Uniaxial Tension Tests Research Articles

An Investigation on Anisotropy Behavior and Forming Limit of 5182-H111 Aluminum Alloy

This paper studies the anisotropy behavior and forming limit of 5182-H111 aluminum alloy by experiments and theoretical analysis. The uniaxial tension tests for three different sheet directions were conducted to investigate the anisotropic properties of Al5182-H111 sheet. Moreover, the Nakajima tests for both rolling and transverse directions were performed to get the forming limit data. The experimental results show that the forming limit curves (FLCs) for rolling direction are much lower than that for transverse direction. In order to further investigate the influence of anisotropic properties on FLCs, the theoretical prediction of FLC was accomplished based on three different hardening laws. The results show that the theoretical FLC with Ghosh hardening law has much better agreement with experimental data. Furthermore, a new FLC was proposed by extending the forming limit from the strain state of uniaxial tension at transverse direction to the strain state of uniaxial tension at rolling direction. The adaptability of new FLC was verified in the Drawing test and Erichsen cupping test.

Characterization of PVA hydrogels’ hyperelastic properties by uniaxial tension and cavity expansion tests

The mechanical behavior of rubber-like materials is dominantly non-linear elastic. Their mechanical response is measured by numerous conventional techniques including the universal loading machines (tension and compression) and indentation. Recently, an unconventional technique based on cavitation rheology has been implemented to measure the mechanics of these materials. The loading mechanism of this technique is different from the axial force–displacement mechanism observed in the other techniques thus the aim of this study is to investigate the difference between the cavity expansion technique and one of the conventional techniques, uniaxial stretching, to understand the difference in material responses under each loading mechanism. PVA hydrogel was used as a representative to hyperelastic materials. The gels were loaded by both techniques, and hyperelastic models (Yeoh, Ogden, and Arruda–Boyce) were used to model their behavior. Finite Element (FE) simulations were performed to reproduce the experimental data. It was observed that the tension stresses generated during the cavity expansion test were similar to those generated in the uniaxial tension to a strain level of 45%, afterward, the cavity tension stresses increased exponentially exceeding those generated in the uniaxial tension. When the von Mises stresses, from both tests, were compared against the major tension stress data, it was observed that the cavity test imposed tension stresses that were twice of those generated during the uniaxial tensile test. In addition, the hoop stresses were significantly larger than the applied pressure. These observations indicate to the equi-biaxial nature of the cavity expansion test.

Modelling-assisted description of anisotropic edge failure in magnesium sheet alloy under mixed-mode loading

An uncoupled fracture criterion based on a simple damage indicator computed using a mean-field crystal plasticity framework is proposed and applied to predict failure of an AZ31 sheet originating from the edges. The damage indicator quantifies the contribution of strain components along the axes of orthotropy leading to material failure. The model is calibrated by uniaxial tension tests. The damage indicator is validated for various mixed-mode deformation histories realized by modified Arcan tests in various loading configurations. The loading history of respective fracture sites obtained from DIC analyses is directly employed to a visco-plastic self-consistent crystal plasticity model to obtain the stress responses. The results indicate that the damage indicator requires – beside the strain history – an input of stress triaxiality, by which an improved predictive accuracy can be achieved. This effect is quantified for various loading scenarios, in which cracks are initiated near or at the edge of the sample.

Anomalous inflation of a nematic balloon

Inflation of an elastomer balloon is one of the most classical and important problems in the field of nonlinear elasticity. Many intriguing phenomena associated with the inflation of elastomer balloons such as snap-through instability and bulge propagation have been often observed and intensively studied. In this article, we report a new phenomenon during the inflation of a cylindrical balloon made from nematic elastomer. We found in the experiment that with a small increment of inflating pressure, the balloon contracts significantly along its axial direction while expands in its radial direction. With further increase of the pressure, the balloon expands mainly in the radial direction while maintains its length almost unchanged. Finally, the balloon expands both in the radial and axial directions abruptly with a tiny increase of inflating pressure, often leading to rupture of the balloon. The inflation behavior of the nematic balloon can be changed when it is subjected to an additional axial load. To quantitatively understand the experiments, we adopt a quasi-convex free energy function of nematic elastomer to derive the relationship between the inflating pressure and its deformation state. We have shown that the anomalous inflation of the nematic balloon is closely associated with the soft elasticity of the nematic elastomer. Our theoretical predictions agree with experimental measurements well with only two material parameters separately determined by uniaxial tension test of nematic elastomer.

Topology optimization design of stretchable metamaterials with Bézier skeleton explicit density (BSED) representation algorithm

A new density field representation technique called the Bézier skeleton explicit density (BSED) representation scheme for topology optimization of stretchable metamaterials under finite deformation is proposed for the first time. The proposed approach overcomes a key deficiency in existing density-based optimization methods that typically yield designs that do not have smooth surfaces but have large number of small intricate features, which are difficult to manufacture even by additive manufacturing. In the proposed approach, Bézier curves are utilized to describe the skeleton of the design being optimized where the description of the entire design is realized by assigning thickness along the curves. This geometric representation technique ensures that the optimized design is smooth and concise and can easily be tuned to be manufacturable by additive manufacturing. In the optimization method, the density field is described by the Heaviside function defined on the Bézier curves. Compared to NURBS or B-spline based models, Bézier curves have fewer control parameters and hence are easier to manipulate for sensitivity derivation, especially for distance sensitivities. Due to its powerful curve fitting ability, using Bézier curve to represent density field allows exploring design space effectively and generating concise structures without any intricate small features at the borders. Furthermore, this density representation method is mesh independent and design variables are reduced significantly so that optimization problem can be solved efficiently using small-scale optimization algorithms such as sequential quadratic programming. Numerical optimization results in three typical tests, uniaxial tension, biaxial tension and shear tests are presented by imposing symmetry on a unit cell for lattice structure design. Numerical examples illustrate that the optimized metamaterials have better stretchability and higher stiffness, and hence the proposed method has great potential of impacting the design of future applications that exploit stretchability of structures.

Deformation mechanisms of the subgranular cellular structures in selective laser melted 316L stainless steel

Selective Laser Melting (SLM) of certain alloys such as Stainless Steel yields a unique microstructure consisting of complex subgranular cell structures produced by ultra-fast cooling during solidification. Owing to the presence of these cell structures, SLM-fabricated 316L Stainless Steel displays a superior yield strength and hardness. Despite the significance of the distinctive cellular structures, their deformation characteristics and the mechanisms responsible for superior mechanical properties are not well-understood yet. In this paper, the mechanical deformation behavior of the complex cellular structures observed in the SLM-fabricated 316L Stainless Steel is investigated by performing a series of molecular dynamics simulations of uniaxial tension tests. The atomic configurations in the computational models of the cell structures are inspired by the SEM images of the microstructure of 316L Stainless Steel. The effects of compositional segregation of alloying elements, distribution of austenite and ferrite phases in the microstructure, subgranular cell sizes, and pre-existing (grown in) nano-twins on the tensile characteristics of the cellular structures are investigated. The highest yield strength is observed when the Ni concentration in the cell boundary drops very low to form a ferritic phase in the cell boundary. However, the flow stress is found to be the highest when both the cell boundary and cell interior are austenitic with higher Ni concentration in the cell interior. Moreover, governed by the initial dislocation density, the subgranular cell size has an inverse relationship with mechanical strength. Finally, the nano-twinned cells exhibit higher strength in comparison with twin-free cells as a result of the dislocation pinning at the twin boundaries. The investigation on the effect of twin spacing reveals a Hall-Petch type phenomenon in which smaller twin spacing strengthens the nano-twinned subgranular cells further.

Microstructure evolution and precipitation characteristics of spray-formed and subsequently extruded 2195 Al-Li alloy plate during solution and aging process

Abstract Al-Li alloys are being successfully employed for manufacturing the structural components of lightweight aircrafts. In this work, the microstructures and precipitation characteristics of the spray-formed and subsequently extruded 2195 Al-Li alloy plate during solid solution and aging treatments were investigated and compared by using different material characterization methods. The grains of the spray-formed alloy were equiaxed without obvious crystallographic textures, whereas in the as-extruded, solution-treated, and aging-treated alloys, the grain morphology changed from being strip shaped near the center of extruded plate to equiaxed near the surface. The grain size increased after the solution treatment, but the aging treatment had no obvious influence on the grain size. A rolling-type texture with a very dominant Brass orientation was observed at the center of the plate, whereas the textures near the surface were rotated, and the crystallographic texture was not sensitive to the solution and aging treatments. The precipitated T1 and θ′ phases, which played a strengthening role in the alloy, gradually formed and developed with increasing aging duration. Besides, the Al-Cu phase in the alloy dissolved and re-precipitated during the heat treatment process, while the Fe-containing particles always remained stable. By applying the uniaxial tension test, a good balance between ultimate tensile strength and elongation was achieved after aging treatment at 170 °C × 6 h.

Evaluation of Crosslink Density Using Material Constants of Ethylene-Propylene-Diene Monomer/Styrene-Butadiene Rubber with Different Nanoclay Loading: Finite Element Analysis-Simulation and Experimental

Nanoclay is used to enhance the mechanical properties of ethylene-propylene-diene rubber (EPDM)/styrene-butadiene rubber (SBR) blends. Sulphur (S), dicumyl peroxide (P), and mixed systems (S + P) were used as crosslinking or vulcanizing agents for the EPDM/SBR nanocomposites. The experimental data of the stress–strain behavior of EPDM/SBR blends with different nanoclay loading have been determined through a tension test. Nonlinear mechanical behaviors of the rubbers are described by strain energy functions in order to assurance that rigid body motions play no role in the constitutive law. The mathematical model such as the Mooney-Rivlin model based on the existence of strain energy density functions depends on the right Cauchy-Green's deformation tensor or Green's strain tensor. The experimental data are fitted to the Mooney-Rivlin model in order to find the rubber material constants. These constants are used to find the crosslinking density. A comparison between the experimental stress–strain behavior and finite element analysis of a uniaxial tension test at different nanoclay loading is presented.

Plasticity of borosilicate glasses under uniaxial tension

AbstractThe mechanical response of multicomponent borosilicate glasses has drawn significant attention in the design of damage‐resistant glasses. In this work, we investigate the plasticity of two borosilicate glasses, Borofloat®33 (Boro33) and N‐BK7®, by implementing a uniaxial tension test using molecular dynamics simulations. A bond‐switching mechanism is found to be responsible for the plastic response of both glasses and is governed by the increasing rate of non‐bridging oxygen (NBO) production during the uniaxial tension. We found that the amount of B4OSi4 linkages in the glass governs the stress drop after yielding, due to its higher tendency to create NBOs compared to Si4OSi4. Also, the initial existence of NBOs weakens the critical stress for breaking the B4‐O bond in B4OSi4, which in turn lowers the yield strength of the glass. The local atomic constraints are analyzed in the two glasses, and high anti‐correlation between the concentration of rigid constraints and plastic deformation is observed.

Aluminum alloy matrix composite reinforced with metallic glasses particles using hot-roll bonding

Composite materials (CM) exhibit high hardness, strength and wear resistance with slightly limited processing properties. The most popular reinforcing components for discretely reinforced composites are carbide, nitride or oxide particles. Amorphous metal materials can be used as an alternative reinforcing component since reinforcement with these particles can ensure improved properties due to higher strength of interfacial bonding between the particles and matrix as compared to traditional reinforcements. A metal matrix composite sheet was obtained based on the Al—5%Zn—5%Ca alloy reinforced by particles of Co 48 Cr 15 Mo 14 C 15 B 6 Tm 2 amorphous metallic glasses with the AA5083 cladding layer. The central layer thickness of the Al—5%Zn—5%Ca alloy reinforced by metallic glass particles covered 60 % of the sheet thickness, and the cladding layer covered 40 % in total. Composite material granules were obtained by mechanical alloying with their subsequent consolidation by hot-roll bonding in the cladding shell at the temperature below the amorphous component devitrification temperature. X-ray and differential thermal analysis showed that metallic glasses retain their amorphous structure after processing in the planetary mill and further consolidation during hot rolling. The microstructure at different steps of composite material production was studied by scanning electron microscopy. Mechanical properties were evaluated by uniaxial tension tests at room temperature. The volume fraction of amorphous particles in the as-rolled state was about |0 %, and their size varied between 2 and 187 pm. The hardness of the obtained composite was 25 % higher as compared to the Al—5%Zn—5%Ca matrix alloy. At the same time, yield strength of the cladded composite material was two times higher than that of the matrix and cladding alloy samples.

Modeling interfacial transition zone of RAC based on a degenerate element of BFEM

To study the properties of the interfacial transition zone (ITZ) of recycled aggregate concrete (RAC), a RAC model was presented that can mesh directly inside the aggregate. The characteristics of the ITZ can be accurately identified and simulated based on this model. Additionally, as for comparative specimens, several projection mesh models of RAC were also set up to prove the accuracy and efficiency of this model. Further, a two-dimensional (2D) element was introduced to simulate the ITZ element, which is degenerated from a three-dimensional (3D) element on the base force element method (BFEM). Hereafter, these RAC models were subjected to a displacement loading to simulate the uniaxial compression and tension tests based on the BFEM. The effect of ITZ thickness on the compressive strength, tensile strength, peak/ultimate strain, and crack development was studied. Besides, the influence of the relative properties between ITZs and the corresponding mortar was also investigated. The obtained results demonstrate that the present model is an efficient way to analyze the influence law of ITZ. Besides, the multiple influencing factors can be fixed via this model; therefore, their effects can be studied by varying one single factor at a time. The degenerate element is a new approach to investigate the element with a large aspect ratio with sufficient accuracy and efficiency.

Analysis of the mechanical response of damaged human cervical spine ligaments

BackgroundCervical spine ligaments that protect the spinal cord and stabilize the spine are frequently injured in motor vehicle collisions and other traumatic situations. These injuries are usually incomplete, and often difficult to notice. The focus of the presented study is placed on analysis of the effect of subfailure load on the mechanical response of the three main cervical spine ligaments: the anterior and the posterior longitudinal ligament and the ligamentum flavum. MethodsA total of 115 samples of human cadaveric ligaments removed within 24–48 h after death have been tested. Uniaxial tension tests along the fiber direction were performed in physiological conditions on a custom designed test equipment. The ligaments were loaded into an expected damage zone at two different subfailure values (based on previously reported reference group of 46 samples), and then reloaded to failure. FindingsThe main effect of a high subfailure load has proven to be the toe elongation change. The toe elongation increase is affected by the subfailure load value. While anterior and posterior longitudinal ligament showed similar changes, the smallest subfailure effect was found in ligamentum flavum. InterpretationsThe normal physiological region of the cervical spine ligaments mechanical response is modified by a high subfailure load. The observed ligament injury significantly compromises ligament ability to give tensile support within physiological spinal motion.

Study on Springback Straightening after Bending of the U-Section of TC4 Material under High-Temperature Conditions.

The thin-walled structures of titanium alloys have peculiar characteristics involving thin curved surfaces, complicated structures, and a poor rigidity. Therefore, bending or twisting distortion frequently occurs in forging, extrusion, drawing, transportation, cooling, and manufacturing. Straightening theory focuses on the straightening curvature or bending moment at room temperature, and a unified analytical model of the straightening curvature, the straightening bending moment, and the straightening stroke, as well as a study on springback straightening under high-temperature conditions, have not been investigated comprehensively. In order to understand the inherent mechanism of springback straightening and quantitative prediction of springback under high-temperature conditions, uniaxial tension tests were carried out to obtain the true stress–strain model of material and stress relaxation under the stress relaxation model. This paper is based on the theory of elastic-plastic mechanics and combines this with the mechanism of stress relaxation to establish springback and residual relative curvature equations of springback. The law of springback straightening is further explored, and springback and residual deflection equations are provided. The results of the study showed that the relative errors of the theoretical residual deflection of the bending deformation and residual deflections obtained by the experiment were less than 20%, with an average absolute error of less than 10%. Therefore, the hardening models adopted can achieve an allowable relative error if hardening parameters are properly selected. The proposed research provides basic data for the prediction of springback straightening, and the design of springback compensation tools can be applied in practical applications.

Pullout Behavior of Bundled Aramid Fiber in Fiber-Reinforced Cementitious Composite.

The tensile performance of fiber-reinforced cementitious composite (FRCC) after first matrix cracking is characterized by a tensile stress–crack width relationship called the bridging law. The bridging law can be obtained by an integral calculus of forces carried by individual bridging fibers considering the effect of the fiber inclination angle. The main objective of this study is to investigate experimentally and evaluate the pullout behavior of a single aramid fiber, which is made with a bundling of original yarns of aramid fiber. The bundled aramid fiber has a nonsmooth surface, and it is expected to have good bond performance with the matrix. The test variables in the pullout test are the thickness of the matrix and the inclined angle of the fiber. From the test results, the pullout load–slip curves showed that the load increases lineally until maximum load, after which it decreases gradually. The maximum pullout load and slip at the maximum load increase as the embedded length of the fiber becomes larger. The pullout load–crack width relationship is modeled by a bilinear model, and the bridging law is calculated. The calculated result shows good agreement with the experimental curves obtained by the uniaxial tension test of aramid–FRCC.

Characterization of Spatially Graded Biomechanical Scaffolds.

Advances in fabrication have allowed tissue engineers to better mimic complex structures and tissue interfaces by designing nanofibrous scaffolds with spatially graded material properties. However, the nonuniform properties that grant the desired biomechanical function also make these constructs difficult to characterize. In light of this, we developed a novel procedure to create graded nanofibrous scaffolds and determine the spatial distribution of their material properties. Multilayered nanofiber constructs were synthesized, controlling spatial gradation of the stiffness to mimic the soft tissue gradients found in tendon or ligament tissue. Constructs were characterized using uniaxial tension testing with digital image correlation (DIC) to measure the displacements throughout the sample, in a noncontacting fashion, as it deformed. Noise was removed from the displacement data using principal component analysis (PCA), and the final denoised field served as the input to an inverse elasticity problem whose solution determines the spatial distribution of the Young's modulus throughout the material, up to a multiplicative factor. Our approach was able to construct, characterize, and determine the spatially varying moduli, in four electrospun scaffolds, highlighting its great promise for analyzing tissues and engineered constructs with spatial gradations in modulus, such as those at the interfaces between two disparate tissues (e.g., myotendinous junction, tendon- and ligament-to-bone entheses).

Effect of Anisotropic Yield Functions on Prediction of Critical Process Window and Deformation Behavior for Hydrodynamic Deep Drawing of Aluminum Alloys

Owing to the reduction of rupture instability and the avoidance of wrinkle defect, the hydrodynamic deep drawing (HDD) process is gradually becoming attractive for fabricating lightweight and complicated products. Meanwhile, since metallic materials present anisotropic deformation behavior, it is necessary to select an appropriate constitutive model for the prediction of plastic deformation behavior of applied material with high precision. In the present research, several anisotropic yield criteria, namely, Hill’48, Yld2000-2d, and BBC2005, were implemented to investigate the effects of yield functions on the prediction accuracy of the critical process window and deformation behavior for the HDD process of 2024 and 5754 aluminum alloys. Material constants in the yield criteria were determined by applying uniaxial and equi-biaxial tension tests and optimizing an error-function using the Levenberg–Marquardt algorithm. Furthermore, the process window diagram was computed utilizing the stress analytical model combined material properties with workpiece geometrical features. Numerical simulation results of predicted material anisotropic parameters, process window, and HDD deformation for aluminum alloys were compared with the experimental data. Through the comparison of diverse yield criteria based on materials’ anisotropic coefficients, critical process window prediction, earing profile, and thickness distribution, it was revealed that the Yld2000-2d and the BBC2005 yield criteria can offer more precise models of material behavior in planar anisotropy properties for the HDD process of 2024 and 5754 aluminum alloys.

Uncertainty propagation in reduced order models based on crystal plasticity

In this work, an uncertainty propagation study is performed based on simulated ensembles of statistical volume elements (SVE) used to inform a reduced order internal state variable model, focusing on model form and uncertainties in numerical simulations. A rate-dependent polycrystal plasticity finite element model (CP-FEM) of cartridge brass is calibrated to room-temperature uniaxial tension testing data of an annealed sample. Model forms are considered with and without the inclusion of back stress in the CP model. Three sizes of SVE are explored for simulation uncertainty effects. Effects are explored with regard to the parameters of a Bammann–Chiesa–Johnson (BCJ) macroscopic viscoplasticity model, calibrated element-wise to SVEs from an ensemble modeled using CP-FEM simulations exploring quasi-static uniaxial tension. Variability in stress–strain response at multiple defined length-scales is measured. The selection of model form is discussed. The numerical simulation uncertainty of the model reduction process is quantified.

Effect of nickel on the kinematic stability of retained austenite in carburized bearing steels – In-situ neutron diffraction and crystal plasticity modeling of uniaxial tension tests in AISI 8620, 4320 and 3310 steels

The presence of kinematically metastable retained austenite in the microstructure of bearing components can significantly affect the macro and micro-mechanical material response. In the present work, the influence of Ni on the stability of the retained austenite within three different grades of high carbon bearing steel using in-situ neutron diffraction is investigated. For the first time, the results show that presence of Ni increases the stability of the austenite in the elastic regime whereas the transformation rate remains unaffected. Crystal plasticity finite element (CPFE) modeling was used to study the deformation in these three steels and shows that the predominant factor causing the difference in mechanical behavior of these steels is the austenite stability. The elastic and plastic response of the matrix martensite was found to be identical among all specimens while the austenite demonstrates similar elastic behavior but remarkably different stabilities under monotonic loading.

Relationship between local damage and macroscopic response of soft materials highly reinforced by monodispersed particles

A rubberlike matrix highly filled with spherical micrometric glass beads is submitted to uniaxial tension tests until break. X-ray tomography imaging performed on the material while submitted to uniaxial tension reveals early debonding at the matrix/filler interfaces at the poles of the particles followed by void coalescence creating damage localization. The latter causes a downturn of the macroscopic stress-strain response. These phenomena are analyzed further with three-dimensional finite element simulations, where 64 spherical beads are distributed randomly in a periodic cell. A simple version of the Tvergaard-Hutchinson cohesive-zone model allows to reproduce all the experimental trends well. The effects of the three parameters involved are analyzed, and three different types of macroscopic behaviors are observed corresponding to three different microstructure damages. The value of the initial stiffness of the interface, limited by numerical convergence, has little effect on how the local damage evolves but has a significant impact on the overall macroscopic stress values. The local damage is strongly dependent on the critical strength and the separation failure displacement, and the adhesion energy may be considered as a resulting parameter of the two previous ones. The interfacial critical strength appears to have a significant impact on the damage initiation, either spread across the structure for low values, or localized for high values. Increasing the interface separation failure displacement delays the possible loss of adhesion to a higher strain and preserves the integrity of the composite material.

Heterogeneous Nature of Calcium Silicate Hydrate (C-S-H) Gel: A Molecular Dynamics Study

Structure and mechanical properties of Calcium silicate hydrate (C-S-H) at a molecular level act as “DNA” of cement-based construction materials. In order to understand loading resistance capability of C-S-H gel, research on molecular dynamics (MD) was carried out to simulate the uniaxial tension test on C-S-H model along x, y, and z directions. Due to the structure and dynamic differences of the layered structure, the C-S-H model demonstrates heterogeneous mechanical behavior. On an XY plane, the cohesive force can reach 4 GPa which is mainly provided by the Ca-O and Si-O ionic-covalent bonds. The good plasticity of calcium silicate sheet is attributed to the silicate branch structure formation and the recovery role of interlayer calcium atoms. However, in z direction, C-S-H layers connected by the unstable H-bonds network, have the weakest tensile strength 2.2 GPa. This results in the brittle failure mode in z direction. The relatively low tensile strength and poor plasticity in z direction provides molecular insights into the tensile weakness of cement materials at macro-level.