In this paper, the degradation and failure of local sharp-dented 6061-T6 aluminum alloy tubes with different diameter-to-thickness ratios subjected to cyclic bending were investigated. Three different diameter-to-thickness ratios of 16.5, 31.0 and 60.0 were considered. For smaller diameter-tothickness ratios, the moment-curvature relationship rapidly became a steady loop from the first bending cycle. The dent depth had almost no influence on the moment-curvature relationship. As for the ovalization-curvature relationship, it exhibited an increasing and ratcheting way along with the number of bending cycles. A larger dent depth in the tube caused a larger ovalization. Moreover, a larger dent depth led to a more asymmetrical appearance of the ovalization-curvature relationship. In addition, for a certain diameter-to-thickness ratio, five non-parallel straight lines corresponding to five different dent depths were discovered for the controlled curvature-number of cycles required to ignite a failure relationship on a log-log scale. Finally, a theoretical model was proposed in this study for simulating the aforementioned relationship. It was found that the experimental and simulated results agreed well.