Study On Meso-Damage Model And Ductile Fracture Criterion In Metal Forming Processes

Metal forming technology plays an important role in the manufacturing industry. It has the merits of high productivity, stable quality and effective utilization of raw material. The mechanical properties of the metal are also improved in the forming processes. In metal forming, workability or formability of material is one of the focusing subjects, which is mainly controlled by damage and ductile fracture. With the development of computer technology and the plasticity, numerical simulation for metal forming process by finite element method (FEM) is more and more widely used. The distribution of stress and strain in metal forming process can be precisely obtained by the numerical simulation, but the prediction of ductile fracture in metal forming process is still immature, which is a bottleneck problem restraining the application of numerical simulation in metal forming process.Ductile fracture in metal forming process is the cumulative result of damage, and usually follows a multi-step failure process involving nucleation of microscopic voids, growth of voids, localization of plastic flow, coalescence of voids and occurrence of macro-fracture. Meso-damage model physically reflects the phenomenon that microscopic structural deterioration leads to failure or fracture of metallic material. However, it is based on idealized hypotheses, and some parameters, which could not be directly determined by experiments, are involved, therefore it is not widely used in engineering field. To determine the reasonable value of the parameters is the precondition of utilization of the model in engineering analysis. On the other hand, in metal forming ductile fracture criteria are usually adopted to predict ductile fracture of the workpiece. Existing ductile fracture criterion is established for specific metal forming process, while the diversity of metal forming processes requires the wide application range of ductile fracture criterion. The research on meso-damage model and ductile fracture criterion and its application in finite element (FE) simulation of metal forming processes are carried out in the dissertation, the main contents are:A set of error function is established to evaluate the accuracy of the simulation on deformation behavior of metal by Gurson-Tvergaard-Needleman (GTN) damage model, then the values of the parameters in the model are reasonably selected, void nucleation parameters are also introduced and optimized.Based on the principle of minimum energy, an analytical model of voids coalescence is propoed, the relationship between the critical strain to fracture and the void volume fraction is derived. Void volume fraction in fracture area of specimens in uniaxial tension experiment is measured, and it is confirmed that the relationship between the critical strain to fracture and the void volume fraction, calculated by using the proposed model, agrees better with the experimental results than that calculated by two-dimensional model.The applicable range and accuracy of six ductile fracture criteria often used in engineering analysis are compared and evaluated. The basic mechanisms of ductile fracture are divided into two modes (tension-type mode and shear-type mode) according to the observation of experimental results. A unified ductile fracture criterion is proposed for wide applicable range. The comparison of experimental results, including tension, compression, torsion, tension after torsion, with numerical analysis results confirms the validity of the newly proposed ductile fracture criterion on different deformation path. The new ductile fracture criterion is then used on predicting ductile fracture in sheet metal and bulk metal forming processes. Deep drawing process of pre-strained sheet metal by uniaxial tension is carried out, which confirms that the newly proposed ductile fracture criterion more accurately predicts ductile fracture compared with forming limit diagram (FLD). Forward extrusion process with different rates of area reduction in cross-section is carried out, which shows that ductile damage of tension-type and shear-type mode simultaneously exist in the extruding samples, the new ductile fracture criterion also accurately predicts the occurrence of surface crack on the samples.