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Consequences of Tight Radius Bending of Steel Plate

June 12, 2014

Structural steel is widely used in the form of plates cold bent into the desired shape for such items as brackets and connectors.  Several standards such as ASTM A6 and ASTM A143 use an inner radius of two times the thickness of the plate as the minimum allowable.  The modification of the steel properties in this case is minimal, as shown by, for instance, A. C. Bannister in the European Steel Technical Report EU22056EN.  But tighter radii can have more severe consequences, since larger amounts of plastic strain are generated both at the inside and outside surface of a tight bend.  This can reduce the fracture toughness of the steel.  Too tight a radius can cause cracks in the outside of the bend during the bending process.  But cracking can also happen at the inside of the bend.  This can happen when subsequent strain relief occurs, such as during post-weld heat treatment or hot-dip galvanizing.  The crack grows when very localized strain relief is favored over more general relief.  For very tight bends, the form of the face of the bending die may have an important effect, since surface grooves imparted to the steel by the die can make the localized strain relief be preferred.