Evaluating Polymer Fatigue Limits under Cyclic Mechanical Loading
Polymer fatigue limit evaluation requires separating mechanical crack growth from hysteresis heating by controlling strain amplitude, frequency, and thermal dissipation.
Fatigue Notch Factor
Meaning
Ratio of the fatigue limit of an unnotched specimen to the fatigue limit of a notched specimen of the same material under cyclic loading conditions. The fatigue notch factor accounts for the reducing effect of surface discontinuities like holes, grooves, or fillets on the fatigue strength of a component over millions of operational cycles. This ratio ceases to be meaningful when the material is subjected to static loads rather than cyclic stress because static failure mechanisms differ fundamentally from fatigue propagation.
Numerical Ratio
Mathematical calculations yield a value that is always greater than or equal to unity. Calculating the fatigue notch factor involves dividing the unnotched fatigue limit by the notched fatigue limit. This value is typically lower than the theoretical stress concentration factor due to localized plastic deformation at the notch root.
Stress Concentration
Geometry changes create localized stress spikes that act as initiation sites for fatigue cracks under cyclic loads. A high fatigue notch factor indicates that the component is highly susceptible to premature failure at these geometric features. Standard designs minimize sharp corners to reduce these risks.
Material Sensitivity
Ductile alloys tend to show less sensitivity to notches than high-strength, brittle materials under cyclic conditions. Low values for the fatigue notch factor are common in metals that can distribute localized stresses through plastic flow. Selecting tough alloys helps mitigate the negative effects of unavoidable geometric changes.