This Is the Comptest Conference in Lausanne s1

26th Annual International SICOMP Conference Gothenburg, 1-2 June 2015

ENERGY-ABSORPTION OF LIGHTWEIGHT HYBRID-CONSTRUCTION SANDWICH MATERIALS: MODEL IDENTIFICATION AND VIRTUAL TESTING Luigi Gigliotti 1, Silvestre T. Pinho1 1Department of Aeronautics, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK

Email: [email protected], [email protected] Web page: www.imperial.ac.uk/aeronautics/research/pinholab/sandwich.html Keywords: Sandwich, TFC, Damage-tolerant design, Virtual testing, Post-damage response, Crushable foams

ABSTRACT The economical and environmental efficiency of cars is of paramount importance for the automotive industry 1. The latter is therefore concerned with investigating and applying strategies towards the weight-reduction of modules and sub-assemblies. Among these, the use of composite components, which can be up to 50% lighter than metal components, is envisioned to be one of the most promising strategies to achieve such weight-reduction and, hence, to reduce the overall carbon footprint of the automotive industry. In electric cars, this would lead to battery-power saving and increase the vehicle’s range. With their superior flexural stiffness-to-weight ratio, improved stability and high energy absorption, composite sandwich structures consisting of a lightweight foam core material bonded to two thin composite facesheets are finding increasing use in a broad range of sectors, including the aerospace and automotive industries [1]. In the latter, owing to the excellent energy-absorption capacity of the core, sandwich structures are used to prevent injuries to the occupants as side/front impact protection systems. Furthermore, sandwich structures are of interest for primary car structures such as the cars’ floor and roof. Numerous studies demonstrate that composite sandwich structures are highly susceptible to localized transverse loads such as those occurring during low-velocity impact [2]; in foam-cored sandwich panels, the resulting local damage consists of a region of crushed core accompanied by a residual dent in the impacted composite facesheet. Such residual dent stems from the local tensile stress underneath the load introduction point; at the damage boundary area, instead, a compressive stress is observed, which represents a possible source of further damage growth upon subsequent reloading. Furthermore, the core needs to be designed such that the largest possible amount of kinetic energy is dissipated while maintaining the impact force at a minimum, resulting in a harmless deceleration in the event of a collision, and in a reduced local damage in case of low-velocity impact of an external object [3]. The ratio between the energy absorbed at each level of deformation and the corresponding maximum stress can be regarded as the efficiency of the core material [4]; for an enhanced passengers’ safety and to lengthen the component’s life, the degradation of the foam core energy-absorption properties and efficiency needs to be minimized. Therefore, the thorough understanding of the post-crushing response of foam materials and the development of affordable manufacturing techniques aimed at enhancing their efficiency and energy-absorption properties are envisioned to be crucial to broaden the use of composite sandwich structures for automotive applications. In this work, the post-crushing compressive behaviour of PMI closed-cell foams for damage-tolerant applications (Rohacell HERO) is experimentally investigated, and the findings of this analysis provide the required insight for devising a novel material model including the appropriate post-crushing constitutive relationships. Moreover, this work investigates the improvement in the energy-absorption properties of a lightweight hybrid-construction sandwich material obtained using the innovative tied foam core (TFC) technology [5]. The proposed material model hinges on (i) the treatment of the crushing process as a phase transformation phenomenon, (ii) the semi-empirical determination of the compressive response of the crushed material and (iii) on the differential behavior of the latter when subjected to compressive loading and unloading. The model relies solely on material data from monotonic compressive tests, thus avoiding the need for performing lengthy compressive tests including several unloading/reloading cycles. The model is shown to accurately capture the residual strain upon unloading, as shown in Fig. 1(a); if phenomena of visco-elastic/plastic relaxation are neglected, such residual strain defines the residual dent resulting from an impact event. Furthermore, both the energy-absorption (Fig. 1(b)) and the efficiency at different residual strain levels (Fig. 1(c)) are correctly predicted by the presented model. To enhance the mechanical properties in the through-the-thickness direction of composite sandwich constructions, several reinforcement techniques have been proposed [6]. The TFC technology investigated in this work consists of a modified tying process, where rovings – comprised of untwisted fiber monofiles such as carbon, glass or organic material – are stitched through foam in the desired arrangement to create a preform, and are then snipped off. This dry preform is placed between two facesheets and infused with resin, with the remaining roving segments becoming

1 The average carbon emissions from a car are nearly equal to 15 g/km, leading to a 2,250 kg/year average total emission. To offset the carbon emission from a single car, approximately 2,000 trees would need to be planted. L. Gigliotti, S.T. Pinho micro-stiffeners – or pins. Prior studies [7] demonstrate that the TFC technology combines high manufacturing economy and rate of standard stitching processes with the outstanding mechanical properties and damage tolerance characteristic of other reinforcement techniques, e.g. using pultruded pins. Furthermore, previous investigations [8] indicate that TFC-reinforced sandwich structures show an improved fatigue behavior, as well as damage- resistance/tolerance and a crack-stopping behavior attributable to the pins. Experimental results confirm that both the compressive stiffness and strength of the reinforced sandwich construction are significantly improved by the presence of TFC-reinforcements, see Fig. 2(a); as expected, the latter are more effective for larger values of the stitching angle (measured with respect to the in-plane direction). As a result, an enhanced energy-absorption capacity is exhibited by the reinforced sandwich construction, with the beneficial effect of TFC-reinforcements being even more evident when the performances of the crushed material are of interest, as shown in Fig. 2(b). The results suggest that the improvements in the energy-absorption properties of the TFC- reinforced core are not determined, not exclusively by the higher compressive stiffness and strength, but also by the interaction of the broken pins with the crushed foam material. This confirms that the reinforced foam materials represent a more damage-tolerant alternative to the corresponding unreinforced materials, especially for larger values of damage. Interestingly, unlike the energy-absorption properties, the efficiency of the crushed foam material seems insensitive to the presence of TFC-reinforcements, as shown in Fig. 2(c).

(a) (b) (c)

Fig. 1. Material model for the post-crushing response of polymeric foam cores. The proposed model is capable to accurately capture the residual strain upon unloading (a), the degradation of energy-absorption properties (b) and of efficiency (c) of crushed foam materials. In (b) and (c), full lines are obtained experimentally while dashed lines are predicted with the proposed model.

(a) (b) (c)

Fig. 2. Energy-absorption properties of TFC-reinforced foam cores. Compared to the unreinforced structure, TFC-reinforced cores exhibit higher compressive stiffness and strength (a). Furthermore, improved energy-absorption properties are observed for the TFC-reinforced material (b), and the benefits provided by the TFC-reinforcements increase at larger values of deformation. This enhanced behavior is achieved without efficiency loss (c). References [1] D. Zenkert, The handbook of sandwich construction, Engineering Materials Advisory Services Ltd., London, UK, 1997. [2] P. Schubel, J.-J. Luo, I. Daniel, Composites A, Vol.38(3), pp.1051 - 1057, 2007. [3] M. Avalle, G. Belingardi, R.Montanini, Int J Impact Eng, Vol.25, pp. 455 – 472, 2001. [4] J. Miltz, O. Ramon, Polym Engng Sci, Vol. 30(2), pp. 129 -133, 1990. [5] Innovative armouring technology for foam materials in high-performance fibre plastic composite applications, Innovation Report Vol. 2, pp.12-13, 2013. [6] D. Cartie’, N. Fleck, Compos. Sci. Technol., Vol. 63(16), pp. 2401 - 2409, 2003. [7] G. Endres, Innovative sandwich constructions for aircraft applications, 9th Int. Conf. Sandwich Struct., Pasadena (USA), 2010. [8] G. Endres, Sandwich material for highest demands in structural applications – Tied Foam Core, SAMPE EUROPE 34th International Conference and Forum, Paris (FR), 2013.