Determination of the thicknesses of pipe elbows on the basis of expressions resulting from EU-Directive

The relations to calculate the maximum value of relative strains, which occur in a process of bending of tubes on benders, in stretched layers of tubes, are presented in this work on the basis of EU-Directive concerning production of pressure equipment. It has been shown that for large deformations that occur during bending of the pipes on knees, logarithmic strain measures (real) and relative strain measures give different values of the relative distribution of strain and different wall thicknesses in the bending zone. Reverse expressions were also derived to calculate the initial wall thickness of the tube to be bent, in order to obtain the desired wall thickness of knee after bending.