Crash worthiness of foam-cored sandwich

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1 Crash worthiness of foam-cored sandwich panels with GRP inserts J.J. Carruthers**, M.S. Found\ and A.M. Robinson" "Advanced Railway Research Centre, University of Sheffield ^SIRIUS Department of Mechanical Engineering, University Abstract The use of structural composites for crashworthy applications has been investigated for foam-cored sandwich panels with integral GRP tubes and frusta. The panels were tested under both quasi-static flatwise and edgewise compression and evaluated in terms of their energy absorption properties and failure mechanisms. Under flatwise compression, panels with inserts which failed by stable progressive brittle fracture exhibited the best specific energy absorptions and compared favourably with the performance of aluminium honeycomb sandwich panels. Under edgewise compression, panels with inserts which prevented separation of the face plates offered the most predictable and repeatable performance. 'Present address : Hexcel Composites Ltd, Duxford, Cambridge CB2 4QD, UK

170 Structures Under Shock and Impact Introduction In recent years there has been a growing acceptance of the importance of crashworthiness of structures used in the transport industries.

2 170 Structures Under Shock and Impact Introduction In recent years there has been a growing acceptance of the importance of crashworthiness of structures used in the transport industries. The careful design of energy absorbing crush zones ensures that vehicles collapse in a controlled manner which dissipates safely the kinetic energy and limits the accelerations transmitted to the occupants. Sandwich panel structures have the potential to dissipate substantial amounts of collision energy through both local core crushing and global panel deflection. Whilst aluminium honeycomb sandwich panels are well established it has been demonstrated that FRP composites can be designed to provide energy absorption capabilities which are superior to those of metals when compared on a weight-for-weight basis^. Provided that the crushing mechanisms can be controlled such that the FRP fails in a stable, progressive manner then very high levels of energy can be absorbed. Whilst some progress has been made in establishing the influence of material, geometric and experimental parameters on the energy absorption of FRP tubes^ the practical application of this knowledge has been limited by the ability to reproduce the behaviour within real structures. Hence, design methodologies and manufacturing techniques need to be developed which will enable the viable production of actual crashworthy structures. This paper described one approach to the development of structural crashworthy composites. It is based on the use of a foam-cored sandwich panel with integral energy absorbing GRP inserts. The function of the inserts, which were in the form of tubes and hollow conical frusta, was to control the failure loads (and hence the energy absorption capability) of the panels. Quasi-static crush tests were undertaken on foam-cored sandwich panels with and without GRP inserts and the performance compared with aluminium honeycomb sandwich panels. By identifying the principal failure mechanisms and ascertaining their relative contribution to the overall energy absorption, recommendations for optimising crashworthiness can be developed. 2 Experimental 2.1 Materials and Specimen Geometries The aluminium honeycomb sandwich panels consisted of a hexagonal 3003 commercial grade aluminium honeycomb of density 83 kg/m\ cell size 6.35 mm and thickness of 25.4 mm bonded to 0.55 mm thick face plates of NS4 H6 aluminium alloy using a high strength, hot curing epoxy

3 Structures Under Shock and Impact 171 adhesive. Flat square specimens measuring 120 x 120 x 26.5 mm were cut from large panels. The basic composite sandwich panels consisted of a rigid closed-cell polyurethane foam of density 120 kg/m* and thickness of 25 mm faced with glass reinforced polyester laminates of 3 mm thickness. Moving outwards from the core, the laminates were constructed from a layer of 450 g/m^ continuousfibremat, a layer of 2336 g/nf [0/45/90/-45] non-crimp quadriaxial mat and an 80 g/nf surface veil. The foam was moulded into solid blocks then cut to size to produce the cores which were laid-up in groups of six with their glass mat facings and consolidated using the resin transfer moulding (RTM) process. After curing the samples were cut into specimens measuring 100 x 100 x 31 mm. In addition several foam cores had four GRP inserts fabricated from [±45] braided glass fibres in a polyester resin as shown schematically in Figure 1. These ran the entire thickness of the core with their longitudinal axes perpendicular to the sandwich facings. The fibres at the ends of each braided insert were merged with those of the face plate laminate so as to provide a mechanical tie between opposing facings. It has previously been shown* that this arrangement inhibits separation of the face plates, even after core debonding, and enhances the mechanical properties of panels, particularly with respect to shear stiffness and strength. A number of different insert geometries were tested (see Table 1), although there was no variation within a given specimen. The geometries of the conical frusta were chosen in accordance with Mamalis et al's* recommendations for designing FRP frusta which fail by high energy progressive crushing. Similar mean diameters were then selected Glass Reinforced Polyester Facings Rigid Polyurethane Foam Core Braided Glass Reinforced Polyester Insert Figure 1: Section through composite sandwich panel with GRP inserts.

4 172 Structures Under Shock and Impact for the tubular inserts. The sandwich panels with GRP inserts were also moulded using the RTM process and the finished specimens were cut to the same size as those from the basic panels. The position of the four inserts was 20 mm from the centre of the specimen measured horizontally and vertically. Table 1 The different geometries of GRP insert investigated Insert Type Semi- Apical Angle Length (mm) Inside Diameter (mm) Outside Diameter (mm) Fibre Volume Fraction (%) Tube Tube Frustum Frustum Wide End = 19.4 Wide End = Wide End = 20.6 Wide End = Test Procedure The aluminium honeycomb sandwich panels were compressed in the flatwise direction whilst the foam-cored sandwich panels were subjected to flatwise and edgewise compression tests. The tests were conducted in a Mayes 100 kn servo-electric test machine and the specimens crushed at a uniform rate with the load-displacement characteristic being recorded for each. In all cases the crushing area of the platens was larger than the test specimens. Further details of materials and experimental techniques can be found in reference^. 3 Results 3.1 Aluminium Honeycomb Sandwich Panels Figure 2 shows the flatwise crushing behaviour of the aluminium honeycomb sandwich panels for which the principal deformation mechanism is that of local buckling of the honeycomb cell walls. There is an initial linear-elastic region which terminates in a peak load at the onset of plastic collapse. The crush load then remains approximately constant as each cell fails by a progressive folding mechanism and starts to increase again once the core is fully crushed. This behaviour has also been observed by Porter^. It is thought that the face play no significant role in the crushing process when the specimen is smaller than the test plates used.

5 Structures Under Shock and Impact 173 Transactions on the Built Environment vol 32, 1998 WIT Press, ISSN Aluminium Honeycomb Sandwich Panel - - Rigid Polyurethane Foam Cored Sandwich Panel 5 2 Percent Core Crush Figure 2: Compressive responses of an aluminium honeycomb sandwich panel and a foam-cored sandwich panel. 3.2 Basic Composite Sandwich Panels Also shown in Figure 2 is the flatwise crushing resistance of a foam-cored sandwich panel without inserts. After the initial linear-elastic behaviour there is a large uniform response in which the load increases slowly with displacement followed by a more rapid increase towards densification of the core. Such a response is typical of polymer foams*, and is associated with the gradual collapse of the cells within the foam indicating that the properties of the sandwich panels are controlled by the bulk core material. 3.3 Foam-cored Sandwich Panels with Inserts Flatwise Compression All specimens exhibited an initial linear-elastic response due to bending and stretching of the cells within the rigid polyurethane foam and the elastic compression of the GRP tubes and frusta. Specimens with GRP inserts display a stiffer response than those without as shown in Figure 3. At crush distances of beyond approximately 15 mm the collapse load of all the specimens increases rapidly with displacement and the cores become fully crushed. At intermediate crush distances of 1-15 mm the load-displacement response of the panels varies considerably with insert geometry and produces either a uniform response with an initial peak load or a non-uniform response. These compare with the uniform response of panels with no inserts as discussed in Section 3.2. The uniform response with an initial peak load represents the characteristic behaviour of sandwich panels with 16 mm outside diameter

6 174 Structures Under Shock and Impact 20.6 mm wide outside diameter frusta 16 mm outside diameter tubes No inserts Crush Distance (mm) Figure 3: Load-displacement characteristics typical of the three principal failure categories. tabular inserts or 21.5 mm wide outside diameter conical inserts. Audible cracking accompanied the termination of the initial peak load suggesting failure of the GRP inserts. The load then drops and assumes a largely uniform response, similar to that typical of foams, but at a higher average crush level. These observations, supported by x-ray analysis, indicate an initial catastrophic failure of the GRP inserts rather than the onset of controlled progressive crushing. Apart from the initial fracture, there was no further evidence of brittle fracture hence the general uniformity in the load-displacement characteristic. It is believed that the majority of the specimens which failed in a catastrophic manner did so because of inconsistencies in the fibre distribution within the GRP tubes and frusta. The non-uniform response represents the characteristic behaviour of the remainder of the specimens, i.e. those with 15 mm outside diameter and 20.6 mm wide outside diameter frusta. Following initial failure, the load-displacement characteristics of those specimens exhibit pronounced serrations and there was audible cracking throughout the crush event. These observations are consistent with the progressive crushing of the inserts which was also supported by x-ray analysis. The serrations in the load-displacement characteristic are caused by the stick-slip nature of the brittle fracture processes Edgewise Compression The load-displacement characteristic of all specimens are of the same general form as shown in Figure 4. Each exhibit an initial linear-elastic response which terminates in a very high peak load before dropping. This peak load coincides with the failure of one of the face plates. All

7 Structures Under Shock and Impact 175 Transactions on the Built Environment vol 32, 1998 WIT Press, ISSN No Face Plate Separation Face Plate Separation Crush Distance (mm) Figure 4: Load-displacement characteristics of sandwich panels under edgewise loading. specimens then proceed to buckle, with the outside of one of the facings bent into a concave profile and the outside of the other in a convex manner. After this initial failure, crushing continues at low load until the specimens rotate or slip out of the grips. Progressive crushing with useful energy absorption did not generally occur since the majority of the loading was taken by the thin facings rather than by the core material Specimens exhibit one of two types of load-displacement response depending on whether the face plates separate. Those specimens which exhibit face separation have a lower initial peak load than those which do not. Catastrophic face plate separation is accompanied by a sharp drop in crushing load followed by an uneven load-displacement characteristic. Specimens whose facings remain intact exhibit a less dramatic drop in peak load and a subsequently much more uniform response. The post-failure response and structural integrity of the sandwich panels is found to be greatly influenced by the GRP inserts. These are supposed to inhibit face plate separation by effectively tying opposing facings together. In practice this is achieved with varying degrees of success. Specimens with no inserts and those with conical inserts have complete separation of one facing. With the conical inserts this always involves the facing attached to the narrow ends of the frusta where the forces acting to separate the facings are concentrated over a relatively small interface region. No face plate separation is observed for specimens with tubular inserts and the panels crush quasi-progressively from one end. They retain their structural integrity and demonstrate the most predictable and repeatable load-displacement characteristics.

176 Structures Under Shock and Impact 4 Energy Absorption Capability Having related the failure mechanisms of the different types of sandwich panel to their load-displacement response their energy

8 176 Structures Under Shock and Impact 4 Energy Absorption Capability Having related the failure mechanisms of the different types of sandwich panel to their load-displacement response their energy absorption capabilities were then assessed. Only useful absorption was considered and therefore the response of panels to edgewise compression was ignored and for flatwise compression tests that beyond crush distances of 15 mm was not taken into account. The amount of energy absorbed by each specimen was calculated from the area under its load-displacement characteristic. These absolute energy absorption were then normalised by mass in order to make a direct comparison between specimens. All the sandwich panels with GRP inserts absorbed more energy than those without. Specimens with 20.6 mm wide outside diameter frusta exhibited the highest mean energy absorption with an increase of 69% over panels with no inserts. However in terms of mass specific energy absorption only two of the insert geometries provided significant improvements over the basic composite sandwich. Hence for most specimens any increase in energy absorption by virtue of the inserts was offset by their higher mass. The two insert geometries which did show improvements were the 15 mm diameter tubes and the 20.6 mm wide diameter frusta with increases in mean mass specific energy absorption of 12 and 34% respectively. These were also the only geometries to crush in a stable, progressive manner and demonstrate the energy absorption potential of the brittle fracture processes. In general specimens with GRP inserts exhibited a much wider spread in recorded specific energy absorption than those without. However, the specimen with 20.6 mm wide outside diameterfrustadid provide a level of consistency which was comparable to the sandwich panels without any inserts. It is believed that this was due to the manufacturing benefits of this particular geometry which helped to ensure that these inserts had a high consistency in their fibre distribution and hence in their mechanical properties. Repeatability is an important aspect of crashworthy design : a minimum level of performance must be guaranteed and, on a larger structural scale, there will be a need to predict and ensure a preferred sequence of collapse. Figure 5 compares the crushing behaviour of specimens with the 20.6 mm wide outside diameter frusta with the aluminium honeycomb sandwich panel. Whilst the average crushing stress levels of both materials are similar the foam-cored sandwich panel with the GRP frusta has a much more uniform characteristic and does not require a high peak load to initiate crushing. However the aluminium honeycomb sandwich

9 Structures Under Shock and Impact 177 Aluminium Honeycomb Sandwich Panel Foam Cored, 20.6 mm Wide Outside Diameter Frusta O) c 1 ' (J Percent Core Crush Figure 5: Comparison between foam-cored sandwich panel with highest energy absorption and aluminium honeycomb sandwich panel. panel has an initial peak load approximately twice the average crushing level. Table 2 provides a comparison between the mean normalised energy absorptions of the two materials. Whilst the specific energy absorption of the panels with the GRP frusta is slightly lower than that of the aluminium honeycomb sandwich, this situation is reversed for the energy absorbed per unit volume. This suggests that the foam-cored sandwich panels provide a very "space efficient" energy absorption capability. Table 2 Comparison between the mean normalised energy absorptions of the specimens with 20.6 mm wide outside diameter frusta and the aluminium honeycomb sandwich panel Sandwich Panel AJuminium Honeycomb Foam-Cored, 20.6 mm Wide Outside Dia. Frusta Specific Energy Absorption (kj/kg) Energy Absorbed per Unit Volume (kj/nf) Conclusions Foam-cored sandwich panels with integral GRP tubes and frusta have been tested under quasi-static flatwise and edgewise compression. Under

178 Structures Under Shock and Impact flatwise compression, panels with inserts which failed by stable progressive crushing exhibited the best specific energy absorption and compare favourably with

10 178 Structures Under Shock and Impact flatwise compression, panels with inserts which failed by stable progressive crushing exhibited the best specific energy absorption and compare favourably with the performance of aluminium honeycomb sandwich panels. Conical inserts were found to offer the most repeatable energy absorption performance as their geometry assisted in ensuring consistency of manufacture. Under edgewise compression, the sandwich panels with tubular inserts, which prevented face plate separation, fail quasi-progressively from one end and produced the most repeatable failure characteristics. Whilst overall no one type of insert geometry was found to give the best performance under both loading conditions it has been demonstrated that foam-cored sandwich panels with GRP inserts are competitive with aluminium honeycomb sandwich panels for crashworthy applications. 6 References 1. Thornton, P. H., Energy absorption in composite structures, Journal of Composite Materials, 13, pp , Schmueser, D. W. and Wickliffe, L. E., Impact energy absorption of continuous fiber composite tubes, Journal of Engineering Materials and Technology, 109, pp , Hull, D., A unified approach to progressive crushing of fibrereinforced composite tubes, Composites Science & Technology, 40, pp , Richardson, M. O. W., Robinson, A. M., Eichler, K. and Moura Branco, C., Mechanical behaviour of a new stress dissipating composite sandwich structure, Cellular Polymers, 13, pp , Mamalis, A. G., Manolakos, D. E., Viegelahn, G. L., Sin Min Yap and Demosthenous, G. A., On the axial crumpling of fibre-reinforced thin-walled conical shells, International Journal of Vehicle Design, 12, pp , Carruthers, J. J., Some Aspects of the Energy Absorption of Composite Materials, PhD thesis, University of Sheffield, Porter, J. H., Utilising the crushing under load properties of polypropylene and polyethylene honeycomb to manage crush energy, SAE, paper No , Gibson, L. J. and Ashby, M. F., Cellular Solids : Structure & Properties, Pergamon Press, Oxford, 1988.

Energy absorption capability of low-cost composite materials

Energy absorption capability of low-cost composite materials Anders Sjögren SICOMP AB, P O Box 104, SE-431 22 Mölndal, Sweden Åke Nylinder Polytec Composites Sweden AB, P O Box 302, SE-341 26 Ljungby,