Fatigue crack growth in selectively reinforced titanium metal matrix composites

Abstract

The fatigue crack growth resistance of a SiC fibre reinforced Ti6Al4V (wt%) composite plate that has been clad on each side with a monolithic layer of Ti6Al4V (wt%) has been assessed. Primarily the growth of mode I through thickness fatigue cracks starting in the monolithic material has been investigated both at room temperature and at a temperature of 400°C. However, at room temperature, as the crack grows towards the fibre reinforced region, fibre matrix interface debonding occurs in the uppermost layer of fibres. Fatigue cracks also initiate and grow transversely in the ‘mode II’ direction, in the matrix ligaments in the uppermost layer of fibres, that is, they grow in the fibre axis direction along matrix ligaments between the fibres. The growth of the mode I fatigue crack also accelerates when such transverse damage occurs and continues until the remaining unreinforced matrix ligament shears back to the growing mode I fatigue crack tip by monotonic ductile rupture. At a test temperature of 400°C transverse crack growth is less severe and there is no ligament of premature ductile shear. Failure is now defined when the growing mode I crack reaches the uppermost layer of fibres. Hole drilling experiments carried out at room temperature show the cladding to be in residual tension. In combination with the stress field ahead of the growing mode I crack and the modulus mismatch between the cladding and reinforced regions, this residual stress promotes premature transverse damage. The behaviour of this material at a test temperature of 400°C, when the residual stress will be reduced, indicates the importance of such residual stresses because now transverse crack growth occurs less readily.