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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy at Loughborough University.