MASTER THESIS Biomimetic potential of sponge ... - IAP/TU Wien

can be considered to start at order 2, where the elements constituting a structured material
show a discernible structure themselves at a smaller length scale. Accordingly, adding up the
number of length-scales with a distinct structure indicates the hierarchical order of a
material.
Lakes (1993) demonstrated that hierarchical architecture can yield remarkable increase
in strength. Contrasting primary and secondary honeycomb structures, made of paper and
glue, he found that the secondary structure (honeycombs cemented together to form a
larger honeycomb) was 3.8 times more resistant to compression while the densities in both
models were equal. This observation can easily be extended to other parameters related to
strength and toughness. This can be exemplified by the strength-to-density ratio of a
hierarchical foam that increases with hierarchical order (Figure 8b). Interestingly, one single
mechanism has been proposed to be the most essential reason for hierarchical structuring in
natural materials: deflect cracks that might threaten the integrity of a structure by opening
up new surfaces, making it energetically unfavourable for the crack to travel (Currey, 2005).
Other authors, however, see this as one among many mechanisms (Mayer, 2005) (see
below).
Hierarchically structured materials in nature show many interesting properties that are
related to phenomena at the nanoscale. Nano-crystalline materials (crystal size < 1 µm) are
“superelastic” due to the large interface areas between individual crystals, i.e. they can
deform significantly without fracturing. Furthermore, this conformation entails very short
diffusion distances, making processes, like sintering, that require high temperatures in most
materials possible at lower temperatures. (Lakes, 1993) In spicules of sponges, this process
occurs at the temperature of the surrounding sea- or lake water (Müller et al., 2009b)(Wang
et al., 2012c).
Equally due to the small length scale of the lowest hierarchical levels in biominerals,
another crack-inhibiting effect arises. For most minerals, the Griffith criterion that accounts
for crack-like flaws and localized stress concentration in the mineral structure governs failure
(Griffith, 1921). However, below a certain mineral size, the so called Griffith length, failure is
governed by the theoretical strength of a given material, i.e. the material is insensitive to
pre-existing cracks or flaws. Since the theoretical failure criterion of a mineral is much higher
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