Science Focus Topic 5 Notes: How Structures Fail
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Forces acting on structures can cause them to fail to perform their function.
Failure can occur if the force is too strong for the structure's
design or if the force is acting on a vulnerable part of the
structure (that part of the structure that will likely fail the most often).
A lever is a device that can change the amount
of force needed to move an object. When a force is applied to the effort arm, a large force, which can move the
structure, is created. This can be intentional - like when a crowbar is used to move a heavy rock, or it can be
unintentional - like when a gust of wind knocks down a flagpole.
Shear - minor weaknesses in a material can cause failure because the
particles move farther apart and are less attracted to each other. This can be caused by compression.
Bend or Buckle - compression can also cause a material to bend
and buckle - like a pop can that is stepped on. To prevent this reinforcements - stringers and ribs - are used
to strengthen the structure (fig. 4.46)
Torsion - Twisting can cause material failure. When sections
of the structure slide past each other the structure can crack or break in two. When the twisting action makes
the structure unusable (even though it is not broken) it has failed because it has lost its shape.
Making Use of Stress
Knowing that materials fail when external forces are applied can be useful.
(Crash Test Dummies)
Buckle - Car bumpers are designed to buckle in a collision - as
the metal fails, it absorbs some of the energy of the impact, which protects the occupants of the vehicle. Blades
of grass on a sports field buckle as players land, which absorbs some of the impact forces on the players body.
Shear - Shear pins are used in outboard motors to prevent failure
of the motor (when the propeller gets tangled in weeds, a shear pin breaks and the propeller becomes disengaged
with the motor and gears. The clutch and automatic transmission in a vehicle take into account shear forces, which
enable parts to slip past each other and produce a smooth ride.
Twist - Spinning wheels twist cotton or wool fibres so they
lock together - making them strong enough to make cloth. Controlled twisting can also be useful in hair braids,
ropes and telecommunication cables.
Metal Fatigue (Definition - The phenomenon leading to fracture
under repeated or fluctuating stress. Fatigue fractures are progressive beginning as minute cracks and grow under
the action of fluctuating stress.)
Metal breaks down over time and extended use. (They get bent and twisted over and over). The particles
in the metal move further apart and have less attraction to each other. When a crack develops it weakens the metal
- metal fatigue - and can eventually fail enen if a small force
Now it is possible
to obtain a uniform and repeatable texture on metal to create a unique and aestheic architectural effect, using