348
10 Mechanical Properties of Biological Materials
materials from which marine organisms are composed. The values, listed in
Table 10.1, have been collected from various sources (Alexander, 1968; Wainwright et at., 1976; Vincent, 1982; Denny, 1988, 1993; Schmidt-Nielsen, 1989;
Cere and Timoshenko, 1991). Following Denny (1988), they are grouped into
three classes: crystalline composites, fibres and rubbers.
Crystalline Composites. These materials fail in tension at relatively low
values of strain. Examples are concrete, stone, glass, cast iron, and many
other man-made materials. The strength of biological crystalline type materials
depends on the amount of organic matter in the material. For example, the
shells of most molluscs are composed of calcium carbonate with a small amount
of organic material (1-4%), mainly protein. There are a number of types of shell
materials, which differ in structure and composition. Sometimes a combination
of these types of materials appears in a single shell. Thin shells are usually
made from a combination of prismatic material (polygonal columns, 100-200
f.1m x several mm long), nacre (flat tablets in sheets, 0.3 f.1m thick), or foliated
material (long, thin crystals in overlapping layers), while very thick shells are
built from crossed lamellar (plywood like lamellae, 20-40 f.1m thick) material
(Vincent, 1982).
The strength of mollusc shell material depends on the relative contribution
of the various shell material components. For example, the ultimate bending
stress for the prismatic component is 140 MN/m 2 , while for the nacre component it is 220 MN/m 2 . Therefore, in Table 10.1, the range of at, a c , ab
and E values are a result of various compositions of the particular material
components.
The breaking stresses of strong, relatively non-porous aragonite mollusc shells
are about 220 MN/m 2 . However, for porous crystalline stony skeletons such as
hard corals, the strength is much smaller. In the previous section, for Acropora
formosa we found that ab ~ 22 MPa. A similar result was obtained for three
specimens of an unidentified seleractinian by Eliat, Israel, namely 27, 27, and
29 MN/m 2 (Wainwright, 1976).
Material strength drops considerably when some holes are present, according
to Ryskevitch's empirical formula:
a = ao exp( -np),
(10.25)
where ao is the strength of non-porous material, p is the porosity, and n is a
constant between 4 and 7. Assuming the mean porosity as 50% and taking
aD = 200 MN/m 2 , we obtain the strength of coral material equal to 16 MN/m 2
and 27 MN/m 2 , for n = 5 and n = 4, respectively. These values are very
close to the observed values reported above. Bucher et at. (1998) measured
the porosity for branch tips from a range of Acropora species. They used
some modification of traditional methods including soaking the dried samples
in acetone to displace air from skeletal voids. Resulting porosity ranged from
33 to 70%.
10 Mechanical Properties of Biological Materials
materials from which marine organisms are composed. The values, listed in
Table 10.1, have been collected from various sources (Alexander, 1968; Wainwright et at., 1976; Vincent, 1982; Denny, 1988, 1993; Schmidt-Nielsen, 1989;
Cere and Timoshenko, 1991). Following Denny (1988), they are grouped into
three classes: crystalline composites, fibres and rubbers.
Crystalline Composites. These materials fail in tension at relatively low
values of strain. Examples are concrete, stone, glass, cast iron, and many
other man-made materials. The strength of biological crystalline type materials
depends on the amount of organic matter in the material. For example, the
shells of most molluscs are composed of calcium carbonate with a small amount
of organic material (1-4%), mainly protein. There are a number of types of shell
materials, which differ in structure and composition. Sometimes a combination
of these types of materials appears in a single shell. Thin shells are usually
made from a combination of prismatic material (polygonal columns, 100-200
f.1m x several mm long), nacre (flat tablets in sheets, 0.3 f.1m thick), or foliated
material (long, thin crystals in overlapping layers), while very thick shells are
built from crossed lamellar (plywood like lamellae, 20-40 f.1m thick) material
(Vincent, 1982).
The strength of mollusc shell material depends on the relative contribution
of the various shell material components. For example, the ultimate bending
stress for the prismatic component is 140 MN/m 2 , while for the nacre component it is 220 MN/m 2 . Therefore, in Table 10.1, the range of at, a c , ab
and E values are a result of various compositions of the particular material
components.
The breaking stresses of strong, relatively non-porous aragonite mollusc shells
are about 220 MN/m 2 . However, for porous crystalline stony skeletons such as
hard corals, the strength is much smaller. In the previous section, for Acropora
formosa we found that ab ~ 22 MPa. A similar result was obtained for three
specimens of an unidentified seleractinian by Eliat, Israel, namely 27, 27, and
29 MN/m 2 (Wainwright, 1976).
Material strength drops considerably when some holes are present, according
to Ryskevitch's empirical formula:
a = ao exp( -np),
(10.25)
where ao is the strength of non-porous material, p is the porosity, and n is a
constant between 4 and 7. Assuming the mean porosity as 50% and taking
aD = 200 MN/m 2 , we obtain the strength of coral material equal to 16 MN/m 2
and 27 MN/m 2 , for n = 5 and n = 4, respectively. These values are very
close to the observed values reported above. Bucher et at. (1998) measured
the porosity for branch tips from a range of Acropora species. They used
some modification of traditional methods including soaking the dried samples
in acetone to displace air from skeletal voids. Resulting porosity ranged from
33 to 70%.