Pyramidal Core Sandwich Research Articles

Local damage and imperfection sensitivities of pyramidal core sandwich cylinders

Local damage and imperfections may affect the loading capacity of truss core sandwich cylinders. This study develops a numerical model to investigate the effects of missing lattice struts, unbound nodes, openings, and eigenmode-shape imperfections on the buckling and post-buckling behavior of pyramidal core sandwich cylinders. Results from the numerical model show that the pyramidal core sandwich cylinder is sensitive to missing struts and unbound nodes when local buckling is the main failure model. Critical loads show a gradual decrease as the opening damage increases. Pyramidal core sandwich cylinders are not sensitive to eigenmode-shape imperfections compared to the equivalent single-walled shells.

Heat transfer property of C/SiC composite pyramidal lattice core sandwich structures

An explicit analytical model for predicting the heat transfer in C/SiC composite pyramidal lattice core sandwich structures is proposed with considering the radiation emitted from the struts. Temperature gradient in thickness direction of the lattice core sandwich structuresis considered in the proposed model. The predictions are compared with the published experimental data and analytical results. It indicates the effect of the radiation emitted from the struts on the equivalent thermal conductivity becomes more significant at high temperature and cannot be overlooked. The present model is more accurate in estimating the high-temperature equivalent thermal conductivity of pyramidal lattice core sandwich structures than those models without considering the radiation of struts. The effects of geometry, solid emissivity, and temperature on the equivalent thermal conductivity of pyramidal lattice core sandwich structures are well discussed.

Predicting the equivalent thermal conductivity of pyramidal lattice core sandwich structures based on Monte Carlo model

An analytical model is presented to predict the equivalent thermal conductivity of the pyramidal lattice core sandwich structure. The equivalent thermal conductivity of the pyramidal lattice core sandwich structure is attributed to both conduction and radiation. The equivalent thermal conductivity caused by heat conduction is evaluated based on Fourier's law. The configuration factor is calculated by the Monte Carlo method considering the effect of radiation in all directions. The predictions are compared with the previous analytical and experimental results. It reveals that the present model is more accurate in estimating the equivalent thermal conductivity for the pyramidal lattice core sandwich structures. The effects of the geometrical parameters, solid emissivity and temperature on the equivalent thermal conductivity of the pyramidal lattice core sandwich structures are well discussed.

Bending behavior of lightweight C/SiC pyramidal lattice core sandwich panels

Due to the excellent mechanical and chemical properties at ultra-high temperature, ceramic matrix composite structures have potential applications for thermal protection systems of hypersonic vehicles. This paper presented a PIP method to fabricate C/SiC pyramidal lattice core sandwich panels (LCSPs). Their bending behaviors with different core angles were experimentally studied firstly. Then the critical failure loads of different failure modes, including facesheet crushing or wrinkling, core member crushing, core shear failure and interlayer delamination, were theoretically established to construct the failure mechanism maps. The influence of geometric parameters, such as the length and the core angle of the core strut, and the thickness of the facesheet, on the failure mode was also analyzed. In addition, further failure process under bending load was investigated by finite element analysis (FEA) method. Elastic-damage constitutive model and cohesive element were applied to modeling the behavior of C/SiC composite and interlayer delamination between the sheet of the core and the facesheet. Theoretical and FEA results agreed well with experimental data. This work is believed to be helpful to understand the bending mechanism C/SiC pyramidal LCSPs.

Design and compressive behavior of a wood-based pyramidal lattice core sandwich structure

Eco-friendly wood-based engineering materials have often received considerable interest worldwide because of their high specific strength/stiffness and biodegradability. To design and enhance an axially compressive member of a wooden structure, a pyramidal lattice core sandwich structure was fabricated with larch sawn timber as the face sheet and birch dowel as the core by using a slotting and adhesive bonding approach. A flatwise compressive experiment was conducted to reveal the compressive behavior and failure modes of the structure. The enhanced designs of the face sheet and the core were based on the failure mechanism and mechanical properties of raw materials. Theoretical analysis and finite element method were applied to predict the structural compressive behavior. Comparison of theoretical, numerical, and experimental results suggested good agreement in the structural compressive response. The results revealed that the reasonable design of the face sheet and the core played an important role in the compressive response and failure modes of the wood-based pyramidal lattice core sandwich structure.

Thermal and thermomechanical performance of actively cooled pyramidal sandwich panels

Previous research has proved that pyramidal core sandwich panel is an excellent kind of passive Thermal Protection Systems (TPSs) for hypersonic aerospace aircrafts suffered from in-service environment. To explore its further insulation performance and load-bearing capability, four kinds of actively cooled pyramidal core sandwich panels are designed and investigated in this paper. Convection flow mechanism for cooling channels is described in details. Influences of the coolant velocity, channel height ratio and flow direction of the two layers on thermal performance are firstly studied and discussed in an appropriate way. Analysis of thermomechanical performance is then performed. The effect of the cover board thickness on thermomechanical performance is also studied. In consequence, a light-weight, structural and thermal integrated thermal protection system (ITPS) has been designed to protect hypersonic aircrafts from intense radiation heat flux in service.

Mechanical behaviors of C/SiC pyramidal lattice core sandwich panel under in-plane compression

C/SiC composite lattice-core sandwich panels combined with thermal insulation and load-bearing capacities are considered as the most promising candidates for thermal protection system (TPS). In this study, C/SiC pyramidal lattice-core sandwich panel with different inclination angles were fabricated using a compression molding and precursor infiltration and pyrolysis (PIP) method. In-plane compressive experiments are conducted to study the failure behavior of these sandwich panels. The analytical failure modes including elastic buckling, face sheet wrinkling, face sheet crushing and interlayer delamination are established to construct the mechanism maps. The effect of geometrical parameters on failure modes are symmetrically and analytically studied. Due to the limits of the cost, virtual tests are supplemented by finite element method (FEM). Face sheet crushing is experimentally observed for almost all specimens with different inclination angles, which is in good agreement with analytical predictions. Numerical simulation results show that interlayer delamination occurs at the attachment between face sheet and lattice core after elastic buckling and core shear buckling. This paper gives us some fundamental understanding of the mechanical response and failure mechanism of C/SiC composite lattice-core sandwich panels.

High temperature mechanical properties of lightweight C/SiC composite pyramidal lattice core sandwich panel

Lattice core sandwich panels, fabricated from metal or resin matrix composites, can not be used under high temperature. Here, we devise a novel pyramidal lattice core sandwich panel fabricated from ceramic matrix C/SiC composite which can be used up to 1600°C. We evaluate the high temperature mechanical properties through experiments, theoretical analysis, and finite element analysis. Under room temperature, 1200°C and 1600°C, the compressive strengths are as high as 12.70, 5.35 and 2.45MPa, and the modulus are 630, 260 and 120MPa, respectively. The specific bending strengths are up to 260.7, 168.7 and 152.1MPa/(g·cm3). We also exclusively reveal that with increasing temperature, the critical relative density for the failure models shows significant reduction, indicating service temperature increment enables the original fracture failure to transfer to the buckling of the core bars. Moreover, we develop a finite element analysis model which can well calculate the damage evolution, failure mechanism and models observed in experiments. The failure model under compression is the fracture of the core bars while the bending failure is shear failure of the core bars. These results indicate that the C/SiC sandwich panel integrates high temperature resistance, lightweight characteristic and robust mechanical properties.

Influence of manufacturing defects on modal properties of composite pyramidal truss-like core sandwich cylindrical panels

Defects can easily appear in composite lattice truss core sandwich structures during the complex preparation process, which may significantly affect the structural response and decrease the load-carrying capability. The purpose of this paper is to investigate the manufacturing defect sensitivity of modal vibration responses of carbon fiber composite pyramidal truss-like core sandwich cylindrical panels by modal experiments and finite element analysis. Defects including debonding between face sheets and truss cores (DFT), truss missing (DTM), face sheet wrinkling (DFW) and gap reinforcing (DGR) are introduced into the present intact specimen artificially and modal testing is conducted to study their dynamic behavior under free-free boundary conditions. Finite element models consistent with the experiments are then developed to further study the effect of defect extents, locations and forms on the modal parameters of the present sandwich cylindrical panels. Results indicate that the degree of sensitivity of natural frequencies of the present sandwich cylindrical panels mainly depends on the vibration modes, defect extents, locations and forms. In addition, damping loss factors are much more sensitive than their corresponding frequencies. Some conclusions and essential mechanisms are summarized, which is helpful to vibration-based non-destructive evaluation (NDE) of such kind of composite lattice sandwich structures.

Thermal and Thermomechanical Performances of Pyramidal Core Sandwich Panels Under Aerodynamic Heating

According to the particular aerodynamic heating loads which hypersonic aerospace aircrafts suffered from in-service environment, a lightweight integrated thermal protection system (ITPS) named pyramidal core sandwich panel is designed. This is considered not only as an insulation structure but also a load-bearing structure. Compared to traditional thermal protection systems (TPSs), the sandwich panel has simultaneous lightweight, load-bearing, and excellent thermal protection property. The finite-element heat transfer analysis for the pyramidal core sandwich structure is performed, and the distributions of temperature in the structure are presented. Then sequential coupling method is adopted to analyze the thermomechanical performance of the structure and presentations of field of stress and displacement under aerodynamic and thermal load are provided. A comparison between corrugated-core sandwich panels and pyramidal core sandwich panels from the perspectives of heat insulation, strength, and mass is carried out. The results indicate that the particular performance of pyramidal-core structure is superior to that of corrugated-core structure.

Design and analysis of integrated thermal protection system based on lightweight C/SiC pyramidal lattice core sandwich panel

Thermal protection system (TPS) plays the key role to successful development of hypersonic vehicles. Here, a novel structurally and thermally integrated thermal protection system (ITPS) based on the lightweight C/SiC pyramidal core lattice sandwich panel is proposed. This ITPS integrates advantages of low areal density and high temperature resistance up to 1600°C. Heat transfer characteristics and compressive responses of the C/SiC sandwich panel are established in advance. The results demonstrate that filling alumina fibers in the pore significantly reduce the effective thermal conductivity from 2.45–4.83W/m°C to no more than 0.7W/m°C. The critical relative density is determinated for the failure models under aerodynamic pressure load. Meanwhile, an analysis procedure of the ITPS is exclusively established under typical aerodynamic heat flux and pressure load. With fulfillment of both temperature and mechanical constraints, minimum areal density is obtained. Compared with current metal corrugated core ITPS, the ITPS proposed here significantly raises the temperature limitation up to 1600°C and reduces the areal density up to 35%, and is very promising for potential application in hypersonic vehicles.

Material combinations and parametric study of thermal and mechanical performance of pyramidal core sandwich panels used for hypersonic aircrafts

A novel kind of lightweight integrated thermal protection system, named pyramidal core sandwich panel, is proposed to be a good safeguard for hypersonic aircrafts in the current study. Such system is considered as not only an insulation structure but also a load-bearing structure. In the context of design for hypersonic aircrafts, an efficient optimization should be paid enough attention. This paper concerns with the homogenization of the proposed pyramidal sandwich core panel using two-dimensional model in subsequent research for material selection. According to the required insulation performance and thermal–mechanical properties, several suitable material combinations are chosen as candidates for the pyramidal core sandwich panel by adopting finite element analysis and approximate response surface. To obtain lightweight structure with an excellent capability of heat insulation and load-bearing, an investigation on some specific design variables, which are significant for thermal–mechanical properties of the structure, is performed. Finally, a good balance between the insulation performance, the capability of load-bearing and the lightweight has attained.

Behavior of pyramidal lattice core sandwich CFRP composites under biaxial compression loading

Pyramidal lattice core sandwich composites all made of CFRP is believed to have superior characteristics, as compared to the conventional composite and sandwich plates. Several articles addressed the behavior of this type of composite in the literature. However, its response under biaxial compression loading and its notch sensitivity was not covered within the current state-of-the art.In the current paper, numerical simulation tools were used to examen the failure mechanisms and the notch sensitivity of the pyramidal lattice core sandwich CFRP composite. The simulations adopted the LaRC failure criteria and property degradation rules. The model took into account the fiber tension and compression, the matrix tension and compression and the fiber matrix shear out failure mechanisms. The problem was solved with and without holes in order to examen the notch sensitivity. The model was validated against experimental results from the literature and showed good agreement. The results showed that these types of structures are less sensitive to notches, as compared to the traditional composites, which may guarantee the use of these structures in bolted joint and similar applications.

Energy absorption and low velocity impact response of polyurethane foam filled pyramidal lattice core sandwich panels

In this paper, the polyurethane foam filled pyramidal lattice core sandwich panel is fabricated in order to improve the energy absorption and low velocity impact resistance. Based on the compression tests, a synergistic effect that the foam filled sandwich panels have a greater load carrying capacity compared to the sum of the unfilled specimens and the filled polyurethane block is found. Moreover, the energy absorption efficiency of foam filled sandwich panels with higher relative density (2.58% and 3.17%) lattice cores is lower than that of the unfilled specimens when the compressive strain is small, while it exhibits superior when the compressive strain arrives at about 0.25, and the superiority enlarges as the strain increase. However, the energy absorption of foam filled sandwich panels owning lower relative density (1.83%) lattice cores is inferior to that of the unfilled specimens. During the low velocity impact tests, it is found that the contact duration between the impactor and the sandwich specimens is shorter and the impact peak load has a slight increase for the foam filled specimens.

Torsion of carbon fiber composite pyramidal core sandwich plates

A combined theoretical, experimental and numerical investigation of carbon fiber composite pyramidal core sandwich plates subjected to torsion loading is conducted. Pyramidal core sandwich plates are made from carbon fiber composite material by a hot compression molding method. Based on the Classical Laminate Plate Theory and Shear Deformation Theory, the equivalent mechanical properties of laminated face-sheet are obtained; based on a homogenization concept combined with a mechanical of materials approach, the equivalent in-plane and out-of-plane shear moduli of pyramidal core are obtained. A torsion solution is derived with Prandtl stress function and can be used in the sandwich plate with orthotropic face-sheets and orthotropic core. The influences of material properties and geometrical parameters on the equivalent torsional stiffness are explored. In order to verify the accuracy of the analytical torsion solution, experimental tests of sandwich plate samples with different face-sheet thicknesses are conducted and two types of finite element models are developed. Good agreements among analytical predictions, finite element simulations and experimental evaluations are achieved, which prove the validity of the present derivation and simulation. The proposed method could also be applied in design applications and optimization of the pyramidal core sandwich structures.

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.

Titanium alloy lattice truss structures

A high-temperature forming and diffusion bonding method has been investigated for the fabrication of modified pyramidal lattice core sandwich structures from a titanium alloy. A periodic asymmetric hexagonal perforation pattern was cut into thin Ti–6Al–4V sheets which were then folded along node rows by a combination of partial low-temperature bending followed by simultaneous hot forming/diffusion bonding to form sandwich panel structures with core relative densities of 1.0–4.1%. The out-of-plane compression and in-plane longitudinal shear properties of these structures were measured and compared with analytical estimates. Premature panel failure by node shear-off fracture was observed during shear testing of some test structures. Node failures were also initiated at stress concentrations at the truss–facesheet interface. A liquid interface diffusion bonding approach has been investigated as a possible approach for reducing this stress concentration and increasing the truss–facesheet interfacial strength.

Imperfection sensitivity of pyramidal core sandwich structures

Lightweight metallic truss structures are currently being investigated for use within sandwich panel construction. These new material systems have demonstrated superior mechanical performance and are able to perform additional functions, such as thermal management and energy amelioration. The subject of this paper is an examination of the mechanical response of these structures. In particular, the retention of their stiffness and load capacity in the presence of imperfections is a central consideration, especially if they are to be used for a wide range of structural applications. To address this issue, sandwich panels with pyramidal truss cores have been tested in compression and shear, following the introduction of imperfections. These imperfections take the form of unbound nodes between the core and face sheets—a potential flaw that can occur during the fabrication process of these sandwich panels. Initial testing of small scale samples in compression provided insight into the influence of the number of unbound nodes but more importantly highlighted the impact of the spatial configuration of these imperfect nodes. Large scale samples, where bulk properties are observed and edge effects minimized, have been tested. The stiffness response has been compared with finite element simulations for a variety of unbound node configurations. Results for fully bound cores have also been compared to existing analytical predictions. Experimentally determined collapse strengths are also reported. Due to the influence of the spatial configuration of unbound nodes, upper and lower limits on stiffness and strength have been determined for compression and shear. Results show that pyramidal core sandwich structures are robust under compressive loading. However, the introduction of these imperfections causes rapid degradation of core shear properties.

Structural response of pyramidal core sandwich columns

An experimental and analytical investigation is carried out to examine the in-plane compressive response of pyramidal truss core sandwich columns. The identified failure mechanisms include Euler buckling, shear buckling and face wrinkling. The operative mechanism is dependent on the properties of the bulk material and geometry of the sandwich columns and analytical formulae are derived for each of these modes. Failure maps are constructed for sandwich columns made from an elastic ideally-plastic material and AISI 304 stainless steel which has a strongly strain hardening response. Pyramidal core sandwich columns made from 304 stainless steel have been designed using these mechanism maps and the measured responses are compared with the analytical predictions. Finally, optimal single layer and multi-layer pyramidal sandwich column designs that minimize the weight for a given load carrying capacity are calculated using the developed analytical models for the failure of the sandwich columns. The results demonstrate that pyramidal core sandwich columns outperform the currently used hat-stiffened column design.