Dynamic crash responses of bio-inspired aluminum honeycomb sandwich structures with CFRP panels

Accepted Manuscript Dynamic crash responses of bio-inspired aluminum honeycomb sandwich structures with CFRP panels Yinghan Wu, Qiang Liu, Jie Fu, Qing Li, David Hui PII:

S1359-8368(17)30980-0

DOI:

10.1016/j.compositesb.2017.03.030

Reference:

JCOMB 4964

To appear in:

Composites Part B

Received Date: 12 September 2016 Revised Date:

28 November 2016

Accepted Date: 17 February 2017

Please cite this article as: Wu Y, Liu Q, Fu J, Li Q, Hui D, Dynamic crash responses of bio-inspired aluminum honeycomb sandwich structures with CFRP panels, Composites Part B (2017), doi: 10.1016/ j.compositesb.2017.03.030. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Dynamic Crash Responses of Bio-Inspired Aluminum Honeycomb Sandwich Structures with CFRP Panels Yinghan Wua, Qiang Liua,b*, Jie Fua, Qing Lic, David Huid a School of Engineering, Sun Yat-Sen University, Guangzhou City, 510006, China State Key Laboratory of Advanced Design and Manufacture for Vehicle Body, Hunan University, Changsha, 410082, China c School of Aerospace, Mechanical and Mechatronic Engineering, Sydney University, Sydney, NSW 2006, Australia d Department of Mechanical Engineering, University of New Orleans, USA

Abstract

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Nature has provided us with extraordinary resources to tackle design challenges facing in modern society nowadays. The multistate structures inspired by animal shell have proven effective to improve the impact resistance of composite laminate. This study aims to identify

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the crash responses and crashworthiness characteristics of bio-inspired sandwich structures composed of carbon fiber reinforced plastic (CFRP) panels and aluminum honeycomb. The crash responses, failure mode as well as the effects of core side length, height and impact velocity on peak load and energy absorption were explored herein. The differences of crashworthiness characteristics between the CFRP aluminum honeycomb sandwiches and

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bare CFRP panel were quantified. Two typical load-displacement relations, namely singlepeak and double-hump curves, were observed in the tests. It was noted in the energydisplacement curve, where the slopes corresponding to the failure stages of the upper and lower face-sheets, were greater than that in the honeycomb failure stage, indicating that the

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bare aluminum honeycomb was of lower energy absorption capacity than the CFRP facesheet. By comparison, the honeycomb filling was an effective way to improve the impact

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resistance of CFRP structure, yielding higher energy absorption and lower peak load during the impact. It was also found that the crashworthiness characteristics were more sensitive to the core length than to the core height; and the specific energy absorption (SEA) varied insignificantly with the increase in the core height. It was noted that the peak load, absorbed energy and SEA increased significantly under high impact velocity. Keywords: CFRP; Bio-inspired; Aluminum honeycomb; Crashworthiness; *Corresponding author. Tel.: +8620 39332766; fax: +8620 39332766. E-mail address: [email protected] (Q. Liu)

ACCEPTED MANUSCRIPT 1. Introduction The development of bio-inspired lightweight and high strength structures offers new research opportunity and methodological alternatives for addressing many of grand challenges in engineering. Biological materials, particularly structural composites, which exhibit remarkable mechanical properties [1], have drawn increasing attention from numerous

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researchers over the recent years. Honeycomb-like sandwich architectures have been widely observed in nature materials, such as turtle shell [2] or beetle forewings [3] (Fig. 1), which comprise of several layers including two exterior face-sheets and inner cores. These natureinherent materials are found to be effective to enhance the energy absorption capability, and

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have gained growing popularity in high performance applications for crashworthiness [4]. The carbon fiber reinforced plastics (CFRP) face-sheets are increasingly applied in aircraft and automobile industries attributable to their lightweight, high strength, extraordinary

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corrosion resistance and manufacturing flexibility [5-7]. Nevertheless, composites are typically rather sensitive to the dynamic impact; often a minor and invisible damage could even lead to significant reduction in structural strength and stiffness [8-10]. To tackle this problem thorough understanding of their mechanical behaviors and crashing responses would be of critical importance to more extensive and reliable applications of such composites [11].

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For bio-inspired sandwich configuration, aluminum honeycomb signifies a promising lightweight core material. Although aluminum honeycomb core can be combined with different panel materials hypothetically, majority of the existing studies have focused on aluminum face-sheets for crashworthiness in literature. For example, Goldsmith and Sackman

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[12] determined the energy dissipation and force transmission characteristics of aluminum honeycomb sandwiches with aluminum face plates for bearing dynamic loading. Paik et al. [13] evaluated the strength of aluminum sandwich panels with aluminum honeycomb core in

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quasi-static three-point bending, axial compression and lateral crushing loads; and they identified that an increase in core thickness delayed the initiation of plastic deformation, offering substantial increase in ultimate and crushing strengths. Crupi et al. [14] analyzed the static and low-velocity crash responses of two aluminum honeycomb face sandwich structures with different cell sizes. They [15] compared the static and low-velocity impact responses of aluminum foam and honeycomb sandwiches; and revealed that the collapse of honeycomb sandwiches was strongly influenced by the cell size in low-velocity impact tests. Jen et al. [16] studied the two-stage cumulative bending fatigue behavior of adhesively bonded aluminum sandwich panels with local indentation; they [17] also investigated the static and fatigue strengths of adhesively bonded aluminum honeycomb sandwich beams subjected to four2

ACCEPTED MANUSCRIPT point bending tests at a temperature range from -25 to 75 °C; and they showed that the ultimate loads and fatigue strengths decreased as temperature increased. Zhu et al. [18] investigated the structural responses of sandwich panels loaded by blasts; and reported that metallic sandwich panels with a cellular core such as honeycomb were of considerable capability of dissipating energy by a large plastic deformation under impact/blast loading.

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Yahaya et al. [19] compared the resistance of sandwich panels with different aluminum honeycomb cores, air sandwich panels and monolithic plates subjected to foam projectile impact; and they found that the honeycomb sandwich panels outperformed the air sandwich panels and the monolithic plates within an impulse range of 2.25 to 4.70 kNsm . Abbadi et

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al. [20] conducted an experimental test of fatigue for honeycomb sandwiches with or without artificial defects; and they found that the lifetime with the defects had no much effect on the static behavior of the structure. Foo et al. [21] developed a three-dimensional (3D) finite

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element (FE) model of the honeycomb sandwich structure in ABAQUS; and showed that it enabled further understanding of the parameters affecting the initiation and propagation of impact damage. Li et al. [22] conducted a blast test to investigate the blast-resistance of aluminum honeycomb sandwich structures with different heights and cell sizes; and further created the FE model for simulating the dynamic responses.

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There have also been some studies on sandwich structures comprised of the aluminum honeycomb and composite panels. Thomsen and Banks [23] performed a series of experimental tests on different configurations of CFRP/honeycomb sandwich panels in compression; and they found that an improved model of the intra-cell buckling could provide

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more accurate predictions than the classic design formulae. Hazizan and Cantwell [24] reported that the flexural modulus of the composite face-sheets and the shear modulus of the aluminum honeycomb core did not exhibit any strain-rate sensitivity in the conditions

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investigated. Shin et al. [25] analyzed the impact response of composite sandwich panels for low floor of a Korean bus; and found that glass face-sheet sandwich plates had a better resistance to impact compared with the aluminum face-sheets. Langdon et al. [26] investigated the response of honeycomb core sandwich panels under blast loading and concluded that the panels with composite face-sheet exhibited smaller residual displacements than the aluminum face-sheet. Shi et al. [27] studied the interfacial bonding and structural performance of carbon-fiber and aluminum-honeycomb sandwich panels; and showed that the short aramid fiber interleave method could prevent interfacial debonding under both bending and compression. Shi et al. [28] also conducted three point bending tests on carbon fiber sandwiches with different types of cores and suggested that the honeycomb filled orthogrid 3

ACCEPTED MANUSCRIPT core sandwich with carbon fiber face-sheet could provide better structural properties for thin walled structures. Ryan et al. [29] conducted hypervelocity impacts on CFRP/Aluminum honeycomb sandwich panel used in satellite structures; and found the new empirical Ballistic Limit Equation (BLE) provided a good approximation to ballistic performance prediction of stand-alone sandwich panel structures. Meo et al. [30] modeled the impact damage on a range

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of sandwich panels; and they suggested that the numerical simulation using LS-DYNA3D was effective for predicting the threshold of impact damage and delamination. Zhu and Chai [31] studied the damage and failure modes of composite sandwich panel subject to quasistatic indentation and low velocity impact. Ivanez et al. [32] simulated the responses of

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composite sandwich beams with honeycomb core subjected to low-velocity impact using ABAQUS/Explicit code; and revealed that the core played a critical role in the energy absorption of the sandwich beams at a low impact velocity.

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Despite the aforementioned works, there have been limited studies on the aluminum honeycomb sandwich structures with the CFRP panel compared to those with metallic panels. Further, few works have paid attention on the contribution of aluminum honeycomb filler on the crash responses of the CFRP structures. This study aims to investigate the crash responses of the bio-inspired sandwich composites with different geometries of honeycomb and

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different low impact velocities. The typical load-displacement and energy absorptiondisplacement curves are obtained first. The differences of crash responses between the sandwich CFRP structures and bare CFRP panels are then evaluated. The effects of the honeycomb structures and impact velocity on the energy absorption characteristics are

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explored in detail.

2. Experiments

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2.1 Preparation of specimens The configurational design of specimen was inspired by a typical honeycomb sandwich material in nature, such as the turtle shell and beetle. As shown in Fig. 1, turtle shell is potentially a bio-mimicking composite structure and essentially a sandwich composite comprising three distinct regions (two outer shells and one inner core). The outer hard shells act as a face-sheet of the sandwich composite serving as a shielding layer, and the foam-like core material between two outer shells behaves as shock absorber (the right picture in Fig. 1a). Fig. 1b plots the micro-cross section of the trabecula-honeycomb microstructure in the forewings of a beetle. The beetle forewing is a three-tiered structure, which consists of a void honeycomb-like trabeculae core, and two thinner and tougher laminations. 4

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Fig. 1 Honeycomb sandwich structures in nature: (a) turtle shell and (b) beetle forewings

Fig. 2 Schematic of tested specimens

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Honeycomb sandwich structures are widely existed in nature and exhibit considerable loading-bearing advantages under impact. It is therefore interesting to explore the crash responses of such bio-inspired structure. The configurations of the bio-inspired tested sandwich specimens consisting of two hard sub-surface layers and inner honeycomb soft core, are shown in Fig. 2. In this study, the honeycomb core was made of AA3003 aluminum alloy and the two face-sheets were fabricated by Toray plain weave carbon fiber T300/epoxy prepreg. The volume fraction of fibers was around 56%. The average tensile strength and Young’s modulus, determined from the tests on the 3-ply laminates according to ASTM D3039-76, was 590 MPa and 56 GPa, respectively [33].

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ACCEPTED MANUSCRIPT The CFRP 3-ply panels were pre-cured and bonded to the honeycomb core with epoxy adhesive film at a temperature of 120 oC for 1 hour [34]. In this study, the thicknesses of facesheet and honeycomb core were taken as the constants of 0.6 mm and 0.07 mm, respectively. Three sets of different parameters were considered herein: the first was that the side wall length of honeycomb core L equaled to 2.5, 5 and 7.5 mm; the second was that the core height

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H equaled to 10, 20 and 30 mm; the third was that the impact velocity equaled to 0.8, 1.2 and 1.6 m/s, respectively. For comparison, the 6-ply CFRP panels were also fabricated and sectioned to the same dimension as the sandwich specimens (i.e. 80×80 mm). The exact dimensions and testing results of the specimens described here were summarized in Table 1.

1.6 1.6 1.6 1.6 1.2 1.2 1.2 1.2

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Weight of Displacement First peak specimen (mm) load (kN) (g) 20.5 13.08 0.88 20.5 14.20 0.97 20.5 14.12 0.89 20.5 13.80 0.91 0.62 0.05 20.5 34.40 0.91 20.5 34.69 1.04 20.5 32.94 0.93 20.5 34.01 0.96 0.94 0.07 20.5 41.53 1.06 20.5 41.52 0.99 20.5 41.17 1.05 20.5 41.41 1.03 0.20 0.04 27.7 12.23 1.66 27.7 12.20 1.70 27.7 12.29 1.67 27.7 12.24 1.68 0.05 0.02 17.9 42.65 1.02 17.9 43.24 0.94 17.9 42.87 1.04 17.9 42.92 1.00 0.30 0.05 17.6 18.85 0.96 17.6 18.73 0.98 17.6 18.53 0.94 17.6 18.71 0.96 0.16 0.02 22.6 47.26 1.00 22.6 45.97 0.91 22.6 46.55 1.00 22.6 46.59 0.97 0.65 0.05 13.5 5.60 1.75 13.5 5.74 1.64 13.5 5.86 1.60 13.5 5.73 1.66 0.13 0.08

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Table 1 Geometry, impact velocity and crashworthy characteristics of the tested specimens

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1.23 1.10 1.23 1.19 0.07 1.41 1.25 1.19 1.28 0.11

1.03 0.97 1.07 1.02 0.05 1.83 1.86 1.85 1.85 0.02 0.97 0.88 1.01 0.95 0.07

0.61 0.58 0.58 0.59 0.02 0.56 0.55 0.59 0.57 0.02 0.74 0.74 0.75 0.74 0.00 1.23 1.23 1.23 1.23 0.00 0.48 0.48 0.48 0.48 0.00 0.87 0.87 0.88 0.87 0.01 0.45 0.46 0.46 0.46 0.00 1.18 1.16 1.13 1.15 0.03

Impact energy(J) 7.98 8.21 8.18 8.12 0.13 19.11 19.22 19.37 19.24 0.13 30.84 30.84 30.74 30.81 0.06 15.06 15.04 15.06 15.05 0.01 20.65 20.80 20.70 20.72 0.08 16.37 16.36 16.29 16.34 0.04 21.48 21.27 21.35 21.37 0.10 6.61 6.64 6.60 6.62 0.02

Abosrbed SEA(J/g) energy(J) 7.91 8.11 8.11 8.05 0.12 18.91 19.03 19.11 19.02 0.10 20.62 21.25 21.42 21.10 0.42 14.95 14.97 14.95 14.96 0.01 17.91 18.47 18.44 18.27 0.32 16.35 16.33 16.29 16.32 0.03 21.24 21.04 21.11 21.13 0.10 6.02 6.04 6.10 6.05 0.04

0.39 0.40 0.40 0.39 0.01 0.92 0.93 0.93 0.93 0.00 1.01 1.04 1.04 1.03 0.02 0.54 0.54 0.54 0.54 0.00 1.00 1.03 1.03 1.02 0.02 0.93 0.93 0.93 0.93 0.00 0.94 0.93 0.93 0.94 0.00 0.45 0.45 0.45 0.45 0.00

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1.14 1.21 1.31 1.22 0.08 1.27 1.24 1.25 1.25 0.01

15.37 15.18 15.00 15.19 0.19 27.16 27.23 27.17 27.19 0.04

15.34 15.03 14.99 15.12 0.19 14.50 14.62 15.09 14.74 0.31

1.14 1.11 1.11 1.12 0.01 1.07 1.08 1.12 1.09 0.02

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C6V2-1 C6V2-2 C6V2-3 Average S.D C6V3-1 C6V3-2 C6V3-3 Average S.D

2.2 Testing procedure

Dynamic axial impact tests of sandwich specimens were conducted in Instron 9250HV drop-weight test machine with a mass of 18.41 kg and a maximum drop height of 1.25 m. The

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corresponding impact velocities (0.8, 1.2 or 1.6 m/s) can be generated with different drop heights controlled by the test machine automatically. Fig. 3 shows the setup of the dropweight impact test, where the impactor with a hemispherical nose of 20 mm diameter was

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used. The specimen was placed between two the pneumatic clamps with a middle hole of 40 mm diameter, then the impactor free fell onto the specimens along two smooth column guides through the center hole, and two sensors were used to record data. A velocity sensor mounted at the side column recorded the initial velocity of the impactor. A force sensor attached to the tip of impactor recorded the load-time response, through which other impact parameters, such

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as displacement, velocity and acceleration, can be derived from the load-time and velocitytime responses [35]. Two pneumatic-controlled rebound arrestors near the guides were used to

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prevent multiple impacts.

Fig. 3 Schematic of testing setup for dynamic impact test 2.3 Finite element analysis (FEA) 7

ACCEPTED MANUSCRIPT Transient dynamic FEA was performed using the LS-DYNA finite element (FE) code to simulate the failure process of the sandwich structure, which was difficult to observe in details experimentally, as shown in Fig. 4. The CFRP face-sheets were modeled using MAT 54 in LS-DYNA, and they necessitate a progressive failure model for shell elements using a modified Chang-Chang failure criterion, which is capable of predicting tensile and

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compressive fiber failure, as well as tensile and compressive matrix failure. The aluminum honeycomb was modeled using an orthotropic elastic-plastic model MAT 3. There are three types of contact defined between the impactor, face-sheets and honeycomb, namely automatic single surface, automatic surface to surface and tied nodes to surface (as illustrated in Fig. 4).

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The adhesive joints between the face-sheets and core were modeled using the tie-break contact, which was used to tie the edges of adjacent shells together. The master and slave nodes were tied by means of a normal and a shear mathematical relationship. The impact

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velocity, mass and boundary conditions were prescribed consistently with the experimental

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Fig. 4 Schematic of finite element model

3. Results and Discussion

3.1 Load-displacement curves 3.1.1 Typical load-displacement curve The contact load can be measured as the compression exerted on the impactor by the specimens, which is one of the key factors quantifying structural responses to the impact loading [36]. The load-displacement (L-d) curve can be obtained from the load-time and displacement-time curves recorded by the sensors. It is noted that there are two typical load8

ACCEPTED MANUSCRIPT displacement curves as shown in Fig. 5. The first is characterized in a single-peak curve (black curve), which was observed in the lower impact energy tests. When the impactor contacted the upper face-sheet of the specimen, the load started to increase almost linearly with the displacement. The local face-sheet experienced primarily in membrane stretching; and the honeycomb underneath the face-sheet started to crush progressively. Then the load

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reached the peak before the initiation of fracture took place on the upper face-sheet (A in Fig. 5). After that, the load started to drop and the crack grew along fracture zone to the surrounding in the upper face-sheet. At the end, the load dropped to zero when the impactor stopped.

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The second is characterized in a double-hump curve (red curve), with the two significant peaks (B and C) in different stages of the curves. Under the higher impact energies, the load gradually dropped from the first peak (B) to a nearly constant value, causing a penetration

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hole in the upper face-sheet. Then honeycomb continued to buckle and crushing, providing a stable resistant load (i.e. the plateau D as pointed in curve). When the impactor reached the lower face-sheet, the resistant load started to increase approximately linearly, reached the second peak (C) as in Fig. 5; and then dropped to zero, with a similar failure mechanism to that in the first cycle. It is interesting to note that the second peak load was higher than the

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first peak in the impact test of sandwich structures with the 3-ply CFRP face-sheets, due to the combination of the frictional resistance between the penetrated impactor and honeycomb as well as between the impact and the face-sheets. Similar finding was observed in the aluminum foam sandwich panels subjected to quasi-static indentation loads in literature. For

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example, Ruan et al. [37] compared the effects of face-sheet thickness of aluminum foam sandwich panels and showed that the first peak load was lower than the second one for the thin face-sheets, whereas the first and second peaks were approximately equal for the thick

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face-sheets.

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Fig. 5 Typical load-displacement curves in dynamic impact tests

3.1.2 Comparison of load-displacement curves

Fig. 6a plots the load-displacement curves of specimens with different side lengths (L). There was only one peak load in the curve of specimen with the length of L=2.5 mm due to its upper face-sheet damage and core crushing. The curves for the specimens with the length of 5 and 7.5 mm were of the double-hump shapes, which corresponded to a lower first peak load

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and farther displacement, attributable to the failure not only in the upper face-sheet but also in the lower face-sheet. Fig. 6b plots the load-displacement curves of the specimens with different heights (H). It is noted that these three curves were of fairly similar double-hump shape, but the second peak decreased with the increase in height of aluminum honeycomb.

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The other feature is that the length of plateau increased with the increase in the height. Fig. 6c plots the load-displacement curves of the same specimens (i.e. L=5 and H=20) under the

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different impact velocities. The curve for the specimen under the impact velocity of 0.8 m/s had only the first peak load representing the low impact energy. The other two curves were of a similar double-hump shape due to the failure of the upper and lower face-sheets under the high impact energy.

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(H) and (c) different impact velocities (V).

3.2 Energy-displacement curves

3.2.1 Typical energy-displacement curve

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Fig. 7 plots the three typical energy-displacement curves observed in the tests. In the first type of curve as shown, the energy absorption increased almost linearly with the displacement,

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which was attributable to that the impactor could only reach the upper face-sheet with a shallow indentation depth into the honeycomb in the impact process. The second type of curve was divided into the two phases of energy absorption, which was observed only in the specimens with a low core height. The upper face-sheet and a portion of honeycomb absorbed impact energy in the first phase, then the impactor continued to penetrate until reached the lower face-sheet even if the upper face-sheet was not completely destroyed, in which both the upper and lower face-sheets contributed on energy absorption, thus leading to the superposition of energy absorption and further rise of the curve in the second phase. The third type of curve represented three stages and was most common in this study. The first stage was similar to that in the second type curve, where only the aluminum honeycomb 11

ACCEPTED MANUSCRIPT absorbed energy after the upper face-sheet completely penetrated, making the energy absorption slope lower. In the third stage, the impactor contacted with the lower face-sheet; and then the lower face-sheet played a key role to absorb energy, thus stacking energy and making the energy absorption slope higher in the third stage. It was noted that the slope in the first and third stage was greater than that in the second stage, indicating that the bare

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Fig. 7 Typical energy-displacement curves in dynamic impact tests

3.2.2 Comparison of energy-displacement curves Fig. 8a plots the energy-displacement curves of specimens with different lengths (L). The curve of specimen with the length of L=2.5 mm was the first type, in which the high-density

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honeycomb presented the excellent impact resistance. In this case the impactor could not reach the lower face-sheet under the same impact energy. When the core length increased to 5

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and 7.5 mm, the impactor penetrated the top face-sheet and further reached the lower facesheet, where the curve changed to the third type. The difference of curves at the second and the third stage indicated the difference of impact resistance attributable to the core size. Fig. 8b plots the energy-displacement curves of specimens with different heights (H). The height of the curve increased with the honeycomb height, indicating that the height could help increase the capacity of energy absorption of the sandwich structures. Note that only the curve of the specimen with the smallest height of 10 mm presented the second type, implying that the impactor was easy to reach the lower face-sheet due to the short height of honeycomb filler.

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ACCEPTED MANUSCRIPT Fig. 8c plots the energy-displacement curves of the same specimens under the different impact velocities (V). It was found that the absorbed energy increased with the impacted energy for the same structures. Whereas the increase in the energy absorption when the lower face-sheet was penetrated (difference between the V3 and V2) was not so significant as to that

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Fig. 8 Comparison of energy-displacement curves: (a) different lengths, (b) different heights and (c) different impact velocities

3.3 Failure mechanics

Finite element analysis was performed to replicate the failure process. The numerical results of specimen under impact and experimental results of impacted specimen are shown in Fig. 9. The whole deformation process can be summarized as the following three stages: (a) failure of upper face-sheet, (b) damage of aluminum honeycomb core; and (c) failure of lower face-sheet. These failure stages are consistent with Taraghi and Fereidoon’s findings [38] in the nanocomposite foam-core sandwich panel.

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Fig. 9 Failure mechanism of honeycomb sandwich structure

As shown in Fig. 9a, depending on the magnitude of the impacting impulse, the upper face-sheet progressed up to a maximum deformation before it started to fracture from excessive local bending and stretching, leading to fiber breakage and resin matrix cracking in the central point. Further increasing the impulse promoted the fracture to propagate along the

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periphery of the projectile on the upper face-sheet, leading to the cross fiber fracture in the impacted area (Figs. 9b and 9f). At the same time, the core near the upper face-sheet started to buckle under further compression.

After the upper face-sheet was completely penetrated, the honeycomb core buckled

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progressively and presented the main impact resistance (Fig. 9c). The deformation of the honeycomb core included three modes: the core crushing, core shear and core buckling, which

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were similar to those observed in the low-velocity impact scenarios [39]. It is interesting to note that the main failure mode of the specimens with a smaller length and height of honeycomb exhibited the progressive crushing of core. With the increase in the side length of core, the deformation mode included core crushing, shear and global buckling (Fig. 12a), and the proportion of core shear increased with the height. Similar to the upper face-sheet, the lower face-sheet was deformed by local bending and stretching (Figs. 9d and 9e), which led to different degrees of penetration at a high impact velocity [40]. When the impact energy was low, the lower face-sheet would appear fracture zone and a slightly visible bulge due to the elastic recovery ability of CFRP panel (Fig. 9g);

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ACCEPTED MANUSCRIPT but under the higher impact energy, the lower face-sheet would appear diamond-shaped fracture attributable to the fiber breakage around the pitting (Fig. 9h).

3.4 Effects of honeycomb filling Fig. 10 plots the crashworthiness characteristics between the bare 6-ply CFRP panel and

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aluminum honeycomb (H=20 and L=5) CFRP sandwich structures. It is clear that the peak and average loads of the sandwich structure were much smaller than those of the 6-ply CFRP panel. The peak loads of sandwich structures decreased by 45.2%, 31.2% and 21.0% respectively under the impact velocities of 0.8, 1.2 and 1.6 m/s; and the average loads

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decreased by 48.7%, 53.3% and 40.8%, respectively. It is shown in Fig. 10b that the sandwich structures absorbed more energy than the 6-ply CFRP panel, increasing around 33.1%, 25.8% and 43.1%, respectively under the impact velocity of 0.8, 1.2 and 1.6 m/s. This means that the

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aluminum honeycomb filler was truly effective to enhance the crashworthiness characteristics of CFRP panel. Note that adding the extra weight from the aluminum honeycomb, the SEA values of sandwich structures were lower than that of the bare CFRP panel. In literature, Sadowski and Zhang [41, 42] demonstrated that the proper choice of core materials could remarkably improve both static and dynamic properties of the sandwich structures. 24

H20L5 2.0

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Fig. 10 Comparison of H20L5 and C6: (a) peak and average loads, and (b) energy absorption 3.5 Effect of core geometric parameters 3.5.1 Effects of core side length Fig. 11 plots the crashworthiness characteristics of the specimens with three different side lengths (L=2.5, 5 and 7.5 mm) but the same height (H=20 mm) and the same impact velocity (V=1.2 m/s). In Fig. 11a, the peak and average loads decreased with the increasing core length. The honeycomb with side length of L=2.5 mm provided a stronger resistance to penetration, leading to invasion on the upper face-sheet only and crushing of the top part of 15

ACCEPTED MANUSCRIPT honeycomb core. Correspondingly, such a denser core exhibited a great peak load and experienced smaller damage profile in the upper face-sheet and core, implying that a greater side length could result in more severe damage. In literature, Foo et al. [21] also reported that a smaller cell size could improve the compression resistance and cushioning properties of aluminum and paper core in sandwich structures under impact test.

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Nevertheless, the energy absorption and SEA of sandwich structure showed different trends. In Fig. 11b, it is interesting to note that despite different core side length with 2.5, 5 and 7.5 mm respectively, the corresponding energy absorption were 14.96, 19.02 and 18.27 J respectively, and the corresponding SEA were 0.54, 0.93 and 1.02 J/g respectively. The

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specimens with the middle side length (L=5.0) exhibited the highest energy absorption; and furthermore the SEA increased with the core length (Fig. 11b). This is because not only the upper face-sheet, but the honeycomb and lower face-sheet played the key roles for the energy

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absorption, leading to the different degrees of penetration in the lower face-sheet. As shown in Fig. 12a, different with the specimen of H20L2.5V2, a visible damage was observed in the lower face-sheet in the specimens of H20L5V2 and H20L7.5V2. The specimen of H20L7.5V2 showed the approximate absorbed energy with the specimen of H20L5V2, but a slightly higher SEA attributable to the lighter weight (17.9 g and 20.5 g, respectively, listed in

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Table 1). Another interesting finding was that the penetration shape of the face-sheets. With the core length increased, the damage area of the upper face-sheet varied from the round to hexagonal shape, whereas the lower face-sheet showed the consistent shape of diamond. Furthermore, as the core length increased, the damage area of the upper and lower face-sheets

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increased.

For the case of the denser honeycomb, higher strengthening effect could be achieved, but the structures may lose its weight efficiency [7]. Consequently, selection of appropriate side

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length for honeycomb is critical to achieve higher energy absorption and weight efficiency in practical lightweight structure design.

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Fig. 11 Effects of side length on: (a) peak and average loads, and (b) energy absorption

Fig. 12 The profile of specimen after impact: (a) length, (b) height and (c) velocity group

3.5.2 Effects of core height 17

SEA (J/g)

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The absorbed energy of honeycomb sandwich increased with the core height, attributable to the increase in the crushing extent of the honeycomb (shown in the section of impacted specimen in Fig. 12b). The absorbed energy increased almost linearly with the height of honeycomb cores. Interestingly, the value of SEA was not sensitive to the increase in core

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height, showing almost a constant SEA in Fig. 13b. This was also identified in Paik’s [43] study where the core height was not an influential parameter on the crushing behavior of

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aluminum sandwich panels with aluminum honeycomb core.

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Fig. 14 Effects of impact velocity on: (a) peak and average loads, and (b) energy absorption

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ACCEPTED MANUSCRIPT 3.6 Effects of impact velocity Fig. 14 plots the crashworthiness characteristics of the specimens under three different impact velocities (V=0.8, 1.2 and 1.6 m/s) but with the same core length (L=2.5 mm) and the same core height (H=20 mm). The effect of the impact velocity on the crashworthiness characteristics was significant. The peak load, average load, absorbed energy and SEA

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increased consistently with the impact velocity. It was attributable to the different degrees of the failure in the specimens under different impact velocities. As shown in Fig. 12c, higher impact velocity led to a greater damage area and severe deformation in the upper and lower face-sheets. In contrast, only a concave area in the upper face-sheet was observed under the

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low impact velocity of V=0.8 m/s. The penetration of upper face-sheet, the crushing of honeycomb and penetration of lower face-sheet occurred sequentially to absorb the impact energy under the higher impact velocities (V=1.2 and 1.6 m/s), implying that the sandwich

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structure with aluminum honeycomb was sensitive to impact velocity. In literature, similar conclusions were drawn in a series of low velocity impact tests on aluminum honeycomb sandwich structure with aluminum face-sheets [24].

4. Conclusions

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The dynamic crash responses and crashworthiness characteristics of bio-inspired aluminum honeycomb sandwich CFRP structure were investigated experimentally and numerically in this study. The crash responses, such as contact load, energy absorption histories and failure mode were explored. The effects of core side length, height and impact

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velocity on peak load and energy absorption were investigated. The differences of crashworthiness characteristics between aluminum honeycomb CFRP sandwiches and bare

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CFRP panel were compared. Within the limitations of the study, the following conclusions can be drawn:

(1) Two distinct types of load-displacement curves and three different types of energydisplacement curves were observed through the impact tests. Different from the single-peak curves observed in the short core length and low energy groups, the double-hump curves were obtained in the specimens when both the upper and lower face-sheets were penetrated. It was noted that in the energy-displacement curve, the slopes in the first and third stage were greater than that in the second stage, indicating that the bare aluminum honeycomb was of lower energy absorption capacity than the CFRP face-sheet.

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ACCEPTED MANUSCRIPT (2) It was shown that the upper CFRP face-sheet, aluminum honeycomb core and lower CFRP face-sheet failed in a sequential manner. The failure of the CFRP face-sheet included the matrix cracking and fiber breakage, leading to different damage modes, such as indentation, round penetration and irregularly-shaped penetration. The failure of aluminum honeycomb included the crushing, shear and buckling in the core.

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(3) The CFRP panels filled with the aluminum honeycomb core exhibited substantial improvements in crashworthiness. The peak loads of sandwich structures decreased by 21.0% to 45.2%, and the energy absorption increased by 25.8% to 43.1%, compared to those of bare CFRP panels.

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(4) Core side length was more sensitive to the crashworthiness characteristics of sandwich structures than the core height. The peak and average loads decreased; whereas the energy

but not with the core height.

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absorption increased with the core length and height. The SEA increased with the core length,

(5) The impact velocity had considerable effect on the crashworthiness characteristics of such sandwich structures, leading to different peak load, energy absorption and SEA attributable to the different degrees of failure in the upper face-sheet, aluminum honeycomb

Acknowledgements

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core and lower face-sheet.

This investigation was financially supported by the National Natural Science Foundation of China (51675540), the Outstanding Young Scholars of Guangdong Province

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(2015TQ01X371), the Pearl River S&T Nova Program of Guangzhou (2014J2200005), the Natural Science Foundation of Guangdong Province (2015A030313016), the Natural Science Foundation of Hunan Province (2016JJ3039) and the China Postdoctoral Science Foundation

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(2015M582323).

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