Understanding Stress and Strain

Understanding mechanics is nothing to do with mathematics, it is the reality of what is actually happening and why it is happening.

Stress and strain

A lot of confusion with these, even with engineers. Basically, stress can be considered in relation to a car spring. Once the spring is fitted to the car it is in a state of stress, but even after years of going over bumpy roads and constantly being in  a state of stress, if you take it off the car it should retain its original length. However, if its length has reduced, then strain has taken place.

Strain is damage caused by too much stress.

A stress fracture is a strain caused by too much stress.  The stress applied has exceeded the design stress of the bone structure.

A pulled ligament is a strain caused by too much stress. The stress applied has exceeded the design stress of the ligaments.

All mechanisms whether in engineering or biology are designed to operate within certain stress levels. As long as we stay within these ‘design’ parameters the mechanism should operate quite happily. If we exceed these parameters then strain is likely to occur.

Note that both strain and sprain have basically the same meaning.

Note: You would think that the people who made shock absorbers and dampers should know the difference. The shock absorbers on a car are the springs. What are sold to the public as ‘shock absorbers’ are in fact dampers, they dampen the oscillations that are created by the springs. Without the dampers your car would bounce along the road like a kangaroo. This is why DAMPERS are so important on a car. A spring absorbs the energy of a wheel passing over a bump in the road and temporarily stores it as stress in the spring. Immediately after passing over the bump the spring attempts  to release this stored energy as quickly as possible, (which would cause the car to bounce). The damper prevents this happening by slowing down the rate at which the stored energy is released. In biology numerous muscles are used to act as dampers, acting as controls to prevent excess body movements that would be dangerous or inconvenient.

The kangaroo utilises a good example of the same basic principles, its legs storing energy in the form of stress in the leg muscles, the stored energy being used to help to power its next jump.

Author – Brian Williams.