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by Steven Nutt
Journal of Composite Materials
Lattice materials can be designed through their microstructure while concurrently considering fabrication feasibility. Here, we propose two types of composite lattice materials with enhanced resistance to buckling: (a) hollow lattice materials fabricated by a newly developed bottom-up assembly technique and the previously developed thermal expansion molding technique and (b) hierarchical lattice materials with foam core sandwich trusses fabricated by interlocking assembly process. The mechanical performance of sandwich structures featuring the two types of lattice cores was tested and analyzed theoretically. For hollow lattice core material, samples from two different fabrication processes were compared and both failed by nodal rupture or debonding. In contrast, hierarchical lattice structures failed by shear buckling without interfacial failure in the sandwich struts. Calculations using established analytical models indicated that the shear strength of hollow lattice cores could be...
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2014, Materials and Design
2011, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
Cellular materials with hollow lattice truss topologies exhibit higher compressive strengths than equivalent structures with solid trusses owing to their greater resistance to plastic buckling. Consequently, hollow trusses have attracted interest as the cores for sandwich panels. Finite-element calculations are used to investigate the elastic–plastic compressive collapse of a metallic sandwich core made from vertical or inclined circular tubes, made from annealed AISI 304 stainless steel. First, the dependence of the axial compressive collapse mode upon tube geometry is determined for vertical tubes with built-in ends and is displayed in the form of a collapse mechanism map. Second, the approach is extended to inclined circular hollow tubes arranged as a pyramidal lattice core; the collapse modes are identified and the peak compressive strength is determined as a function of geometry. For a given relative density of hollow pyramidal core, the inclined tube geometry that maximizes pe...
2003, Composites Science and Technology
Composite Structures
Demand has been growing for structural systems utilizing new materials that are more durable and require less maintenance during the service lifetime. In particular, sandwich composite structures attract attention due to many advantages such as light weight, high strength, corrosion resistance, durability and speedy construction. In this study, three designs of glass reinforced composite sandwich structures, namely boxes (web-core W1), trapezoid and polyurethane rigid foam, are fabricated using new generation of two-part thermoset polyurethane resin systems as matrix materials with vacuum assisted resin transfer molding (VARTM) process. The stiffness, load-carrying capacity and compressive strength were evaluated. Core shear, flatwise and edgewise compression tests were carried out for these three models. The mechanical response of three designs of sandwich structures under flexural loading were analyzed using commercial finite element method (FEM) software ABAQUS. The simulation re...
2014, Energy and Buildings
2014, Composite Structures
2019, Composite Structures
A full mechanical characterisation of three types of 3-D printed lattice cores was performed to evaluate the feasibility of using additive manufacturing (AM) of lightweight polymer-based sandwich panels for structural applications. Effects of the shape of three selected lattice structures on the compression, shear and bending strength has been experimentally investigated. The specimens tested were manufactured with an open source fused filament fabrication-based 3-D printer. These sandwich structures considered had skins made of polypropylene (PP)-flax bonded to the polylactic acid (PLA) lattice structure core using bi-component epoxy adhesive. The PP-flax and the PLA core structures were tested separately as well as bonded together to evaluate the structural performance as sandwich panels. The compression tests were carried out to assess the in-plane and out of plane stiffness and strength by selecting a representative number of cells. Shear band and plastic hinges were observed during the in-plane tests. The shear and three-point bending tests were performed according to the standard to ensure repeatability. The work has provided an insight into the failure modes of the different shapes, and the force-displacement history curves were linked to the progressive failure mechanisms experienced by the structures. Overall, the results of the three truss-like lattice biopolymer non-stochastic structures investigated have indicated that they are well suited to be used for potential impact applications because of their high-shear and out of the plane compression strength. These results demonstrate the feasibility of AM technology in manufacturing of lightweight polymer-based sandwich panels for potential structural uses.
2011, International Journal of Solids and Structures
2014, Experimental Mechanics, 55(1): 27-40
There is a great deal of interest in understanding the mechanics in pin-reinforced composite sandwich structures. In particular, characterizing the relationship of the mechanics in smaller to larger specimens due to changes in the pin response can expedite the development of more advanced models, as well as the assessment of new materials and processing conditions. In this investigation, specimens with a conventional symmetric pin configuration anchoring into each facesheet have been experimentally characterized under compressive loading. The effect of specimen size on the compressive response has been investigated through the number of active pins under loading. The stress–strain response of these structures exhibit initially high compressive stiffness followed by a significant load shedding and flow stress. The transition in the mechanical behavior is attributed to the failure of reinforced pins under a combination of axial compression and transverse bending due to their orientation relative to the facesheets. As the size of a specimen is decreased, the effective stiffness and strength decrease due to a decrease in effective pin stiffness. It was found that these effects on the compressive stress can be scaled through a stress scaling factor derived from the change in the structure of the specimen due to the number of active versus inactive pins per unit of area, which is scaled by the pin density related to the pin basis and spacing for the corresponding pin configuration. Using Digital Image Correlation (DIC), it was then possible to account for these size effects on compressive strain directly through the pin deformations. A stiffness scaling factor could then be determined from a model of a beam in bending under axial and transverse loading that was correlated to the DIC displacements to determine the effective pin stiffness and corresponding stiffness scaling factor for strains. A stretched exponential relationship was also found to describe the scaling of the effective pin stiffness with the number of active pins. The compressive stress–strain response was then corrected to show that similar scaled properties could be obtained for the compressive modulus. The Poisson’s ratio was also obtained from the DIC strains averaged over the full specimen, and a scaling relation similar to that for the effective pin stiffness was developed.
Natural structural materials which feature hierarchical architectures, like bone and glass sponge skeletons, often display remarkable mechanical properties. Employing the principle of hierarchy can create self-similar architected metamaterials across multiple length-scales, but the strengthening mechanisms remain to be fully understood. In the present study, self-similar hierarchical octet-truss lattice materials were fabricated via additive manufacturing and deformed in uniaxial compression. Experimental results indicated that the mechanical properties of such hierarchical lattice materials were not determined by relative density, unlike those of non-hierarchical ones, but varied with strut slenderness ratios in the two hierarchical levels. In terms of specific strength and stiffness, hierarchical architected structures do not necessarily outperform non-hierarchical structures. To explain the underlying mechanisms of these phenomena, analytical models considering effects of complex nodal microstructure were established. The upper and lower bounds of strength for the hierarchical lattice materials were deduced and compared with that for the non-hierarchical materials; these comparisons suggested that the hierarchical construction could be used to access unique mechanical properties that are unachievable in traditional materials. Additional levels of hierarchy beyond the second order could be similarly analyzed. This study discerns how hierarchical architecture can be used to access the unique properties of lattice materials, provides insight into the role of design in regulating the mechanical properties of such mechanical metamaterials.
2021, Journal of the Mechanics and Physics of Solids
Nature's materials are generally hybrid composites with superior mechanical properties achieved through delicate architectural designs. Inspired by the precipitation hardening mechanisms observed in biological materials as well as engineering alloys, we develop here dual-phase mechanical metamaterial composites by employing architected lattice materials as the constituent matrix and reinforcement phases. The composite metamaterials made from austenitic stainless steel are simply fabricated using selected laser melting based additive manufacturing. Using quasi-static compression tests and simulation studies, we find that strength and toughness can be simultaneously enhanced with the addition of reinforcement phase grains. Effects of reinforcement phase patterning and connectivity are examined. By fully utilizing the energy dissipation from phase-boundary slip, an optimized dual-phase metamaterial is designed with the maximum slip area, where every truss unit in the matrix phase is completely surrounded by reinforcement phase lattices; this material exhibits a specific energy absorption capability that is ~2.5 times that of the constituent matrix phase lattices. The design rationale for dissipative dual-phase meta-materials is analyzed and summarized with a focus on phase pattering. The present digital multi-phase mechanical metamaterials can emulate almost any of nature's architectures and toughening mechanisms, offering a novel pathway to manipulate mechanical properties through arbitrary phase-material selection and patterning. We believe that this could markedly expand the design space for the development of future materials.
2016
mainly due to the fact that they are light weight, have a better strength to weight ratio and to an extent corrosion resistant. Each application might involve different fabrication technique and selection of appropriate foam core and facings. In this study, three different core densities of PU foam combined with glass fiber facings are fabricated using vacuum bagging technique. Because of their complex nature of fabrication by adding epoxy resin between facings and core makes the evaluation of their mechanical characteristics quite difficult to predict. Since core material being the weakest part of the sandwich composite, studies are carried out to determine the effect of change in mechanical properties by increasing the core thickness and density. Further, the mechanical response of these sandwich composites are studied by carrying out flatwise, edgewise compressive test and flexural strength on a Mecmesin MultiTest 10 –i testing equipment as per ASTM standards. It is found that wi...
2015, Eksploatacja i Niezawodnosc - Maintenance and Reliability
2012, Engineering Structures
2012, EPJ Web of …
2000, Composites Part A-applied Science and Manufacturing
This paper deals with an innovative integrated hollow (space) E-glass/epoxy core sandwich composite construction that possesses several multi-functional benefits in addition to the providing lightweight and bending stiffness advantages. In comparison with traditional foam and honeycomb cores, the integrated space core provides a means to route wires/rods, embed electronic assemblies, and store fuel and fire-retardant foam, among other conceivable benefits. In the current work, the low-velocity impact (LVI) response of innovative integrated sandwich core composites was investigated. Three thicknesses of integrated and functionality-embedded E-glass/epoxy sandwich cores were considered in this study—including 6, 9 and 17 mm. The low-velocity impact results indicated that the hollow and functionality-embedded integrated core suffered a localized damage state limited to a system of core members in the vicinity of the impact. The peak forces attained under static compression and LVI were in accordance with Euler's column buckling equation. Stacking of the core was an effective way of improving functionality and limiting the LVI damage in the sandwich plate. The functionality-embedded cores provided enhanced LVI resistance due to energy additional energy absorption mechanisms.
Future lightweight military vehicles will demand increasingly mass-efficient structures to satisfy design requirements such as increased mobility and survivability. One possible family of lightweight structures entails sandwich panels consisting of solid facesheets and a low-density core. The sandwich structures can provide both structural strength and stiffness, with load-bearing potential. In this study, sandwich panels consisting of metallic facesheets and a pyramidal truss core manufactured from either 6061-T6 aluminum or 316L stainless steel are investigated. The structures were subjected to panel bending and in-plane compression testing to explore the effects of relative core density and process parameters. The mechanical response and failure mechanisms were categorized for the different structures and materials. The processing challenges associated with fabricating larger panels are also discussed.
2001, International Journal of Solids and Structures
Journal of Composites Science
Composite panels with polyurethane (PU) foam-core and facing materials, such as gypsum, engineered wood or some composite materials, are being used as structural members in building construction. This paper reviews and summarises major research developments, and provides an updated review of references on the structural performance of foam-filled building composite panels from 1998 to 2017. The review revealed that previous studies on the structural performance of foam-filled building composite panels could be categorised into five themes; namely, energy absorption and dynamic behaviour; bending and shear behaviour, edgewise and flatwise compressive/tensile behaviour; delamination/deboning issues; and finally some miscellaneous issues. These categories comprise approximately 30%, 40%, 11%, 11% and 8% of related studies over the last two decades, respectively. Also, over the past five years, the number of relevant studies has increased by ~400% relative to the previous similar period...
2019, Engineering Journal
2019, Composite Structures
This study investigates the impact response and damage mechanisms of composite sandwich structures in arctic condition. Carbon fiber reinforced composite sandwich panels with polyvinyl chloride (PVC) foam core are subjected to low-velocity impact at extreme low temperatures, representative of the harsh arctic environmental condition. Force-time history curves evidently show that test temperature has a significant influence on the impact damage behavior. Specimens impacted at extreme low temperature (−70 °C) exhibit less strength, and higher susceptibility to damage, resulting in severe penetration by the impactor. X-ray micro-computed tomo-graphy technique is employed to reveal multiple complex impact damage modes. Specifically, results from this work elucidate arctic temperature influence on detrimental failure mechanisms: large facesheet-core debonding, extensive composite facesheet delamination, significant core shearing and crushing, and severe facesheet fiber fracture. This work provides an important fundamental knowledge for future design of naval composite sandwich structures with enhanced impact performance at low-temperature arctic condition.
2019, International Journal of Solids and Structures
The present work investigates, using a corotational finite element formulation considering large displacements and rotations, geometric imperfections and an elasto-plastic material constitutive law, the nonlin-ear behavior of a pyramidal space truss under vertical and horizontal loads. Additionally novel analytical closed-form solutions for the nonlinear equilibrium paths, natural frequencies and critical parameters are derived for comparison purposes and investigation of the influence of geometric and material parameters on the truss stability. This geometry has an immediate practical interest since these structures are currently used in many present-day applications, either as main structural component or as a constitu-tive element. Four types of nonlinear instability phenomena are investigated theoretically and numerically: saddle-node (limit point) bifurcation, pitchfork bifurcation, individual Eulerian bars buckling and elasto-plastic buckling. Through a detailed parametric analysis the interaction between these buckling phenomena, using the FE tool, is investigated. In addition, the effect of elastic supports is considered. The influence of these phenomena on the load carrying capacity of the structure and its imperfection sensitivity is duly discussed in the present work. Finally, a detailed parametric analysis of a large roof composed of pyramidal truss units is conducted and the influence of the geometric parameters on its bistable behavior and load carrying capacity is investigated.
Journal of Sandwich Structures & Materials
Buckling and crushing behavior of foam-core hybrid composite sandwich columns under edgewise compressive load is dealt in this study. Composite laminates with different stacking sequence configurations made of glass and Dyneema-woven fabrics and AL 2024-T3 sheets were used in combination of polyvinyl chloride foam core to manufacture the specimens. Effects of face sheet thickness and stacking sequence configuration, slenderness ratio, boundary conditions, and sandwich reinforcement with through-thickness resin pins on the buckling and crushing behavior of the specimens were investigated. The results revealed that using the resin pins changes the unstable Euler buckling mode to a more stable progressive end-crushing and significantly increases the buckling load, specific buckling load, and energy absorption capability, which are highly favorable. Also, the results showed that in the specimens with fiber metal laminates, the major failure modes are face sheet-core debonding and face s...
2018
In this study, a new core design is introduced for sandwich composite structures. Its strength and failure behavior are investigated via three-point bending tests. E-glass-fiber-reinforced epoxy resin is selected as the material for both the core and the face sheets. The core has an egg-crate shape. Acoustic emission (AE) method is used to detect the progression of damage. Signals due to elastic waves caused by activated damage mechanisms are investigated in order to identify the corresponding failure modes. A finite element model of the sandwich structure is developed to predict the failure behavior of the specimens under the loading conditions in the tests. A promising agreement between the results of the finite element model and the experiments is observed. The force-deflection-relation, the failure load as well as the region where damage initiates are accurately predicted.
2011, Materials & Design
2005, Applied Composite Materials
2010, Applied Composite …
2001, Composite structures
2017, Thin-Walled Structures
Sandwich beams offer designers a number of advantages, as the high strength to weight ratio, flexibility, high bending and buckling resistances. Sandwich construction results higher natural frequencies than none sandwich constructions, also it developed an adaptive tuned vibration absorber. In the present work, the natural frequencies and mode shapes of the sandwich beam structure are calculated under different boundary conditions. Three models are created using MSC-PATRAN/NASTRAN software, 1D beam, 2D shell and 3D solid. The results for AL solid beam, CPVC solid beam, and AL-CPVC sandwich construction beam were obtained and compared with the analytical results. The results show a good agreement between the finite element models and analytical models for AL solid beam, CPVC solid beam with less than 2% error. For AL-CPVC sandwich construction beam the analytical solution is over predict the natural frequencies with 27% for the first mode and increases with increasing the number of modes to reach 40% at the fourth mode. The effect of material application and the boundary conditions are studied.
2019
We consider a sandwich plate with face sheets made of unidirectional fiber-reinforced composite with fibers being either glass, or carbon or aramid and the core made of balsa wood loaded by a blast pressure, and find optimal geometries and materials for maximizing the first failure load. While analyzing the problem, we assume that the areal density is fixed and use the Nest-Site Selection optimization algorithm, a third-order shear and normal deformable plate theory, a one-step stress recovery scheme, and the Tsai-Wu failure criterion. We also delineate the effect on the first failure load of inertia forces and uncertainties in values of various parameters. For a sandwich plate optimally designed for the first failure load, we find the ultimate load by progressively degrading elasticities of failed elements. We find that the optimal single-core sandwich designs are symmetric about the mid-surface with thick face sheets and the optimal two-core sandwich designs have a thin middle fac...
2015, Composites Part B: Engineering
Materials
Structural systems developed from novel materials that are more durable and less prone to maintenance during the service lifetime are in great demand. Due to many advantages such as being lightweight as well as having high strength, corrosion resistance, and durability, the sandwich composites structures, in particular, have attracted attention as favorable materials for speedy and durable structural constructions. In the present research, an experimental investigation is carried out to investigate the flexural response of sandwich beams with a pre-cracked core-upper facesheet interface located at one end of the beam. During the development of the sandwich beams, an initial pre-cracked debond was created between the core and facesheet by placing a Teflon sheet at the interface. Both three-point and four-point flexural tests were conducted to characterize the flexural behavior of the sandwich beams. The effects of the loading rate, core thickness, and placement of the initial interfa...
Applied Sciences
This paper presents a novel meta-functional auxetic unit (MFAU) cell designed to improve performance and weight ratio for structural bridge bearing applications. Numerical investigations were conducted using three-dimensional finite element models validated by experimental results. The validated models were exposed to compression and buckling actions to identify structural failure modes, with special attention placed on the global behaviours of the meta-functional auxetic (MFA) composite bridge bearing. This bearing uses an unprecedented auxetic sandwich core design consisting of multiple MFAU cells. Numerical predictions of the elastic local critical buckling loads of the MFAU cell were in excellent agreement with both the analytical and experimental results, with an observed discrepancy of less than 1%. These results demonstrate that local buckling failures of MFAU cells can potentially be incurred prior to yielding under compression due to their slenderness ratios. Surprisingly, ...
Polymers
This work focuses on the manufacturing and characterization of highly environmentally friendly lightweight sandwich structures based on polylactide (PLA) honeycomb cores and PLA-flax fabric laminate skins or facings. PLA honeycombs were manufactured using PLA sheets with different thicknesses ranging from 50 to 500 μm. The PLA sheets were shaped into semi-hexagonal profiles by hot-compression molding. After this stage, the different semi-hexagonal sheets were bonded together to give hexagonal panels. The skins were manufactured by hot-compression molding by stacking two Biotex flax/PLA fabrics with 40 wt% PLA fibers. The combined use of temperature (200 °C), pressure, and time (2 min) allowed PLA fibers to melt, flow, and fully embed the flax fabrics, thus leading to thin composite laminates to be used as skins. Sandwich structures were finally obtained by bonding the PLA honeycomb core with the PLA-flax skins using an epoxy adhesive. A thin PLA nonwoven was previously attached to t...
2015, Composite Structures