Page 439 - 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. 423

A DAMAGE MODEL FOR ESTIMATING LOW CYCLE FATIGUE
LIVES UNDER NONPROPORTIONAL MULTIAXIAL LOADING

Takamoto ITOH and Toshiki MIYAZAKI
Department of Mechanical Engineering, Fukui University,
9-1, Bunkyo 3-chome, Fukui 910-8507, Japan

ABSTRACT
This paper develops a damage model for the evaluation of low cycle fatigue lives under complex
cyclic multiaxial loadings. One of the authors has proposed the equivalent strain parameter for
the life prediction of the nonproportional low cycle fatigue. This strain parameter can evaluate
the dependence of fatigue lives on strain history and material, and correlates the fatigue lives
within a small scatter band for 15 kinds of proportional and nonproportional strain paths.
However, the parameter was only applicable to the life prediction under a limited
nonproportional strain history, so that some modifications were required. In this study, a simple
damage model for the life prediction is proposed by combining the equivalent strain parameter
with Miner’s law in order to apply it to the life prediction under more complex nonproportional
loadings. The applicability of the proposed model is examined for the life evaluation of
nonproportional low cycle fatigue for different materials; type 304 stainless steels, copper,
aluminum alloys, chromium-molybdenum and carbon steels, which were obtained from
different research institutes. The model can correlate most of the fatigue data within a factor of
two scatter band and has a potential to become a good damage model for nonproportional low
cycle fatigue.

KEYWORDS

Life prediction, Low cycle fatigue, Multiaxial loading, Nonproportional loading, Damage model,
Additional hardening, Path and material dependencies, Slip system

INTRODUCTION

The ASME Code Case N-47 [I] has been frequently used as a design criterion for
nonproportional low cycle fatigue (LCF), but recent studies [2-81 have shown that the Code Case