Aluminium Design and Construction John Dwight

Figure 7.3 Typical relation between local buckling strength and slenderness for internal
elements, plotted non-dimensionally.
(see Section 7.1.3). The pattern for outstands is similar, but with a different
ß-scale (about one-third). The scatter shown in the figure results from
random effects including initial out-of-flatness and shape of the stressstrain
curve.
Curve E indicates the stress at which buckling would begin to occur
for an ideal non-welded plate, having purely elastic behaviour and
zero initial out-of-flatness. For very slender plates, this stress, known
as the elastic critical stress (� cr ), is less than the mean applied stress � m
at which the plate actually collapses. In other words, thin plates exhibit
a post-buckled reserve of strength, In the USA the specific terms ‘buckling’
and ‘crippling’ are used to distinguish between � cr and � m . The elastic
critical stress � cr , which is readily found from classical plate theory, is
given by the following well-known expression:
(7.2)
in which the buckling coefficient K may be taken as follows for elements
that are freely hinged along their attached edges or edge:
Internal element K=4
Outstand K=0.41.
It is found that welded plates (i.e. ones with longitudinal edge welds)
perform less well than non-welded plates, with strengths tending to
fall in the lower part of the scatter-band in Figure 7.3. This is due to
HAZ softening, and also the effects of weld shrinkage (locked-in stress,
distortion). Because of the HAZ softening, sm for compact welded plates
(low ß) fails to reach f , unlike the performance of non-welded ones.
°
Copyright 1999 by Taylor & Francis Group. All Rights Reserved.