1. Introduction

The increasing demand for environmentally friendly, dense, stiff, and super lightweight products in manufacturing has necessitated the creation of natural fiber-reinforced polymer composites (NFRPC) for diverse applications in the construction, manufacturing, and automotive sectors. Composites are materials formed by combining two or more constituent materials to produce a single material with enhanced properties. Composite materials exhibit superior qualities by merging materials with distinct physical or chemical characteristics. The reinforcement and matrix elements determine the properties of composite materials. The matrix material, which is continuous, bonds and strengthens the composite, ensuring quality surface properties. Various matrix materials, including metallic, polymeric, and ceramic, can be employed. Reinforcements are crucial for enhancing the mechanical properties of the matrix, providing efficiency and reliability to the composites. Composite reinforcements can be nonfibrous or fibrous, with fibrous composites reinforced by natural fiber (NF) or synthetic fiber. Composite materials span multiple fields, including nonstructural, structural, electrical, biological, biomedical, electrochemical, and thermal applications, all aimed at improving human productivity and living standards. In the automotive industry, metals are extensively used to manufacture vehicle frame components. The weight of the materials used in automobile manufacturing significantly impacts fuel consumption. This research endeavors to rigorously assess a hybrid composite consisting of banana fiber (BF) and nano alumina as reinforcing materials within a polyester matrix. Rising environmental considerations, global climate change, waste disposal issues, decreasing fossil materials and assets, and increasing oil prices have resulted in a need for new and innovative materials that are ecologically responsible, low density, high strength, and cost-effective. Green products are increasingly being promoted for sustainable development [1]. In recent decades, the focus of research and engineering has shifted from conventional large materials to fiber-reinforcement polymer-based materials. Heavyweight metal ions were replaced with polymers reinforced with NF/synthetic fiber such as various animals, plants, glass, wood, and carbon fibers because of their unique benefits of superior strength-to-weight ratio, higher corrosion resistance, and greater fracture toughness [2]. NFRPC derived from agricultural waste and forestry are attracting the attention of scientists, engineers, professionals, and researchers more than synthetic fiber-reinforced composites [3]. NFs derived from natural resources have several benefits over synthetic reinforcement fibers, including low cost, nontoxicity, equivalent strength, and low density, as well as minimal disposal of waste issues [4]. NF’s structure and chemical composition are heavily influenced by soil type, plant age, climatic conditions, and the unique features of every plant variety [5]. BF is a lingo-cellulosic fiber that is low in density, absorbs well, is thin and soft, and has an appealing impression in aesthetics [6]. It is the best fibrous material with comparatively excellent mechanical properties and a low manufacturing cost because it is derived from agricultural remaining waste. The fiber can be obtained in various ways, including manual operation with an iron bamboo/ribbon scrubber, mechanical means with a machine, chemically with chemicals, naturally with steeping, and biochemically with enzymes.

Fibers are thread-like strands derived from plants, animals, minerals, or man-made materials. Fibers are also used in composite materials and can be matted into sheets to make paper or felt. The interest in using natural fiber as reinforcement in composites has grown dramatically in recent years. Automotive and aerospace manufacturers continued developing unique forms of herbal fibers, particularly flax, hemp, sisal, banana, and bio-resin structural features for their indoor parts [7]. Natural fiber-reinforced composites (NFRCs) are commonly used in interior paneling for aircraft and automobiles, as well as household tables, chairs, window frames, laptop cases, and other consumer items. NFRCs are valued for their sustainable development and lightweight compared to comparable conventional materials, making them particularly common in the automotive sector [5]. Prasad et al. [8] investigated the physical properties of an epoxy composite reinforced with chemically treated sugarcane bagasse fiber. Kumar and Singh [9] investigated the mechanical properties of an epoxy composite based on bidirectional basalt fiber mats. The samples were created using the hand lay-up method, with varied orientations of 450, 600, and 900 and different mechanical properties. The 900-fiber orientation yielded the best mechanical properties, while the 450-fiber orientation yielded the worst. Yahaya et al. [10] investigated the effect of fiber orientation on hybrid composite mechanical properties. Prasad et al. [11] investigated the impact of fiber packing and a wide range of other physical and mechanical properties of a polyester composite fortified with BFs. Karthick et al. [12] studied the impact, tensile, hardness, and flexural characteristics of BF-reinforced epoxy hybrid composites with varying percentages and lengths of fiber. Venkateshwara et al. [13] investigated a BF-reinforced epoxy composite’s SEM, tensile, water absorption, flexural, and impact characteristics. Composite samples of fiber length (5, 10, 15, and 20 mm) and weight ratio (8, 12, 16 and 20) were constructed. According to the data, the ideal fiber length and weight percentage for flexural, tensile, and impact strength are 15 mm and 16%, respectively. In that order, the highest values were 16.39 MPa, 57.53 MPa, and 2.25 J/m. The water absorption of the composite is controlled further by fiber content than fiber length, and the value is about 5% for all samples. An SEM picture revealed that raising the fiber concentration over 16% results in poor interfacial adhesion between the matrix and fiber. According to their findings, the fundamental parameters influencing composite material qualities are fiber length and concentration of content.

Parre et al. [14] examined the effect of the alkali treatment on the mechanical possessions of BF. For 24 h, all fibers were treated with NaOH concentrations of (0%, 1%, 3%, 5%, 7%, and 9%). Fibers that have been alkali-treated showed improved chemical, thermal, and morphological properties of the composite. The treatment successfully removed non-cellulosic elements and contaminants from the surface. An optimum NaOH concentration was 5%. Khan et al. 17 studied the impact of alkali chemical treatments somewhat on the mechanical characteristics of even a BF-reinforced epoxy composite material.

Nanoparticles are now considered promising fillers for improving a polymer composite’s mechanical and physical characteristics [15]. The elastic modulus and tensile strength of an alumina nanoparticle/carbon fiber of glass-reinforced epoxy composite were investigated by Mohanty et al. [16]. According to Mohanty, among the most frequently utilized engineered ceramic ingredients are Al₂O₃ microparticles. Consequently, its utilization is limited because of its high brittleness and particle aggregation throughout manufacture.

Fathy et al. [17] examined an experimental and statistical behavior occurring in a unidirectional glass fiber-reinforced epoxy composite filled with silica and alumina nanoparticles at four weight percentages (0.5, 1.0, 2.0, and 3.0 wt%). The tensile examination evaluated an effect that occurred in nanoparticles. Swain et al. [18] reported that the incorporation of 1wt% of nano alumina into a polyurethane matrix increased the tensile strength by 50%, and the addition of silica nanoparticles by 41% significantly raised throughout tensile strength there at the same concentration. Prasad et al. [19] investigated an impact on nano titanium oxide by improving a flax fiber-reinforced epoxy composite’s thermal, mechanical, and water absorption behavior. A composite specimen was developed using compression molding. As reported by researchers [20], incorporating zinc oxide into a glass fiber polymeric composite enhanced the flexural strength up to 62.12% at 3 wt% of ZnO when compared to the unfilled specimens. Increases throughout ZnO loading also enhanced hardness, thermal stability, and impact strength. Rostamiyan et al. [21] studied the effect of silica and clay of nano and within the fiber orientation on the strength of flexural epoxy/nano of SiO2/fiber of glass/nano clay hybrid composite. Response surface methodology (RSM) was utilized to optimize and maximize flexural strength. Wang et al. [22] investigated the effect of grafting flax fiber containing nano TiO2 on the bonding and tensile characteristics of unidirectional and single-fiber armored epoxy. Ramezanzadeh et al. [23] investigated the influence of zinc oxide nanoparticles on mechanical characteristics and tried to cure the behavior of epoxy nano composites.

Fong et al. [24] investigated the mechanical characteristics of sugarcane bagasse fiber armored plate epoxy amalgamation composite. The hand lay-up approach created the composite sample with the short fiber of varying weight fractions and 1% nano silica-reinforced epoxy composites. Flexural and tensile strengths were evaluated toward the ultimate result on particulate filler and upon performance occurring in the composite. Ramakrishnan et al. [25] examined both mechanical and free vibrational characteristics occurring in untreated and treated jute fiber and nano clay reinforced epoxy composite. Alkali treatment improved the bonding between fibers and matrix, strengthening composites [26]. Sansevieria trifasciata fiber (STF) has high cellulose content and is a potential reinforcement for polymer composites.

Alkali treatment was found to increase the cellulose content by 51.8%, enhancing the fiber’s surface characteristics and promoting better wettability and fiber–matrix bonding [27]. Microsized luffa natural fibers (MLNFs) sourced from Vietnam were subjected to sodium hydroxide treatment to enhance the fracture toughness of epoxy resin 828. The study investigated the impact of NaOH concentration, temperature, and duration on composite strength. Optimal treatment using 6% NaOH at 70°C for 6 h led to a 93% rise in the critical-stress-intensity factor and a 44.6% increase in Izod impact strength [28]. Various composite fabrication methods have been applied, with numerous studies focusing on the structural characteristics and properties assessed through different characterization techniques [29]. Investigations have also addressed fiber treatments aimed at improving mechanical properties, manufacturing techniques, hybrid composite performance, laminate configurations, and the broad range of applications for natural fiber composites [30]. A comprehensive analysis of retting methods, chemical modifications, surface treatments, and the characterization of natural fibers has been conducted [31]. Chemically extracted fibers were treated using NaOH, stearic acid, benzoyl peroxide, and potassium permanganate. The composition of cellulose, hemicellulose, lignin, and waxes was determined using standard TAPPI procedures. The FT-IR technique helped to elucidate molecular bonding characteristics, crystallinity, and their association with the fiber structure [32]. A bio-based polymer composite incorporating a high loading of natural fibers was developed using short BFs as reinforcement and epoxy as the matrix. Approximately 77 wt. % of BFs was integrated into the epoxy composite via a pressure-induced fiber dipping method [33]. The dynamic mechanical behavior of BF-reinforced polyester composites was analyzed, focusing on the effects of fiber content, temperature, and frequency. Properties such as component interaction, system morphology, and interfacial bonding were key factors in determining performance [34]. Hybrid composites comprising short, randomly oriented BFs and sisal fiber in polyester were fabricated with varying fiber loadings (0.20–0.50 Vf). The relative volume fraction of each fiber type was systematically altered at different loadings [35]. The production of an adsorbent using lime juice-activated banana peel (Musa paradisiaca) has also been demonstrated. X-ray diffraction (XRD) analysis revealed that both untreated and treated samples exhibited a characteristic cellulose peak between 2θ = 18° and 25° [36]. Reducing biomass waste to the nanoscale is expected to significantly influence biofuel production regarding efficiency, yield, and cost. However, current technologies remain inefficient and are limited to small-scale, energy-intensive applications. Thus, there is an urgent need to develop scalable particle size reduction technologies [37]. In the case of unidirectional sisal-glass hybrid laminates, the work of fracture (WOF) improved from 80.2 to 228 kJ m⁻² as the glass volume fraction increased. A further rise in WOF was observed when the glass core was positioned towards the tensile side of the laminate [38].

Kumar et al. [39–43] explored the development of sustainable epoxy-based hybrid composites by utilizing waste Prosopis juliflora (PJ) bark powder and jute fabric, with a focus on understanding the effect of fabric stacking sequences on mechanical and thermal behaviors. The study demonstrated that the incorporation of 15 wt% PJ bark powder significantly improved tensile, flexural, impact strength, hardness, and thermal stability of the composites. The observed enhancements are attributed to improved fiber–matrix interfacial bonding, making these bio-based composites viable candidates for diverse industrial and automotive applications.

It has been observed that much research has been carried out in developing composites based on natural fibers. However, the optimization of process parameters at which these composites possess the best characteristics is rarely carried out through multiresponse optimization techniques. Although the individual parameter affecting the output has been explored in the past, common parameters causing the desired effect have not been explored. So, in this manuscript, the development of natural fiber-based composites has been carried out with multiresponse optimization of their characteristics through gray relational analysis (GRA).