Reverse loading tests of steel tube under biaxial stress states

Biaxial loading and reverse loading tests were performed using seamless carbon steel tubes. Biaxial stress components in the axial and circumferential directions were applied to the tubular specimens using a servo-controlled multiaxial tube expansion testing machine developed by Kuwabara and Sugawara (2013). The tubular specimens were loaded under linear tensile stress paths. Contours of plastic work were measured in the principal stress space, and the differential hardening (DH) behavior was observed; the shapes of the contours of plastic work changed with an increase in plastic work. In addition, small uniaxial tensile specimens were fabricated from the mother tube wall in axial and hoop directions, and tension–compression reverse loading tests were performed to quantitatively evaluate the Bauschinger effect of the test material. Moreover, bilinear stress path experiments were performed to investigate the effects of axial prestraining on the change in the Bauschinger effect; compressive preloading in the axial direction (first loading) was followed by the application of linear stress paths in the first quadrant of the principal stress space (second loading). The measured Bauschinger effect in the second loading was different from those measured in the uniaxial reverse loading tests for the as-received material. The material model will be utilized to improve the accuracy in the numerical analyses of the cold working processes for fabricating steel tubes