Polymers exhibit viscous and elastic characteristics, or viscoelasticity, a powerful characteristic that offers distinct advantages. However, before choosing polymeric fasteners, designers must evaluate the effects of stress relaxation in polymers, which can cause joint failure over time.
Stress relaxation is a gradual decrease in stress in response to consistent effort. This means that there is less tendency for polymeric materials to return to their original shape after the filler is removed. Therefore, stress relaxation is an important design factor to consider for assemblies where a polymer fastener experiences sustained forces during application.
For example, when using snap-on mounts, it is important to evaluate tension relaxation. Such effects may be irreversible because modification of the binding material is a molecular phenomenon, like breaks or reorientations in molecular chains in polymers.
While stress relaxation examines how stress varies, fluency is a measure of an increase in tension. Both are used to determine dimensional stability:
- Stress relief: Constant effort is applied and stress decreases over time.
- Creeping: Constant tension is applied and deformation increases over time.
Stress relaxation can be difficult to measure accurately and the specific form of strain response varies depending on the type or grade of polymer. Unfortunately, material data sheets may provide insufficient information to describe a fastener's long-term stress relieving potential. Hands-on testing is recommended to ensure the fastener meets design expectations.
As with most fasteners, the performance of the material must also be considered in relation to the substrate. Improper judgment in fastener selection can result in loosening or pulling out of the fastener, which can lead to joint failure upon assembly. For example, in medical applications, stress relief is of particular concern because assembly failures can lead to leaks at fluid connections.
