QUANTITATIVE ANALYSIS OF APPARENT FATIGUE LIMITS OF NOTCHED SPECIMENS

In this work, the apparent fatigue limits of notched specimens were analyzed quantitatively from acomprehensive point. Three-point bending fatigue limits of smooth specimens and three kinds of notchedspecimens of quenched and then high temperature tempered 35CrMo steel are determined. The stress distributionsof notched specimens are calculated by finite element analysis software ANSYS. The experimental results areanalyzed with the ``micro-meso-process theory of fatigue sourcegeneration'': though the generation of fatiguesource appears in individual weak grains as a result of dislocation movements, it must satisfy certain deformationharmonization condition as well as probability condition, therefore a``meso-yielding region'' containing quite afew grains must be formed. According to above analyses, the apparent fatigue limit of specimens is the maximumstress (expressed as nominal stress) required to create a critical meso-yielding region on the dangerous section ofspecimen when it is bearing alternating loads. The analyses also show that when take the critical dimension of themeso-yielding region, x_W, as 11 grains, the compositive error between calculated apparent fatigue limits ofnotched specimens and their measured values reaches the least. Under this condition, the errors of calculated onesare all under 5\%, so x_W can be considered as a characteristic parameter of material. The study above alsoelucidates the physical essence of concentration factor of fatiguestress k_W. In engineering practice, apparentfatigue limits of notched specimens can be predicted according tofatigue limits of material (\sigma_W), x_W and thestress distributions on the notch section.