A Comparative Analysis of the Impact Behavior of Honeycomb Sandwich Composites ^†

Abstract

The increasing need for materials that are both lightweight and strong in the aerospace and automotive sectors has driven the extensive use of composite sandwich structures. This study examines the impact response of honeycomb sandwich composites fabricated using the vacuum-assisted resin transfer molding (VARTM) technique. Two configurations were analyzed, namely carbon–honeycomb–carbon (CHC) and carbon–Kevlar–honeycomb–Kevlar–carbon (CKHKC), to assess the effect of Kevlar reinforcement on impact resistance. Charpy impact testing was conducted to evaluate energy absorption, revealing that CKHKC composites exhibited significantly superior impact resistance compared to CHC composites. The CKHKC composite achieved an average impact strength of 70.501 KJ/m², which is approximately 73.8% higher than the 40.570 KJ/m² recorded for CHC. This improvement is attributed to Kevlar’s superior toughness and energy dissipation capabilities. A comparative assessment of impact energy absorption further highlights the advantages of hybrid Kevlar–carbon fiber composites, making them highly suitable for applications requiring enhanced impact performance. These findings provide valuable insights into the design and optimization of high-performance honeycomb sandwich structures for impact-critical environments.

1. Introduction

Hybrid composite structures, such as honeycomb sandwich composites, offer an optimal combination of light weight [1], high strength [2], and energy absorption [3]. Consisting of a lightweight honeycomb core sandwiched between durable face sheets, these structures exhibit superior mechanical performance under various loads [4]. The integration of carbon fiber and Kevlar enhances impact resistance, toughness, and structural integrity, making them essential in aerospace [5], automotive [6], marine [7], and defense applications [8]. Their exceptional impact resistance ensures suitability for aircraft fuselages, UAVs, crash structures, and protective gear, reinforcing their role in high-performance engineering [9].

Djellab et al. [10] studied the low-energy impact of honeycomb sandwich composites, showing that carbon face sheets improved impact resistance and flexural strength by 157.14% and 45.72%, respectively, compared to glass. Zaman et al. [11] examined the impact performance of GFRP pipes, reporting an energy absorption of 6 J for a six-layer GFRP configuration. Topkaya et al. [12] found that face sheet thickness significantly affected impact strength in honeycomb sandwich composites, with 0.5 mm carbon fiber sheets perforating at 10 J of energy. Song et al. [13] studied the tensile and bending performance of woven carbon–aramid/epoxy hybrid composites and found that it is greatly influenced by the lamination position and stacking sequence, which play a key role in enhancing strength and stiffness while minimizing material usage. Castellanos et al. [14] studied the response of woven carbon/vinyl ester sandwich composites to low-velocity impacts under cold marine environments, finding that lower temperatures increased stiffness and brittleness, reducing residual strength and causing severe perforation at −25 °C and −50 °C under high impact energy. Sarvestani et al. [15] analyzed the impact energy absorption and damage mechanisms of woven CFRP composites using experiments and LS-DYNA simulations, identifying strain-rate effects, progressive damage, and failure modes while proposing strategies to enhance impact resistance. Lin et al. [16] studied the behavior of sandwich composite panels with shear-thickening gel-filled honeycomb cores under low-velocity impact conditions, finding that STG filling reduced penetration depth and enhanced impact resistance, as confirmed by experimental and CEL-based numerical analyses. Zhao et al. [17] studied the ballistic impact behavior of hybrid carbon/Kevlar epoxy laminates, finding that Kevlar back layers enhanced impact resistance in thinner laminates, while shear plugging dominated in thicker ones. Ahmed et al. [18] studied the high-velocity impact response of carbon/Kevlar hybrid 3D woven composites, demonstrating that their FE model, using Hashin failure criteria and cohesive contact, accurately predicted damage mechanisms, residual velocities, and the role of z-yarns with low computational cost. Lebaupin et al. [19] investigated the low-energy impact resistance of flax/PA11 composites, revealing that quasi-isotropic layups offered the highest peak load with minimal damage, while sandwich-like layups absorbed more energy but sustained greater damage. Deng et al. [20] analyzed the dynamic response of composite honeycomb sandwich panels under strong impacts, demonstrating that greater core height and density improve impact resistance by minimizing shock wave propagation.

Kottapalli et al. [21] investigated the impact properties of flax–jute hybrid composites, analyzing fiber orientation and stacking sequences. Their findings highlight the influence of hybridization on impact resistance, advancing biodegradable alternatives to low-strength plastics. Wang et al. [22] examined the synergistic effects and failure modes of carbon/Kevlar HFRP laminates subjected to bending-after-impact loading, finding that the [K3C3] configuration maximized residual bending strength, while [KCC] laminates offered superior impact protection. Zaman et al. [23] examined the compressive behavior of hand-laminated GFRP pipes in the hoop direction, reporting a maximum compressive strength of 6.88 MPa and a maximum load capacity of 6.797 kN. Feng et al. [24] analyzed the impact response of composite sandwich structures, revealing that wood cores exhibited higher contact force than foam cores, with both structures undergoing surface damage, core deformation, and rear panel failure, offering insights for optimizing core material selection. Charkaoui et al. [25] investigated the study of low-velocity impact behavior in hybrid core sandwich panels and revealed that strut-based cores provided enhanced resistance to penetration. Silicone-filled cores reduced damage by 60%, while unfilled spring cores had a 100% perforation risk. You et al. [26] analyzed carbon/Kevlar hybrid composites, finding that the carbon core limited delamination, enhancing impact resistance. Higher temperatures weakened the matrix and fiber interface, reducing mechanical performance.

Despite extensive research on hybrid composite configurations, the impact performance of carbon–honeycomb–carbon (CHC) and carbon–Kevlar–honeycomb–Kevlar–carbon (CKHKC) sandwich structures remain largely unexplored. These composite configurations offer significant potential for high-performance applications, yet their energy absorption capabilities under impact loading require further investigation. This study addresses this research gap by evaluating the Charpy impact energy absorption characteristics of CHC and CKHKC composites, providing valuable insights into their structural behavior and potential applications in demanding environments.

2. Methodology

The research methodology involves the selection of materials for the composite sandwich structure, the fabrication process, the design of test specimens according to ASTM standards, and the various testing protocols that will be employed to evaluate the impact performance of composite sandwich structures. The procedure is depicted in the flowchart in Figure 1.

2.1. Materials

The core material used was Nomex aerospace honeycomb, featuring a cell size of 0.06 mm and a height of 4 mm. The face sheets were made from 210 g/m² 1 × 1 plain weave 3K carbon fiber fabric, provided by Easy Composites (Longton, Stoke-on-Trent, United Kingdom). Additionally, Kevlar 49 aramid fiber, also obtained from Easy Composites, was integrated into the composite structure. For bonding the carbon fiber face sheets to the honeycomb core, Pakfiber epoxy resin was utilized, mixed with a hardener in a 2:1 ratio following the manufacturer’s guidelines. Figure 2 presents a visual representation of the materials used in fabricating the sandwich composite structure.

Table 1. Material properties of the carbon fiber, Nomex honeycomb, Kevlar, and epoxy.

Properties	Carbon Fiber	Nomex Honeycomb	Kevlar	Epoxy
Density (g/cm3)	1.79	0.72	1.45	1.08–1.12
Tensile Strength (MPa)	4120	2500	2900	70.0–80.0
Modulus of Elasticity (MPa)	234,000	5000	130,000	2500–3500
Elongation (%)	1.80	3–5	2.3	6.0–10.0

[27] summarizes the material properties of the carbon fiber, Nomex honeycomb, Kevlar, and epoxy.

2.2. Fabrication Process

A composite sandwich panel, with and without Kevlar reinforcement, was manufactured using the vacuum-assisted resin transfer molding (VARTM) technique. Before initiating the fabrication process, all essential materials—including a glass slab, vacuum pump, wax, carbon fiber, Kevlar, honeycomb core, peel ply, mesh, plastic sheet, distribution pipes, and epoxy resin—were carefully prepared sourced from Easy Composites Ltd, Stoke-on-Trent, United Kingdom. The process began by thoroughly cleaning the glass slab and applying a wax coating to prevent epoxy adhesion. Each material component, including carbon fiber, Kevlar, honeycomb, peel ply, mesh, plastic sheet, and distribution pipes, was precisely weighed. The resin content was determined to be 40% of the total weight of these materials. To ensure uniformity, an epoxy mixture of resin and hardener in a 2:1 ratio was prepared, adhering to manufacturer recommendations.

In the first configuration, the composite structure consisted of a single layer of honeycomb core enclosed between two carbon fiber layers. The second configuration incorporated an additional Kevlar layer on both sides of the honeycomb. Fabric sheets and mesh were placed over the layers, and distribution pipes were aligned along the inlet and outlet sides. The perimeter was sealed using sealing tape, leaving a small gap for air evacuation. The entire setup was then covered with a plastic sheet and sealed under pressure. A vacuum pump was attached to the outlet to create a vacuum, while the resin was introduced from one end of the flow pipe to ensure uniform infusion. After the resin fully permeated the fibers and honeycomb structure, both ends were sealed, and the composite was left to cure under vacuum for 10 h. Once cured, the plastic sheet was removed, and the composite panel was detached from the glass slab. An overview of the setup and fabrication process is depicted in Figure 3.

2.3. Design of Test Specimen

The test specimens were designed following ASTM D256 standards to ensure accurate and reliable mechanical property evaluation. Each impact specimen featured a centrally located 45-degree notch to facilitate controlled fracture initiation. Adherence to these standardized specifications enhances the consistency and robustness of mechanical testing, allowing for precise material performance assessment and meaningful comparisons. The dimensions for impact test specimens are illustrated in Figure 4.

2.4. Charpy Impact Testing

The impact strength of the specimens was evaluated using a Shimadzu Charpy impact testing machine, following the methodology outlined in previous studies [28]. The test was performed with a free-fall angle of 151° (α) and a corresponding post-impact angle (β). The hammer used for testing had a weight of 22.023 N and a moment arm length of 0.363 m. Each specimen featured a notched cross-sectional area of 2.99 × 10⁻⁵ m². A schematic diagram illustrating the impact testing setup is provided in Figure 5.

$$\mathrm{Charpy}\mathrm{Impact}\mathrm{Value}={\displaystyle \frac{E}{A}}$$

(1)

$$E=WD\left(\mathit{cos}\beta -\mathit{cos}\alpha \right)$$

(2)

where

E represents the energy absorbed during impact.

W signifies the mass of the impact hammer.

D denotes the moment arm, defined as the distance between the axis and the hammer’s center of gravity.

β corresponds to the angle observed after the hammer has completed its motion.

α refers to the initial angle before the hammer is released.

Figure 5. (a) Shimadzu test machine. (b) Sample holder (front view). (c) Sample holder (side view). (d) Impact testing sample.

Figure 5. (a) Shimadzu test machine. (b) Sample holder (front view). (c) Sample holder (side view). (d) Impact testing sample.

3. Results and Discussion

Toughness is a critical parameter in evaluating the performance of newly developed composite structures, as it quantifies the energy a material can absorb before failure. The Charpy impact test is a widely used method for directly measuring toughness. The Charpy impact testing was conducted with a free-fall angle of 151° (α) and a corresponding post-impact angle (β) outlined in

Table 2. Experimental results from the Charpy impact testing.

Title 1	β	α	Cos(β) − Cos(α)	E (J)	E (KJ/m2)	Average (KJ/m2)
C-H-C 1	134	151	0.17996	1.439	48.127	40.570
C-H-C 2	139	151	0.11991	0.959	32.073
C-H-C 3	136	151	0.15528	1.241	41.505
C-K-H-K-C 1	126	151	0.28683	2.293	76.688	70.501
C-K-H-K-C 2	129	151	0.24530	1.961	65.585
C-K-H-K-C 3	128	151	0.25896	2.070	69.230

.

As illustrated in Figure 6, the CKHKC sandwich structure exhibits greater resistance to impacts compared to the CHC configuration, while the CHC composite, consisting solely of carbon, behaves as a brittle material due to carbon’s inherent stiffness and low toughness. However, the addition of Kevlar enhances the composite’s toughness by improving energy absorption and crack resistance.

The CKHKC sandwich structure achieved an average maximum impact strength of 70.501 KJ/m², while the CHC configuration recorded 40.570 KJ/m². This reflects an approximate 73.8% increase in impact resistance for the CKHKC structure compared to the CHC panels. This substantial enhancement highlights the effectiveness of Kevlar reinforcement in improving the impact strength of honeycomb sandwich composites.

4. Conclusions

This study investigated the impact performance of carbon–honeycomb–carbon (CHC) and carbon–Kevlar–honeycomb–Kevlar–carbon (CKHKC) sandwich structures using Charpy impact testing. The results demonstrated that the CKHKC configuration exhibits significantly higher impact resistance compared to the CHC structure. Specifically, the CKHKC composite achieved an average impact strength of 70.501 KJ/m², which is approximately 73.8% higher than the 40.570 KJ/m² recorded for CHC. This notable improvement highlights the effectiveness of Kevlar reinforcement in enhancing the energy absorption capabilities of honeycomb sandwich composites. The findings underscore the potential of CKHKC structures for applications requiring superior impact resistance, such as in the aerospace, automotive, and defense industries.

The following are suggested directions for future research:

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Conduct SEM analysis to study the microstructure and failure modes of CKHKC composites.

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Investigate the bending behavior of CKHKC structures under flexural loads.

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Evaluate the long-term durability of CKHKC composites under cyclic or environmental conditions.