Characterisation of fatigue crack growth using digital image correlation measurements of plastic CTOD

Crack tip opening displacement (CTOD) was measured using digital image correlation (DIC) and resolved into elastic and plastic CTOD components via an offset compliance technique. The plastic CTOD range gave a single linear correlation for fatigue crack growth rate at stress ratios of 0.1 and 0.6 in compact-tension (CT) specimens of Grade 2 titanium. A sensitivity analysis was performed to find the optimum position in the crack wake to make the CTOD measurements. The results are the first time (to the authors knowledge) that plastic CTOD has been found directly from experimentation and demonstrate that plastic CTOD is a useful alternate characterising parameter for fatigue crack growth to the stress intensity factor defined by Irwin.