The influence of glass fiber on fracture behavior of isotactic polypropylene
Introduction
Today the demand of polypropylene increases and a large amount of work have been carried out over the past few years to understand the fracture mechanism of glass fiber (PP/GF) composites. This is due to the very fast growing rate of polypropylene composite applications in automotive and other industries. Although the role of dual character of polypropylene (its ability to both shear yield and craze) during deformation or fracture cannot be neglected. There are many studies which have concentrated on the mechanical behavior of PP/GF composites [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. In fact, two main approaches can be distinguished in these research works. The first approach concerns the adhesion at the fiber/matrix interface and role of the coupling agent [5], [6], [7]. The second approach pays attention to the morphology of the spherulitic structure and its interaction with glass fibers [8], [9], [10], [11], [12], [13].
In spite of the importance of fracture mechanism on mechanical behavior of the materials, there are limited studies dealing with the fracture mechanism in PP/GF composites [14], [15]. These research works have mostly been focused on the interface debonding due to poor adhesion between fiber and matrix. In addition, dual behavior of polypropylene during fracture has not been considered. Therefore, this field of study still requires further investigations to reveal the details of fracture in PP/GF composites. Besides, according to author's knowledge there is no evidence to show that investigation of fracture mechanism via both three- and four-point bending tests has been performed.
The main aim of this work is to elucidate the role of glass fiber on fracture behavior of polypropylene. To do so, three- and four-point bending experiments have been employed, also optical and electron microscopic techniques have been used to evaluate fracture phenomena in PP/GF composites.
Section snippets
Experimental material
A series of PP composites was used in this study. A PP composite with 30 wt.% GF was obtained from Polykemi Company. The initial average length and diameter of GF were approximately 6 mm and 13 μm, respectively. In order to obtain the other composites, using a Windsor SX-30 single extruder (Windsor Machine and Stamping, Windsor, Ontario, Canada), the original composite has been diluted with a pure isotactic polypropylene (designated 080) with MFI=8 g/10 min, produced by Imam Khomeini Complex.
Results and discussion
Table 1 summarizes the results of the DSC tests. These show that incorporation of the GF into PP led to an increase in crystallization temperature, and a decrease in the heat of melting of the composites [ΔHm (J/g Co)]. Increasing of crystallization of PP can be attributed to nucleating effect of the glass fiber. Reduction of the heat of melting is caused because of substitution of PP by GF. But as it can be seen, the heat of melting of the PP phase in PP composites is almost independent of the
Conclusion
To investigate the fracture mechanism of polypropylene, which is reinforced with glass fibers, three- and four-point bending tests were performed. The results are summarized as bellow.
- 1
Addition of GF to PP causes an increase in fracture toughness.
- 2
Craze initiation forms at the end of glass fiber and its propagation is dictated by glass fiber.
- 3
Crazes are propagated by moving towards the interface following craze thickening and fiber breakage.
- 4
Observation of damage zone in front of pre-crack and
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