In high toughness ductile materials crack initiation is generally followed by a region of stable cracking. Such regions have been conveniently characterized in terms of crack growth resistance curves. Both J-integral and crack tip opening displacement (CTOD) concepts are successfully used to describe ductile crack growth using J-Aa and CTOD-Aa R-curves, respectively [1-6]. The current trend is to determine both the crack initiation toughness (JI ,6.) and crack growth toughness (tearing modulus or crack tip opening angle) from the slngle R-curve. While the former is obtained at the intersection point of the R-curve with the theoretical blunting line, the latter is given by the average slope of the regression line. However, the use of CTOD in the R-curve analysis has been limited in comparison to the J-integral. Unless directly measured by replica technique [7], CTOD is generally obtained following standard clip gauge method as given in BS 5762 [8] where