Abstract
A simple testing device for evaluating the formability of metal tubes is proposed. In this testing device a tubular specimen is subjected to internal hydraulic pressure to bulge freely with both ends sealed simply using O-rings; thus it bulges under a near uniaxial tension stress state in the circumferential direction. Formability tests using the hydraulic bulging device are performed for eight kinds of electric resistance welded tubes with different mechanical properties, thickness and manufacturing processes. The manufacturing processes of the tubes are classified into three groups: as-rolled, as-rolled and normalized and cold-drawn and normalized. It is found that the uniformity of initial thickness distribution of the tubes significantly depends on the manufacturing process and that the more uniform the thickness, the higher the formability (the limit strain in the circumferential direction) of the tube. The cold-drawn and normalized tubes exhibit anomalous bulging behavior; they do not bulge axisymmetrically and do not burst at the center of the tube. The anomalous behavior of the cold drawn and normalized tubes is successfully reproduced by a FEM analysis taking account of the inhomogeneous thickness and work hardening characteristics of the tube.
