The fiber reinforced plastics (FRP) are now available for increased use in structural materials. FRP are, however, generally characterized by a considerable scatter in mechanical properties. Understanding the sources and nature of these variabilities is the key to better utilization of these composite materials. This paper deals with the two typical types of glass fiber reinforced composites (GRP) which are reinforced by plain-woven glass cloth and chopped strand glass mat. The main results obtained in this work are summarized as follows;
(1) The mixture law of strength is applied to the modeled laminate which consists of three parts; the first part is the unidirectional layer aligned parallel to the load direction, the second is the layer which does not contribute to tensile strength and the third is the matrix layer. The mixture laws of the average and the variation of tensile strength are developed under an assumption that the fracture strengths of the first and third layers and the fiber volume content are random variables.
(2) The average and the coefficient of variation of fracture strength in the first and third layers can be obtained by use of the bundle theory and the weakest link theory. The coefficient of variation of fiber volume content is given by the combination of the coefficient of variation of volume of reinforced material and that of thickness of laminate.
(3) The calculated results of the average of fracture strength are found to agree well with the experimental whether each of the random variables are independent or not. But the coefficient of variation of strength is dependent on the correlation of each variables.