# Definitions for Fatigue

 Endurance Limit As the alternating stress gets smaller, the material can take more stress cycles before it fails due to fatigue. The endurance limit is the highest alternating stress that does not result in fatigue failure. In other words, if the alternating stress is equal to or lower than the endurance limit, the number of stress cycles to cause failure becomes very large (practically infinite). The endurance limit is usually defined for zero-mean alternating stresses. The endurance limit is also called the fatigue limit. Some metals do not have a measurable endurance limit. Alternating Stress The alternating stress is defined as |(σmax - σmin)| / 2 where σmax and σmin are the maximum and minimum stresses respectively. In Simulation, you can set the stress component to calculate the alternating stress as: the Stress intensity (P1 - P3), the equivalent stress (von Mises), or the maximum absolute principal stress (P1). Stress Range Stress = (σmax - σmin) Mean Stress Mean stress = Sm = (σmax + σmin)/2 Stress Ratio Stress ratio R = σmin/σmax Fatigue Life Fatigue life, at a given alternating stress level and mean stress, is the number of cycles required to cause failure due to fatigue. Infinite Life Number of cycles where the fatigue strength ceases to decrease. In other words, the number of stress cycles required to cause failure is practically infinite. Fatigue Strength The stress at which fatigue failure occurs after a given number of loading cycles.