EFFECTS OF PREEXISTENT CRACK IN DOUBLE AND GRADIENT COATINGS ON THE CRACK EXTENSION INTO FIBER AND INTERFACIAL DEBONDING

When a crack is formed on a fibre surface by the premature fracture of the coating, crack extension into the fibre or interfacial debonding between fibre and coating occurs, affecting the strength of the coated fibre. In the present work, the influence of pre-existent crack in do uble and gradient coating layers on the crack extension and interfacia l debonding was studied to find the condition to improve the strength of the coated fibre. It is shown that, in both types of coating, (i) t he energy release rates for crack extension into the fibre and for int erfacial debonding become low when the inner coating portion adjacent to the fibre has low Young's modulus, while they become high when the inner portion has a high Young's modulus, and (ii) the ratio of the en ergy release rate for debonding to that for crack extension into the f ibre is approximately 0.3. These results suggest that the reduced fibr e strength by crack extension into the fibre in the case of strong int erfacial bonding can be raised by double and gradient coatings with re duced Young's modulus of the inner coating portion. Alternatively it c an be increased by weakening the interface so that the critical energy release rate for debonding is less than 0.3 times the critical energy release rate for crack extension into the fibre.