Improvement of the impact behaviour of foam core sandwich through the use of a cork layer as impact shield (original) (raw)
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Journal of Sandwich Structures and Materials, 2021
Impact events are common in everyday life and can severely compromise the integrity and reliability of highperforming structures such as sandwich composites that are widespread in different industrial fields. Considering their susceptibility to impact damage and the environmental issues connected with their exploitation of synthetic materials, the present work aims to propose a bio-based sandwich structure with an agglomerated cork core and a flax/basalt intraply fabric as skin reinforcement and to address its main weakness, i.e. its impact response. In-service properties are influenced by temperature, therefore the effect of high (60 °C) and low (-40 °C) temperatures on the impact behavior of the proposed structures was investigated and a suitable comparison with traditional (polyvinyl chloride) (PVC) foams was provided. The results highlighted the embrittlement effect of decreasing temperature on the impact resistance of the sole cores and skins and of the overall structures with a reduction in the perforation energy that shifted, in the last case, from 50-60 J at-40 °C up to more than 180 J at 60 °C. A maleic anhydride coupling agent in the skins hindered fundamental energy dissipation mechanisms such as matrix plasticization, determining a reduction in the perforation threshold of all composites. In particular, neat polypropylene (PP) skins displayed a perforation energy of 20 J higher than compatibilized (PPC) ones at 60 °C, while agglomerated cork sandwich structures at 60 °C were characterized by a perforation threshold higher of at least 50 J.
Impact behavior of sandwich structures made of flax/epoxy face sheets and agglomerated cork
Journal of Natural Fibers, 2018
The unremitting quest of natural and renewable materials able to replace their synthetic counterparts in high-performance applications has involved also sandwich structures. In this regard, the aim of this work is to characterize the impact response, in both high-and low-velocity conditions, of green sandwich structures made of agglomerated cork as core and flax/epoxy laminates as face sheets. Both bare cork, flax skins, and complete sandwich structures were subjected to impacts at three different energy levels representing the 25%, 50%, and 75% of the respective perforation thresholds. A gas gun was instead used to assess the high-velocity impact behavior of these green sandwich structures and evaluate their ballistic limit. This study shows that the buckling of cell walls of agglomerated cork enables to tailor the damage extension through-the-thickness in low-velocity impacts compared to traditional synthetic foams coupled with a considerable amount of energy absorption.
Experimental study of the low-velocity impact behaviour of primary sandwich structures in aircraft
Composites Part A: Applied Science and Manufacturing, 2009
The susceptibility of sandwich structures to localised (impact) damage is one of the main reasons why the sandwich concept is not yet used in large primary aircraft structures of airliners. The objective of this work is to experimentally investigate the damage tolerance of representative composite sandwich panels for primary aircraft structures. Instrumented low-velocity impact tests were performed on sandwich specimens consisting of carbon Non-Crimp Fabric/epoxy facings and a Rohacell (PMI) foam core. Both internal and external damage resulting from these impact events was evaluated. The foam core material has a considerable influence on the amount of damage detected by ultrasonic TTU C-scan. CAI tests however showed that this core damage has no significant influence on the residual compressive strength of the specimens.
Mechanical Behavior of Sandwich Structures using Natural Cork Agglomerates as Core Materials
Journal of Sandwich Structures and Materials, 2009
Cork is a material of great value to the Portuguese economy. Unfortunately, its use is still restricted to traditional areas, with the agglomerate form in particular not being used to its full potential. The objective of this article is to analyze the viability of using cork-based material as core materials in sandwich structures in aeronautical and aerospace applications. The use of cork-based material is proposed because of its isolation properties (both thermal and acoustic) and there is no significant performance loss, when compared with the currently used materials. It presents other advantages, as well as, less wastage of energy in manufacturing and a better environmental integration, both in the transformation stage and in the end of life recycling stage. The objective of this work is to study the mechanical behavior of different sandwich specimens, with carbon/epoxy faces, and cores of different cork agglomerates and their comparison with the results obtained with similar sp...
Dynamic impact behavior of syntactic foam core sandwich composites
Journal of Composite Materials, 2019
Sandwich composites and syntactic foams independently have been used in many engineering applications. However, there has been minimal effort towards taking advantage of the weight saving ability of syntactic foams in the cores of sandwich composites, especially with respect to the impact response of structures. To that end, the goal of this study is to investigate the mechanical response and damage mechanisms associated with syntactic foam core sandwich composites subjected to dynamic impact loading. In particular, this study investigates the influence of varying cenosphere volume fraction in syntactic foam core sandwich composites subjected to varying dynamic impact loading and further elucidates the extent and diversity of corresponding damage mechanisms. The syntactic foam cores are first fabricated using epoxy resin as the matrix and cenospheres as the reinforcement with four cenosphere volume fractions of 0% (pure epoxy), 20%, 40%, and 60%. The sandwich composite panels are th...
Behaviour of sandwich structures with cork compound cores subjected to blast waves
Engineering Structures, 2013
Sandwich structures can often sustain large deformations under constant load enabling them to absorb significant amounts of energy. The mechanical properties of cork (e.g. low density and high specific stiffness and strength) suggest that this material-and its compounds-may have excellent properties when acting as core in energy absorbing sandwich systems and structures. Cork is a natural material with a cellular structure (closed cell). After reaching yield stress, cork exhibits a region of almost constant stress for increasing strains until densification is reached, allowing it to absorb considerable amounts of energy. Within the scope of the present work, two micro-agglomerated cork (MAC) compounds are incorporated as cores in sandwich structures with 5754-H22 aluminium alloy face sheets. Samples with constant thickness of the face sheets and different core thicknesses are tested. These structures are fixed on a 4-cable ballistic pendulum and subjected to blast waves originated from the detonation of 30 g of high explosive (C4) at a fixed stand-off distance (300 mm). The deflection of the front and back face sheets is measured as well as the transmitted impulse and movement of the pendulum. The effects on the structural response of the core thicknesses and core densities are determined. A linear dependence between the relative core thickness reduction and the initial core thickness is determined for both MAC compounds. A value of %11% was obtained for the relative thickness reduction, strongly indicating the possibility of energy dissipation by the core, most probably due to crushing of the cellular structure of cork.
Processing and high strain rate impact response of multi-functional sandwich composites
Composite structures, 2001
Sandwich composites are ®nding increasing applications in aerospace, marine and commercial structures because they oer high bending stiness and lightweight advantages. Currently, foam and honeycomb core sandwich composites are widely used in structural applications. However, aordability continues to be the driver to develop sandwich constructions that can be processed at lower costs and containing integrated design features. This paper considers sandwich constructions with reinforced cores by way of three-dimensional Z-pins embedded into foam, honeycomb cells ®lled with foam, and hollow/space accessible Z-pins acting as core reinforcement. These designs oer added advantages over conventional constructions load bearing by enabling functions such as ability to route wires, mount electronic components, increase transverse stiness, tailor vibration damping, etc. With the assumption that these sandwich constructions would be part of a larger structure, impact damage is often of concern. This paper deals with: (a) processing of sandwich composites using out-of-autoclave cost-eective liquid molding approach, and (b) investigation of the high strain rate impact (164±326/s) response of the sandwich composite structures. Wherever applicable, comparisons are made to traditional foam core and honeycomb core sandwich constructions. Ó
Low velocity impact response of glass fiber reinforced aluminium foam sandwich
The aim of this study was the analysis of the bending and the low-velocity impact response of aluminium foam sandwich reinforced by the outer skins made of glass fiber reinforced epoxy matrix and the results were compared with those obtained for aluminium foam sandwiches without glass fiber skins. Static bending tests were carried on panels with the same nominal size at different support span distances in order to analyze the collapse modes and their capacity of absorbing energy, while the energy amount absorbed under dynamic loading was evaluated by means of impact tests. The experimental investigation has particular importance for applications which require lightweight structures with a high capacity of energy dissipation, such as transport industries. 1 Introduction Sandwich structures, consisting of glass fiber reinforced plastic (GFRP) skins bonded onto low density cores, offer great potential for use in various high performance composite structures which are nowadays widely used in aerospace, marine, automobile, windmills, transport, shipbuilding, defense because of their high specific stiffnesses and strengths, excellent thermal insulation, acoustic damping, fire retardancy, ease of machining and forming among others. It is important to choose high-quality core material in the optimal design of sandwich. Most current sandwich structures are based on polymeric foams (such as PVC, PUR) and aluminium honeycomb bonded to GFRP skins. Recently a great number of metal foams have been developed to replace polymer foams in applications where multifunctionality is important. For instance, acting as a structural component in a sandwich composite but also as an acoustic damper, fire retardant or heat exchanger [1]. As a new multi-function engineering material, aluminium foams have many useful properties such as low density, high stiffness, good impact resistance, high energy absorption capacity, easy to manufacture into complex shape, good erosion resistance, etc. [2, 3]. This fact opens a wide range of potential applications for sandwich structures with aluminium-foam core. Aluminium foam sandwiches (AFS) [4, 5], obtained by combining metal face sheets with a lightweight metal foam core, are suitable for applications in automotive industry and ship construction
Impact response of aluminum foam core sandwich structures
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2011
Sandwich panels, comprised of metallic foam core and face sheets, are widely used to withstand impact and blast loadings. Based on the actual application requirements, the performance can be optimized with the proper combination of face sheets design. In this paper the impact responses of aluminum foams with various tailored face sheets, whose behavior represents elastic, elastic-ideally plastic and elastic-plastic strain work hardening, were investigated experimentally. The experiment was carried out using hemispherical indenters on blocks of aluminum foam with and without the face sheet. Competing failure modes for the initiation of failure are discussed based on comparison of energy absorption capacity. Results show that increase in thickness of foam and the use of face sheet enhances the impact energy absorption capacity. The type of face sheet not only affects the energy absorption capacity but also the failure mode for the foam blocks. Aluminum foam blocks with stainless steel sheet are strong enough to withstand the pre-designated impact loading without penetration damage. At the same time, this study also provides a comparison of the impact performance, in terms of impact energy and failure mode, among blocks with different face sheets under the low velocity impact.