Evaluations of Mechanical Properties and Residual Strength of Drilled Glass Fiber Reinforced Polymer (GFRP) Composites

Irina, M.M.W a, Tan, C.L.b, Azmi, A.I.c, Leong, K.W.d, Radzi, M.N.Me

School of Manufacturing Engineering, University Malaysia Perlis (UniMAP), Pauh Putra Campus, 02600 Arau, Perlis, Malaysia

airinawong90@live.com.my, bchyelih@gmail.com, cazwaniskandar@unimap.edu.my, dkwleong@unimap.edu.my, emohdnazmi@unimap.edu.my

Keywords: Glass fiber; Mechanical properties; Vacuum-assisted resin transfer moulding.

Abstract. This paper investigates the mechanical properties of drilled and non-drilled glass fiber reinforced polymer (GFRP) composites of plain woven and stitch bi-axial ±45° fabrics. Vacuum-assisted resin transfer moulding was employed to fabricate the composite panels. In the first stage of this study, mechanical properties such as tensile strength, flexural strength, and volume fraction of the composites were determined by per ASTM standards. Later on, the fabricated GFRP composites were drilled in order to investigate the delamination damage. Based on this issue, residual strength was evaluated after drilling process using constant feed rate and spindle speed but different drill bit geometries, which include twist drill bit and step drill bit. Experimental results showed that plain woven performed better in term of mechanical properties and residual strength after drilling process. In addition, residual strengths of drilled composites using step drill bit exhibited superior performance than that of the twist drill.

Introduction

In common practice, fiber reinforced polymer (FRP) composites are produced by two or more phases of different materials. This is mainly aimed to achieve specific properties that cannot be found in other individual materials. One phase is known as the reinforcing phase, whilst the other one, which is embedded in it, is known as the matrix [1].

Until recently, a number of research studies have been carried out to manipulate the orientations of the fibers using different architectures. The purpose is to develop the better mechanical performance of the composites, which are less weight, more strength and low cost. Kumar et. al. [2] conducted a research on the effect of angle ply orientation on tensile properties of bi-directional woven fabric glass epoxy composite laminate. The results showed that the 0˚ fiber orientation has high strength when compared to other orientations for the same dimension in the test specimen and load applied.

Although composite components are often made by near-net shape, the need for secondary machining processes such as drilling and milling processes are inevitable. One of the most common machining operations is the drilling process. This basic process is known as making holes on composite parts that required for subsequent assembly operation. Navid et al [3] asserted that dynamic loads such as impact and drilling process caused significant reductions in the composites stiffness and strength. Therefore, study on residual strengths after drilling process are essential, in order to give the better ideas or suggestions to the manufacturer in selecting the appropriate cutting tool when performing drilling on FRP composites. The purpose of this study is to investigate and compare the effect of the fiber architecture and mechanical behavior of Glass fiber reinforced polymer (GFRP) composites. In addition to that, the residual strengths of GFRP composite with different fiber architecture after drilling process have also been investigated.

Materials and fabrication methods

The selected fiber reinforcements were plain woven (0/90°) and stitch bi-axial (±45°) E-glass fibers, Fig. 1. Both fibers have the same E-glass fabric tow size of 3k but different areal weight (g/m2), which

Applied Mechanics and Materials Vol. 660 (2014) pp 270-274 Submitted: 03.07.2014Online available since 2014/Oct/31 at www.scientific.net Revised: 13.08.2014© (2014) Trans Tech Publications, Switzerland Accepted: 13.08.2014doi:10.4028/www.scientific.net/AMM.660.270

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 130.207.50.37, Georgia Tech Library, Atlanta, USA-09/12/14,14:07:56)

is 200 g/m2 and 400 g/m2 for plain woven and stitch bi-axial respectively. The matrix used was Epoxy Amite 100, with 103 slow hardener.

Fig.1. Close-up views of the E-glass reinforcement; (a) plain woven fiber and (b) stitch bi-axial fiber

During the fabrication process, layers of mould release agent were applied on the surface of the

glass mould for easy release of the composite panel after fully cured. The E-glass fiber piles (300 mm x 300 mm) were then stacked on the flat glass mould with orientations: [0/90°]21 for plain woven and [±45°]10 for stitch bi-axial fiber. Once the arrangement of laminates has been completed, vacuum pressures were then applied to draw out all the air inside and compacted the fiber preform. The compaction pressure was in the range of 11-17 mBar. Following that, the resin was prepared by mixing the epoxy and hardener in the portion of 4:1 and then infused into the mould cavity. The infused panel was left to cure for 24 hours in room temperature under vacuum pressure. The final cured panel was then cut into required specimen sizes for different mechanical tests.

Sample characterizations

Volume fraction

Five test specimens of 25 mm x 25 mm were prepared from different fabricated laminate panels according to ASTM D3171 for burn-off tests. The masses of the samples with crucible were measured and recorded to nearest 0.1 mg using Electronic balance meter, while the density of specimen was measured by Densimeter. The samples were then placed into the preheat Nabertherm heat treatment machine and maintained at 500°C for 5 hours. After the resins have been fully charred, the remaining weights of fiber reinforcements in its crucible were measured. Once all the data have been measured, the volume fraction was calculated using Eq.1.

100 (1)

Where: = volume percent of reinforcement in the specimen

= final mass of specimen after combustion

= initial mass of specimen before combustion

= density of the composite specimen

= density of the reinforcement

Tensile test

Five rectangular tensile specimens of 250 mm x 25 mm were prepared from different fabricated panels in accordance to ASTM D3090 for woven specimens and ASTM D3518 for stitch bi-axial fiber. The loading rate on the Shimadzu Universal Testing Machine (UTM) was set to 2 mm/min for the tensile tests.

Flexural test

Meanwhile, for flexural tests, five rectangular flexural specimens of 200 mm x 25 mm were also prepared from different fabricated laminate panels according to ASTM D790 with loading rate of 12.93 mm/min using the Shimadzu UTM.

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Residual tensile test

For the residual testing, five residual tensile samples with drilled holes were tested using UTM. The holes were drilled in the tensile specimen using two different drill point geometries namely twist drill and step drill. Both drills have same drill diameter and point angle, which are 8 mm and 85° respectively. Five specimens were tested and the average value was taken for analysis.

Results and discussion

Fiber volume fraction of the GFRP composites

Quatity consistency of reinforcements in FRP composites is often determined by fiber volume fraction and density. Results after the burn-off testes are as depicted in Table 1. The results of woven and stitch bi-axial composites showed consistent thickness, density and volume fraction. According to Alam et al [4], the densities of FRP composites are mainly depend on the type of fiber and matrix used. Meanwhile, it is evident that the current results revealed the fiber volume fraction achieved by VARTM process is higher than that of wet-hand lay-up process [5]. The conclusion that can be drawn is that the characterization performance for both woven and bi-axial composites was consistence.

Table 1. Specification of fiber reinforcements.

Batch

Woven Fiber Stitch Bi-axial (±45°) Fiber

Thickness (mm)

Density (g/cm3)

Volume fraction (%)

Thickness (mm)

Density (g/cm3)

Volume fraction (%)

1 3.20 1.914 71.55 3.30 1.821 68.84

2 3.30 1.876 69.00 3.30 1.802 66.91

3 3.30 1.854 68.34 3.25 1.818 67.13

4 3.20 1.870 68.42 3.30 1.801 66.18

5 3.30 2.028 72.55 3.30 1.825 66.62

Mean 3.26 (±0.05) 1.908 (±0.06) 69.97 (±1.74) 3.29 (±0.02) 1.813 (±0.01) 67.14 (±0.91)

Tensile properties

The results of tensile mechanical properties for woven and bi-axial fiber are shown in Table 2. As expected, the woven fiber composite has the highest tensile strength compared to the bi-axial composite. This is due to the loading direction of the 0° fabrics, which support majority amount of load, thus makes it stiffer and stronger than bi-axial composites [6]. These results were higher than that of previous by [2, 7], in which the composites were fabricated by VARTM and hand lay-up processes.

Table 2. Tensile properties for different batch of GFRP specimens

On the other hand, Fig. 2 shows the stress-strain curves for all five samples of woven and stitch bi-axial composites. Stress-strain curve reveals that the woven composite has directly ruptured without experience any necking process as the metallic material. This is expected as woven fiber composites do not have any plastic region, so it can be characterized as brittle material. It is also exhibited that the tensile strength of the bi-axial fiber composite is only about one third of the woven fiber composite.

Sample

Woven Fiber Stitched Bi-axial (±45°) Fiber

Ultimate Tensile Strength (MPa)

Tensile Modulus (GPa)

Ultimate Tensile Strength (MPa)

Tensile Modulus (GPa)

1 317.86 26.72 111.74 2.78 2 362.32 23.49 111.49 2.99

3 313.51 20.41 113.03 3.13

4 394.23 22.09 108.44 3.24

5 363.14 33.03 112.44 3.17

Mean 350.21 (±30.48) 25.15 (±4..45) 111.37 (±1.70) 3.06 (±0.16)

272 Advances in Mechanical, Materials and Manufacturing Engineering

Fig. 2. Tensile stress-strain curves for woven and stitch bi-axial composites

Flexural properties

The average flexural properties data for woven and bi-axial composites are depicted in Table 3. Woven fiber composite has the higher ultimate flexural strength and flexural modulus compared to that of the bi-axial composite. This highest ultimate flexural strength indicated that the composite laminates require supreme stress to rupture. Besides that, the flexural modulus of woven laminates indicated that the ability of the composite laminate to bend prior to fracture is 20.39 GPa. It is important to note that the result of flexural properties were comparable with the experimental work by the Chawla [8].

Table 3. Flexible properties for different batch of GFRP specimens

Sample

Woven Fiber Bi-axial (±45°) Fiber

Ultimate Flexural Strength (MPa)

Flexural Modulus (GPa)

Ultimate Flexural Strength (MPa)

Flexural Modulus (GPa)

1 304.64 21.25 168.80 11.79 2 341.64 20.07 130.09 10.61 3 312.58 19.68 128.60 10.36 4 320.17 21.15 120.26 8.73 5 311.88 19.80 104.51 7.85

Mean 318.18 (±12.72) 20.39 (±0.67) 130.45 (±21.22) 9.87 (±1.40)

Residual tensile properties

The average residual tensile strength data for woven and bi-axial for both types of drill are shown in Table 4. As expected, woven composite performed better compared to the bi-axial for ultimate residual strength and flexural modulus. This is attributed to the alignment of fibers, which is damaged after the hold is drilled. Different direction of shear is occurred even though the force is applied at same direction, Fig. 3. Other than that, when compare to the type of drill bit used, residual strength after drilling with step drill exhibited slightly higher than that of the twist drill. Therefore, combination of woven composite laminates with step drill performs the best for residual properties.

Table 4: Average residual tensile strength for woven and bi-axial composites with twist drill and step drill

Laminates Residual Tensile Strength (MPa) Residual Tensile Modulus (GPa)

Twist drill

Woven 172.91 (±15.61) 10.58 (±1.55)

Bi-axial 63.47 (±4.00) 2.14 (±0.31)

Step drill

Woven 191.12 (±9.75) 16.03 (±1.65)

Bi-axial 65.05 (±1.98) 2.14 (±0.22)

For woven fibers, Fig. 3(a), it can be observed that the 0° fabrics are on the loading direction. Thus, the strength and stiffness of woven composite are higher than in the bi-axial composites. Based on this result, it can be concluded that the direction of shear will be damaged according to the architecture of fiber used.

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Fig. 3. Direction to shear for (a) woven composite and (b) bi-axial composite.

Concluding Remarks

Mechanical performance for GFRP woven composites is better than the stitch bi-axial composite in terms of tensile and flexural strengths. Thus, GFRP woven composite are preferable, but it is only suitable for applications that required below than 350 MPa in strength. Meanwhile, results of residual properties of GFRP woven composites also show better than bi-axial composites in terms of residual tensile strength for both twist and step drill. Therefore, woven composite laminates are more suitable to be used because it can endure a high degree of load in its direction of fiber. Even though some portion of the composite is noticeable to be damaged, it also can resist the load and perform better than bi-axial composite. However, by comparing the type of drill bit used, residual strength after drilling with a step drill bit is slightly higher than twist drill.

References

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Advances in Mechanical, Materials and Manufacturing Engineering 10.4028/www.scientific.net/AMM.660 Evaluations of Mechanical Properties and Residual Strength of Drilled Glass Fiber Reinforced

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