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10 Mechanical Properties of Biological Materials
buoyancy strategies or generate dynamic lift. However, prior to considering the
locomotion of aquatic animals, we will firstly discuss the mechanical properties
of biological material from which marine organisms are constructed.
10.2 Definition of Mechanical Properties of Biological
Materials
Mechanics of materials is a branch of applied mechanics dealing with the behaviour of solid bodies subjected to various types of loading. Many methods
used in the mechanics of materials are very useful for the determination of
mechanical properties of biological material. Therefore, we will start with definitions of material properties and its behaviour commonly used in mechanics.
10.2.1 Stress and Strain
Let us consider a prismatic bar that is loaded by axial forces, P, at the ends
(Fig. lD.1a). The axial forces, directed towards the bar, produce a uniform
compression of the bar. When forces, P, are directed out of the bar, the bar
is said to be in tension. In both cases, the intensity of force (the force per unit
area) is known as the stress and is commonly denoted by the letter (J. Thus,
if the cross-sectional area of the bar is A, the stress becomes:
P
(J = A'
(10.1)
in which stress (J is expressed in the same units as pressure, namely newtons
(N) per area in square metres (m 2 ) or pascals (Pa).
Depending on force direction, the resulting stresses are compressive or tensile stresses. If stresses act in a direction perpendicular to the surface, they
are referred to as normal stresses. Stresses acting parallel to the surface
are known as shear stresses. By convention, tensile stresses are defined as
positive and compressive stresses as negative.
When the bar is loaded axially, it becomes longer when in tension and shorter
when in compression. The change in length or elongation per unit length is
known as strain and is usually denoted by the letter f. The relationship
between stress, (J, and strain, E, illustrates the behaviour of various materials
as they are loaded statically in tension or compression. For some materials,
when the load is slowly removed, they exactly follow the same curve back to
the origin (see Fig. lO.lb). Such a property of a material, where it returns
to its original dimensions after unloading, is known as elasticity, and the
material itself is said to be elastic. The stress-strain curve from point 0 to A
in Fig. 10.1 b does not need to be linear in order for the material to be elastic.
In Fig. 1O.lb it is shown that in an initial region on the stress-strain diagram,
many materials behave both elastically and linearly. This means that stress,
(J, is linearly proportional to the strain E, i.e.:
10 Mechanical Properties of Biological Materials
buoyancy strategies or generate dynamic lift. However, prior to considering the
locomotion of aquatic animals, we will firstly discuss the mechanical properties
of biological material from which marine organisms are constructed.
10.2 Definition of Mechanical Properties of Biological
Materials
Mechanics of materials is a branch of applied mechanics dealing with the behaviour of solid bodies subjected to various types of loading. Many methods
used in the mechanics of materials are very useful for the determination of
mechanical properties of biological material. Therefore, we will start with definitions of material properties and its behaviour commonly used in mechanics.
10.2.1 Stress and Strain
Let us consider a prismatic bar that is loaded by axial forces, P, at the ends
(Fig. lD.1a). The axial forces, directed towards the bar, produce a uniform
compression of the bar. When forces, P, are directed out of the bar, the bar
is said to be in tension. In both cases, the intensity of force (the force per unit
area) is known as the stress and is commonly denoted by the letter (J. Thus,
if the cross-sectional area of the bar is A, the stress becomes:
P
(J = A'
(10.1)
in which stress (J is expressed in the same units as pressure, namely newtons
(N) per area in square metres (m 2 ) or pascals (Pa).
Depending on force direction, the resulting stresses are compressive or tensile stresses. If stresses act in a direction perpendicular to the surface, they
are referred to as normal stresses. Stresses acting parallel to the surface
are known as shear stresses. By convention, tensile stresses are defined as
positive and compressive stresses as negative.
When the bar is loaded axially, it becomes longer when in tension and shorter
when in compression. The change in length or elongation per unit length is
known as strain and is usually denoted by the letter f. The relationship
between stress, (J, and strain, E, illustrates the behaviour of various materials
as they are loaded statically in tension or compression. For some materials,
when the load is slowly removed, they exactly follow the same curve back to
the origin (see Fig. lO.lb). Such a property of a material, where it returns
to its original dimensions after unloading, is known as elasticity, and the
material itself is said to be elastic. The stress-strain curve from point 0 to A
in Fig. 10.1 b does not need to be linear in order for the material to be elastic.
In Fig. 1O.lb it is shown that in an initial region on the stress-strain diagram,
many materials behave both elastically and linearly. This means that stress,
(J, is linearly proportional to the strain E, i.e.: