1. Introduction
Polymer composites are used in many structural applications. Thus, it is crucial to promote green composites comprised of natural fibers to conserve the environment for future generations. The green composite produces fewer carbon emissions during destruction than a synthetic composite since green ingredients were employed to make it. As a result, there is less environmental contamination and a more stable, green ecosystem. Due to the use of natural fiber composites in load-bearing applications, including automotive, construction and others, researchers have been studying natural fiber composites under static and dynamic loads [
1]. Due to the structural instability of composite structures, the structure will attain a failure in various ways, such as fatigue failure and buckling failure. Due to the axial compression pressure or heat load, composite constructions are frequently susceptible to buckling failure [
2]. The natural fiber polymer composite constructions may experience an axial compression load throughout their service since they are employed in a variety of load-bearing applications [
3]. A natural fiber-reinforced polymer composite needs to be able to bear stresses and maintain stability throughout its full life cycle. Large compressive stresses, however, might be applied to the structure, leading to a buckling failure. Buckling-related failures of natural fiber-reinforced polymer composite structures occur at loads many times below the material’s yield strength [
4]. It is commonly known that a structure’s buckling is influenced by its slenderness ratio. Several studies have been conducted to investigate the mechanical properties and other physical parameters of natural fiber-reinforced polymer composites [
5]. It is crucial to analyze the structural performance of novel materials throughout service in addition to their characterization. Hence it is important to examine the buckling behavior of natural fiber-reinforced polymer composite structures to enhance their usage in various applications [
6].
The buckling properties of laminated composite plates were studied utilizing a plate with internal holes, shear deformation, local effects, nonlinear stress-strain behavior, sandwich construction with other materials, hygro-thermal effects, external stiffeners, post-buckling behavior and early faults [
7]. They identified the improvements during buckling loads of a rectangular plate. Additionally, the effects of laminated composite plates with different boundary conditions, thickness, aspect ratios and membrane stiffness on the buckling loads were studied [
8]. The modified rectangular plate specimen was subjected to buckling load to evaluate the effectiveness of composite plates subjected to bi-axial stress [
9]. For numerical simulations, shell elements such as Shell181 and Shell281 are suitable to model rectangular plates. The plate/shell concept theory reduces the plate issue to a surface model with thickness-dependent strain and deformation suppositions for shell components [
10]. The buckling performance of a thin-walled carbon/epoxy laminated circular cylindrical composite shell under combined axial and torsional loading is evaluated both analytically and experimentally. It was found that the stiffness eccentricity played a major role in the amount of axial buckling load than that of the combined load [
11]. A laminated composite cylinder with cut-outs was subjected to buckling and post-buckling examination. The combined effect of internal pressure and compression was examined. The results showed that the buckling load of a compression-loaded cylinder was highly influenced by internal pressure, cut-out and orientation. Thus, buckling analysis is essential for detecting localized delamination failure or structural instability in plate structures [
12]. Natural fiber-reinforced composite beams’ buckling and free vibration behaviors were experimentally subjected to axial compression. A numerical study using the finite element technique was used to validate a critical buckling load that was calculated experimentally. It was shown that the number of layers increased the composite laminate’s buckling strength. It was also discovered that the critical buckling load was impacted by the weaving pattern of a woven fabric, with the basket-type weaving model providing a higher buckling strength [
13]. To explore the buckling and vibration properties of a partly or internally cracked natural fiber-reinforced composite plate with corner point supports, a new symplectic analytical methodology integrated with the finite element method has been developed. According to the authors, a plate with an interior fracture would reduce the critical buckling load and the natural frequency more than a plate with a surface crack [
14]. Various parametric studies were also performed using the proposed finite element model by considering the different parameters such as the ply orientation, the aspect ratio and the number of layers. The elastic constants were determined for the numerical simulation using a new theoretical approach, especially for the natural fiber-reinforced polymer laminated composites [
14].
Plant fibers are comprised of cellulose, lignin and hemicellulose, with numerous microtubules aligned along the fiber. Fiber stiffness and strength were directly related to the primary constituent of plant fibers, cellulose. Response Surface Methodology was used to set up different combinations by considering different factors. Numerical and experimental studies were also performed on the dynamic properties of uniform plant fiber-reinforced polymer laminated composite plates, where the elastic constants of the composite lamina were used [
15]. The alkali treatment method is one of the simplest, most cost-effective procedures for improving the bonding of natural fibers to epoxy resin. Several researchers looked at the effect of the coupling agent sodium hydroxide in alkali treatment. The findings demonstrated that varying concentrations of sodium hydroxide have distinct effects on fiber surface morphology [
16,
17,
18]. Sodium hydroxide was used as an alkali treatment because it converts fibers’ smooth surface to a rough surface by making undulation on the surface. It indicated that the matrix in the cell wall was removed after alkali treatment. The author reported that the pore sizes and void ratios vary depending on cellulose origin and as well as treatment history. These pores are important for dissolution because it provides spaces for the diffusion of solvent chemicals into fibers. Na-Cellulose-I lattices were formed when excess sodium hydroxide was removed. Cellulose-II, having a more stable lattice in comparison to Cellulose-I, was formed by rinsing the cellulose with water and removing the sodium ions from the cellulose [
19,
20,
21,
22]. The fiber is affected by the alkaline treatment in two ways. As a result, mechanical interlocking was improved, and the surface roughness and cellulose exposure to reaction sites were both increased. It was observed that mechanical interlocking has a long-term influence on fiber mechanical behavior, notably the strength and stiffness of fibers [
23,
24,
25].
Kabir et al. [
26] studied the surface treatments of natural fibers to maximize the composites’ bonding strength and stress transferability. The overall mechanical properties of natural fiber-reinforced polymer composites were highly dependent on the morphology, aspect ratio, hydrophilic tendency and dimensional stability of the fibers. Harish et al. [
27] evaluated the mechanical properties of coir-reinforced composites. The tensile results showed the suitability of coir fiber for low load-bearing applications. Rashed et al. [
28] studied the tensile strength of jute fiber-reinforced composites. The effects of parameters, such as alkali treatment (compared to no treatment), fiber size (1, 2 and 4 mm) and fiber loading (5, 10 and 15 wt%) on the tensile strength were considered. They analyzed the tensile behavior and performed fractographic observations. Akil et al. [
29] reviewed kenaf fiber-reinforced composites. Kenaf fibers are readily available and are used as reinforcement in various ways. NaOH-treated kenaf fiber increases the tensile and flexural properties of epoxy composites, but the thermal resistance decreases compared to untreated kenaf fiber-reinforced composites. Senthilkumar et al. [
30] evaluated the mechanical and vibrational properties of pineapple leaf fiber-reinforced composites. They prepared the PALF polyester composites by the hand lay-up process and then compressed the materials using a compression testing machine. They analyzed the tensile, flexural and vibration results and found that an increase in PALF reinforcement increases the mechanical strength and reduces the damping ratio of the composites. They suggested that 45 wt% PALF composites are better suited for structural applications.
Ozbek [
31] investigated the effect of silica nanoparticles (NS) on the buckling characteristics of Kevlar/epoxy fiber-reinforced composite laminates. The results revealed that decreases in the length of samples resulted in significant increases in axial and lateral buckling characteristics. Rozylo et al. [
32] studied the failure phenomenon of carbon fiber-reinforced plastic columns with three distinct lay-up patterns of the composite. It was observed that the dominant failure form occurred in the region representing the end sections of the composite structures (short 180 mm columns). Moreover, the delamination phenomenon usually occurs just before structural failure. Debski et al. [
33] investigated the effect of eccentric compressive load on the stability, critical states and load-carrying capacity of thin-walled composite Z-profiles. Compressive fiber damage began in Plies 2 and 7 in the squeezed composite structure. The damage process evolved into a complex failure mechanism, causing the structure to lose its load-carrying capability when all mixed damage initiation requirements were met in the damaged area.
Although many researchers studied the mechanical and flexural characteristics of different fiber/polymer combinations, they still have limited knowledge related to the buckling behavior of composites reinforced with natural cellulose microfibrils. However, they have not carried out optimization methods such as the response surface approach to determine the ideal critical buckling load for natural fiber composites. In the present study, the critical buckling stress of a composite reinforced with cellulose microfibrils is analyzed using Box--Behnken response surface design, and the desired parameter result is optimized. The research flowchart is mentioned in
Figure 1. The impact of three different parameters, including fiber volume% (
w/
w), fiber diameter (µm), and sodium hydroxide % (
w/
w) on buckling behavior (dependent variable), is investigated.
2. Methodology
Raw banana stem fibers were processed to micron size, acquired from local vendors (ECO Green unit, Coimbatore, TN, India), and used as a filler. Chemicals such as NaOH, HCl and demineralized water were used to remove cellulose microfibrils (SRL Chemicals, Sigma Aldrich, and NICE Chemicals, Chennai, TN, India). Epoxy LY556 and hardener HY951 were purchased from S.M. Composites, Chennai, TN, India.
Raw banana fibers were cut to 4–5 mm lengths and prewashed with demineralized water to remove dirt and impurities. Then, the fibers were air-dried for two days to remove excess moisture. Fully dried banana fibers were powdered by the pulverizing process (Saral Pulverizer, Gujarat, India) and size separation was performed using a sieve shaker.
Figure 2 describes the procedure for the chemical treatment of powdered banana fiber to prepare cellulose microfibrils for reinforcement of epoxy composites. The chopped banana fibers were pretreated with different
w/
w percentages of sodium hydroxide (NaOH) solution for 2 h. Then, the fibers were washed several times with distilled water. The pretreated banana fibers were hydrolyzed using a 1M HCl solution at 80 °C ± 5 °C for 2 h. Then, the fibers were washed several times with demineralized water. The acid-hydrolyzed fibers were treated again with a 2% (
w/
w) NaOH solution for 2 h at 60 °C ± 5 °C. The acid-alkali-treated fibers were washed several times with demineralized water until the pH reached 7. These acid-alkali-treated fibers had more cellulose microfibrils and less pectin, hemicelluloses and lignin.
The Euler critical formula is utilized for predicting the buckling load of the supporting plate with different boundary parameters. Euler’s Critical Load Formula for Plate is presented in Equations (1) and (2).
where E represents Young’s modulus of Text plates, ν represents Poisson’s ratio, k
c = a/b Buckling Coefficient, h- thickness, D- Flexural rigidity of the plate per unit length and N
xcr-critical buckling load. The plate thickness, width, Young’s modulus and Poisson’s ratio are a = 100 mm, E = 5249.27 MPa, v = 0.34, h = 0.75 mm and a/b = 1, respectively. One can theoretically calculate the critical buckling load using this Euler critical formula and these property values. The critical value factor in this expression represents the Euler load for a strip of unit width and length ‘a’. The second factor, ‘b’, denotes the proportion of greater stability gained by the continuous plate compared with that of an isolated strip.
Table 1 shows that the epoxy reinforced with 20% NaOH-treated filler (run no. 14) exhibits higher tensile stress and modulus compared to the other samples.
Response Surface Methodology
In the study, a sophisticated statistically verified prediction model is used to conduct and evaluate trials to determine the optimal combination and the impact of various elements, such as a change in sodium hydroxide %, a change in fiber diameter and a change in volume percentage, on the buckling characteristics. The experimental design in this work employs the Response Surface Methodology’s Box--Behnken design.
Table 2 lists the process parameters and the three levels at which each parameter is evaluated. Box--Behnken design is used to generate higher-order response surfaces using fewer required runs than a standard factorial technique [
34]. The design uses the twelve middle edge nodes and three center nodes to fit a 2nd order equation.
In MINITAB software, these levels and factors are utilized to frame the different combinations and represent the results, as shown in
Table 3.