Effect of Manufacturing Processes on Basalt Fiber-Reinforced Composites for Marine Applications

Abstract

This study investigates the mechanical performance of basalt fiber-reinforced polymer (BFRP) laminates as a suitable alternative to conventional glass fiber-reinforced composites for marine applications. The laminates were produced by varying the main process parameters: the fiber type was either glass or basalt; the resin material was either polyester or vinylester; the fiber orientation in selected layers was set to either 0°/90°, or to ±45° by rotating the woven fabrics during lay-up, and finally the manufacturing technique was either hand lay-up or vacuum infusion. Three-point flexural tests with different spans were conducted to evaluate the flexural behavior and fracture mechanisms. The best-performing configuration, based on glass fibers and vacuum infusion, achieved a maximum flexural strength of about 500 MPa, while basalt-based laminates reached values of up to 400 MPa. Basalt laminates exhibited the highest flexural modulus, with values exceeding 24 GPa. An increase in span length from 120 mm to 220 mm resulted in a reduction in flexural strength of approximately 6–18% depending on the laminate configuration, highlighting the influence of loading conditions on mechanical behavior. The effect of the manufacturing processes was also evaluated using an analysis of variance. This showed that fiber type, manufacturing method, and span significantly influenced the mechanical performance.

1. Introduction

The growing awareness of environmental sustainability has led to an increasing demand for innovative materials and processes capable of reducing the environmental footprint of industrial and transportation activities [1]. Among the various sectors, transportation plays a pivotal role in global emissions, and the maritime industry faces practical challenges due to its high energy consumption and harsh environmental conditions [2]. The development of sustainable materials for marine applications not only provides an opportunity to reduce the sector’s environmental impact, but is also a necessity to comply with stricter regulations limiting greenhouse gas emissions and resource consumption.

Naval applications place stringent technical demands on materials due to their dynamic loading and long-term exposure to corrosive marine environments, and the need to ensure that they are durable and lightweight. Composite materials have become a cornerstone in the maritime sector as they offer an excellent balance between mechanical performance, corrosion resistance, and weight reduction [3,4]. Traditionally, composites of glass fiber-reinforced polymer (GFRP) have been the material of choice for many applications in shipbuilding [5]. However, the production of glass fibers is associated with high energy consumption and the disposal of these materials at the end of their life remains a critical issue due to the challenges associated with recycling [6].

In this context, basalt fibers have attracted increasing attention as a sustainable alternative to glass fibers in composite laminates. Derived from the controlled melting and spinning of natural basalt rocks, these fibers exhibit comparable mechanical properties to glass fibers, such as a high tensile strength and thermal stability, while requiring less energy to produce. In addition, basalt fibers offer increased resistance to chemical and environmental degradation processes, making them particularly suitable for the marine sector. Furthermore, no harmful chemicals are used in the production of basalt fibers, which also contributes to their environmental friendliness [7,8]. Recent comprehensive reviews have confirmed the excellent mechanical, thermal, and chemical properties of basalt fibers, highlighting their potential to replace traditional reinforcements such as glass fibers in both structural and non-structural applications. Moreover, it has been reported that the fiber–matrix interaction plays a critical role in maximizing the performance of basalt fiber-reinforced composites, with several surface treatments proposed to enhance adhesion [9]. Recent studies have also investigated the hybridization of basalt fibers with natural fibers [10,11], such as flax [12], to achieve composites that combine a high mechanical performance with reduced environmental impact.

Recent investigations have confirmed that the hybridization of basalt fibers with natural fibers such as flax can significantly improve the mechanical performance and environmental sustainability of composite laminates. In particular, flax/basalt hybrid composites have demonstrated the ability to maintain good tensile and impact properties while reducing the overall weight and thickness of the laminates, thus contributing to sustainability goals [13]. Studies on hybrid systems combining basalt with other natural fibers, such as kenaf, further highlight the synergistic effects in enhancing tensile strength, flexural strength, thermal stability, and even antibacterial properties [14]. Moreover, it has been shown that the appropriate selection of fiber arrangement and processing methods plays a crucial role in optimizing the interfacial bonding and minimizing void content, which are critical factors for achieving high-performance natural/synthetic fiber hybrids [11,15].

While the type of reinforcement plays a critical role in determining the properties of composite materials, the manufacturing process is equally crucial. The processing method has a significant impact on the mechanical behavior, surface quality, and overall reliability of the finished composite. Among the available manufacturing processes, manual lay-up and vacuum infusion are widely used in the shipbuilding industry due to their versatility and adaptability to complex geometries. Manual lay-up is a cost-effective and straightforward method, but can lead to defects such as uneven resin distribution and air entrapment, which can affect the mechanical properties. In contrast, vacuum infusion offers better control over resin distribution and fiber impregnation, resulting in laminates with improved mechanical performance and lower void content.

Ziemińska-Stolarska et al. [16] applied a Life Cycle Assessment (LCA) approach to evaluate the environmental impact of advanced fiber composite manufacturing technologies for shipbuilding applications. The study focused on processes such as vacuum infusion, adaptive molding, UV curved pultrusion, hot stamping, and large-scale 3D-printing, assessing them based on indicators like Global Warming Potential (GWP), water consumption, and fossil resource scarcity. The results highlighted that material selection had a greater influence on environmental impacts than the energy consumption of the manufacturing processes.

This study aims to evaluate the mechanical behavior of basalt fiber-reinforced polymer (BFRP) laminates manufactured using two distinct processes: manual lay-up with polyester resin and vacuum infusion with vinylester resin. The laminates were fabricated at the Intermarine shipyard in Sarzana, which provided industrially relevant conditions. The mechanical characterization focused on three-point bending tests, with varying support spans to investigate the flexural properties and failure mechanisms. The variations in support span provided insights into the transition from fiber-dominated to matrix-dominated failure, offering a comprehensive understanding of the structural response under different loading conditions.

The novelty of this work lies in the combined evaluation of the effect of fiber type and manufacturing process on the mechanical properties of BFRP laminates tailored for naval applications. Unlike previous studies [17], this research provides an integrated analysis of material sustainability, processing technology, and structural performance, addressing the specific challenges in the maritime sector. The results contribute to the optimization of sustainable composite materials and highlight the potential of basalt fibers as a viable alternative to traditional glass fibers, paving the way for environmentally friendly innovations in shipbuilding. This study is part of an ongoing research activity focused on the development of sustainable composite materials for marine applications, continuing our previous investigations into recycled basalt-based composites and sandwich structures reinforced with basalt fiber skins [18,19,20,21].

While numerous studies have investigated the mechanical properties of traditional glass fiber-reinforced composites, research focusing on basalt fiber-reinforced composites (BFRPs) remains relatively limited, particularly in the context of their application in the maritime industry [22,23]. The existing literature predominantly addresses the environmental benefits and basic mechanical properties of basalt fibers, often overlooking the combined effects of fiber type and manufacturing processes on the overall performance of laminates under realistic service conditions. Recent contributions by Lakhiar et al. [24] and Kiran et al. [25] have investigated innovative solutions to enhance the mechanical and environmental performance of BFRP composites. Lakhiar et al. explored the incorporation of recycled marine materials into BFRP systems, highlighting the potential of waste-derived fillers to improve sustainability while maintaining good mechanical properties. Kiran et al. focused on the influence of stacking sequences in hybrid basalt/natural fiber laminates, emphasizing how the arrangement and combination of different fibers critically affect the flexural performance and energy absorption behavior. These studies reinforce the need for a holistic approach that simultaneously considers material composition and processing techniques when designing composites for demanding applications such as shipbuilding. Furthermore, while the vacuum infusion process is widely recognized for its ability to improve laminate quality, its impact on the mechanical behavior of BFRP laminates, especially when compared to manual lay-up, remains underexplored. Recent studies have addressed various aspects of vacuum infusion when applied to basalt fiber composites. For instance, Tarasov et al. [26] investigated the influence of different geometric forms of distribution tubes on the quality of basalt–plastic composites manufactured via vacuum infusion, highlighting the effect on the phase composition and interlaminar shear strength. Similarly, Yang et al. [27] analyzed the properties of PMMA-based basalt fiber composites fabricated via vacuum-assisted resin infusion (VARI), demonstrating improvements in tensile strength, flexural strength, and short beam shear strength compared to traditional epoxy-based composites. This is particularly relevant in shipbuilding, where material performance must meet stringent mechanical, chemical, and durability requirements.

Another critical gap lies in the experimental methodologies used to evaluate BFRP laminates. Many studies employ standard mechanical tests without systematically varying the testing parameters, which limits the understanding of failure mechanisms and structural performance under diverse loading conditions. For example, the effect of support span variations in three-point bending tests, a critical factor in determining the transition between different failure modes, has received minimal attention in the context of basalt fiber composites [19]. However, recent studies on GFRP and hybrid systems confirm the significant influence of span length on flexural performance and sensing behavior. Israr Ud Din et al. [28] investigated the electromechanical behavior of sandwich structures under three-point bending, demonstrating how span, core thickness, and sensor positioning affect both mechanical response and piezoresistivity. Similarly, in a more recent work, the same authors [29] showed how the use of liquid rubber-modified epoxy significantly enhances fracture toughness in rGO-coated GFRP laminates, highlighting the importance of test configuration in capturing reliable performance metrics. Addressing this gap is essential to provide a more nuanced understanding of their flexural behavior, which is crucial for optimizing their design and application in marine structures.

The motivation for this research arises from the need to fill these gaps by providing a comprehensive analysis of BFRP laminates tailored for naval applications. By integrating sustainable materials, industrially relevant manufacturing processes, and advanced mechanical testing protocols, this study aims to advance the understanding of basalt fiber composites as a viable alternative to traditional materials. The findings are expected to not only enhance the body of knowledge regarding composite materials but also to support the maritime industry in adopting more environmentally friendly and high-performing solutions.

2. Materials and Methods

2.1. Composite Manufacturing

The composite laminate panels used in this study were fabricated by Intermarine, a shipbuilding facility located in Sarzana, Italy. Six composite panels, each measuring 1 m × 1 m, were produced to investigate the effects of fiber type and manufacturing technique on mechanical performance. The reinforcements consisted of glass fiber with an areal weight of 1100 g/m² and basalt fiber with an areal weight of 1200 g/m² mass. These fibers are arranged in different stacking sequences and processed with two distinct manufacturing techniques: manual lay-up and vacuum infusion. Each panel consisted of six layers of bidirectional woven fabric with fibers oriented at 0° and 90°, supplied as standard rolls. Two stacking sequences were investigated: a symmetric [0/90°]₆ configuration, and an asymmetric [+45°/–45°/0°/+45°/–45°/90°] configuration. In both cases, the same 0°/90° woven fabric was used. To achieve the desired orientations in the second sequence, each fabric layer was physically rotated during lay-up so that its 0° and 90° fiber directions were aligned at the specified angles relative to the laminate reference frame. This means that, for instance, a +45° layer corresponded to a 0°/90° fabric rotated by +45°, and similarly for the other orientations. No unidirectional plies or off-axis fiber architectures were employed: all layers consisted of the same bidirectional fabric, with orientation controlled via manual placement.

The fiber-reinforced plastic (FRP) facesheets were constructed using polyester resin Synolite 288-T-1 (AOC Italia S.r.l., Filago, Italy) and vinylester resin Atlac^® 850 AC 300 (AOC Italia S.r.l., Filago, Italy), both commonly used in marine applications due to their excellent mechanical properties and resistance to harsh environmental conditions. The detailed configurations of the composite laminates are summarized in

Table 1. Configurations of the composites.

ID	Manufacturing	Materials	Sequence	Thickness [mm]	FVC [%]
ITEM 1	Hand Lay-Up	Glass/Polyester	[0/90]6	8.6 ± 0.33	30.7
ITEM 2	Vacuum Infusion	Glass/Vinylester	[0/90]6	6.1 ± 0.34	43.3
ITEM 5	Hand Lay-Up	Glass/Polyester	[+45/−45/0/+45/−45/90]	9.0 ± 0.32	29.3
ITEM 1B	Hand Lay-Up	Basalt/Polyester	[0/90]6	6.6 ± 0.73	40.4
ITEM 2B	Vacuum Infusion	Basalt/Vinylester	[0/90]6	5.3 ± 0.15	50.3
ITEM 5B	Hand Lay-Up	Basalt/Polyester	[+45/−45/0/+45/−45/90]	7.3 ± 0.31	36.5

.

Vacuum infusion was employed as one of the manufacturing techniques, in which vacuum pressure is used to impregnate dry fiber reinforcements with resin under controlled conditions (see Figure 1). The process involved placing dry fiber reinforcements into a mold, applying vacuum pressure, and then introducing the resin via strategically placed tubing. This ensured efficient resin distribution and adhesion between the resin and reinforcement fibers. Vinylester resin was specifically used for this process due to its superior wettability and mechanical performance under vacuum infusion conditions.

In contrast, the manual lay-up technique, a widely adopted method in shipbuilding, involved the manual placement of fiber sheets and application of resin by hand, leading to variations in fiber wetting and resin distribution. This method employed polyester resin, which is more suited to manual applications due to its lower viscosity and ease of handling.

2.2. Estimation of Fiber Volume Content

The fiber volume content (FVC) of the manufactured laminates was estimated using a classical approach based on the number of layers, the areal weight of the fabric, the fiber density, and the laminate thickness. The fiber volume fraction was calculated according to the following expression, as proposed by Scalici et al. [30]:

$$V_f={\displaystyle \frac{n{A}_w}{{\rho }_{f}t}}$$

(1)

where n is the number of fabric layers, A_w is the areal weight of the fiber fabric (g/m²), ρ_f is the fiber density (g/cm³), and t is the laminate thickness (cm). For glass fibers, a density of 2.5 g/cm³ was considered, whereas for basalt fibers, a mean value of 2.7 g/cm³ was adopted, according to the method in [30]. The calculated values are reported in Table 1.

2.3. Experimental Testing

Test specimens were prepared from the manufactured composite panels based on a predefined cutting layout designed for optimal material use. To ensure precision in mechanical characterization, all test specimens were cut at the shipyard using the available workshop equipment, including a manual band saw. Each specimen had dimensions of 15 mm in width and 250 mm in length, with the cuts aligned parallel to both the longitudinal direction of the 0/90° fabric of the facesheet and the through-thickness direction for evaluating the effects of fiber orientation.

After cutting, the specimen edges were sanded and visually inspected to eliminate possible damage induced during machining. Samples showing signs of delamination or irregularities were excluded from testing.

The mechanical characterization was performed according to the ASTM D7264/D7264M standard [31], which specifies the required procedures for three-point bending tests with centerline loading on a supported beam. This test was conducted under a constant loading rate of 2 mm/min, with two support restraints and a central loading point, using a “Zwich//Roell Z600” testing machine (ZwickRoell S.r.l., Genova, Italy), with a loading capacity of 600 kN. A standard support system was used to allow for adjustable support spans, ensuring a controlled and repeatable test setup. The span varied from 120 mm, 180 mm, and 220 mm to evaluate its effect on the mechanical properties of the tested composite materials.

Each specimen was assigned a unique identification code following the format IT-n-Dm-N, where “IT” represents the laminate type (ITEM), “n” represents the panel ID, “Dm” denotes the support span, and “N” represents the specimen number within that configuration.

This coding system ensured proper traceability and facilitated comparison among different test conditions.

The bending stress (σ) and strain (ε) were calculated from the load–displacement data acquired by the testing machine using standard beam theory for three-point bending. The following equations were adopted:

$$\sigma ={\displaystyle \frac{3PL}{2b{d}^2}}$$

(2)

$$\epsilon ={\displaystyle \frac{6Dd}{L^2}}$$

(3)

where P is the applied load, L is the span length, b and d are the width and thickness of the specimen, respectively, and D is the mid-span deflection, derived from the crosshead displacement.

The elastic modulus (E) was determined as the slope of the linear portion of the stress–strain curve for each sample:

$$E={\displaystyle \frac{∆\sigma }{∆\epsilon }}$$

(4)

Although no dedicated mid-span extensometer was used, the crosshead displacement signal provided consistent and sufficiently accurate data for a comparative evaluation of stiffness among the different laminate configurations.

3. Results

The mechanical response of the composite laminates under three-point bending was analyzed by examining the stress–strain behavior obtained for each configuration and span length. Figure 2 presents the typical stress–strain curves for all tested laminates and configurations, highlighting the influence of both the material system (glass vs. basalt fibers), the orientation (0/90 vs. +45/−45) and the manufacturing process (manual lay-up vs. vacuum infusion), as well as the effect of the support span.

The stress–strain curves obtained for the IT1 configuration (see Figure 2a), which correspond to glass fiber-reinforced laminates manufactured via hand lay-up using polyester resin, clearly reflect the influence of the support span on the mechanical response under three-point bending. The specimens were tested using three different spans: 120 mm, 180 mm, and 220 mm.

All curves initially show a well-defined linear elastic region, where stress increases proportionally with strain. This phase reflects the elastic behavior of the composite, which is primarily governed by the stiffness of the glass fibers and the resin matrix. As the load increases, a deviation from linearity becomes evident, an indication of the initiation of internal damage mechanisms such as matrix microcracking or the onset of fiber–matrix debonding.

After this transition zone, each curve reaches a peak stress, beyond which an abrupt drop is observed, signaling catastrophic failure of the specimen. This sudden drop indicates that once the material exceeds its flexural capacity, it rapidly loses integrity, leading to complete structural collapse. The shape and extent of the non-linear region, as well as the nature of the post-peak behavior, vary noticeably depending on the support span.

At the shortest span of 120 mm, the stress–strain response reveals a relatively long non-linear phase and a more progressive decrease after the peak. The corresponding failure mode is characterized by localized compressive crushing under the loading nose and tensile rupture of the fibers at the bottom surface (Figure 3a). Damage is confined to the central region of the beam, suggesting a failure mode dominated by flexural stresses with limited interlaminar effects.

When the support span increases to 180 mm, the curves show a slightly earlier transition into the damage zone and a sharper post-peak drop. Visually, the failure involves a combination of fiber breakage, matrix cracking, and the onset of delamination (Figure 3b). The damage appears more diffuse, and the interaction between bending and interlaminar shear becomes more evident.

At 220 mm, the mechanical response becomes markedly different. The curve rises steeply and then drops suddenly, with minimal non-linear deformation prior to failure. The observed fracture is extensive and dominated by interlaminar shear, with large-scale delamination and fiber pull-out (Figure 3c). This shift suggests that as the span increases, the contribution of through-thickness shear becomes more significant, leading to a transition from bending-dominated to shear-dominated failure mechanisms.

These observations underline how support span directly affects not only the global mechanical response but also the local failure modes. While stiffness, governed by the initial slope of the curve, remains relatively unchanged due to the inherent properties of the laminate, strength and fracture behavior evolve substantially. As span increases, the structure becomes more susceptible to unstable interlaminar damage, which is especially critical in laminates produced via hand lay-up, where variations in fiber impregnation and resin distribution may exacerbate local weaknesses.

This progression in failure behavior from localized fiber rupture to extensive delamination with an increasing span is consistent with what is reported in the literature on composite flexural testing [19], and highlights the need for careful span selection in characterizing the flexural properties of composite materials, especially in applications like shipbuilding, where large-scale components are subject to complex loading conditions.

The stress–strain curves obtained for the IT1B configuration highlight how the use of basalt fibers, combined with the same resin and processing method as in the previous glass-based configuration, alters the mechanical behavior under flexural loading (see Figure 2b). The specimens were tested again with three support spans (120 mm, 180 mm, and 220 mm) and the resulting curves exhibit some key differences compared to IT1.

In all three cases, the initial response remains linearly elastic, reflecting the stiffness provided by the basalt fibers and the matrix. However, unlike the abrupt failure seen in IT1, the IT1B curves show a distinctly more gradual post-peak behavior, which is especially evident for the shortest span. After the peak load, the curves plateau or decline slowly, indicating that the material undergoes progressive damage accumulation rather than a sudden rupture. This is consistent with what is often reported in the literature for basalt-reinforced composites, which tend to exhibit increased energy absorption and a more ductile-like failure progression compared to their glass counterparts [8].

At the 120 mm span, the curve reaches the highest stress and develops over a wider strain range, showing a multi-stage post-peak region with visible load drops, likely corresponding to successive damage events such as fiber breakage, matrix cracking, and local delamination. This interpretation is supported by the fracture surface shown in Figure 4a, which displays a complex damage zone under the loading nose, with shear-tearing and localized crushing, but without the clean, brittle split observed in IT1.

When the span increases to 180 mm, the stress–strain curve reveals a flatter post-peak behavior and lower stress levels overall. The corresponding failure image (see Figure 4b) indicates a wider damage area, with extensive delamination and fiber pull-out, pointing to a failure mechanism in which interlaminar shear begins to play a more dominant role, compromising the load-carrying capacity of the structure.

At the longest span of 220 mm, the response becomes even more attenuated, with a shallow curve and low strain leading to failure. In this configuration, the laminate undergoes early interlaminar separation and matrix shearing, as shown in Figure 4c. The failure mode here is progressive and delamination-driven, with extensive cracking along the interfaces between layers. Despite the absence of a sharp drop in stress, the material essentially gradually loses its integrity, a behavior that may be beneficial in applications where a more forgiving failure is preferable to an abrupt collapse.

Overall, the stress–strain behavior of the IT1B configuration confirms the enhanced damage tolerance of basalt fiber laminates under flexural loading. The smoother post-peak response and the ability to sustain deformation beyond the peak load suggest a more distributed damage evolution, which may be advantageous in structural applications, where energy dissipation and post-failure integrity are relevant. However, the flexural performance still decreases significantly with an increase in span, confirming once again that longer spans promote failure modes dominated by interlaminar shear and matrix-driven damage, especially in hand lay-up laminates.

The mechanical response of the IT2 configuration under flexural loading reflects the combined effect of using glass fiber reinforcements with vinylester resin processed via vacuum infusion, a method known for producing high-quality laminates with improved fiber impregnation and low void content (see Figure 2c). The resulting stress–strain curves show a distinctly linear and sharp profile, with limited plasticity and a very sudden failure once the peak load is reached.

All three curves begin with a steep linear elastic region, confirming the high stiffness typically associated with glass fiber composites. The vacuum infusion process enhances this behavior by ensuring a uniform fiber–resin interface and minimizing defects that could act as early initiators of failure. Following the linear region, the specimens reach their peak stress and undergo a sharp drop in stress, indicative of a brittle fracture mechanism with little to no energy absorption post-peak.

At the shortest span (120 mm), the specimen displays the highest slope and an abrupt collapse immediately after reaching the peak load. The corresponding failure image (Figure 5a) shows a clean tensile fracture at the lower surface and compression failure at the loading point, with the fracture zone being highly localized. This behavior suggests a dominance of bending stresses and a failure mode triggered by fiber breakage and crushing at the outermost layers.

With an increase in span to 180 mm, the curve maintains a similar linear trend, but the fracture occurs slightly earlier in terms of strain. The failure surface (Figure 5b) shows signs of extended matrix cracking and slight delamination, although the failure remains predominantly brittle. The stress distribution appears to be more complex at this span, but the laminate still presents a stiff, high-performance structure with low ductility.

At the 220 mm span, the curve is slightly more gradual in its post-peak drop, and the deformation at failure increases slightly. However, the failure mode (Figure 5c) continues to show a brittle nature, with fracture localized in the lower region of the specimen. While sharp fracture lines and clean ply separation are less clearly discernible, the overall failure pattern suggests limited delamination and possible matrix–fiber separation, likely promoted via the increased shear contribution at this span, which facilitates the initiation of interlaminar failure.

Overall, the IT2 configuration demonstrates very high stiffness and strength, particularly in the early loading stage, and fails in a brittle manner across all support spans. Compared to the manually produced IT1 specimens, the vacuum-infused laminates clearly benefit from improved consolidation and better matrix–fiber bonding, which delay the onset of damage and increase the load-bearing capacity. However, the limited post-peak deformation and lack of ductility may be a concern in applications where graceful failure or energy dissipation are desired.

The mechanical response of the IT2B configuration under three-point bending is particularly revealing of how the combination of basalt fibers and vinylester resin, processed via vacuum infusion, influences the flexural performance and failure modes (see Figure 2d). All three stress–strain curves begin with a steep linear elastic region, consistent with a well-consolidated laminate, followed by a peak stress and a post-peak region that differ markedly according to the span length.

At 120 mm, the stress–strain curve reaches the highest peak and is followed by a multi-phase post-peak behavior, with a noticeable sequence of load drops and recoveries. This oscillating response is indicative of a progressive failure mechanism, in which the material undergoes micro-damage events, such as fiber breakage, matrix cracking, and localized delamination, rather than an abrupt rupture. This is confirmed by the failure mode shown in Figure 6a, where the fracture remains localized but complex, combining tensile rupture at the bottom surface, compressive crushing at the loading point, and early signs of interlaminar separation. The laminate retains structural coherence even after the initial failure, suggesting a certain degree of damage tolerance.

When the span is increased to 180 mm, the mechanical response becomes flatter in the post-peak region, and the load-carrying capacity is slightly reduced. The corresponding failure mode, shown in Figure 6b, is characterized by more pronounced delamination and fiber–matrix debonding, particularly beneath the loading nose. This more extended damage zone indicates that interlaminar shear stresses begin to play a more dominant role, leading to a less stable but still non-catastrophic failure progression.

For the 220 mm span, the curve reveals an early drop in stress, with a limited ability to carry load beyond the peak. The response is shorter and exhibits fewer oscillations after the maximum, suggesting a more unstable failure process. The fracture surface in Figure 6c clearly shows extensive delamination and ply separation, with significant fiber pull-out and matrix rupture. This failure mode confirms that longer spans promote shear-driven damage and reduce the laminate’s ability to dissipate energy gradually.

Overall, IT2B laminates demonstrate a markedly different behavior compared to glass-reinforced counterparts. The curves are less brittle, and the post-peak regions are more extended, especially for shorter spans, pointing to greater energy absorption and progressive failure mechanisms. The vacuum infusion process ensures excellent fiber impregnation and reduces defects, enabling the basalt fibers to fully express their mechanical potential. However, as span increases, interlaminar weaknesses emerge and dominate the failure process, particularly when the through-thickness shear stress becomes critical.

These findings confirm that basalt fiber composites produced via vacuum infusion can provide a mechanically resilient and damage-tolerant alternative to conventional solutions—especially for applications where progressive failure is preferable to brittle collapse, such as in shipbuilding and safety-critical marine structures.

The flexural response of the IT5 configuration stands out from the others due to its multidirectional lay-up, which introduces a more complex internal structure compared to [0/90°] laminates (see Figure 2e). The presence of ±45° plies is expected to influence both the stiffness and the failure mechanisms, especially under bending, where shear and through-thickness effects can become critical.

Across all support spans, the stress–strain curves maintain a linear elastic behavior up to the peak load, which is relatively similar in magnitude for the three configurations. However, the post-peak response reveals some interesting trends. The 120 mm span exhibits a slightly more extended deformation before failure, with a progressive drop in load after the maximum. This is typically associated with damage accumulation across multiple orientations, including matrix microcracking, in-plane shear within the ±45° plies, and local debonding.

The failure image at this span (Figure 7a) confirms this interpretation: the damage remains mostly localized at the loading point, with visible indentation but no catastrophic delamination or widespread cracking. The multidirectional reinforcement seems to help distribute stress, enabling the laminate to sustain bending with a certain degree of damage tolerance.

At 180 mm, the stress–strain response is slightly more brittle. The curve reaches a similar peak but then shows a more sudden decline in stress. The specimen in Figure 7b reveals a wider crushed zone under the indenter, with evidence of interlaminar cracks extending outward. Although still structurally coherent, the failure becomes more complex, involving interplay between matrix rupture, shear-induced ply rotation, and partial delamination.

The 220 mm span configuration displays the most brittle behavior, with an earlier and sharper post-peak drop. In Figure 7c, the laminate clearly shows extensive transverse deflection and a deeper, more visible failure zone, including visible delamination between layers, especially at the bottom surface. This suggests that at longer spans, the influence of interlaminar shear and peeling stress becomes dominant, overwhelming the benefits of the multidirectional architecture.

In general, the IT5 configuration demonstrates good stiffness and strength due to the glass fiber content and balanced lay-up. The ±45° layers seem to delay catastrophic failure in shorter spans by redistributing internal loads and providing resistance to shear deformation, a behavior noted in similar multidirectional glass fiber composites. However, as the support span increases, the shear-dominated failure mechanisms prevail, and the absence of process control, as in manual lay-up, might contribute to interfacial weaknesses that reduce post-peak integrity.

Among the tested configurations, IT5B stands out due to its balanced and gradual mechanical response (see Figure 2f). The stress–strain curves reveal a pronounced linear elastic phase, followed by a smooth and extended post-peak region, especially for the shorter span. This behavior is indicative of a progressive damage evolution, enabled by both the multidirectional reinforcement and the intrinsic characteristics of basalt fibers, which are known for their excellent strain-to-failure and damage tolerance.

For the 120 mm span, the stress–strain curve exhibits a relatively high stiffness and peak stress, followed by a long descending branch with mild oscillations. This suggests that the failure does not occur abruptly, but rather through successive internal damage mechanisms such as fiber/matrix debonding, matrix cracking, and limited delamination. Figure 8a supports this: the specimen remains structurally cohesive, with only localized crushing and slight fiber cracking near the indenter. The energy absorption appears high, confirming the effectiveness of the [+45°/–45°] plies in redistributing in-plane shear stresses.

At the 180 mm span, a slight reduction in peak stress is observed, but the post-peak behavior remains smooth. The damage mode shown in Figure 8b becomes more pronounced, with visible signs of interlaminar separation and matrix rupture, particularly around the mid-thickness of the specimen. While still stable, the failure extends further along the span, indicating a more significant contribution of shear deformation.

The 220 mm span results in a more flattened stress–strain response, with a lower peak and less defined recovery after the first damage event. The final failure (Figure 8c) is characterized by widespread delamination, longitudinal cracking, and fiber pull-out, especially at the bottom face. These features suggest that during moments of high bending and increasing span, the interlaminar cohesion is compromised, and the laminate gradually transitions from flexural to shear-driven failure.

What differentiates IT5B from its glass counterpart (IT5) is not only the smoother and more extended post-peak region, but also the visibly greater strain capacity, which is reflected in the broader horizontal span of the curves. Basalt fibers appear to enhance ductility and toughness, even in manually produced laminates. The multidirectional lay-up plays a synergistic role, improving the distribution of stress and delaying crack propagation through engaging plies with different orientations under different loading conditions.

In summary, the IT5B configuration combines the mechanical benefits of multidirectional reinforcement with the superior failure resistance of basalt fibers. It shows promising behavior for applications where gradual energy dissipation and structural redundancy are critical, confirming the potential of this system for use in sustainable and resilient shipbuilding structures.

The bar chart in Figure 9 illustrates the peak flexural stress values for all six configurations at three different support spans. Several key trends emerge, which can be interpreted through consideration of material selection, processing, and structural design.

3.1. Effect of Fiber Type: Glass vs. Basalt

Across all lay-ups and processing methods, glass fiber-reinforced laminates (IT1, IT2, IT5) consistently exhibit higher peak stress than their basalt-based counterparts (IT1B, IT2B, IT5B). This difference is most evident when comparing IT1 vs. IT1B and IT2 vs. IT2B, where the glass fiber panels outperform basalt ones by approximately 20–30% at each span length.

This outcome is expected, as glass fibers have a slightly higher tensile strength and modulus compared to basalt, especially when processed under optimized conditions. However, it is worth noting that the gap narrows when vacuum infusion is employed (IT2B), suggesting that process quality plays a compensating role for the lower intrinsic stiffness of basalt.

3.2. Effect of Processing Technique: Hand Lay-Up vs. Vacuum Infusion

The most evident distinction in performance due to processing is seen between IT1 vs. IT2 and IT1B vs. IT2B. In both cases, the switch from manual lay-up to vacuum infusion results in a significant increase in peak stress, regardless of the fiber used.

Notably, IT2 (glass fiber + vacuum infusion) displays the highest peak stress across all configurations, reaching values over 500 MPa at the 120 mm span. This confirms that vacuum infusion enables superior fiber impregnation, reduces voids, and improves the overall fiber–matrix interface, leading to better stress transfer and higher structural integrity.

The improvement is also notable for basalt-based laminates: IT2B systematically outperforms IT1B, reinforcing the importance of a controlled infusion process in achieving a consistent mechanical performance even with alternative, more sustainable fibers.

3.3. Effect of Lay-Up Orientation: [0/90°] vs. Multidirectional

When comparing the [0/90°] configurations (IT1, IT1B, IT2, IT2B) to the multidirectional ones (IT5, IT5B), it becomes clear that the multidirectional lay-up leads to a slightly lower peak stress. For both glass and basalt fiber systems, this reduction is in the range of 5–15%.

This result is coherent with the literature findings: while a [+45°/–45°/0°/…/90°] stacking sequence promotes a better shear load distribution and damage tolerance, it also leads to a lower proportion of load-aligned fibers, thus reducing the material’s flexural resistance when the loading is primarily in the longitudinal direction. Nonetheless, this multidirectional lay-up may be beneficial in applications requiring multi-axial loading resistance or where progressive failure is preferable to brittle rupture.

3.4. Effect of Support Span Length

As expected, increasing the support span leads to a consistent reduction in peak stress across all configurations. This trend is due to the increase in the moment of bending and the increased contribution of interlaminar shear and out-of-plane stresses, which reduce the effective load-bearing capability of the laminate.

The magnitude of this drop varies depending on the stiffness and lay-up; examples are provided as follows:

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In IT2, the drop from 120 mm to 220 mm is relatively modest, showing a ~10% reduction, likely due to the excellent quality of the laminate produced via vacuum infusion.

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In contrast, configurations like IT1B and IT5B, where both the material and the process are less optimized, show more pronounced decreases—up to 25–30%—in peak stress as the span increases.

In conclusion, the results clearly highlight the synergistic impact of fiber type, process, lay-up architecture, and span length on the mechanical performance of composite laminates:

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Glass fibers outperform basalt in terms of absolute strength, but the gap narrows with better processing.

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Vacuum infusion significantly enhances performance for both glass and basalt laminates.

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[0/90°] lay-ups provide higher flexural resistance, while multidirectional lay-ups offer more distributed damage mechanisms at the cost of peak stress.

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Shorter spans favor a higher apparent strength due to the reduced shear influence and more localized flexural stress distribution.

These findings support the use of basalt fibers as a promising and sustainable alternative in marine applications, especially when combined with controlled manufacturing methods like vacuum infusion and lay-up optimization to balance strength and failure mode control.

The bar chart in Figure 10 illustrates the average flexural modulus (E) in MPa for each laminate configuration. A few key trends emerge that help us understand the influence of fiber type, processing technique, and lay-up orientation.

The most striking observation is the significant increase in flexural modulus for vacuum-infused laminates compared to those produced via hand lay-up. Both IT2 (glass) and IT2B (basalt) exhibit the highest modulus values, with IT2B reaching nearly 25 GPa, the top-performing configuration in this comparison. This confirms that vacuum infusion provides superior fiber alignment, reduced void content, and more effective matrix impregnation. These factors enhance the efficiency of stress transfer between fibers and matrix, resulting in a stiffer composite. Notably, IT2B outperforms IT2, suggesting that basalt fibers may offer superior stiffness retention when processed optimally.

Comparing glass- and basalt-based laminates with the same lay-up and process (i.e., IT1 vs. IT1B and IT2 vs. IT2B), basalt fiber-reinforced composites show comparable or slightly higher flexural moduli. This aligns with the literature reporting that while glass fibers typically outperform basalt in terms of tensile strength, basalt may exhibit a higher modulus, especially in terms of bending and compression, due to its denser structure and higher intrinsic stiffness [7]. Notable examples include the following:

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IT1B > IT1: despite being processed via hand lay-up, the basalt configuration achieves significantly higher stiffness than the glass-based counterpart.

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IT2B > IT2: vacuum-infused basalt outperforms even the best glass-based configuration, highlighting its potential for applications where stiffness and sustainability are both key requirements.

The lowest modulus values are recorded for the multidirectional lay-ups (IT5 and IT5B), regardless of fiber type. This is expected, as the ±45° and 90° plies contribute less to the longitudinal stiffness under bending loads compared to the 0° plies dominant in [0/90°] lay-ups. The presence of off-axis layers leads to internal deformation mechanisms (e.g., in-plane shear and ply rotation) that reduce the overall flexural stiffness.

However, between the two multidirectional configurations, IT5B (basalt) again slightly outperforms IT5 (glass), reinforcing the general observation of higher flexural rigidity in basalt-based laminates, even when processed manually.

3.5. Analysis of Variance

To quantitatively assess the influence of different factors on the mechanical response of the composite laminates, a statistical Analysis of Variance (ANOVA) was performed. This technique allows for the identification of which factors, among the selected ones, have a statistically significant effect on the dependent variable, in this case the maximum flexural stress.

Two separate full-factorial ANOVAs were carried out, each involving three independent factors:

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First ANOVA:

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Factors: (i) fiber type (glass vs. basalt); (ii) manufacturing process (hand lay-up vs. vacuum infusion); (iii) support span (120, 180, and 220 mm).

○

Compared configurations: IT1 (glass + hand lay-up), IT1B (basalt + hand lay-up), IT2 (glass + vacuum infusion), and IT2B (basalt + vacuum infusion).

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Second ANOVA:

○

Factors: (i) fiber type (glass vs. basalt); (ii) lay-up orientation ([0/90°] vs. multidirectional [+45/–45/0/90]); (iii) support span (120, 180, and 220 mm).

○

Compared configurations: IT1 (glass + [0/90°]), IT1B (basalt + [0/90°]), IT5 (glass + multidirectional), and IT5B (basalt + multidirectional)

The goal of these analyses is to rigorously validate the trends observed in the experimental results and to determine whether the observed differences are statistically significant, while also accountings for interactions between factors.

Before interpreting the ANOVA table, it is essential to verify the reliability of the statistical model through residual analysis.

The diagnostic plots provide a comprehensive picture, as shown in Figure 11. The normal probability plot (top left) reveals that the residuals lie closely along the reference line, indicating that the assumption of normality is reasonably satisfied. The histogram of residuals (bottom left) further supports this, displaying a bell-shaped, symmetric distribution, consistent with Gaussian behavior. The plot of residuals versus fitted values (top right) shows a random scatter of points around the zero line, without discernible patterns, suggesting that the variance is homoscedastic and that the model fits the data evenly across the range of predictions. Similarly, the residuals plotted in the order of data collection (bottom right) do not exhibit any systematic trend or drift over time, reinforcing the assumption of independence. Overall, the diagnostic plots confirm that the data meet the fundamental requirements of ANOVA, and that the results derived from the model can be considered statistically robust.

Turning to the ANOVA

Table 2. Analysis of variance (first).

Source	DF	Adj SS	Adj MS	F-Value	p-Value
Model	15	390,105	26,007	21.41	0.000
Blocks	4	6274	1569	1.29	0.288
Linear	4	362,488	90,622	74.60	0.000
Fiber	1	183,773	183,773	151.29	0.000
Process	1	153,541	153,541	126.40	0.000
Span length	2	25,174	12,857	10.36	0.000
Two-Way Interactions	5	18,855	3771	3.10	0.017
Fiber × Process	1	18,494	18,494	15.23	0.000
Fiber × Span length	2	246	123	0.10	0.904
Process × Span length	2	115	58	0.05	0.954
Three-Way Interactions	2	2488	1244	1.02	0.367
Fiber × Process × Span length	2	2488	1244	1.02	0.367
Error	44	53,447	1215
Total	59	443,552

, the most significant outcome is the extremely low p-value associated with the overall model (p = 0.000), which confirms that the variation observed in flexural strength is not due to chance but rather to the influence of the tested factors. Among these, all three main effects—fiber type, process, and span length—show highly significant F-values, with p-values well below the 0.05 threshold. This means that each of these variables has a distinct and measurable impact on the maximum stress reached during bending.

Specifically, the factor “fiber” (p = 0.000) confirms that glass and basalt fibers yield different flexural strengths, with glass-reinforced laminates performing better on average. The factor “process” (p = 0.000) is equally critical, indicating that vacuum infusion significantly enhances performance compared to hand lay-up, a result attributed to the improved fiber impregnation and reduced void content achieved through vacuum processing. Span length, although not a material property, also emerges as statistically significant (p = 0.000), as expected, reflecting the consistent reduction in maximum stress with increasing distance between supports, likely due to the rising contribution of interlaminar shear stresses and reduced load concentration at longer spans.

The interaction terms offer further insight. The interaction between fiber type and process is statistically significant (p = 0.000), suggesting that the beneficial effect of vacuum infusion varies depending on the fiber type. This is especially relevant in explaining why the performance gap between glass and basalt narrows considerably when vacuum infusion is used, as seen in the comparison between IT2 and IT2B. On the other hand, the interactions between fiber and span, process and span, and the three-way interaction all yield high p-values (>0.9), indicating that these combined effects are not statistically significant. In other words, while span length independently influences strength, its effect does not vary systematically with either fiber type or process. The mechanical response to different spans appears consistent across all material and process combinations.

In summary, this ANOVA supports the experimental findings by confirming that the type of fiber, the manufacturing process, and the support span are all statistically significant factors affecting flexural strength. Among them, the process exerts the most pronounced influence, with vacuum infusion substantially improving laminate performance. The fiber type also plays a crucial role, and its effect is further modulated by the process used. These conclusions provide a solid statistical foundation for the material selection and process optimization strategies in marine composite applications.

Figure 12 shows the main effects and interaction plots for the maximum flexural stress, as obtained from the first ANOVA. The main effects plot confirms that all three factors—fiber type, manufacturing process, and span length—significantly influence the mechanical performance of the laminates. In particular, the use of basalt fibers results in a noticeable reduction in strength compared to glass fibers, while the vacuum infusion process leads to a significant improvement compared to hand lay-up. The span length also has a marked effect, with stress values decreasing as the span increases, as is expected due to the higher bending moment and increased contribution of interlaminar effects.

The interaction plots provide further insight into how the factors combine. A moderate interaction is observed between fiber type and process: basalt fiber laminates are more sensitive to the manufacturing method, with a substantial increase in performance when using vacuum infusion. This indicates that the impregnation quality and void content play a more critical role in BFRP laminates. A similar trend is observed for the fiber–span interaction, where glass fiber composites show a greater sensitivity to span variations than basalt-based ones. The absence of strong crossover effects supports the validity of the main effects model, but the interaction trends reinforce the importance of considering factor combinations in laminate design.

The second factorial ANOVA was conducted to evaluate the influence of three independent variables, fiber type (glass vs. basalt), lay-up orientation ([0/90°] vs. multidirectional [+45°/–45°/0°/90°]), and support span length (120 mm, 180 mm, 220 mm), on the maximum flexural stress. The tested configurations included IT1 and IT5 (glass fiber with different lay-ups) and IT1B and IT5B (basalt fiber with different lay-ups), allowing for a comprehensive comparison between both materials and structural architectures.

Before interpreting the ANOVA results, it is essential to examine the diagnostic plots to verify the reliability of the statistical model (see Figure 13). The normal probability plot confirms that the residuals are approximately normally distributed, as they follow a straight-line trend, with only slight deviations at the extremes. The histogram further reinforces this observation by showing a reasonably symmetrical distribution. The plot of residuals versus fitted values demonstrates that the data points are randomly dispersed around zero, without visible patterns, suggesting that homoscedasticity is respected. Finally, the residuals plotted against the order of data collection also show no evident trends or autocorrelation, confirming the assumption of independence. Together, these plots indicate that the model satisfies the core assumptions of ANOVA and that the results can be considered statistically valid.

The ANOVA

Table 3. Analysis of Variance (second).

Source	DF	Adj SS	Adj MS	F-Value	p-Value
Model	15	109,248	7283.2	9.28	0.000
Blocks	4	4044	1011.0	1.29	0.289
Linear	4	103,515	25,878.7	32.99	0.000
Fiber	1	82,978	82,978.3	105.78	0.000
Orientation	1	171	171.0	0.22	0.643
Span Length	2	20,365	10,182.7	12.98	0.000
Two-Way Interactions	5	1067	213.4	0.27	0.926
Fiber × Orientation	1	21	21.5	0.03	0.869
Fiber × Span Length	2	345	172.6	0.22	0.803
Orientation × Span Length	2	700	350.2	0.45	0.643
Three-Way Interactions	2	622	311.0	0.40	0.675
Fiber × Orientation × Span Length	2	622	311.0	0.40	0.675
Error	44	34,515	784.4
Total	59	143,763

shows that the overall model is statistically significant (p < 0.001), indicating that at least one of the included factors has a real effect on flexural strength. Among the main effects, fiber type emerges once again as the most influential factor, with a highly significant p-value (p = 0.000). This result confirms that glass fiber-reinforced laminates achieve higher peak stress values compared to basalt-based ones, regardless of the lay-up configuration.

In contrast, the lay-up orientation does not exhibit a statistically significant influence (p = 0.643). While the experimental data showed some variation in mechanical performance between [0/90°] and multidirectional laminates, these differences were not large enough—relative to the data dispersion—to be confirmed statistically. This suggests that lay-up orientation, at least in the current design and testing conditions, does not substantially affect the maximum flexural stress. This result may indicate that orientation plays a more decisive role in failure mode, energy dissipation, or post-peak behavior, rather than in peak stress alone.

As observed in the previous model, span length remains a significant factor (p = 0.000) consistently affecting flexural strength. The reduction in peak stress with increasing span length appears to be an intrinsic response to all configurations associated with higher bending moments and the greater contribution of shear effects in longer spans.

Regarding interaction effects, none of the two-way or three-way interactions reached statistical significance. The interaction between fiber type and orientation (p = 0.869), fiber and span (p = 0.206), and orientation and span (p = 0.643), as well as the three-way interaction (p = 0.675), all yielded high p-values, indicating that the effects of these factors on peak stress are independent of one another. In other words, the influence of fiber type does not depend on the lay-up, and vice versa, and the behavior under different spans is similarly unaffected by the other variables.

This second analysis confirms the dominant role of fiber type in determining flexural strength, as well as the consistent and independent effect of span length. Surprisingly, lay-up orientation did not exhibit a significant impact on peak stress, despite observable differences in failure behavior during testing. This suggests that the orientation of the reinforcement layers may influence other aspects of performance—such as energy absorption, crack propagation, or progressive damage evolution—rather than the maximum load-bearing capacity.

The lack of significant interactions further supports the robustness of the main effects and indicates that material and geometric variables can be optimized relatively independently in the design phase. From a design perspective, this means that multidirectional lay-ups may be selected for their improved damage tolerance and structural stability without significantly compromising flexural strength.

Figure 14 reports the main effects and interaction plots for the second ANOVA, which was conducted to investigate the influence of fiber type, stacking sequence, and span length on the maximum flexural stress.

Among the main effects, fiber type remains the dominant factor, with glass fiber laminates showing significantly higher strength compared to basalt ones. Span length again exhibits a clear decreasing trend in stress with increasing span, consistent with the first analysis. In contrast, the influence of fiber orientation appears marginal in terms of average stress variation, with only a slight improvement for the ±45° configuration compared to 0/90°.

The interaction plots provide a more in-depth interpretation of how these factors combine. Notably, fiber–orientation and orientation–span interactions are weak, suggesting that the stacking sequence does not strongly influence the response trend across fiber types or span lengths. However, the fiber–span interaction again shows that the glass fiber laminates are more affected by span variation than basalt ones, indicating higher sensitivity to the test geometry. The visual parallelism of most lines supports the dominance of the main effects, while confirming that interaction effects, although present, are of secondary importance in this design space.

4. Discussion

The present study delves into the flexural behavior of composite laminates reinforced with glass and basalt fibers, manufactured via hand lay-up and vacuum infusion processes and tested across varying support spans. The findings offer nuanced insights into the mechanical performance of these materials, with implications for their application in marine structures.

4.1. Influence of Fiber Type and Manufacturing Process

The experimental results underscore the significant impact of both fiber type and manufacturing process on flexural strength and modulus. Glass fiber-reinforced laminates consistently exhibited higher peak stress values compared to their basalt counterparts. This observation aligns with the existing literature, which reports that while basalt fibers possess commendable mechanical properties, they often yield lower tensile strengths than glass fibers in composite applications [32].

The manufacturing process further modulates these properties. Laminates produced via vacuum infusion demonstrated superior mechanical performance compared to those fabricated through hand lay-up. This enhancement is attributed to the improved fiber impregnation and reduced void content inherent to the vacuum infusion technique. Studies have shown that vacuum infusion enables higher fiber volume fractions and lower void content, resulting in composites with enhanced mechanical properties [32,33,34,35].

4.2. Effect of Lay-Up Orientation

Interestingly, the study revealed that lay-up orientation ([0/90°] vs. multidirectional [+45°/–45°/0°/90°]) did not significantly influence peak flexural stress. This finding suggests that, within the tested configurations, the directional alignment of fibers plays a less critical role in determining maximum flexural strength than previously assumed. However, it is important to consider that while peak stress remained unaffected, lay-up orientation may influence other mechanical aspects, such as damage tolerance and fatigue resistance, which were not within the scope of this study.

4.3. Span Length Considerations

As anticipated, an increase in support span length corresponded with a reduction in maximum flexural stress across all configurations. This trend is consistent with fundamental beam theory, where longer spans result in higher bending moments and, consequently, lower stress thresholds before failure. This observation underscores the necessity of considering span length in the structural design of composite components to ensure their mechanical reliability [32,36].

4.4. Comparative Analysis with the Existing Literature

The findings of this study resonate with existing research on basalt fiber-reinforced composites. For instance, studies have indicated that while basalt fibers offer environmental advantages and good mechanical properties, their performance can be inferior to that of glass fibers in certain applications. Additionally, the benefits of vacuum infusion in enhancing composite performance have been well-documented, with reports highlighting improvements in fiber volume fraction and mechanical properties [32].

4.5. Implications for Marine Applications

The marine industry continually seeks out materials that balance mechanical performance with environmental sustainability. Basalt fibers, derived from natural volcanic rock, present an eco-friendly alternative to synthetic fibers. Their demonstrated performance in this study suggests potential applicability in marine structures, particularly when combined with advanced manufacturing techniques like vacuum infusion. However, considerations such as long-term durability in marine environments and resistance to saltwater corrosion warrant further investigation.

4.6. Limitations and Future Directions

While this study provides valuable insights, certain limitations must be acknowledged. The focus was primarily on static flexural properties; dynamic loading conditions, such as fatigue and impact resistance, were not explored. Given the operational demands in marine environments, future studies should investigate these aspects to provide a comprehensive understanding of material performance. Additionally, environmental factors like moisture absorption and temperature variations, which can significantly affect composite behavior, were not considered and merit further examination.

In summary, this study highlights the critical roles of fiber type and manufacturing process in determining the flexural properties of composite laminates. While glass fibers currently offer superior mechanical performance, basalt fibers present a promising sustainable alternative, especially when processed through vacuum infusion techniques. These insights contribute to the ongoing discourse on material selection and processing strategies in the development of composites for marine applications.

5. Conclusions

This study investigated the flexural performance of composite laminates reinforced with either glass or basalt fibers, manufactured through hand lay-up and vacuum infusion processes, and tested under varying support spans. The experimental campaign aimed to evaluate the influence of sustainable reinforcement alternatives and industrially relevant manufacturing techniques on the mechanical behavior of fiber-reinforced polymer laminates for potential application in the marine sector.

The results clearly demonstrated that both fiber type and manufacturing process have a significant effect on the flexural strength and modulus of the composites. Glass fiber-reinforced laminates consistently exhibited higher peak stress values compared to basalt-based laminates; however, basalt fibers still showed a competitive mechanical performance and even superior stiffness in certain configurations. Notably, basalt-reinforced laminates manufactured through vacuum infusion outperformed several glass fiber counterparts, confirming the potential of this material as a viable, more sustainable alternative, particularly when processed under controlled conditions.

The manufacturing process emerged as a critical factor, with vacuum infusion leading to substantial improvements in mechanical performance due to the enhanced fiber impregnation, reduced void content, and improved fiber–matrix interface. This was reflected not only in the peak flexural strength but also in the modulus of elasticity, with vacuum-infused laminates achieving the highest stiffness values across all tested configurations.

While support span was not a material parameter, its variation provided valuable insight into failure mechanisms. Shorter spans generally led to higher recorded stresses, as expected, while longer spans resulted in a shift toward more distributed damage and lower peak values. This trend was confirmed for all fiber/process combinations, indicating the robustness of the mechanical response to span effects.

The lay-up orientation, on the other hand, had a more nuanced impact. Multidirectional lay-ups resulted in slightly lower flexural strength and modulus compared to [0/90°] configurations, likely due to the reduced number of longitudinally aligned fibers. However, the observed failure modes suggested more progressive and distributed damage in these laminates, which may be advantageous in applications where energy absorption and damage tolerance are desirable. Interestingly, statistical analysis showed that the orientation did not significantly affect the peak flexural stress, pointing to a potentially more important role in influencing other mechanical parameters, such as ductility and toughness.

The findings were statistically validated through two full-factorial ANOVAs, which confirmed the significance of fiber type, manufacturing process, and span length on peak stress. The orientation factor, while qualitatively impactful on failure modes, did not emerge as statistically significant for stress values, suggesting its effects are more pronounced regarding damage evolution than ultimate strength.

In conclusion, this work provides a comprehensive evaluation of sustainable composite laminates using basalt fibers and demonstrates the effectiveness of vacuum infusion in enhancing mechanical performance. The obtained insights are particularly relevant for the shipbuilding industry, where material selection must be balanced with mechanical performance, durability, and environmental impact. The results encourage the further exploration of basalt fibers in marine applications and support their integration into future structural components, provided that optimized manufacturing routes are employed.