THE INFLUENCE OF SURFACE-DEFECTS ON THE FORMING-LIMIT DIAGRAM OF SHEET-METAL

In order to study the effects of surface defects, which an one of the causes of initial inhomogeneity of sheet thickness, on the forming-lim it diagram (FLD), uniaxial tensile, in-plane stretching and hydraulic bulging tests were performed using soft and hard pure aluminum sheets, by introducing an artificial machined notch. The results of these exp eriments showed that the existence of a critical depth of surface defe cts alpha(e) was found, up to which fractures do not develop and the f orming-limit does not decrease. Specifically, the forming-limit strain did not decrease for defect depths of up to 2 and 0.5% of the thickne ss for soft and hard sheets, respectively. Numerical calculations were performed also using the Marciniak-Kuczynski (M-K) model to evaluate the effects of defect depth theoretically and it was clarified that th e rate of decrease of limit strain with respect to the increase of def ect depth can be expressed qualitatively. However, quantitatively, onl y the calculated results for the effects of the defect depth on the li mit strain of the soft sheets for the uniaxial tension case and those from the equi-biaxial tension case agreed with the results obtained by experiment. Moreover, a method for the estimation of alpha(e) was dev eloped using the M-K model together with forming-limit strain data obt ained by experiment. From these results, it is concluded that it is po ssible to decrease the rejection rate and reduce the material cost by applying less strict standards when inspecting the surface conditions of sheets in production shops, if the presence of defects is not a ser ious problem in terms of external appearance. (C) 1997 Elsevier Scienc e S.A.