Anisotropic Mechanical Response Research Articles

Crystal plasticity finite element modeling of twin band formation and evolution together with the macroscale mechanical responses of hexagonal metals

Deformation twinning is one of the major plastic deformation modes of hexagonal metals. In this study, a new system of crystal plasticity finite element models is developed, with the cooperative interactions of dislocation slip and deformation twinning comprehensively involved. The obtained stress-strain curves for several differently-orientated Mg single crystals match well with experimental results, and the formation and thickening features of twin bands are captured. Simulation results reveal that (1) some thin twin bands are firstly formed in the shearing direction of the twinning plane, because of the much lower twinning resistance than that in the normal direction; (2) the softening and hardening of the twinning resistance play a critical role in the anisotropic mechanical response. By optimizing the twinning activation behaviors, the favorable interactions of twinning and dislocation slip can occur and the plastic deformation ability can be improved.

On the Anisotropic Mechanical Response of Ti6Al4V Sheet at High Strain Rates

In traditional sheet metal forming processes, a thin sheet metal is deformed at relatively low speeds. However, more and more the speed of production processes is increased, very often to take advantage of the beneficial implications that high strain rates can have on the formability of certain metals. In this work, the effect of strain rate on the anisotropic behavior of Ti6Al4V is investigated performing tensile tests in different orientations, namely 0o, 45o and 90o, with respect to the rolling direction (RD), and in-plane shear experiments along RD. Three nominal strain rates, i.e., , 0.5 and 1000, are considered. A novel high-speed bulge (HSB) test developed at Ghent University, designed to deform sheet metals in nearly equibiaxial loading conditions at high strain rates, is used for the dynamic bulge test. The experimental results, presented in this paper, clearly show the need of including the effect of strain rate, and eventually temperature, into the formulation of the yield function in order to correctly model and predict deformation processes performed at high speed.

Phase transformation and incompatibility at grain boundaries in zirconia-based shape memory ceramics: a micromechanics-based simulation study

Zirconia-based shape memory ceramics (SMCs) exhibit anisotropic mechanical response when undergoing elastic deformations as well as during austenite–martensite phase transformation. This behavior results in different types of strain incompatibility at grain boundaries, which we study here using a micromechanical model. A single-crystal model is implemented to provide a full mechanistic three-dimensional description of the anisotropic elastic as well as martensitic transformation stress–strain response, including non-Schmid behavior caused by the significant volume change during martensitic transformation. This model was calibrated to and validated against compression tests of single-crystal zirconia micro-pillars conducted previously, and then used to model bi-crystals. Upon the introduction of a grain boundary, the simulation provides detailed information on the nucleation and evolution of martensite variants and stress distribution at grain boundaries. We identify bi-crystal configurations which result in very large stress concentrations at very low deformations due to elastic incompatibility, as well as others where the elastic incompatibility is relatively low and stress concentrations only occur at large transformation strains. We also show how this approach can be used to explore the misorientation space for quantifying the level of elastic and transformation incompatibility at SMCs grain boundaries.Graphical abstractMicromechanics models provide insights on grain boundary elastic and phase transformation strain incompatibility in shape memory zirconia

Modeling of the tension-compression asymmetry reduction of ECAPed Mg-3Al-1Zn through grain fragmentation

In this work, a physically based model accounting for grain-to-grain interaction and grain refinement mechanisms is proposed to predict the anisotropic mechanical response and the texture evolution in ECAPed Mg-3Al-1Zn. The proposed model couples two approaches: crystal plasticity (CP), including twinning, and continuum dislocation dynamics (CDD). A grain refinement mechanism is also integrated into the model in order to predict the formation of refined grains during severe plastic deformation. A robust parameter identification method is proposed, in which experimentally reported process parameters are calibrated to fit the simulated mechanical behavior, texture evolution, and deformation systems-related activities. The anisotropic behavior evolution of the Mg-3Al-1Zn hot-rolled plate is examined by predicting the mechanical behavior, dislocation evolution, and slip/twin systems activities of the ECAPed material. The coupled CP-CDD model predicts grain size reduction with an average grain size which is in agreement with the experimentally measured values. Furthermore, the model generated textures are in accordance with those reported in the literature. Finally, a good agreement between the experimental and predicted true tensile stress–strain curves up to the ultimate tensile strength for both routes 4A and 4 K along extrusion and flow directions was found.

Effects of SiC content on the mechanical and thermophysical properties of 3D Cf/SiC–Al composites

3D Cf/SiC–Al composites were achieved through the pressure infiltration of liquid Al–Si alloy into porous 3D Cf/SiC preform, which was produced by different cycles of precursor infiltration and pyrolysis. The effect of silicon carbide volume fraction on the microstructure, anisotropic mechanical response, and thermophysical characteristics of the 3D Cf/SiC–Al composites was investigated. The results demonstrated that the initial microstructure of 3D Cf/SiC can be retained, and the obtained Cf/SiC–Al composites presented remarkable anisotropy characteristics. As the silicon carbide ceramics content increased from 12.5 vol% to 41.8 vol%, the thermal conductivity and thermal expansion coefficient of 3D Cf/SiC–Al composites decreased, whereas the bending strength initially increased and then decreased in the Z-direction. The bending strength perpendicular (Z) to the carbon cloth layer of 3D Cf/SiC–Al composites was higher than that of parallel (X–Y) to the carbon cloth layer. However, a significant anisotropy in the thermal conductivity values was the opposite. The 3D Cf/SiC–Al composite with 1ow ceramic content (17 vol%) had a higher thermal conductivity in the X–Y direction (64 W m−1 K−1) than in the Z-direction (34 W m−1 K−1). The thermal expansion coefficient of all the 3D Cf/SiC–Al composites along the X–Y direction also decreased initially and then increased in the range of 100–450 °C, which presents low expansion characteristics.

Anisotropic mechanical response, high negative thermal expansion, and outstanding dynamical stability of biphenylene monolayer revealed by machine-learning interatomic potentials

Recently, the two-dimensional (2D) biphenylene network with a specific arrangement of four-, six-, and eight-membered carbon rings has been fabricated over the gold surface via a two-step polymerization technique (Science 372(2021), 852). Inspired by the aforementioned experimental advance and exciting physics of full-carbon 2D lattices, for the first time, we herein employ machine-learning interatomic potentials (MLIPs) to explore the mechanical properties, failure behavior, dynamical stability, and thermal expansion of the biphenylene monolayer. The remarkable accuracy of the developed MLIP-based models is concluded by comparing the predicted direction-dependent uniaxial stress-strain relations and failure mechanism of the biphenylene monolayer with those obtained by density functional theory simulations. Analysis of phonon dispersion relations reveals an outstanding dynamical stability of the biphenylene monolayer. Similarly to graphene, the biphenylene network also exhibits a negative thermal expansion, but with around twice the value of graphene at room temperature. We also studied the temperature effect on the tensile strength and failure strain of the biphenylene monolayer. The presented results provide a useful vision concerning the thermo-mechanical properties of the 2D biphenylene network.

Metamaterials with enhanced mechanical properties and tuneable Poisson’s ratio

This paper reports a structural modification of an auxetic metamaterial with a combination of representative re-entrant and chiral topologies, namely, a re-entrant chiral auxetic (RCA) metamaterial. The main driving force for the structural modification was to overcome the undesirable properties of the RCA metamaterial such as anisotropic mechanical response under uniaxial compression. Additively manufactured polyamide 12 specimens via Multi Jet Fusion were quasi-statically compressed along the two in-plane directions. The experimental results confirmed that the modified structure was less sensitive to the loading direction and the deformation was more uniform. Moreover, similar energy absorptions were obtained when the modified metamaterial was crushed along the two in-plane directions. The energy absorption per unit volume was improved from 390 to 950 kJ m−3 and from 500 to 1000 kJ m−3 compared with the RCA when they were crushed along the X and Y directions, respectively. The absorbed energy per unit mass (specific energy absorption) also improved from 1.4 to 2.9 J g−1 and from 1.78 to 3.1 J g−1 compared with that of the RCA under the axial compression along the X and Y directions. Furthermore, parametric studies were performed and the effects of geometric parameters of the modified metamaterial were numerically investigated. Tuneable auxetic feature was obtained. The energy absorption and Poisson’s ratio of the modified metamaterial offer it a good candidate for a wide range of potential applications in the areas such as aerospace, automotive, and human protective equipment.

Evaluating the effects of porosity on the mechanical properties of extrusion-based 3D printed concrete

Elements fabricated by extrusion-based 3D concrete printing (3DCP) display anisotropic mechanical properties when subject to loading conditions orientated in principle cartesian axis directions. Various studies show that the mechanical characteristics of 3DCP components are lower than their mould-cast counterparts, reportedly due to the existence of weaker, more porous, interfacial joints in-between filament layers. To promote the widespread adoption of 3DCP, the elastic response and strength parameters of 3DCP elements must be fully comprehended. Numerous attempts have been made to improve the mechanical properties of 3DCP using fibre reinforcement. At the root of all mechanical properties are the microstructural morphology and constituent characteristics of the mixture. It has been shown that the 3DCP process influences the ensuing microstructural morphology, and it is known that constituent type, constituent proportion, porosity content and void topology affect a porous medium's bulk strength and elasticity. Therefore, the correlation between such observations warrants additional investigation concerning the influence of porosity on the anisotropic mechanical properties of 3DCP components. In this research, the effects of porosity metrics such as total defect content; 3D void topology (shape, size, and orientation); pore spatial, size and compactness distributions; and interconnectivity are quantitively investigated for a fibre-reinforced printable concrete (FRPC) mixture via X-ray computed tomography and related to the elasticity, compressive strength and observed fracture patterns of mould-cast and concrete printed specimens consisting of the same fibre reinforced printable concrete matrix. The experimental findings indicate higher porosity, on average and at interlayer locations, in 3DCP samples as well as alterations in the spatial and topological attributes of voids in 3DCP samples. Furthermore, it is shown that both reductions in the elastic modulus and compressive capacity can be physically linked to the porosity metrics detected in 3DCP elements. Elasticity is dominated by porosity content and compressive capacity by the interrelation between porosity content, loading direction relative to the interlayer orientation, stress concentrations at void boundaries, and the increased deformability of the composite configuration. From the consideration and interpretation of these aspects, the anisotropic mechanical response of 3DCP samples is elucidated, and potential remedies are proposed.

Analysis of optimum stacking sequence of GFRP composite laminate under axial loading condition

Composites' popularity/use in the aerospace, automobile, and defense industries has lately expanded because to its cheaper production costs, light weight, higher fracture toughness, low thermal expansion, corrosion resistance, and better control over thermo-mechanical properties. Because of their fiber orientations, FRC have a strong anisotropic mechanical response. Present work is aimed in shaping the most favorable stacking sequence of the GFRP composite laminate under axial loading situations; these orientations create a variety of failure modes, which are more complex. The studies used MATLAB and simulation methods (ANSYS) to evaluate stress, strain, and deformation values in order to better understands and predict the behavior of GFRP Composite laminates with various unidirectional fiber orientations under plane stress conditions.

Halogen Bonded Network Modulating the Mechanical Property Elastic and Plastic Bending in Nonconventional Molecular Solid Solutions

Anisotropic mechanical response of a material to an applied external stress results in bending of organic crystals in particular directions. This phenomenon is essentially dictated by intermolecular interactions. In this work, we have tactically designed solid solutions of two non-isostructural molecular crystalline phases, 4-bromo-3-chlorophenol (4BR, I41/a) and 3-bromo-4-chlorophenol (3BR, P21/c)─an exception to the Kitaigorodsky rule. Single crystals of 4BR show elastic bending, whereas 3BR crystals are brittle in nature. The solid solutions of 4BR and 3BR in 1:1 and 1:2 stoichiometric ratios attain a unique solid solution crystal structure (P21/c, Z′ = 2). In response to mechanical stimuli, we observed elastic bending in 1:1 solid solution crystals and plastic bending in 1:2 solid soltion crystals. A detailed investigation combining laboratory single crystal X-ray diffraction, powder X-ray diffraction, thermal analysis, and periodic DFT calculations reveals intriguing structural features of these solid solutions and explores the role of halogen bonding in modulating the mechanical property from elastic (1:1) to plastic (1:2) with stoichiometric variation.

Anisotropic Mechanical Response and Strain Localization of a Metallic Glassy-Fiber-Reinforced Polyethylene Terephthalate Fabric.

Optimizing the mechanical properties of composites through microstructural design has been a long-standing issue in materials science. In this study, we reinforced a typical polymer, i.e., polyethylene-terephthalate-woven fabric, with a type of Fe-based metallic glassy fiber (MGF) with an extremely large Young’s moduli. The MGF-reinforced fabrics, with three different fiber bundle orientations (0°, 45°, and 90°), were investigated by in situ electron-microscopy mechanical testing techniques in conjunction with a digital image correlation (DIC) technique. The fabrics exhibited a pronounced anisotropic mechanical response, and the associated characteristics were verified to depend on the fiber bundle orientation relative to the external load. Furthermore, localized strains near the intersections of the fiber bundles were found to be much higher than the global strain. It is confirmed that the restriction from warp to weft is the dominant factor influencing strain localization during deformation. Our results are enlightening for understanding the fracture mechanisms of composites.

Influence of dislocations on the apparent elastic constants in single metallic crystallites: an analytical approach

Intricate knowledge of dislocation networks in metals has proven paramount in understanding the constitutive behaviour of these materials but current experimental methods yield limited information on the characteristics of these networks. Recently, the isotropic anelastic response of metals has been used to investigate complex dislocation networks through the well-known phenomenon that the observed elastic constants are influenced by dislocations. Considering the dependence of the behaviour of a Frank-Read (FR) source on its initial dislocation character and using discerning characteristics of dislocations, i.e. Burgers vector, line sense and slip system, the present paper takes dislocation character, crystal structure and dislocation network geometry into account and obtains the anisotropic mechanical response for a generic Poisson’s ratio. In this work, the tensile test tangent moduli and yield points are presented for spatially uniform and nonuniform dislocation distributions across slip systems. First, the reversible shear strain of the FR source is derived as a function of initial dislocation character. The area swept by a mobile and initially straight dislocation segment pinned at both ends is given as an explicit function of the line stress. Secondly, the anisotropic anelastic strain contribution of FR sources to the total pre- and at-yield strain in single crystallites is calculated. For a given normal stress and superposition of the principal infinitesimal linear elastic lattice strain and anelastic dislocation strain, the tangent moduli are presented. The moduli and the inception of plastic flow have a notable dependence on initial dislocation character, spatial dislocation distribution and loading direction.

Microscale compressive behavior of hydrated lamellar bone at high strain rates

The increased risk of fracture in the elderly associated with metabolic conditions like osteoporosis poses a significant strain on health care systems worldwide. Due to bone's hierarchical nature, it is necessary to study its mechanical properties and failure mechanisms at several length scales. We conducted micropillar compression experiments on ovine cortical bone to assess the anisotropic mechanical response at the lamellar scale over a wide range of strain rates (10−4 to 8·102 s−1). At the microscale, lamellar bone exhibits a strain rate sensitivity similar to what is reported at the macroscale suggesting that it is an intrinsic property of the extracellular matrix. Significant shear band thickening was observed at high strain rates by HRSEM and STEM imaging. This is likely caused by the material's inability to accommodate the imposed deformation by propagation of thin kink bands and shear cracks at high strain rates, leading to shear band thickening and nucleation. The post-yield behavior is strain rate and direction dependent: hardening was observed for transverse oriented micropillars and hardening modulus increases with strain rate by a factor of almost 2, while axially oriented micropillars showed strain softening and an increase of the softening peak width and work to ultimate stress as a function of strain rate. This suggests that for compression at the micrometer scale, energy absorption in bone increases with strain rate. This study highlights the importance of investigating bone strength and post-yield behavior at lower length scales, under hydrated conditions and at clinically relevant strain rates. Statement of significanceWe performed micropillar compression experiments of ovine cortical bone at two different orientations and over seven orders of magnitude of strain rate. Experiments were performed under humid condition to mimic the natural conditions of bone in a human body using a newly developed micro-indenter setup. The strain rate sensitivity was found to be of a similar magnitude to what has been reported for higher length scales, suggesting that the strain rate sensitivity is an intrinsic property of the bone extracellular matrix. In addition, localized shear deformation in thick bands was observed for the first time at high strain rates, highlighting the importance of investigating bone under conditions representative of an accident or fall at several length scales.

Ab initio study of anisotropic mechanical and electronic properties of strained carbon-nitride nanosheet with interlayer bonding

Due to the noticeable structural similarity and being neighborhood in periodic table of group-IV and -V elemental monolayers, whether the combination of group-IV and -V elements could have stable nanosheet structures with optimistic properties has attracted great research interest. In this work, we performed first-principles simulations to investigate the elastic, vibrational and electronic properties of the carbon nitride (CN) nanosheet in the puckered honeycomb structure with covalent interlayer bonding. It has been demonstrated that the structural stability of CN nanosheet is essentially maintained by the strong interlayer σ bonding between adjacent carbon atoms in the opposite atomic layers. A negative Poisson’s ratio in the out-of-plane direction under biaxial deformation, and the extreme in-plane stiffness of CN nanosheet, only slightly inferior to the monolayer graphene, are revealed. Moreover, the highly anisotropic mechanical and electronic response of CN nanosheet to tensile strain have been explored.

Phase Transformation and Incompatibility at Grain Boundaries in Zirconia-Based Shape Memory Ceramics: A Micromechanics-Based Simulation Study

Zirconia-based shape memory ceramics (SMCs) exhibit anisotropic mechanical response when undergoing elastic deformations as well as during austenite-martensite phase transformation. This behavior results in different types of strain incompatibility at grain boundaries, which we study here using a micromechanical model. A single-crystal model is developed to provide a full mechanistic three-dimensional description of the anisotropic elastic as well as martensitic transformation stress-strain response, including non-Schmid behavior caused by the significant volume change during martensitic transformation. This model was calibrated to and validated against compression tests of single-crystal zirconia micro-pillars, prior to being used to model bi-crystals. Upon the introduction of a grain boundary, the simulation provides detailed information on the nucleation and evolution of martensite variants and stress distribution at grain boundaries. We identify bicrystal configurations which result in very large stress concentrations at very low deformations due to elastic incompatibility, as well as others where the elastic incompatibility is relatively low and stress concentrations only occur at large transformation strains. We also show how this approach can be used to explore the misorientation space for quantifying the level of elastic and transformation incompatibility at SMCs grain boundaries.

A continuum model and simulations for large deformation of anisotropic fiber–matrix composites for cardiac tissue engineering

Cardiac patch therapies promise to restore heart function and lower the risk of heart failure after heart attack. Fiber–matrix engineered tissue scaffolds have gained significant attention due to their tunable micro-structures, providing nonlinear mechanical properties similar to native anisotropic heart tissues. Mechanical properties of engineered scaffolds directly affect the stress fields generated inside and around the tissue scaffolds and have significant impact on the tissue functionality. Currently, biomedical cardiac patches are designed through experimentation and there exists a need for an accurate model that will allow micro-structural design optimization and analysis of effectiveness of the implanted patches. We have developed a three-dimensional large strain continuum model that can predict nonlinear, anisotropic mechanical response of engineered tissue scaffolds that have two orientation families of fibers inside a bulk hydrogel matrix. We have validated the predictive capability of our continuum model for the fiber–matrix composite using selected experiments and a suite of detailed finite element analysis that incorporated the micro-structural details of the composites. Comparing the continuum model predictions (1 element) against the representative volume micro-structural geometry finite element simulations (with greater than 4,00,000 elements), we show that the proposed model can accurately predict nonlinear mechanical behavior of highly anisotropic tissue scaffolds in both the longitudinal and transverse directions, as a function of the critical design parameters inter-fiber angle and fiber spacing. We show that the model can also capture native heart tissue’s anisotropic large strain mechanical response. We implemented our model in the finite element software Abaqus by writing a user material subroutine UANISOHYPER and demonstrated its predictive abilities by conducting a full three-dimensional analysis of engineered tissue patch application on an infarcted heart.

Effect of size and orientation on stability of dislocation networks upon torsion loading and unloading in FCC metallic micropillars

At the continuum length scale, mechanical properties of metals show relatively weak orientation dependence; however, they exhibit strong anisotropic behaviors as the size of sample decreases to micron and nanometer length scales. In this study, three-dimensional dislocation dynamics (DD) simulations are performed to investigate the orientation-dependent plasticity in submicron face-centered cubic (FCC) micropillars subjected to torsion. Accommodating results from atomistic modeling, updated surface nucleation schemes in DD models have been developed for three orientations ([001], [101], and [111]), allowing investigation of the dislocation microstructure evolution and the corresponding anisotropic mechanical response upon torsional loading and unloading. The DD simulation results show that the coaxial and hexagonal networks formed in [101] and [111] oriented nanopillars, respectively, exhibited excellent plastic recovery, while the rectangular network formed in the [001] crystal orientation was more stable and did not experience as much plastic recovery.

A combined experimental and modeling study revealing the anisotropic mechanical response of Ti2AlN MAX phase

Ti2AlN MAX phase with a hexagonal crystal structure exhibits great potential as structural material for operation under harsh environments due to its excellent mechanical performance. For a reliable application, a comprehensive understanding of the mechanical behavior, and in particular of the anisotropic properties is needed. Thus, in this study, we combined nanoindentation and electron-backscatter diffraction experiments to correlate elastic modulus and hardness of Ti2AlN to the crystallographic orientation. We used two different modeling approaches to better understand, validate, and in the long run to predict the anisotropic mechanical behavior of MAX phase materials. While we observed consistent trends in both experiments and modeling, elastic modulus and hardness showed different dependencies on the crystal orientation.

Anisotropic and size-dependent mechanical responses of free-standing Ni-Nb metallic glass thin film

Via in-situ tensile test, we demonstrated a brittle-to-ductile transition of Ni-Nb metallic glass (MG) thin films when the film thickness decreased from 363 nm to 87 nm. The elastic strain limit of 4.7% and apparent work hardening behavior were seen for free-standing 87 nm-thick film. Contrary to quenched MGs, anisotropic mechanical response existed in MG thin films with nanocolumnar structure owing to the deteriorated bonding strength of column boundaries.

Magnetic field-structuring as versatile approach to shape the anisotropic mechanical response of composite foams

The use of a high amount of reinforcing phase improves the mechanical performance of a polymeric composite structure. In composite foams this approach is associated to difficulties in developing a regular cellular structure, due to the hindering effect of such high amount of non polymeric phase on the bubble growth process. Additionally, the compressive response of foams produced with conventional forming processes is isotropic, and cannot fully exploit the reinforcing potential of the reinforcement. To overcome this limitation, the reinforcing phase should be aligned along preferential directions but available industrial methods are costly post-processing procedures that can induce damages in the cellular structure. Recently, several research proposals investigated the use of a contactless magnetic field to build reinforcing structures in polymeric foams, but they were mainly focused on the linear elastic region of the compressive response, while the large strain behaviour was not under control. In this paper, a process based on a time-variable magnetic field is exploited to manage in a simple and fully adaptable way the shape of the stress-strain response of the foam (up to at least 60% strain) during the foaming process. The resulting composite foam is characterized by mechanical anisotropy with a homogeneous and regular cellular morphology. This goal is achieved by imparting suitable geometrical features to the field-structured reinforcing aggregates by means of the proper time-profile and intensity of a magnetic field during the production process.