By Julie Miller

Polymers exhibit both viscous and elastic characteristics, or viscoelasticity —  a powerful feature that offers distinct advantages. However, before choosing polymer fasteners, designers must weigh the effects of stress relaxation in polymers, which may cause joint failures over time.

Stress relaxation is a gradual decrease in stress in response to constant strain. This means there’s a decreased tendency for polymer materials to return to their original shape after removal of loading. Therefore, stress relaxation is an important design factor to consider for assemblies, where a polymer fastener undergoes sustained forces during application.

For example, when using snap-fit assemblies, it is important to assess stress relaxation. Such effects can be irreversible because the change to the fastener material is a molecular phenomenon — such as breaks or reorientations in molecular chains in the polymers.

While stress relaxation examines how stress varies, creep is a measure of an increase in strain. Both are used to determine dimensional stability:

• Stress relaxation: Constant strain is applied and the stress decreases with time.
• Creep: Constant stress is applied and the strain increases with time.

Stress relaxation can be challenging to measure accurately, and the specific form of strain response differs with each type or grade of polymer. Unfortunately, material data sheets may provide insufficient  information to describe a fastener’s long-term potential for stress relaxation. Practical testing is recommended to ensure the fastener meets design expectations.

As with most fasteners, the material’s performance must also be considered as it relates to the substrate. Misjudging the fastener choice can result in fastener loosening or pull-out, which may lead to joint failure in the assembly. For example, in medical applications, stress relaxation is particularly concerning because failure of the assembly could lead to leaks in fluid connections.