Tensile and compressive quasi-static behaviour of 40% short glass fibre - PPS reinforced composites with and without geometrical variations
Introduction
Using Short Fibre Reinforced Composites (SFRCs) in structural components is becoming increasingly popular, thanks to their significantly high strength to weight ratio, their low manufacturing cost, and the possibility to mould them into complex geometries also reaching high production rates.
Polyphenylenesulphide (PPS) is a linear semi-crystalline thermoplastic polymer with repeated thiophenyl units and is widely used as a special engineering plastic offering good high temperature resistance, chemical resistance and dimensional stability. Thanks to its low viscosity, PPS can be moulded with high percentages of fillers and reinforcements, which are able to overcome the material inherent brittleness. These fillers and reinforcements can increase the material strength, surface properties, dimensional stability and electrical properties [1], [2], [3], [4], [5], [6], [7], [8], [9]. Therefore, PPS-SFRCs are widely used to manufacture structural components for the automotive and aerospace fields. In view of this, several papers can be found in the open literature concerning the load bearing characteristics of short fibre reinforced PPS composites. Among those works, worth of mentioning is that of Karger-Kocsis et al. [1] who analysed the relations between microstructure and Mode I fracture toughness of injection-moulded unfilled, short Glass fibre (GF) reinforced, and Carbon fibre (CF) reinforced PPS composites. They first pointed out that the microstructure is highly influenced by the processing conditions as well as by the presence of the impact modifier used, namely a modified Polypropylene. Then, the authors noticed that the Mode I fracture toughness KIC increased by a factor of about 2 when the PPS matrix is reinforced with the 30% wt of fibres, regardless of the fibre type [1]. Mode I fracture toughness tests and the associated damage mechanisms were analysed by Zhang et al [3], by carrying out three-point bending tests on CF-PPS and on CF-PES-C (polyarylether sulphone with cardo side groups) short fibre composites, with different volume fractions. They demonstrated that the main energy dissipation mechanisms were plastic deformation on the matrix, interfacial debonding and fibre pull-out. Concerning the Mode I fracture toughness, the authors noticed that CF-PPS composites experienced higher KIC values with respect to CF-PES-C composites.
Tensile, compressive, flexural and impact properties of 20% wt GF-PPS SFRCs were investigated by Lou et al. [2], with particular reference to the influence of environmental effects, as the high temperature and exposure to hot water for various periods of time, from one day to six months. They found that after an initial rapid drop, the properties levelled off at about 80% of those measured at room temperature.
Vieille et al [5] compared the static mechanical performances of notched and plain epoxy- and PPS- carbon fabric reinforced laminates at room temperature and 120 °C. The notched geometry consisted of a 5-mm-diameter-central hole, and two different lay-ups were studied, namely [0/45/0/45/0/45/0] and [45]7; the damage evolution was investigated by means of the Digital Image Correlation technique. It was found that the PPS matrix accommodated the over stress close to the hole with a higher level of efficiency as compared to the epoxy matrix, thanks to its ductility [5]. Later, Vieille et al [6] proposed a physically-based model, derived from linear elastic fracture mechanics criteria, for the static strength assessment of brittle or quasi-brittle notched laminates, starting from the static strength of plain laminates and their Mode I fracture toughness. The proposed model was successfully compared with the experimental data obtained by carrying out quasi-static tensile tests on quasi-isotropic carbon fibre woven reinforced PPS laminates weakened by a central hole, tested at temperatures higher than the glass transition one, enhancing the matrix ductility and the fracture toughness. Differently, Wang et al. [8] investigated the compressive behaviour of notched and plain carbon woven-ply PPS thermoplastic laminates at different temperatures, focusing the attention also on the associated failure modes. It was found that, in the case of plain specimen, the compressive strength decreased as the temperature increased, due to matrix softening. Concerning the notched specimens with a 6 mm central hole, the authors noticed that the strength decreased slightly compared with that of plain specimens and they justified this result considering the damage mechanisms observed close to the hole, which accommodate the over stresses induced by the notch.
To the authors’ knowledge, comparatively far less attention was devoted to the quasi-static tensile and compressive strength of notched GF-PPS short fibre reinforced composites. The influence of the presence of holes on the load bearing capacity and the chemical corrosion effects of random oriented 40% wt GF-PPS SFRCs composites were investigated by Yilmaz et al [4]. They carried out tensile static tests to study the influence of some geometric parameters (namely the hole diameter, the specimen width and the distance between the centre of the hole and the specimen edge) and the presence of a corrosion environment, like 10% HNO3– and saturated NaCl-solution. It was concluded that the load bearing performance strictly depends on both geometric parameters and environmental factors. In the present paper, instead, the tensile and compressive static strength of GF-PPS SFRCs in the presence of notches are investigated, with the main aim to:
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provide new experimental results useful for engineers involved in the structural design with SFRCs;
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understand the effect of the stress concentration, the loading condition and the fibre orientation angle on the damage mechanisms and the overall strength.
To this end, specimens having different notch geometries and fibre orientations were milled starting from 40% wt. short glass fibre-PPS reinforced injection moulded plates. The choice of obtaining the specimens from an injection moulding plaque, instead of directly moulding them, was driven by the necessity of having a uniform fibre distribution along the specimen axis, not affected by the specimen geometry itself [10]. Moreover, to avoid the influence of some features (i.e. notches) on the polymer flow, which can generate weld lines or fibre non-uniform distribution, the notches were milled using an ultra-precise milling centre. Finally, the machining of small features on SFRCs is supported by the common-practice in Industry when high accuracy and precision are required [11], [12].
Later on, static tensile and compressive tests were carried out on the manufactured specimens with the aim to carefully study the notch sensitivity, the effect of the notch root radius, the fibre orientation angle and the loading condition (tension or compression).
Taking advantage of a digital microscope and a Scanning Electron Microscope (SEM), a thorough damage analysis was carried out, investigating the damage mechanisms both at the macroscopic and microscopic scale, in order to better understand and explain the significant effect of the above-mentioned parameters on the strength of notched components.
Eventually, the experimental data related to the tension tests on notched specimens were re-analysed using a Generalised Stress Intensity Factor approach recently proposed in the literature [13], showing the suitability of such a criterion to summarise the fracture data from notched specimens made of SFRCs.
Section snippets
Material
The material subject of the present investigation was a short fibre reinforced Polyphenylenesulphide, containing 40% wt. glass fibres with a nominal diameter of 10 μm (designation: GF40-PPS), provided by Solvay [14].
Quasi-static tensile and compressive tests were carried out on plain and notched specimens, obtained by machining injection moulded 200 mm × 200 mm plates (see Fig. 1) having 1.8 -mm-thick and 4-mm-thick, as specified later. Injection moulding was carried out by using a Battenfeld
Fibre length distribution and fibre orientation
The fibre length distribution is illustrated in Fig. 6. The diagram plots the fibre lengths divided into 0.4 mm-classes against their frequency. It can be observed that the majority of fibre lengths ranged between 0.16 mm and 0.24 mm. Fibre breakage mechanism is a well-known phenomenon in injection moulding and it is mainly attributed to friction and the high shear rates at which the polymer flow is subjected during plastication and injection, respectively [18], [19]. Quantitative measurements
Failure modes and damage analysis
The observed damage mechanisms are reported in the following sections. The main aim of this damage investigation was to thoroughly analyse the damage mechanisms both at the macroscopic and microscopic scale, in order to better understand and explain the significant effect of the fibre orientation angle and the loading condition on the fracture behaviour exhibited at the macroscale by the tested specimens and presented in the previous section.
Summary of the experimental data in terms of critical Generalised stress Intensity factors
In the previous literature, the prediction of the static strength of notched components made of composite materials has been tackled using different approaches. Without the ambition of being overarching, worth of mentioning here are those based on averaging methods, fracture mechanics, finite fracture mechanics, notch mechanics, and cohesive zone modelling (see, among the others [5], [6], [37], [38], [39], [40], [41], [42], [43] and references quoted therein). The literature on this topic is
Conclusions
The static tensile and compressive strength of notched and plain 40% wt. glass fibre- Polyphenylenesulphide (40GF-PPS) short fibre reinforced composite was analysed experimentally and the associated damage mechanisms were investigated both at the macroscopic and microscopic level. To this end, plain and notched specimens (with notch radius ranging from 0.25 mm to 10 mm and notch depth ranging from 2 to 10 mm) were manufactured with two different fibre orientation angles (as measured with
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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