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.
Stress Ratio R
Meaning
Dimensionless parameter defined as the ratio of the minimum applied stress to the maximum applied stress in a cyclic fatigue test. The value of the stress ratio R determines the nature of the fatigue cycle, indicating whether the material experiences fully reversed, pulsating, or tension-tension loading, which significantly affects the fatigue life of the tested specimen. This parameter does not apply to static loading conditions where stress does not fluctuate over time.
Numerical Influence
Negative values of this ratio indicate that the specimen undergoes alternating tension and compression during each cycle. Setting the stress ratio R to zero represents a zero-to-tension cycle where the minimum stress is zero. This value has a strong effect on the crack growth rate.
Testing Condition
Standardized laboratory protocols require maintaining a constant ratio throughout the duration of the fatigue test. Operating with a specific stress ratio R allows researchers to isolate the effects of mean stress on material degradation. This ratio is controlled through automated hydraulic systems.
Failure Effect
High ratios generally lead to faster crack propagation because the crack tip remains open during a larger portion of the cycle. Under conditions where the stress ratio R is negative, crack closure effects can slow down the fatigue damage process. This behavior is analyzed to design fail-safe structures.