Low-velocity impact resistance of composite sandwich panels with various types of auxetic and non-auxetic core structures

Highlights

• Polyurethane foam and PLA plastic cellular honeycombs are used as core material for composite sandwich panels.

• Auxetic core design could provides greater impact resistance and energy absorption capability.

• Non-auxetic cores could have advantages over the auxetic ones at small deformation.

• Arrowhead and hexachiral honeycomb cores are mostly recommended under impact loading.

Abstract

This work describes the low-velocity impact behavior of composite sandwich panels with different types of auxetic (negative Poisson’s ratio) and non-auxetic prismatic core structures. Sandwich panels have been manufactured with carbon/fiber epoxy composite face sheets, polyurethane rigid foam core or 3D printed PLA plastic cellular honeycombs head (hexagonal, re-entrant, hexachiral and arrowhead). The material properties of the constituents have been determined via tensile and compression tests. The cellular core topologies have the same wall thickness and number of cells (39x4, except for the hexachiral topology). A rigid striker with a hemispherical head tip is dropped on the specimens with a speed of 2.6 m/s. Explicit finite element (FE) models are validated by the experimental results. Parametric numerical analyses using the validated FE have been carried out with different impact energies of 10, 20, 30, 40, 50, 60 and 76 J to identify the best core designs. The results show that non-auxetic cores could have advantages over the auxetic ones at small deformation (impact energy is equal to 10 J) thanks to the larger contact surface and higher thickness of the cellular structure. The auxetic core, however, provides greater impact resistance and energy absorption capability as the impact energy increases due to the larger densification and lower indentation during collapse. The arrowhead-based panels in particular possess 25%, 13% and 11% larger crash efficiency than the other samples for impacts with 50, 60 and 76 J. The hexachiral lattice provides the best performance at 10, 20 and 30 J, and also possesses advantages over the other cellular configurations (except for the arrowhead core) in the case of 40, 50, 60 and 76 J impact loading. As a result, the arrowhead and hexachiral configurations are those mostly recommended for applications involving impacts under large deformations.

Introduction

Composite panels have a wide range of uses in the aerospace, marine/offshore industries and ground transportations due to their lightweight characteristics, acoustic insulation, high strength, high stiffness, corrosion resistance and wear resistance, etc. Increasing strength, energy absorption and reducing the weight of the structural components simultaneously are among the primary aims of engineering. The usage of sandwich panels has become popular in the aeronautics and aerospace industry, marine/offshore industries, ground transportations, and military applications thanks to its high specific strength, stiffness, tailorability, and stability properties against impact loading [1], [2], [3], [4], [5], [6], [7], [8]. They are composed of two thin stiff, and strong composite face sheets and thick lightweight core materials. They can protect structural integrity while absorbing large amounts of impact energy. Low-velocity impact protection has a particular interest in engineering applications. It induces more localized damage than quasi-static compression and less localized damage than high-velocity impact loading [9]. The deformation mechanism under high and low-velocity impact conditions is likely to be governed by localized material damage rather than global buckling.

The traditional approach is to bond composite laminates with the honeycomb or foam core. The energy absorption of the brittle composite laminates is mainly governed by elastic deformation. Deformation mechanism and transverse damage resistance are poor due to lack of reinforcement through the thickness direction [10]. Notably, carbon fiber/epoxy resin (CFRP) composites can absorb energy by material damage such as fiber breakage, matrix cracking, delamination rather than yielding [11]. The geometrical configuration has a key role in being able to absorb more energy and have more impact resistance. The interface angle, stacking sequence, ply orientation, ply thickness, and material type are the important parameters for the manufacturing composite laminates. Strait et al. [12] indicated that the stacking sequence has a significant effect on the impact resistance by comparing the energy to maximum load response of the cross-ply, quasi-isotropic, and [0/$\pm$45] lay-ups. Wang et al. [13] showed composite laminates with angle-ply stacking sequences can keep greater residual tensile strength than cross-ply stacking sequences. In addition, the relatively small thickness of laminates is prone to buckling under compression loading due to elastic instabilities [14].

Foams and cellular honeycombs are extensively used as core material for sandwich structures under impact loading [3]. The impact response of the foam core sandwich constructions is mostly related to the density and modulus of foam [4]. Heavier cores increase the local rigidities, which can annihilate the potential advantages of the core under low-velocity impact. CoDyre et al. [15] have indicated that by doubling and tripling the polyisocyanurate foam core density, the flexural strength of flax fiber-reinforced polymer skinned sandwich panels increases by 82% and 213%, respectively. The ultimate impact energy of those sandwich panels increases with the core density (32, 64 and 96 kg/m³) and face thickness (1, 2 and 3 layers of flax fabric) [16]. Sharma et al. [5] noted that PU core failure was governed by shear strain and delamination between the face sheet and core. The core material increased the support surfaces for bonding with the face sheets. Sharaf and Fam [17] have also observed that the vicinity of the supports could provide a significant effect on the delamination between skin and core and on the core shear of composite sandwich panels with orthogonal GFRP ribs and polyurethane foam core. Njuguna et al. [6] examined the impact response of PU core sandwich panels by filling the core with nanoparticles. This way increased the number of PU cells with the smaller dimensions and anisotropy index and led to higher peak loads. Zhang et al. [7] used a hybrid core material with the constituents of PU foam and pyramidal truss members to enhance the impact resistance and energy absorption of composite panels. The load-carrying capacity of the foam-filled sandwich structures with truss members was developed thanks to the synergistic effects of the core constituents. Crupi et al. [18] compared the low-velocity impact response of honeycomb and foam for the aluminum sandwiches and observed different collapse mechanisms. The collapse mechanism of honeycomb sandwiches is represented by the buckling of cells, while foam core sandwiches collapsed for the foam crushing.

The impact resistance and energy absorption capacity of the foam core were strongly related to foam quality, while the mechanical performance of the honeycomb core tended to cell size. Geometric configurations of corrugated cores had a significant influence on the energy absorption capability of the composite sandwich panels under low-velocity impact, unlike high-velocity impact where the sinusoidal and arc-shaped cores possessed lower energy absorption due to lower out-of-plane stiffness [19]. Jiang and Hu [20] fabricated a novel multilayer orthogonal structural composite with auxetic effect consisting of PU foam and reinforcement structures and then compared low-velocity impact performance with the non-auxetic one. They exhibited auxetic composite could provide better energy absorption. Xiong et al. [21] have presented a comprehensive review of the current state of the art on sandwich structures with foam-based and prismatic core constructions.

The polylactic acid (PLA) used here as core material of the cellular honeycombs is most common among polymers used in FDM 3D printing because of its low melting temperature, easy printing, good mechanical properties and biodegradability [22]. However, its equivalent mechanical properties are strongly dependent on the parameters of the manufacturing process (infill ratio, tool path and speed). Manufacturing conditions such as room temperature and melting temperature also affect its crystallization history [23], [24]. Subramaniam et al. [25] indicated that the yield strength, ultimate tensile strength and elastic modulus of the PLA they studied were 26.1, 32.9 and 807.5 MPa for optimum raster angle and infill density, respectively. Kain et al. [26] showed that the infill orientation and mechanical properties of the specimen printed with the angle of 0°, 15° crossed, 30° crossed, 45° crossed, 60° crossed and 75° had a direct interaction but no exclusive correlation. Yao et al. [27] found that the tensile strength of the PLA samples increased with the infill orientations by comparing the specimens printed with the seven different angles in the range of 0° to 90°. The ultimate tensile strength of the specimens changed between 26.6 and 55.9 MPa for the layer thickness of 0.1 mm. Therefore, the printing parameters should be taken into account to increase the mechanical properties of a 3D printed part.

Cellular honeycombs can be classified as auxetic and non-auxetic according to the sign of Poisson’s ratio. Auxetic materials that have a negative Poisson’s ratio (NPR) under tensile or compressive loading could improve resilience, shear resistance, heat insulation, vibration damping, energy absorption, indentation, and impact resistance. Here we focus on re-entrant [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], arrowhead [43], [44], [45], [46], [47], [48], [49], [50], [51] and chiral honeycombs [52], [53], [54], [55], [56], [57], [58], [59], which are the typical unit cell configurations of auxetic materials. The re-entrant configuration possesses the butterfly shape morphology [28]. Sarvestani et al. [37] presented the re-entrant configurations could get higher energy absorption capacity over hexagonal and rectangular cellular cores. Yang et al. [38] exhibited the re-entrant auxetic sandwich panels could achieve higher impact performance considering the combination of the reduction in peak load and increase in energy absorption capacity comparing to the hexagonal, rhombic and octahedral sandwich panels. The material of the re-entrant honeycomb core flowed into the impact region. It caused material concentration due to the negative Poisson’s ratio effect, which increased the impact resistance of the sandwich panels [39], [40].

Larsen et al. [47] first proposed the auxeticity of the double arrow-head (DAH) honeycombs. The interest in DAH honeycomb has increased thanks to its auxetic properties remarkably. Wang et al. [48] indicated that the stiffness and auxeticity of DAH auxetic structure increased with the increase of compression strain. The deformation pattern of the DAH configuration was governed by bending of the ribs rather than axial and shear deformation [49], [50]. Lim [51] exhibited the Poisson’s ratio of DAHs could be controlled by changing linkage lengths or half angles. Similar to the re-entrant and DAH configurations, hexachiral auxetic structures have also drawn increasing attention in recent years [52], [53], [54], [55], [56], [57], [58], [59]. Prall and Lakes [52] first proposed hexachiral honeycomb on the basis of the theoretical expressions under small deformation. The deformation mechanism of the chiral honeycombs could be identified by rotation of cylinders and flexing of ligaments [53].

This work presented here examines the low-velocity impact response of composite sandwich panels with a series of auxetic and non-auxetic core structures. To the best of the authors’ knowledge, the concepts evaluated in this work are not present in the open literature. We describe the design, manufacturing, characterization, drop weight tests, and Finite Element Analyses (FEA) of the sandwich structures with the constituents of carbon fiber face sheets and polyurethane rigid foam core or PLA plastic cellular core. The cellular cores here possessing different honeycomb topologies (hexagonal, re-entrant, DAH and hexachiral) were 3D printed with the same core thickness, cell wall thickness, and number of cells. Drop tests were carried out at the speed of 2.62 m/s with an impact energy of 76 J. Then, the FEA results were validated by comparing with the experimental results, and then impact the performance of the same models were analyzed at the different impact energies. Their response to localized impact loading was assessed in terms of dent depth, damaged surface, and impact resistance at different impact energy levels. The impact response of the structures with different cores was likely to be completely different. The results showed that using auxetic core structures improved the impact performance of composite sandwich panels. The auxetic designs used in this work provide an increase of the impact resistance of the specific sandwich panels developed here. Moreover, we provide a first in terms of comprehensive analysis of the advantages and disadvantages of the selected common auxetic cores over the non-auxetic ones by evaluating deformation patterns and crash efficiencies of the associated sandwich structures under the low-velocity impact.