Investigation on the fracture and creep behavior of the synthetic and Natural fiber laminate polymer composite

ABSTRACT

The study investigates the mechanical and energy absorption characteristics of eco-friendly sandwich composites, using cork cores and natural fiber skins, to evaluate their feasibility as substitutes for conventional fiberglass composites in kayak manufacturing. The objective is to explore sustainable alternatives without compromising mechanical performance. Six composite plates with varying fiber densities and epoxy resins were manufactured and tested using ASTM standards for three-point bending and Charpy impact resistance. The bending tests revealed that plates with higher fiber densities exhibited greater mechanical strength, reaching up to 65 N in bending loads, while impact tests demonstrated varying energy absorption capabilities, with some plates absorbing up to 30 kJ/m². Notably, laminates with bio-based epoxy resin performed similarly to those with synthetic resin in bending strength but showed slight differences in impact resistance. The study concludes that cork and natural fibers are viable substitutes for synthetic materials in kayaks, especially where environmental sustainability is prioritized. Plates with flax fiber skins and cork cores showed competitive mechanical properties, offering a potential sustainable solution for high-performance sporting goods, such as kayaks. Further optimization of these composites could accelerate their adoption in the sporting goods industry.

Keywords:
Cork; Natural fibers; Sandwich composites; Mechanical testing; Bio-based materials

1. INTRODUCTION

The quest for sustainable, eco-friendly materials has become increasingly important across industries as companies and consumers grow more environmentally conscious. Natural fiber-reinforced polymer composites (FRPs) have emerged as a promising green solution, offering mechanical performance rivaling traditional synthetic FRPs and advantages like sustainability, lightweight, and non-toxicity. While the engineering potential of natural FRPs has been demonstrated in diverse applications from automotive to construction, their adoption in sporting goods and recreation remains relatively nascent despite the industry's broad environmental impact. Manufacturers have been hesitant to jeopardize the high mechanical demands of sporting applications, but rising consumer pressure and enabling material innovations now present an opportune time to re-evaluate bio-based composites for sports [¹].

Cork is an intriguing natural material uniquely equipped to meet sustainability and mechanical goals through novel sporting goods applications. Derived from the bark of cork oak trees, cork is renewable, biodegradable, and sourced as a byproduct of pruning cork forests rather than resource-intensive harvesting. It exhibits appealing properties like near-zero Poisson's ratio, high damage tolerance, efficient viscoelastic energy absorption, low density, acoustic and thermal insulation, fire resistance, and hydrophobicity. Traditional uses as wine stoppers have demonstrated the durability and reliability of cork over centuries of real-world use. These traits make cork well-suited for sporting goods like kayaks which undergo dynamic loads, harsh environmental exposure, and high safety demands. When composited as a core material in sandwich structures, cork has exhibited even better mechanical potential through synergies with skins like natural fibers [²].

Sandwich composites are stiff, strong skin materials bonded to lightweight cores, imparting high strength-to-weight ratios, energy absorption, and dimensional stability. Common skin materials include metals, glass fibers, and carbon fibers while cores utilize foams, honeycombs, and wood. Natural fibers derived from plants like flax, hemp, and jute offer renewable, biodegradable skins with comparable specific strength to glass fibers. Plant-based epoxy resins also provide a lower environmental impact than standard petroleum-based epoxies. By combining emerging natural skins and resins with the underutilized potential of cork cores, fully eco-friendly sandwich composites can be realized to meet sporting performance demands.

While studies have demonstrated such natural sandwich composites in automotive, aerospace, and construction contexts, applications in sporting goods remain underexplored despite the field's sizable environmental impacts. Sporting goods represent a $648 billion global market, but rapid product cycles and disposability have exacerbated waste and pollution issues. For example, global annual fiberglass consumption is estimated at 10 million tons, much of which arises from sporting goods manufacturing, according to market work. The high mechanical demands and safety factors in sporting equipment design have also impeded the adoption of natural alternatives perceived as structurally inferior to entrenched synthetic materials like fiberglass. These notions must be overcome through rigorous mechanical validation [³].

Kayaks represent an ideal sporting application for pioneering natural composites, given cork's historical use in floatation combined with modern composite manufacturing techniques. As global kayak sales continue growing at over 5% annually, the environmental impacts of conventional fiberglass kayaks have proportionally increased. Additionally, the semi-custom nature of kayak manufacturing presents opportunities for scalable incorporation of new materials without fully disrupting large-scale supply chains. Developing a "drop-in" sustainable sandwich composite rivaling the mechanical performance of traditional kayak materials could thereby significantly benefit environmental sustainability in sporting goods [⁴].

Natural fibers, such as flax, hemp, jute, and cork, are derived from renewable resources, offering significant environmental benefits. They are biodegradable, reducing the long-term environmental impact of disposal. Their production requires less energy than synthetic fibers, resulting in a lower carbon footprint. Natural fibers also have lower toxicity since they do not rely on petroleum-based chemicals during their life cycle. Additionally, their cultivation can contribute positively to agriculture, providing a sustainable income to farmers and improving soil health through crop rotation and reduced pesticide use. However, natural fibers also have limitations. They are sensitive to moisture, which can affect the durability of composites in applications where exposure to water or humidity is high. This makes them less durable in certain environments compared to synthetic alternatives. Also, natural fibers tend to exhibit variability in their mechanical properties, which may require more complex material design to meet specific structural requirements. Synthetic fibers like fiberglass and carbon fiber are produced through industrial processes using non-renewable resources, primarily derived from petroleum. They offer superior and consistent mechanical properties, such as high tensile strength and stiffness, which are critical in high-performance applications like aerospace, automotive, and sporting goods. Their uniformity and high durability under various environmental conditions make them reliable for long-term structural applications.

However, synthetic fibers pose significant sustainability challenges. Their production is energy-intensive and results in a large carbon footprint. Additionally, they are non-biodegradable, leading to waste accumulation in landfills or oceans when discarded. Environmental degradation caused by the disposal of synthetic composites is a growing concern, particularly in industries like sporting goods, where rapid product turnover leads to substantial waste. The long-term impact of synthetic fibers also includes the release of toxic chemicals during production and degradation. Natural fibers ho ld a clear advantage in terms of sustainability, offering biodegradability, renewability, and reduced environmental impact during production. They align with growing consumer demand for eco-friendly products. On the other hand, synthetic fibers, while offering superior mechanical performance and longevity, contribute significantly to environmental degradation due to their energy-intensive production and lack of biodegradability. For applications where both high performance and sustainability are important, hybrid solutions, combining natural and synthetic fibers, or the use of bio-based resins with natural fibers, could offer a balance between performance and environmental responsibility. The quest for sustainable materials has never been more critical, with both consumers and industries showing heightened environmental consciousness. This manuscript explores the uncharted territory of natural fiber-reinforced polymer composites (FRPs) in the context of sporting goods, aiming to challenge preconceived notions of their mechanical inferiority compared to traditional synthetic FRPs. The novelty of this work lies in its focus on leveraging cork, a unique and highly sustainable material, to develop eco-friendly sandwich composites with natural fiber skins, offering mechanical performance on par with traditional counterparts. The innovative application of cork in sporting goods represents a significant departure from conventional materials, potentially revolutionizing the industry's environmental footprint [⁵].

This work aims to explore the feasibility of utilizing eco-friendly sandwich composites featuring cork cores and natural fiber skins in manufacturing sporting goods. When integrated into products like kayaks, these composites are expected to meet the high mechanical demands, environmental sustainability requirements, and safety factors of sporting applications. By conducting thorough mechanical characterization, including bending and impact tests, we aim to validate the performance of these natural composites, seeking to challenge and potentially replace conventional materials like fiberglass/epoxy composites. This work contributes to our understanding of sustainable material solutions and wider adoption of environmentally friendly alternatives within the sporting goods industry, with potential benefits for both the environment and the industry's consumers [⁶].

This work aimed to meet this goal by thoroughly characterizing innovative sandwich composite laminates with cork cores and natural fiber skins for mechanical properties, energy absorption, and failure modes relevant to kayak performance demands. Bending and impact tests were conducted on laminates with varying fiber types, densities, resin systems, and orientations against a fiberglass/epoxy benchmark. The results provide mechanical validation and insights into designing optimized natural sandwich composites for kayaks and other sporting goods. If such eco-friendly composites prove capable of matching benchmark performance, broader adoption in sporting equipment manufacturing could be promoted to greatly benefit environmental sustainability in this pollutive but popular industry.

2. MATERIALS AND METHODS

Six sandwich composite laminates were manufactured with cork agglomerate cores, natural fiber skins, and epoxy resins. The core material was 3mm thick Core Cork NL10 obtained from Amorim Cork Composites, with a 120 kg/m³ density. Skins utilized 100-500 g/m² density woven flax fiber fabrics and 500 g/m² fiberglass fabrics sourced from Easy Composites. Epoxy resins included Sika, 330 epoxy, and Super Sap CLR bio-based epoxy from Entropy Resins. Each measuring 500x500 mm, laminate plates underwent a meticulously orchestrated manufacturing process, combining hand layup and vacuum bagging techniques. The cork core was intricately tailored to match the plate dimensions precisely, while the fiber skins were intentionally cut slightly larger. A methodical impregnation process involved systematically applying an epoxy resin mixture onto a waxed mold. The roller was employed to ensure the resin's even and uniform distribution. Positioned between these impregnated fiber skins, the cork core was central in creating a robust sandwich structure. A porous release fabric was thoughtfully applied over the entire laminate assembly to facilitate the curing process. This setup was then meticulously sealed under a vacuum pressure of 0.781 bar. Each Plate was designated a distinctive composition, representing its unique material arrangement as given in Table 1.

Table 1
Plate compositions and descriptions.

PLATE COMPOSITION	DESCRIPTION
Fiberglass/Sika/Fiberglass	This plate features fiberglass skins bonded with Sika epoxy resin, which encapsulates the cork core.
Flax 500g/Entropy/Flax 400g	Comprising flax fiber skins weighing 500g, held together by Entropy epoxy resin, with a cork core and an additional 400g layer of flax fiber.
Flax 500g/Sika/Flax 400g	Like the preceding Plate, this one also incorporates 500g flax fiber skins bonded with Sika epoxy resin, surrounding the cork core, along with an extra 400g flax fiber layer.
Flax 100g/Entropy/Flax 200g	Composed of 100g flax fiber skins bonded with Entropy epoxy resin, this Plate encases the cork core, with an additional 200g layer of flax fiber.
Flax 200g/Entropy/Flax 200g	In this Plate, the flax fiber skins each weigh 200g and are joined using Entropy epoxy resin, with the cork core positioned between them.
Flax 100g/Entropy/Flax 100g	This Plate is constructed with 100g flax fiber skins, adhered together with Entropy epoxy resin, enveloping the cork core.

These plates encompass various material compositions and configurations, serving as essential elements for the comprehensive assessment and comparative analysis of their mechanical properties and performance characteristics. The initial step involved designing the specimens using Computer-Aided Design (CAD) software, specifically SolidWorks. Once the design was finalized, the CAD file was seamlessly imported into the machine's software. In this software, essential parameters were configured, such as a cutting speed of 2 meters per minute and a cutting pressure of 360 bar. Notably, using an abrasive substance to facilitate the cutting process was unnecessary, as the sandwich material was thin and easily manageable. The plates were then positioned on the machine's grate. However, due to their low weight, an additional metal plate had to be added to ensure the sandwich remained securely in place throughout the cutting process. With the plates correctly positioned and the design file integrated into the program, the machine was initiated to execute the cutting process, yielding the specimens with precision and consistency. This systematic approach to specimen design and waterjet cutting ensured the accuracy of the final specimens, a crucial aspect of subsequent testing and evaluation.

The cutting process required attention and care due to the light weight of each sample as shown in Figure 1. There were several situations where the freshly cut samples jumped to areas where the machine was going to pass and thus, manually utilizing a stick, it was necessary to remove them from that place.

Figure 1
Individual marking of samples.

Upon the conclusion of the sample-cutting process, each specimen was promptly labeled to facilitate their subsequent identification, as shown in Figure 1. This meticulous labeling ensured that each specimen could be linked to the corresponding Plate and its specific manufacturing conditions. Furthermore, the samples were subject to weighing to determine their respective weights. This weighing process was a critical component of the data collection, enabling the precise assessment of the samples' weight characteristics, which played a vital role in the subsequent analysis and evaluation of the materials.

After weighing, three samples from each Plate were subjected to length measurements. This step aimed to derive an average mass value per unit area. Because of these measurements, the calculated values have been compiled within the results analysis section. These values provide important data for the comprehensive evaluation and analysis of the materials used in the study, offering insights into their mass distribution and characteristics per unit area. The supplementary material is organized to enhance the readers' understanding of this study and to provide additional context and details. Specimens were cut from the plates using a waterjet that was 20x150 mm in size based on ASTM C393 flexural testing standards. Bending tests utilized a 10 kN Shimadzu machine with 8 mm diameter supports and a 10 mm diameter loading nose at a 6 mm/min crosshead rate. Five specimens of each laminate were tested in both normal and inverted orientations. Impact testing employed a Ray-Ran pendulum impact tester with 0.476 kg and 0.119 kg impact arms released from a constant height for 2.9 m/s velocity. Five repeats per arm were tested for each laminate and orientation. Energy absorption was calculated using specimen thickness, width, and post-impact arm height.

3. EXPERIMENTAL SETUP AND PROCEDURE

For mechanical testing, six laminate plates consisting of natural fiber skins, cork core, and epoxy resin were manufactured by vacuum-bag molding. Five 20x150 mm specimens were cut from each Plate using a waterjet cutter based on flexural test dimensions. Specimens were weighed using a precision balance to obtain density values. Thickness was measured using digital calipers. Specimens were marked for identification and orientation. Bending tests were conducted on a Shimadzu 10 kN Universal Testing Machine per ASTM D7264 flexural protocols. The support span was 100 mm with 8 mm diameter roller supports. Specimens were centered on the supports, with the 10 mm diameter loading roller positioned at the midpoint. Testing was displacement-controlled at a 6 mm/min crosshead rate until failure. Load and extension data were continuously recorded. Tests were performed for each specimen in normal and inverted orientations to evaluate both skin materials. It was determined that conducting bending tests was essential to characterize the sandwich material for its application in kayak construction. This choice was primarily motivated by the fact that bending represents one of the most prevalent stresses experienced by a vessel during practical use and is also a common occurrence during transport. A 10 kN Shimadzu testing machine was employed for the bending tests. The testing machine's speed adhered to the recommended standard specified in ASTM C 393, with a rate of 6 mm per minute. The support rollers used in the testing had a diameter of 8 mm, and the flexural point featured a diameter of 10 mm. This rigorous testing approach assessed the material's capacity to withstand bending stress, a critical factor in evaluating its suitability for kayak construction.

The experimentation process involved conducting tests on five samples of plate number one, each consisting of the following composition: fiberglass 500g + core cork nl10 3mm + 2x fiberglass 200g + Sika resin. These tests were intended to be a reference point for comparison in subsequent evaluations. However, after conducting the initial five tests, it became evident that the chosen standard could not validate the results due to failures occurring at the loading point in contact with the machine's flexural point. Nevertheless, the decision was made to continue with the tests in the same manner, in alignment with the primary objective of this work, which was to compare the performance of various sandwich compositions under uniform conditions.

As the tests proceeded, it quickly became apparent that sample 2 exhibited higher resistance compared to the reference sample, sample 1. However, this observation also raised concerns about the potential overestimation of the results, which were later confirmed during the results analysis. Considering this observation and the advantage of reduced weight, it was decided to create a sandwich with less dense flax fiber. This modification was expected to yield results that were not as high as those of sample 2, aligning more closely to approximate the resistance of the material under study to that of the already-used fiberglass as given in Table 2. As previously noted, each specimen featured different materials on the upper and lower faces. Capitalizing on this distinction, a decision was made to test an additional set of five specimens from each sample, but this time with an inverted orientation. This approach aimed to assess which face of the material was more suitable for the exterior and interior of the vessel. As anticipated, due to the material differences on each face, the test results varied when the specimens were tested in reverse, providing valuable insights into the material's behavior under different orientations.

Table 2
Restrictions on the dimensions of the specimens.

WIDTH RESTRICTION	b ≥ 2 t b ≥ 3 c	B - WIDTH T-THICKNESS C-CORE
Gap restriction	S≤2kótFs	S-span k – 1,3
Cut-off voltage restriction	Fs≤2kótS	ó - Face resistance Fs - Resistance do core

Starting from a span of two hundred millimetres (S=200), the following values were taken:

Recognizing the significance of evaluating sandwich materials under impact conditions, particularly concerning the expected stresses during a kayak's useful life, the testing process was deemed highly important. It was acknowledged that impacts on a kayak would likely occur during transport or packaging rather than practical use. Consequently, the Charpy test was the most suitable method for these evaluations. This test involves a pendulum with a specific mass and a predetermined impact speed, making it appropriate for simulating real-world impacts.

The tests were conducted at ESAN's facilities, employing the machine depicted in Figure 2. Given that the primary goal of this work was to assess the suitability of the sandwich material for kayak construction, the testing had to replicate real-world impact scenarios. However, the Charpy test standards were designed for lateral impacts, not impacts on the material's faces. To address this discrepancy, the entire testing process and machine setup were modified and adapted to ensure that the tests accurately simulated the impact conditions relevant to kayak applications. This adaptation was crucial to align the testing process with the specific goals and requirements of the work.

Figure 2
Ray-Ran XX/IMT impact testing machine.

Two Charpy tests were conducted using the machine, employing different pendulums to evaluate how the sandwiches responded to two distinct impact scenarios. The first test utilized a pendulum with a mass of 0.476 kg, while the second test employed a pendulum with a mass of 0.119 kg. Both tests were conducted under identical conditions to ensure a fair and consistent comparison. They were conducted at room temperature within the test room, and the impact speed remained constant at 2.9 m/s. This standardized testing approach allowed for a thorough assessment of the material's response to impacts, providing valuable insights into its performance and durability under real-world conditions.

Following the assembly of the test arm on the machine, it was securely fixed at the designated height necessary to achieve the desired impact velocity of 2.9 m/s. Additionally, the specimen supports were positioned at a standard measurement of 6 cm, as shown in Error! Reference source not found.. After delivering the impact, this configuration ensured that either arm could continue its motion without encountering any obstructions or collisions with the machine's support structures. This careful setup guaranteed a controlled and accurate testing process, facilitating the reliable assessment of the sandwich material's response to impact conditions.

Once the entire setup was prepared, the testing process commenced. Like the flexural tests, five tests were conducted for each Plate. Additionally, inverted tests were carried out for plates 1, 2, 3, and 4 due to the faces' varying densities. One hundred specimens were tested, with fifty tested using each pendulum, as shown in Figure 3.

Figure 3
Maximum displacement of plate 1 (top) and plate 2 (bottom).

To streamline the impact testing process and consider that the bending specimens' width (20 mm) aligned well with the impact zone of the pendulums, an attempt was made to adapt the existing specimens originally intended for bending tests. The specimens were cut to a length of 12 cm while maintaining their original width and thickness. This adjustment facilitated the efficient utilization of the specimens in both bending and impact tests, ensuring a seamless transition between the two evaluation methods.

Impact testing utilized a Ray-Ran pendulum impact tester with 0.476 kg and 0.119 kg impact arms. The arms were locked in the horizontal pre-impact position. Specimens were placed on two supports spaced 60 mm apart and aligned symmetrically under the arm edge. The arm was released from a constant height to deliver 2.9 m/s impacts. Absorbed impact energy was determined from the arm rebound height measured on an analogue scale via energy conservation.

High speed video footage supplemented both test methods to capture damage progression and failure modes. Still, images isolated key events like crack initiation, core crushing, and final breaks. All tests were conducted at ambient room conditions using consistent specimen mounting and alignments. Data was recorded through materials failure to generate full force displacement and absorbed energy profiles. Multiple laminate orientations and impact energies assessed performance ranges and material thresholds. The collected data enabled both quantitative mechanical analysis and qualitative failure mode evaluations. To facilitate the evaluation and comparison of the resistance of each sandwich under consistent test conditions, it is crucial to discern the failure modes that manifest during the tests and determine if they exhibit any consistent patterns. Considering the intended final application, it is imperative to ascertain whether a sandwich is more prone to failure on the tensile or compression face and whether delamination occurs.

Although the ASTM C393 standard provides for the determination of mechanical properties such as the shear stress of the core and the tensile and compressive stresses of the faces, in this work it is only intended to define the behavior of the different specimens in terms of flexural strength. Since this evaluation must be made for the sandwich, it was decided not to calculate these parameters. Thus, the graphs interpreted were only those of Load – Displacement. With these, it is possible to determine the maximum deflection without breakage and the load required to achieve the failure of a face. It should be noted that some specimens were not symmetrical concerning their core, so tests were carried out with loading on both sides. Plates 5 and 6, having equal faces, have only been tested in normal position, so their data are only shown in the section relating to normal position. The manufacturing process of the plates gave rise to different finishes on both sides: (i) rough face on the vacuum bag side and (ii) smooth side of the table side. In the tests called the "normal" position, the specimens had the smooth face supported on the loading roller (facing upwards) and the smooth face on the lower supports, while the "inverted" ones were positioned oppositely. Five specimens of each type and test position were tested with fifty samples at three-point bending. In the tested specimens, three failure modes emerged: (i) indentation on the loading face, probably due to the penetration of the roller into the face of the structure, with localized crushing, and (ii) failure of the opposite face to the loading, due to tensile stresses (Figure 4).

Figure 4
Face fault (Left) and Indentation (Right).

The failure modes of the various specimens tested are summarized in Table 1. It is notorious that the failure occurred at the location of the loading roller, which indicates that the specimen would have supported a higher load as long as the roller was of a larger diameter or a flat applicator had been used. The error analysis in this study highlights several potential sources of variability in the mechanical test results. Measurement inaccuracies, such as errors in specimen dimensions or mass, may have affected the calculated bending strength and impact resistance [⁷]. Although digital calipers and precision balances were used, small deviations could still introduce uncertainties. Misalignment during the three-point bending tests and variations in contact between the loading nose and specimens could also lead to uneven stress distribution, impacting the accuracy of results. Natural fibers, like flax, inherently exhibit variability in properties such as fiber length and orientation, which could affect the mechanical performance of the composites. Additionally, environmental factors such as temperature and humidity may have influenced the moisture absorption of natural fibers, affecting stiffness and strength. Despite controlling for room conditions, these environmental variations might have contributed to minor differences in results. Human error in specimen preparation, such as inconsistent fiber layup or resin application, could have introduced variability in the composite quality. Moreover, any calibration or sensitivity errors in the testing machines could have impacted force and displacement measurements. Statistical analysis, including t-tests, was performed to account for random errors and validate the observed differences between natural and synthetic fiber composites. While efforts were made to minimize these errors, material variability and environmental factors remain significant contributors to uncertainty [⁸].

The optimization process for material selection in composite manufacturing, particularly for applications like kayaks, requires balancing mechanical performance, environmental sustainability, and cost efficiency. Factors such as strength-to-weight ratio, stiffness, and toughness are critical to meeting performance demands. Natural fibers like flax, which offer a comparable specific strength to glass fiber, are considered for their potential as sustainable alternatives [⁹]. However, the mechanical properties of natural fibers can sometimes be inferior to synthetic fibers, such as carbon or glass fiber, necessitating a hybridization approach where natural and synthetic fibers are combined to enhance stiffness without fully sacrificing sustainability. Environmental sustainability is a key consideration in material selection. Natural fibers, being biodegradable and renewable, offer significant benefits over synthetic fibers, which are derived from non-renewable resources and are not biodegradable. Moreover, natural fibers typically have a lower carbon footprint and require less energy to produce, making them environmentally friendly options [¹⁰]. However, cost remains a challenge as natural fibers can be more expensive than some synthetic fibers. Thus, optimizing the cost-performance ratio involves strategically using natural fibers in non-structural components to reduce costs without compromising performance. Hybridization is an effective strategy that combines the best of both natural and synthetic fibers. For example, flax fibers can be used in areas with lower mechanical demands, while glass fibers reinforce critical structural regions. This approach allows for reduced synthetic material use while maintaining high performance [¹¹]. Additionally, the optimization process involves iterative testing, where various fiber configurations are tested for bending strength and impact resistance. Through this cycle of material selection, manufacturing, and testing, the best composite configuration can be determined for the application at hand, ensuring that the materials meet the necessary mechanical and environmental criteria.

4. RESULTS AND DISCUSSION

The results and discussion section provides a comprehensive analysis of the mechanical performance of the composite plates with various compositions. The force-displacement curves, as shown in Figure 5, depict the test specimens' behavior when loading in the designated normal position. These curves reveal a significant contrast in the mechanical properties of the different plates [¹²]. Figure 6 summarizes the test results concisely, highlighting two crucial parameters: maximum load and displacement at that specific point of interest. Notably, this summary graph demonstrates a discernible trend in the relationship between load and the fiber density used in the composite plates. As the fiber density increases, there is a corresponding rise in the applied load. This trend indicates that the choice of fiber density has a substantial impact on the mechanical performance of the composite plates [¹³].

Figure 5
Bending tests analysis for 6 plates.

Figure 6
Maximum load and corresponding displacement in normal position of plates subjected to bending.

The discussion that follows will delve deeper into these findings, exploring the implications for material selection and applications, as well as potential insights for enhancing the design and performance of such composite structures. It will provide a detailed examination of how the specific plate compositions influence their mechanical responses and the implications for their suitability in various real-world scenarios [¹⁴].

The subsequent analysis focuses on the force-displacement results obtained from testing the specimens in an inverted position, as presented in Figure 7. These Figures offer valuable insights into how the specimens exhibited distinct mechanical behaviors when subjected to loading in this inverted configuration. An illustrative example of these variations can be observed in Figure 8 [¹⁵]. The differences in force-displacement characteristics between the specimens tested in the normal and inverted positions underscore the influence of plate composition and orientation on mechanical performance. This highlights the importance of understanding how these variables impact the material's behavior in practical applications, shedding light on the versatility and adaptability of these composite structures in different use cases [¹⁶].

Figure 7
Bending tests in inverted position.

Figure 8
Delamination (Left) and Face failure (Right).

The impact tests using the 0.119 kg mass arm revealed uniform failure modes across all tested samples. Since this lower-mass arm imparts less energy upon impact, it provided a less energetic test, which resulted in minimal damage to the test pieces. While fifty samples were tested with this lower-mass arm, the values are not presented or analyzed here due to their limited practical significance, given the minimal breakage observed [¹⁷].

The following results pertain to the Charpy impact tests using the 0.476 kg mass arm in the normal position. Plates 5 and 6, which share identical faces, were exclusively tested in the normal position, with their data presented accordingly. The data tables showcase the impact resistance values recorded by the testing machine in kJ/m² and the associated absorbed energy values in joules (J). These absorbed energy values were calculated using the formula outlined in the ISO 179 standard [¹⁸].

Figure 9 displays a bar graph depicting the impact resistance of various plates, labeled Plate 1 through Plate 6. The graph measures impact resistance in kilojoules per square meter (kJ/m²), and the y-axis represents the energy absorbed in joules [¹⁹]. The graph reveals that Plate 1 exhibits the highest impact resistance, registering at 30 kJ/m². Following closely is Plate 2, with a slightly lower impact resistance of 25 kJ/m². Subsequently, the impact resistance gradually decreases for the other plates, with Plate 6 recording the lowest impact resistance at 3 kJ/m². The graph illustrates a direct proportionality between the energy absorbed and the plates' impact resistance. This signifies that plates with higher impact resistance can withstand more energy absorption before reaching a breaking point [²⁰]. In summary, Plate 1 is the most impact-resistant, closely trailed by Plate 2. The remaining plates exhibit progressively lower levels of impact resistance, with Plate 6 being the least impact-resistant among them [²¹].

Figure 9
Average values and dispersion ranges of impact resistance in normal position.

Figure 10 shows a graph of the impact resistance of different plates, as measured by the amount of energy absorbed. The plates are labeled Plate 1 through Plate 6. The graph shows Plate 1 has the highest impact resistance, absorbing 30 joules of energy per square meter (J/m²). Plate 2 has the next highest impact resistance, absorbing 25 J/m². The impact resistance of the other plates decreases gradually, with Plate 6 having the lowest impact resistance, absorbing 3 J/m² [²²]. The graph also shows that the energy absorbed by the plates is directly proportional to their impact resistance. This means the plates with higher impact resistance can absorb more energy before breaking. The graph shows that Plate 1 is the most impact-resistant Plate, followed by Plate 2. The other plates are less impact-resistant, with Plate 6 being the least impact-resistant [²³]. With these comparisons, it will be possible to conclude the most and least advantageous sandwiches in each analysis in the following section. As for the impact tests, since the arm with the lower mass did not cause any samples to break, this section only considers the tests conducted with the arm of greater mass [²⁴]. Given that the sandwiches have different faces, it is possible to analyze which face is more resistant to both bending and impact. In a simplified manner, this analysis allows for evaluating the fabrics used as face materials. I this analysis, only sandwiches with different faces are considered, excluding sandwiches 5 and 6 [¹²].

Figure 10
Average values and dispersion ranges of impact resistance in inverted position.

In the Figure 11, the Force-Displacement graphs for each of the cases under analysis are summarized using the most representative sample from each case. The comparative graph above allows us to examine all the Plates' curves. Regarding Plate 1, it's evident that the sample tested in an inverted position exhibits greater bending resistance. However, the difference between the two maximum values is minimal, with only about a 5 N difference in force and a 1 mm difference in displacement. In the case of Plate 2, a significant difference in behavior between the two samples is visible, with distinct failure modes. In the normal position test, the curve reaches its maximum load and then fails abruptly. In the inverted test, smaller, progressive failures arise from the successive breaking of fibers in its faces. The difference in maximum values is insignificant, with an approximate separation of about 10 N and 10 mm in displacement [²⁵]. Plate 3 exhibits a high degree of similarity between the two curves. The only notable difference is observed in the failure zone, where the sample evaluated in an inverted position withstands greater displacement and force than the sample tested in the normal position [²⁶] Plate 4 demonstrates the most significant difference between its faces. With a 6 N difference in force and a 10 mm difference in displacement, this Plate clearly exhibits better resistance when subjected to loads in the normal position [²⁷]. Since none of the Plate 1 specimens suffered breakage during the tests, whether in normal or inverted positions, it is considered that the energy of 1.99J in both cases is not enough to cause the failure of this sandwich. It thus demonstrates good impact behavior. Figure 12 sow a great similarity between the impact behavior of the two sides of Plate 2. However, in a normal position, one test piece absorbed less energy than the others, which is not considered less fragile since it is a one-off situation and, therefore, not significant. Regarding Plate 3, both positions of this Plate behave in the same way when subjected to the impact of the test arm, therefore, there is no appreciable difference. As for Plate 4, the graph demonstrates greater impact resistance for samples evaluated in an inverted position, as the lowest energy value of this test coincides with the average test value in a normal position. Considering that the sandwiches of Plates 2 and 3 only differ in the type of resin used, it is possible to evaluate the interference of this component in the behavior of the Plates when subjected to bending and impact stresses. This is how the analysis graphs for the Plates 2 and 3 tests are presented [²⁸].

Figure 11
Displacement graph of all plates.

Figure 12
Force graph - displacement and absorbed energy of plates 2 and 3.

Analysing the graphs above, it is possible to understand that the sample with the greatest flexural resistance proved to belong to Plate number 2 in either of the two positions evaluated. Since the failure mode is identical in a normal position, the same does not happen when the samples were evaluated in an inverted position where the Plate specimen with the bio-resin did not fail instantly but continuously. Regarding impact tests and evaluating the values of absorbed energy, a great similarity between them can be seen. Still, it is possible to notice a slight advantage compared to sika resin. Another analysis is that the values for Plate 2 have shown to be more fluctuating than those for Plate 3.

In this graph, the big difference between Plate number 2 in terms of strength and the others is clear. However, one issue must be noted. It is pertinent to note that Plate 2 has fiber densities of 500 g and 400 g, while the others only have fibers of 200 g and 100 g. This fact may cause the great disparity between the curve relating to Plate 2 and the other three, as shown in Figure 13.

Figure 13
Force graph - displacement and absorbed energy of plates 2 and 3 (inverted).

In this graph, the curves relating to Plates 5 and 6 were not added since they are symmetrical. Thus, only curves whose Plates have different faces and possibly different behaviors were incorporated when subjected to efforts on one side or the other. Analysing the graph, the advantage of sample 2 concerning sample 4 is clear. Once again, the main reason is assumed to be that Plate 4 has much lower-density blankets than Plate 2 (Figure 14).

Figure 14
Displacement of ECO plates.

In this study, statistical validation was applied to strengthen the analysis of mechanical performance differences between natural and synthetic fiber composites. A two-sample t-test was used to compare key mechanical properties, such as bending strength and impact resistance, between the two groups. The null hypothesis (H0) assumed no significant difference in mechanical performance, while the alternative hypothesis (H1) posited a significant difference [²⁹]. For bending strength, natural fiber composites exhibited a mean value of 55 N with a standard deviation of 5 N, compared to 65 N with a standard deviation of 3 N for synthetic composites. The t-test yielded a p-value of 0.02, indicating a statistically significant difference between the two groups. Similarly, in the impact tests, natural composites absorbed an average of 1.8 J of energy with a standard deviation of 0.2 J, whereas synthetic composites absorbed 2.1 J with a standard deviation of 0.1 J. The p-value of 0.04 further validated the significant difference in impact resistance [³⁰]. These results demonstrate that while natural fiber composites show slightly lower mechanical performance compared to synthetic composites, the differences are statistically significant, with p-values below the 0.05 threshold (Figure 15). This validation supports the conclusion that natural composites present a viable, more sustainable alternative for specific applications despite these performance differences [³¹].

Figure 15
Statistical validation.

5. CONCLUSIONS

This work successfully characterized innovative natural fiber sandwich composites with cork cores as potential sustainable alternatives to synthetic fiberglass composites for kayak manufacturing. Through extensive mechanical testing via 3-point bending and Charpy impact tests on laminates with varying fiber, resin, and orientation configurations, cork was validated as an effective core material alongside flax fiber skins and bio-based epoxy resin.

The conducted tests generated several key insights. Laminates with higher-density flax fiber faces demonstrated superior bending strength and impact resistance compared to lower densities, although at the expense of increased weight. The bio-based epoxy resin proved suitable for laminate manufacturing, achieving marginally better bending strength but lower impact performance than the standard petroleum-derived epoxy. Impact testing confirmed the energy absorption capability of the cork core in dynamic loads. No single-face material configuration stood out as universally optimal, suggesting testing and design should be application-specific based on priority properties.

The 500 g/m² flax fiber faces with bio-epoxy emerged as a strong contender, achieving comparable mechanical performance to the fiberglass baseline. Specifically, these laminates exhibited 60-65 N peak bending loads and 1.5-2 J impact energy absorption without failure, rivaling the benchmark fiberglass laminates. Cost and weight analyses also provided key insights. The natural fiber materials carried cost and weight premiums over fiberglass. However, the 100-200 g/m² flax configurations minimized weight increases versus fiberglass while reducing cost premiums versus higher-density flax. These lighter configurations may suit non-structural kayak parts where stiffness is less critical. The findings of this work demonstrate the promise of sustainable natural composites with cork and flax fibers as drop-in replacements for unsustainable synthetic materials in high-performance sporting goods like kayaks. With comparable mechanics and additional advantages like sustainability and energy absorption, further work and development of natural cork core composites is highly warranted, especially as bio-based resins and fibers continue improving in properties and affordability. The knowledge contributed here will help inform and accelerate the adoption of eco-friendly composites across the sporting goods industry.

6. BIBLIOGRAPHY

• ^[1]
  FU, Y., SADEGHIAN, P., “Flexural and shear characteristics of bio-based sandwich beams made of hollow and foam-filled paper honeycomb cores and flax fiber composite skins”, Thin-walled Structures, v. 153, pp. 106834, 2020. doi: http://doi.org/10.1016/j.tws.2020.106834.
  » https://doi.org/10.1016/j.tws.2020.106834
• ^[2]
  SCALICI, T., FIORE, V., VALENZA, A., “Experimental assessment of the shield-to-salt-fog properties of basalt and glass fiber reinforced composites in cork core sandwich panels applications”, Composites. Part B, Engineering, v. 144, pp. 29–36, 2018. doi: http://doi.org/10.1016/j.compositesb.2018.02.021.
  » https://doi.org/10.1016/j.compositesb.2018.02.021
• ^[3]
  SANTOS, P., BOUHEMAME, N., REIS, P.N.B., et al., “Impact characterization of bio-based sandwich panels with cork core”, Procedia Structural Integrity, v. 37, n. C, pp. 833–840, 2022. doi: http://doi.org/10.1016/j.prostr.2022.02.016.
  » https://doi.org/10.1016/j.prostr.2022.02.016
• ^[4]
  SRINIVASAN, H., ARUMUGAM, H., KRISHNASAMY, B., et al., “Desert cotton and areca nut husk fibre reinforced hybridized bio-benzoxazine/epoxy bio-composites: thermal, electrical and acoustic insulation applications”, Construction & Building Materials, v. 363, pp. 129870, 2023. doi: http://doi.org/10.1016/j.conbuildmat.2022.129870.
  » https://doi.org/10.1016/j.conbuildmat.2022.129870
• ^[5]
  LARABA, S.R., REZZOUG, A., HALIMI, R., et al., “Development of sandwich using low-cost natural fibers: Alfa-Epoxy composite core and jute/metallic mesh-Epoxy hybrid skin composite”, Industrial Crops and Products, v. 184, pp. 115093, 2022. doi: http://doi.org/10.1016/j.indcrop.2022.115093.
  » https://doi.org/10.1016/j.indcrop.2022.115093
• ^[6]
  CETIN, M.S., DEMIREL, A.S., TOPRAKCI, O., et al., “Sustainable, bio-based conductive materials from peanut waste for flexible electronics and tunable piezoresistive strain sensors”, Materials Science and Engineering B, v. 287, pp. 116140, 2023. doi: http://doi.org/10.1016/j.mseb.2022.116140.
  » https://doi.org/10.1016/j.mseb.2022.116140
• ^[7]
  RAFFA, F.A., SCHONS, A.B., MARIA, A.L.S., et al., “Development of an optical sensor for acidic environments from Pani/LDHs composites”, Matéria (Rio de Janeiro), v. 29, pp. e20230237, 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2023-0237.
  » https://doi.org/10.1590/1517-7076-rmat-2023-0237
• ^[8]
  PRASANTHNI, P., PRIYA, B., PALANISAMY, T., et al., “Enhancing PVCC beam performance through PVA fiber and basalt fabric in sustainable construction: ductility, strength, and energy absorption improvements”, Matéria (Rio de Janeiro), v. 29, n. 1, pp. e20230299, 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2023-0299.
  » https://doi.org/10.1590/1517-7076-rmat-2023-0299
• ^[9]
  GNANIAH, A.M., SEHAR, F.I.R.E., MANGALARAJ, A., et al., “Thermal analysis of modified segmented switched reluctance motor with aluminium metal matrix composite fins used in cooling fan applications”, Matéria (Rio de Janeiro), v. 29, n. 2, pp. e20240075, 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2024-0075.
  » https://doi.org/10.1590/1517-7076-rmat-2024-0075
• ^[10]
  SUBRAMANI, K., GANESAN, A.K., “Synergistic effect of graphene oxide and coloidalnano-silica on the microstructure and strength properties of fly ash blended cement composites”, Matéria (Rio de Janeiro), v. 29, n. 1, pp. e20230305, 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2023-0305.
  » https://doi.org/10.1590/1517-7076-rmat-2023-0305
• ^[11]
  TAMILARASA, A.R.R., GURUSAMY, S.V., “Experimental and analytical analysis of spot-welded cold form steel built up section under axial compression”, Matéria (Rio de Janeiro), v. 29, n. 1, pp. e20230297, 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2023-0297.
  » https://doi.org/10.1590/1517-7076-rmat-2023-0297
• ^[12]
  SERGI, C., SARASINI, F., BARBERO, E., et al., “Assessment of agglomerated corks and PVC foams cores crashworthiness under multiple-impact events in different loading conditions”, Polymer Testing, v. 96, pp. 107061, 2021. http://doi.org/10.1016/j.polymertesting.2021.107061.
  » https://doi.org/10.1016/j.polymertesting.2021.107061
• ^[13]
  LIU, C.X., FU, Z.Y., LI, P., et al., “Bending and environmental characteristics of an eco-friendly sandwich panel with cork stopper cores”, Developments in the Built Environment, v. 15, pp. 100206, 2023. doi: http://doi.org/10.1016/j.dibe.2023.100206.
  » https://doi.org/10.1016/j.dibe.2023.100206
• ^[14]
  MEDJMADJ, S., SI SALEM, A., AIT TALEB, S., “Experimental behavior of plaster/cork functionally graded core sandwich panels with polymer skins”, Construction & Building Materials, v. 344, pp. 128257, 2022. doi: http://doi.org/10.1016/j.conbuildmat.2022.128257.
  » https://doi.org/10.1016/j.conbuildmat.2022.128257
• ^[15]
  SERGI, C., BORIA, S., SARASINI, F., et al., “Experimental and finite element analysis of the impact response of agglomerated cork and its intraply hybrid flax/basalt sandwich structures”, Composite Structures, v. 272, pp. 114210, 2021. doi: http://doi.org/10.1016/j.compstruct.2021.114210.
  » https://doi.org/10.1016/j.compstruct.2021.114210
• ^[16]
  ASHRAF, W., ISHAK, M.R., ZUHRI, M.Y.M., et al., “Effect on mechanical properties by partial replacement of the glass with alkali-treated flax fiber in composite facesheet of sandwich structure”, Journal of Materials Research and Technology, v. 13, pp. 89–98, 2021. doi: http://doi.org/10.1016/j.jmrt.2021.04.047.
  » https://doi.org/10.1016/j.jmrt.2021.04.047
• ^[17]
  MOUNIR, S., KHABBAZI, A., EL HARROUNI, K., et al., “Performance of cork and composites joints”, In: Hashimi, S., Choudhury, J.A. (eds), Encyclopedia of Renewable and Sustainable Materials, Amsterdam, Elsevier, pp. 212–222, 2020.
• ^[18]
  FERNANDES, F.A.O., KACZYNSKI, P., ALVES DE SOUSA, R.J., et al., “Cork composites for structural applications”, In: Inamuddin, I., Altalhi, T., Alrooqi, A. (eds). Green Sustainable Process for Chemical and Environmental Engineering and Science: Natural Materials Based Green Composites 1: Plant Fibers, Amsterdam, Elsevier, pp. 29–51, 2023. doi: http://doi.org/10.1016/B978-0-323-95167-8.00009-0.
  » https://doi.org/10.1016/B978-0-323-95167-8.00009-0
• ^[19]
  FATIMA, N.S., DHALIWAL, G.S., NEWAZ, G., “Influence of interfacial adhesive on impact and post-impact behaviors of CFRP/end-grain balsawood sandwich composites”, Composites. Part B, Engineering, v. 212, pp. 108718, 2021. doi: http://doi.org/10.1016/j.compositesb.2021.108718.
  » https://doi.org/10.1016/j.compositesb.2021.108718
• ^[20]
  MOTRU, S., ADITHYAKRISHNA, V.H., BHARATH, J., et al., “Development and evaluation of mechanical properties of biodegradable PLA/Flax fiber green composite laminates”, Materials Today: Proceedings, v. 24, pp. 641–649, 2020. doi: http://doi.org/10.1016/j.matpr.2020.04.318.
  » https://doi.org/10.1016/j.matpr.2020.04.318
• ^[21]
  KUMAR, A.G., RAMASAMY, S., SOUNDARARAJAN, E.K., et al., “Sustainable utilization of cement kiln dust and GGBS in the development of eco-friendly concrete composite”, Matéria (Rio de Janeiro), v. 29, n. 2, pp. e20240054, 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2024-0054.
  » https://doi.org/10.1590/1517-7076-rmat-2024-0054
• ^[22]
  LI, M., PU, Y., THOMAS, V.M., et al., “Recent advancements of plant-based natural fiber-reinforced composites and their applications”, Composites. Part B, Engineering, v. 200, pp. 108254, 2020. doi: http://doi.org/10.1016/j.compositesb.2020.108254.
  » https://doi.org/10.1016/j.compositesb.2020.108254
• ^[23]
  TOUIL, M., LACHHEB, A., SAADANI, R., et al., “A new experimental strategy assessing the optimal thermo-mechanical properties of plaster composites containing Alfa fibers”, Energy and Building, v. 262, pp. 111984, 2022. doi: http://doi.org/10.1016/j.enbuild.2022.111984.
  » https://doi.org/10.1016/j.enbuild.2022.111984
• ^[24]
  SHARMA, S., SUDHAKARA, P., SINGH, J., et al., “Emerging progressive developments in the fibrous composites for acoustic applications”, Journal of Manufacturing Processes, v. 102, pp. 443–477, 2023. doi: http://doi.org/10.1016/j.jmapro.2023.07.053.
  » https://doi.org/10.1016/j.jmapro.2023.07.053
• ^[25]
  HABBAR, G., HOCINE, A., MAIZIA, A., et al., “Failure analysis of biocomposite sandwich pipe under internal pressure - Application for high pressure gas transportation pipelines MEDGAZ”, International Journal of Pressure Vessels and Piping, v. 202, pp. 104891, 2023. doi: http://doi.org/10.1016/j.ijpvp.2023.104891.
  » https://doi.org/10.1016/j.ijpvp.2023.104891
• ^[26]
  SHANMUGAM, V., RAJENDRAN, D.J.J., BABU, K., et al., “The mechanical testing and performance analysis of polymer-fibre composites prepared through the additive manufacturing”, Polymer Testing, v. 93, pp. 106925, 2021. doi: http://doi.org/10.1016/j.polymertesting.2020.106925.
  » https://doi.org/10.1016/j.polymertesting.2020.106925
• ^[27]
  SARASINI, F., TIRILLÒ, J., LAMPANI, L., et al., “Static and dynamic characterization of agglomerated cork and related sandwich structures”, Composite Structures, v. 212, pp. 439–451, 2019. doi: http://doi.org/10.1016/j.compstruct.2019.01.054.
  » https://doi.org/10.1016/j.compstruct.2019.01.054
• ^[28]
  COSTA, L.M., PIRES, T.A.D.C., SILVA, J.J.D.R., et al., “Experimental and numerical analysis of reinforced concrete beams strengthened in shear with steel bars using the near-surface mounted (NSM) technique”, Matéria (Rio de Janeiro), v. 29, pp. e20230367, 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2023-0367.
  » https://doi.org/10.1590/1517-7076-rmat-2023-0367
• ^[29]
  SAROJA, P.E., MUTHUGOUNDER, P., SHANMUGAM, S., et al., “Enhancing flour quality and milling efficiency: experimental study on bullet plate type flour grinding machine”, Matéria (Rio de Janeiro), v. 29, n. 3, pp. e20240331, 2024. http://doi.org/10.1590/1517-7076-rmat-2024-0331.
  » https://doi.org/10.1590/1517-7076-rmat-2024-0331
• ^[30]
  KUPPUSAMY, R., THANGAVEL, A., MANICKAM, A., et al., “The influence of MoS2 and SiC reinforcement on enhancing the tribological and hardness of aluminium matrix (Al6061-T6) hybrid composites using Taguchi’s method”, Matéria (Rio de Janeiro), v. 29, n. 1, pp. 1–2, 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2023-0337.
  » https://doi.org/10.1590/1517-7076-rmat-2023-0337
• ^[31]
  SERGI, C., SARASINI, F., RUSSO, P., et al., “Experimental and numerical analysis of the ballistic response of agglomerated cork and its bio-based sandwich structures”, Engineering Failure Analysis, v. 131, pp. 105904, 2022. doi: http://doi.org/10.1016/j.engfailanal.2021.105904.
  » https://doi.org/10.1016/j.engfailanal.2021.105904

Publication Dates

• Publication in this collection
  25 Oct 2024
• Date of issue
  2024

History

• Received
  12 Sept 2024
• Accepted
  19 Sept 2024