Observation of 3D shape and propagation mode transition of fatigue cracks in Ti–6Al–4V under cyclic torsion using CT imaging with ultra-bright synchrotron radiation

In the present study, micro computed tomography (μCT) imaging by using ultra-bright synchrotron radiation X-ray (SR μCT), which is generated at a facility located in Japan and called SPring-8 (Super Photon ring-8 GeV), is used to observe torsion fatigue crack propagation behavior in round bars. It is well known that the transition of crack propagation from the shear mode to the tensile mode takes place in the above case. The transition condition, however, was not discussed in detail in the past, since the crack growth behavior under the surface cannot be provided precisely by the conventional techniques, such as optical microscope, scanning electron microscope, and atomic force microscope. A compact torsion fatigue-testing machine has been recently developed to conduct fatigue tests and measurements of SR μCT concurrently at Spring-8. Therefore, the shape of torsion fatigue cracks can be evaluated quantitatively and nondestructively, and the transition of crack propagation from the shear mode to the tensile mode can be observed three-dimensionally. The condition of the transition of crack propagation is discussed by considering the three-dimensional shape of cracks.