1, high cycle fatigue (high cycle fatigue). The stress level acting on the parts is low, and the failure cycles are generally higher than the fatigue of 104 ~ 105. The fatigue of springs and transmission shafts falls into this category.
2. Low cycle fatigue (low cycle fatigue). The stress level acting on the parts is high, and the failure cycle times are generally lower than the fatigue of 104 ~ 105, such as the fatigue of pressure vessels and gas turbine parts. Practice shows that fatigue life is scattered, so statistical analysis must be carried out to consider the survival rate (that is, reliability).
The fatigue life Np with a survival rate of P (such as 95%, 99% and 99.9%) means that the fatigue life of individuals with P in the matrix (population) is longer than Np. And the failure probability is equal to (1-p). The S-N curve obtained by routine fatigue test is a curve with p = 50%. The S-N curve of P corresponding to each survival rate is called p-S-N curve.
Extended data
Fatigue is a common symptom, which also happens from time to time in healthy people. There are four main reasons for this:
First, modern people work hard;
Second, the usual physical condition is not very good or there are basic diseases;
Third, sudden or emergency situations, such as entrance exams, can also cause fatigue;
Fourth, seasonal factors, such as fatigue at the turn of winter and spring and at the turn of summer and autumn.
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