Blast resistance of auxetic and honeycomb sandwich panels: Comparisons and parametric designs
Introduction
Nowadays, the importance of smart materials and structures for energy-absorption in extreme events is growing more and more. Moreover, the design of structures both light in weight and able to mitigate both blast and localised impacts is a complex challenge [1], [2], [3], [4], [5], [6], [7], [8]. The use of auxetic structures –i.e. characterised by negative Poisson’s ratio– is an interesting opportunity to solve all these issues. These auxetic materials present an interesting and unique behaviour; they contract when compressed and expand if stretched [9]. Consequently, many mechanical properties, such as fracture toughness, indentation resistance, shear modulus and vibration absorption show enhanced performances [10], [11], [12], [13].
Negative Poisson’s ratio materials and structures have been investigated for protective purposes since their discovery. Many researchers looked into the behaviour of auxetic and conventional foams, demonstrating that auxetic foams present higher yield strength, lower stiffness and better energy absorption [14], [15], [16], [17], [18], [19]. Other studies of auxetic structures in composite panels demonstrated improvements in static indentation, specifically in terms of stiffness, low impact velocities and resistance to fibre pull-out with localisation of damage, therefore overall requiring less maintenance [20]. Sandwich panels with certain auxetic cores have been analysed under static and dynamic loadings, including blast-induced shockwaves. Reduction of deformation, localisation of damage [21], [22], [23], better flexure response [24], [25] –with lower effective shear modulus and higher maximum effective shear strain and better energy absorption have been obtained in different studies [21], [22], [26], [27], [28], [29], [30].
In general, cellular structures have shown better properties than the conventional monolithic materials, such as higher strength to weight ratio and energy absorption [31]. It is necessary to understand the benefits of auxetic composite panels compared to typical honeycomb composite structures for their performances against extreme loadings. Honeycomb structures have been used in a wide range of applications, from shock absorption to high temperature dissipation [31], [32], [33], [34]. Specifically, they have been used for structural applications, due to their impact resistance and energy absorption properties. Analytical [35], [36], experimental [37], [38] and numerical [39], [40] studies have been conducted to evaluate their mechanical behaviours. In-plane behaviour has also been investigated [36], [39], [41], [42], [43], [44], [45], where analyses show the importance of the cell-wall geometrical configurations.
The design and manufacture of lightweight composite structures for protective purposes has been a main interest for both defence and civilian applications, such as the protection of critical infrastructures or light weight armoured vehicles (Fig. 1). The sandwich structure, which is composed of crushable cores between two monolithic facet panels, is one of the most effective approaches for blast and impact protection. The frontal panel helps to distribute the impulsive load over the crushable core, dissipating a large amount of imparted energy, slows the shockwave and, therefore, prevents critical failure. A large number of studies on sandwich panels showed their better performance when compared with monolithic structures of similar areal mass [33], [44], [46], [47], [48], [49].
From the manufacturing perspective, the 2D-based structures are simpler and less expensive to fabricate [50], [51], [52] compared to the three dimensional ones. In order to manufacture a 3D truss-lattice structure, more complex design processes are required, such as 3D printing or deformation forming –with perforated sheets punched to obtain the right shape. Instead, it is possible to manufacture these 2D composite panels through stamping or profile-rolling of sheet-metal blanks [27], slotting together steel sheets [53] or 3D printing. As the manufacturing process can be complex and expensive, the necessity of developing an initial design and understanding the dynamic responses of the hybrid auxetic composite sandwich panels is paramount.
In this paper, we will develop a numerical model to simulate hybrid auxetic composite sandwich panels under blast loadings and we will show a comparison with a conventional honeycomb sandwich panel. The well-known 2D re-entrant NPR structure [10] will be used and modelled in multiple layers for the core, and sandwiched between two metallic facets. Numerical models will be validated with analytical results. The last section will present a comparison of auxetic and conventional structures and some parametric studies on their geometric parameters to observe and explain their effect on blast resistance performance.
Section snippets
Unit cell descriptions
The two different types of unit cells investigated in this work are presented in Fig. 2. In particular, the honeycomb unit cell (HU) is one of the most typical cellular structures (Fig. 2b), while the auxetic unit cell (AU) with a re-entrant shape (Fig. 2a) is considered as the most popular structure exhibiting negative effective Poisson’s ratio. Both structures have simple designs, which make them easier to manufacture and maintain at minimum cost. Due to the cellular structures, these panels
Material characterisation and models
Various materials have been used for the facets, the auxetic core and the concrete base. As the HACPs are analysed under high-speed impacts, it is necessary to also describe their rate-dependency for low-to-high strain rates. For the containment case, a ballistic steel, AISI 4340 steel alloy –with higher yield strength and lower ductility– is used and its properties are obtained from previous studies [62].
Aluminium alloy 5083-H116 is used for the auxetic structure as it is characterised by high
Blast resistance performances of baseline HACP and best EPP
The baseline model of the auxetic panel (HACP) is presented here and compared with the best result from an equivalent honeycomb panel (EPP) with positive Poisson’s ratio. The baseline HACP is defined through the following shape factors: θ = 50°; rL = 0.5; N = 5, while the EPP is characterised by: θ = 130°; rL = 2.5; N = 5. This EPP is chosen to have similar relative density compared with the HACP. The differences in relative density will be taken in account using a normalisation. A blast load of 300 g TNT
Influences of geometrical parameters: angles θ and ratios rL
Different auxetic unit cells can be obtained by changing the geometrical parameters indicated in Fig. 2. In particular, angles θ and ratios rL largely vary the effective Poisson’s ratio of the unit cell and, consequently, of the structure. Different HACPs are obtained changing these parameters, with different relative densities and behaviours. The height of all the structures is kept constant to 100 mm. In this section, three representative combinations of angles θ (30°, 50° and 70°) and ratios r
Conclusions
Numerical investigations of the dynamic responses and energy absorbing properties of hybrid auxetic composite panels (HACPs) and equivalent honeycomb panels (EPPs) under blast loadings were conducted. The Poisson’s ratio and general mechanical behaviours were controlled by changing the geometrical parameters θ and rL. Convergence studies and comparison with analytical results were conducted to validate the numerical model. An empirical model for describing the relationship between the
Acknowledgements
This research is sponsored by the Australian Research Council linkage grant LP150100906. Authors also greatly appreciate the MIFRS and MIRS scholarships from the University of Melbourne.
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