THE ESTIMATION OF SURFACE HARDENING INFLUENCE ON THE ENDURANCE LIMIT OF CYLINDRICAL PARTS OF VARIOUS DIAMETERS UNDER STRESS CONCENTRATION CONDITIONS
The influence of surface plastic hardening on the endurance limit of cylindrical parts of various diameters (10 - 50 mm) made of steel 20 with stress concentrators under bending is investigated experimentally. We consider the semicircular notches with radii of 0.3; 0.5 and 1 mm. The distribution of residual stresses in smooth specimens is determined experimentally by the rings and strips method. For the notched specimens it is determined numerically by calculation the sum of additional residual stresses due to the redistribution of residual loadings, after advancing hardening and residual stresses of smooth specimens. The distributions of axial residual stresses in smooth specimens and in the smallest cross section of specimens with concentrators are given. It was found that for predicting the endurance limit of the surface-hardened cylindrical parts under the stress concentration it is preferable to use the mean integral criterion of residual stresses. It is shown that after treatment of surface by microbeads and running roller in specimens with stress concentrators the endurance limit value increased by 30 ÷ 100% compared to the non hardened specimens. The experimental and calculated values of the endurance limits are given.