Failure mechanism and forming limit of tube axial compressive process

Available online at www.sciencedirect.comsCIENCE@oIRECT.Transactions ofNonferrous MetalsSociety of ChinaScienceTrans. Noferrous Met. Soc. China 16(2006) s785-s790Presswww.csu.edu.cn/ysxb/Failure mechanism and forming limit of tube axial compressive processSUN Zhi-chao(孙志超), YANG He(杨合)School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, ChinaReceived 10 April 2006; accepted 25 April 2006Abstract: Based on minimum energy principle for plastic forming, tearing and buckling failures mechanisms are explored andcriteria for them are developed by theoretical analysis and experiment. Combined with finite element software developed forminglimit and effects of process parameters on failures are investigated and proper parameters for stable forming are determined. Theresults show that: 1) The failures and forming limit are mainly determined by geometry and materials parameters of tube blank, filletradius or half conical angle of die. For the process under fllet die, there exists a maximum fllet radius within which no tearingfailure happens, and a maximum radius and a minimum radius range within which no buckling failure happens. For the process underconical die, there exists a maximum and minimum half conical angle range within which no tearing and buckling failures occur.2) For both forming processes, the higher the value of material strain hardening exponent or the lower the value of relative thickness,the more impossible for tearing and buckling failures to occur, and the larger the ranges of fllet radius and half conical angle. Theexperiment results verify the reliability and practicability of this research.Key words: tube axial compressive forming; forming limit; tearing failure; buckling failurewith finite element simulation[8] forming limit and1 Introductioneffects of process parameters on failures are investigatedand reasonable ranges for process parameters to realizeTube axial compressive, as shown in Fig.l, is anstable forming are discussed.advanced .formingprocess for manufacturingcomplicated thin-walled parts with high strength/mass2 Tube axial compressive process and failureratio, high efficiency, and good flexibility for sizemodeschanging[1, 2]. However tube axial compressive is a caseof forming behavior between flaring and bucklingFig.1 shows typical tube axial compressive formingfailures, only meeting certain forming condition can theprocess under conical and fillet die， in which a .tube curl to form double-walled tube. There exist tearingthin-walled tube, axially compressed over the die,and buckling failures concerning forming limit duringtransforms from zone ab into three zones bc, cd, and de,the process, which have an essential effect on then order. The process may be developed further tsuccessful reality of forming process, the size anproduce inside-out inverting-forming and to obtain aprecision degree of part formed[3- 6]. Some research iscomplicated part such as the double-walled tube on thedone on the tube axial compressive process anright in Fig.l. The tube blank can be divided into threedeformation mechanisms by experiment, theoreticalzones such as pre-deforming zone, deforming zone, andanalysis[2- 7] and FE simulation[8, 9] Also the impactdeformed zone. During the process the pre-deformingenergy absorption characteristics and fracture in dynamiczone undergoes press and deforming zone undergoes pull,loading[10- 12] are investigated, but the report andso the tearing and buckling may occur as shown in Fig.2.research work on failures and forming limit of thisprocess are relatively lack. So in this paper tearing and3 Failure mechanisms and criteriabuckling failures mechanisms are explored and criteriafor them are developed for tube axial compressive3.1 Tearing fail中国煤化工onforming under both fllet and conical dies. CombinedDuring the:MYHCNM HGnsion formingCorresponding author: SUN Zhi-chao; Tel: +86- 29-884602 12-802; Fax: +86-29-88495632; E-mail: zcsun@nwpu.edu.cn.s786SUN Zhi-chao, et al/Trans. Nonferrous Met. Soc. China 16(2006)n+1-nrp + (doto/4)n+2(rp)n+2 <[do -to)e”W50.0C..-d]/2(1)Pre-deformingFor the process under conical die we havezonedo1-cosa--sin2 a>2o.Deformed/zone2√todoJ]2n+2 (n+2)(2)Deforming+Vl.(n+1)(√3n)"(do -to)- doa> cos~1-+√-2n(1-n/2-N),(3)(a)N=[-2v/tod0121n+2(n+2) (4)∠∠“50H0N59 (do -to)-do0|' Pre-deforming3.2 Buckling failure mechanism and criterionDeformed zoneexceeds tube stable limit, the buckling failure may occur.The buckling failure is affected by tube and die geometryDeforming zoneparameters, material parameters and contact condition,for the given tube blank die geometry parameters are keyfactors. Based on the energy method the bucking wave ismodeling and the critical press Por is deduced as follows:b)πσoFig.1 Sketch of tube axial compressive forming: (a) Tube axial(13/2121013/2+7号compressive forming under fllet die; (b) Tube axial(5)compressive forming under conical dieFor the stable forming process without buckling it isnecessary to meet P