Aluminium Design and Construction John Dwight

for so doing. In welded members, they are based on the strength of the
parent metal, even though the material in the heat-affected zone (HAZ)
is weakened due to the welding. The latter effect is looked after by
taking a reduced (‘effective’) thickness in the softened region, as explained
in Chapter 6.
The stress most used in member design is p o . In steel codes, this is
taken equal to the yield stress, and it seems reasonable in aluminium
to employ the equivalent value, namely the 0.2% proof stress f o . This is
generally satisfactory, but problems can arise with ‘low-n’ materials
having a very rounded stress-strain curve (f u f o ) . For these, the use of
p o =f o will result in a small amount of irrecoverable plastic strain at
working load, which may not be acceptable.
To allow for this, we propose that when f u > 2f o a reduced value
should be taken for p o , as shown in Table 5.3. This expression has been
designed to limit the plastic component of the strain at working load
to a value of about 0.0002, i.e. one-tenth of the proof stress value (0.2%),
assuming that the stress s then arising is 0.65p o . Such an approach
seems reasonable. The BS.8118 rule, which we believe to be over-cautious,
is compared with ours in Figure 5.3. The estimated plastic strains at
working load as given by the two methods are plotted in Figure 5.4,
based on the Ramberg-Osgood stress-strain equation (4.3).
For the stress p a , we generally follow the British Standard and take the
mean of proof and ultimate stress (although some codes take p a =f u ). Again,
there is a problem with low-n material, although a greater degree of
plastic strain can now be tolerated, as we are concerned with yielding at
a localized cross-section of the member. In our method, we take a cut-off
Figure 5.3 Relation between limiting stresses (p o , p a ) and material properties (f o ,f u ).
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