Free vibration analysis of jute/kenaf/banana hybrid biocomposites: effects of various stacking sequences

This work provides an insight of the free vibration behaviour based on experimental modal analysis of hybrid natural fibre reinforced polymer composites fabricated via compression moulding method. This study aims to study the impact of different polymer matrices (vinyl ester and epoxy) and stacking order of different natural fibres (Jute, Kenaf and Banana) on the composite’s free vibration characteristics. Jute-Kenaf-Banana-Epoxy (JKBE) matrix composites exhibited enhanced free vibration properties compared to single fiber reinforcements, except the banana/epoxy composites. Additionally, Jute-Kenaf-Banana-Vinyl ester (JKBV) composites demonstrated improved free vibration properties in comparison to single fiber reinforcements, with the exception of kenaf/vinyl ester composites. Kenaf-Banana-Jute-Jute-Banana-Kenaf-Epoxy (KBJJBKE) and Kenaf-Banana-Jute-Jute-Banana-Kenaf-Vinyl ester (KBJJBKV) hybrid composites showed the highest natural frequencies of 68.36 Hz and 97.66 Hz, respectively. However, there was no significant improvements in the natural frequencies of Jute-Kenaf-Banana-Kenaf-Banana-Jute-Epoxy (JKBKBJE) and Jute-Kenaf-Banana-Kenaf-Banana-Jute-Vinyl ester (JKBKBJV) hybrid composites. The highest damping factor was observed for JKBKBJE (0.153) and JKBKBJV (0.224) hybrid composites. These hybrid composites with better free vibration properties shall be a potential candidate in the automobile interior applications.


Introduction
Natural fiber reinforced hybrid composites represent a pioneering approach in composite materials, integrating the unique advantages of natural fibers with other reinforcing materials to create versatile, eco-friendly, and high-performance composites [1,2].This strategy involves combining various natural fibers such as jute, kenaf, flax, hemp, or bamboo with other synthetic or natural reinforcements, including glass, carbon, or aramid fibers, within a compatible matrix material [3,4].Natural fibers are known for their sustainability and eco-friendliness.Jute, particularly valued for its robustness and cost-effectiveness, contributes a notable tensile strength and stiffness to the composite [5,6].Kenaf, renowned for its exceptional flexibility and impact resistance, reinforces the composite with better toughness and durability [7,8].Meanwhile, Banana fibers are known for their good elongation and enhanced ductility [9,10].
The concept of interply hybrid composites with natural fibers involves integrating these fibers within specific layers or plies of the composite structure, thereby combining their inherent advantages with other reinforcing materials [11].This integration can be within the same layer or alternating layers within the composite stack, enabling the creation of a composite material with tailored properties [12].Interply hybrid composites with natural fibers find applications across various industries including automotive, aerospace, construction, and consumer goods [13].They can be utilized in structural components, panels, interior elements, and other areas where a combination of lightweight, strength, and sustainability is achieved [14,15].Chandramohan et al studied the influence of combining sisal, banana and roselle fibers with bio epoxy to form hybrid composites.They found that sisal/roselle/epoxy hybrid reinforcements, are best suited for making a range of automobile accessories, such as seat covers, indicator covers and rearview mirrors for two-wheelers.According to their research, a variety of automotive components, including door handles, door panels, roofs and dash boards etc may be made from hybrid fiber reinforced composite material [16].
The free vibration analysis of composite materials investigates the inherent vibrational behavior when subjected to natural oscillations of any external forces or excitations [17].This focuses on determining the fundamental parameters such as natural frequencies, mode shapes and damping characteristics, offering critical insights into the dynamic response and structural integrity of the composites [18].Key aspects of free vibration analysis in composites include natural frequency and damping properties [19].Natural frequencies represent the inherent oscillation rates at which the composite structures vibrate when disturbed and left to oscillate freely.Understanding these frequencies is crucial as they determine the system's resonant behavior and vulnerability to dynamic loads [20,21].Damping factor measures the rate at which vibrational energy dissipates within the composite material.Characterizing the damping properties is essential for evaluating the material's ability to absorb and dissipate vibrational energy, thereby leading to the decay of vibrations over time [22,23].
A number of variables, including fiber length, orientation, stacking order and fiber/matrix interface may affect the natural frequency of fiber-reinforced composites.The free vibration properties of hybrid composites made of banana fiber and coconut sheath with polyester matrix was investigated using two distinct fiber stacking sequences and surface treatment techniques [10].According to the results, the skin-core type of coconut/banana/coconut fiber reinforced polyester composites exhibited better natural frequency owing to their increased stiffness [10].In another study, the free vibration properties of hybrid composites with short sansevieria cylindrica/ coconut sheath fiber reinforced polyester matrix was examined.Results revealed that the coconut sheath/sansevieria cylindrica/ coconut sheath hybrid polyester composites exhibited a higher value of natural frequency which was attributed to the higher stiffness of the composites [24].Senthilkumar et al [25] explored the free vibration characteristics of bamboo/basalt/bioepoxy composites with different fiber layering sequences of the fibers.They found that when bamboo fiber was positioned on the top layer a higher natural frequency was obtained whereas, the incorporation of basalt fibers in top layer improved the damping properties.Ismail et al [26] investigated the modal analysis of hybrid composites with kenaf/bamboo reinforced with epoxy matrix with varying weight percentages of the fibers.The results revealed that the highest natural frequency and damping factor was found for the composites with 30 wt% of banana and 70 wt% of kenaf reinforcements.
From the literature survey it was found that there is a limited investigation of free vibration behavior of natural fibers such as jute, kenaf, and banana when combined to form hybrid composites.The free vibration properties of epoxy and vinyl ester-based biocomposites have received less investigations in comparison.Furthermore, there is a noteworthy lack of research on the free vibration behavior of hybrid composites, particularly those with a six-layer interply stacking order and reinforced with jute, kenaf, and banana fibers in conjunction with epoxy and vinyl ester.In this study, the free vibration analysis of interply jute, kenaf, and banana fiber mats reinforced with two distinct matrices, such as epoxy and vinyl ester has been investigated and compared.

Fabrication of inter ply hybrid composites
The JKBE and JKBV composites were fabricated by compression molding method to reinforce Jute-Kenaf-Banana fiber within the epoxy and vinyl ester matrices.In a 30 × 30 × 0.3 cm 3 mould, the fiber mats of kenaf, banana, and jute were layered according to the designated stacking arrangement (shown in figures 1 and 2) and impregnated with the matrices.The weight of different fibers used for fabricating the hybrid composites are presented in table 1.Later, the mould was squeezed in a hot press for 60 min at 200 bar pressure and 100 °C.The composite laminates were then subjected to post-curing at 100 °C for 25 min after being taken out of the mould.Finally, the JKBE composite laminates were removed and cut as per the ASTM standards for testing purposes.A similar procedure was followed for the fabrication of JKBV composites.The void content in each of the composites was measured and is listed in table 2.

Characterization of free vibration analysis
The study of a structure's inherent properties is known as modal analysis, and it is frequently used to assist in the design of parts for the automobile, aerospace, and even sports industries [27].The characteristics of natural   frequency and mode shape are essential for noise and vibration applications.These characteristics can be determined through impulse hammer testing, extracting information from observed frequency response functions.The impact hammer approach is most commonly used in the case of light-weight composite structures.In this work, the impact hammer method was employed to conduct a modal study on the Jute-Kenafbanana fiber reinforced hybrid composites.The accelerometer (Kistler model 8778A500) was attached at the end of the composite specimen as shown in figure 3.In accordance with ASTM E756-05 [28,29] guidelines, a composite specimen measuring 200 × 20 × 3 mm was utilized.Higher frequencies were obtained by using the impulse hammer (Kistler model 9722A500) with sharp hardened tip.A data collecting system and an ICP conditioner were used to record the accelerometer's displacement signal.One adapter was used to measure the hammer's reaction to the sample, while the other was used to capture the output signal from the accelerometer.One sample for each composite type was employed in the free vibration test.Figure 3 depicts the free vibration test specimen and the experimental setup.The damping factor values for each composite was determined using the half-power bandwidth method.This involves analyzing the Frequency Response Function (FRF) curves obtained from the Fast Fourier Transform (FFT) analyzer.The calculation of damping values was carried out based on the equation (1).

Free vibration analysis of JKBE hybrid composites
The effect of stacking sequences on the damping and natural frequency of JKBE hybrid composites is presented in figure 4 and table 3. The results showed that the banana fiber reinforced composites and hybrids with banana in the skin layer (top layer) possessed the highest natural frequency in all three modes.This could be attributed to the exceptional mechanical properties, such as high stiffness and low density of the banana fibers.These characteristics enhance the structural integrity and dynamic response of the composite, resulting in elevated natural frequencies compared to other materials [9].Low-density materials often exhibit higher natural frequencies, as the mass involved in the vibration is lower.Further, KBJJBKE had the highest natural frequency in mode I, followed by the JKBKBJE, KJBBJKE, JE and KE composites.This is because of KBJJBKE hybrid composites had highest tensile strength and stiffness [9].The frequency versus amplitude curve depicted in figure 5 reveals that KBJJBKE composites exhibit the highest natural frequency.However, JKBKBJE hybrid composites possess the highest natural frequencies (58.59 Hz, 419.9 Hz and 908.2 Hz) in all three modes.This could be ascribed to the arrangement and orientation of jute fibers within the composite, as well as the stacking sequence, can influence the composite's overall stiffness and, consequently, its natural frequencies.Kenaf fiber reinforced hybrid composites displayed the lowest fundamental natural frequency of 48.83 Hz when compared  It appears that the fundamental natural frequencies (58.59 Hz) of the hybrid composites JE, KJBBJKE, and JKBKBJE are similar, indicating that they have similar vibrational behavior.From a scientific perspective, this could indicate that the structural arrangement and mechanical properties of the different hybrid configurations which lead to resonance at comparable frequencies.This shared frequency response may be attributed to the specific combination of fibers and the resulting composite structure.Also, the identification of similar fundamental natural frequencies in KBJJBKE and BJKKJBE (68.36 Hz) hybrid composites suggests a consistent vibrational behavior across these diverse material configurations.Such observations are valuable for understanding the composite's dynamic response and may indicate a certain level of predictability in their vibrational characteristics.
Many studies have shown that specific strength and fiber orientation are among the elements that influence the damping behaviour of composite laminates [19].The fundamental damping factor for epoxy composites made of jute, kenaf and banana at varying layer sequences is displayed in figure 4. The results showed that JKBKBJE hybrid composites possessed the greatest damping factor value (0.15357).The synergy between jute fibers and other reinforcing fibers in the hybrid composite may enhance damping properties.The BJKKJBE hybrid composites have the lowest values of damping factors (0.05080, 0.00773, and 0.00341) in all three modes which could be due to the inherent low damping characteristics of banana, jute, and kenaf fibers.Potential challenges in this aspect could be achieving effective interfacial bonding between fibers and epoxy matrix, as well as suboptimal fiber orientation or distribution within the composite structure.

Free vibration analysis of JKBV hybrid composites
Figure 6 and table 4 illustrate the damping and natural frequency of JKBV hybrid reinforcements.According to the findings, kenaf fiber composites (KV) and the hybrid composites (KBJJBKV and KJBBJKV) with kenaf fiber on the top layer had the greatest natural frequency of 117.2 Hz, 97.66 Hz and 87.89 Hz respectively.Kenaf fibers might possess characteristics such as high stiffness, low density, and effective damping, contributing to an enhanced structural response and higher natural frequencies in the composites.Additionally, the strategic use of kenaf fibers as the skin layer may further optimize the composite's dynamic behavior, leading to the observed increased natural frequencies.Further, figure 7 illustrates that kenaf-vinyl ester composites exhibited the highest natural frequency.Out of all the composites, BJKKJBV hybrid composites showed the lowest natural frequency of 58.59 Hz.This composite configuration, possibly featuring a structure promoting flexibility or having lower stiffness, contributes to a lower natural frequency in comparison to other hybrid composites [11].The highest natural frequency of vinyl ester-based hybrid composites was observed in the subsequent order: KBJJBKV > KJBBJKV > JKBKBJV > BJKKJBV.
Different stacking sequences can influence the stiffness and damping properties of the composite.Damping behavior changed consistently with changes in the fiber stacking order.Figure 6 and table 4 displayed the damping factor of JKBV hybrid composites.BV composites and JKBKBJV hybrid composites displayed the maximum damping factor values of 0.48332 and 0.22481, respectively.The enhanced damping factor might be attributed to the fiber's porous nature.The lowest value of damping factor (0.04355) was found in KJBBJKV hybrid composites.It could be caused by a weak binding at the contact.The highest damping factor values was found in mode 1 damping compared to mode 2 and mode 3. Different modes of vibration may involve different damping mechanisms.The material's ability to dissipate energy through internal friction, viscoelasticity, or other damping mechanisms may vary between modes.

Comparison on free vibration analysis of JKBE and JKBV hybrid composites
It was found that the laminate's various fiber and resin compositions had an impact on damping and natural frequency.The highest natural frequency of 117.2 Hz was observed in mode I for kenaf-vinyl ester laminate.The rise in natural frequency of kenaf fiber composites was induced by a considerable stress transfer at the interface.Furthermore, the KBJJBK hybrid laminate exhibits an enhanced natural frequency for both epoxy and vinyl ester composites.Specifically, the first three modes of highest natural frequencies of BJKKJBE hybrid composites are 68.36Hz, 449.2 Hz, and 1015.6 Hz.This could be attributed to the better stiffness of the fiber-reinforced composites.The BJKKJBV hybrid composites exhibit the lowest natural frequency among vinyl ester-based composites, which is likely due to a combination of factors such as lower stiffness of the constituent materials, suboptimal fiber arrangement, and potential challenges in achieving effective interfacial bonding within the composite structure.These factors collectively contribute to a reduced ability to resist deformation and result in the observed lower natural frequencies [9,11].
From the damping results, banana fiber reinforced epoxy and vinyl ester composites exhibited the highest damping factor values of 0.291 and 0.483 while the jute/epoxy and kenaf/vinyl ester composites showed the lowest damping values 0.033 and 0.13.The banana fiber's porous structure might possibly be the reason for an increased damping.In hybrid composites, BJKKJBE and KJBBJKV possessed the lowest damping values of 0.05 and 0.043.The interaction and compatibility between different fibers within the composite influence its damping behavior.In cases where the materials do not form strong interfacial bonds or do not have optimal compatibility, it can result in reduced damping behavior.JKBKBJE and JKBKBJV hybrid composites exhibited the highest damping factor of 0.153 and 0.224.It could be due to their changes in fiber surface morphology.Epoxy based hybrid composites exhibited the low damping compared to vinyl ester-based hybrid composites.This is because of the epoxy resins typically possess a stiffer and more rigid nature compared to vinyl ester resins.Tables 3 and 4 displays the observed damping factor for each composite (Modes 1, 2, and 3).The mode 1 damping showed the highest value while the mode 3 damping factor exhibited the lowest damping value in both epoxy and vinyl ester-based composites.In mode 1, the fibers might be aligned or structured in a way that facilitates energy dissipation more efficiently compared to mode 3.This could be due to the preferred alignment of fibers along the direction of the primary applied load, enhancing damping in that specific mode.

Conclusions
In this study, hybrid composites were fabricated using a combination of jute, banana, and kenaf fiber mats in epoxy and vinyl ester matrices by compression moulding technique.The free vibration characteristics of the hybrid composites JKBE and JKBV were investigated.It was discovered that the layering sequence had a significant effect on the dynamic properties of the hybrid composites, including damping and natural frequency.In both epoxy and vinyl ester-based composites, higher natural frequencies were found for the same stacking design (KBJJBK).The JKBKBJE and JKBKBJV hybrid composites obtained higher damping values due to their higher energy dissipation.Moreover, epoxy-based hybrid composites exhibited the least damping compared to vinyl ester-based hybrid composites.KBJJBK hybrid composites exhibited better free vibration properties in both epoxy and vinyl ester matrix systems which is suitable for the automobile interior parts.

Figure 3 .
Figure 3. Free vibration test (i) specimens and (ii) experimental setup with the specimen.

Figure 4 .
Figure 4. Damping factor and natural frequency of JKBE hybrid reinforcements.

Figure 6 .
Figure 6.Damping factor and natural frequency of JKBV hybrid reinforcements.

Table 1 .
Different weight percentages of JKBE and JKBV hybrid composites.

Table 2 .
Void content percentage of JKBE and JKBV hybrid composites.

Table 3 .
Damping factor and natural frequency of JKBE hybrid reinforcements with three modes (M1, M2 and M3).
to all other composites.A poor fiber-matrix bonding or inconsistencies in fabrication might have affected stiffness and natural frequencies.

Table 4 .
Damping factor and natural frequency of JKBV hybrid reinforcements with three modes (M1, M2 and M3).