Damage assessment and lifing of continuous fibre-reinforced metal-matrix composites

Abstract

The fatigue response of a continuous silicon carbide (SCS-6) fibre-reinforced Ti-6Al-4V metal-matrix composite in the presence of a sharp precrack has been studied in single-edge notched test-pieces in bending. Crack growth rates have been measured for different values of span-to-width ratio ( s/W) at ambient temperature and at a test temperature of 550°C in air by the use of a direct current potential difference technique. It was found that in most cases the observed crack growth rates initially decreased with increasing crack length (and hence increasing nominal applied stress intensity range). Effects of frequency on fatigue crack growth rates at 550°C in air have also been identified. In general, crack growth rates are increased at 550°C only at low frequencies, relative to the crack growth rates measured at ambient temperature. Based on observations to date it has been shown that fatigue cracks grown at a large span-to-width ratio propagate to failure more rapidly than cracks grown at small span-to-width ratios for equivalent initial nominal stress intensity ranges. Metallographic sections through the composite indicate that the improved fatigue life observed at low values of s/W ratio may be attributable to debonding at the fibre/matrix interface, which is deduced to delay the onset of fibre failure.