Experimental Investigation on Mechanical and Free Vibration Characteristics of Elastomer-Embedded Natural-Rubber-Filled GFRP Laminates for Anti-Vibration Mounts ^†

Abstract

The present work investigates the influence of natural rubber (NR) on the mechanical properties and free vibration characteristics of elastomer-embedded NR-filled GFRP laminates for anti-vibration (AV) mounts. The tensile, flexural, and impact strength values of the preferred hybrid laminates are evaluated as per ASTM standards. To estimate vibration characteristics such as the modal frequency and damping of the hybrid laminates, a free vibration study is carried out under the fixed-free boundary condition. Based on the experimental results, the effect of NR filling in an epoxy matrix of elastomer-centric GFRP laminates is thoroughly investigated for its application in AV mounts.

1. Introduction

The mitigation of induced vibration in automobiles, aircraft, and heavy machinery is of great concern today when aiming to achieve the desired stability, durability, and reliability. During the operation of vehicles or machinery, the induced vibrations cause discomfort and damage to the various internal components. Anti-vibration mounts (AV mounts) are used to alleviate the effect of vibration in all these applications to reduce the wear and tear of joints and moving components [1]. AV mounts are made of elastomeric materials like silicone, polyurethane, polybutadiene, neoprene, etc., to absorb the vibrations of the machinery [2]. An elastomer is a type of polymer material that can deform under tension but can then revert to its original shape when the force is removed. Elastomers are mostly made of viscoelastic polymers that have a low elastic modulus and high failure strain due to weak intermolecular forces [3]. In an earlier investigation, Mago et al. [4] found that composite laminates made of bamboo-bio-char-embedded natural rubber composites had high elasticity and excellent vibration absorption properties. Malinova et al. [5] observed that the ratio of elastomers used in the production of composites can significantly impact the properties of the resulting material. For example, increasing the number of elastomers in a composite can increase its elasticity and toughness, but may also reduce its strength and stiffness. Sokolova et al. [6] discussed various methods of surface modification, such as chemical treatments, plasma treatments, and the addition of coupling agents, which lead to improvements in the mechanical strength, thermal stability, and electrical conductivity of the material. S Barrera et al. [7] found that the addition of eggshell fillers to natural rubber composites improved the tensile strength, hardness and vibration damping properties of the composites, which is important for vibration isolation applications.

Polyurethane has higher suppressing characteristics regarding the deformation of engine mounts when compared to natural rubber [8,9]. The static and dynamic mechanical properties improve significantly when adding fillers to epoxy resin, resulting in a low damping ratio [4]. In the present study, PU rubber is selected as an elastomer and micro-sized natural rubber particles are used as a filler material for the epoxy matrix. The current paper aims to find the tensile, impact, flexural strength values and vibrational resistance of the PU elastomer-unit-centered NR-filled GFRP composite laminates.

2. Materials and Methods

2.1. Raw Materials Preferred

In the present study, an elastomer unit is made by using polyurethane (PU) rubber and pine fabric, and strengthened by natural rubber (NR)-filled glass-fiber-reinforced polymer (GFRP) composite laminates. Pine fiber is a natural fiber extracted from the inner bark of pine trees and has a range of properties that make it suitable for structural applications. PU is a type of polymer that is formed by reacting a polyol with a polymeric isocyanate used in various industries, including construction, automotive, furniture, and footwear. NR is a polymer substance made from the latex of rubber trees, and has exceptional flexibility and resilience. Epoxy resin is a type of synthetic thermosetting polymer that is created through a chemical reaction between epoxide monomers and hardening agents, typically amines or anhydrides. It is widely used in industrial and commercial applications, including coatings, adhesives, and composites. Glass fiber is a type of synthetic fiber that is strong, lightweight, and commonly used in construction, transportation, and manufacturing. Figure 1 shows photographs of the various raw materials used in the present study.

2.2. Fabrication of Elastomer Unit Centered NR Filled GFRP Laminates

Elastomers made of PU rubber are most widely used in making anti-vibration mounts because of their ability to absorb high amounts of energy during high impact and vibration. To improve the stiffness of the AV mounts, the elastomeric units are embedded in glass-fiber-reinforced polymer (GFRP) composite laminates. To promote the application of natural fiber, a pineapple fabric is introduced in between the two layers of the PU rubber. The pine fabric in the middle and a PU rubber layer on either side of it are considered an elastomeric unit. This elastomeric unit is reinforced with two layers of glass fabric on either side to make it a structural unit applicable in dynamic conditions. Further, to enhance the damping nature of epoxy resin, natural rubber (NR) particles are blended with resin in different proportions. Hence, the effect of natural rubber as a filler in GFRP laminates embedded with an elastomeric unit on mechanical and free vibration characteristics is investigated in the present study.

Four types of composite specimens are prepared using varying weight percentages of NR particles blended with epoxy matrix, as illustrated in

Table 1. Symbol used, layering arrangement, and images of the four types of elastomeric-unit-centered NR-filled GFRP laminates fabricated by the hand layup method.

Layering Arrangement	Photograph Showing the Four Types of Specimen Prepared	Description
		(a) SPEC 1—1 Wt% NR filled Epoxy (b) SPEC 2—2 Wt% NR filled Epoxy (c) SPEC 3—3 Wt% NR filled Epoxy (d) SPEC 4—4 Wt% NR filled Epoxy

. NR particles are blended with epoxy resin at a 1%, 2%, 3%, and 4% weight proportion. In all four types of NR-filled GFRP laminates, PU rubber and pine fabric elastomeric units are kept as the core layer, and two layers of glass fabric are placed on the outer side. All types of composite laminates are prepared by the hand layup method with uniform thickness. After fabrication, the laminates are cured for more than 48 h. Table 1 shows the symbol used, the layering arrangement, the percentage of NR particles added, and photographic images of the four types of natural-rubber-filled GFRP laminates fabricated.

3. Testing of Preferred Elastomeric-Unit-Centered NR-Filled GFRP Composite Laminates

The mechanical properties of the preferred elastomeric-unit-centered NR-filled GFRP laminates are tested according to ASTM standards.

3.1. Tensile Test

The standard specimen size for the tensile test according to ASTM standard D3039 is 250 mm × 25 mm × t mm, where t is the thickness of the specimen. During the tensile test, the specimen is mounted in a universal testing machine, and a known force is applied to the ends of the specimen, causing it to stretch or elongate. The force is increased gradually until the specimen fractures, while the deformation and stress of the specimen are continuously measured. Figure 2a shows a typical tensile test carried out on elastomeric-unit-centered NR-filled GFRP laminates in a UTM.

3.2. Flexural Test

The standard size of the specimen used for the flexural test according to ASTM standard D790 is 127 mm × 12.5 mm × t mm, where t is the thickness of the specimen. During the test, the composite specimen is placed on two knife-edged supports, and an external force is applied to the center of the test specimen. The amount of deflection and the stress in the specimen are continuously measured as the force is increased. Figure 2b shows a typical flexural test carried out on elastomeric-unit-centered NR-filled GFRP laminates in a UTM.

3.3. Impact Test

The impact test involves mounting the composite specimen in a work-holding device, with a notched V-shaped groove at one end. A pendulum with a known weight and height is then released from a specific height, and it swings down to strike the specimen at the notch. The impact strength of the material is determined by dividing the energy absorbed by the specimen by its cross-sectional area at the point of impact. The standard specimen size for the Izod impact test according to ASTM standard D256 is 63 mm × 125 mm × t mm, where t is the thickness of the specimen. Figure 2c depicts a typical Izod impact test carried out on elastomeric-unit-centered NR-filled GFRP laminates in an impact test machine.

3.4. Free Vibration Study

Figure 3 illustrates the experimental arrangement of the free vibration study conducted for the elastomeric-unit-centered NR-filled GFRP composite specimen under fixed–free boundary conditions according to ASTM standard E756. The free vibration study of all preferred hybrid laminates is carried out using the impulse hammer method. The composite specimen, with a size of 250 mm × 25 mm × t mm, is placed under a cantilever boundary condition. External force is applied to the specimen using an instrumented impact hammer (Kistler-9722A500) and the vibration response is measured by an accelerometer (Kistler-8766A500). Both the force and acceleration signals are fed into a data acquisition system (NI-ATA9234) for normalization and then processed by software [DEWEsoft 2022.4] to obtain the frequency response function (FRF) plots. From the FRF plots of various composite specimens, the fundamental natural frequency and percentage of damping values are evaluated and reported [10].

4. Results and Discussion

4.1. Mechanical Properties of Elastomeric-Unit-Centered NR-Filled GFRP Laminates

Figure 4 shows a comparison of the load–displacement plots of the various types of elastomeric-unit-centered NR-filled GFRP laminates obtained from the tensile test. During the axial loading condition, the applied load is equally shared by the individual layers of the hybrid laminates and therefore each layer deforms with a different strain rate. From the comparison graph, it is understood that, for the same loading condition, the strain rate of deformation of various laminates differs according to the percentage of NR particles blended with the matrix material. Though the purpose of blending the NR particles in the epoxy matrix is to improve the damping property of GFRP hybrid laminates, it also improves the stiffness of the matrix, and thereby increases the overall stiffness of the laminates. This characteristic behaviour causes variations in the tensile strength of the different types of hybrid laminates [11].

Table 2. Experimentally evaluated tensile, flexural and impact strength of various types of elastomeric-unit-centered NR-filled GFRP laminates.

Specimen	Tensile Strength (MPa)	Flexural Strength (MPa)
SPEC 1	20.70667	42.33
SPEC 2	21.45333	47.41
SPEC 3	23.33333	71.12
SPEC 4	32.42667	88.05

shows the experimentally evaluated tensile, flexural and impact strength of the various types of elastomeric-unit-centered NR-filled GFRP laminates used in the present study. The tensile strength values of the different hybrid laminates shown in Table 2 confirm that the inclusion of NR particles in the epoxy resin improves the strength and stiffness of the laminates.

Figure 5 shows a comparison of the load–displacement plots of the four types of elastomeric-unit-centered NR-filled GFRP laminates obtained from the flexural test. During the flexural testing of FRP composite laminates, the load is applied in the thickness direction, i.e., perpendicular to the longitudinal fabric plane. Therefore, the flexural strength and modulus of the laminates are significantly controlled by the outer layer of the laminates. Since the outer layers of all preferred laminates are made of glass fabric with an NR-blended epoxy matrix, the flexural strength of hybrid laminates increases according to the weight percentage of NR added to the matrix [12]. The results shown in Table 2 also confirm the typical behaviour of the NR-filled GFRP laminates considered in the present study under transverse loading conditions.

Figure 6 shows a comparison of the impact strength of the different elastomer-unit-centered NR-filled GFRP laminates considered in the present study. From Figure 6, it is observed that there is no variation in the impact strength of the various hybrid laminates considered. In FRP laminates, the suddenly applied load is most prominently tackled by the reinforced fiber present in the laminates [13]. Since all types of hybrid laminates considered in the present study are made up of the same set of glass fiber layers on either side of the laminate, there is no variation in the impact energy absorbed by the different elastomer-unit-centered NR-filled GFRP laminates. Thus, the suddenly applied load in the thickness direction does not cause any variation in the impact energy of the preferred hybrid laminates.

4.2. Free Vibration Analysis of Elastomeric-Unit-Centered NR-Filled GFRP Laminates

To understand the dynamic behaviour of the preferred elastomer-unit-centered NR- filled GFRP laminates, a free vibration study is carried out under fixed–free boundary conditions. Figure 7 shows a comparison of the FRF plots of the various hybrid laminates considered. It is observed from Figure 7 that the modal frequency response of various types of hybrid laminates is controlled by the percentage of NR particles blended with the epoxy matrix [14]. Figure 8 shows the experimentally evaluated modal frequency values of the preferred hybrid laminates. Figure 8 reveals that the percentage of NR particles blended with the matrix material improves the stiffness of the laminates, and therefore the modal frequency values increase accordingly. The experimentally measured flexural strength values shown in Table 2 also support this characteristic behaviour of the preferred elastomer composite laminates.

Figure 9 shows the experimentally evaluated percentage of damping values for the preferred hybrid laminates. In all the hybrid laminates, the center elastomer unit consists of pine fabric and PU rubber is a major cause of the damping property [15], since PU rubber is viscoelastic in nature and has a high strain rate. The slight variation in the damping values for the preferred laminates is due to the inclusion of NR particles in the epoxy matrix [16]. From the experimentally evaluated mechanical properties and free vibration characteristics, it is confirmed that the inclusion of NR particles in elastomer-unit-centered GFRP laminates improves the strength and the stiffness, which will be beneficial for structural applications.

5. Conclusions

In the present study, an attempt is made to promote the use of elastomer-centric and NR-filled GFRP laminates in the AV mounts used in automobiles, aircraft and heavy machinery. The elastomer unit, which is made of pine fabric and PU rubber, is kept as core in making the various types of GFRP laminates. To improve the overall stiffness of the laminates, NR particles are blended with epoxy resin at different wt% for fabricating four types of hybrid laminates. Static mechanical properties such as tensile strength, flexural strength and impact strength are experimentally evaluated for all types of hybrid laminates. Free vibration study is carried out under fixed-free boundary condition to understand the dynamic behaviour and stability of the preferred hybrid laminates. The following conclusions are arrived based on the mechanical and vibration testing of four types of elastomeric-unit-centered and NR-filled GFRP laminates:

• The tensile strength of the preferred elastomer composite laminates varies according to the wt% of NR particles blended with the epoxy matrix, since the inclusion of NR particles in the epoxy matrix marginally improves the tensile modulus of the laminate.
• The introduction of NR particles in the epoxy resin effectively increases the flexural modulus, and therefore the variation in the flexural strength among the preferred elastomer composite laminates is more significant.
• Since the impact strength of the composite laminates is majorly governed by the strength of the reinforced fiber, the inclusion of NR particles at different wt% in the matrix material caused no variation in the impact strength of the various types of elastomer composite laminates considered.
• The addition of NR particles to the epoxy matrix aids in the improvement of the overall stiffness of the laminates, and therefore the modal frequency and damping values of the preferred elastomer hybrid laminates marginally vary according to the variation in the wt% of NR particles in the epoxy resin.
• The increase in the mechanical strength and overall dynamic stiffness of the preferred elastomer-unit-centered and NR-filled GFRP laminates proves that the present form of hybrid laminate arrangement is suitable for making AV mounts.