Page 418 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

8 Chapter 16 Sheet-Metal Forming Processes and Equipment
TABLE l6.3

Minimum Bend Radius for Various Metals at Room Temperature
Condition
Material Soft Hard
Aluminum alloys 0 6T
Beryllium copper 0 4T
Brass (low-leaded) 0 2T
Magnesium 5 T 13T
Steels
Austenitic stainless 0.5T 6T
Low-carbon, low-alloy, and HSLA 0.5T 4T
Titanium 0.7T 3T
Titanium alloys 2.6T 4T

however, depends on the radius and the bend angle (as described in texts on me-
chanics of materials). An approximate formula for the bend allowance is
L), = a(R + /QT), (16.2)

L, = air +
where a is the bend angle (in radians), T is the sheet thickness, R is the bend radius,
and le is a constant. In practice, la values typically range from 0.33 (for R < ZT) to
6 =
0.5 (for R > ZT). Note that for the ideal case, the neutral axis is at the center of the
sheet thickness, 12 = 0.5, and, hence, (16.3)

Minimum Bend Radius. The radius at which a crack first appears at the outer
fibers of a sheet being bent is referred to as the minimum bend radius. It can be
shown that the engineering strain on the outer and inner fibers of a sheet during
bending is given by the expression 1 (16.4)

Thus, as R/T decreases (that is, as the ratio of the bend radius to the thickness be-
comes smaller), the tensile strain at the outer fiber increases and the material eventu-
ally develops cracks (Fig. 16.17). The bend radius usually is expressed (reciprocally)
in terms of the thickness, such as ZT, 3T, 4T, and so on (see Table 16.3). Thus, a 3T
minimum bend radius indicates that the smallest radius to which the sheet can be
bent without cracking is three times its thickness.
There is an inverse relationship between bendability and the tensile reduction of
the area of the material (Fig. 16.18). The minimum bend radius, R, is, approximately,
50
R = T T - 1), (16.5)

where r is the tensile reduction of area of the sheet metal. Thus, for r = 50, the mini-
mum bend radius is zero; that is, the sheet can be folded over itself (see hemming,
Fig. 1623) in much the same way as a piece of paper is folded. To increase the bendabil-
ity of metals, we may increase their tensile reduction of area either by heating or by
bending in a high-pressure environment (which improves the ductility of the material;