A system is developed to automatically measure crack length and crack opening load in real-time without changing any testing condition, by using an unloading elastic compliance method. The performance of the developed system was verified through constant amplitude crack growth testing. The system is a very simple system composed of a personal computer, an analogue electrical subtraction circuit, a stepping motor, a stepping motor driver, a PIO board and applications software. Crack closure and growth behaviour of surface fatigue cracks in 7075-T651 aluminium alloy is investigated under pure bending loading, by using the automated measurements system. Special attention is focused on the difference of closure behaviours between through-thickness cracks and surface cracks, between bending loading and axial loading and between at the maximum crack depth point and at the surface intersection point. The crack opening ratio at the maximum crack depth point of the surface cracks is 17% greater than the crack opening ratio of the side-grooved through-thickness cracks. The crack opening ratio is 12% greater at the maximum crack depth point than at the surface intersection point. This result is the same as the result under axial loading. The crack opening ratio of the surface cracks under bending loading is greater than under the axial loading in both directions. A model is suggested to predict the surface fatigue crack growth under bending loading. The prediction model provides conservative estimation for fatigue life within factor of two. The crack geomerty prediction errors between predicted result and experimental data are less than 5%.
