JOURNAL OF SCIENCE & TECHNOLOGY • No 94 - 2013 2D SIMULATION OF INTERNAL FRACTURE DURING HOT CROSS-WEDGE ROLLING PROCESS MO PHONG SO 2D HIEN TU'ONG PHA HUY VAT LI$U TRONG CONG NGHE CAN NEM NGANG HaiD.V*, Tuan V.V, GiangN.T Hanoi University of Science and Technology- Vietnam Received March 08, 2013; accepted April 27, 2013 ABSTRACT Internal fracture, known as Mannesmann effect, is a phenomenon regularly occurring i forming processes such as rotary tube piercing, cross-wedge rolling, etc It is the mam reas reduced production quality and precision Although there have been a number of publications topic, the scientific knowledge about fracture mechanisms leading to the phenomenon is still this study, the Johnson-Cook damage model is applied to investigate the internal material frac 2D cross-wedge rolling process The fracture mechanisms related to the Mannesmann effec discussed in this adicle Besides, tt is also shown that the rolling velocity highly affects the app of sudace defects and the occurrence of internal fracture in the billet Keywords: Internal fracture, FEM, Johnson-Cook Model, Cross Wedge Rolling TOM TAT Hieu dng Mannesmann la hipn tupng fgo cic khuyit tit rdng t^i tam cOa sin phim tron trinh can ngang, cin nem ngang, va day li nguyin nhin lam anh hwang l&n t&i ca tinh cua si Do vi$c nghien cwu ca chi hinh thinh khuyit t$t t^i tim phdt li rat quan trpng Da cd mdt s6 ciru nhim giii thich vin di nhien, nhOng til li$u, cdng bo vi ca chi phi buy vit Hiu d Crng Mannesmann giy van cdn han chi Chinh vi viy nghien ciiu nay, md hinh John da duvc stf dung di dt/bio trwacsi/phi ht^y vit liiu hiiu i>ng fvlannesmann gay qu cin nim ngang tren md hinh 2D Ca chi gSy nen cic hiin tupng phi buy v$t lipu dupc phin tich nghiin ciru niy Benh canh dd, kit qui nghiin ciru cOng cho thiy, toe dd cin cd inh hu&ng l&n hinh thinh va phat triin pha huy tren be mit cQng nhw t^i tim cua vit cin Introduction , * J J *u J If In recent decades, the cross-wedge rolling /i-ô/D\ u ã u • A J (CWR) technique has greatly improved and _ , *I c • emerged as an innovative metal forming technique because of many advantages such as high productivity, material saving, better strength characteristic and simplicity for process automation Because of the complexity of forming and kinematics of the material flow in CWR, numerical simulation procedures have not been clearly established yet Thus, experimental research is still dominant in this field Besides the lack of accurate theories, control of the process to obtain products without defects, such as void and crack initiation [1] is difficult Furthermore, during the manufacturing process, internal fracture or Mannesmann effect (ME) may occur in the workpiece However, this phenomenon is not fully understood either Therefore, analyzes of the ME occuiring in CWR have always received much attention Man> experimental studies have attempted to explain this effect through r• ^ •, r, • m mechanisms of materia fai ure Smimov [i] , • • • , • ^ JL claimed that the cavity formation was caused o) , , , , • f^l^ and tens.le stresses ,ns,de the xvorkpTO, Other studies also pointed ottt that exces« '^"="'^ ="^''== combined with low cycle fatiglt T"f P™"'°""8 * e growth of micro-cracks, '^"'''"S'° ' " « " ' ' ' ' ft'lu^e [3-7], From a numerical modeling point of view, several publications focused on the internal fracture of the workpiece [1,8-11] but aacl initiation and failure criterion were not considered, which limits the impact of these numerical results However, numericil predictions of the ME were achieved in seveni studies on cross-roll piercing (CRP) Erman's pioneering work [12] focused on the analysisof material flow and strain distribution dirt*! deformation using the finite element methtKi (FEM) to interpret the ME Mori et al [13, HI JOURNAL OF SCIENCE & TECHNOLOGY * No 94 - 2013 predicted the ME through Oyane's damage model [15] Different approaches for the numerical simulations of the piercing process were proposed For example, Komori [16] developed a model to investigate the effects of the rolling conditions on the ME according to the author's steady-state formulation Yang [17] assumed a solution based on the Arbitrary Langrangian Eulerian (ALE) formulation that provided good results However, in these studies, the forming processes simulations were performed assuming plastic deformation only but fracture was not considered In order to get a better understanding of the ME in CRP and CWR process through numerical simulations, the established model should consider crack initiation due to the ME developing along the billet axis Complete simulations carried out according to this framework and allowing the prediction of the ME were developed in a number of studies [18-21] However, the knowledge about tracture mechanisms leading to the ME, in particular, the ME in CWR, is still limited In the present study, the 2D CWR process is simulated in order to investigate the ME The Johnson-Cook fracture model is used to predict the crack initiation The simulations are performed using the commercial finite element code Abaqus [22] Material Model In this study, the Johnson-Cook approach is adopted to describe the material behavior during cross-wedge rolling at high temperature Johnson-Cook material model In 1985, Johnson and Cook [23] proposed a dynamic plastic flow model that includes the effects of strain, strain-rate hardening and temperature softening as follows: {A+B£"\\ + C\n\ -r' r] (I) where (T is the flow stress, e is the equivalent plastic strain, s'/s' is the dimensionless strain rate and S^ is a reference strain rate A, B, C, n and m are empirical parameters to determine using experimental test results The first term of the right hand side in Eq (1) represents the quasi-static stress-strain relationship at room temperature The second term accounts for the effects of strain rate hardening The influence of the temperature is included in the third term where T is the homologous temperature defined as [24] /^ For T < Tr For r