Page 190 - Marine Structural Design

166 Part II Ultimate Strength

(a) Initial mode
a-a' b-b*

L",!'
(C) Subsequent mode - 2

Figure 9.8 Local Buckling in Cosine Mode

Pure Bending Test for Small Scale Specimens
Loads vs. load-line displacement relationships are plotted in Figure 9.11. In the case of BD1
specimen, breaking of a specimen occurred from a bolt-hole near the end before local buckling
took place. In all specimens, cross-sectional flattening is observed as the load increases.
Furthermore, deformation of a ripple pattern with two or three half waves begins to grow near
the ultimate strength. The ultimate strength seems to be attained by the cross-sectional
flattening and the formation of ripples. In the case of thin-walled tubes, the bottom of one
wave of the ripple suddenly changes to a dent near the ultimate strength, and the load carrying
capacity decreases. Contrary to this, the ripple deformation grows after the ultimate strength is
attained in the case of thick-walled specimens. Then, the ripple suddenly changes to a local
dent.
It is not clear, whether the initiation of local buckling is due to the formation of a ripple pattern
or the formation of a dent. However, much attention has to be paid to the formation of a dent,
since this causes a sudden drop in the load carrying capacity.
The formation of new dents after the initial dent has formed is almost the same as in the case
of eccentric axial compression.