Energy absorption of SMC/balsa sandwich panels with geometrical triggering features

The influence of triggering topologies on the peak load and energy absorption of sandwich panels loaded in in-plane compression is investigated. Sandwich panels with different geometrical triggering features are manufactured and tested experimentally. Damage initiation in panels with grooves is investigated using finite element models.     

Pyramidal lattice sandwich structures with hollow composite trusses

Pyramidal lattice sandwich structures with hollow composite trusses were fabricated using a thermal expansion molding approach. Composite lattice structures with three relative densities were fabricated with two fiber architectures and the out-of-plane compression properties were measured and compared. Lattice cores with a fraction of carbon fibers circumferentially wound around the hollow trusses (Variant 2) exhibited superior mechanical properties compared with similar structures comprised of unidirectional fibers (Variant 1). The out-of-plane compressive properties of composite pyramidal lattice structures in Variant 2 were well-matched by analytical predictions. Unusual strain hardening behavior was observed in the plateau region for Variant 2, and the energy absorption capabilities were measured and compared with the similarly constructed silicone rubber–core truss pyramidal lattice structures (Variant 3). The energy absorption per unit mass of selected hollow truss composite lattice structures reported here surpassed that of both hybrid truss counterparts (Variant 3) and hollow truss metallic lattice structures.     

Sound radiation of orthogonally rib-stiffened sandwich structures with cavity absorption

A comprehensive theoretical model is developed for the radiation of sound from an infinite orthogonally rib-stiffened sandwich structure filled with fibrous sound absorptive material in the partitioned cavity, when excited by a time-harmonic point force. The vibrations of the rib-stiffeners are accounted for by considering all possible motions. Built upon the concepts of dynamic density and bulk modulus, both frequency dependent, an equivalent fluid model is employed to characterize the absorption of sound in the fibrous material. Given the periodicity of the sandwich structure, Fourier transform technique is employed to solve the series of panel vibration equations and acoustic equations. In the absence of fibrous sound absorptive material, the model can be favorably degraded to the case of an infinite rib-stiffened structure with air or vacuum cavity. Validation of the model is performed by comparing the present model predictions with previously published data, excellent agreements are achieved. The influences of air–structure coupling effect and cavity-filling fibrous material on the sound radiation are systematically examined. The physical features associated with sound penetration across these sandwich structures are interpreted by considering the combined effects of fiberglass stiffness and damping, the balance of which is significantly affected by stiffener separation. The proposed model provides a convenient and efficient tool for the factual engineering design of this kind of sandwich structures.     

Experimental investigation of energy-absorption characteristics of components of sandwich structures

Two series of experiments are performed to investigate the dynamic response of various essential components of a class of sandwich structures, under high-rate inertial loads. One consists of dynamic inertia tests and the other involves dynamic impact tests. A split Hopkinson bar apparatus is modified and used for these experiments.     

The effect of processing conditions on the energy absorption capability of composite tubes

Structures capable of absorbing large amounts of energy are of great interest, particularly in the automotive and aviation industries, in an effort to reduce the impact on passengers in the case of a collision. The energy absorption properties of composite materials can be tailored, thus making them an appealing option as a substitute of more traditional materials in applications where energy absorption is crucial. In this research, the effect of the processing conditions (with or without vacuum) on the specific energy absorption capacity of composite tubes was investigated. Tubes of circular and square cross sections were fabricated using orthophthalic polyester resin and plain weave E-glass fabric with fibers oriented at 0°/90°, with respect to the tube axis. Test specimens consisting of tube segments were prepared and tested under quasi-static compression load. Test results indicate that, among the conditions considered, tubes of circular cross section fabricated under applied vacuum display the highest level of specific energy absorbed. Ultimately, this investigation demonstrates the potential for tailoring the energy absorption properties of composite materials through controlled processing conditions.     

Improving the bending strength and energy absorption of corrugated sandwich composite structure

The bending strength, stiffness and energy absorption of corrugated sandwich composite structure were investigated to explore novel designs of lightweight load-bearing structures that are capable of energy absorption in transportation vehicles. Key design parameters that were considered include fibre type, corrugation angle, core-sheet thickness, bond length between core and face-sheets, and foam inserts. The results revealed that the hybridization of glass fibres and carbon fibres (50:50) in face-sheets was able to achieve the equivalent specific bending strength as the facet-sheets made entirely of carbon fibre composites. Increasing the corrugation angle and the core sheet thickness improved the specific bending strength of the sandwich structure, while increasing the bond length led to a reduction in the specific bending strength. The hybrid composite coupons with foam insertion showed medium energy absorption, ranging between the glass fibre and the carbon fibre composite coupons, but the highest crush force efficiency among all designs.     

Impact behavior and energy absorption of paper honeycomb sandwich panels

Dynamic cushioning tests were conducted by free drop and shock absorption principle. The effect of paper honeycomb structure factors on the impact behavior was analyzed. Results of many experiments show that the dynamic impact curve of paper honeycomb sandwich panel is concave and upward; the thickness and length of honeycomb cell-wall have a great effect on its cushioning properties; increasing the relative density of paper honeycomb can improve the energy absorption ability of the sandwich panels; the thickness of paper honeycomb core has an up and down fluctuant effect on the cushioning properties; with the increase of the thickness of paper honeycomb core, the effect dies down; flexible corrugated paperboard as liners can improve the compression resistance and cushioning properties of paper honeycombs. The research results can be used to optimize the structure design of paper honeycomb sandwich panel and material selection for packaging design.     

Energy absorption of braiding pultrusion process composite rods

Composite body structures are now commonly used in road and rail vehicles, ships and submarines, aircraft and spacecraft due to their capability to effectively absorb high kinetic energy to weight ratio. One such structure designed as an energy device with pre-determined properties is a braided pultruded process (BPP) composite rod of either circular or square cross-section.

This paper reports the results of an investigation on circular BPP rods and unidirectional pultruded process rods in epoxy matrix subjected to compressive loading. Test results depict BPP rods to have superior properties in comparison to the unidirectional rods in terms of energy absorption capability that is manifested through well-defined progressive crushing failure mechanisms. Generally the rods' fracture and complete failure mechanisms show distinct creation of buckling zone, followed by generation of fronds as the wedge area increases with every augmentation of applied load. Fracture morphology related to overall performance characteristics is discussed through the step-by-step analysis of microphotography. The specific energy absorption property is shown to be best achieved in carbon/carbon (C/C) BPP followed by glass/carbon (G/C) rod combination and then the glass/glass (G/G) BPP rods. The latter (G/G), although worst performer of all the rods in terms of energy characteristics, still outperforms the documented best tubes made of Kevlar fibres, steel and aluminium. On average, the carbon/carbon (C/C) BPP rod's specific energy absorption is between 35% and 55% more than the nearest comparable tubes.     

Ballistic impact experiments of metallic sandwich panels with aluminium foam core

Metallic sandwich structures with aluminium foam core are good energy absorbers for impact protection. To study their ballistic performance, quasi-static and impact perforation tests were carried out and the results are reported and analysed in this paper. In the experiments, effects of several key parameters, i.e. impact velocity, skin thickness, thickness and density of foam core and projectile shapes, on the ballistic limit and energy absorption of the panels during perforation are identified and discussed in detail.     

A new hybrid concept for sandwich structures

Sandwich structures are considered as optimal designs for carrying bending loads and can be either metal (aluminium faces and honeycomb or metal foam cores) or polymer structures (composite faces with polymer foam cores). In this paper, a new hybrid sandwich structure has been developed by combining most of the advantages of metallic and polymeric materials while avoiding some of their main disadvantages. For this new concept metal sheets are used at the outer surfaces to maximize rigidity while introducing in between lightweight cores adhesively bonded to keep the whole structure together. Furthermore, composite or wood layers may be used as intermediate layers to improve impact resistance. Potential methods for the manufacturing of this new structure are based on compression under vacuum. The results include the study of several panel configurations theoretically based on Finite element analysis and on the modified simplified equations and experimental results in the most representative cases of the study.     

Dynamic energy absorption characteristics of hollow microlattice structures

Hollow microlattice structures are promising candidates for advanced energy absorption and their characteristics under dynamic crushing are explored. The energy absorption can be significantly enhanced by inertial stabilization, shock wave effect and strain rate hardening effect. In this paper we combine theoretical analysis and comprehensive finite element method simulation to decouple the three effects, and then obtain a simple model to predict the overall dynamic effects of hollow microlattice structures. Inertial stabilization originates from the suppression of sudden crushing of the microlattice and its contribution scales with the crushing speed, v. Shock wave effect comes from the discontinuity across the plastic shock wave front during dynamic loading and its contribution scales with v². The strain rate effect increases the effective yield strength upon dynamic deformation and increases the energy absorption density. A mechanism map is established that illustrates the dominance of these three dynamic effects at a range of crushing speeds. Compared with quasi-static loading, the energy absorption capacity at dynamic loading of 250 m/s can be enhanced by an order of magnitude. The study may shed useful insight on designing and optimizing the energy absorption performance of hollow microlattice structures under various dynamic loads.     

Multiscale approach for the design of composite sandwich structures for train application

In the present work a multiscale approach is considered for the design of composite sandwich structures for a roof of railway vehicle. The procedure consists in different steps that start from cost/benefit analysis on materials and their manufacturing process and cycle up to analysis of sub-components and entire structures. Each step is characterized by experimental, theoretical and numerical studies. The design activities herein presented count experimental campaigns able to characterize both the properties of novel sandwich material, manufactured expressly for transportation industry, the sandwich and joint behaviors. Analytical and numerical approaches have been used to validate and optimize the structural layout. Finite element analysis has been also performed on a test article to verify the “new” sandwich roof in regard to structural requirements suggested by European Code.     

Energy absorption diagrams of paper honeycomb sandwich structures

It is very important to evaluate the cushioning properties of paper honeycomb sandwich structures for optimizing pack design. The energy absorption diagram is one method to characterize the cushioning properties of materials. In this paper, we investigate energy absorption and develop energy absorption diagrams for paper honeycomb sandwich structures. Based on static compression experiments, the compressive stress–strain curve is simplified into three sections: linear elasticity, plateau and densification. By considering the factors associated with the structure of paper honeycombs, the energy absorption model is obtained and characterized by the thickness‐to‐length ratio of the honeycomb cell wall. Both theory and experiment show that the compression energy absorption capability increases with the increasing thickness‐to‐length ratio of the honeycomb cell wall, and a good agreement is achieved between the theoretical and experimental energy absorption curves. The proposed method to develop an energy absorption diagram for paper honeycomb sandwich structures can be used to characterize the cushioning properties and optimize the structures of paper honeycomb sandwiches. Copyright © 2008 John Wiley & Sons, Ltd.     

Thermal characteristics of composite sandwich structures for machine tool moving body applications

The thermal deformation affects the accuracy of a precision machine tool. There are various heat sources in machine tools such as motors, spindle units, and friction in LM-guide systems. In this work, the thermal characteristics of composite sandwich structures for machine tool parts were investigated both by FEM analysis and experiment. Then the machine tool slide of a high speed CNC milling machine was designed and manufactured with composite sandwich structures combined with a welded steel structure––a hybrid machine tool structure. In addition, the reliability of adhesive joints between the composite sandwich and the steel structure was investigated in terms of adhesive joint strength and thermal stress induced by heat generation of linear motors.     

In-plane effective elastic properties of a novel cellular core for sandwich structures

Within this paper an analytical model is presented for the calculation of the in-plane effective elastic properties E_x and E_y of a novel cellular structure which is proposed to be used as a core in sandwich structures. The proposed cellular core may represent a less expensive and easily to produce alternative to the already known cellular structures used for the construction of sandwich structures. The developed analytical model is validated through experimental tests. The results obtained by analyzing the theoretical model show a good agreement with the tests. The structure topology is studied using a parameterized unit cell and it is shown the way in which the in-plane stiffness depends on the geometric parameters and relative density of the core.     

Concepts for enhanced energy absorption using hollow micro-lattices

We present a basic analysis that establishes the metrics affecting the energy absorbed by multilayer cellular media during irreversible compaction on either a mass or volume basis. The behaviors at low and high impulse levels are distinguished through the energy dissipated in the shock. The overall mass of an energy absorbing system (comprising a cellular medium and a buffer) is minimized by maximizing the non-dimensional dissipation per unit mass parameter for the cellular medium, Λ≡Umρs/σY$\Lambda \equiv U_{\text{m}}{\rho }_{\text{s}}/{\sigma }_{\text{Y}}$, where U_m is the dissipation per unit mass of the cellular medium, ascertained from the area under the quasi-static compressive stress/strain curve, σ_Y the yield strength of the constituent material and ρ_s the density of the material used in the medium. Plots of Λ$\Lambda$ against the non-dimensional stress transmitted through the medium, σtr/σY${\sigma }_{\text{tr}}/{\sigma }_{\text{Y}}$ demonstrate the relative energy absorbing characteristics of foams and prismatic media, such as honeycombs. Comparisons with these benchmark systems are used to demonstrate the superior performance of micro-lattices, especially those with hollow truss members. Numerical calculations demonstrate the relative densities and geometric configurations wherein the lattices offer benefit. Experimental results obtained for a Ni micro-lattice with hollow members not only affirm the benefits, but also demonstrate energy absorption levels substantially exceeding those predicted by analysis. This assessment highlights the new opportunities that tailored micro-lattices provide for unprecedented levels of energy absorption for protection from impulsive loads.     

Parametric study on design of composite–foam–resin concrete sandwich structures for precision machine tool structures

In this paper, sandwich structures for micro-EDM machines are optimized by using parametric study varying composite geometries and parameters like stacking sequence, thickness and rib geometry. The structures are composed of fibre reinforced composites for skin material and resin concrete and PVC foam (Closed cell, Divinycell) for core materials. Column structure was designed by a beam with cruciform rib and performance indices such as static bending stiffness (EI) and specific bending stiffness (EI/ρ) for dynamic stability are examined by controlling the thickness and stacking sequence of composites. For the machine tool bed, which usually has a plate shape, was designed to have high stiffness in two directions at the same time controlling stacking sequence and rib geometry; that is, rib thickness and number of ribs. The sensitivity of design parameters like rib thickness and composite skin thickness was examined and the optimal condition for high stiffness structure was suggested. Finite element analysis was also performed to verify the static and dynamic robustness of the machine structure. L-shaped joint for combining bed and column of the micro-EDM machine was proposed and fabricated using adhesive bonding. The dynamic performance such as damping characteristics was investigated by vibration tests. From the results optimal configuration and materials for high precision micro-EDM machines are proposed.     

水下目标吸声材料和结构的研究

全面了解和掌握水下吸声材料及结构在应用频率范围内其不同温度、压力等环境下的声学性能,对于水下目标结构的声隐身设计具有重要意义.结合近年来水下目标吸声材料和结构的应用及研究,从材料声学设计的角度,讨论了水下吸声材料的声学特性和研究进展.还对水下吸声结构的形式、吸声机理和应用情况进行了归纳和评述.最后预测了水下目标吸声材料和结构的发展趋势.     

Energy absorption of a thin-walled cylinder with ribs subjected to axial impact

We performed experimental and theoretical analyses that show a thin-walled cylinder with stiff ribs can be used as a structural element to improve or adjust energy absorption characteristics. We conducted impact crushing tests using several different cylinders with ribs. The experimental results showed that the axisymmetric and non-axisymmetric crushing modes were dependent on not only the cross-section size but also on the distances between the ribs. A critical distance between the ribs was found to exist for generating axisymmetric and non-asxisymmetric crushing modes and it was more than double the wavelength of axisymmetric wrinkles regardless of cylinder size. The mean crushing forces of the axisymmetric modes were found to be roughly 1.3 times larger than those of the non-axisymmetric modes. The theoretical results based on plastic hinge behavior showed good agreement with the experimental results. The effects of material and cylinder size on the crushing behavior of a cylinder with ribs were expressed using approximate mathematical equations. The critical distance between ribs for generating axisymmetric or non-axisymmetric crushing mode was also expressed approximately. Stiff ribs appropriately spaced in a cylinder were found to be effective in absorbing a large amount of energy with a short crushing deformation.