In order to predict the flattening rate of the cross-section accurately during the tube bending,the generation principle,the solution and the influence factor of the cross-section flattening were studied.On the basis of the plane-stress and the assumption that the plastic volume is constant,three-dimensional strain formulas were established in consider of the cross-section flattening.Considering the wall-thickness change,the approximate calculation formulas of short axis flattening rate were deduced,with the outer diameter and the inner diameter as parameters.Because different materials have different cross-section flattening rates,a material correction factor was introduced to modify the formula based on experiments.Finally,the validity of the theoretical formulas was proved according to the calculation and the experiment results,which can provide a reference for the forming quality prediction in tube bending.
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