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10.3.2 Deformation Mechanisms
The foam has an extremely complex cell structure with geometrical imperfections
and hierarchical features. From the virtual CT slices shown in Fig.
10.4
a and b, it
is clearly seen that the cell shape deviates from regular ellipsoids and polyhedra.
Moreover, the cell walls themselves are somewhat porous, being significantly
thicker at the interconnecting portions. Figure
10.4
a and b show the cell defor-
mations within the central cross-section of the 3D volume. The compressive
deformation is markedly inhomogeneous, being characterised by the formation and
development of localised deformation bands. As the compressive strain increases,
some 'weak' cells collapse prematurely, presumably due to their inferior load
bearing capacity, marking the beginning of localised deformation (see the con-
figuration at a strain of 7.7 % in Fig.
10.4
). Deformation then becomes concen-
trated in these cells which contribute most to the subsequent increase in the
compressive strain, indicating the development of a deformation band (see the
configuration at a strain of 17.8 % in Fig.
10.4
). This process continues until new
deformation bands are formed. Such deformation on the cell level corresponds to
the plateau regime in the stress-strain curve observed in the macro measurement.
Therefore, in common with previous observations [
11
], the meso-scale cause of
the plateau stress, which represents the compressive strength of the foam, is the
progressive cell collapse occurring in the localised deformation bands.
The prematurely collapsing 'weak' cells play a crucial role in the localised
deformation mechanism. However, the exact cause of the premature collapse has
not been unequivocally identified. Based on the observation of cell deformation
within the cross-section, Bart-Smith et al. [
11
] identified two critical cell mor-
phologies, i.e. ellipsoidal cells with T-shaped wall intersections and cells with
appreciably curved walls, for the weak cells, but these morphologies cannot be
easily distinguished in the present experimental observations. Our results reveal
that the weak cells susceptible to collapse actually have a wide variety of shapes
and sizes. Consequently, existing simple (mostly 2D) geometric criteria are unli-
kely to describe them, especially when their 3D nature is considered. It is ques-
tionable whether a 2D geometry can describe the 3D cell structure of a foam, since
the 2D geometric characteristics of one foam cell may change significantly from
one cross-section to another. For instance, a 'small cell' observed in one cross-
section may be part of a large cell and the 2D curvature does not necessarily
represent the 3D one. Therefore, the observation of 3D cell deformation is
important in establishing a better understanding of the local collapse behaviour.
The deformation bands in the central XZ and YZ planes are shown in Fig.
10.4
a
and b as the dotted lines. The comparison between them indicates that the location
and orientation of the bands depend on the plane selected. The band in the XZ
plane is close to the top and is slightly inclined. By contrast, the band in the
YZ plane is closer to the bottom and is significantly inclined. The cells outside the
bands essentially retain their original shape, even when the compressive strain has
reached 17.8 %. To inspect the 3D features of these deformation bands, translucent
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