Today's KNOWLEDGE Share:Plastic Fatigue

Today's KNOWLEDGE Share:

Plastic Fatigue

I recently completed a failure analysis where the component cracked through fatigue. Fatigue is one of the primary plastic component failure mechanisms, along with impact, creep, environmental stress cracking, and molecular degradation.

Fatigue failure is the formation and propagation of cracks due to intermittent, repetitive or cyclic loading. Most fatigue failures are associated with cyclic loads that are significantly below the yield strength of the plastic material. Fatigue is sometimes referred to as dynamic fatigue in order to distinguish it from creep, which is at times called static fatigue.

“The failure occurs due to the cyclic nature of the load which causes microscopic material imperfections (flaws) to grow into a macroscopic crack (initiation phase). The crack can then propagate to a critical size that results in structural or pressure boundary failure of the component.” George Antaki, Ramiz Gilada, in Nuclear Power Plant Safety and Mechanical Integrity, 2015

Fatigue cracks commonly initiate at stress concentrations, structural discontinuities, or flaws within the molded component. Fatigue cracks can also propagate from existing macroscopic cracks, initiated through another failure mechanism, such as impact or molecular degradation.

Fatigue results in the progressive weakening of a component with increasing time under dynamic loading conditions, such that loads to be supported satisfactorily for short durations produce failure after long durations. Essentially, the intermittent, repetitive or cyclic loading results in a decay in the strength of the component. The fatigue strength of a plastic material depends on the period of action of the applied force. The applied stress can be compressive or tensile. This can result in various situations where the maximum and minimum stress of the dynamic loading are:

·   Tension - Tension

·   Tension - Neutral

·   Tension - Compression

·   Compression - Neutral

·   Compression - Compression

This is illustrated in the graphic below from “Fatigue and Tribological Properties of Plastics and Elastomers” Second Edition by Laurence McKeen.

In general, the higher the mean applied stress, the faster that cracking will initiate and propagate.

Source:Jeffrey A.Jansen

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