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The Effect of Cold Roller-Bending on the Mechanical & Geometric Properties of Steel Hollow Sections / Jonathon Richards
Swansea University Author: Jonathon Richards
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DOI (Published version): 10.23889/Suthesis.52473
In recent years there has been a significant increase in the use of steel hollow sections; in particular, where applications require a more aesthetic appeal.  As advances have been made in manufacturing and technology, the cost of hollow sections has decreased, however; it is still greater than t...
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In recent years there has been a significant increase in the use of steel hollow sections; in particular, where applications require a more aesthetic appeal.  As advances have been made in manufacturing and technology, the cost of hollow sections has decreased, however; it is still greater than that of hot-rolled open sections, such as Universal beams (I-beams). However, despite having a greater cost per tonne than open sections, hollow sections are being utilised increasingly due to a number of properties that make them an obvious choice for both aesthetically pleasing and utilitarian structures alike. Tata Steel Tubes is one of Europe's leading manufacturers of welded steel tubes and pipes. They produce many thousands of tonnes of hot-finished and cold-formed tubes each year that are used in a diverse range of applications including the construction sector. Another key sector is energy & power which relies heavily on conveyance tubes. This project investigates the cold-roller-bending of plain carbon steel hollow sections for use in construction applications. The process of cold-roller-bending is widely considered to be an art rather than a science and therefore relatively little is known about the effects of the process on the physical and mechanical properties of the steel. In many cases it is assumed to be negligible and therefore ignored by the standards and many of the guidance notes. Thus, the aim of this exercise was to determine and compare the effects of the cold bending process on the properties of hot-finished and cold-formed, plain-carbon steel, high-frequency induction (HFI) welded tubes. There is no current EU, British or American standard that specifies changes in the mechanical properties brought about by the bending process or a maximum allowed curvature. This is because the properties are considered to largely remain the same as the original unbent material and that any alterations in these properties are made up for by the shape of the section.– This project refutes this assumption. Hot-finished and cold formed hollow sections were curved using roller-bending equipment to a range of strains with increasing curvature. The project has examined all of the requirements outlined by a structural engineering calculation, including the tensile strength, impact resistance, hardness and geometric resistances of the cross section. This has been performed in the hope of determining the most suitable section for the cold-bending process. The testing has determined that the process of cold-roller bending can be detrimental to many of the properties detailed above. All of the mechanical, mechanical and geometric properties examined in the experimentation were affected in some way. For example, the cross-sectional geometry of the section can be significantly altered by the bending process as the stresses involved distort the cross section of the tube. This leads to differences in the geometric properties when compared to the original unbent section. These properties include the sections resistance to bending and torsion. The tensile properties of the sections are also affected. During testing it was determined that cold-roller bending of steel leads to significantly reduced yield strength where the material is in compression during bending. This reduction in the elastic capacity of the section has been determined to have been caused by the Bauschinger effect; a phenomenon that occurs in many metals that leads to a reduction in capacity to resist stress and therefore yield stress of a section is reduced. The effect is observed when the section has underdone pre-strain beyond the materials yield point in the opposite direction to the current deformation. Further investigation around the Bauschinger effect revealed that it occurs when the section is first elongated and then compressed as well as where the section is compressed before elongation. For a number of applications, in Civil design, the plastic properties are not normally considered as a structure is not commonly designed to operate within its plastic region when in service. However, sections that have been curved using cold-roller bending will yield at significantly reduced stresses and therefore the plastic properties should be considered, particularly if the section is being used in a position where the bending loads are being reversed (from that of bending), for example, as an archway. Furthermore, this reduced capacity to resist bending will lead to spreading of the archway when loaded from above. This will lead to increased lateral load being exerted on a support structure which must be considered when designing a structure using curved steelwork. It was observed that both the hot-finished and cold-formed sections experienced a similar reduction in the yield strength during tensile testing (at the intrados) however; hot-finished sections regained their properties with further cold deformation more rapidly than the equivalent cold-formed product. This observation, coupled with improved impact resistances and superior geometric properties lead to the conclusion that hot-finished sections are considered to be more suitable for the cold-roller bending process.
A selection of third party content is redacted or is partially redacted from this thesis.
Roller-bending, Bauschinger, Tubes, Hollow sections
Faculty of Science and Engineering