Experimental evaluation of shielding effect on growing fatigue cracks under overloads using ESPI

• First time that ESPI is used to quantify the effect of shielding and overloads.
• Crack opening loads at the overload event proportional to the overload magnitude.
• Post-overload retardation of crack growth proportional to the overload magnitude.
• CJP model useful in fracture mechanics when plasticity plays a significant role.
• Results remove some controversy associated with crack closure by quantitative data.

The effect of single cycle overloads, ranging in size from 10% to 50%, on fatigue crack growth behaviour in compact tension specimens subject to constant amplitude loading with an R-ratio of 0.6 has been investigated using electronic speckle pattern interferometry. The resultant displacement fields have been used to evaluate crack opening and closing loads and effective stress intensity factors using the Christopher–James–Patterson model that takes explicit account of the effect of crack tip and wake plasticity on the singularity-dominated elastic fields surrounding the crack. The results show that the crack opening loads and effective stress intensity factors at the overload event are proportional to the magnitude of the overload and that period of post-overload retardation of crack growth is also proportional to the magnitude of the overload. These findings demonstrate the usefulness of the CJP model and should help enhance understanding of plasticity-induce closure phenomena.