Effects of Notch Depth and Direction on Stability of Local Sharp-Notched Circular Tubes Subjected to Cyclic Bending

Cyclic bending of tubes leads to progressive ovalization of the tube cross-section, and persistent cycling causes catastrophic buckling of the tube. This paper presents the response and stability of 5U5304 stainless steel tubes with local sharp-notched depths of 0.2, 0.4, 0.6, 0.8, and 1.0 mm and notch directions of 0 degrees, 30 degrees, 60 degrees, and 90 degrees under cyclic bending. The experimental results reveal that the moment-curvature relationship first exhibits cyclic hardening and then a steady loop after a few cycles. Because the notches are small and localized, notch depth and direction show minimal influence on the moment-curvature relationship. In contrast, the ovalization-curvature relationship demonstrates an increasing and ratcheting pattern along with the bending cycle, whereas notch depth and direction show a strong influence on this relationship. Finite-element analysis via ANSYS is used to simulate the moment-curvature and ovalization-curvature relationships, and an empirical model is proposed to simulate the relationship between the controlled curvature and number of cycles required to ignite buckling. The experimental and analytical data agree well with each other.