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10 Mechanical Properties of Biological Materials
An important factor in reducing the strength of coral structure is bioerosion
of coral skeletons (Highsmith, 1981). Bioerosional damage to corals primarily depends on the amount of skeletal surface not covered by live tissue as
most excavations in coral skeletons occur within 2 cm of a dead surface. Even
small amounts of bioerosion in the basal region of corals result in a substantial
decrease of skeletal attachment strength.
Thnnicliffe (1979) examined the Caribbean stony coral Acropora cervicornis
and found that 75% of the corals had sponge holes at their bases. The most
abundant boring sponge was Cliona aprica. The breaking strength of corals
with basal erosion by boring sponges was one order of magnitude lower than
the 'healthy' coral strength.
Density of massive coral skeleton varies during the course of a year. The resulting annual density bands have been recognized as a retrospective means of
analyzing coral growth and as a possible source of proxy environmental information for shallow tropical waters (Barnes and Lough, 1993; Lough et al., 1996).
The comparison of density for forty colonies of Porites in the central Great Barrier Reef, Australia, showed that maximum density varies from 1.47x 10 3 kg/m 3
to 2.23x 10 3 kg/m 3 , while minimum density changes from 0.82x 10 3 kg/m 3 to
1.62x 10 3 kg/m 3 . Average overall density of skeleton is about 1.4x 10 3 kg/m 3
(Lough and Barnes, 1992). Variations of skeleton density through the crosssection of coral cause changes in skeleton strength. However, at present there
are no sufficient data to develop the relationship between these two quantities.
Fibres. This material consists of a base material (or matrix) in which are
embedded high strength fibres or filaments. The resulting composite material is much stronger than the base material. For example, glass and resin
are each substantially brittle. However, when combined into an engineering
material known as 'fibreglass', they show very high strength (see Table 10.1).
Fibreglass, synthetic textile fibres, as well as animal fibres such as collagen,
chitin and keratin, contain both crystalline regions, where the molecules are
arranged in an orderly pattern, and amorphous regions, where they are arranged randomly. The crystalline component provides material with a stiffness
with Young's modulus of the order of 1 GPa (see Table 10.1). The material
stiffness is only apparent when the fibre is loaded in tension. Under other
loading modes, the fibre material buckles quickly. For example, when a tensile
stress is applied at right angles to the fibres, they contribute little or nothing
to the strength and stiffness. To provide some isotropy of material properties,
the fibres are arranged in some sort of matrix. An example of the connective
collagen fibres in a mainly polysaccharide matrix is the body wall mesogloea
of sea anemones. The mesogloea of Metridium senile is composed of about
86% water, 5% salt, 6.7% collagen, and 2% matrix (Vincent, 1982). Young's
modulus of collagen is of the order of 1-2 GN/m 2 and the elastic strain limit
is about 0.04. However, the mesogloea of Metridium strains up to 3 times its
initial length. This means that collagen in the matrix is discontinuous and the
mechanical properties are mainly controlled by the matrix modified by collagen.
10 Mechanical Properties of Biological Materials
An important factor in reducing the strength of coral structure is bioerosion
of coral skeletons (Highsmith, 1981). Bioerosional damage to corals primarily depends on the amount of skeletal surface not covered by live tissue as
most excavations in coral skeletons occur within 2 cm of a dead surface. Even
small amounts of bioerosion in the basal region of corals result in a substantial
decrease of skeletal attachment strength.
Thnnicliffe (1979) examined the Caribbean stony coral Acropora cervicornis
and found that 75% of the corals had sponge holes at their bases. The most
abundant boring sponge was Cliona aprica. The breaking strength of corals
with basal erosion by boring sponges was one order of magnitude lower than
the 'healthy' coral strength.
Density of massive coral skeleton varies during the course of a year. The resulting annual density bands have been recognized as a retrospective means of
analyzing coral growth and as a possible source of proxy environmental information for shallow tropical waters (Barnes and Lough, 1993; Lough et al., 1996).
The comparison of density for forty colonies of Porites in the central Great Barrier Reef, Australia, showed that maximum density varies from 1.47x 10 3 kg/m 3
to 2.23x 10 3 kg/m 3 , while minimum density changes from 0.82x 10 3 kg/m 3 to
1.62x 10 3 kg/m 3 . Average overall density of skeleton is about 1.4x 10 3 kg/m 3
(Lough and Barnes, 1992). Variations of skeleton density through the crosssection of coral cause changes in skeleton strength. However, at present there
are no sufficient data to develop the relationship between these two quantities.
Fibres. This material consists of a base material (or matrix) in which are
embedded high strength fibres or filaments. The resulting composite material is much stronger than the base material. For example, glass and resin
are each substantially brittle. However, when combined into an engineering
material known as 'fibreglass', they show very high strength (see Table 10.1).
Fibreglass, synthetic textile fibres, as well as animal fibres such as collagen,
chitin and keratin, contain both crystalline regions, where the molecules are
arranged in an orderly pattern, and amorphous regions, where they are arranged randomly. The crystalline component provides material with a stiffness
with Young's modulus of the order of 1 GPa (see Table 10.1). The material
stiffness is only apparent when the fibre is loaded in tension. Under other
loading modes, the fibre material buckles quickly. For example, when a tensile
stress is applied at right angles to the fibres, they contribute little or nothing
to the strength and stiffness. To provide some isotropy of material properties,
the fibres are arranged in some sort of matrix. An example of the connective
collagen fibres in a mainly polysaccharide matrix is the body wall mesogloea
of sea anemones. The mesogloea of Metridium senile is composed of about
86% water, 5% salt, 6.7% collagen, and 2% matrix (Vincent, 1982). Young's
modulus of collagen is of the order of 1-2 GN/m 2 and the elastic strain limit
is about 0.04. However, the mesogloea of Metridium strains up to 3 times its
initial length. This means that collagen in the matrix is discontinuous and the
mechanical properties are mainly controlled by the matrix modified by collagen.