High-temperature off-axis fatigue behaviour of unidirectional carbon-fibre-reinforced composites with different resin matrices

The off-axis fatigue behaviour of unidirectional carbon-fibre-reinforced plastics has been studied at high temperature from a phenomenological point of view, with a particular emphasis on the influence of matrix properties. Tension–tension fatigue tests were performed at 100°C on plain coupon specimens of AS4/PEEK and T800H/polyimide for various off-axis angles. The fatigue behaviour of these systems was compared with that of T800H/epoxy for the same loading condition. The off-axis S–N relationships for AS4/PEEK and T800H/epoxy plotted on logarithmic scales were almost linear over the range of fatigue life up to 106 cycles, and such linearity rapidly degraded near 104 cycles for T800H/polyimide. The normalized off-axis S–N curves plotted using the cyclic elastic strain range were separated into two groups associated with fibre and the other off-axis directions, regardless of the composite systems. The strain-range basis plots for the fibre direction of the three kinds of system were almost in agreement. However, the off-axis elastic strain range of AS4/PEEK for 0°<θ⩽90° was significantly higher, compared with the results on T800H/epoxy and T800H/polyimide; the latter two systems showed almost the same strain range. The S–N data plotted on the fatigue strength ratio basis were approximately represented using two straight lines which were associated with fibre and the other off-axis directions over a range of fatigue life, regardless of the systems. An early degradation of the off-axis fatigue performance of T800H/polyimide was due to a distribution of huge resin-rich regions produced during fabrication process. Finally, it was demonstrated that the off-axis fatigue behaviours of these unidirectional composites were favourably described using the classical fatigue failure criteria and a fatigue damage mechanics model developed.