Influence of various factors on fatigue resistance
Stress concentration. Under the influence of variable loads, stress concentration reduces the endurance limit for various materials.
The effect of stress concentration on the endurance limit depends on the sensitivity of the material to the stress concentration and is taken into account in calculations using an effective concentration factor.
The effective concentration coefficient is the ratio of the endurance limit of a sample without a stress concentrator to the endurance limit of a specimen with a stress concentrator, made of the same material and having the same transverse dimensions of the working part as the first sample:
Effective concentration coefficients for normal stresses and tangential stresses are greater than unity and are determined through theoretical concentration coefficients and as follows:
where are the coefficients of the material's sensitivity to the stress concentration.
The sensitivity of the material to the stress concentration depends primarily on the properties of the material and increases with an increase in the tensile strength. Therefore, the use of high-strength materials at variable loads ns is always expedient.
For alloy steels ; for carbon
As experience shows, the sensitivity coefficient q also depends on the dimensions of the part and its shape. Therefore, in practical calculations it is more appropriate to use the effective coefficients and , found experimentally. The values of these coefficients for some types of stress concentrates are given in Chap. 4 in Table. 4.17-4.19.
The scale factor. The effect of the scale factor on the endurance limit is estimated in calculations by the coefficient , which is the ratio of the endurance limit of a smooth specimen of a given diameter D to the endurance limit of a standard sample with a diameter d :
The coefficient k f j is determined from the table. 4.20.
State of the surface. Surface defects reduce the fatigue resistance of the part. It has been established by experiments that the limit of endurance of samples with a polished surface is higher than that of polished ones, and in grinded ones it is higher than that of cutters, etc.
The effect of surface cleanliness on the endurance limit is estimated by the coefficient , equal to the ratio of the endurance limit of a specimen with a given surface treatment to the endurance limit of the same sample, but with a carefully ground surface:
Coefficient К /, is determined from the graph presented in Ch. 4 in Fig. 4.71.
Temperature. Tests of samples from steel and light alloys have shown that at moderate temperatures (up to 200 ° C) the endurance limit does not change significantly. At high temperatures, the fatigue curve for steels does not have a horizontal asymptote. In this case, assessments are made only taking into account the limit of limited endurance. At low temperatures, the endurance limit rises.
The external environment. At variable loads, corrosion significantly reduces fatigue resistance, especially light alloys.
In steels, the lower the endurance limit for corrosion, the greater the higher strength steel.
In the presence of a corrosive medium, the test time also affects the fatigue resistance; the destructive number of cycles decreases with a decrease in the frequency of changes in stress, and the fatigue curve, even in ferrous metals, does not have a horizontal asymptote. The quantitative characteristics of the reduction in endurance depend on the aggressiveness of the external environment.
The influence of a corrosive medium is taken into account in calculations by the coefficient
where - the limit of endurance in the presence of an aggressive environment. Values of the coefficient K are given in [10, 17].
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