On the relationship between crack initiation toughness and crack growth toughness

Abstract

Prior studies of the relationship between the crack initiation toughness (characterized by the critical value J Ic of the J integral and the crack growth toughness (characterized by the tearing modulus T R), in a variety of microstructures representing very different compositions, have suggested that an increase in the value of the former parameter generally leads to a monotonic increase in the latter. In this work, some new results, derived from controlled experiments on the effect of composition and microstructure on J Ic and T R are presented for a number of high purity alloys belonging to the AlLiCuZr system. It is shown that microstructural characteristics, which are generally considered to be detrimental to fracture initiation toughness J Ic can lead to a substantial improvement int he tearing modulus T R. The underaged tempers of the material with the highest lithium content show the largest resistance to crack growth (as indicated by T R), even though their crack initiation toughness J Ic values are the lowest among the underaged tempers of alloys in the same AlLiCuZr system exhibiting “globally similar” failure mechanisms. We rationalize such results on the basis of microstructurally induced variations in crack path.