板材の穴拡げにおける変形限界ひずみ

抄録

The bore-expanding process is performed on commercially pure aluminum (Al-0 and Al-1/4H), and 60/40 brass(Bs-0 and Bs-1/4H), and an aluminum alloy (A5182-0). Except for in the case of Al-1/4H, breakage (or the localized neck) starts at the rim of the bore and extends obliquely to the rim circumference. The issue of limiting strain for this case is raised to some extent when compared with that in uniaxial tension. We refer to this phenomenon as “strengthening due to constraint effect by surrounding material”. To verify this point, strain distributions along the radial line are measured to determine whether or not the steep negative gradient of hoop strain exists. The constraint effect is greater for a steeper gradient. As for Al-1/4H, breakage starts at the point 3 mm from the rim edge and extends along the radial line. The strain gradient around this point is almost 0 and thus little constraint effect exists. Moreover the state consists almost entirely of in-plane strain with no radial strain. This is why breakage starts at an inner point. It is emphasized that most breakage conditions derived thus far for a uniform field should be employed not at one point but over a range for breakage judgement, especially in the case where the localized neck extends along the direction of negative strain gradient. The effect of planar anistropy on the direction of the starting point of the breakage is discussed. It is also emphasized that appearance of the localized neck (localized-type instability)may precede diffuse-type instability in bore-expansion with a small-diameter bore, and that localized-type instability predominates to induce breakage. Finally, the critical bore diameter such that the punch penetrates through the sheet without breakage is discussed.