Issue 49

M. Abbadeni et alii, Frattura ed Integrità Strutturale, 49 (2019) 282-290; DOI: 10.3221/IGF-ESIS.49.28 287 An evaluation of the forming severity for the two processes is obtained by comparing the numerical results for the strain distributions with the experimental FLD of the used material. The FLD is plotted in a principal strain reference. Major strain which is the largest in algebraic value is on the ordinate. Minor strain which is the smallest in algebraic value is on the abscissa. Although the major strain is always positive because it is obtained by stretching, the minor strain is may be either positive in the case of stretching or negative in shrink drawing case. The upper region of the FLD represents necking and fracture. The safe region in which no fracture is expected is the region under the FLD. Numerical major and minor strains distribution and experimental FLD of the used blank material are presented in Fig. 5. For the two processes, the material exhibits slightly different principal strains and the produced parts have not the same formability. The results prove that, until 15 mm of punch displacement, the parts can be formed without fracture. For CDD process, beyond this punch displacement (Fig. 6), excessive thinning followed by a fracture can occur on the punch corner where the most dangerous points are located. From these results, it can be concluded that the use of the fluid pressure gives better formability and drawing ration. According to Sadegh-yazdi et al. [21], formability can be improved using an optimum fluid pressure path for radial and cavity pressures in HDD process. Figure 5 : Experimental FLD and numerical results for principal strains distribution after 15 mm of punch displacement. The quality of the formed part not only depends on the formability but also depends on other factors such as thinning. To study the influence of the fluid pressure on the blank thickness distribution, a comparison of the blank thicknesses for the two processes is carried out and the result is shown in Fig. 7. It can be shown that the thickness distribution is significantly influenced by the presence or not of the fluid in the cavity. As shown, there is a tendency of thinning at the punch corner in the CDD process with a minimum thickness of 0.81 mm. When compared with CDD process, less thinning is observed in HDD process. The decrease in thinning tendency in the HDD process is due to the presence of the pressurized fluid, as it acts as an effective lubrication which facilitates the flow of the blank. And also, the force generated by the fluid pressure in the cavity acts as uniform stress during the forming operation. As reported in the literature [22], the use of the pressure force, in the HDD process, reduces product defects like thinning. More uniformity of thickness distribution can be obtained by using the initial bulging of the blank [23]. C ONCLUSION n this paper, The HDD process and CDD process have been simulated by means of the FE software package ABAQUS/Explicit. The plastic deformation distribution and the formability of the formed part were discussed. The obtained results indicate that the fluid pressure under the blank plays an important role in the lubrication effect, the I