Mechanical behavior of the sandwich structures with carbon fiber-reinforced pyramidal lattice truss core

Sandwich panel construction with carbon fiber-reinforced pyramidal lattice truss is attracting more and more attention due to its superior mechanical properties and multi-functional applications. Pyramidal lattice truss sandwich panels made from carbon fiber reinforced composites materials are manufactured by hot-pressing. The facesheets are interconnected with truss cores, the facesheets and truss cores are manufactured in one manufacturing process without bonding. The buckling and splitting of truss member is observed in the compressive and shear tests and no nodal failure is observed. The predicted results show that the mechanical behavior of the pyramidal lattice truss core sandwich panels depends on the relative density of core and the material properties of truss members.     

Mechanical response of metallic honeycomb sandwich panel structures to high-intensity dynamic loading

Explosive tests were performed in air to study the dynamic mechanical response of square honeycomb core sandwich panels made from a super-austenitic stainless steel alloy. Tests were conducted at three levels of impulse load on the sandwich panels and solid plates with the same areal density. Impulse was varied by changing the charge weight of the explosive at a constant standoff distance. At the lowest intensity load, significant front face bending and progressive cell wall buckling were observed at the center of the panel closest to the explosion source. Cell wall buckling and core densification increased as the impulse increased. An air blast simulation code was used to determine the blast loads at the front surfaces of the test panels, and these were used as inputs to finite element calculations of the dynamic response of the sandwich structure. Very good agreement was observed between the finite element model predictions of the sandwich panel front and back face displacements and the experimental observations. The model also captured many of the phenomenological details of the core deformation behavior. The honeycomb sandwich panels suffered significantly smaller back face deflections than solid plates of identical mass even though their design was far from optimal for such an application.     

Response of metallic pyramidal lattice core sandwich panels to high intensity impulsive loading in air

Small scale explosive loading of sandwich panels with low relative density pyramidal lattice cores has been used to study the large scale bending and fracture response of a model sandwich panel system in which the core has little stretch resistance. The panels were made from a ductile stainless steel and the practical consequence of reducing the sandwich panel face sheet thickness to induce a recently predicted beneficial fluid-structure interaction (FSI) effect was investigated. The panel responses are compared to those of monolithic solid plates of equivalent areal density. The impulse imparted to the panels was varied from 1.5 to 7.6 kPa s by changing the standoff distance between the center of a spherical explosive charge and the front face of the panels. A decoupled finite element model has been used to computationally investigate the dynamic response of the panels. It predicts panel deformations well and is used to identify the deformation time sequence and the face sheet and core failure mechanisms. The study shows that efforts to use thin face sheets to exploit FSI benefits are constrained by dynamic fracture of the front face and that this failure mode is in part a consequence of the high strength of the inertially stabilized trusses. Even though the pyramidal lattice core offers little in-plane stretch resistance, and the FSI effect is negligible during loading by air, the sandwich panels are found to suffer slightly smaller back face deflections and transmit smaller vertical component forces to the supports compared to equivalent monolithic plates.     

Behaviour of fibre–metal laminates subjected to localised blast loading: Part I—Experimental observations

This two-part article examines the behaviour of aluminium alloy–glass fibre-reinforced polypropylene-based fibre–metal laminates (FMLs) subjected to localised explosive blast loading. Part I presents observations from the experiments on samples of varying thickness and material distribution, and investigates the influence of stacking configuration. This extensive study examines panels which have between two and five layers of aluminium, and up to eight plies of composite between each pair of aluminium layers. Diamond and cross-shaped back face damage is observed and varies according to panel thickness. Pitting, global displacement and ring buckling of the front face are also discerned. Some observations are related to wave propagation effects. Part II reports a quantitative analysis of the experimental data. Expressed in terms of non-dimensional parameters, front and back face displacements fall within one plate thickness of a linear trend line. The threshold impulse for the onset of tearing is found to increase linearly with panel thickness.     

加载速率对点阵铝力学行为及吸能特性的影响

目的研究基体材料和加载速率对点阵铝力学性能和吸能特性的影响规律。方法针对工业纯铝、6063铝合金为基体的点阵铝在3种不同的加载速率下进行压缩力学试验。结果加载速率从2mm/min增加到250 mm/min时,点阵纯铝的屈服强度增加了2 MPa,点阵6063铝合金的屈服强度增加了7.6 MPa;加载速率从250 mm/min增加到500 mm/min时,点阵纯铝的屈服强度变化不大,而点阵6063铝合金的屈服强度增加了8.2 MPa;当加载速率一定时,点阵6063铝合金的屈服强度要大于点阵纯铝。结论点阵6063铝合金的力学性能和单位体积吸能随着加载速率的增大而增大,并且点阵6063铝合金的力学性能和吸能特性要大于点阵纯铝。     

复合材料点阵夹芯结构平压性能不确定分析与优化

考虑一体化成型工艺制备的复合材料点阵夹芯结构及其不确定性, 采用区间向量实现不确定参数定量化, 建立复合材料点阵夹芯结构平压性能区间分析模型。考虑结构功能状态判断的模糊性, 分别在不考虑设计容差与考虑设计容差情形下, 建立了不确定平压载荷作用下含区间参数模糊可靠性分析与优化模型。研究结果表明: 材料参数及结构参数不确定性, 特别是设计容差对复合材料点阵夹芯结构平压性能影响明显, 因此在工程优化中不仅需要充分考虑材料参数与外部载荷等不确定性, 而且需要充分重视传统不确定设计方法中未计及的设计容差的影响。本研究实现了理论成果与工程应用的有机结合, 为工程领域复合材料点阵夹芯结构平压性能分析与优化提供有效理论方法。     

Compressive and shear buckling analysis of metal matrix composite sandwich panels under different thermal environments

Combined inplane compressive and shear buckling analysis was conducted on flat rectangular sandwich panels using the Rayleigh-Ritz minium energy method with a consideration of transverse shear effect of the sandwich core. The sandwich panels were fabricated with titanium honeycomb core and laminated metal matrix composite face sheets. The results show that slightly slender (along the unidirectional compressive loading axis) rectangular sandwich panels have the most desirable stiffness-to-weight ratios for aerospace structural applications; the degradation of buckling strength sandwich panels with rising temperature is faster in shear than in compression; and the fiber orientation of the face sheets for optimum combined-load buckling strength of sandwich panels is a strong function of both loading condition and panel aspect ratio. Under the same specific weight and panel aspect ratio, a sandwich panel with metal matrix composite face sheets has a much higher buckling strength than one having monolithic face sheets.     

Face sheet debonding in CFRP/PMI sandwich structures under quasi-static and fatigue loading considering residual thermal stress

The face sheet debonding behaviour under quasi-static and fatigue loading in sandwich structures consisting of Carbon Fibre Reinforced Polymer face sheets and a Polymethacrylimid foam core is investigated. The sandwich structure is tested under global mode I and global mode II loading using the Single Cantilever Beam test and the Cracked Sandwich Beam test. Because of the different thermal expansion behaviour of the face sheets and the foam core thermal stresses occur already after the manufacturing process. The impact of these temperature loads on the crack propagation behaviour is investigated via evaluating the experiments numerically with Finite Element Analysis and Virtual Crack Closure Technique.     

Mechanical behaviour of CFRP sandwich structures with tetrahedral lattice truss cores

A method of manufacturing carbon fibre reinforced polymer (CFRP) tetrahedral lattice truss core sandwich structure by thermal expansion silicon rubber mould was developed. The sandwich structure was manufactured integrally without secondary bonding and the silicon rubber mould can be made mass-production with low cost in this approach. The intrinsic property of the CFRP was fully exploited because of carbon fibres aligned in the axial orientation of the truss member. The mechanical properties of CFRP tetrahedral lattice truss core sandwich structures were investigated by flatwise compression and shear test. The experimental results indicate that CFRP tetrahedral lattice truss core sandwich structures have higher weight-specific compressive strength than some metal truss cores, and are competitive with conventional honeycombs.     

Yield surfaces and micro-failure mechanism of block lattice truss materials

The stiffness and the strength of block lattice truss materials were derived, as well as polyhedrical yield surfaces. Tension yield, compression yield and compression buckling of struts are the three main micro-failure mechanisms of the lattice materials. It is shown that when the relative density of the lattice is smaller than a critical value micro compression buckling of struts will dominate the macro failure mode of the material under macro shearing loading or even macro tensile loading. The lattice truss materials may be optimal designed according to their stacking mode of struts and the critical relative density.     

Hybrid carbon fiber composite lattice truss structures

Carbon fiber reinforced polymer (CFRP) composite sandwich panels with hybrid foam filled CFRP pyramidal lattice cores have been assembled from linear carbon fiber braids and Divinycell H250 polymer foam trapezoids. These have been stitched to 3D woven carbon fiber face sheets and infused with an epoxy resin using a vacuum assisted resin transfer molding process. Sandwich panels with carbon fiber composite truss volumes of 1.5–17.5% of the core volume have been fabricated, and the through-thickness compressive strength and modulus measured, and compared with micromechanical models that establish the relationships between the mechanical properties of the core, its topology and the mechanical properties of the truss and foam. The through thickness modulus and strength of the hybrid cores is found to increase with increasing truss core volume fraction. However, the lattice strength saturates at high CFRP truss volume fraction as the proportion of the truss material contained in the nodes increases. The use of linear carbon fiber braids is shown to facilitate the simpler fabrication of hybrid CFRP structures compared to previously described approaches. Their specific strength, moduli and energy absorption is found to be comparable to those made by alternative approaches.     

Compression and bending performances of carbon fiber reinforced lattice-core sandwich composites

To restrict debonding, carbon fiber reinforced lattice-core sandwich composites with compliant skins were designed and manufactured. Compression behaviors of the lattice composites and sandwich columns with different skin thicknesses were tested. Bending performances of the sandwich panels were explored by three-point bending experiments. Two typical failure mechanisms of the lattice-core sandwich structures, delaminating and local buckling were revealed by the experiments. Failure criteria were suggested and gave consistent analytical predictions. For panels with stiff skins, delamination is the dominant failure style. Cell dimensions, fracture toughness of the adhesives and the strength of the sandwich skin decide the critical load capacity of the lattice-core sandwich structure. The mono-cell buckling and the succeeding local buckling are dominant for the sandwich structures with more compliant skin sheets. Debonding is restricted within one cell in bending and two cells in compression for lattice-core sandwich panels with compliant face sheets and softer lattice cores.     

负泊松比点阵结构填充薄壁管轴向压缩行为及吸能特性

薄壁填充结构具有轻量化、高比吸能的优点,被广泛应用于航空航天、汽车、轨道交通等工程领域。负泊松比结构在受到冲击时力学性能会逐渐增强,因此本文基于双箭头型负泊松比点阵提出一种新型薄壁填充管吸能结构,通过准静态压缩实验和有限元数值模拟方法研究了新型填充管在压缩载荷作用下的变形失效模式与力学响应。建立了平均碰撞力的理论预测模型,并通过有限元分析验证了模型可靠性,在此基础上研究了负泊松比点阵结构的细观设计参数对填充管抗压缩性能的影响规律。结果表明,填充管在压缩载荷作用下的失效模式为局部屈曲失效,相比于单一薄壁管与点阵结构,填充管具有更好的抗压缩性能;通过参数分析明确了通过增加胞元杆件壁厚和下支撑杆夹角,能显著提高填充管抗压缩性能,这将为后续负泊松比点阵填充结构的抗冲击优化设计提供重要参考。     

Consideration of bending and buckling behaviors of monolayer and multilayer graphene sheets

Graphene is a two-dimensional carbon based material. Remarkable mechanical, thermal and electrical properties of graphene make it as promising material for advanced applications; nevertheless, majority of its mechanical properties are still unknown. This research investigates buckling and bending behaviors of monolayer and multilayer armchair and zigzag graphene sheets. Bending stiffness, critical buckling force per unit length and critical strain of graphene sheets have been measured by molecular dynamic simulation method. Zigzag graphene sheet shows higher bending stiffness than armchair sheet. Van der Waals interaction between graphene sheets has an improving effect on the stability of middle layers. Cross-linkages reduce the buckling force per unit length and the buckling strain of multi layer graphene sheets.     

Mechanical behavior of carbon fiber reinforced polymer composite sandwich panels with 2-D lattice truss cores

Composite sandwich structures with lattice truss cores are attracting more and more attention due to their superior specific strength/stiffness and multi-functional applications. In the present study, the carbon fiber reinforced polymer (CFRP) composite sandwich panels with 2-D lattice truss core are manufactured based on the hot-pressing method using unidirectional carbon/epoxy prepregs. The facesheets are interconnected with lattice truss members by means of that both ends of the lattice truss members are embedded into the facesheets, without the bonding procedure commonly adopted by sandwich panels. The mechanical properties of the 2-D lattice truss sandwich panels are investigated under out-of-plane compression, shear and three-point bending tests. Delamination of the facesheets is observed in shear and bending tests while node failure mode does not occur. The tests demonstrate that delamination of the facesheet is the primary failure mode of this sandwich structure other than the debonding between the facesheets and core for conventional sandwiches.     

Indentation study of Z-pin reinforced polymer foam core sandwich structures

Z-pin reinforced foam core sandwich panels with composite face sheets, supported on a rigid base and subjected to quasi-static indentation using spherical indenter was studied in this paper. The effects of configurations of Z-pin, including inclination angle and pinning density, on the load–indentation response were studied, and the resulting damage modes were investigated. The effect of inclination angle of pin on the load–indentation behavior is not notable compared with those of Z-pinning density and Z-pin configuration. The collapse of Z-pinned foam core is due to the buckling of pin, and the pin buckling is significantly dependent on the location of indenter. An approximate solution was developed based on the principle of minimum potential energy to simulate the indentation damage response of Z-pin reinforced foam core sandwich. The analytical predictions compare well with the experimental results.     

Dynamic response of a multilayer prismatic structure to impulsive loads incident from water

The dynamic crush response of a low relative density, multilayered corrugated core is investigated by combining insights from experiments and 3D finite element simulations. The test structures have been fabricated from 304 stainless steel corrugations with 0°/90° lay-up orientation and bonded by means of a transient liquid phase method. Characterization of the dynamic crushing of these structures has revealed that at low rates, interlayer interactions induce a buckling-dominated soft response. This softness is diminished at high rates by inertial stabilization and the response of the structure transitions to yield-dominated behavior. Unidirectional dynamic crushing experiments conducted using a dynamic test facility reveal a soft response, consistent with lower rate crushing mechanisms. The 3D simulation predictions of crushing strain, pulse amplitude/duration and impulse delivery rate correspond closely with the measurements. The application of core homogenization schemes has revealed that by calibrating with a multilayer unit cell, high fidelity continuum level predictions are possible. Moreover, even simplified hardening curves based on equivalent energy absorption provide remarkably accurate predictions of the crush strains and the impulse transmitted through the core. The multilayered structures investigated here significantly reduced the transmitted pressures of an impulsive load.     

Quasi-isotropic bending responses of metallic sandwich plates with bi-directionally corrugated cores

In order to reduce anisotropic behaviors of sandwich plates with open channel cores under the bending load, bi-directionally corrugated cores were introduced. Bi-directionally corrugated core has two additional design parameters related with a corrugation pass than uni-directionally corrugated core, so that its properties with respect to core orientations can be controlled. Sandwich plate with bi-directionally corrugated core is designed optimally so that beam buckling of face sheets is reduced drastically and anisotropic buckling behavior in the face sheets is minimized. The cores fabricated by a sectional forming process were bonded with face sheets by adhesive bonding. Three-point bending experiments were carried out with respect to core orientations. It has been shown from the experiments that sandwich plates with bi-directionally corrugated cores exhibit quasi-isotopic bending behaviors and structural performances in sandwich plates.     

Compression and impact testing of two-layer composite pyramidal-core sandwich panels

Quasi-static uniform compression tests and low-velocity concentrated impact tests were conducted to reveal the failure mechanisms and energy absorption capacity of two-layer carbon fiber composite sandwich panels with pyramidal truss cores. Three different volume-fraction cores (i.e., with different relative densities) were fabricated: 1.25%, 1.81%, and 2.27%. Two-layer sandwich panels with identical volume-fraction cores (either 1.25% or 2.27%), and also stepwise graded panels consisting of one light and one heavy core, were investigated under uniform quasi-static compression. Under quasi-static compression, load peaks were identified with complete failure of individual truss layers due to strut buckling or strut crushing, and specific energy absorption was estimated for different core configurations. In the impact test, the damage resulting from low-velocity concentrated impact was investigated. Our results show that compared with glass fiber woven textile truss cores, two-layer carbon fiber composite pyramidal truss cores have comparable specific energy absorptions, and thus could be used in the development of novel light-weight multifunctional structures.     

Effects of material heterogeneities on the compressive response of thiol-ene pyramidal lattices

A process of directed UV photo-curing was previously developed for producing periodic thiol-ene lattices, with potential for use in lightweight structures. The present study probes the compressive response of two families of such lattices: with either one or two layers of a pyramidal truss structure. The principal goals are to assess whether the strengths of the lattices attain levels predicted by micromechanical models and to ascertain the role of lattice heterogeneities. These goals are accomplished through characterization of the lattice geometries via X-ray computed tomography and optical microscopy, measurements of the mechanical properties of the constituent thiol-ene and those of the lattices, and strain mapping on the lattices during compressive loading. Comparisons are also made with the properties of the thiol-ene alone, produced in bulk form. We find two lattice heterogeneities: (i) variations in strut diameter, from smallest at the top surface where the incident UV beam impinges on the monomer bath to largest at the bottom surface; and (ii) variations in physical and mechanical properties, with regions near the top surface being stiffest and strongest and exhibiting the highest glass transition temperature. Finally, we find that the measured strengths of the lattices are in accord with the model predictions when the geometric and material property variations are taken into account in the micromechanical models.