Static and fatigue properties of UD carbon fibre composites at 77 K

The static longitudinal strength of unidirectional CFRP is determined by the fibres selected. The static shear strength (torsion of thin-walled, hoop-wound tubes), however is controlled by the fibre/matrix combination. Highest strengths are achieved with a tough, thermoplastic matrix (PEEK) which yields an optimum fibre/matrix bond (AS4/PEEK). By contrast, the fatigue live of UD-composites, both under cyclic or shear threshold loading, is determined by matrix-controlled damage mechanisms. A progressive damage mechanism and long fatifue lives are observed for composites with rigid duroplastic matrices (epoxies). Unfavourable shear fatigue properties of the thermoplastic PEEK-matrix may probably lead to premature failure of the composite. It will be shown that the shear properties of the matrix, i.e. the load transmission between two adjacent fibres, predominate under both types of loading. Hence, the fatigue limit is a matrix controlled property