Experimental Investigation on the Mechanical Properties of Woven Glass–Polyester–Polypropylene Fiber-Reinforced Epoxy Hybrid Composites ^†

Abstract

Natural composites find application in various fields because of their low specific weight and low investment cost. But due to their inherent nature, natural composites have lower strength and tend to absorb moisture, which makes them weak. In this work, woven glass, mono-bi-filament polypropylene, and polyester fibers in an epoxy matrix were developed with four and five different stacking layers of texture utilizing the hand-layup procedure. However, understanding the directional dependence of material properties is necessary for the application of these new materials. Three distinctive plates were fabricated for the purpose of the investigation. The laminated plates were tested on a universal testing machine (UTM) and a flexible test setup to examine the mechanical properties of the polymer fiber. By adding short fibers such as polypropylene, polyester fibers in a random manner improved the mechanical strength of the polymer composite compared to the other fiber types such as woven glass fiber sheets and woven polypropylene sheets placed in the middle of the composite. This is because short polymer fibers bond well with epoxy resin and have very good bonding strength.

1. Introduction

Composite laminates’ high in-plane specific strength and stiffness have prompted substantial research in a range of industries, including aerospace, automotive, marine, and civil engineering. Composite material selection is influenced by specific application needs. Widely employed in a variety of applications, glass fiber-reinforced polyester (GFRP) woven laminates are valued for their accessibility and low cost [1]. Continuous filament glass fibers have excellent mechanical properties and are widely used in applications that need a low bulk factor and strong strength. With intermediate properties like minimal water absorption and strong corrosion resistance, epoxy resins—often manufactured from Araldite and epoxy LY-556 (bisphenol-based epoxy)—are suitable for a range of composite applications [2]. Glass fiber-reinforced epoxy resin is appropriate for usage in a range of dinghie, yacht, and workboat components due to its exceptional strength, durability, and resistance to marine conditions [3]. Fiber-stacked polymer composites, comprising glass fiber, polyester, polypropylene, bi-filament, and mono-filament mixtures, significantly enhance the mechanical properties of hybrid composites [4]. Natural fibers often have worse mechanical properties than synthetic materials like glass, polyester, and polypropylene fibers. Furthermore, applications requiring moisture resistance may encounter challenges due to their hydrophilic nature [5]. Since the fiber stacking order and manufacturing procedures have a significant impact on the mechanical properties of hybrid composites, hybridization can effectively address these issues [6,7,8,9,10,11]. In order to achieve the required mechanical properties for specific applications, polymer composites usually comprise a resin matrix and a carefully chosen reinforcing element. Numerous technological and structural applications commonly employ thermoset resins, such as polyester, silicone, epoxy, phenolic, polyamide, and others [5]. Sayed et al. investigated the mechanical performance of a hybridized polymer composite composed of glass fiber and jute using different stacking configurations. The matrix material is polyester resin and MEKP binder. The results of the study suggest that a hybrid composite that incorporates non-woven glass fiber with a unique stacking sequence could be used in furniture, car interiors, and interior design [5]. By using epoxy resin and graphite filler to reinforce glass and sisal fibers, Suresh Kumar et al. investigated the mechanical characteristics and wear resistance of hybrid composite materials. They found that whereas composites reinforced with individual fibers demonstrated better mechanical qualities, the weight fraction of glass, sisal, graphite, and epoxy resin significantly affected the mechanical properties [12]. Ref. [13] investigated the fiber stacking order and mechanical properties of hand-lay-up polymer hybrid composites. After creating five distinct types of ten-layer composite laminates, they concluded that optimizing the stacking sequence significantly increases the strength of the polymer composite.

There has been a lot of research on woven glass fiber composites, but not much on hybrid composites that combine woven glass fiber with polyester, polypropylene, bi-filament, and mono-filament. By investigating several fabric layers’ stacking sequences and employing epoxy resin, this work focuses on the creation of such hybrid composites. The aim of this study was to analyze how these stacking patterns affect the composites’ mechanical characteristics and highlight their possible appropriateness for marine applications.

2. Materials and Methods

2.1. Materials

The resin employed in this investigation was epoxy LY-556, which is based on bisphenol A and has a density of 1.20 g/cm³ at 250 °C and a viscosity of 10,000–12,000 MPa. It was combined with an Araldite HY 951 hardener, which has a density of 0.95 g/cm³ at 25 °C, at a weight ratio of 2:1. Weaved glass fiber sheets with a density of 0.93 g/cm³, polyester fiber with a density of 1.38 g/cm³, and polypropylene fiber with a density of 0.9 g/cm³ were purchased from Kalyani Polymers Pvt. Ltd. (Chennai, India). in India and used as the applied fibers.

2.2. Fabrication of Hybrid Composite Specimen A

The glass fiber mold was coated with LY-556 resin (56.6 wt%) and Araldite HY951 epoxy hardener (5.66 wt%) after being treated with a mirror glass releasing agent. Hybrid composite specimen A was prepared using the hand lay-up technique and crushed. Three woven glass fiber pieces were trimmed into 300 × 300 mm² samples before being used. In order to enable resin wetting, the first layer of woven glass fiber sheet (6 wt%) was laminated and compressed using a metal roller. A second layer of mono- and bi-filament polypropylene short fiber (12.12 wt%) was laminated and again pressed until it was fully wet after the epoxy matrix mixture was added. After adding the epoxy matrix mixture, a third layer of 6 wt% woven glass fiber sheet was laminated and compressed once more until it was fully wet. After adding the epoxy mixture, a fourth layer of 6.6 wt% polyester short fiber was laminated and compressed once more until it was fully wet. The fifth layer of woven glass fiber sheet (6 wt%) was laminated and again compressed until it was fully wet after the epoxy matrix mixture was added. The resin to fiber ratio was 1.7:1. The composite sample was made up of woven glass fibers that were orientated in both directions (0°/90°) and intermediate layers of small fibers that were oriented randomly. The composite was compressed under 8 N for an entire night at room temperature after being vacuum-bagged at 20 °C. After the laminate composite had set for 12 h, it was further cut into test sections that met ASTM 638 [14] for tensile specifications, ASTMD 790 [14] for flexural specifications, and ASTMD 2344 [14] for shear specifications. The hybrid composite sample composition is shown in

Table 1. Creation of a hybrid composite material using epoxy, woven glass, polyester, and polypropylene fiber.

Samples	Epoxy Matrix (wt.%)	Woven Glass Fiber (wt.%)	Mono and bi Filament Polypropylene Short Fiber (wt.%)	Polyester Fiber (wt.%)	Woven Polypropylene Fiber (wt.%)
Epoxy	100	0	0	0	0
A	62.6	18	12.12	6.6	0
B	62.6	12	12.12	6.6	6
C	68.56	12	12.12	6.6	0

, and a schematic design of the hybrid composite preparation procedure is shown in Figure 1.

2.3. Fabrication of Hybrid Composite Specimen B

A coating of araldite-HY951 epoxy hardener (5.66 wt%) and epoxy-LY-556 resin (56.6 wt%) was placed in a glass fiber mold coated with a mirror glass release agent. The hand lay-up approach was used to prepare hybrid composite specimen B, which was then compressed. Before being used, 300 mm × 300 mm samples were cut from two plies of woven glass fiber sheets and one ply of woven polypropylene sheets. A metal roller was used to laminate and press the first layer of woven glass fiber sheet (6 wt%) in order to prepare it for resin wetting. After adding the epoxy matrix mixture, a second layer of (12.12 wt%) mono- and bi-filament polypropylene short fiber was laminated and compressed once more until it was fully wet. A third layer of woven polypropylene sheet (6 wt%) was laminated and again compressed until it was fully wet after the epoxy matrix mixture was added. After adding the epoxy mixture, a fourth layer of 6.6 wt.% polyester short fiber was laminated and compressed once more until it was fully wet. After adding the epoxy matrix mixture, a fifth layer of 6.6 wt.% woven glass fiber sheet was laminated and compressed once more until it was fully wet. The resin to fiber ratio was 1.67:1. The composite sample was made up of woven glass fibers that were orientated in both directions (0°/90°) and intermediate layers of small fibers that were oriented randomly. The composite was compressed under 8 N for an entire night at room temperature after being vacuum-bagged at 20 °C. After the laminate composite had set for 12 h, it was further cut into test sections that met ASTM 638 for tensile specifications, ASTMD 790 for flexural specifications, and ASTMD 2344 for shear specifications.

2.4. Fabrication of Hybrid Composite Specimen C

A layer of LY-556 resin (62.33 w%) and Araldite HY951 epoxy hardener (6.23 w%) was placed in a glass fiber mold that had been treated with a mirror glass releasing agent. The hand lay-up approach was used to prepare hybrid composite specimen C, which was then compressed. Two plies of woven glass fiber sheets were cut into 300 × 300 mm samples prior to use. In order to enable resin wetting, the first layer of woven glass fiber sheet (6 w%) was laminated and compressed using a metal roller. A second layer of mono- and bi-filament polypropylene short fiber (12.12 wt%) was laminated and again pressed until it was fully wet after the epoxy matrix mixture was added. After adding the epoxy mixture, a third layer of 6.6 wt% polyester short fiber was laminated and compressed once more until it was fully wet. After adding the epoxy matrix mixture, a fourth layer of 6 wt% woven glass fiber sheet was laminated and compressed once more until it was fully wet. The resin to fiber ratio was 2.23:1. The composite sample was made up of woven glass fibers that were orientated in both directions (0°/90°) and intermediate layers of small fibers that were oriented randomly. The composite was compressed under 8 N for an entire night at room temperature after being vacuum-bagged at 20 °C. After the laminate composite had set for 12 h, it was further cut into test sections that met ASTM 638 for tensile specifications, ASTMD 790 for flexural specifications, and ASTMD 2344 for shear specifications. The hand layout schematic is displayed below in Figure 2, Figure 3, Figure 4 and Figure 5.

3. Experiment

3.1. Tensile Testing Method

We utilized the universal testing machine with 10 tons in compliance with ASTM D638. A speed of 2 mm per minute was used for the test. Normal characteristics were intended to be the tractable properties of distinct composites. Three samples, each measuring 3.32 × 12.76 × 50 mm³, were tested. The example placed in the UTM for the tensile test is shown in Figure 6, Figure 7 and Figure 8. The specimens were tested for different strengths, and

Table 2. Results of mechanical test.

PROPERTIES	SAMPLE A	SAMPLE B	SAMPLE C
Ultimate tensile strength (Mpa)	39	18.6	37
Youngs modulus (Mpa)	629.33	789	291.14
Bulk modulus (Mpa)	320.76	347.88	122.843
Flexural strength (Mpa)	41	36	47
Flexural strain	0.0524	0.0867	0.0877
Shear strength (Mpa)	15.9	15.1	11.2

shows the results.

3.2. Failure Mode of Tensile Tests

The specimen’s failure investigation showed that there was delamination between the woven glass fiber sheet layers, as shown by the obvious separations along the fiber–matrix interface. Weak fiber–matrix bonding was also indicated by fiber pull-out, which is the partial or complete separation of individual fibers from the matrix, as shown in Figure 6, Figure 7 and Figure 8.

3.3. Shear Testing Method

In accordance with ASTM D2344, the test was performed using a universal testing apparatus with a 10-ton capacity. Three samples measuring 3.02 mm by 25.61 mm by 50 mm were examined on the laminated composite. The example implemented in the UTM for the shear test is displayed in Figure 9, Figure 10 and Figure 11.

3.4. Failure Mode of Shear Tests

The specimen showed both delamination and fiber pull-out upon testing. Along the fiber–matrix interface, delamination was visible, especially close to the borders where the layers were clearly separated. Furthermore, inadequate fiber–matrix bonding was indicated by fiber pull-out, in which individual fibers separated from the matrix and protruded from the surface as shown in Figure 9, Figure 10 and Figure 11.

3.5. Flexural Testing Method

The test was performed using a 200 KN capacity flexural testing apparatus in accordance with ASTM D790. For flexural testing, the test samples are cut to the appropriate dimensions in accordance with ASTM D790 requirements. The 3.6 mm × 27.6 mm × 50 mm test specimen geometry, as stated in the aforementioned specification for balanced symmetric (0/90°), was used. A strain rate of 0.5 mm/min is used for the test. A three-point bend configuration was used for the flexural test. A spacing distance of 100 mm was maintained between two supports. The composite’s ultimate load-bearing capacity was noted. Figure 12, Figure 13 and Figure 14 depict the sample in the flexural testing apparatus for flexural testing.

4. Results

Figure 15 shows the tensile strength of a woven glass/polyester/polypropylene fiber-reinforced epoxy hybrid composite. Specimen A contains three woven glass fiber sheets with an equal amount of mono and bi filament polypropylene short fiber and polyester short fiber; specimen B contains two woven glass fiber sheets with an equal amount of mono and bi filament polypropylene short fiber; and sample C contains two woven glass fiber sheets with an equal amount of mono and bi filament polypropylene short fiber. It was observed that specimen A exhibited the highest ultimate tensile strength of 39 MPa, followed by sample C at 37 MPa, while sample B had the lowest ultimate strength of 18.6 MPa. This indicates that the combination of fibers and epoxy matrix in sample A contributed to a higher tensile strength compared to sample C, and it also indicates that the variation in the stacking sequence has a great influence on the tensile strength.

The shear strength in Figure 16 shows the shear strength of different-layer hybrid composites. It can be observed that in table A, sample A exhibits a higher shear strength than sample B, and sample A exhibits the highest shear strength (15.9 MPa), followed by sample B (15.1 MPa) and sample C (11.2 MPa). Shear strength reflects the material’s ability to withstand shear stresses. Sample A demonstrates superior resistance to shear stress compared to samples B and C.

The flexural strength of various four- and five-layer stacking configurations of the hybrid composite is displayed in Figure 17. The table showed that samples A and B, which had five layers of hybrid composite, yielded worse results compared to sample C, which had four layers. It was discovered that the interfacial zone in fiber-reinforced composites is crucial for load transfer between the fiber and the network, which in turn affects mechanical qualities like strength. The samples’ extreme layers experienced the highest stress during the flexural test. The maximum stress shared by the woven glass fiber sheets was achieved by positioning the glass fiber at the ends. Therefore, compared to specimens A and B, specimen C had a comparatively higher flexural strength.

Figure 18 shows the mechanical characteristics of several examples with differing layer arrangements. Our composite formulation, which combines woven glass fibers with mono- and bi-filament polypropylene fibers, provides a cost-effective and mechanically robust solution while preserving production simplicity. Compared to traditional composites, which depend on a single fiber type and may limit performance in some applications, this method offers a considerable advantage.

5. Conclusions

(1)

The discrepancies seen in the mechanical properties of the samples can be ascribed to variances in the fiber composition, orientation, matrix material, fabrication technique, and interfacial bonding between the materials.

(2)

Sample C is a potentially good option for structural applications that require a mix of these qualities because it typically demonstrates a balance between tensile strength, flexural strength, and stiffness.

(3)

Additional research on the fracture surfaces and microstructural characterization would shed light on the underlying mechanisms controlling the mechanical behavior of the hybrid composites.

The above investigation on polymer fibers shows that short, discontinuous polymer fibers such as polypropylene and polyester, oriented in a random manner, improve the mechanical strength of composites more than other types of fiber such as woven glass fiber sheets and woven polypropylene sheets placed in the middle of the composites. This is because short polymer fibers bond well with epoxy resin and yield very good bonding strength.