Composite is a synergistic combination of two or more micro-constituents that differ in physical form and chemical composition and are insoluble in each other. Composite materials have successfully substituted the traditional materials in several light weight and high strength applications. The reasons why composites are selected for such applications are mainly their high strength-to- weight ratio, high tensile strength at elevated temperatures, high creep resistance and high toughness. Typically, in a composite, the reinforcing materials are strong with low densities while the matrix is usually a ductile or tough material. If the composite is designed and fabricated correctly it combines the strength of the reinforcement with the toughness of the matrix to achieve a combination of desirable properties not available in any single conventional material. The strength of the composites depends primarily on the amount, arrangement and type of fiber and/or particle reinforcement in the resin.
Polymers are classified broadly into two groups: thermoplastics and thermosets. The most commonly adopted thermoplastics include polypropylene (PP), polyethylene and poly vinyl chloride (PVC); while phenolic resin, epoxy and polyester resins are some of the most used thermosetting polymer matrices. Particulate filled polymer composites are being used extensively in various fields due to their low production costs and the ease with which they can be formed into complex shapes. Besides, they behave isotropically and are not as sensitive as long fiber composites to the mismatch of thermal expansion between the matrix and the reinforcement [100, 101]. Generally, particulate fillers are used in polymers for a variety of reasons such as cost reduction, improved processing, density control, optical effects, thermal conductivity, modified electrical and magnetic properties, flame retardancy, improved hardness and wear resistance. Hard particulate fillers consisting of ceramic or metal particles and fiber-fillers made of glass are being used these days to improve the performance of polymer composites to a great extent [102]. Various kinds of polymers and polymer matrix composites reinforced with metal particles have a wide range of industrial
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applications such as heaters, electrodes [103], composites with thermal durability at high temperature etc. [104]. Similarly, ceramic filled polymer composites have also been the subject of extensive research in last two decades. When silica particles are added into a polymer matrix, they play an important role in improving electrical, mechanical and thermal properties of the composites [105, 106]. The mechanical properties of particulate filled polymer composites depend strongly on the particle size, particle-matrix interface adhesion and particle loading. Sumita et al. [107] underlined the interest of replacing micro-scale silica by its nano-scale counterpart, since nano-scale silica particles possess superior mechanical properties. Smaller particle size yields higher fracture toughness also for calcium carbonate filled high density polyethylene (HDPE) [108]. Similarly, epoxy filled with smaller alumina tri- hydrate particles shows higher fracture toughness [109]. Thus, particle size is being reduced rapidly and many recent studies have focused on how single- particle size affects mechanical properties [110, 111].
Yamamoto et al. [112] reported that the structure and shape of silica particle have significant effects on the mechanical properties such as fatigue resistance, tensile and fracture properties. Nakamura et al. [113, 114] discussed the effects of size and shape of silica particles on the strength and fracture toughness based on particle-matrix adhesion. Usually the strength of a composite strongly depends on the stress transfer between the particles and the matrix [115]. For well-bonded particles, the applied stress can be effectively transferred to the particles from the matrix resulting in an improvement in the strength. However, for poorly bonded micro-particles, reduction in strength is found to have occurred. Nicolais and Nicodemo [116] studied the effect of particle shape on tensile properties of glassy thermoplastic composites. While most of these investigations have focused either on the particle shape or on particle size, the study made by Patnaik et al. [117] reported that the mechanical properties of polyester based hybrid composites are highly influenced also by the type and content of the filler materials. Padhi et al. [118] reported on processing,
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characterization and wear analysis of short glass fiber-reinforced polypropylene composites filled with blast furnace slag particles. They also predicted and simulated the erosion wear behavior of these composites. Tagliavia et al. [119] reported analysis of flexural properties of composites filled with hollow particles. They studied the flexural properties of hollow-glass particle filled vinyl ester composites, which are used in marine applications. Weidenfeller et al. [120] made a detailed study on cooling behaviour of particle filled polypropylene composites during injection molding process. Hassan et al. [121] studied morphological and mechanical properties of carbonized waste maize stalk as reinforcement for eco-composites. Omar et al. [122] investigated on the particle size dependence on the static and dynamic compression properties of polypropylene/silica composites. Recently, Gupta and Satapathy [123] have reported the processing, characterization and wear analysis of borosilicate glass microsphere (BGM) filled epoxy composites.
Lauke and Fu [124] reported the theoretical modeling for the fracture toughness of particulate-polymer composites by considering a simple geometrical model of particle-particle interaction in a regular particle arrangement. They also discussed the influence of structural properties such as particle volume fraction and matrix mechanical properties on fracture toughness. Jerabek et al. [125] studied filler/matrix-debonding and micro-mechanisms of deformation in particulate filled polypropylene composites under tension. In their approach, they introduced a novel method for the detection of debonding using volume strain measurements, which takes into account the dilatational and deviatoric behaviour of the neat matrix polymer and the composite. Bishay et al. [126] studied electrical, mechanical and thermal properties of polyvinyl chloride (PVC) composites filled with aluminum powder. Agrawal and Satapathy [127] developed a heat conduction model and investigated on thermal conductivity enhancement of AlN/epoxy composites. They further investigated thermal and dielectric properties of epoxy and polypropylene reinforced with micro-sized AlN particles [128].
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