What is elastic and visco-elastic behavior . How do plastics behave differently than elastic materials
Plastics unlike Metals are visco elastic in nature.
To understand visco elasticity we need to first understand the concept of elasticity.
Elasticity is the ability of a material to deform proportionately to the load applied and go back to its original state when the load is removed. Like a spring which is acted by a force.
But if a Dash pot or damper (mechanical device that resists motion via viscous friction) is connected to the spring in parallel then the behaviour of the system differs.
The Load deflection in loading and unloading has an internal area on the curve which is Energy lost during the Load-Unload cycle. In Elastic materials the energy is not lost but in Visco elastic materials the energy is lost in deflection due to inherent damping behaviour.
This is called visco-elasticity. Different plastics exhibit varying levels of visco-elasticity.
Its common to observe permanent deformation after loading and unloading cycle in plastics and elastomers (like Rubbers)
Plastics and Elastomers dissipate heat due to hysteresis and this visco-elastic behaviour .
Excessive loading cycles produces more heat and higher temperatures reduce the load bearing ability of the materials and modify other mechanical properties.
When designing with plastics its important to note this visco elastic nature especially when the loads are sufficiently high to degrade performance.