Failure analysis carried out by various researchers has shown that fracture models based on the Crack Tip Opening Angle (CTOA) criterion which have been calibrated based on data from large C(T) and M(T) specimens can be transferred successfully to cracked aircraft fuselage structures and other low constraint structures for residual strength predictions [
]. One major difficulty with this method is the experimental measurement of CTOA [
] either in real structures or in a laboratory scale test. This could be the limiting factor that prevents the more extensive use of this promising fracture parameter. There are a number of ways to obtain CTOA data but the most appropriate and reliable method has yet to be agreed.