Bendability Assessment of Double-Walled Tubes With Additively Manufactured Core Structure

Abstract Double-walled tubes offer significant advantages such as leak prevention, thermal insulation, corrosion resistance, and the ability to withstand high pressure, making them essential in the oil and gas, chemical, aerospace, and marine industries. Major manufacturing challenges arise when bending is required, e.g., for applications like heat exchangers. This study focuses on bending double-walled tubes, made up of E235 steel for both the outer and inner tubes (cold drawn and seamless), with an additively manufactured 316L stainless steel core structure serving as a spacer or core. The pure bendability of these tubes is investigated through experimental and numerical methods. The bending moments from experiments align well with numerical results, establishing the minimum bending radius for different tube and core combinations. As the ratio of the outer-tube-diameter-to-core-thickness increases, so does the minimum bending radius. Tube hydroforming is also employed, to improve the assembly before bending. The burst pressure is determined for each tube–core combination. A key factor affecting the burst pressure is the corner radius of the core structures. The experiments show that hydroforming has the potential to enhance the bendability of double-walled tubes. Numerical simulations indicate that combining hydroforming with simultaneous bending leads to a significantly improved minimum bending radius.