The energy-absorbing characteristics of composite tube-reinforced foam structures

Abstract

This paper reports the findings of a research study investigating the energy-absorbing characteristics of polymer foams reinforced with small carbon fibre reinforced epoxy tubes. Initial attention focuses on establishing the influence of tube diameter on the specific energy absorption (SEA) characteristics of the chamfered CFRP tubes. Here, it is shown that the SEA of the tubes increases rapidly with decreasing diameter/thickness ratio, with the highest values being close to 93 kJ/kg. Similar tests were conducted at dynamic rates of strain, where it was observed that the measured values of SEA were lower than the corresponding quasi-static data, possibly due to rate-sensitive effects in the delamination resistance of the composite material. In the next stage of the investigation, the composite tubes were embedded in a range of polymer foams in order to establish the influence of both tube arrangement and foam density on the crush behaviour of these lightweight structures. In addition, a limited number of blast tests have been undertaken on structures based on these core materials. Here, extensive crushing of the composite tubes was again observed, suggesting that these structures should be capable of absorbing significant energy when subjected to this severe loading condition. Finally, the results of these tests are compared with previously-published data from studies on a range of different cores materials. Here, it has been shown that the energy-absorbing characteristics of these systems exceed values associated with other core materials, such as aluminium honeycombs, polymer honeycombs and metal foams.

Introduction

The superior energy-absorption and crashworthiness properties of composite materials has, in recent years, attracted the attention of a range of sectors, including those associated with the automotive and aerospace industries. Extensive testing on various types of tubular structure has shown that composite materials can offer extremely high values of specific energy absorption (SEA) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. For example, in a detailed review of energy-absorption in composite structures, Jacob et al. [3] determined that only 0.66 kg of a high-performance thermoplastic matrix composite is required to absorb the energy of a 1000 kg car travelling at 15.5 m/s (35 mph). Published values for the SEA of widely-used composites, such as carbon fibre reinforced epoxy, generally fall in the range 50–80 kJ/kg [4], [5], but can be as high as 110 kJ/kg [6].

Several studies have focused on the influence of strain-rate on the energy-absorbing capacity of composite tubes, with the results of these studies being somewhat contradictory [5], [13], [14]. For example, Schmuesser and Wickliffe [13] reported reductions in energy absorption of up to 30% following tests on carbon glass and Kevlar fibre tubes based on a [0°, ±45°] configuration. In contrast, Thornton [5] observed very little change in the SEA of such tubes over a wide range of loading rates.

A number of workers have investigated the influence of tube geometry on energy absorption [9], [10], [11]. Thornton and Edwards concluded that for a given fibre stacking sequence, glass, carbon and Kevlar fibre reinforced circular tubes out-perform their square and rectangular counterparts [9]. Mamalis et al. [10] studied the crushing characteristics of a range of glass fibre reinforced composite structures with circular, square and conical cross-sections. They found that circular tubes offered the highest values of energy-absorption, with the crashworthiness of conical structures decreasing with increasing cone angle. Farley [12] investigated the influence of specimen size on the energy-absorbing capability of carbon and Kevlar reinforced epoxy cylindrical tubes and observed that the ratio of the inner diameter of the tube to that of its thickness, (D/t), greatly influences the specific crushing stress (SCS) of the tube. It was shown that the value of SCS increased by approximately 180% as the value of D/t was decreased from 120 to 3.8. This increase in crushing response at lower values of D/t was attributed to a reduction in interlaminar cracking. Here, it was argued that the buckling load of the fibre bundles increases with a reduction in the number and length of these interlaminar cracks [12].

Given that the SEA of composite tubes increases significantly with reducing D/t ratio, it is likely that structures based on an array of small tubes in a low density foam could represent an attractive option in the search for new, lightweight energy-absorbing structures. Since they are based on simple cylindrical tubes that are widely available in the market place, tubular-reinforced sandwich structures should offer a number of potential benefits, including a relative ease of fabrication of complex and curved structures, superior energy-absorbing characteristics and a relatively low cost. Such structures could also offer other attractive characteristics, such as an ability to control the crushing load during compression, e.g. through the use of embedded tubes of different length, as well as the possibility to produce curved core geometries for more complex structures.

This paper investigates the properties of a range of tube-reinforced foams for use in lightweight energy-absorbing structures. Following an initial study to characterise the quasi-static and dynamic behaviour of the individual tubes and simple tube/foam configurations, a limited number of blast tests are conducted on tube-reinforced foams. Foam has been selected as the substrate (rather than honeycombs) since it is very easy to introduce circular holes into a foam and a foam substrate fully surrounds and supports the tube, which is not the case for a honeycomb-type structure. Finally, the properties of multi-tube systems are compared with those offered by other types of core material.