Effect of 3D-Printed Hexagonal Honeycomb Core Density of PLAWood Based Subjected to Low-Velocity Impact

Journal of Natural Fibre Polymer Composites (JNFPC) 3(2) (2024), 3 Research Paper Effect of 3D-Printed Hexagonal Honeycomb Core Density of PLA- Wood Based Subjected to Low-Velocity Impact F. Nur Ainin1, M.D. Azaman1,2,*, M.S. Abdul Majid1,2, M.J.M. Ridzuan1,2, L. M. Ferreira3,4 and C. A. C. P. Coelho5 1 Faculty of Mechanical Engineering & Technology, Universiti Malaysia Perlis, 02600 Arau, Perlis, Malaysia. 2 Centre of Excellence Frontier Materials Research, Universiti Malaysia Perlis, 02600 Arau, Perlis, Malaysia. 3 Grupo de Elasticidad y Resistencia de Materiales, Escuela Técnica Superior de Ingeniería, Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain. 4 Escuela Politécnica Superior, Universidad de Sevilla, c/Virgen de África 7, 41011 Sevilla, Spain. 5 Unidade Departamental de Engenharias, Escola Superior de Tecnologia de Abrantes, Instituto Politécnico de Tomar, Rua 17 de Agosto de 1808, 2200-370 Abrantes, Portugal. * Correspondence: [email protected] Abstract: Additive manufacturing (AM) technology has become the preferred method for fabricating lightweight sandwich composite structures, due to its ability to produce complex designs rapidly. However, these structures are susceptible to performance decline and potential damage, especially under impact loading in engineering applications. This study investigates the low-velocity impact characteristics of 3D-printed hexagonal honeycomb cores made from wood-filled polylactic acid (PLA) with unit cell sizes of 6, 8, and 10 mm. Energy absorption and failure mechanisms were assessed through drop-weight impact testing at an energy level of 11 J, with results analyzed using a stereo microscope. The findings demonstrate that unit cell size significantly impacts the performance of sandwich composite structures. Smaller unit cells increase core density, leading to enhanced energy absorption capabilities. The medium-density 8 mm unit cell is identified as the optimal structure for lightweight materials, offering efficient energy absorption and intermediate failure modes. While lighter than the high-density 6 mm unit cell, the 8 mm unit cell absorbs a comparable amount of energy (8 mm: 9.22 J, 6 mm: 9.61 J). Furthermore, this medium-density cell outperforms the low-density 10 mm unit cell, which absorbs only 7.44 J, due to its intermediate stiffness that better resists substantial deformation compared to the lower-density structure. Keywords: Additive Manufacturing; Sandwich Composite Structure; Low-Velocity Impact; Energy Absorption; Failure Mechanism 1. Introduction Additive manufacturing (AM) is a computer-controlled technique for producing structures from three-dimensional (3D) computer-aided design (CAD) model data through the layer-by- layer deposition of materials, as defined by ASTM-I F2792: Standard Terminology for Additive Manufacturing Technologies [1]. This advanced technology significantly outperforms traditional manufacturing methods, including conventional machining. Rapid prototyping, additive techniques, additive layer manufacturing, layered manufacturing, and freeform fabrication are terms associated with AM, a novel manufacturing technology highlighted in previous research [2], [3]. Recent advancements in additive manufacturing for lightweight sandwich composite structures have garnered attention from the aerospace, automotive, marine, and wind energy sectors. These structures provide a favorable ratio of flexural stiffness to weight, effective thermal insulation, and significant energy absorption capacity [4]. Composite materials are frequently utilized in applications; however, it is essential to recognize that sandwich structures are susceptible to impact damage, leading to significant reductions in stiffness and residual strength [5]. A case study of the Boeing 747 indicated that 13% of repairs were attributed to Low-Velocity Contact from Foreign Body Impact, derived from a sample of seventy-one aircraft with over 600 repair characteristics (damage) [6]. Although 3D printed structures present several benefits, including reduced costs and enhanced design flexibility, their adoption in the aerospace and automotive sectors remains limited due to uncertainties regarding their mechanical performance under impact conditions [7]. Citation: Nur Ainin, F.; Azaman, M.D. Abdul Majid; M.S., Ridzuan, M.J.M.; Ferreira, L. M.; and Coelho, C. A. C. P. Effect of 3D-Printed Hexagonal Honeycomb Core Density of PLA-Wood Based Subjected to Low-Velocity Impact. Journal of Natural Fibre Polymer Composites (JNFPC) 2024, 3(2), 3. Academic Editor: Harussani M.M. Received: 15th June 2024 Accepted: 4th December 2024 Published: 31st December 2024 2821-3289 JNFPC 2024, 3(2), 3 2 of 9 Numerous studies are being performed to develop sandwich structures that increase structural efficiency by reducing the weight of the structure while maintaining its impact-resistance strength. As a case in point, honeycomb cores, commonly employed in sandwich panels by engineering applications, are recognized for their exceptional energy absorption capability, as well as their favorable mechanical properties and low-density characteristics. Thus, according to prior findings [8], [9], [10], sandwich core geometry greatly influences energy absorption and failure mechanisms. Hou et al. [11] investigated the low-velocity impact resistance of 3D-printed auxetic (re-entrant honeycomb) and non-auxetic (diamond lattice and conventional honeycomb) lattice composites. The re-entrant topology exhibits reduced energy absorption capacity while demonstrating enhanced robustness and endurance. Consequently, when the impact energy is sufficient, the re-entrant panel operates effectively regarding force reduction and energy dissipation. Wang et al. [8] employed 3D printing to evaluate the impact characteristics of sandwich panels featuring lattice structures in comparison to those with solid structures at a low impact velocity of 2.48 m/s. Experiments indicate that sandwich panels with a lattice structure demonstrate greater energy absorption effectiveness compared to solid sandwich panels. Lattice structures facilitate greater deformation during impact, thereby enhancing energy absorption capabilities. Graded lattice cores enhance the energy absorption capacity of sandwich panels. He et al. [12] examined the dynamic behavior of foamcore composite sandwich plates under low-velocity impact through drop-weight impact tests and numerical simulations. The investigation revealed that the top faceplates of specimens with significant foam-core thickness are vulnerable to low-velocity impacts at different energy levels, while the increased foam-core thickness significantly reduces the deformation of the internal plates. Akatay et al. [13] assessed the impact behavior of honeycomb sandwich structures utilized in aircraft interiors subjected to repeated impacts. Based on the results of low-velocity single-impact tests, the total penetration impact energy was measured at 110 J, indicating that the sample experienced penetration from a single impact. Alternatively, the sample experienced the lowest impact energy of 5 J, leading to minimal matrix cracking. Increasing the impact energy level significantly expanded the number of impacts that resulted in complete perforation of samples. Ozen et al. [14] evaluated the low-energy impact response of woven carbon fiber reinforced plastic (CFRP) composite sandwich panels with thermoplastic honeycomb and re-entrant cores under three different impact energies: 20, 40, and 70 J. The in-plane re-entrant core-based composite sandwich panel demonstrates superior impact strength and energy dissipation compared to both in-plane and out-of-plane honeycomb core-based composite sandwich panels. Sarvestani et al. [15] conducted a theoretical and experimental study to assess the low-energy impact response of rectangular, hexagonal, and auxetic (re-entry) sandwich core specimens. Their experimental and theoretical findings indicate that auxetic patterned cores produced by 3D printing may be advantageous for composite sandwich panels in terms of energy absorption. Minimal experimental research has been conducted in this area addressing sandwich structures composed of natural fiber-filled polymers, widely known as biocomposites with excellent mechanical properties. The current focus on biodegradable materials aims to achieve environmental friendliness, exceptional stiffness, and high strength in sandwich panels, comparable to traditional materials. According to previous studies, Smardzewski et al. [16] determined the compression and low-velocity impact resistance of wood-based sandwich panels featuring auxetic lattice cores, utilizing 3D printing with LayWood bio-composite filament, as indicated by prior research. The findings demonstrated a proportional relationship between the energy absorbed by samples featuring high-density fiberboard face sheets and the energy of the impactor. The energy absorption of panels with high-pressure laminate face sheets decreases. Daver et al. [17] reported on the design and development of cork-poly (lactic acid) (PLA) biodegradable filaments for the characterization of mechanical properties using fused deposition modelling (FDM). The results indicate that the incorporation of cork granules into the cellular structure decreased tensile strength while enhancing impact strength. Essassi et al. [18] investigated the cyclic fatigue bending behavior of 3D-printed bio-based sandwich structure composites featuring auxetic cores. The results indicate that core density significantly influences the fatigue performance of sandwich composites, with lower core density associated with extended fatigue life. It emphasizes the impact of stiffness deterioration on fatigue strength. Ayrilmis et al. [19] studied the effect of the face/core layer ratio on the mechanical properties of 3D-printed wood/polylactic acid (PLA) green biocomposite panels featuring a gyroid core. This study recommends gyroid-structured composites with a skin thickness of 2 mm or greater, as they exhibit superior mechanical properties compared to composites with thinner skins. Previous studies highlight the potential of biocomposites for sustainable, high-performance sandwich structures and emphasize the importance of material composition and structural design in optimizing mechanical properties and energy absorption. JNFPC 2024, 3(2), 3 3 of 9 Current research work addresses the research gap in understanding the energy absorption capacities and failure modes of 3D-printed hexagonal honeycomb cores with varying geometries, specifically under low-velocity impact conditions. By focusing on PLA-Wood material with unit cell sizes of 6, 8, and 10 mm, the investigation aims to evaluate the impact resistance and identify the types of damages and failure modes in the sandwich panel structure according to ASTM D7136 with an impact energy input of 11 J. The study's objectives include assessing the influence of different geometries on impact resistance, understanding the mechanical behavior of PLA-Wood under impact, and conducting a comparative analysis to determine the optimal unit cell size for maximum energy absorption and minimal damage, thereby contributing valuable insights into the field of composite materials and impact-resistant structures. 2. Materials and Methods 2.1. Fabrication of Sandwich Composite Structures The specifications of the hexagonal honeycomb core in both 2D and 3D CAD models, encompassing the unit cell thickness, are depicted in Figure 1(a), whereas the length, width, and height are presented in Figure 1(b). The Ender 3 Pro 3D printer from Shenzhen Creality 3D Technology Co., Ltd employs FDM technology to additively produce architected samples featuring a full infill density of 100%. Figure 2 illustrates three distinct unit cells of 3D-printed hexagonal honeycomb cellular cores (6, 8, and 10 mm) created with SUNLU brand PLA + 18-20% Wood filament. The relevant core configurations are presented in Table 1. Each sample was bonded using a chosen face sheet material of carbon fiber fabric, employing an epoxy resin mixture of EpoxAmite 100 and 103 Slow Hardener under a vacuum bagging process to enhance the 3D-printed core into sandwich composite structures. Figure 1. (a) Thickness of unit cell; (b) Dimensions of the core Figure 2. The unit cell sizes of 3D-printed PLA-Wood hexagonal honeycomb core structure. (a) 6 mm; (b) 8 mm; (c) 10 mm Table 1. The parameters of hexagonal honeycomb core structure Size of Unit cell (mm) Mass (g) Density (g/cm3) 6 19 0.4398 8 17 0.3935 10 15 0.3472 (a) 1 mm 12 mm 60 mm 60 mm (b) (a) (b) (c) JNFPC 2024, 3(2), 3 4 of 9 2.2. Mechanical testing The low-velocity impact tests were conducted on sandwich composite structures featuring various unit cell sizes of 3D-printed hexagonal honeycomb core, utilizing a drop weight impact tester machine, model IM10R manufactured by IMATEK Ltd, in accordance with ASTM D7136. A hemi-spherical impactor, measuring 10 mm in diameter and weighing 0.71 kg, was utilized in the study. The sample was secured between two parallel rigid clamps featuring a central hole during the impact test, as illustrated in Figure 3. The experimental testing was carried out with a chosen impact energy of 11 J, and the software system provided the results of the data experiment, enabling the user to acquire various impact characteristics including energy absorption, impactor displacement, and the recorded contact force during impact. The calculation of absorbed energy for each sample was derived from the area under the curve representing force versus displacement. Following the impact test, the failure mode was assessed through the use of a stereo microscope to evaluate and analyze the surface damage. A comprehensive analysis was carried out to determine the configuration selected for the sandwich composite structure. Figure 3. Drop-weight impact system 3. Results and Discussion 3.1. Force-displacement Figure 4 illustrates the load-displacement results from low-velocity impact tests performed on honeycomb structures featuring various unit cells (6, 8, and 10 mm) and exposed to an impact energy of 11 J. The phenomenon observed during the impact event distinctly illustrates that the load exhibits an initial linear trend with displacement at certain local peaks, which can be linked to the continuous deformation of the face sheet until the impactor attains a penetration level similar to the initial peak load (Fd). The extent of a quick load drop is influenced by the severity of the fracture in the upper face sheet and the resulting decrease in resistance to penetration by the impactor. The maximum peak force (Fmax) and displacement (Dmax)occur when the impacting mass engages with the core layers. The stress momentarily decreases as the impactor advances, followed by a brief rise due to localized peaks caused by the strain hardening of the core layers. The fundamental framework will take in the impact energy. The impactor has the potential to induce perforation or full penetration of the structure when the impact energy is substantial; however, rebounding is permissible if the impact energy does not adequately support the core structure. Sunith et al. [5] indicate that composite materials undergo damage at various stages, such as contact, impact, deformation, rebound, perforation, and ultimately penetration when exposed to lowvelocity impact [5]. The testing of the hexagonal honeycomb core reveals significant variations in performance based on the unit cell size. For unit cells measuring 6, 8, and 10 mm, the initial peak load (Fd) values are 1.50, 0.87, and 0.76 kN, respectively. The maximum peak load (Fmax) observed for the cores to withstand impact loads shows similar trends, with values of 1.91, 1.32, and 1.12 kN for the 6, 8, and 10 mm unit cells. Additionally, the maximum displacement (Dmax) increases with unit cell size, reaching 5.46 mm for the 6 mm unit cell, 5.53 mm for the 8 mm unit cell, and 7.00 mm for the 10 mm unit cell. These findings highlight the influence of unit cell size on the honeycomb core's ability to resist peak loads and displacement under impact conditions. The outcome significantly affects the rigidity of the core structure, characterized by the differing dimensions of unit cells, or alternatively, the density of each composite core structure. As the unit cell sizes decrease, the maximum peak load generally increases while the maximum displacement tends to decrease. The Hemispherical Impactor Force Base Clamps Sandwich sample JNFPC 2024, 3(2), 3 5 of 9 stiffness of a sandwich composite structure exhibits a higher value at high density (unit cell 6 mm) compared to medium density (unit cell 8 mm) or low density (unit cell 10 mm). Aloyaydi et al. [20] indicated in their study that structures with high density (i.e., 100% infill density) exhibit superior load-carrying capacity when contrasted with low-density PLA samples (i.e., 40% infill density). In this instance, the high-density structure (unit cell 6 mm) is not relevant for the advancement of a lightweight sandwich structure. Therefore, a detailed examination of low and medium-density structural cores will be conducted for subsequent analysis and comparison. Figure 4 illustrates that the 8 mm unit cells exhibit greater maximum loads (Fmax), and reduced deformation (Dmax) compared to the 10 mm unit cells. The medium density (unit cell 8 mm) seems to exhibit intermediate stiffness, whereas the low-density (unit cell 10 mm) appears to demonstrate low stiffness. Figure 4. Force–displacement curves of the sandwich composite structures with hexagonal honeycomb core under impact energy 11 J 3.2. Energy Absorption The energy absorption of a sandwich panel subjected to an impact load is typically quantified as the total area under the force-displacement curve. This measure reflects the energy delivered upon impact and serves as a critical indicator of damage in sandwich composite structures. Figure 5 illustrates the energy absorption characteristics of hexagonal honeycomb core sandwich panels with varying unit cell sizes (6, 8, and 10 mm) subjected to a low-velocity impact with an energy of 11 J. The recorded energy absorption values are 9.61, 9.22, and 7.44 J for the 6-, 8-, and 10-mm unit cells, respectively, showing consistent results across the different configurations. The results reveal that energy absorption is significantly influenced by the configuration of the core structure. As the density of the sandwich structure increases, energy absorption also increases, suggesting that higher core density leads to reduced structural damage and more effective energy dissipation. The 6 mm unit cell, with the highest density, achieves the greatest energy absorption efficiency. However, while this configuration shows superior energy absorption, its compact and solid nature makes it less suitable for developing lightweight sandwich composite structures. Therefore, to strike a balance between high energy absorption and lightweight design, the 8 mm unit cell was chosen. This size offers a combination of high energy absorption, adequate stiffness, and reduced weight. In contrast, the low-density 10 mm unit cells exhibit lower energy absorption, with the impact energy penetrating deeper into the structure. The medium-density 8 mm unit cells, however, demonstrate enhanced energy absorption, with moderate deformation. This configuration strikes an optimal balance between energy dissipation and structural integrity. Additionally, research by Alavi Nia and Kazemi [21] showed that reducing foam density increases porosity, which in turn decreases both tensile and compressive strengths, making it easier for the projectile to penetrate the structure. Consequently, the 8 mm unit cell demonstrates superior performance in absorbing higher impact energies compared to the 10 mm unit cell. This results in more JNFPC 2024, 3(2), 3 6 of 9 effective energy dissipation and reduced deformation, ultimately improving the overall performance of the sandwich composite structure under impact loads. Figure 5. Energy absorption of the sandwich composite structures with hexagonal honeycomb core for impact energy 11 J 3.3. Failure Modes The experimental results are shown in Table 2, illustrating that impacts were applied to the top surface of sandwich structures. The diameter of the damage was assessed, indicating inelastic deformation of the hexagonal honeycomb composite structure. For unit cells measuring 6, 8, and 10 mm, the observed damage diameters are 13.74, 14.07, and 15.37 mm, respectively. The extent of the damage diminished with an increase in the structure's density. The results indicate that the increased core density from smaller unit cells led to the smallest damage diameter value, potentially enhancing structural damage resistance and resulting in the least damaged area. This finding aligns with the observations made by Feng and Aymerich [22], who noted that the enhanced support from a denser and stiffer core during localized indentation impacts leads to a decrease in the overall damage area when compared to cores of lesser density. While unit cells with a 6 mm greater density exhibit enhanced damage resistance, this is not ideal because the lightweight performance of the sandwich structures is constrained. The maximum acceptable unit cell size is 8 mm, characterized by medium density and an intermediate damage diameter. A comprehensive failure mode analysis concentrating on the 8 mm unit cell, selected as the appropriate parameter for this study, can identify the emergence of failure modes. Figure 6 demonstrates the observable impact damage on sandwich composite constructions featuring hexagonal honeycomb cores with unit cells measuring 8 mm, made from PLA-Wood material, when subjected to an impact energy of 11 J. The energy impact illustrated the changes in the structure's response resulting from the densification of the core material. The compression of the affected front face sheet was the primary factor in core densification, while significant densification was observed at the bottom face sheet as a result of impactor tip compression. There was no major penetration of the back face sheet, aside from some bulges and minor cracks. Consequently, fiber breaking occurs later in the impact process compared to matrix cracking and delamination between the face sheet and core, serving as a precursor to the catastrophic penetration mode for the entire sandwich structures. The delamination failure between core layers in a 3D-printed honeycomb core occurred specifically in the region where significant delamination damage began. Reduced debonding might be observed due to core delamination, while an escalation in core delamination indicates significant internal structural damage. Daniel et al. [23] detailed how the delamination impact damage at this point significantly diminishes the strength of the structural panel. Delamination occurs due to transverse impact once a certain energy threshold is achieved, leading to localized core fracture within the cell wall of the honeycomb core. As noted by Hou [24], the compromised cell walls contribute to an increased level of structural instability, which diminishes the peak impact force and causes the core to bend more significantly, leading to a higher incidence of core ruptures. Local distortion occurs because of core breakage, followed by permanent deformation until lower face sheet failure, and the structures lose their load-carrying capacity when they collapse catastrophically under impact loading due to low fracture toughness. JNFPC 2024, 3(2), 3 7 of 9 Table 2. Front damage of sandwich composite structures with hexagonal honeycomb core with different unit cells subjected to impact energy 11 J. Size of Unit Cell (mm) Top Surface Diameter of Damage (mm) 6 8 10 Figure 6. Cross-sectional view of sandwich composite structures with hexagonal honeycomb core with impact energy 11 J for the unit cell of 8 mm. 4. Conclusions This study emphasizes the considerable promise of sandwich composite structures for lightweight applications, primarily due to their outstanding energy absorption properties. By optimizing the architecture of these structures through variations in unit cell sizes and core densities, it is possible to enhance energy absorption while minimizing structural damage. Key findings highlight the impact of core density on structural performance. Increasing core density results in higher maximum force (Fmax) and lower maximum displacement (Dmax) during deformation, suggesting that stiffer structures exhibit enhanced energy absorption characteristics. However, structures with high-density cores, such as those with a 6 mm unit cell, are less suitable for lightweight applications due to their increased weight. Further analysis was conducted on medium-density (8 mm unit cell) and low-density (10 mm unit cell) structures. Among these, the medium-density of 8 mm unit cell emerged as the most effective, offering an optimal balance between lightweight properties, significant energy absorption, and reduced damage compared to lower-density configurations. This balance allows for efficient energy dissipation while maintaining overall structural integrity. Despite these advantages, impact energy can still cause various types of internal and external damage, including densification, delamination between core layers, and core fractures, all of which pose risks 14.07 mm Fiber breakage Core delamination Core breakage Core densification JNFPC 2024, 3(2), 3 8 of 9 to the structural integrity of the sandwich composite. This highlights the need for further design optimization and material selection to minimize such damage while maintaining the desired energy absorption properties for practical applications. Future research should focus on refining core designs and material properties to improve durability without compromising energy absorption performance. Acknowledgments: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors acknowledge the facilities support provided by Advanced Material Processing & Design, Faculty of Mechanical Engineering & Technology, Universiti Malaysia Perlis. References [1] ASTM-I F2792-12A, “Standard Terminology for Additive Manufacturing Technologies,” 2012 Accessed: Aug. 11, 2022. [Online]. Available: https://www.astm.org/f2792-12a.html [2] S. Mellor, L. Hao, and D. 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