Meaning
Maximum deformation a polymer component can sustain over its intended service life without experiencing structural failure or excessive permanent creep. Long term strain limit is a critical design parameter for plastic parts that are subjected to constant or repeated loads. This limit is much lower than the short term yield strength found on a material datasheet, as polymers are prone to slow deformation over time.
Engineers use this value to ensure that the part will maintain its shape and function for the duration of its use. The limit depends on the type of resin, the operating temperature and the presence of any chemicals that might accelerate the degradation of the material. It represents the boundary between a reliable design and one that is likely to fail prematurely.
Creep Resistance
When a plastic part is under a constant load, the molecular chains slowly slide past each other, leading to a gradual increase in strain. This process, known as creep, continues as long as the load is applied and can eventually lead to the total failure of the part. The long term strain limit is the point beyond which the rate of creep becomes unpredictable or leads to a loss of structural integrity.
To determine this limit, materials are tested under constant load for thousands of hours at different temperatures. The results are used to create creep rupture curves, which show how the material behaves over long periods of time. Designers use these curves to choose the right material and wall thickness for their application.
A material with high creep resistance can sustain a higher strain limit over a longer period.
Service Lifetime
The intended life of the product is a major factor in determining the appropriate strain limit. A part that is only expected to last for a few months can be designed with a higher limit than one that must last for twenty years. For long life applications, like those found in the automotive or construction industries, the strain limit is usually set very conservatively to ensure a high level of safety.
Overestimating the limit can lead to parts that warp or crack before their intended end of life, resulting in costly repairs or recalls. Conversely, underestimating the limit can lead to over designed parts that are heavier and more expensive than they need to be. The final design must balance these competing factors to create a product that is both reliable and cost effective.
Every part is designed to stay within its strain limit throughout its entire service life.
Safety Margin
To account for variations in material properties and environmental conditions, designers always include a safety margin when setting the long term strain limit. This margin provides a cushion against unexpected loads or higher than normal temperatures that could accelerate the creep process. The size of the safety margin depends on the criticality of the application and the potential consequences of a failure.
For medical devices or structural components, the safety margin is typically very high. The margin also accounts for the fact that a moulded part may have internal stresses or weld lines that make it weaker than the material used in a test bar. By staying well below the maximum limit, the designer ensures that the part will remain stable and functional even under less than ideal conditions.
The final strain limit is a conservative value that guarantees the long term performance of the component.
