SiC連続繊維強化Ti-24Al-11Nb基複合材料の安定き裂成長過程

抄録

Stable fracture behavior in a Ti-24Al-11Nb matrix composite reinforced unidirectionally with continuous SiC fibers, SCS-6, was studied by using the smooth and notched specimens with different notch length and specimen width. Special attention was paid to the understanding and the evaluation of the relationship between stable crack growth and damage tolerance. It was found that the stable crack growth occurred according to the following procedures: (i) first matrix cracking and/or interface debonding near the notch tip, which were induced and promoted by the residual stress and the β-phase depleted zone near the interface, (ii) the increase of density of matrix cracking, resulting in crack face bridging by intact fibers, (iii) the break of some bridging fibers, and finally (iv) unstable fracture, accompanying with the coalescence of cracks and with fiber pull-out. A significant increase in fracture resistance with stable crack growth was observed, in which fiber bridging was found to play the most important role. Not only the critical stress intensity factor but also the critical J-integral value at which unstable fracture occurred were inadequate as fracture criteria, because they did not show a unique value, or varied with the initial crack length, a_i and with the relative ratio, a_i/W. But instead, the significance of the slope in R-curve, which must give the developing rate of fiber bridging with stable crack growth, was identified as a useful material parameter which represents the degree of damage tolerance of composites.