Lightweight Core Research Articles

Computational analysis of sandwich-structured composites with an auxetic phase

A sandwich-structured composite is a special class of composite materials that is fabricated by attaching two thin but stiff layers to a lightweight but thick core. Composites analyzed in this paper consist of two different materials: auxetic and structural steel. The optimization criterion is minimum compliance for the load case where the frame's top boundary is downward loaded. Outer layers are made of steel while the middle layer is two-phase solid material composite. Only the middle layer is optimized by means of minimization of the objective function defined as the internal strain energy. In the first part of this paper we study the application of the solid isotropic material with penalization (SIMP) model to find the optimal distribution of a given amount of materials in sandwich-structured composite. In the second part we propose a multilayered composite structure in which internal layers surfaces are wavy. In both cases the total energy strain is analyzed.

Equivalent mechanical properties for cylindrical cell honeycomb core structure

Structural members made up of two stiff, strong skins separated by a lightweight core are known as sandwich panels. The separation of the skins by the core increases the moment of inertia of the panel with little increase in weight, producing an efficient structure against in-plane and out-of-plane loadings. In general, the mechanical behavior of cellular structures is given by the effective elastic modules, which are also dependent on the structure topology and different geometric parameters. This paper presents analytical equations for equivalent stiffness properties of a cylindrical cell honeycomb core. The cylindrical cells are obtained by connecting bended plate strips using adhesion or laser-welding. The stiffness properties and effective elastic modules for in-plane and out-of-plane compression and shear loadings are derived using an energy based approach. The analytical models are validated by 3D finite element method based on solid elements. The obtained mechanical properties are used to assess the stress and strain state of a sandwich beam in three-point bending. The present investigation shows that the analytical equations can predict the stiffness coefficients and respectively the effective elastic modules with very good accuracy.

Fatigue in the core of aluminum honeycomb panels: Lifetime prediction compared with fatigue tests

In comparison to their weight, honeycomb composite structures have a high bending stiffness, which makes them very suited for every application such as airplanes, railway-cars and vehicles, where little weight is important. The sandwich panels consist of two thin and stiff aluminum face sheets, which are bonded to a thick and lightweight aluminum honeycomb core. These structures are subjected to dynamic loading. However, in the literature, there are hardly any fatigue properties of the honeycomb core described. The fatigue properties of the core are investigated using the finite element method and experiments. Depending on the load application, the honeycomb core fails either through core indentation or shear failure. For a fatigue prediction, both failure modes have to be investigated. Additionally, the physical behavior of the honeycomb core is depending on the orientation of the core. Hence, fatigue tests were conducted in three directions of the core: the stiffest direction, the most compliant direction and the direction with the highest stresses. A three-point bending test setup was built to study the fatigue properties of the honeycomb core. Several fatigue tests were carried out with a load ratio of R = 0.1 (maximum load 10 times bigger than minimum load) and the fatigue diagrams being illustrated. Additionally, food-cart roller tests (wheels of a cart rolling in a circle on a floor panel) were done to dynamically test the panels in every angle. The sandwich structures were modeled with the ANSYS finite element software. The simulations, which were used to determine the stress amplitudes in the specimens, are described later. In addition, buckling analyses were used to examine core indentation failure. Based on these simulations, failure predictions can be made. The fatigue life of the examined specimens is successfully approximated in this manuscript, with the lifetime analysis being based on the FKM guideline (error less than 14% in load amplitude).

Dynamic Response of Local Indentation of Sandwich Constructions under Low-Velocity Impact

Sandwich constructions with a lightweight core are more and more widely applied, which are liable to local indentation when subjected to concentrated loading. Dynamic response of sandwich constructions under low-velocity impact is predicted based on the quasi-static indentaion law in our previous study, and the equations of motion are obtained based on damping-mass models. The analytical solutions show a good agreement with numerical ones using the ABAQUS code.

Thermal elongation of sandwich panels

Sandwich cladding panels with two thin metal faces separated by a lightweight core are well-established cladding products. There have been several instances recently in which the thermal elongation or contraction of sandwich panels has been cited as a possible cause of leakage or local damage in a roof. Historically, the industry has always designed sandwich panels for proper combinations of conventional loading together with a temperature gradient across the panel. This temperature gradient gives rise to ‘thermal bow' which can cause stresses of the same order of magnitude as those from wind or snow load. As a result of this, designers have generally dismissed longitudinal thermal movements from consideration because it is argued that they are taken out by ‘thermal bow' rather than by linear expansion or contraction. Conversely, it is not unusual for roofing and cladding ‘experts' to argue either that thermal bow has little influence on longitudinal thermal movements or that, although it may help in the summer case, it can be of no benefit when the panels try to contract in winter. Surprisingly, there does not appear to have been any previous attempt to determine the actual thermal expansion or contraction of multi-span sandwich panels and appropriate calculation procedures are described for the first time in this study. Perhaps unsurprisingly, it transpires that neither side is correct. Thermal bow results in a significant reduction of longitudinal temperature movements, but it does not eliminate them completely. The formal calculations are not trivial, but it is possible to make a relatively simple estimate of the movements arising in a long panel which has several approximately equal spans.

Efficient Implementations of NTRU in Wireless Network

NTRU is a lattice-based public key cryptosystem featuring reasonably short, easily created keys, high speed, and low memory requirements, seems viable for wireless network. This paper presents two optimized designs based on the enhanced NTRU algorithm. One is a light-weight and fast NTRU core, it performs encryption only. This work has a gate-count of 1175 gates and a power consumption of 1.51 μW. It can finish the whole encryption process in 1498 μs at 500 kHz. As such, it is perfect for wireless sensor network. Another high-speed NTRU core is capable of both encryption and decryption, with delays of 16,064 μs and 128,010 μs in encryption and decryption respectively. Moreover, it consists of 25,758 equivalent gates and has a total power consumption of 59.2 μW (it will be reduced greatly if low power methods were adopted). This core is recommended to be used in base stations or servers in wireless network.

Wrinkling of Sandwich Materials with Nonorthogonal Plain Woven Composite Facesheets

Fabric is often attached on lightweight core to form a sandwich plate. When complex sandwich structure is manufactured, the fabric facesheets are in a state of in-plane tension or shear deformation, and the angle between the warp and weft yarns is altered. An analytical model to calculate the wrinkling load of sandwich plates with nonorthogonal plain woven composite facesheets is established, and the effects of various parameters are demonstrated. Analysis results indicate that the core modulus and thickness, fiber modulus, and yarn orientation of facesheets can influence the facesheets’ wrinkling behavior of sandwich plate on different levels.

Theoretical modeling of guided wave propagation in a sandwich plate subjected to transient surface excitations

A fast and efficient two-dimensional (2D) semi-analytical model based on global matrix method is developed to study the general characteristics of guided wave propagation in a honeycomb composite sandwich structure (HCSS) subjected to time-dependent transient surface excitations. The HCSS used in this study has an extremely lightweight and thick nomex honeycomb core, which is sandwiched between two thin graphite woven composite skins. The homogenized material properties of the skin and the core are considered to be elastic and quasi-isotropic in nature. Far-field time history of surface displacements are calculated for vertical and horizontal tone-burst surface excitations that are representative of thickness and radial mode of vibrations of piezoelectric transducers, respectively. Results are compared with those obtained from Finite element modeling (FEM) in LS-DYNA showing good agreement. Wavelet transform is then performed on the time-domain signal to obtain the group velocities of the propagating modes for their accurate identification on the basis of the theoretical dispersion curve. It is found that the response signal is dominated by the first anti-symmetric mode for a vertical excitation, whereas, the signal characteristics are multimodal in nature with dominating higher order symmetric and anti-symmetric modes for a horizontal excitation. The model is expected to be helpful for appropriate guided wave mode tuning and rapid analysis of data for experimental detection of disbonds in these novel structures.

Core material effect on wave number and vibrational damping characteristics in carbon fiber sandwich composites

A sandwich composite is typically designed to possess high bending stiffness and low density and consists of two thin and stiff skin sheets and a lightweight core. Due to the high stiffness-to-weight and strength-to-weight ratios, sandwich composite materials are widely used in various structural applications including aircraft, spacecraft, automotive, wind-turbine blades and so on. However, sandwich composite structures used in such applications often suffer from poor acoustic performance. Ironically, these superior mechanical properties make the sandwich composites “excellent” noise radiators. There is a growing interest in optimizing and developing a new sandwich composite which will meet the high stiffness-to-weight ratio and offer improved acoustic performance. The focus of this study is to investigate the structural–vibrational performance of carbon-fiber face sheet sandwich composite beams with varying core materials and properties. Core materials utilized in this study included Nomex and Kevlar Honeycomb cores, and Rohacell foam cores with different densities and shear moduli. The structural–vibrational performance including acoustic and vibrational damping properties was experimentally characterized by analyzing the wave number response, and structural damping loss factor ( η ) from the frequency response functions, respectively. It was observed that the relationship between the slopes of the wave number data for frequencies above 1000 Hz is inversely proportional to the core material’s specific modulus ( G / ρ ). The analysis also showed the importance of using a honeycomb core’s effective properties for equal comparison to foam-cored sandwich structures. Utilizing analytical modeling, the loss factors of the core materials ( β ) was determined based upon the measured structural loss factors ( η ) for a frequency range up to 4000 Hz. It was determined that low shear modulus cores have similar material damping values to structural damping values. However as the core’s shear modulus increases, the percent difference between these values is found to increase linearly. It was also observed that high structural damping values correlated to low wave number amplitudes, which correspond to reductions in the level of noise radiation from the structure.

Shear stresses in honeycomb sandwich plates: Analytical solution, finite element method and experimental verification

Honeycomb composite structures are used in airplanes, railway cars and vehicles. The sandwich panels consist of two stiff face sheets of aluminium, which are bonded to a very lightweight honeycomb core of aluminium. Compared to normal plates, sandwich panels have a very high stiffness and simultaneously a low weight. The core of these structures is mainly subjected to shear stresses. The shear stresses depend strongly on the angle of the load application. The distribution and the level of the shear stresses are investigated using analytical calculations. The load direction which induces highest stresses in the honeycomb core is derived. This direction is not the W-direction, which is the most compliant one. When doing finite element simulations of honeycomb cores, often the core is homogenized in order to reduce the calculation time. In this article, some equations are derived in order to calculate the real shear stresses from the shear stresses of the homogeneous core. The equations are validated by finite element simulations and partially by tests. Three-point bending tests and additionally some Food Cart Roller Tests were conducted in order to test the panels in different angles.

Light-weight honeycomb core sandwich panels containing biofiber-reinforced thermoset polymer composite skins: Fabrication and evaluation

The application of biofiber based paper-reinforced polymer (PRP) composites as skin materials for light-weight sandwich panel constructions was explored. Various sandwich panels with PRP composite skins and a commercial resin-impregnated aramid paper honeycomb core of different cell sizes and core heights were fabricated in the laboratory. The effects of honeycomb core height and cell size on the flexural properties of the lab-made sandwich panels were evaluated. The flexural moduli and strengths of the lab-made panels were compared to the reported values for three existing commercial products used for automotive load floor applications. The lab-made PRP composite/honeycomb core sandwich panels had comparable bending rigidity and flexural load bearing capability but lower areal weights when compared to the commercial products suggesting that PRP composites have the potential to be used as an alternative to glass fiber-reinforced polymer composites as skin materials in sandwich panel fabrication.

The influence of core properties on the perforation resistance of sandwich structures – An experimental study

The aim of this study is to investigate the perforation resistance of a range of foam-based sandwich structures. Nine foams, based on a crosslinked PVC, a linear PVC and PET, have been combined with thin glass fibre reinforced plastic skins to produce a range of lightweight sandwich structures, Initially, the mechanical properties of the different foams are characterised. Here, a new test geometry is used to evaluate the toughness characteristics and strengths of the foams under shear loading, a condition similar to that encountered during the impact perforation event.The influence of the plastic collapse stress of the foam in determining the failure thresholds of the front and rear composite skins is established. Here, an existing model has been used to successfully predict failure of the top surface composite skin in the sandwich structures. In addition, the force associated with perforating the lightweight core has been shown to be strongly dependent on the shear strength of the polymer foam. Finally, the perforation resistance of the sandwich structure has been shown to be closely linked to the Mode II work of fracture of the foam material. Here a unique relationship has been established between these two parameters, with all of the experimental points lying on one curve.

A comparative study of the impact properties of sandwich materials with different cores

Sandwich panels are made of two high strength skins bonded to either side of a light weight core and are used in applications where high stiffness combined with low structural weight is required. The purpose of this paper is to compare the mechanical response of several sandwich panels whose core materials are different. Sandwich panels with glass fibre-reinforced polymer face sheets were used, combined with five different cores; polystyrene foam, polypropylene honeycomb, two different density Balsa wood and Cork. All specimens were subjected to low velocity impact and their structural response (Force-displacement curves) were compared to quasistatic response of the panel tested using an hemispherical indenter.

A New FEM Approach for Simulation of Metal Foam Filled Tubes

The combination of thin metal cases or tubes with a filling made of metal foams is interesting and promising for many applications in mechanical engineering. Components made of an outer hollow thin compact metal structure and a cellular lightweight core are especially suited to energy absorption applications. In order to allow for an efficient product/process design with a concurrent engineering approach, reliable and computationally affordable finite element method (FEM) calculations are required by both product and process engineers. The structural performance of these complex composite parts must be numerically predicted, in order to find the optimal combination of outer structure and metal foam properties. While FEM simulation at large deformations of bending, crushing, etc. of thin sheets and tubes is state of the art, the accurate FEM simulation of the mechanical behavior of metal foams cannot be considered fully established. In this paper the three most common methods for FEM simulation of metal foam materials are discussed: (a) homogenization approach, (b) realistic reconstruction of tomographic data, and (c) repetition of standard unit cells. A new effective approach is proposed, suited for simulation of composite, metal foam filled, structures of realistic dimensions. The approach is based on meshing the metal foam by replicating a unit cell made of 32 triangular shell elements, and then by randomizing the nodal position in order to emulate the intrinsic homogeneity of foam morphology. The method is validated by means of different experimental tests. The results show that the proposed method correctly predicts the behavior of foam structures in axial compression. The method slightly overestimated the actual load registered in three point bending tests. Several improvements are described and discussed in the paper, such as randomization of nodal positions of the mesh, in order to reduce the overestimation of forces. An FEM approach for the simulation of large deformations of metal foam filled metal structure (e.g., tubes) suited for the design of realistic large dimensions structural components has been presented. The proposed method shows some innovative features with respect to the available scientific literature, such as a configuration based on octahedral unit cells with low number of triangular shell elements. Randomization of nodal positions of each unit cell has been implemented as a method for better representing the intrinsic variability of metal foams and for reducing the stiffness of the simulated structure.

Steel–Concrete–Steel sandwich slabs with lightweight core — Static performance

This paper investigates the static performance of Steel–Concrete–Steel (SCS) sandwich slabs which consist of an ultra-lightweight concrete core sandwiched between two steel plates. Special J-hook connectors have been developed to develop composite action between the concrete core and the two steel plates. The core is made of lightweight concrete of density less than 1450 kg/m 3. Laboratory tests were carried out on eight SCS sandwich slabs under centrally applied patch load. Test results showed that the mode of failure and crack pattern of SCS sandwich slabs were very similar to those of reinforced concrete slabs especially when the concrete core and steel plates act in a fully composite manner. Flexural and punching are the primary modes of failure. After flexural yielding, membrane action developed in the slab due to the effectiveness of J-hook connectors in maintaining composite action which further increases its load carrying capacity after flexural yielding. Theoretical models are proposed to predict the flexural and punching resistance and a good correlation with test results is obtained. A large deflection analysis considering plate membrane action is also proposed to predict the force deflection relation of SCS sandwich slabs.

Putting Faulty Cores to Work

Necromancer, a robust and heterogeneous core coupling execution scheme, exploits a functionally dead core to improve system throughput by supplying hints regarding high-level program behavior. Necromancer partitions a chip multiprocessor system's cores into multiple groups, each of which shares a lightweight core that can be substantially accelerated using execution hints from the faulty core.

Necromancer

Aggressive technology scaling into the nanometer regime has led to a host of reliability challenges in the last several years. Unlike on-chip caches, which can be efficiently protected using conventional schemes, the general core area is less homogeneous and structured, making tolerating defects a much more challenging problem. Due to the lack of effective solutions, disabling non-functional cores is a common practice in industry to enhance manufacturing yield, which results in a significant reduction in system throughput. Although a faulty core cannot be trusted to correctly execute programs, we observe in this work that for most defects, when starting from a valid architectural state, execution traces on a defective core actually coarsely resemble those of fault-free executions. In light of this insight, we propose a robust and heterogeneous core coupling execution scheme, Necromancer, that exploits a functionally dead core to improve system throughput by supplying hints regarding high-level program behavior. We partition the cores in a conventional CMP system into multiple groups in which each group shares a lightweight core that can be substantially accelerated using these execution hints from a potentially dead core. To prevent this undead core from wandering too far from the correct path of execution, we dynamically resynchronize architectural state with the lightweight core. For a 4-core CMP system, on average, our approach enables the coupled core to achieve 78.5% of the performance of a fully functioning core. This defect tolerance and throughput enhancement comes at modest area and power overheads of 5.3% and 8.5%, respectively.

Static and fatigue load performance of a gfrp honeycomb bridge deck

This paper investigates the performance of a glass fiber-reinforced polymer (GFRP) bridge deck under static and fatigue load cycles. The bridge deck has a sandwich panel configuration, consisting of two stiff face shells separated by a light-weight honeycomb core. The deck was manufactured using a hand lay-up technique. In this study, a full-size panel that had the same design as an actual bridge deck was tested. The experimental data are analyzed and compared to the results of finite element analysis. The data obtained have indicated that the failure of the system is governed by the delamination of the face shells from the honeycomb core, and the failure behavior is pseudo-ductile even though the material itself is brittle. Hence, the design of such a deck panel should be based on the shear strength of the face–core interface. However, the shear strength can depend significantly on the workmanship in the fabrication process. For design, if the interface shear strength can be reliably identified, the maximum shear stress should be no greater than 15% of the shear strength to avoid fatigue damage under the service load condition.

Characterization of Integrated Functionally Gradient Syntactic Foams

Light weight high strength composites can be obtained by reinforcing resin with fillers such as hollow or solid particles and fibers. Composites were fabricated using microballoons (hollow particles) called syntactic foams. These foams can be used in various low density applications such as buoyancy aid materials for deep sea exploration and aerospace vehicles. These foams are usually utilized as light weight core materials for sandwich structures. The present study explores the procedure to fabricate functionally gradient syntactic foams (FGSFs) and further analyze their mechanical properties. The FGSFs produced are gradient structures consisting of four layers with four different types of microballoons, namely, S22, S32, S38, and K46, each having different wall thickness. The volume fraction of all microballoons is maintained constant at 60% to maintain light weight structures. Several FGSF specimens having similar density are fabricated with different layer arrangements. The different layers are integrated before major solidification takes place. Quasistatic compression testing is then performed on the cured FGSF samples using MTS-810 servohydraulic machine. Compressive strength and energy absorption values for each arrangement are compared. The stress plateau in integrated FGSF composites extends from 10% to 60% strain compared with plain syntactic foams. The integrated FGSF shows increment in yield strength and energy absorption compared with adhesively bonded FGSF. It is found that the compressive strength and energy absorption of integrated FGSF composites can be varied based on arrangement of the layers.

新しく開発した軽量コアパネルの曲げ剛性の評価

新しく開発した軽量コアパネルの曲げ剛性の評価