In-plane Compression Research Articles

Pressure Dependence of Intra- and Interlayer Excitons in 2H-MoS2 Bilayers.

The optical and electronic properties of multilayer transition metal dichalcogenides differ significantly from their monolayer counterparts due to interlayer interactions. The separation of individual layers can be tuned in a controlled way by applying pressure. Here, we use a diamond anvil cell to compress bilayers of 2H-MoS2 in the gigapascal range. By measuring optical transmission spectra, we find that increasing pressure leads to a decrease in the energy splitting between the A and the interlayer exciton. Comparing our experimental findings with ab initio calculations, we conclude that the observed changes are not due to the commonly assumed hydrostatic compression. This effect is attributed to the MoS2 bilayer adhering to the diamond, which reduces the in-plane compression. Moreover, we demonstrate that the distinct real-space distributions and resulting contributions from the valence band account for the different pressure dependencies of the inter- and intralayer excitons in compressed MoS2 bilayers.

Numerical modelling of DMLS Ti6Al4V(ELI) polygon structures

Numerical modelling is particularly advantageous for analysing structures with complex behaviour. It is used to predict the mechanical properties of structures. Analytical modelling, on the contrary, has limited capacity for predicting the behaviour, particularly of structures, because it is based on mathematical equations that do not always exactly represent the geometry of the model. In such cases, numerical modelling is used for predicting structural bending, axial deformation, and buckling behaviour. This study documents numerical modelling of different types of polygon structures. To reduce computation costs, planar and extruded Ti6Al4V(ELI) hexagonal shell structures were used to predict stresses in the out-of-plane and in-plane directions. This was followed by numerical modelling of different types of planar polygon structures to predict their load-bearing capacity and stiffness. Thereafter, the hexagonal polygon was subjected to out-of-plane and in-plane uniaxial compression loads. This was done to compare the bending and buckling behaviour of finite element (FE) models to analytical models. The numerical and analytical results were then compared to determine how the ratio (t/L) of the wall thickness (t) and length of the polygon members (L) influenced the effective stiffness of the hexagonal polygon. The triangular polygon was seen to have the greatest load-bearing capacity and stiffness of all polygons that were modelled. The hexagonal model was observed to generate deformations due to compression, similar to those reported in literature. The critical buckling loads for the analytical honeycomb (HC) models were found to be below the yield stress for (1-, 1.125-, and 1.25-mm wall thicknesses) and above the yield stress for all FE HC models, respectively. The effective stiffness of the HC models were observed to increase with the increasing (t/L) ratio, for both the numerical and analytical models.

Laminar hypersonic boundary layer flow over planar compression ramp with sharp leading edge and upstream influence

The present work aims to study the effects of shock wave — boundary layer interaction in 2D hypersonic flow at the sharp leading edge of a flat plate and a flat plate followed by a compression ramp. A key parameter in this work is the hypersonic interaction parameter χ, defined as χ≡CM∞3/Re∞1/2. This flow is modeled as a perfect gas and its behavior is studied over various compression ramp configurations, ranging from 0°–6°, using a semi-analytic reduced-order model. In this shock-layer analysis we focus on characterizing the viscous boundary layer for an adiabatic flow over an insulated wall, improving on existing semi-analytic methods by obtaining a non-singular solution, and using the Tangent Wedge approximation to match the pressure on the boundary layer edge. Upstream influence resulting from the compression ramp is also investigated with newly developed correlations.

Numerical study on hybrid metallic sandwich structures subjected to air burst

This study presents a numerical investigation on the use of previously unexplored hybrid sandwich protective structural configurations with aim to resist airblast loading. A honeycomb, corrugated sheet, and woven interlaced core configurations are used to develop the hybrid system. An experimental result reported in the previous literature is used for validation of the finite element analysis models using Abaqus/Explicit finite element software with a conventional weapon blast related parametric codes. Then an extended numerical study is conducted further on various hybrid sandwich inner core topologies for maximizing protection against an air blast induced shock wave. The study demonstrated that a protective system, combining square honeycomb metallic cores with X-shaped corrugated sheets and fortified against in-plane compression and buckling, showed excellent shielding capabilities against airblast loading. This configuration also exhibited the highest energy dissipation, the least effective plastic strain, minimal back sheet displacement, and a favorable damage profile.

Geometrical imperfection sensitivity on in-plane compressive modulus and strength of honeycomb core and deviations from isotropy

Honeycomb core sandwich may suffer from face/core debonding. Candidate debond fracture test specimens such as End-Notch Flexure (ENF) and Mixed Mode Bending (MMB) contain unbonded regions of the sandwich where the core is required to carry in-plane com-pressive load. In-plane modulus and compressive strength of honeycomb core are not pro-vided by core producers and must be estimated from unit cell models assuming perfect hexagonal cells. In this study it is shown that geometrical imperfections, such as deviations in the angle between cell walls and curved cell walls have a strong influence on modulus and strength because of deviations from transverse isotropy. A range of Nomex honeycomb cores were tested under in-plane compression. Modulus and strength ratios ( L/W) were found to vary from 0.3 to 3.24 and 0.96 to 1.8, respectively. Analysis based on unit cell models verified that deviations of the inclined cell wall angle, θ, from 30 ◦ is a major cause for orthotropy. Rounded corners and curved walls were analyzed using finite element analysis. Both principal elastic moduli of the core ( E L and E W) were reduced by the curvature while the Poisson’s ratios remained largely unaffected. Implications of the results for design of MMB and ENF fracture test specimens are discussed.

Postbuckling Strengths of Composite Laminates with a Central Circular/Elliptical Cutout Under In-Plane Combined Loads

This paper deals with the effect of cutouts (i.e., circular and elliptical) aspect ratio on the buckling and postbuckling strengths, and failure characteristics of a simply-supported quasi-isotropic composite laminate subjected to in-plane shear and uni-axial compression combined with in-plane shear loads. The present study is carried out using self developed finite element program. The finite element formulation is based on the first order shear deformation theory and von Karmans assumptions to incorporate geometric nonlinearity. The resulting nonlinear algebraic equations are solved by Newton-Raphson method. The postbuckling strength of the laminate is defined in terms of the first-ply failure load of the lamina predicted by tensor polynomial form of the 3-D Tsai-Hill criterion. It is observed that for maximum buckling and first-ply failure strengths of the laminate with a cutout, it is preferred to have elliptical shape of the cutout as compared to the circular shape of the same area. It is also found that irrespective of load conditions and cutout shapes and sizes, shear load direction has significant effects on buckling and postbuckling stiffness of the laminate; although buckling stiffness is greater under negative shear, but the laminate shows stiffer postbuckling response under positive shear load.

Study on Mechanical Properties of Carbon Nanotube Reinforced Composites.

In this study, carbon fiber composite laminates were modified by carbon nanotube films. In-plane and out-of-plane compression tests were carried out in a wide strain rate range (10-3-103/s). Results display that the out-of-plane compressive properties are improved by CNT interlaminar toughening because CNT can hinder the propagation of interlayer cracks; however, the dynamic in-plane compression performance is decreased due to the lack of resin in CNT film that leads to delamination inside of CNT film in advance. To optimize the material preparation process, two methods were used to prepare the mode I fracture test: (a) curing the prepreg by autoclave process; and (b) curing of resin preform by vacuum resin-transmitted molding (VARTM). Results showed that CNT prolonged the crack propagation path and improved the interlaminar fracture properties when the preform was infiltrated with resin and cured by VARTM. In addition, it was found that the interlaminar thickness was almost linear with the number of CNT layers.

A Poisson's ratio sign-switching mechanical metamaterial with tunable stiffness

An innovative Poisson's ratio (PR) sign-switching stiffness-changing mechanical metamaterial with potential applicability in automotive and construction industries is presented. The metamaterial that is designed by modifying and combining hexagonal and re-entrant unit cells is fabricated via additive manufacturing and explored using experiments and finite element analysis (FEA). The metamaterial shows excellent specific energy absorption (SEA) properties in comparison with the hexagonal lattice structure, exhibiting approximately 25% higher SEA. Parametric studies reveal that the auxeticity of the metamaterial and its PR sign-changing properties can be tuned by modifying the design parameters. Also, the load-carrying capacity of the metamaterial can be tuned along with its stiffness-changing properties by modifying the unit cell parameters. The geometric parameters of the internal concave unit cells significantly affect the auxeticity of the structure and the strains in which PR sign-switching and effective Young's modulus change take place. Finally, an enhanced form of the metamaterial is presented and analyzed via FEA, showing superior effective Young's modulus change ratio and improved stability under in-plane compression. The parametric studies show that the proposed metamaterial exhibits stiffness-changing property under transverse compression and PR sign-switching behaviour under transverse tension.

Impact of inverse unequal height strut structure on the functional performance of an additively manufactured cardiovascular stent

Recently, additive manufacturing (AM) has been investigated as an innovative method to manufacture stents due to its capability in producing complex and customized structures. In this paper, the cardiovascular stents of M-type and N-type with inverse unequal height strut structure and N-type with equal height strut structure were designed and manufactured by Selective Laser Melting (SLM). Following surface polishing, balloon expansion, plane compression and three-point bending experiments were carried out to evaluate the mechanical performance of the stent. The stents designed with inverse unequal height strut structure showed higher radial support performance and lower radial recoil when compared to the stents with uniform design. This study proved the feasibility of SLM in rapid manufacturing of cardiovascular stents that can be used for performance evaluation in design stage.

Nanofiber Interleaved Composites for Aerospace Applications

This paper extended the authors previous work of demonstrating the concept of polymer nanofiber interleaving to enhance the toughness of fiber reinforced carbon/epoxy composite laminates. In this study, in-plane tension, compression, interlaminar shear, and Mode I fracture and fatigue onset life properties were studied. Results demonstrated that by adding 1% weight of nanofibers in the form of interlayer in-plane tension and compression properties (strength, modulus, and Poisson’s ratio) of interleaved composites remained the same as base composites. Whereas polymer nanofiber interleaving increased interlaminar shear strength by 4%, fracture toughness by 150% and fatigue threshold energy release rate by 67%

Numerical Analysis of the Impact Resistance of Composite A-Shaped Sandwich Structures.

This paper focuses on the finite element analysis simulation of the impact properties of composite sandwich structures made of carbon fiber-reinforced polymer lamina. In the existing studies, the composite sandwich structures with A-shaped cores have superior mechanical properties under quasi-static plane compression loads compared to W-shaped, Y-shaped, and X-shaped cores. However, there is limited research on the impact resistance of this structure. This paper studied the resistance of a composite A-shaped core structure to ballistic impact. Using ABAQUS/explicit finite element analysis software, ballistic impact tests for the composite A-shaped core structure were simulated based on the Hashin and Yeh failure criteria with a progressive damage model introduced in the user-defined subroutine VUMAT. First, the composite Y-shaped core sandwich structure was verified via experiments and simulations to determine the accuracy of the method, and then the composite A-shaped sandwich structure was subjected to a series of ballistic impact simulations. With varied impact velocity, the damage to the front and rear face sheet and cores via ballistic loads was simulated to illustrate the overall dynamic response process of the sandwich structure. Subsequently, a curve was fitted using a ballistic limit velocity equation, which was used as the criterion to evaluate the impact resistance of the composite A-shaped core structure. The results showed that, under the same relative density and the same number of component layers, the ballistic limit velocity of the composite A-shaped core sandwich structure was bigger than the composite Y-shaped core sandwich structure. The composite A-shaped core structure had 12.23% higher ballistic limit velocity than the composite Y-shaped core, indicating the impact resistance capabilities of the A-shaped core structure. In addition, the impact location's effect on the impact response was investigated.

Experimental and numerical evaluation of tessellated lattice structure under inplane compression loading

The lattice structures are conformable and adaptable technology for saving weight of the components and energy absorption applications. In present work, three different basic lattices known as cubic(C), body centered cubic (B) and face centered cubic (F) lattices are considered and arranged in the layered pattern to withstand the in-plane quasi static compression in-plane loading for energy absorption ap-plication. The lattice structures are maintained constant volume and developed through tessellation. These structures are produced by VAT polymerization 3D printer and allowed to test under quasi static compression in-plane loading. The characterization of structures has been evaluated through stress strain distribution and crushing behavior to asses and analysis the energy absorption, axial collapse and failure modes. Moreover, the impact of each basic lattice behavior of a tessellated structure and its role in enhancing the mechanical properties has been studied under quasi-static in-plane loading. The validation of experimental results has been verified through ANSYS numerical simulation results. the behavior of each lattice and configuration effect of tessellation are studied with the experimentation results and validated through numerical simulation.

Investigations on the quasi-static/dynamic mechanical properties of 3D printed random honeycombs under in-plane compression

The aim of this study was to reveal the effects of inertia and base material strain rate on the mechanical properties of 3D-printed random honeycombs. First, random honeycombs were established with various meso-structural parameters based on the Voronoi technique and then manufactured by 3D printing. Quasi-static compression tests were performed using a universal test machine. The results indicated that the random honeycombs exhibited a more uniform deformation mode which led to a gentle stress–strain curve when compared with the regular honeycombs. Furthermore, the strain rate effect of the base material was determined by Split Hopkinson Pressure Bar (SHPB) tests. Finite element models considering the base material strain rate were verified by the quasi-static compression and drop weight impact tests. Dynamic simulations were carried out to determine the critical velocity between the deformation mode and clarify its mechanisms. The second critical velocities increase as the cell size and cell wall thickness increased or as the strain rate effect of the base material is considered. The effects of inertia and base material strain rate on the mechanical properties were discussed. The dynamic stress enhancement is dominated by the strain rate effect of the base material and the inertia effect in the random and shock modes, respectively. Finally, an empirical model that includes the inertia effect and strain rate effect of the base material was proposed based on the 1D shock wave theory and the Johnson–Cook material model.

Damage sensitivity studies of composite honeycomb sandwich structures under in-plane compression and 4-point bending: Experiments and numerical simulations

Honeycomb sandwich structures, especially those with carbon-fiber panels and Nomex-honeycomb cores, are widely employed in automobiles and aircraft due to their excellent mechanical properties. Here, in-plane compression and 4-point bending experiments, as well as FEM simulations, are performed for a composite honeycomb sandwich structure with face-core de-bonding defects or impact damage to evaluate the damage sensitivity. It is found that the structure exhibits a higher sensitivity to impact damage than to de-bonding defects. Impact damage caused by an energy of 3 J–5.5 J can alter the failure mode under both in-plane compression and 4-point bending loads, and reduce in-plane compression strength by more than 50 % and 4-point bending strength by 15 %. In comparison, de-bonding defects of the same size can change the failure mode and decrease the strength by only 10 % under in-plane compression. The developed FEM model predicts all failure modes and residual strengths obtained from experiments. The simulation results show that the structural characteristics of producing concaves in the facesheet and core are the dominant contributors to structural failure. Our model, covering all geometric and structural details of the honeycomb core, provides a useful tool for studying the mechanical behaviors of such structures.

Mechanical and energy absorption properties of 3D-printed honeycomb structures with Voronoi tessellations

3D printing technology is the new frontier in building construction. It is especially useful for making small structures within a short period. Full construction, including interior partitions and exterior façades, can be achieved with this technology. This paper proposes a parametric Voronoi tessellations model for quickly generating and fabricating 3D-printed hexagonal honeycomb partitions for interior design. Comprehensive experimental testing was conducted to characterize the mechanical properties and investigate the energy absorption characteristics of the proposed 3D-printed hexagonal honeycomb while comparing it to alternative hexagonal honeycomb structures. The tests included tensile testing (ASTM-D638) of the printed Polylactic Acid (PLA) material, especially with the almost total absence of conducted research that reported mechanical properties for 3D printed material with low infill percentages such as 10%. In addition, an in-plane quasi-static axial compression testing of the lightweight honeycomb structures was also conducted on the printed structure with the same low infill percentage. Compared to non-Voronoi honeycomb structures, the Voronoi honeycomb resulted in superior mechanical and energy absorption properties with energy absorption values ranging from 350 to 435 J and crash force efficiency being 1.42 to 1.65.

Application of column buckling theory to steel aluminium foam sandwich panels

In steel structures, a lot of attention is paid to lightweight structures, i.e. reduction of dead load without compromising structural safety, integrity and performance. Thanks to modern steel aluminium foam sandwich panel manufacturing technology a new possibility became available for lightweight structural design. Assessment and understanding of the behaviour of this sandwich panel under in-plane compression or flexure is crucial before its application in steel structures. Column buckling theory is considered and applied to the steel aluminium foam sandwich panel to evaluate its behaviour under in-plane compressive load. In this work, various assumptions are made to generalise Euler’s buckling formula. The generalisation requires modification of the buckling stiffness expression to account for sandwich panel composite properties. The modified analytical expression is verified with finite element simulation employing various material models specific to steel face-plates and aluminium foam as well as various geometric imperfections. Based on this study, it can be concluded that Euler’s buckling formula can be successfully modified and used in the prediction of the load-carrying capacity of a sandwich panel.

Effect of forced assembly on bearing performance of single-lap, countersunk composite bolted joints – Part Ⅱ: Numerical investigation

A progressive damage model with the combination of 3D Hashin criterion and exponential material degradation was developed and validated according to the experimental results of the Part Ⅰ. The effect of forced assembly on stress distribution and bearing damage of single-lap, countersunk composite bolted joints were revealed by utilizing the present model. It is found that the joints with forced assembly show higher contact stress levels. The stress distribution of the countersunk hole tends to be uniform, while that of the non-countersunk hole is more uneven, and the maximum contact stress occurs at the non-countersunk hole. Furthermore, the forced assembly made a bolt sustain additional axial tensile load, introducing high failure risks to the bolts. With forced assembly, the fiber compressive damage and in-plane matrix compressive damage are more serious, and the most serious damage is transferred from the countersunk hole to the non-countersunk hole.

Buckletronics: how compression-induced buckling affects the mechanical and electronic properties of sp2-based two-dimensional materials.

We explore the mechanical and electronic response of sp2-based two-dimensional materials under in-plane compression employing first principles density functional theory-based calculations. Taking two carbon-based graphynes (α-graphyne and γ-graphyne) as example systems, we show that the structures of both two-dimensional materials are susceptible to out-of-plane buckling, which emerges for modest in-plane biaxial compression (1.5-2%). Out-of-plane buckling is found to be more energetically stable than in-plane scaling/distortion and significantly lowers the in-plane stiffness of both graphenes. The buckling also gives rise to in-plane auxetic behaviour in both two-dimensional materials. Under compression, the induced in-plane distortions and out-of-plane buckling also lead to modulations of the electronic band gap. Our work highlights the possibility of using in-plane compression to induce out-of-plane buckling in, otherwise planar, sp2-based two-dimensional materials (e.g. graphynes, graphdiynes). We suggest that controllable compression-induced buckling in planar two-dimensional materials (as opposed to two-dimensional materials, which are buckled due to sp3 hybridization) could provide a route to a new 'buckletronics' approach for tuning the mechanical and electronic properties of sp2-based systems. This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

Investigation of energy absorption performances of a 3D printed fiber-reinforced bio-inspired cellular structure under in-plane compression loading

This article proposes glass-fiber-reinforced bone-inspired cellular structures to enhance energy absorption capability. The elastic modulus of the bone-inspired unit cell is obtained analytically based on the energy method and then employed in Particle Swarm Optimization algorithm to get optimized cellular structures. In the optimized cellular structure, the stiffness is optimized and the energy absorption capacity is investigated. A Fused Filament Fabrication 3D printing process is used to fabricate the cellular structures with continuous glass fiber-reinforced polylactic acid (PLA). In-plane compression tests are performed to investigate the mechanical performance of cellular structures. Finite Element Modeling (FEM) is conducted to analyzed the mechanical performance of the structures. In FEM, the failure criterion is determined using the maximum stress and VUSDFLD subroutine, and the damage growth is modeled by decreasing the mechanical properties. A good agreement between numerical and experimental results was observed. Results demonstrated that the energy absorption in glass-fiber-reinforced PLA is ∼250% higher than in the un-reinforced structure. The optimized cellular structure exhibits a stable prolonged plateau stress region and very high specific energy absorption parameters.

Mössbauer and X-ray Studies of Radiation-Induced Processes in Nb-Zr Alloys Implanted with 57Fe Ions.

The effect of implanting 57Fe ions on the crystal structure of Nb-Zr alloys has been studied using Mössbauer spectroscopy on 57Fe nuclei and X-ray diffraction. As a result of implantation, a metastable structure was formed in the Nb-Zr alloy. The XRD data indicated a decrease in the crystal lattice parameter of niobium; that is, there was a compression of the niobium planes when implanted with iron ions. Mössbauer spectroscopy revealed three states of iron. The singlet indicated a supersaturated Nb(Fe) solid solution; the doublets characterized the diffusion migration of atomic planes and crystallization of voids. It was shown that the values of the isomer shifts in all three states did not depend on the implantation energy, which indicates the invariance of the electron density on the 57Fe nuclei in the studied samples. The resonance lines of the Mössbauer spectra were significantly broadened, which is typical for materials with low crystallinity and a metastable structure that is stable at room temperature. The paper discusses the mechanism of radiation-induced and thermal transformations in the Nb-Zr alloy, which leads to the formation of a stable well-crystallized structure. A Fe2Nb intermetallic compound and the Nb(Fe) solid solution formed in its near-surface layer, while Nb(Zr) remained in the bulk.