Page 120 - Biaxial Multiaxial Fatigue and Fracture

BiaxiaVMultiaxial Fatigue and Fracture.
Andrea Carpinteri, Manuel de Freitas and Andrea Spagnoli (Eds.)
0 Elsevier Science Ltd. and =IS. All rights reserved. 105

LONG-LIFE MULTIAXIAL FATIGUE OF A NODULAR GRAPHITE CAST IRON

Gary B. MARQUIS* and Pgivi KARJALAINEN-ROIKONEN**
lhppeenranta University of Technology, Department of Mechanical Engineering,
P.O. Box 20, FIN 53851 hppeenranta, Finland
**
VTT Industrial Systems, P.O.Box 1705, FIN-02044 VTT, Finland

ABSTRACT

Nodular graphite cast iron is one example of a material that fails in fatigue primarily by the
initiation and growth of Mode I cracks. With this in mind, a tensile critical plane damage
parameter would be expected to correlate life for different stress states. Long life fatigue tests
have been performed on cast nodular graphite iron using uniaxial tension, torsion and equi-
biaxial loading. Results are compared to a Goodman-type fatigue limit criterion previously
developed for this material that successfully correlates data for a variety of stress states:
uniaxial tension with and without mean stresses, fully reversed torsion, cyclic torsion with
mean shear stress, and cyclic torsion with static normal stresses. The criterion includes a
multiaxial correction physically based on the growth of small cracks from notches and small
defects, and is able to account for the detrimental effect of the negative second principal stress
in torsion. The expected benefit of a positive second principal stress in biaxial tension was not
observed in experiments. The equi-biaxial stress state greatly increased the observed tortuosity
of fatigue cracking as compared to the uniaxial or torsion stress states. It is assumed that this
produced a lower fatigue limit because the crack driving force was nearly equal in all
directions thus allowing cracks to link up weaker regions of the complex cast iron
microstructure.

KEYWORDS

Nodular graphite cast iron, fatigue limit, equi-biaxial stress, torsion fatigue, Goodman, critical
plane

INTRODUCTION

Early damage models for multiaxial fatigue were developed primarily empirically using
numerical methods to fit data from two or more stress states. As more knowledge has been
gained about the complex growth mechanisms of cracks in uniaxial and complex strain fields,
newer damage models have been devised which attempt to capture the essential loading