Page 301 - Biaxial Multiaxial Fatigue and Fracture

BiaxiaYMultiaxial Fatigue and Fracture
Andrea Carpinteri, Manuel de Freitas and Andrea Spagnoli (Eds.)
0 Elsevier Science Ltd. and ESIS. All rights reserved. 28 5

THE BACKGROUND OF FATIGUE LIMIT RATIO OF TORSIONAL FATIGUE TO
ROTATING BENDING FATIGUE IN ISOTROPIC MATERIALS AND MATERIALS
WITH CLEAR-BANDED STRUCTURE

Takayuki FLJKUDA' and Hironobu MSITAM'
' Department of Mechanrcal Engineering, Sasebo National College of Technolom.
Sasebo, 857-1193, Japn
Department of Mechanical Engineering, Kyushu Sangyo University
Fukuoka, 813-8503, Japan

ABSTRACT
The fatigue limit ratios of torsional to rotating bending fatigue ( zw/uw) have been determined
in several steels. This ratio was found to be equal to about 0.55 in an annealed rolled carbon
steel ((24.2, 0.45%) and about 0.65 in an annealed cast carbon steel, in a diffusion annealed
carbon steel (C=0.45%) and in a quenched & tempered carbon steel (C=0.45%). The difference
between the two values is due to the microstnrcture. The annealed rolled carbon steels
delivered as round bars have a clear-banded structure of ferrite and pearlite. The other three
materials have no banded structure. Namely, the former materials are regarded as anisotropic,
while the latter ones are considered as isotropic materials. The clear-banded structure in the
axial direction greatly affects the torsional fatigue limit, but it does not affect the rotating
bending fatigue limit. This is due to the fact that, in the carbon steel with a clear-banded
structure, large local strain concentration occurs within the fenite in torsional fatigue but not in
rotating bending fatigue. Because of this fact, the torsional fatigue limit in a banded structure
relatively decreases and, therefore, the value of fatigue ratio T w/ uw becomes small.

KEYWORDS
Fatigue limit ratio, Torsional fatigue, Rotating bending fatigue, Isotropic material, Banded
structure, Carbon steel.

INTRODUCTION
A large number of studies have been made on the fatigue behavior under combined stresses
[I-8]. In particular, Nishihara [2], Gough [3] and Findley [4] have studied the fatigue strength
under torsional and rotating bending combined stresses. They proposed methods according to
which the fatigue limit under general combined stresses is calculated from individual fatigue
limits uw and rW, where uw is the rotating bending fatigue limit and tw is the torsional
fatigue limit.
Since the fatigue limit under general combined stresses is usually discussed according to the