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Continuous Joints
Continuous joints are yet another way that living
organisms achieve simplicity at a macro level.
Examples of continuous joints include the connection
between tendons and bones and ligaments and bones.
At the junction between these different types
of tissue, the living cells merge to form a continuous
joint. In addition, fibers (such as collagen fibers)
extend continuously from one part to the next.
For example, collagen fibers in the bone extend
right into the tendon, thus producing a very tough
connection. A continuous joint is generally more
robust than a mechanical joint because there are
no mechanical fasteners that can become loose.
Also, continuous joints are generally lighter,
and this leads to lower loads due to self-weight
being imposed on the system.
An engineering analogy to the continuous joints
found in creatures is that of the welding of two
dissimilar materials to form a permanent continuous
joint. These permanent joints can enhance reliability
because they are simple and cannot be shaken apart.
It is generally accepted that simplifying interfaces
in engineering systems helps achieve better reliability.
Continuous joints are also feasible with advanced
bonding techniques, and this is now beginning
to be used in the automotive industry in the joining
of aluminum structures. The use of continuous
joints presents a dilemma for designers because
there is a conflict between the advantage of a
simple light joint and the disadvantage of a design
that is difficult to disassemble and repair. However,
nature shows that there are important reliability
benefits in using continuous joints.
Flexible Structures
Living organisms contain structures that are very
strong and flexible. For example, the ligaments
and muscles around joints are very tough and can
withstand significant stretching and distortion.
The flexibility of natural structures often plays
a key role in achieving a robust design. In general,
structural design has one of three objectives:
(i) design for stiffness; (ii) design for strength;
and (iii) design for flexibility. Figure 1 shows
a beam that is designed for each of these three
goals. In general, a stiffness goal leads to a
design that deflects the least distance for a
given load, while the flexible beam deflects the
most. In terms of resistance to a given load,
it can be argued that the stiff beam in Figure
1(a) is the most robust of the three designs because
it deflects the least. However, a flexible beam
(Figure 1(c)) actually has several advantages:
- It deflects so much
that it may become supported by a secondary
structure
-
It is tolerant of significant enforced displacements
-
It is tolerant of significant thermal loads
-
It is tolerant of significant misalignments
in assembly
Therefore, depending on the
nature of loading, flexibility can be a source
of robustness rather than weakness. In the case
of enforced displacements and thermal loads, the
stiff beam is actually the most vulnerable to
breakage because the resulting internal loads
are very high.
Large-Scale Redundancy
Large-scale redundancy is defined here as m
out of n redundancy, where n is
the total number of elements and m is the
number of elements that must work in order for
the system to survive. The advantage of large-scale
redundancy is that very high levels of reliability
can be achieved even though individual elements
may have a modest level of reliability. An example
of large-scale redundancy in nature is found in
the flight feathers of birds. Birds have between
30 and 88 flight feathers in their wings [Ref.
4], but only a certain proportion of these are
required to be in place and functioning for the
bird to fly. The reliability of a system where
m out of n parts must be working
is given by:
The advantage of maximizing n where m/n
is a constant, for elements that have an individual
reliability of 0.9, is shown in Table 1. The table
shows that having 1-in-2 redundancy (i.e., parallel
redundancy) improves the reliability from 0.9
to 0.99, which is a large improvement but does
not produce a very high level of reliability.
However, by making n very large, the reliability
becomes very high indeed. Since flying birds have
more than 30 flight feathers on their wings, they
achieve high levels of reliability even though
the reliability of an individual feather may not
be that large. One example of m out of
n redundancy may be found in the lighting
systems of large work areas that are served by
a large number of individual lights. In such cases,
the lighting system is often tolerant of several
individual light failures and, therefore, the
overall reliability is high.
Adaptive
Control
The autonomic control system within living creatures
has a remarkable ability to perform adaptive control.
One example is the control of the mammalian heart
rate. The heart is able to beat at widely varying
rates, depending on the particular demand. The
advantage of adaptive control in this case is
that the power generation of the system is always
set at an optimum level. Adaptive control has
recently been applied to the control of aircraft
by developing algorithms that take into account
the changes in the aircraft due to fuel loss and
other factors [Ref. 5].
Alternative
Energy Sources
Many creatures in nature have alternative energy
sources in terms of food that they can eat, and
this means that they are not vulnerable to shortages
of specific food types. For example, humans have
teeth that can handle a wide range of vegetables
and meats, and this gives humans a wide range
of options for feeding. The principle of alternative
fuel sources is sometimes applied to engineering
systems where alternative energy sources such
as gas and coal can be used as fuel for power
plants. Being dependent on one fuel can be risky
because of the possibility of sudden price rises
or shortages.
Self-Healing
Living tissue has the remarkable ability to perform
self-repair. In the case of a cut to human flesh,
a self-repair process is carried out including
blood clotting, scab formation and skin growth.
The clotting process is remarkable because it
only occurs at wound sites and would be dangerous
anywhere else. The principle of self-healing has
recently been applied to composite materials with
the use of a large number of distributed and embedded
glue capsules [Refs. 6, 7]. In this system, if
there is a crack in the material, a local capsule
or capsules will burst and release a healing agent.
The liquid molecules come into contact with a
catalyst that is also embedded within the polymer
matrix, causing the healing agent to polymerize.
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