Page 259 - Biaxial Multiaxial Fatigue and Fracture

Biaxialhiultiaxial Fatigue and Fracture
Andrea Carpinteri, Manuel de Freitas and Andrea Spagnoii (Eds.)
0 Elsevier Science Ltd. and ESIS. All rights reserved. 243

THE MULTIAXIAL FATIGUE STRENGTH OF SPECIMENS CONTAINING
SMALL DEFECTS

Masahiro END0
Department of Mechanical Engineering, Fukuoka Universiv,
Jonan-ku, Fukuoka 814-0180, Japan

ABSTRACT

A criterion for multiaxial fatigue strength of a specimen containing a small defect is proposed.
Based upon the criterion and the & parameter model, a unified method for the prediction
of the fatigue limit of defect-containing specimens is presented. In making this prediction, no
fatigue testing is necessary. To validate the prediction procedure, combined axial and torsional
loading fatigue tests were carried out using smooth specimens as well as specimens containing
holes of diameters ranging from 40 to 500 pm which acted as artificial defects. These tests were
conducted under in-phase loading condition at R = -1. The materials investigated were annealed
0.37 % carbon steel, quenched and tempered Cr-Mo steel, high strength brass and nodular cast
irons. When the fatigue strength was influenced by a defect, the fatigue limit was determined by
the threshold condition for propagation of a mode I crack emanating from the defect. The
proposed method was used to analyze the behavior of the materials, and good agreement was
found between predicted and experimental results. The relation between a smooth specimen and
a specimen containing a defect is also discussed with respect to a critical size of defect below
which the defect is not detrimental.

KEYWORDS

Multiaxial loading, fatigue thresholds, small defects, small cracks, 6 parameter model,
steels, brass, cast irons.

INTRODUCTION

Over a number of years a great deal of effort has been expended in the attempt to establish
reliable predictive methods for the determination of the fatigue strength under both uniaxial and
multiaxial loading conditions. However, prior to the 1970’s, the methods proposed did not
provide a useful mean for the analysis of materials which contained either non-metallic
inclusions or small flaws that are usually encountered in engineering applications. This was in
part because most of the proposed methods were applicable only to two-dimensional cracks or