FATIGUE-STRENGTH OF NOTCHED SPECIMENS HAVING NEARLY EQUAL SIZES OF FERRITE

In this study, rotating bending fatigue tests were conducted on circum ferential notched large and small specimens of three kinds of carbon s teel with nearly equal (20 mu m) ferrite grain sizes but different vol ume fractions of pearlite. The effect of microstructures on the fatigu e strengths sigma(WS) (the limit stress for slip bands), sigma(W1) (th e limit stress for macrocrack initiation), and sigma(W2) (the limit st ress for fracture) is discussed by comparing these values in three kin ds of carbon steel. The main results newly obtained are as follows. (1 ) Since sigma(WS) is localized to a highly specified region, variation of K-t sigma(WS) (maximum stress repeated at the notch root in the li mit stress for slip bands) is small, irrespective of notch root radius . (2) If the grain size of ferrite is nearly equal, variation of sigma (W1), is small irrespective of pearlite under the threshold amount of pearlite. The threshold volume fraction of pearlite is about 50%. (3) In medium-carbon steel, sigma(W2) increases with increasing grain size of ferrite. (4) Notch root radius at the branch point, rho(0), varies depending on the grain size of ferrite. (5) The values of K-t sigma(w 1) (maximum stress repeated at the notch root in the limit stress for macrocrack initiation) and K-t sigma(w2) (maximum stress repeated at t he notch root in the limit stress for fracture) can be determined by t he notch root radius rho alone, independent of geometrical conditions.