Anisotropic Mechanical Properties Research Articles

An anisotropic elasto-viscoplastic-damage model for Selective Laser Sintered food

Selective Laser Sintered food exhibits anisotropic mechanical properties that are characteristic for its layer-by-layer manufacturing process. A constitutive model that is capable of describing the anisotropic elasto-viscoplastic-damaging behavior of the material, which was experimentally characterized by compression testing in two distinct principal orientations, is presented. Damage is modeled by applying the effective stress concept in combination with a viscoplastic flow rule that effectively regularizes damage localization. The model allows for quantitative predictions of the mechanical response of the printed food and is a valuable tool for assessing texture properties of food with Selective Laser Sintering.

Highly Anisotropic Thermal Conductivity of Three-Dimensional Printed Boron Nitride-Filled Thermoplastic Polyurethane Composites: Effects of Size, Orientation, Viscosity, and Voids.

Extrusion-based three-dimensional (3D) printing techniques usually exhibit anisotropic thermal, mechanical, and electric properties due to the shearing-induced alignment during extrusion. However, the transformation from the extrusion to stacking process is always neglected and its influence on the final properties remains ambiguous. In this work, we adopt two different sized boron nitride (BN) sheets, namely, small-sized BN (S-BN) and large-sized BN (L-BN), to explore their impact on the orientation degree, morphology, and final anisotropic thermal conductivity (TC) of thermoplastic polyurethane (TPU) composites by fused deposition modeling. The transformation from one-dimensional axial alignment in the extruded filament to two-dimensional alignment (horizontal and vertical alignment) in the stacking filament of BN sheets is observed, and its impact on anisotropic TC in three directions is clarified. It is found that L-BN/TPU composites show a high TC of 6.45 W m-1 K-1 at 60 wt % BN content along the printing direction, while at a lower content (<40 wt %), S-BN/TPU composites exhibit a higher TC than L-BN/TPU composites. Effects of orientation, viscosity, and voids are comprehensively considered to elucidate such differences. Finally, heat dissipation tests demonstrate the great potential of 3D printed BN/TPU composites to be used in thermal management applications.

A Numerical Study of Hydraulic Fracture Propagation Geometry in a Layered Shale Reservoir

An unconventional shale reservoir commonly develops multiple layers and shows strong anisotropy in mechanical properties, which has a great effect on hydraulic fracture propagation geometry. The most common mechanical properties are elastic modulus, Poisson’s ratio, tensile strength, and formation permeability. Therefore, the extended finite element method- (XFEM-) based cohesive zone model (CZM) is applied to analyze the effect of these mechanical properties on both the fracture propagation geometry and breakdown pressure in a layered shale reservoir after verifying the present numerical method by published analytical results. The parametric analysis indicates that the stiff or soft outer layers limit the fracture propagation width, while promoting the fracture propagation length. Higher Poisson’s ratio and formation permeability in outer layers narrow the fracture propagation width in middle layer. Poisson’s ratio contrast between different layers almost has no significant effect on the fracture propagation length and breakdown pressure. The hydraulic fracture propagation geometry presents the trend of “shorter and wider” with the increase of the tensile strength in outer layer. For asymmetric specimen with different mechanical parameters in each layer, hydraulic fracture shows an asymmetric propagation behavior, and hydraulic fracture preferentially propagates to the layer with higher elastic modulus, Poisson’s ratio, formation permeability, and low resistance.

Measuring mechanical anisotropy of the cornea with Brillouin microscopy

Load-bearing tissues are typically fortified by networks of protein fibers, often with preferential orientations. This fiber structure imparts the tissues with direction-dependent mechanical properties optimized to support specific external loads. To accurately model and predict tissues’ mechanical response, it is essential to characterize the anisotropy on a microstructural scale. Previously, it has been difficult to measure the mechanical properties of intact tissues noninvasively. Here, we use Brillouin optical microscopy to visualize and quantify the anisotropic mechanical properties of corneal tissues at different length scales. We derive the stiffness tensor for a lamellar network of collagen fibrils and use angle-resolved Brillouin measurements to determine the longitudinal stiffness coefficients (longitudinal moduli) describing the ex vivo porcine cornea as a transverse isotropic material. Lastly, we observe significant mechanical anisotropy of the human cornea in vivo, highlighting the potential for clinical applications of off-axis Brillouin microscopy.

Effect of Al–5Ti–1B grain refiner on microstructure and properties of arc-additive-manufactured Al–Mg alloy

In this study, a strip-shaped Al–5Ti–1B grain refiner was employed to produce Al–Mg alloy using cold metal transfer arc additive manufacturing. The microstructure and mechanical properties of the straight-wall samples (SWSs) were analyzed. The results indicated that the Al3Ti phase occurred in SWS with Al–5Ti–1B, and acted as heterogeneous nucleation sites during solidification. Formation of Al3Ti particles led to the transformation of the columnar grains between the layers into equiaxed grains and refined the grains at the center of the layers. Furthermore, the Al3Ti particles increased the fraction of low-angle grain boundaries by 8.2%. In addition, the anisotropy in mechanical properties (i.e. tensile strength and elongation) was reduced significantly. Also, the low hardness zone between the layers was eliminated. Thereby, improving the mechanical properties of the additive-manufactured Al–Mg alloy samples.

Microstructural and Hardness Behavior of H13 Tool Steel Manufactured by Ultrasound-Assisted Laser-Directed Energy Deposition

Metal additive manufacturing (AM) by Laser-Directed Energy Deposition (L-DED) usually results in the formation of textured columnar grains along the build direction, leading to anisotropic mechanical properties. This can negatively affect the intended application of the product. Anisotropy can be eliminated by modifying the material through an additional exposure to ultrasound (US-assisted) during the L-DED process. In this paper, a multi-track sample was manufactured from AISI H13 (TLS Technik, Bitterfeld-Wolfen, Germany) tool steel by a US-assisted (28 kHz) L-DED process using a specially designed cooling system. The study also included post-process annealing and quenching with the tempering heat treatment of the modified steel, resulting in the retention of the properties, as confirmed by hardness measurements. XRD analysis was used to measure the structural parameters of the unit cell, and the hardness properties were measured in two directions: longitudinally and parallel to the deposition direction. It was found that US-assisted L-DED allows us to obtain a more isotropic structure with an equal size of the coherent scattering region in two printing directions, and to reduce the residual stresses in the material. The anisotropy of the hardness was significantly reduced, with 636 and 640 HV found between the XY and XZ planes. Based on the obtained hardness data, it should be noted that some of the heat treatments studied herein can also result in a decrease in the anisotropy of the properties, similarly to the US-assisted effect.

Microstructure characterization and mechanical behaviour of laser additive manufactured ultrahigh-strength M54 steel

Ultra high-strength M54 steel blocks were fabricated by laser metal deposition. The microstructure and mechanical behavior of the material were investigated systematically. The microstructure of the as-deposited M54 steel is anisotropic; the cross-section (XOY plane) has a cellular structure, whereas the longitudinal section (XOZ and YOZ planes) shows a mixture of alternating cellular and columnar forms. Compositional segregation is present at the cell walls (interdendritic regions) in the as-deposited state, resulting in retained austenite at the cell walls. The cross-sectional XOY plane contains 10.08% austenite, whereas the XOZ and YOZ planes contain 24.59% and 22.4% austenite, respectively. The retained austenite at the cell wall (interdendritic region) has low thermal and mechanical stability and disappears after the cryogenic treatment or is transformed into martensite during a tensile test. The as-deposited samples show anisotropic mechanical properties. The transverse samples exhibit stronger transformation-induced plasticity (TRIP) and work hardenability with a lower yield strength of 662 MPa and higher ultimate strength of 1982 MPa, corresponding to a higher amount of retained austenite in this direction. The longitudinal ultimate strength and yield strength are 1832 MPa and 997 MPa, respectively. The ductility and toughness are also largely anisotropic, and their reduction in the transverse direction is only 1/3 of that in the longitudinal direction. The Vickers hardness of the microstructure increases slightly from the bottom to the middle and upper part of the sample due to less thermal cycling in the upper part.

Determination of in-plane anisotropy of rolled sheets, taking into account the effect of strain intensity

The results of experimental studies of the mechanical properties anisotropy of rolled sheets made of NKh260YD steel are presented. Tension was applied to flat specimens cut in directions of 0, 45, and 90 degrees with respect to the rolling direction. A dividing grid was applied to the samples with a diamond indenter in the area of the estimated length on an instrumental microscope. Based on the results of measurements of the samples linear dimensions before and after deformation in the places where the dividing grid was applied, the logarithmic deformations were calculated over the width and thickness of the sample in each of the considered sections. The results of measurements and calculations showed the uneven intensity of deformations in its various sections. For further processing, data in sections under conditions of uniform deformation were used. The anisotropy coefficients were determined as the ratio of logarithmic strains over the width and thickness of the sample. The tension was carried out in three stages, which made it possible to establish the dependences of the anisotropy coefficients on the strain intensity, which were used to assess the degree of in-plane anisotropy of the material under study. Graphic dependences are presented illustrating the nature of the change in the anisotropy coeffi cients and the degree of anisotropy of the material under study depending on the intensity of deformation. A significant dependence of the HX260YD steel planar anisotropy degree on the magnitude of the strain intensity is shown, which must be taken into account when developing technological processes for production of critical articles from anisotropic sheet material by plastic shaping operations.

Anisotropy of microstructures and mechanical properties in FeCoNiCr0.5 high-entropy alloy prepared via selective laser melting

Anisotropy of microstructures and mechanical properties in FeCoNiCr0.5 high-entropy alloy prepared via selective laser melting

A study on anisotropy in wire arc additively manufactured Inconel 625 multi-layered wall and its correlation with molten pool thermal history

In the present study, the variation in molten pool thermal history with layer number during wire arc additive manufacturing of Inconel 625 wall was investigated. Further, its effect on the evolution of microstructure and anisotropy in mechanical and corrosion properties was reported. During the deposition process, the molten pool thermal history was monitored using a non-contact type IR pyrometer operating at 1.6 μm wavelength. A total number of 40 layers were deposited, building a wall of 60 mm height. With an increase in layer number, the molten pool lifetime and cooling rate were found to increase and decrease, respectively, resulting in coarse grains and increased elemental segregation or Laves phase formation. To investigate its effect on mechanical properties, the samples were collected in a skewed fashion along the height with orientation in the deposition direction. The tensile specimens collected close to the substrate exhibited better strength and ductility, while the samples from the top location of the wall exhibited a relatively brittle mode of fracture, which was investigated by carrying out the fracture surface analysis using SEM. Corrosion test was also conducted along the height of the wall, wherein the samples close to substrate exhibited better corrosion resistance due to refined microstructure and low elemental segregations. Further, EDS, XRD analysis and hardness test were carried out to investigate the elemental composition, variation in phases and hardness with layer number, respectively.

Digital Light Processing 3D Printing of Enhanced Polymers via Interlayer Welding.

Digital light processing (DLP) 3D printing is advantageous in high printing efficiency and printing resolution for fabricating complex structures across various applications. However, the layer-by-layer curing manner of DLP leads to weak interlayer adhesion and the anisotropic mechanical properties of printed objects. Here, linear polymers are introduced into commercial resins to weld the interlayer by the diffusion and entanglement of linear polymers after DLP printing via heat treatment. This introduction of linear polymers not only shows a strengthening and toughening effect on the printed objects, but also has no negative impact on the DLP printability. The tensile strengths of objects containing 4.7wt% polycaprolactone can reach up to ≈500% of that of neat samples in any printing direction. This simple strategy by adding linear polymers into printing resins provides an effective access to prepare DLP printed objects with improved mechanical properties as well as ensure printing resolution and printing efficiency.

Influence of anisotropy in mechanical properties of muddy aquatic sediment on CH4 bubble growth direction and migration pattern

Gas-charged sediments of shallow water bodies are significant sources of atmospheric methane (CH4), an important greenhouse gas. They are a source of a permanent concern for their contribution to destabilization of coastal and aquatic infrastructure. Past accounts of gas bubbles developed in shallow muddy aquatic sediments and in their surrogates have reported a controversial occurrence of vertical as well as horizontal bubbles morphologies. Our study suggests an explanation of the apparent conundrum about preferred orientations of bubbles in muddy sediments. It is conducted by employing mechanical reaction–transport numerical model, which couples CH4 diffusion-led expansion of gas bubble and elastic-fracturing mechanical sediment response to its growth. Muddy sediment is assumed to exhibit a transverse anisotropy in fracture toughness (describing an easiness of breaking the inter particle bonds), attributed to partial or full alignment of plate-like clay particles. Bubbles growing in isotropic sediment develop a vertically oriented morphology and start their ascent once reaching their mature sizes. Under an increasing measure of anisotropy, the bubbles grow preferentially horizontally. These bubbles can coalesce with neighboring ones and form interconnected permeable horizontal gas networks, as observed in some lab experiments. For the first time, our results suggest that anisotropy-led lateral bubble growth can also play a crucial role in accumulating gas reserve from long distances around large and small scale outlets in aquatic sediments. Moreover, in contrast to the vertical bubbles, horizontal bubbles tend to be stationary, thus being responsible for high gas storage capacity of anisotropic sediments.

Geomechanics evolution integrated with hydraulic fractures, heterogeneity and anisotropy during shale gas depletion

Due to reservoir depletion, the reservoir geomechanical parameters, such as stresses, permeability, are changing during shale gas production. A coupled flow-geomechanics model was built to investigate time-lapse geomechanical responses in production before infill well placement of Sichuan Basin shale gas reservoir. In the modelling, hydraulic fractures were simulated from microseismic data and mapped from a geological model to build the flow model. An interface code was improved to build the geomechanical model properties from the geological model, as well as communicates the data between finite difference grids in the reservoir simulator and finite element grids in the geomechanical simulator. In addition, the anisotropy of physical properties and rock mechanical properties was integrated in the flow model and the geomechanical model. Stress-dependent porosity and permeability models were embedded as well. Case studies show that stress evolution and re-orientation are affected by both the trajectory and fractures of producing wells, as well as the heterogeneity of geomechanical properties. Besides, the effect of parallel wells production should be taken into consideration based on the wells-spacing.

Revealing the Anisotropic Structural and Electrical Stabilities of 2D SnSe under Harsh Environments: Alkaline Environment and Mechanical Strain.

As a promising thermoelectric and semiconducting material, the stability of two-dimensional tin selenide (SnSe) under harsh environments is significant for its practical applications. Here, focusing on the key procedures in the device fabrication process, we report the anisotropic structural and electrical stabilities of SnSe under an alkaline environment and mechanical strain. Due to the anisotropic mechanical properties, the SnSe flakes can naturally form long-straight {011} edge planes during the mechanical exfoliation process. Such a cleavage tendency provides an effective crystal orientation identification method to uncover the orientation-dependent properties. We find that the single-crystalline SnSe flakes experience an anisotropic degradation process with the preferable {011} dissolution planes in the alkaline environment and can be gradually transformed to be polycrystalline consisting of SnSe2, Sn, and Se nanocrystals. SnSe flakes present an anisotropic electromechanical response with a gauge factor value that reaches ∼-460 under the uniaxial strain along the ⟨011⟩ directions. Our revealed structural and electrical stability of SnSe under harsh environments can provide guidance for the device design, fabrication, and performance evaluation.

A Rough Discrete Fracture Network Model for Geometrical Modeling of Jointed Rock Masses and the Anisotropic Behaviour

The geometry of the joint determines the mechanical properties of the rock mass and is one of the key factors affecting the failure mode of surrounding rock masses. In this paper, a new rough discrete fractures network (RDFN) characterization method based on the Fourier transform method was proposed. The unified characterization of the complex geometric fracture network was achieved by changing the different Fourier series values, which further improved the characterization method of the RDFN model. A discrete element numerical calculation model of the complex RDFN model was established by combining MATLAB with PFC code. Numerical simulation of the anisotropic mechanical properties was performed for the RDFN model with a complex joint network. Based on the results, the geometry of the joint network has a significant influence on the strength and failure patterns of jointed rock masses. The failure modes of the opening are highly affected by the orientation of the fracture sets. The existence of the rough fracture sets could influence the failure area of different excavation situations. The study findings provide a new characterization method for the RDFN model and a new characterization approach for stability analysis of complex jointed rock masses.

Programmable Anisotropic Hydrogel Composites for Soft Bioelectronics.

Fabrication of hydrogel composites embedded with aligned 1D nanoparticles has shown substantial growth over the past 5 years. Direct ink printing technology (DIW) has been used in this work to create the alignment of the 1D nanoparticles due to the shear gradient of the pseudoplastic precursor (2-hydroxyethyl methacrylate (HEMA) with thickening agents). Orderly distributed 1D particles constructing anisotropic nanostructures endow the hydrogel composite with unique mechanical, electric, or electromechanical coupling properties. Quasi-static uniaxial tensile test, electric resistivity, and piezoresistivity measurements have been conducted for investigating the mechanical, electric, and electromechanical coupling properties of the hydrogel composites, respectively. Based on the experimental results, it can be speculated that the developed printing process is able to fabricate hydrogel composites with programmable anisotropic mechanical, electric, and electromechanical properties. The products pumped out from this work have the potential of being substrates for soft devices and may have a great impact on the fields of flexible bioelectronics.

Investigation of anisotropic mechanical, electronic, and charge carrier transport properties of germanium-pnictogen monolayers

Recently, novel two-dimensional (2D) GeP and GeAs systems have been fabricated by mechanical exfoliation and utilized in various applications. These developments have brought the 2D germanium-pnictogens, C2/m-GeX (X = N, P, As, Sb, and Bi) structures into the limelight. In this study, we systematically investigate the structural, mechanical, electronic, and charge carrier transport properties of GeX monolayers by using first-principles methods. Our results show that the considered systems are dynamically stable and possess anisotropic physical properties. Examined structures are found to be flexible, and their mechanical strength and stiffness decrease down the group-V, in line with the trends of the bond strength, cohesive energy, charge transfer, and electron localization function. Additionally, the zigzag in-plane direction is mechanically superior to the armchair direction. The electronic band structure calculations based on HSE06 hybrid functional with the inclusion of spin–orbit coupling indicate that GeX monolayers are either direct or quasi-direct semiconductors with band gaps lying within the infrared and visible spectrum. The estimated charge carrier mobilities are highly anisotropic and also differ significantly depending on the structure and carrier type. These unique properties render GeX monolayers as suitable 2D materials for flexible nanoelectronic applications.

Alleviating plastic anisotropy of boron modified titanium alloy by constructing layered structure via electron beam directed energy deposition

It is widely known that the coarse columnar grains and grain boundary α phases are inevitably formed during the wire-based additive manufacturing (AM) process, which usually lead to the performance anisotropy and deterioration of mechanical properties. In this study, a layered structure was built via alternately depositing Ti-6Al-4 V layers and boron-modified Ti-6Al-4 V layers based on dual-wire electron beam directed energy deposition (EB-DED). It was demonstrated that the anisotropy of mechanical properties effectively was reduced. The tensile test results showed that the anisotropy of ultimate tensile strength and fracture elongation of the layered structure were relatively lower compared with those reported for the homogeneous titanium alloys prepared with the same process. Quasi-isotropic mechanical properties were obtained for the as built layered structures. Additionally, the products with layered structures indicated a good match between tensile strength and plasticity. This work provided a new perspective for the exploration of preparing titanium alloy with isotropic high performance by AM, and may further broaden the application range of additive manufacturing titanium alloys.

Cellular Microbiaxial Stretching Assay for Measurement and Characterization of the Anisotropic Mechanical Properties of Micropatterned Cells.

Characterizing the mechanical properties of single cells is important for developing descriptive models of tissue mechanics and improving the understanding of mechanically driven cell processes. Standard methods for measuring single-cell mechanical properties typically provide isotropic mechanical descriptions. However, many cells exhibit specialized geometries in vivo, with anisotropic cytoskeletal architectures reflective of their function, and are exposed to dynamic multiaxial loads, raising the need for more complete descriptions of their anisotropic mechanical properties under complex deformations. Here, we describe the cellular microbiaxial stretching (CμBS) assay in which controlled deformations are applied to micropatterned cells while simultaneously measuring cell stress. CμBS utilizes a set of linear actuators to apply tensile or compressive, short- or long-term deformations to cells micropatterned on a fluorescent bead-doped polyacrylamide gel. Using traction force microscopy principles and the known geometry of the cell and the mechanical properties of the underlying gel, we calculate the stress within the cell to formulate stress-strain curves that can be further used to create mechanical descriptions of the cells, such as strain energy density functions. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Assembly of CμBS stretching constructs Basic Protocol 2: Polymerization of micropatterned, bead-doped polyacrylamide gel on an elastomer membrane Support Protocol: Cell culture and seeding onto CμBS constructs Basic Protocol 3: Implementing CμBS stretching protocols and traction force microscopy Basic Protocol 4: Data analysis and cell stress measurements.

Post-build thermomechanical processing of wire arc additively manufactured stainless steel for improved mechanical properties and reduction of crystallographic texture

Wire-arc additive manufacturing (WAAM) was used to fabricate 304 L stainless steel plates for the purpose of characterizing texture and anisotropic behavior in the as-built parts, followed by post-build rolling and heat-treating to modify mechanical properties and reduce texture. The plates were made using a robotic controlled gas metal arc (GMA) welding system designed for high deposition rates and were characterized using uniaxial tensile testing, macro- and micro-hardness, and optical microscopy. In addition, resonant ultrasound spectroscopy (RUS) was used for the first time of WAAM plates to measure the anisotropic elastic modulus, shear modulus and Poisson’s ratio. Post processing consisted of mechanical rolling the walls to 25% and 40% strains followed by heat treating at 650 °C, 850 °C and 1050 °C. Results show that anisotropy of mechanical and physical properties is present in the starting microstructure and persists in the rolled plates prior to heat treating, with elastic modulus in the build direction of 50% of the polycrystalline wrought value, and shear modulus and Poisson’s ratio of 200% of the polycrystalline value. Anisotropy also persists for all samples stress relieved at 650 °C, and in samples annealed at 1050 °C without any prior rolling strain. Heat treating has shown the reduction in texture and anisotropy only in the rolled plates. Complete elimination of texture and anisotropy was observed for WAAM plates rolled and annealed at 1050⁰C, and for plates rolled to 40% and recrystallized at 850 °C. Recrystallization of the rolled plates at temperatures 850 °C or higher increases yield strength, ultimate strength and elongation to failure relative to the as-built parts, with properties that equal or exceed typical wrought or cast alloys of similar composition. These results demonstrate that WAAM can be used to build custom sized semi-finished parts and blanks that can later be mechanically formed, and heat treated into high quality finished components.