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Posted by admin on July 24th, 2012

Fatigue has to be considered when designing plastic parts that are subject to Repeated loading. The cyclical loading application is relatively infrequent and there is a long time between applications. If the loading is cyclical, use the proportional limit for design calculations. If the loading is repeated at short intervals and for long periods, use the S-N (stress vs. number of cycles) curves as the design criterion.

S-N curves

The S-N curves are obtained by tests run in bending, torsion, or tension, at a given constant frequency, temperature, and amplitude of loading. The stress at which the plastic will fail in fatigue decreases with an increase in the number of cycles, as shown in Figure 11. With many materials, there is an endurance limit (corresponding to the stress at the level-off section) below which stress level fatigue failure is unlikely to occur.

FIGURE 11. A typical flexural fatigue (S-N) curve with the Endurance limit below which the repeated load is unlikely to cause fatigue.

Fatigue phenomenon

Depending on the stress level, repeated loading to a relatively low stress level may not show complete recovery after each cycle. In addition, as the number of load and unload cycles increases, and the interval between loading decreases, micro-cracks on the surface or other physical defects could develop and over time lead to a decrease in overall toughness and eventual failure.

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