the model revealed that the weakening of the grain size effect with increasing current density in the polycrystalline superalloy was caused by the combined variations in dislocation interaction, shear modulus, and strengthening mechanisms. The abnormal evolution of the surface effect with the current density was captured by the proposed constitutive model, which demonstrates that the electric current can promote the grain size effect in the multicrystalline superalloy.

Network topology and stability of homologous multiblock copolymer physical gels.

The mechanical properties of physical gels generated by selectively swelling a homologous series of linear multiblock copolymers are investigated by quasistatic uniaxial tensile tests. We use the slip-tube network model to extract the contributions arising from network crosslinks and chain entanglements. The composition dependence of these contributions is established and considered in terms of simulations that identify the probabilities associated with chain conformations. Dynamic rheology provides additional insight into the characteristics and thermal stability of the molecular networks.

Temperature-Dependent Macroscopic Mechanical Behaviors and Their Microscopic Explanations in a Medium Mn Steel

Plastic instability (e.g., Luders band and PLC band) is a phenomenon frequently observed in cold-rolled medium Mn steels. In this study, through a number of quasi-static uniaxial tensile tests with digital image correlation (DIC) measurement of the strain rate field, we found the medium Mn steel (Fe-0.14C-7.14Mn-0.23Si wt. pct with approximately 33 vol pct of retained austenite and 67 vol pct of ferrite) exhibited distinct competition/coordination between the Luders band and the PLC band at different deformation temperatures: (1) at 25 °C and 55 °C, a Luders band was followed by several PLC bands; (2) at 100 °C, fracture occurred right after the Luders band formation, without the presence of a PLC band; (3) at 250 °C, only PLC bands were observed. We propose that these macroscopic behaviors can be explained by the temperature-dependent conditions for the formation/appearance and propagation of Luders bands and PLC bands. The propagation of Luders bands becomes difficult when the temperature increases as less hardening is provided by martensitic transformation. In contrast, the appearance and propagation of PLC bands become easier when the temperature increases and/or the plastic strain becomes large (16.5 and 18.9 pct in this material at the deformation temperatures of 25 °C and 55 °C, respectively), as the difference in the average velocities of solute atom diffusion and dislocation gliding is reduced. Either condition for Luders bands or for PLC bands will be satisfied at different deformation temperatures or plastic strains, thus leading to the competition/coordination between them.

Characterization of non-linear mechanical behavior of the cornea

The objective of this study was to evaluate which hyperelastic model could best describe the non-linear mechanical behavior of the cornea, in order to characterize the capability of the non-linear model parameters to discriminate structural changes in a damaged cornea. Porcine corneas were used, establishing two different groups: control (non-treated) and NaOH-treated (damaged) corneas (n = 8). NaOH causes a chemical burn to the corneal tissue, simulating a disease associated to structural damage of the stromal layer. Quasi-static uniaxial tensile tests were performed in nasal-temporal direction immediately after preparing corneal strips from the two groups. Three non-linear hyperelastic models (i.e. Hamilton-Zabolotskaya model, Ogden model and Mooney-Rivlin model) were fitted to the stress–strain curves obtained in the tensile tests and statistically compared. The corneas from the two groups showed a non-linear mechanical behavior that was best described by the Hamilton-Zabolotskaya model, obtaining the highest coefficient of determination (R2 > 0.95). Moreover, Hamilton-Zabolotskaya model showed the highest discriminative capability of the non-linear model parameter (Parameter A) for the tissue structural changes between the two sample groups (p = 0.0005). The present work determines the best hyperelastic model with the highest discriminative capability in description of the non-linear mechanical behavior of the cornea.

Experimental and numerical analysis of the heat generated by plastic deformation in quasi-static uniaxial tensile tests

This study evaluates the temperature variation observed in quasi-static uniaxial tensile tests, due to the heat generated by plastic deformation. The AA6016-T4 aluminium alloy was the material selected, considering different values of crosshead velocity (from 0.01 mm/s up to 1 mm/s). The temperature variation was also evaluated during the stress relaxation test. A finite element model of the uniaxial tensile test is presented, which takes into account the heat generated by plastic deformation, as well as the effect of the heat losses to the environment (convective heat transfer coefficient) and to the grips (interfacial heat transfer coefficient). The numerical results show that the predicted temperature variation is almost independent of the selected heat transfer coefficients. On the other hand, the temperature rise is influenced by the Taylor–Quinney coefficient. The comparison between experimental and numerical temperatures shows that the Zehnder model (increasing Taylor–Quinney coefficient) provides more accurate results than the Aravas model (decreasing Taylor–Quinney coefficient). Nevertheless, the evolution of the Taylor–Quinney coefficient defined by the Zehnder model assumes a constant value for the hardening coefficient, which does not fit the hardening behaviour observed for this aluminium alloy.