Topic 5: How Structures Fail

- If a great enough force acts on a structure, it will fail- Small forces acting in vulnerable places can cause damage- Materials and structures will buckle, bend, twist and shear when they fail.

Student notes

Levers- A device that can change the amount of force

needed to move an object- Consists of a long arm that rests on a pivot or

fulcrum- When an external force is applied to the lever, a

large enough force is created to lift a heavy load.

An unintentional lever action can damage a structure

Ex: a strong wind

Strong winds can exert an external force that bends the frame of the structure of a pole. Even a slight bend can exert a large force on one side of the pole’s base. The base acts as a lever. As the external force pushes down against one side of the base, the other side pushes up against the load of the pole. Eventually the bolts holding the frame to the base weaken and the pole falls over.

- External forces can cause internal forces in the structure

Shear

- When a solid material is compressed, a crack can enlarge or break apart- One section may shear (slide over another section along the weakness)

Eg. Gravity acting on land

Bend or Buckle

- Compression forces cause material to bend on the inside curve and snap or pull on the outside curve

Eg: stepping on a pop can, pushing ends of paper together

All thin panels tend to bend and buckle when they are compressed. Shell structures that use thin panels to support their entire load (boats and planes) are reinforced to prevent buckling.

Torsion

- Twisting forces cause material failure- Brittle structures shear when twisted- Flexible structures shear less but lose their shape

Although the structure is not broken, it has lost it’s shape, which is a form of failure.

Metal Fatigue

- Metals weaken when they are bent and twisted over time- Particles are moved apart. The force holding them together becomes weaker- Cracks develop and eventually the materials fail

Making Use of Stress

- knowing that materials fail when external forces are applied can be useful.

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.

Shear - Shear pins are used in outboard motors to prevent failure of the motor (when the propeller hits, cuts, logs or gets tangled in weeds)

Twist - Spinning wheels twist cotton or wool fibres so they lock together - making them strong enough to make cloth.