Material Selection Requirements

• In addition to the magnitude, you must consider how long the load will be applied.
  
  
• Plastics will "Creep" (continue to deflect/deform) when subjected to a constant load for a long time.
  
  
• For parts subjected to long term loading, effects of creep must be considered to ensure that the deflection does not exceed the allowable limits over time.
  

• When subjected to constant strain, the resulting stress in the plastics diminishes over time. Imagine a metal insert pressed into a boss. As time goes on, the plastic boss material grips the insert with less and less force. Eventually the insert may become too loose, resulting in a failed joint.
  

• Bayer MaterialScience provides creep and stress relaxation curves for its materials.