2.1. Matrix and Reinforcement Preparation

The polyester resin was utilized as the matrix material for the composite, and methyl ethyl ketone peroxide was utilized as the accelerator. M. N. Polyester (India) Private Limited, situated in Coimbatore, Tamil Nadu, India, was the supplier of the accelerator. The natural fiber used for this investigation is extracted from a fragrant screwpine plant, obtained from an indigenous crop from Kanyakumari District, Tamil Nadu, India. The fibers can be extracted from both stems and prop roots of the fragrant screwpine plant. Figure 1(a) shows the photograph of the fragrant screwpine plant and Figure 1(b) shows the stem and prop roots of the fragrant screwpine plant.

In this research study, the fully grown prop roots of the fragment screwpine plant were selected for fiber extraction. The selected prop roots were cut from the shrub and then shortened to a thin sheet of approximately 4-5 mm. The shortened sheets were examined to remove the fractured or spoiled sheets. The filtered thin sheets were dried in direct sunlight for 7–10 days, and then the sheets were soaked in water. Figure 2(a) shows the thin sheet of prop roots dried under sunlight. After soaking, the sheets were rolled using a small ballpin. Finally, the FSPFs were extracted from the dried sheet, rewashed, and dried under sunlight for 8–12 h [24]. Figure 2(b) shows the extracted FSPF from the root.

2.2. Mold Preparation

The FSPFs were reinforced with polyester resin to manufacture a novel composite. The preparation of polymer composite material was performed by the hand lay-up technique. For this process, a stainless-steel mold of dimensions 200 × 200 × 4 mm was employed. The mold consists of three layers, namely, the top, bottom, and middle layers, which are easily fastened using a nut and bolt and are shown in Figure 3. During the preparation of the polymer composite, the fibers were spread on the bottom plate and then the resin was poured. The top plate was used as a cover, and the nuts and bolts were adjusted to compress the polymer composite to avoid debris in the composite part during curing.

2.3. Fabrication of the Composite

For the preparation of composites, the fibers were chopped to 3 and 9 mm for diameters varying from 0.1 to 0.3 mm. The mold plates were cleaned and dried before fabricating the composites. The chopped fiber was laid randomly over the mold before applying polyester resin. After properly arranging the fiber, an adequate amount of polyester–resin mixture was applied, and the plates were tightened. The composite material with a mixture of fibers and polyester resin was compressed and kept for a curing time of 12 h to fabricate FSPF-reinforced polymer matrix composites. Following the completion of the curing practice, the examination samples were sliced into the requisite amounts in accordance with the ASTM specifications. The composites were prepared for different weight percentages of fibers (10, 20, 30, and 40), and the mechanical characterizations were performed.

2.4. Examining Composite Samples

After fabrication, the assessment specimens were tested for numerous mechanical tests at room temperature as per the ASTM standards using a Universal Testing Machine (UTM) (Make: INSTRAN, Model: 5500 R). The standards followed are ASTM-D638 for the tensile experiment with the assessment speed of 5 mm/min, ASTM-D790 for the flexural experiment, and ASTM-D256 for the impact experiment. In every instance, the total of the consequences is determined by taking the average of five different samples. The cut tensile, flexural, and impact test specimens of various weight percentages are shown in Figure 4.

3.1. Microstructure Evaluation

The microstructure is used to find the distribution of fiber reinforcements and weight fractions in the polymer composite. Figure 5 shows the HRSEM image of the 30 wt.% and 9 mm fiber length–strengthened FSPF-strengthened polymer matrix composite. Figure 5(a) shows the longitudinal view of the FSPF-strengthened polymer matrix composite. It revealed the existence of fiber with limited exterior impurities or debris. Danish et al. [25] obtained similar findings in their 20 wt.%-strengthened bamboo polymer matrix composite. Figure 5(b) shows the transverse view of the FSPF-strengthened polymer matrix composite.

3.2. Mechanical Property Evaluation

The outcome of the tensile, flexural, and impact experiments conducted as per the ASTM standards is discussed in this section. Figure 6 demonstrates the comparison of tensile (TS), FS, and IMS of the different weight percentages of FSPF-reinforced polymer matrix composite at room temperature.

The significant increase in strength modulus that gives composites a chance to be used in practical applications gives natural fiber-reinforced polyester composite material its significance. Figure 6(a) illustrates the TS comparison of the fabricated composite. After reaching a certain point, the TS can be enlarged by altering the fiber length and load percentage. Conversely, after that point, the TS will decline due to the insufficient interfacial relationship among the fiber and the matrix substance. The results indicated that the maximum TS at the 3 and 6 mm fiber distances is 34.68 and 37.2 MPa, respectively. Therefore, the ideal weight percentage of fiber for getting maximum TS in both fiber lengths is 30% to achieve the highest TS. This could be due to the stress transfer among the matrix and well-distributed fibers, which may increase load-bearing capability. At 3 and 6 mm fiber lengths, the fiber–matrix interface supports efficient load transfer, and 30% fiber content provides appropriate reinforcement without fiber aggregation or poor wetting. Furthermore, the TS may decrease owing to poor interfacial bonding, fiber pull-out, or stress concentration after this point. Mahesh et al. [26] noticed a similar trend of TS in the Terminalia arjuna and Moringa oleifera fiber–strengthened epoxy polymer composite.

Figure 6(b) illustrates the FS of composites at various weight percentages to facilitate comparison. A steady rise in the fiber loading causes the material’s FS to steadily increase over time, and the maximum value was reached at 30 wt.% (44.52 MPa). This is all considered when the fiber length is 3 mm. As fibers carry the load, stiffness and bending resistance rise, increasing FS. However, at a fiber length of 9 mm, the maximum values reach 20 wt.% (46.81 MPa), after which they gradually decrease. This could be due to the fiber tangling, poor dispersion, and reduced matrix–fiber adhesion, lowering load transfer efficiency. Hence, the optimum volume fraction of 3 mm fiber length is 30 wt.%, and the 9 mm fiber measurement is 20 wt.%. Subramonian et al. [27] noticed a similar trend of FS in the bagasse fiber–strengthened polypropylene composites.

The IMS of a composite reflects its ability to resist a sudden impact. It also refers to the amount of energy that is necessary to completely break a specimen. Based on the findings of the experiment, it can be observed that the IMS of the fiber rises steadily for both the 3 and 9 mm measurements. The concentrated IMS of 17 J/m is achieved at 30 wt.%, and then decreases, as shown in Figure 6(c).

3.3. Hirsch’s Theoretical Analysis

A lot of different theoretical analyses have been put forward to model the TS of composites using various factors. One of the most important tools for figuring out the tensile properties of the randomly aligned fibers in a polymer matrix composite is the Hirsch model.

As per this prototype, the TS of the composite is given by the following equation:

()

The tensile properties of the created composite, matrix, and fiber are denoted by the symbols σ_c, σ_m, and σ_f, respectively. Specifically, the volume fractions of the fiber and matrix are denoted by the letters V_f and V_m, respectively, and x is a number that ranges from 0 to 1 [5]. The magnitude of the x number in equation (1) is what governs the quantity of stress that is transferred among the fiber and the matrix. In the current learning, to acquire the greatest fit value with the investigational value, the x value is chosen as 0.2, 0.13, 0.1, and 0.08 for 3 mm fiber length and 0.22, 0.15, 0.11, and 0.09 for 9 mm fiber length. It is also stated that as the fiber length is increased, stress transfer among the fiber and matrix has also amplified. The little “x” magnitude designates reduced stress transfer among the FSPF and polyester matrix, caused by reduced interfacial bonding among fiber and matrix. Hirsch’s model for Sansevieria cylindrica fiber/polyester composites was determined to have a parameter “x” of 0.1 by Sreenivasan et al. [28], which indicated reduced stress transfer among the Sansevieria cylindrica fiber and the polyester matrix. Regarding the tensile properties of the composite, the weight percentages of the FSPF were 10, 20, 30, and 40 wt.%, and they are in good accord with Hirsch’s modeling. The experimental value is contrasted with the theoretical TS numbers in Figures 7(a) and 7(b). Based on the t-test, the letter “ZZ” means that there is no significant modification between the theoretically expected and actual TS. Theoretical TS values from Hirsch’s model are shown in Table 1, along with actual values.

3.4. Statistical Model

One of the most important and useful statistical methods for modeling and studying numerical data is regression analysis. The dependent variables, also known as responses, and the independent variables, should both be present in numerical statistics (also called predictors). The mechanical characteristics of the composites made of fiber-reinforced polymer composite can be calculated using this formula. In addition, it is possible to establish a quantitative connection between mechanical properties and fabrication parameters by using a nonlinear regression analysis. The mechanical properties of the FSPF-strengthened polymer matrix composite material have been predicted using various prediction methods over the past few decades in relation to various process parameters. With the aid of the Statistical Package for Social Sciences (SPSS), the tensile, flexural, and impact power values are predicted in the current research. For the purpose of developing mathematical models based on the experimental data obtained from the mechanical studies, regression analysis was utilized. The relationship between the process parameters (fiber length and fiber content) and the response variables (TS, FS, and IMS) can be represented in the nonlinear regression analysis in the following form:

()

where k, x, and y are the parameters that are always the same. FL is the fiber length in millimeters, while FC is the fiber content in weight percent. TS, FS, and IMS are the TS, FS, and IMS, respectively, in MPa. The developed regression equations for TS, FS, and IMS are given as follows:

()

()

()

The coefficient of determination (R²) for TS, FS, and IMS was established to be 0.820, 0.784, and 0.728, respectively, in the developed nonlinear regression model. R² is a statistical measure which gives evidence about the goodness of fit of a developed model. The value of R² measures how well the data fit the regression model. The higher magnitudes of R² represent smaller differences between the experimental and predicted measurements. The value of R² can take any value between 0 and 1. This indicates that the regression line is a perfect fit for the data that have been provided. It was noted, based on the measurements of average absolute percentage errors (Table 2) and R², that the statistical model was seen to agree with the experimental performances when it was used for the prediction of the mechanical properties of natural fiber composites. These properties include TS, FS, and IMS. Figure 8 presents a comparison of the values of the properties that were anticipated and those that were observed.

3.5. HRSEM Morphology of Composite Specimen

The HRSEM was utilized to investigate the fracture exterior of the composite specimen in order to assess the fiber pullout, fiber splitting, debonding, matrix cracking, and fiber matrix interaction. Figure 9 shows the HRSEM of the tensile fractured surface of the 30 wt.%-reinforced FSFR polymer composite with a fiber length of 9 mm. From Figure 9(a), poor dispersion of fibers and fiber attrition on the fracture surface of the composite were witnessed. These features suggest poor interfacial bonding, resulting in reduced mechanical properties. The fiber pullout hole, which can be seen in Figure 9(b), demonstrates that the rank of bond among the natural fibers and the resin matrix is low when stress is applied to the material. The fibers can be easily extracted from the composite as a result of this occurrence, which results in the formation of extensive holes. The composite containing short fibers did not exhibit good mechanical properties since fiber debonding and fiber pullout contribute to energy dissipation during fracture that occurs in the composite. Sanjita et al. [29] observed similar failure patterns in their bamboo fiber–strengthened polypropylene composites.