The role of mean stress

To draw a Smith s fatigue strength diagram, the stress amplitude at the fatigue limit (te is measured for different values of the mean stress The 

Lowercase subscripts are used for the prescribed quantities, e. g., the mean stress 7m, while uppercase subscripts are used for the resulting quantities for endurance, e. g., the stress amplitude 7e. Or, in other words, for a given mean stress 7m, we search the fatigue limit 7e. 

According to this, the relation between Oa and corresponds to a straight line through the origin, with a slope that depends on the R ratio, as drawn in figure 10.27(a). 

Furthermore, the condition (iMax Rp niust hold to avoid yielding. The curve constructed in this way serves as a reasonable and conservative approximation for ductile materials in its range of validity. 

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