FE Analysis Of Composed Dies Stress In Cold Extrusion Process And Study On Fatigue Life Of Dies

Die stress is one of the main factors causing damage of cold-extrusion dies. The mechanics of fatigue has not been studied deeply due to the complex processes in die stress alternation. Base on the detail reviews of the development of materials forming and dies stresses analysis used finite element method (FEM), in this work, the cold forming process of AISI 1010 billet is simulated by using the commercial finite element analysis package, DEFROM^TM-3D. For a comparison with the distribution of stress/strain in die, two FEM modeling approaches are simulated, which are integral die and composed die with pre-stress. The main results can be list as followed:(1) From consideration of the die as an elasto-plastic body, the FEM results are credible. It is showed that the finite element method used in die stresses analysis is feasible and effective. According to the results, both the peak of circumferential stress and radial stress are all appeared in the corners area of the cavity die. Serious stress concentration and continuous cyclic high stress contribute to cracks initiation and growth and then lead to die failure.(2) Because the Lame equation can't describe die stress accurately, it is impossible to get the optimization of the cold extrusion composed die design as base on this formula. By FEM, it is easy to obtain all the accurate mechanical information, such as die stress, strain, stress concentration, circumferential stress, radial stress and effective stress etc. Obviously, the FEM analysis is paramount and effective for the prediction of fatigue life and reduction of high stress within the tools. And it can save a lot of cost in experimentation.(3) According to the results of numerical simulation, stress distribution within theshrink-fitted die is more perfect than the integral one. Circumferential tensile stress is decreased greatly and the effective stress is lower. Then, strength of die will be improved extremely.In this thesis, a fatigue failure model for die life estimation was set up by using local stress-strain approach. Pre-estimate the fatigue life of die will find a new way for cold extrusion die design optimization. According to this estimation equation, life of integral die is not more than 3000 cycles when the composed die with pre-stress is about 10000 cycles. Consideration of dimensional effect and stress gradient effect, index number of fatigue strength was amended. So the model could be appropriated for the estimation of low-cycle fatigue and high-cycle fatigue.Die stresses analysis using FEM and fatigue life model set up can help to determine die quality and can give a good reference for dies development and manufacture.