Journal of the Japan Society for Technology of Plasticity
Online ISSN : 1882-0166
Print ISSN : 0038-1586
ISSN-L : 0038-1586
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Displaying 1-2 of 2 articles from this issue
Regular Papers
  • Yasuhiro KISHIGAMI, Masaki URABE, Yoshikiyo TAMAI, Kosuke SUZUKI, Masa ...
    2024 Volume 65 Issue 758 Pages 31-35
    Published: March 25, 2024
    Released on J-STAGE: March 01, 2024
    JOURNAL RESTRICTED ACCESS

    A sharp character of automobiles is required to express its sporty appearance. A sharp character is, however, prone to a type of defect called the “skid line” as one of the surface defects caused during press forming. Dealing with skid lines requires repeated die repairs, which have been a major problem in the press shops. We clarified the mechanism of skid line formation by continuous surface shape measurement and finite element analysis (FEA) of the formation process of a skid line during press forming. In this study, the effect of material strength on skid line formation was investigated by conducting experiments on skid line reproduction and measuring the skid line profiles, using 270-440 MPa grade sheet steels, which are used for automobile outer panels. The results suggest that the Yield point (YP) of sheet steel has a major effect on skid line formation as shown by the finding that skid lines became more marked with the decrease in material strength. From the test results, the prediction equation of the estimated Δs of skid lines by the blank holder force and YP was obtained.

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  • Yuta KARIYAZAKI, Kentaro SATO, Toru INAZUMI, Yutaka TAKAHASHI
    2024 Volume 65 Issue 758 Pages 36-42
    Published: March 25, 2024
    Released on J-STAGE: March 01, 2024
    JOURNAL RESTRICTED ACCESS

    Fracture is a major issue in the press forming of ultrahigh-strength steel (UHSS) sheets. To avoid fracture, it is helpful to accurately predict necking that appears before fracture. Forming limit diagrams (FLDs) have been used to predict necking. However, conventional methods often fail to predict necking because UHSS sheets have different necking behaviors from mild steel sheets. In this study, three types of FLD are compared, namely, those determined by conventional time-dependent (TD) and position-dependent (PD) methods, and by the developed time- and position-dependent (TPD) method. The results of conventional methods indicate that the necking limit determined by the TD method is very large, and that the accuracy of the necking limit determined by the PD method is unstable under a strain path with high-degree peaks in its strain distribution. Alternatively, the necking limit determined by the TPD method is less affected by high-degree peaks, suggesting that the accuracy of the necking limit is stable. Furthermore, the necking limit determined by the TPD method can also be precise based on the thickness distribution in a panel formed immediately before fracture. From the above reasons, the TPD method is considered to be more appropriate in accurately predicting necking in UHSS panels.

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