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For
the want of a nail, the horse-shoe was lost . . . .
A
part of the roof of a Boeing 737 blew off, after creeping failure of
skin joints. The plane landed safely. One person was
lost. In 1974 the cargo door of a DC-10 blew off because a closure
indicator showed that a bolt was home when it wasn’t. Everyone
died. In 1979 an engine came off a DC-10 because of a faulty
maintenance procedure on the connection between engine pylon and wing. Everyone died. In 1985 the rear dome
of a Boeing 747 failed, destroying the fin and the control systems,
because of a faulty repair to a joint in the dome. Only four people
survived. Deadly and expensive failures can result from simple
faults in joining parts together.
On
a simpler level, if you make a bunt in a glider, and you aren’t strapped in,
your next action will be to pull the rip-cord, assuming that you have a
parachute.
Note,
in the picture at the top of the page, how the joints are wider than the
bars.
These are the chains of the Clifton bridge, near Bristol. Look at the "knees" of a flamingo, or even your own
ankles. The difficulty of making joints, especially moving ones,
was a factor in the long period which elapsed between the invention of
variable sweep by Barnes-Wallis, and its use in actual aircraft, such as
the Tornado, F111, F-18 and B-1. However, numerous aircraft have
had folding wings for use on aircraft carriers.
The
jib of this crane is assembled by bolting together a series of short
sections. These are revealed by the widening of the top member,
especially where the sloping bar holds the whole thing up.
In
spite of all efforts in design and maintenance, pieces occasionally fall
off aircraft. If they fly clear, the aircraft will normally be
able to continue, and the parts hardly ever hit anything significant on
the ground. Planes have flown with a piece of rudder or flap
missing, or decompressed by ejection of a window or failure of a
fuselage part, just as birds and butterflies can fly with bits
missing. But in the rare cases that a part falls on a runway,
if the even more rare event happens where a wheel actually hits the
part, a tyre can be damaged. And in even more rare cases, this
can damage the plane enough to bring it down.
No
matter how strong the parts of a structure, they have to be joined to
other parts, in a way that does not reduce their effectiveness.
This can present problems that are far from trivial. Some airline
passengers probably wonder what stops the wings falling off. In
fact, of course, the wing structure goes right across through, over, or
under, the fuselage, forming a continuous beam, to which the fuselage is
attached. The attachment must transmit the forces in a way that
does not produce too much stress concentration. In fact, the two
wings may be bolted to the central beam. The bolts have to
withstand enormous forces caused by the bending moment. When a
Boeing 747 exploded near Lockerbie, these bolts held, and although most
of the fuselage was shattered, the wings were intact until they hit the
ground.
If
the wings are to move, as in some carrier-based aircraft, or in types
with variable sweep, the weight penalty may be severe. It may be
partially offset by a reduction in the requirement for high-lift devices
for take-off and landing. Imagine the structural needs of the wing
of a bird or beetle, both of which fold the wings when not in use.
To appreciate the difficulties faced by the designers of movable
connections, we need only look at the human ankle, the legs of a horse
or a flamingo, the universal joint in the transmission shaft of a truck,
or the hinges of the blades on a helicopter. And think about how
many problems, especially in old age, and in people who take part in
games and sports, are caused by wear and tear or disease of joints.
Here are some pictures taken
on a very dark morning after a severe storm. Most of the
tree-branches broke at or near the point of attachment. Why do you
think that in so many cases, the branch brought away a long strip of the
trunk, instead of just snapping off?
And
here is the same phenomenon on a much larger scale, showing a common
type of fracture.
This
picture shows a view from above of a gate post made from a tree
trunk. Much of the middle has rotted away, but five conical parts
remain, pointing inwards and downwards. These are the remains of the
anchorages of five branches that formed at that height on the tree, as can
be seen from the "knots" on the outside.
This
tree has split right down the trunk, which is seen to have been
hollow. The break probably started high up at a branch point.
Nature
seldom joins things together in a crude manner. Look at the way that
a tree grows its branches – if you cut through the wood, you see the lines
of force well inside the main branch, showing where the subsidiary branch
grew out. The first two pictures below show how palm leaves grow.
The
other pictures show pieces of wood cut from a place where two branches
grew out, together with a computer simulation. If you look at an old
fallen tree you can often see clearly the flow of the stresses to which
its growth was a response. Given the enormous time-scale of
evolution, we can assume that natural structures represent good
compromises between all the requirements for survival and reproduction.
A plant or an
animal is never strong enough to resist all possible forces, for it
would be at a disadvantage in more usual circumstances. The
evolved structure is strong enough that on average the species
continues. As many as a third of adult gibbons have broken bones
because of misjudgments or breaking branches. Were they stronger,
and therefore heavier, their speed would be reduced, and they would
probably be less successful in feeding, reproducing and escaping. On the
other hand, some male animals, such as elephant seals, have gone to the other, massive, extreme,
and have evolved suitable mating behaviour along with the increase in
size.
Given that most
animals have to reproduce by sexual means, and that many, from spiders
to tigers, are fierce predators, ingenious means have evolved to achieve
coupling without suicide. Nevertheless, in some species such as
mantids, male promiscuity is not usually possible. Plants, being
mainly immobile, have evolved an enormous range of mechanisms to get
their gametes together, often using animals to transport pollen.
In fact, many flowers have probably evolve together with the
corresponding insects and birds.
The same is
true of many mammals and their fleas, which have also evolved together,
so that the fleas cannot live apart from their mammal. Lichens are
symbiotic pairs of fungi and algae. We ourselves contain
mitochondria, which may represent ancient species which have become
almost a part of ourselves. These species bonds may not be
physical, but they are strong nevertheless.
Returning
to more tangible ideas, the
difficulty of transmitting huge forces can be appreciated if we think of
an elephant, the
Eiffel tower, a Saturn rocket, or a Boeing 747. In each case, huge
forces are at some places transmitted through quite small areas.
Stress
concentrations at bolts, rivets, rivet holes, welds, and other joints,
are major sources of concern. Deformed rivets from the Eiffel
tower are on display in the shops, showing dramatically the effects of
continuous long term stress. Look at pictures of the wreckage of
the first Tay bridge to see the effects of poor connections between the
iron piers and the masonry below. Connections do indeed begin with
the foundations, and end only at the top. Examples – Tay
bridge 1 and Tay
bridge 2.
Watch
a weight-lifter and look at the great care taken to ensure the best
possible connection of the hands to the bar. Anyone who has
carried heavy shopping bags will know about this. The connection
formed by a handshake is in fact a symbol of goodwill between people,
and the term is even used in electronics and communication to denote the correct exchange
of information. We sometimes speak of marrying parts
together. Other forms of touching, such as holding hands, kissing and
hugging, are also used by people. Some
common phrases referring to joining are – "coming
apart at the seams", "a screw loose", "hold
on", "losing his grip", "buttoned up",
"unbuttoned", "unhinged", "a
stitch in time saves nine", "stitched up", "Let these persons be joined
together in holy matrimony". Here
are some common requirements –
Within
materials –
Cohesion
– cast iron, spider web, steel, etc
Within
parts –
Reinforced
concrete, pre/post-stressed concrete, fibreglass, tufnol, and other
composites – avoidance of separation, delamination and cracking (see
web-page about cracks, in this web-site).
Between
parts –
Joints
– Compression, Tension, Sliding
fit, Rotating joints
Here
are some commonly used ways of joining and holding things –
Anchor,
Arc welding, Ball and socket, Ball race, Belt, Bluetack, Bobby pin, Bolt, Brazing,
Buckle, Bulldog clip, Bush, Button, Cement, Chain, Chuck, Circlip, Cleat,
Clevis pin, Clip, Clip-board, Clothes peg, Collet, Contact adhesive, Contact welding,
Cotter-pin, Crimp, Crocodile clip, Cyano-acrylate adhesive, Door-bolt, Double sided tape, Dovetail, Dowel, Drawing pin, Drift, Duct
tape, Electromagnet, Electrostatics, Epoxy, Expansion bolt, Explosive bolt, Eye-bolt, Flange,
Friction welding, Gasket, Gecko feet, Glue, Grub-screw, Gummed paper,
Hasp, Hat-pin, Hinge,
Hook, Hook and eye, Impact adhesive, Jubilee clip, Jumar, Karabiner,
Kirby grip, Knot, Lace, Latch, Lock, Lock-washer, Magnet, Magnetic chuck,
Match casting, Mooring rope,
Mortar, Mortice and tenon, Nail, Nut, Olive, O-ring, Paper-clip,
Passepartout, Peg, Picture hook, Pin, Piton, Plain bearing, Plug and
socket, Popper, Pop rivet, Post-it, Rawlplug, Rawlbolt, Redux bonding, Rivet,
Rope, Rubber band, Safety belt, Safety pin, Screw, Sealing wax, Self-tapping screw,
Sellotape, Set screw, Shrink fit, Shrink wrap, Solder, Split pin, Spot
welding, Stamp hinge, Staple, Stitching, Strap, String, Sucker,
Superglue, Surface tension, Tenon, Tendon, Tent peg, Thermite
welding, Tie-clip, Tie-pin, Toggle, U-bolt, Universal joint, Velcro,
Vice, Wedge, Weight, Welding (arc, contact, friction, spot, etc), Wire, Woodscrew, Wringing, Yorkshire, Zip. Within
these groups, think how many types of plugs and sockets, and how many
types of screws, bolts, nuts and washers there are. Some
of these types of fastenings may require water tightness, permeability, gas tightness,
electrical insulation, electrical conduction, heat insulation, heat
conduction, stiffness, flexibility, corrosion resistance, vibration
resistance, pressure resistance, damp resistance, temperature
resistance, sliding, rotation, inspection, reliability in inaccessible
places, and so on. And all parts of a joint must be compatible
with each other, without unwanted effects, which may be binding, slipping,
chemical action, electrolytic action, and
other problems. A joint may need to be reliable for many years,
yet demountable for inspection, modification, or repair. Parts
of the famous iron bridge in Coalbrookdale are connected in ways which
would not be used today, and were probably hardly ever used again. The
designer may have decided that innovation in materials was a sufficient
leap of faith in itself, without inventing new jointing methods.
Certainly the history of projects that have tried to innovate in every
possible way has not always been happy. On the other hand, trying
to invoke a new technology without supporting it with new design and
construction techniques may nullify some of the potential gains. Here
are some examples of attachments in bridges –
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