Impact Performance of Composite Sandwich Shells with Cork Core

Impact Performance of Composite Sandwich Shells with Cork Core L.M. Ferreira1,2[0000−0002−5223−5309], C.A.C.P. Coelho3[0000−0002−4433−2002], and P.N.B Reis4[0000−0001−5203−3670] 1Grupo de Elasticidad y Resistencia de Materiales, Escuela Técnica Superior de Ingeniería, Universidad de Sevilla, Camino Descubrimientos S/N, 41092 Sevilla, Spain 2Escuela Politécnica Superior, Universidad de Sevilla, C/ Virgen de África 7, 41011 Sevilla, Spain [email protected] 3Unidade Departamental de Engenharias, Escola Superior de Tecnologia de Abrantes, Instituto Politécnico de Tomar, Rua 17 de Agosto de 1808 S/N, 2200-370 Abrantes Portugal [email protected] 4Department of Mechanical Engineering, University of Coimbra, CEMMPRE, ARISE, 3030-194 Coimbra, Portugal [email protected] Abstract. Composite sandwich structures have gained significant attention in engineering applications due to their exceptional strength-to- weight ratio, stiffness, and energy absorption capabilities. One promising core material is cork, which possesses desirable characteristics such as low density, high compressive strength, and good energy absorption capacity. This study aimed to compare the impact performance of composite shells with composite sandwich shells featuring an agglomerated cork core, while investigating the influence of semicylindrical shell diameters. The results underscore the significant impact of specimen diameter on the performance of these structures. Larger diameter specimens (106 mm) exhibited notably higher maximum force, albeit with lower maximum displacement, energy absorption, and contact time compared to those with smaller diameter (82 mm). Similar trends were observed in composite sandwich shells. Furthermore, the inclusion of cork in composite sandwich shells marginally reduced strength of the specimens. For identical diameters, both configurations exhibited comparable maximum force and displacement. However, composite sandwich shells with a cork core absorbed less energy than those without a cork core. This behaviour persisted across both shell geometries, with slightly higher percentage differences observed in the larger diameter specimens. Keywords: Composite sandwich shell ·Low-velocity impact ·Mechanical testing. 1 Introduction Composite sandwich structures have gained significant attention in engineering applications due to their exceptional strength-to-weight ratio, stiffness, and en- ergy absorption capabilities. These structures consist of two outer composite face sheets separated by a lightweight core material, providing enhanced mechanical properties and structural integrity. One promising core material is cork, which possesses desirable characteristics such as low density, high compressive strength, and good energy absorption capacity [13, 15, 10]. In recent years, several studies have explored the impact behaviour of composite shells [7, 12, 9, 8, 6] and composite sandwich shells [11, 4]. Specifically for composite sandwich shells, researchers have investigated various core materials, such as foams, honeycombs, and balsa wood, to enhance the impact resistance and energy absorption capabilities. However, the utilisation of cork as a core material in composite sandwich shells for impact applications remains relatively unexplored, despite its promising properties. This study builds upon the existing state of the art by examining the impact performance of composite sandwich shells with a cork core with different diameters and comparing the results with composite shells (without a cork core). The objective is to evaluate their response to impact loading, including factors such as maximum force, maximum displacement, energy absorption, and contact time. The findings of this study contribute to expand the knowledge on the impact behaviour of composite sandwich shells with a cork core, while investigating the effects of varying their diameter. This information can be used in optimising the design and material selection for applications where impact resistance is critical, such as aerospace, automotive, and marine industries. Additionally, the study provides insights into the potential advantages and limitations of using cork as a core material, helping to guide future development and improvement of composite sandwich structures. 2 Materials and experimental procedure Composite semicylindrical shells and composite sandwich semicylindrical shells with an agglomerated cork core, were manufactured with two different diameters using the hand lay-up method. The materials employed to manufacture the shells were the following: (i) plain weave T300 carbon fibre 3k fabric with a mass of 160 g/m2and a density of 1.76 g/cm3, supplied by Toray (as reinforcement); (ii) epoxy resin SR1500 with hardener SD2503, supplied by Sicomin (as matrix); (iii) agglomerated cork NL 20 CORECORK with 2 mm and a density of 0.25 g/cm3, supplied by Amorim Cork Composites (as sandwich core). The stacking sequences and the geometric parameters of the specimens employed in this study are listed in Table 1 and depicted in Fig. 1. Notice that all the composite layers were aligned in the same direction. Following the assembly of the layers, they were placed inside a vacuum bag for 24 hours. Subsequently, a maximum vacuum pressure of 0.5 bar was applied for 9 hours to ensure a consistent fibre volume fraction, uniform thickness, and removal of any air bubbles present within the laminate. In accordance with the supplier’s data-sheet, the post-curing process was conducted in an oven at 60°C for 16 hours. Table 1. Stacking sequence and geometric parameters of the specimens. Layup Sequence Label Thickness (mm) Diameter (mm) Length (mm) [0]8C1 2.1 82 100 [0]8C2 2.6 106 100 [04, Cork, 04]C//C1 4.0 82 100 [04, Cork, 04]C//C2 4.3 106 100 Fig. 1. Tested specimens: (a) Geometric parameters of the semicylindrical shells and impact location; (b) Lay-up sequence of the composite shell and of the composite sandwich shell. Fig. 2. Schematic representation of the experimental setup. The low-velocity impact tests were conducted using an IMATEK-IM10 drop weight testing machine. Detailed information about this impact machine can be found in [5]. A 10 mm diameter impactor, weighing 2.826 kg, was utilised, and the impact was directed at the centre of the samples. The curved edges of the specimens were free, while the straight edges were simply supported (Fig. 2). An impact energy of 5 J was employed, which had been previously determined to induce visible damage without causing perforation of the specimens. The impact tests were performed in accordance with the guidelines outlined in [2]. All tests were conducted at room temperature, and a total of five specimens were tested for each configuration. 3 Results and discussion To evaluate the impact performance of both the composite shells and the composite sandwich shells with cork core with two different diameters, an examination of the representative impact curves was conducted. These impact curves encompassed the force-time, force-displacement, and energy-time results for the various specimen configurations. In addition to the impact curves, parameters such as the maximum force and displacement values, as well as the absorbed energy quantities, were determined. These parameters play an important role in understanding and comparing the performance of the different specimen configurations. Furthermore, the contact time, which provides insights into the duration of impact and energy transfer, was also calculated and considered in the evaluation process. Representative results obtained specifically for the C1 and C2 specimens, which correspond to the composite shells with diameters of 82 mm and 106 mm, respectively, are shown in Fig. 3. Furthermore, in Fig. 4, the corresponding impact curves for the composite sandwich shells, specifically the C//C1 and C//C2 configurations are presented. It is evident from the experimental data that all response curves exhibit distinctive oscillations, which arise due to the propagation of elastic waves resulting from the vibrations of the specimens [14]. These oscillations are directly influenced by the stiffness and mass characteristics of both the test specimen and the impactor [3]. To facilitate the analysis and comparison among the various specimen configurations, the force and energy histories are presented in Fig. 5. These graphical representations provide a visual depiction of the force and energy variations throughout the impact event, enabling a better understanding of the impact performance of the considered configurations/geometries. Additionally, a summary of the most significant results extracted from the impact curves is compiled in Table 2. It encompasses parameters such as maximum force, maximum displacement, absorbed (dissipated) energy, and contact time. The experimental procedures incorporated the utilisation of a high-speed camera, enabling the recording and subsequent visualisation of the specimen’s deformation during the impact event. In Figure 6, a series of sample photos capturing the deformation of the specimens at the point of maximum displacement Fig. 3. Representative force-time, force-displacement and energy-time curves for the C1 and C2 composite shell specimens. Fig. 4. Representative force-time, force-displacement and energy-time curves for the C//C1 and C//C2 composite sandwich shell specimens. Table 2. Summary of the averaged impact results (maximum force, maximum displacement, absorbed energy and contact time) with standard deviations of the tested specimens. Specimen Max. Force (kN) Max. Displac. (mm) Abs. Energy (J) Contact time (ms) C1 1.36±0.08 5.76±0.10 4.74±0.25 12.6±0.12 C2 1.96±0.05 4.20±0.02 3.82±0.38 7.30±0.18 C//C1 1.31±0.22 5.51±0.18 4.38±0.22 9.98±0.34 C//C2 1.76±0.09 4.70±0.05 3.40±0.01 8.47±0.17 Fig. 5. Representative force-time, force-displacement and energy-time curves for all the tested specimens. are presented. These images provide a visual representation of the behaviour exhibited by the various configurations/geometries tested. It is worth noting that the quality of the photos depicting the C//C1 specimens was affected by poor lighting conditions during the testing process. As a result, the clarity and visibility of this particular image was compromised. Fig. 6. Tested specimens at the moment of maximum displacement. From the analysis of the energy histories in Fig. 5 for all the tested configurations, it is clear that the impactor makes contact with the specimens and subsequently rebounds, clearly indicating that the applied impact energy was insufficient to puncture the specimens. It is noteworthy that the initiation of the plateau phase signifies the moment when contact between the impactor and the specimen is lost, indicating the energy absorbed by the specimen [1]. The results highlight the significant impact of specimen diameter on the performance of composite shells and composite sandwich shells. Specifically, larger diameter specimens (C2) experienced a considerably higher maximum force compared to smaller diameter specimens (C1), with a difference of approximately 44.1%. In contrast, C1 exhibited higher maximum displacement (around 27.1%), absorbed more energy (about 19.4%), and had a longer contact time compared to C2 (approximately 19.8%). A similar trend can be observed when comparing the results of composite sandwich shells with different diameters. The percentage differences for the considered parameters were not substantially affected, except for maximum displacement. For example, C//C2 had a 34.9% higher maximum force compared to C//C1, while for the maximum displacement, absorbed energy and impact time, C//C1 experienced higher values than C//C2, with percentage differences of approximately 9%, 20.2% and 16% respectively. Furthermore, when comparing composite shells (C1 and C2) with composite sandwich shells with cork core (C//C1 and C//C2) of the same diameter, it becomes evident that the inclusion of cork reduces resistance and the amount of energy absorbed as damage, albeit to a minimal extent. For instance, C1 and C//C1 exhibit comparable maximum force and displacement values with percentage differences of less than 4.5%. However, C//C1 absorbs less energy and has a shorter contact time compared to C1, with percentage differences of approximately 7.6% and 21.1%, respectively. A similar pattern is observed when comparing C2 with C//C2. In this case, the percentage differences for maximum force, displacement, and absorbed energy are slightly higher, with values of 10.2%, 7.4%, and 11%, respectively. Additionally, C//C2 has a slightly shorter contact time compared to C2, with a percentage difference of about 14.1%. Overall, these results indicate that larger diameters lead to increased strength, while resulting in decreased absorbed energy, contact time and displacement. Furthermore, the composite shells composed solely of carbon fibre exhibit a higher degree of damage and greater strength compared to the composite sandwich shells with a cork core. 4 Conclusions This study compares the impact performance of two types of structures: composite shells and composite sandwich shells incorporating an agglomerated cork core. Moreover, it aims to investigate how varying semicylindrical shell diameters affect the performance of these structures. The findings emphasise the significant influence of specimen diameter on their behaviour. Specifically, when comparing composite shells, those with a 106 mm diameter displayed higher maximum force but lower maximum displacement, energy absorption, and contact time values in contrast to their 82 mm diameter counterparts. A similar pattern was observed in the composite sandwich shells. Additionally, the introduction of cork into composite sandwich shells led to a marginal reduction in impact strength. In cases with identical geometries, both configurations exhibited similar maximum force and displacement. However, composite sandwich shells with a cork core absorbed less energy. This trend was consistent across various shell geometries, with slightly more pronounced percentage differences observed in the larger diameter specimens. Acknowledgments. This research was sponsored by national funds through FCT — Fundação para a Ciência e a Tecnologia, under the project UIDB/00285/2020 and LA/P/0112/2020.