Indian Journal of Engineering & Materials Sciences Vol. 25, August 2018, pp. 315-320

New investigations on higher mechanical properties of woven glass fiber reinforced SMC composites

Mahmut Bingola & Kadir Cavdarb*

aVocational School of Yalova, Yalova University, Yalovo, Turkey bDepartment of Mechanical Engineering, Bursa Uludag University, Bursa, Turkey

Received 21 September 2015; accepted 12 July 2017

There are many different types of glass fibers that can be used in the composites, which are fabricated using a sheet molding compound (SMC) process. The SMC composites produced from different type of glass fibers with unsaturated polyester resin have been investigated based on their microstructure and mechanical properties. The composites produced from woven glass fiber had a better mechanical properties compared with chopped glass fiber reinforced SMC composites. The SEM analysis revealed that resin pull-out formation occurred in the composite reinforced with randomly distributed glass fibers whereas this formation was not detected with woven glass fibers. A good agreement was found between the mechanical analyses and SEM studies for the comparison of random or woven glass fibers reinforcement.

Keywords: Glass fibers, Polymer-matrix composites, SMC composites, Woven fibers)

Composite materials are formed by mixing two or more dissimilar materials in order to control and develop new and improved structures and properties. They widely used in industries such as automotive, aerospace, marine, electric have expectations for strength, hardness, lightweight and chemical resistance. Generally, composite materials are produced by using a combination of high performance reinforcements (inorganic materials such as graphite, oxides, carbides, metals etc. or fiber form of polymeric, ceramic or metals) coupled with organic polymer (thermoset or thermoplastic) matrices. Thermosets composites have many unique properties including exceptional strength, light weight, corrosion resistance, UV resistance, electrical non-conductance, long usage time and cheap raw material costs, and often provide easy wetting of reinforcing fiber and easy forming to final part geometries1-3. Thermoset composites are fabricated either using wet‐forming process, the resin gets cured in the product while the resin, or prepreg process, pre‐fabricated material in semi‐cured form is used to provide shape to the final product4-7. The typical wet‐forming processes include hand layup, bag molding, filament winding, resin transfer molding and pultrusion8,9. The prepreg processes are bulk molding (BMC) and sheet molding compounds (SMC)10. The SMC method is a sheet of

ready-to-mold composites containing uncured thermosetting resins and uniformly distributed short fibers and fillers11. Due to its simplicity, today this method is a widely used technique in the composites industry. Moreover, steel has been replaced with SMC composites in automotive industry. The SMC method provides not only short cycle time and low cost products but also composites with better mechanical properties, perfect surface quality, esthetic and corrosion resistance12. Generally the SMC method consists of polyester or vinyl ester resin, chopped glass fibers, inorganic fillers, additives, and other materials13,14. To achieve successful SMC production, both chemical and mechanical control of the process must be provided. For example the loss in mechanical properties and surface quality is due to the spaces in material on the surface during the production process15. Again, if the fiber ratio and homogeneous distribution are not finely adjusted, the expected mechanical properties and surface cannot be obtained16-18. The ratio of distribution of the fibers in the prepreg materials and maturation are the factors affecting the mechanical properties19-21.

In this paper, a comparative study of the SMC composites prepared from random or woven glass fibers has been reported. For this purpose, prepregs with random glass fibers with different lengths (24, 48 and 65 mm) and woven glass fiber containing six of woven layers were prepared. The SMC parts were

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*Corresponding author (E-mail: cavdar@uludag.edu.tr

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produced under heat (150oC) and constant pressure (80 bars). The mechanical properties of obtained composites were investigated by flexural and tensile tests.

Materials and Methods E-glass fiber with diameter of the bundles of 15 μm

was provided by Cam Elyaf A.S. (SMC3-2400). The glass fibers were cut into 24, 48, and 65 mm lengths and added into resin randomly at a concentration of 20% by weight. A woven fabric in plain construction produced by Fibroteks is used as reinforcement material which has a superficial density of 500 g/m2. An unsaturated polyester resin (PolipolTM 347-BMC-SMC, d=1,118 g/cm3) by Poliya was used in the experiments.

Prepregs are prepared using a special formulation. In this formulation, there are 36% polyester, 10% thermoplastic resin, 30% CaCO3, 20% fiber reinforcement material, and 4% other additives. The prepregs with sample code R24-48-65 means; R as fibers random distribution, 24, 48 and 65 refer to fiber lengths in mm. When woven fabric is used as fiber in prepreg 6W refers to six of woven layers placed on top of other. Mix ratio is the same for all samples (filler to mix by weight = 0.2:1). For this reason, the woven glass fiber was used with six of woven layers. Figure 1 presents (a) random and (b) woven glass fibers and (c) final prepregs.

Figure 2a shows a graphical image of a SMC sheet measured over a surface of 140 × 280 cm2. Mold was equipped with resistance, electrical cabinet and thermocouple was installed for temperature control. In order to have constant pressure on mold, it was connected to a hydraulic press. The SMC molding press (Fig. 2b) and SMC parts (Fig. 2c) which, manufactured at 140-150ºC heat and constant pressure (80 bars) are presented below.

The manufactured plates are cut by diamond saw and specimens are prepared according to ISO527 for tensile strength and ISO178 for flexural strength tests. Tensile and tests were performed with the Shimadzu-AG-I machine at 5 mm/min speed and the Zwick-1446 machine 2 mm/min, respectively. The morphological features of the broken composite surfaces (obtained from tensile and flexural tests) were characterized by scanning electron microscopy (SEM, Carl Zeiss EVO 40) with accelerating voltage of 20 kV. The specimen surfaces were coated with gold palladium and observed under reduced pressure.

Results and Discussion Mechanical properties of the composites prepared

by SMC method usually depend on the type of reinforcing materials, their length and diameter, volume fractions and their orientation in the matrix. In the SMC formulation, chopped-strand mat contains

Fig. 1 — (a) Random glass fiber, (b) woven glass fiber and (c) final version of the prepregs

Fig. 2 — (a) SMC mold, (b) SMC mold connected to the press and (c) manufactured plates

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randomly distributed fibers cut in different lengths are generally used as a reinforcing material. Recently, the woven fibers provide excellent integrity and conformability for advanced composites applications. The advantages of woven fibers compared to randomly distributed counterparts are excellent drapeability, improved mechanical properties and the interlaminar or interfacial strength. In this study, the woven glass fibers were compared with various lengths of randomly distributed chopped fibers in the SMC composite production. In order to compare the obtained composite materials, all prepregs were matured under the identical experimental conditions. To meet these requirements, all SMC composites were produced with a special steel mold under constant heat and high pressure. Firstly, different lengths of randomly distributed glass fibers (24, 48 and 65 mm), which are widely used in literature, have been evaluated in our special formulation (36% polyester, 10% thermoplastic resin, 30% CaCO3, 20%

fiber reinforcement material, and 4% other additives). For the comparison, the woven glass fiber was used with six of woven layers in the formulation to meet same mixing ratio. For a healthy comparison, special efforts on consistency and reproducibility of sample preparations have been paid for all samples. In this case, six samples plates of each type of glass fibers, randomly or woven, were produced and five of them were chosen to cut test specimens with the diamond saw. In order to see the influence on the reinforce materials on composite, blank samples, in the absence of reinforcement material, were also prepared and tested in the similar experimental conditions.

The results in Fig. 3 and Table 1 show that the tensile strength of the composite reinforced with different lengths of randomly distributed glass fibers increased from 48 to 68 N/mm2 by increasing the fiber lengths (from 24 to 65 mm). This increase is continuing into fibers with 65 mm length, further

Table 1 — Average results of tensile and flexural stress (MPa)

Mean Resin R24 R48 R65 6W Tensile stress 5.19 47.95 51.51 67.58 137.29 Flexural stress 22.11 133.57 146.25 153.49 200.19

Fig. 3 — Tensile strength results of final plates prepared by random and woven fibers

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increase in the length causes a drop in the mechanical properties. Furthermore, the composite obtained from the glass fiber with 24 mm length which had lowest tensile strength was ten times stronger than the blank resin sample. These results confirmed that the addition of randomly distributed glass fiber in the polymer matrix ultimately improve the mechanical properties of the composite materials. Furthermore, the composites obtained from woven glass fibers has a better tensile

strength properties (approximately twice as higher than the 65 mm sample) than corresponding composites produced by randomly distributed glass fibers. It was noted that tensile strength values of the composites produced from randomly distributed glass fibers have a greater standard deviation than woven composites. There are several reasons for this including the random alignment of fibers where few or no fibers (inhomogeneous orientation) were oriented in the direction of the applied stress and result in a weak and low-strength structure.

Figure 4 is summary of the tensile strength of curves of the composites and blank resin, which were drawn from average values of five samples. Surprisingly, the composite produced from woven glass fiber (red color) had a double tensile strength value compared to randomly distributed glass fiber composites, in which all fibers were feed with same weight percentage in the prepregs. This significant improvement may be due to the effect of orientation of fibers which has the major role in the mechanical behavior of the composite. The similar trends were also determined with flexural strength results of the composites (Fig. 5 and Table 1).

The SEM images in Fig. 6 show a part of a fractured SMC specimen. The damage mechanism of

Fig. 4 — Stress-strain curves of blank resin, composite samplesproduced from random fibers in different lengths and woven fiber

Fig. 5 — Average results graphs of (a) tensile and (b) flexural stress

Fig. 6 — Random fiber (R65) SMC composites: (a) 135X, (b) 100X and (c) 48X magnifications

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the composites normally proceeds in three steps; first stage is the formation micro cracks, second stage is the separation of fiber and matrix and the last stage is the separation of interface and breaking of fibers22-24. In the previous studies, this damage has been tried to explain by fracture mechanics25,26. Since the chopped fibers were randomly distributed in the polymer matrix, the randomness in orientation of individual neighboring fibers was often not high. The SEM observations for fracture surfaces of the composite produced randomly distributed glass fiber suggest that the pull-out mechanism was acting on tensile strength tests. Further SEM analysis also confirmed that resin pull-out was the dominant failure mechanism for the composite produced randomly distributed glass fibers. The pull-out of fiber structures means that the interface strength between fiber and matrix is not enough in the composites27. As a consequence the pull-out structures of these composites may have been the cause of the lower mechanical properties due to a poor stress transfer efficiency favoring crack initiations.

When comparing the fiber behavior in the composites produced woven glass fiber (woven versus random fibers), a big difference was found. Figure 7 demonstrated the SEM micrograph of woven SMC composite at high resolution; the fibers exhibited good bonding with the resin.

The SEM images also showed the woven fabrics layers placed on top of each other (Fig. 7a), transverse and longitudinal cross-sections of fibers (Fig. 7b) and longitudinal fibers at high resolutions (Fig. 7c). In the woven fiber composites, the fiber de-bonding can be slowed down or stopped by a complex structure of woven fabrics where parts of fiber bundles are oriented in the force direction and hence they can hold the more load. In other words, the woven fibers do not pull out like randomly distributed fibers therefore resist tensile force till break. While the woven glass fibers may elongate under the forces, these forces may not be

sufficient to cause a break of the fibers entirely. Moreover, it is also possible to damage and separate polymer matrix from the woven fibers.

Conclusions The influence of type and length of glass fibers on

the microstructure and mechanical properties of SMC composites based on unsaturated polyester matrix can be summarized as follows:

(i) The tensile strength of the composite reinforced with randomly distributed glass fibers increased from 48 to 68 N/mm2 by increasing the fiber lengths from 24 to 65 mm.

(ii) The composite obtained from the glass fiber with smallest length which had lowest tensile strength was ten times stronger than the blank resin sample.

(iii) The composite produced from woven glass fiber had a double tensile strength (137.29 MPa) value compared to randomly distributed glass fiber composites (67.58 MPa), in which all fibers were feed with same weight percentage.

(iv) The similar results were also obtained with flexural strength tests of the composites.

(v) The SEM analysis revealed that resin pull-out was the dominant failure mechanism for the composite reinforced with randomly distributed glass fibers.

(vi) The fiber pull-out did not observe with the composites reinforced with woven glass fibers. This could be due to the complex structure of woven fabrics where parts of fiber bundles are oriented in the force direction and hence they can hold the more load.

(vii) As a result of these findings, type (randomly or woven), length (24, 48 or 65 mm) and

Fig. 7 — Woven SMC material: (a) 18X, (b) 58X and (c) 100X magnifications

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volume fraction of reinforcement materials are the main factor of changing properties of the SMC composites.

Acknowledgements

This study was supported by SANTEZ Program of Ministry of Science, Industry and Technology with the code 01019.STZ.2011-2. The authors would also like to thank Martur Automotive Seating and Interiors for providing necessary research facilities during the course of this study. References 1 Abrams L M & Castro J M, Polym Compos, 24 (2003)

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