Aluminium Design and Construction John Dwight

Table 4.6 Bendability index � for flat material
appreciated that in terms of practical fabrication the subject of bend
radii is not an exact science.
For any given thickness the minimum 90° bend radius may be
estimated with the aid of Figure 4.5, which gives curves of r/t plotted
against t. The appropriate curve depends on the bendability index � of
the material being bent, values thereof being listed in Table 4.6 for a
range of alloys. The equation to the curves in the figure is:
(4.2)
where: A=0.3�-1.7; B=0.08�+0.5; and t=thickness in mm.
The curves in Figure 4.5 are valid for cold bending in thicknesses
from 1.5 to 10 mm. They apply to 90° bends. For bends beyond 90°, the
minimum radius will increase, especially in the harder tempers. With
work-hardened material, the bendability is slightly better for the H2x
and H3x tempers, than it is for H1x. For heat-treated material in the T4
temper, we have assumed that forming takes place after the material
has artificially aged to its final condition; a smaller r/t will be possible
if it is bent immediately after quenching.
The bending of extruded sections is an altogether more complicated subject,
and the reader is referred to the useful publication Aluminium Extrusions—
A Technical Design Guide, issued by the Shapemakers. (Section 1.6)
4.3.5 Strength variation with temperature
One weakness of aluminium as a structural metal is its fairly rapid falloff
in strength with temperature. On the other hand, it has the advantage
that with decreasing temperature the properties steadily improve, and
there are none of the brittle fracture problems met with in steel. The
tensile strength of 6082-T6 goes down by 70% at 200°C, compared to
room temperature, but increases by 40% at–200°C.
Figure 4.6 shows how tensile strength varies with temperature (T)
for a range of alloys. The curves are plotted from data supplied by the
Aluminium Federation, based on specimens held for a long time at the
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