The characterization and mechanical properties of a series of fibrous hybrid composites

Relevant literature on plain and hybrid unidirectional fibrous composites is critically reviewed and the difficulty of assessing previous work due to insufficient data is emphasized. A systematically varied series of hybrid composites based on vinyl ester resin and unidirectional carbon and E-glass fibre reinforcements is studied and the constituent materials characterized. Particular attention is given to the effect of total and relative fibre volume fractions, geometrical arrangements and fibre surface treatments on the tensile characteristics and interlaminar shear strength of the composites. Certain hybrid tensile specimens exhibited what is termed a 'hybrid effect', their first failure strain being greater than the expected failure strain of the parent carbon composites. This is discussed in terms of the data and information obtained from the tensile and interlaminar shear strength tests and from a study of the tensile fracture surfaces. Theoretical models for the tensile failure of hybrid composites are critically examined. The tensile fracture mode and the importance of the statistical nature of fibre tensile strength are discussed. Modifications are made to existing statistical failure theory which result in two equations for the ratio of the lower bound on hybrid composite first tensile failure strain to that on the tensile failure strain of the lower elongation fibre parent composite. Comparison between the two equations enables the prediction of the composite failure mode. Where appropriate the theories are applied to the experimental results. Factors controlling the initial failure strain are shown to be the relative volume fractions and statistical characteristics of the two fibre types, the fibre ineffective length and the stress concentrations acting on fibres adjacent to a failed fibre.