Unsaturated polyester (UP)/glass fiber/clay composites were prepared by hand layup method. The effect of clay loading on the morphological and mechanical properties of UP/glass fiber composites was investigated in this study. X-ray diffraction (XRD) was used to characterize the structure of the composites. The mechanical properties of the composites were determined by tensile, flexural, unnotched Charpy impact and fracture toughness tests. XRD results indicated that the exfoliated structure was found in the composite containing 2？wt% of clay while the intercalated structure was obtained in the composite with 6？wt% of clay. The tensile strength, flexural strength, and flexural modulus of the composites were increased in the presence of clay. The optimum loading of clay in the UP/glass fiber composites was attained at 2？wt%, where the improvement in in tensile strength, flexural strength, and flexural modulus was approximately 13, 21, and 11%, respectively. On the other hand, the highest values in impact toughness and fracture toughness were observed in the composites with 4？wt% of clay. 1. Introduction Glass fiber reinforced composites have become attractive structural materials not only in weight sensitive aerospace industry but also in marine, armor, automobile, railways, civil engineering structures, sport goods, and so forth. This is attributed to high specific strength and specific stiffness of the glass fiber reinforced composites. Unsaturated polyester (UP) is one of most commonly used polymer matrix with reinforcing fibers for advanced composites applications due to its low cost, easy handling, rigid, resilient, flexible, corrosion resistant, weather resistant, and flame retardant. Montmorillonite clay has received much attention as reinforcing materials for polymer because of its potentially high aspect ratio and unique intercalation/exfoliation characteristics. The small amount addition of montmorillonite clay into polymer matrix exhibits unexpected properties including reducing gas permeability, improved solvent resistance, being superior in mechanical properties and thermal stability, and enhanced flame retardant properties [1]. Typically, clay minerals have a layered silicate structure about 1？nm in thickness and high aspect ratio ranging from 100 to 1500 [2]. Glass fiber is a part of reinforcing materials for reinforced polymer based on single filaments of glass ranging in diameter from 3 to 19 micrometer. Glass fibers show good performance and play a main function in playground equipment, recreational items, piping for corrosive chemicals,
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