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    There have been many studies in this field.Stachowicz1derived formula for calculating theflattening rate of the cross-sectionwhich consid-ered the hardening exponent and the effect of boostforce.E Daxin2predicted the cross-section flatten-ing of bent tube with the plane-strain assumptionthat the tube diameter was a constant.Howeverthetube bending forming mainly depends on the cir-cumferential and axial flowing of the tube wall ma-terials3so the circumferential strain should not beignored.Liu4established theoretical formula oftube cross-section flattening rate based on the princi-ple of virtual force assuming that the tube was in thestate of free deformation under pure bendingwiththe impact of mold during tube bending neglected.In this paperwith consideration of the tubebending characteristicthree dimensional strain for-mulas have been established to describe the forma-tion mechanism of the tubes flattening.The influ-ence factors of the cross-section are analyzedand acorrection factor is introduced to modify the differ-ent flattening rates of different tube materials.1 Analysis of three dimensionalstrain duringtube bending  The tube is bent around the bending center Owith the bending moment M.A and B are the mi-cro-bodies on the tensile side and the compressionsiderespectivelyFig.1.During the initial stageof bending deformationthe tensile strain1is thefirst principal strain of tensile sideand the com-pressive strain2is the first principal strain of com-pression side.While the circumferential strainandthe radial straintare not obviousthe three-dimen-sional strain state is gradually present with the in-creasing of the degree of bending deformation5.Dis the tube diametert1and t2are the thicknesses ofouter wall and inner wall after deformation.Fig.1 Strain distribution of tube bending The radial displacement of outer wall materialswill appear during the tube bending deformationasshown in Fig.2.Fig.2 Cross-section flattening in tube bending It is assumed that the outer semi-circle is astandard semi-ellipse after the cross-section flat-tened.The three-dimensional strain of any point inthe middle layer of the tube can be described aswhere ais the average radial displacement of anypoint on middle surface of the out wallrmis theaverage radius of the tube cross-sectionRis thebending radiust is the original wall-thickness ofthe tubet is the change of the wall-thicknessisthe circumferential direction angle between anypoint and neutral layer on the cross-section.Since the longitudinal tensile stressreachedmaximum on the tube outer layerthe circumferen-tial stress1=036.Meanwhilebecause the wall-thickness is much smaller than the tube radiustheradial stresstcan be neglected.According to theplastic mechanical total theory2--t=t2t--=2-t-.2Then we can obtain the approximate relationship ofthe longitudinal strain and the radial straint==-12.3Assuming that the tube bending follows the plasticcondition of constant volume+t+=0.4Since the short axis flattening rate of cross-sectionreaches the maximum value in the elbowthe aver-age radial displacement amust be maximum.Sosubstitute Eq.1Eq.3=2and rm=D-t2into Eq.4we obtain the maximum value of aamax=D-t2+8 RD-t.5Finallythe maximum changet1of outerwall-thickness and the maximum changet2of in-ner wall-thickness can be calculated as
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