How do temperature changes affect fatigue strength in plastic parts?

Temperature fluctuations can significantly impact fatigue strength. Higher temperatures often soften plastics, reducing their ability to withstand repeated stress, while colder temperatures can make some materials more brittle. Parts exposed to wide temperature swings should be evaluated under realistic thermal conditions to ensure long-term durability.

Can surface finish influence fatigue life in injection molded parts?

Yes, surface finish can affect fatigue performance. Rough surfaces or tool marks may act as initiation points for microcracks under cyclic loading, while smoother finishes generally reduce stress concentrations at the surface. Read more about finish types here.

Does part size or scale change fatigue strength behavior?

Part size can influence fatigue behavior because larger parts may experience non-uniform stress distribution or higher internal stresses during molding. Thicker sections may cool unevenly, affecting molecular structure and fatigue performance. Scaling a design up or down should include reevaluation of fatigue-related risks.

Fatigue Strength in Plastic Parts: Design Considerations

30 Jan 2026

BY nwmcadmin
POSTED IN Product Development
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What is fatigue strength in plastic parts?

Fatigue strength refers to a plastic part’s ability to withstand repeated or cyclic loading over time without cracking or failing. Unlike single-load strength, fatigue strength focuses on how materials behave when stress is applied again and again, even if that stress is well below the part’s maximum strength rating.

In injection molded parts, fatigue strength is especially important for components that flex, snap, hinge, or experience vibration during normal use. A part may appear strong during initial testing but still fail prematurely if fatigue behavior isn’t considered during material selection and design.

Why do plastic parts fail after repeated use instead of breaking right away?

Repeated loading causes microscopic damage to accumulate within the plastic material. Each stress cycle creates small internal cracks or molecular rearrangements that grow over time. Eventually, these defects reach a critical point where the part can no longer carry the load and fails suddenly.

This type of failure can be difficult to predict without understanding fatigue behavior. Because the part doesn’t fail immediately, designers may assume it is overbuilt, only to see issues arise after weeks, months, or thousands of cycles in real-world use.

How does fatigue strength differ between common injection molding materials?

Different plastics respond very differently to repeated stress. Some materials, such as polypropylene and polyethylene, are known for good fatigue resistance and flexibility, making them suitable for hinges and snap features. Others, like rigid styrenics or heavily glass-filled materials, may offer high stiffness but lower fatigue strength.

Material additives also play a role. Fillers, reinforcements, and modifiers can improve strength or heat resistance while reducing fatigue life. Selecting a material based solely on tensile strength or stiffness can lead to poor fatigue performance if cyclic loading is involved.

How do creep and fatigue strength work together in repeated-use applications?

Creep and fatigue often occur simultaneously in plastic parts subjected to repeated or sustained loads. Creep causes gradual deformation under constant stress, while fatigue results from cyclic stress. When combined, creep can increase localized stress levels, accelerating fatigue failure.

For example, a snap-fit that slowly deforms due to creep may experience higher stress during each use cycle. Over time, this combination can significantly shorten the part’s usable life. Designers must consider both behaviors together, especially for parts exposed to heat or long-term loading.

Which design features are most likely to reduce fatigue strength?

Sharp corners, abrupt thickness changes, and poorly supported features are common contributors to reduced fatigue strength. These elements create stress concentrations where cracks are more likely to initiate during repeated loading. Even small geometric details can have an outsized impact on fatigue life.

Living hinges, when designed correctly, are a notable exception. They allow controlled flexing over thousands or even millions of cycles by distributing stress evenly across a thin, flexible section. For more information, check out our more detailed post on living hinges here.

Can gate location and flow direction affect fatigue performance?

Yes, gate location and material flow direction can significantly influence fatigue strength. During injection molding, polymer chains tend to align in the direction of flow. This alignment can make parts stronger in one direction and weaker in another, affecting how they respond to repeated stress.

If a part is repeatedly flexed perpendicular to the flow direction, it may fail sooner than expected. Thoughtful gate placement can help align material flow with anticipated load paths, improving fatigue performance without changing the overall part design.

What types of applications require high fatigue strength in plastic parts?

A few applications include:

Living hinges and snap-fit closures
Medical devices with repeated actuation
Consumer products with moving or flexible components
Industrial enclosures exposed to vibration
Clips, latches, and locking mechanisms
Wearable or handheld devices

When should fatigue testing be considered during product development?

Fatigue testing should be considered as early as possible when a part will experience repeated loading in real-world use. Addressing fatigue strength during the design and material selection phase is far more effective than trying to correct failures after tooling is complete.

Rex Plastics can help evaluate fatigue considerations during product development, from material recommendations and design feedback to mold building and production support. Contact us today for a free quote and advice on your upcoming project.

Frequently Asked Questions

Question	Answer
How do temperature changes affect fatigue strength in plastic parts?	Temperature fluctuations can significantly impact fatigue strength. Higher temperatures often soften plastics, reducing their ability to withstand repeated stress, while colder temperatures can make some materials more brittle. Parts exposed to wide temperature swings should be evaluated under realistic thermal conditions to ensure long-term durability.
Can surface finish influence fatigue life in injection molded parts?	Yes, surface finish can affect fatigue performance. Rough surfaces or tool marks may act as initiation points for microcracks under cyclic loading, while smoother finishes generally reduce stress concentrations at the surface. Read more about finish types here.
Does part size or scale change fatigue strength behavior?	Part size can influence fatigue behavior because larger parts may experience non-uniform stress distribution or higher internal stresses during molding. Thicker sections may cool unevenly, affecting molecular structure and fatigue performance. Scaling a design up or down should include reevaluation of fatigue-related risks.

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Suggested Citation:
Rex Plastics. (2026, January 30). Fatigue Strength in Plastic Parts: Design Considerations. https://rexplastics.com/product-development/fatigue-strength-in-plastic-parts-design-considerations/.