Material Aspects of Tube-Hydroforming

Failure in internally pressurized bent tubes

“…[A1] and [1], the differences between these analyses are minor and, realistically, indistinguishable. Thus, for clarity, at a strain of 0.10, only the straight-tube analysis is shown.…”

“…Since deformation in the bent section occurs in the region of the tube where the wall thickness is unchanged from that of the as-received tube, and since the tube geometry is unchanged by bending, the only factor dictating the deformation site is the wall strength (Eq. [1]). Obviously, the weaker, undeformed wall in the as-received tube deforms at a lower stress and, hence, lower pressure than the stronger prestrained wall in the bent tube, and, hence, deformation would naturally initiate in the straight, undeformed tube section.…”

“…[1] Even before the hydroforming process itself, the tube is fabricated by deforming flat-rolled product by bending it into an "O" shape, welding the edges together, and then further deforming it to develop the desired size and ovality. Thus, the tube-wall properties differ from those of the flat sheet from which it was fabricated and also vary through the thickness.…”

“…[1] There is continuing interest in modeling this process and in performing post mortem analyses of failed processes to understand whether these failures are more related to material characteristics or process complexity. One element of any modeling or analysis process is to relate the material failure to its forming-limit curve.…”

FEM analysis of tube pre-bending and hydroforming

“…To make it even more complicated, for THF processes with an internal pressure as high as 300 MPa or so, the friction effect is large. There is also the important variable of the tube material properties [6]. Sometimes, complicated hydroformed structural parts need a pre-forming operation like pre-bending.…”