Experimental determination of crack tip angles (CTOA) and their applicability to instability prediction in structures

Crack tip opening angles (CTOA) of growing ductile cracks in structural steels were measured from grids printed on the specimen surface. The grids, with underline spacing if 127 micrometres, were printed on the specimen surface using a photographic technique. The pre–cracked specimens were tested in three–point bending, to increasing values of ductile crack growth, in order to establish a J–R curve. The crack tip opening angles were measured from the deformed grids during the tests. The measured values of CTOA correlated well with the analytical calculations based on the theory proposed by Rice and coworkers. A method is suggested to predict instability in structures based on CTOA.