Uniaxial Tension Strain Research Articles

Impact of uniaxial strain on physical properties of zigzag graphene nanoribbons with topological defects

The synergistic regulation mechanism of uniaxial strain, topological defects, edge passivation atom and nanoribbon width on the geometric and electronic structures of zigzag graphene nanoribbons have been studied systematically by first-principles. It is found that the average formation energy and strain energy of X-N 1 N 2-LD-ZGNR (X = H, F and O, as well as, N 1 = N 2 = 3, 4 and 5) increase with the increase of uniaxial strain, and this relationship is also dependent of edge passivation atom species and nanoribbon width. And the edge of 55-LD-ZGNR passivating with O and F atoms is more beneficial than H atom for system stability. The stress–strain curve shows that the limiting strain of zigzag graphene nanoribbon depends on edge passivation atom species and nanoribbon width. The Young’s modulus in the case of ε > 3% and Poisson’s ratio except O-33-LD-ZGNR at ε = 1% of X-N 1 N 2-LD-ZGNR decrease with the increase of the tensile strain, and is dependent of nanoribbon width and edge atom species. And O-55-LD-ZGNR is easier than F-55-LD-ZGNR and H-55-LD-ZGNR to be stretched or compressed. The magnetism is induced in both H-55-LD-ZGNR and F-55-LD-ZGNR, and remains with the increases of uniaxial tension strain. What is more, magnetic property of O-55-LD-ZGNR can be regulated by applying uniaxial strain, and the band gap of the O-N 1 N 2-LD-ZGNR (N 1 = N 2 = 3, 4 and 5) system can be regulated by adjusting the uniaxial tensile strain and nanoribbon width. Our research provides a new method to open the graphene band gap, which can provide some new theoretical guidance for the application of graphene in electronic devices and other fields. The band gap of the O-LD-ZGNDR system is opened as the uniaxial tensile strain increases.

Wide-direct-band-gap monolayer carbon nitride CN2: a potential metal-free photocatalyst for overall water splitting.

Two dimensional metal-free semiconductors with high work function have attracted extensive research interest in the field of photocatalytic water splitting. Herein, we have proposed a kind of highly stable monolayer carbon nitride CN2 with an anisotropic structure based on first principles density functional theory. The calculations of electronic structure properties, performed using the HSE06 functional, indicate that monolayer CN2 has a wide direct band gap of 2.836 eV and a high work function of 6.54 eV. And the suitable band edge alignment, high electron mobility (∼103 cm2 V-1 s-1) and visible-light optical absorption suggest that monolayer CN2 has potential on visible-light photocatalytic water splitting at pH ranging from 0 to 14. Moreover, we have observed that uniaxial strain can effectively control the electronic structure properties and optical absorption of monolayer CN2, which can further improve its solar to hydrogen efficiency from 9.6% to 16.02% under 5% uniaxial tension strain along the Y direction. Our calculations have not only proposed a new type of potential metal-free photocatalyst for water splitting but also provided a functional part with high work function for type-I and scheme-Z heterojunction applied in photocatalytic water splitting.

First-Principle Investigation into Mechanical Properties of Al6Mg1Zr1 under Uniaxial Tension Strain on the Basis of Density Functional Theory

The influences of uniaxial tension strain in the x direction (εx) on the mechanical stability, stress–strain relations, elastic properties, hardness, ductility, and elastic anisotropy of Al6Mg1Zr1 compound were studied by performing first-principle calculations on the basis of density functional theory. It was found that Al6Mg1Zr1 compound is mechanically stable in the range of strain εx from 0 to 6%. As the strain εx increased from 0 to 6%, the stress in the x direction (σx) first grew linearly and then followed a nonlinear trend, while the stresses in the y and z directions (σy and σz) showed a linearly, increasing trend all the way. The bulk modulus B, shear modulus G, and Young’s modulus E all dropped as the strain εx increased from 0 to 6%. The Poisson ratio μ of Al6Mg1Zr1 compound was nearly unchanged when the strain εx was less than 3%, but then it grew quickly. Vickers hardness HV of Al6Mg1Zr1 compound dropped gradually as the strain εx increased from 0 to 6%. The Al6Mg1Zr1 compound was brittle when the εx was less than 4%, but it presented ductility when the strain εx was more than 4%. As the strain εx increased from 0 to 6%, the compression anisotropy percentage (AB) grew and its slope became larger when the strain εx was more than 4%, while both the shear anisotropy percentage (AG) and the universal anisotropy index (AU) first dropped slowly and then grew quickly. These results demonstrate that imposing appropriate uniaxial tension strain can affect and regulate the mechanical properties of Al6Mg1Zr1 compound.

Effect of Pore Defects on Uniaxial Mechanical Properties of Bulk Hexagonal Hydroxyapatite: A Molecular Dynamics Study.

Hydroxyapatite (HAP) is a calcium apatite bioceramic used in various naturally-derived and synthetic forms for bone repair and regeneration. While useful for the regrowth of osseus tissue, the poor load-bearing capacity of this material relative to other biomaterials is worsened by the propensity for pore formation during the synthetic processing of scaffolds, blocks, and granules. Here we use molecular dynamics (MD) simulations to improve the current understanding of the defect-altered uniaxial mechanical response in hexagonal HAP single crystals relative to defect-free structures. The inclusion of a central spherical pore within a repeated lattice was found to reduce both the failure stress and failure strain in uniaxial tension and compression, with up to a 30% reduction in maximum stress at the point of failure compared to a perfect crystalline structure observed when a 30 Å diameter pore was included. The Z axis ([0 0 0 1] crystalline direction) was found to be the least susceptible to pore defects in tension but the most sensitive to pore inclusion in compression. The deformation mechanisms are discussed to explain the observed mechanical responses, for which charge imbalances and geometric stress concentration factor effects caused by pore inclusion play a significant role.

Remarkable response of hollow thermoplastic microspheres-elastomer matrix composites in uniaxial tension

In the last few years, the mechanical response of hollow thermoplastic microsphere-elastomer matrix composites has been investigated. The large majority of the studies focuses on their compressive properties and particularly on the stress-strain response. In the present paper, large strain uniaxial tension experiments are conducted on thermoplastic microsphere filled polyurethane elastomer. Six volume fractions are considered. Thanks to a two-camera setup and digital image correlation measurements, the remarkable volumetric behaviour is highlighted. First, the hydrostatic pressure vs. volume change response during a loading-unloading cycle consists of a hysteresis loop, and its size is directly related to the volume fraction of microspheres in the composite. Second, this loop admits a complex shape with several extrema. This remarkable response can be considered as the macroscopic signature of the complex microstructural phenomena involved during deformation.

Analysis and visualization of traceless symmetric tensors. Application to the Hencky strain tensor for large strain tension–torsion

Cyclic multiaxial loadings of soft materials are usually studied throughout experiments involving machines that prescribe a combination of uniaxial tension and torsion. Due to the large strain framework, classical kinematic analyses of strain in uniaxial tension–torsion are usually very complex. Based on this observation, the present paper proposes a method to both analyze and visualize a strain measure during a duty cycle of uniaxial tension–torsion in large strain: based on the mathematical properties of the Hencky strain tensor h, the method consists in projecting h onto a well-chosen tensor basis, whose constituting elements are described in terms of physical meaning. Thanks to this decomposition, the history of h reduces to the time evolution of a 3-component vector α(t). This vector history can then be visualized as a path in 3D space, rendering very visual the complex kinematic phenomenon. As a second result, an original definition of the mean and the amplitude of a strain path, based on the theory of the Minimum Circumscribed Spheres, is proposed. This definition could be useful for fatigue studies, for instance.

Multi-strain path deformation behavior of AA6016-T4: Experiments and crystal plasticity modeling

The development of backstresses that occur during a strain path change when forming sheet metal renders final part geometry prediction, after springback, difficult using conventional models. Most deformation models do not explicitly account for the influence of these stresses. A more recently developed elasto-plastic self-consistent (EPSC) model incorporates the backstresses to influence the activation of slip systems for more accurate simulations of the complex material response. The current study assesses the performance of the EPSC model for deformation response of AA6016-T4 via multiple biaxial, plane-strain, and uniaxial tension strain paths. The response to complex strain paths was examined by first pre-straining under uniaxial, biaxial, and plane-strain tension, then by loading in uniaxial tension. The EPSC model predictions closely matched experimental results. The model correctly predicted the highest yield stress and sharpest transition from elastic to plastic deformation for uniaxial tension after an initial uniaxial pre-strain. Lower yield stress for uniaxial tension after first pre-straining in biaxial and plane-strain tension is also correctly predicted, along with a smoother transition from elastic to plastic behavior. A linear geometrically necessary dislocation (GND) development, with strain, was observed using high-resolution electron backscattered diffraction (HREBSD) while a quadratic statistically stored dislocation (SSD) development was predicted by the model. The comparison revealed an expected transition from kinematic to isotropic hardening at higher strains. Finally, at higher strain levels the backstress accounted for around 15% of the total subsequent flow stress in all pre-strain cases.

Design of heterogeneous interior notched specimens for material mechanical characterization

Nowadays, virtual predictions are essential in the design and development of new engineering parts. A critical aspect for virtual predictions is the accuracy of the constitutive model to simulate the material behavior. A state-of-the-art constitutive model generally involves a large number of parameters, and according to classical procedures, this requires many mechanical experiments for its accurate identification. Fortunately, this large number of mechanical experiments can be reduced using heterogeneous mechanical tests, which provide richer mechanical information than classical homogeneous tests. However, the test’s richness is much dependent on the specimen's geometry and can be improved with the development of new specimens. Therefore, this work aims to design a uniaxial tensile load test that presents heterogeneous strain fields using a shape optimization methodology, by controlling the specimen's interior notch shape. The optimization problem is driven by a cost function composed by several indicators of the heterogeneity present in the specimen. Results show that the specimen's heterogeneity is increased with a non-circular interior notch. The achieved virtual mechanical test originates both uniaxial tension and shear strain states in the plastic region, being the uniaxial tension strain state predominant.

Strain-engineering on GeSe: Raman spectroscopy study.

Among the IV-VI compounds, GeSe has wide applications in nanoelectronics due to its unique photoelectric properties and adjustable band gap. Even though modulation of its physical characteristics, including the band gap, by an external field will be useful for designing novel devices, experimental work is still rare. Here, we report a detailed anisotropic Raman response of GeSe flakes under uniaxial tension strain. Based on theoretical analysis, the anisotropy of the phonon response is attributed to a change in anisotropic bond length and bond angle under in-plane uniaxial strain. An enhancement in anisotropy and band gap is found due to strain along the ZZ or AC directions. This study shows that strain-engineering is an effective method for controlling the GeSe lattice, and paves the way for modulating the anisotropic electric and optical properties of GeSe.

Micromechanics-based identification of a ductile fracture model for three structural steels

From an engineering point of view, it is beneficial to reduce the number of mechanical tests required to calibrate the plasticity and fracture models of a structural material. In this study, the ductile fracture model for three high-strength steels is identified based on unit cell simulations, metal and porous plasticity modelling, and strain localization analysis combined with a single uniaxial tensile test per material. Finite element simulations of a unit cell model with a spherical void are performed with the matrix material described by metal plasticity and used to calibrate the parameters of the porous plasticity model. Strain localization analyses are conducted using the imperfection band approach with metal plasticity outside and porous plasticity inside the imperfection band. These simulations are first used to determine the nucleation rate in the porous plasticity model giving the experimentally obtained fracture strain in uniaxial tension, and then to compute the fracture locus under proportional loading in generalized axisymmetric tension. By combining the fracture locus with a simple damage accumulation rule and metal plasticity, finite element simulations of ductile fracture in tensile tests on smooth and notched specimens of the three steels are performed. Comparison of the predicted results with existing experimental data shows that the fracture model gives satisfactory estimates of ductility for a wide range of stress triaxiality ratios in steels of different strengths. This study shows the potential of micromechanical analyses in the calibration of fracture models for engineering applications.

Experimental and numerical study on the deformation mechanism of straight flanging by incremental sheet forming

The geometric accuracy is a key quality indicator for the application of flanging by incremental sheet forming (ISF), namely incremental flanging (IF). As a local loading forming process, the deformation process in IF is different from that in conventional flanging formed by press-working operation in which considerable geometric error can be introduced. Thereby, the deformation process of straight flanging by ISF is investigated through both experimental and numerical simulation approaches. First, the effects of different yield functions (Mises and Hill48) and hardening models (A–F, isotropic and kinematic) on the prediction accuracy are investigated. In particular, the Hill48 yield function is not only solved by r-values, but also calibrated by material properties of strain states corresponding to the actual deformation condition of straight flanging by ISF. Among different yield functions, calibrated methods and hardening models, a A–F hardening model with the calibrated Hill48 yield function is found to have the best performance in terms of prediction accuracy of both forming force and cross-section profile. Then, based on this model, the deformation process of the straight flanging by ISF is analyzed. The results show that the strain states of straight flanging by ISF are plane and uniaxial tension strain states. In the forming process of straight flanging by ISF, the whole blank experiences bending-unbending deformation in which the material contacting with the die undergoes reverse loading. In addition, the main deformation modes of straight flanging are bending and stretch deformation through internal energy analysis. Finally, the deformation process of the straight flanging by ISF is theoretical analyzed. It is found that the elastic moment in the reverse loading mode is the main source for geometric error of the straight wall.

Anisotropy of mechanical properties and crystallographic texture in hot rolled AZ31+XSr sheets

Abstract The influences of Sr on the microstructure, texture and mechanical properties including flow and anisotropy behavior of AZ31 alloys are investigated. Slabs containing no, 0.4 and 0.8 wt% of strontium were cast and subjected to hot rolling. Results indicate that Sr reduces the basal texture intensity (23% in the AZ31+0.8Sr alloy) and homogenizes the distribution of strain in uniaxial tension. Furthermore, Sr increases both the strength coefficient and the strain hardening exponent in all directions. In the transverse direction, enhancements are more significant. Moreover, Sr enhances the combination of tensile strength and total elongation; i.e., toughness, whether tests are performed parallel to the rolling, diagonal or transverse directions, to significant extents.

Molecular dynamics simulation of crack growth in pure titanium under uniaxial tension

ABSTRACTThe analysis of crack growth in titanium was performed using molecular dynamics simulation with Embedded Atom Method potentials. The effect of temperature and strain rate on the mechanism of crack growth and the change of microstructure were discussed. After setting an initial crack, the specimen was subjected to uniaxial tension strain up to the total strain level of 0.2 with a constant strain rate. During the period, the shape and the microstructure of crack tip as well as the stress–strain curves were monitored. In the simulation, the gather of voids and stress concentration leading to the crack growth occurred, which are in agreement with experimental results observed by transmission electron microscopy. The transformation from HCP to BCC also occurred at crack tip. The remarkable effect of temperature and strain rate on the growth direction and rate of stacking fault of crack tip was observed. Moreover, initial crack greatly lowered the tension yield point of pure titanium. In the stage of deformation, simulation results showed that loading strain rate and temperature strongly influenced peak stress point, which was increased by the low temperature and high strain, whereas the initial slope of the stress strain curve was independent of loading strain rate.

Effects of strain states on stability of retained austenite in medium Mn steels

Based on uniaxial tensile and plane strain deformation tests, the effects of strain states on the stability of RA (retained austenite) in medium Mn steels, which were subjected to IA (intercritical annealing) and Q&P (quenching and partitioning) processing, were investigated. The volume fractions of RA before and after deformation were measured at different equivalent strains. The transformation behaviors of RA were also investigated. The stability of RA differed across two different transformation stages at the plane strain state: the stability was much lower in the first stage than in the second stage. For the uniaxial tension strain state, the stability of RA corresponded only to a single transformation stage. The main reason was that there were two types of transformations from RA in the medium Mn steel for the plane strain state. One type was that the martensite originated in the strain-induced stacking faults (SISF). The other type was the strain-induced directly twin martensite at a certain equivalent strain. However, for the uniaxial tension state, only the strain-induced twin martensite was observed. Dislocation lines and dislocation tangles were also observed in specimens deformed at different strain states. In addition, complex microstructures of stacking faults and lath-like phases were observed within a grain at the plane strain state.

Width-dependent structural stability and magnetic properties of monolayer zigzag MoS2 nanoribbons

First-principles study based on density functional theory has been employed to investigate width-dependent structural stability and magnetic properties of monolayer zigzag MoS2 nanoribbons (ZZ-MoS2 NRs). The width N = 4–6 (the numbers of zigzag Mo–S chains along the ribbon length) are considered. The results show that all studied ZZ-MoS2 NRs are less stable than two-dimensional MoS2 monolayer, exhibiting that a broader width ribbon behaves better structural stability and an inversely proportional relationship between the structural stability (or the ribbon with) and boundary S–Mo interaction. Electronic states imply that all ZZ-MoS2 NRs exhibit magnetic properties, regardless of their widths. Total magnetic moment increases with the increasing width N, which is mainly ascribed to the decreasing S–Mo interaction of the two zigzag edges. In order to confirm this reason, a uniaxial tension strain is applied to ZZ-MoS2 NRs. It has been found that, with the increasing tension strain, the bond length of boundary S–Mo increases, at the same time, the magnetic moment increases also. Our results suggest the rational applications of ZZ-MoS2 NRs in nanoelectronics and spintronics.

Role of weak particle-matrix interfacial adhesion in deformation and fracture mechanisms of rigid particulate-filled poly(methyl methacrylate)

Toughening glassy thermoplastics such as poly(methyl methacrylate) (PMMA) without sacrificing modulus and thermomechanical stability is a valuable but challenging objective. Rigid particulate fillers have been found to improve toughness of some polymers with complex dependence on matrix ductility, particle size, and particle-matrix interfacial adhesion. We tested the effects of both strong and weak interfacial adhesion on deformation and fracture of a model system comprising PMMA filled with monodisperse 1µm diameter silica spheres. Fracture energy GIC of PMMA was found to increase by over 50% when filled with 1 v% of weakly bonded particles, while the force observed during melt compounding increased by less than 15% and Young's modulus increased systematically with filler loading. However, GIC decreased with filler loading above 1 v%. This behavior is consistent with a modified Kinloch-type model considering localized shear banding and plastic void growth around debonded particles at the crack tip. The ability of the matrix to deform via shear yielding and plastic void growth was confirmed by digital image correlation measurement of volumetric strain in uniaxial tension. We have extended Kinloch's model to account for shortening of the crack tip craze by the particles, which reduces the intrinsic toughness and toughenability of the PMMA matrix. Particles with strong interfacial adhesion generally reduced toughness. The experimental and modeling results suggest weakly bonded particles with size on the order of the crack-tip craze width may provide optimum toughening of glassy thermoplastics.

Role of oxide seed layer in plastic response of epitaxially grown textured metal films

The elastic/plastic mechanical behavior of freestanding polycrystalline {111}-textured Pt films with thicknesses 50–1000 nm and different combinations of grain size and film thickness, grown epitaxially on a 35–50 nm thick polycrystalline (100)-TiO2 (rutile) seed layer, was studied at uniaxial tension strain rates 10−6 - 10 s−1. The mismatch strain between the {111}-Pt films and the underlying (100)-TiO2 seed layer gives rise to an interfacial dislocation network, which, in turn, determines the initiation of plastic deformation in Pt. Experiments showed that the flow stress increases, while the plastic strain accumulation at failure decreases with decreasing Pt film thickness. A modified Thompson model that accounted for the combined effect of film thickness and grain size provided good predictions for the elastic limit of Pt films. However, the yield stress was underestimated by the same model; a Taylor strain hardening model was superimposed to the modified Thompson model to account for additional hardening as a result of dislocation interactions during plastic deformation, which provided good predictions for the evolution of flow stress with plastic strain.

Direct experimental evidence of time-temperature superposition at finite strain for an amorphous polymer network

The time-temperature superposition property of an amorphous polymer acrylate network is characterized at infinitesimal strain by standard dynamic mechanical analysis tests. Comparison of the shift factors determined in uniaxial tension and in torsion shows that both tests provide equivalent time-temperature superposition properties. More interestingly, finite strain uniaxial tension tests run until break at constant strain rate show that the acrylate network exhibits the same time-temperature superposition property at finite strain as at infinitesimal strain. Such original experimental evidence provides new insight for finite strain constitutive modelling of polymer amorphous networks.

Characterization of shape recovery via creeping and shape memory effect in ether-vinyl acetate copolymer (EVA)

Shape recovery in a commercial ether–vinyl acetate copolymer (EVA) was systematically characterized by studying its creep and the thermoresponsive shape-memory effect (SME). The influences of the programming temperature and maximum uniaxial tension strain on the shape-fixity ratio and the shape-recovery ratio were investigated quantitatively. In addition to excellent SME, high elasticity and high creep were observed at around room temperature (with the EVA in the glassy state). The underlying mechanisms for the different shape-recovery phenomena (i.e., creep and the SME) are discussed. Two potential applications utilizing the shape-recovery property and high elasticity of this EVA are presented.

Cohesive zone representation and junction partitioning for crystal plasticity analyses

SUMMARYA novel scheme is presented for incorporating finite thickness cohesive interfaces in virtual grain structures for crystal plasticity finite element (CPFE) analyses of intergranular crack initiation and propagation. A Voronoi tessellation model is used to define the virtual grain structure, with automatically generated nonzero thickness cohesive zones (CZs) representing the grain boundaries and multiple junctions. An efficient grain boundary offsetting algorithm is presented, and issues related to automatically partitioning multiple junctions are discussed. Two feasible junction partitioning schemes are presented, the second of which has the advantage of partitioning junctions using uniform quadrilateral elements and naturally defining their normal and tangential directions. For the second scheme, a rule‐based method is presented that carries out the preliminary meshing of CZ junctions, including data representation, edge event processing, and cut and trim operations. A virtual grain structure modelling system, VGRAIN, is introduced to implement the proposed CZ junction partitioning method and directly generate meshed virtual grain structures with CZ grain boundaries for CPFE studies. To demonstrate the proposed junction partitioning and CZ representation schemes, two finite strain CPFE simulations are presented for plane strain uniaxial tension and three‐point bending, demonstrating large‐scale crack initiation and propagation under shear and opening modes. Copyright © 2012 John Wiley & Sons, Ltd.