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The influence of surface defects on the forming-limit diagram of sheet metal

Journal of Materials Processing Technology
Publication Date
DOI: 10.1016/s0924-0136(97)00136-2
  • Surface Defect
  • Forming-Limit Diagram
  • Aluminum Sheet Metal
  • M–K Model
  • Critical Defect Depth


Abstract In order to study the effects of surface defects, which are one of the causes of initial inhomogeneity of sheet thickness, on the forming-limit 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 experiments showed that the existence of a critical depth of surface defects α e was found, up to which fractures do not develop and the forming-limit does not decrease. Specifically, the forming-limit strain did not decrease for defect depths of up to 2 and 0.5% of the thickness 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 the rate of decrease of limit strain with respect to the increase of defect depth can be expressed qualitatively. However, quantitatively, only the calculated results for the effects of the defect depth on the limit 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 α e was developed using the M–K model together with forming-limit strain data obtained by experiment. From these results, it is concluded that it is possible 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 serious problem in terms of external appearance.

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