Stress factors that can affect the behaviour of plastics over time

When we design components or parts it is important that we consider the different types of stress that may impact their performance over time.

Here are the major stress factors that can affect the behaviour of plastics:

Fatigue

Fatigue occurs in plastic when stress is repeatedly applied to an area. The load does not have to be very great as to weaken or break the material immediately but over time a repeated action, each time applying a small load to an area, will break it down and cause it to fail.

A common example where fatigue is likely to occur is a plastic living hinge. How often the hinge is likely to be flexed will determine the design of the hinge and the choice of polymer.

Creep

Plastic parts have elastic limits and if they are stressed below those limits for a short time they will return to their original shape. If the part is stressed beyond its limit or even below its limit but for a longer period – there is a possibility its shape will deform. This is known as creep.

The amount of stress that is applied and the duration of that stress are factors that affect creep, as does the temperature. Generally, the higher the temperature the higher the chance of creep. Technicians need to factor in these variables when designing parts to reduce the likelihood of creep.

Weathering

Most polymers have a poor long-term resistance to weather and weathering can occur if materials are exposed to temperature, moisture or oxygen. Plastic that is exposed to ultraviolet light for any length of time will also degrade as this will cause a breakdown at molecular level.

It is important that length of exposure to light is a factor in the design and evaluation of weathering tests.