# Strength at variable stresses, Fatigue phenomenon...

## Fatigue phenomenon

In the process of working in the design there are different in sign and magnitude of the voltage. It has been experimentally established that the structural elements subject to the action of alternating stresses are destroyed under the action of stresses less than the proportionality limit.

The destruction of a material under the action of alternating stresses is called fatigue.

Fatigue failure begins with the formation in the weakest part of the part first micro-, and then macrocrack, which with a variable load tend to progressively develop. The development of cracks is particularly intense if the stresses vary not only in magnitude but also in sign.

The nature of the change in stress over time is very diverse. Often designs are tested

Fig. 2.48

the effect of loads randomly changing in time or, as they say, representing a random process.

If the maximum ( or ) and ( or xmjn) the stress values ​​are numerically equal to each other but opposite in sign, the voltage variation cycle is called the symmetric (Figure 2.48, a). If the maximum and minimum stresses in modulus are not equal to each other, then the cycle is called asymmetric (Figure 2.48, b).

The degree of asymmetry of the cycle is characterized by the asymmetry coefficient

(2.102)

A cycle whose minimum voltage is zero is called a zero (pulsating) (Figure 2.48, c).

asymmetry coefficient of the symmetric cycle , and for the zero cycle . In the case of constant stresses

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The value of is called the amplitude, and - the average voltage of the cycle.

It has been experimentally established that the number of cycles at which destruction occurs depends not only on the magnitude of the maximum (in absolute value) voltage, but also on the amplitude of the voltage fluctuation

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