Page 399 - Biaxial Multiaxial Fatigue and Fracture

BiaxiaVMultiaxial Fatigue and Fracture
Andrea Carpinten, Manuel de Freitas and Andrea Spagnoli (Eds.)
Q Elsevier Science Ltd. and ESIS. All rights reserved. 383

A MULTIAXIAL FATIGUE LIFE CRITERION FOR NONSYMMETRICAL AND
NON-PROPORTIONAL ELASTO-PLASTIC DEFORMATION

Mauro FJLIPPINI' , Stefan0 FOLETTI',
Ioannis V. PAPADOPOULOS2 and Cetin Morris SONSIN03

' Dipartimento di Meccanica, Politecnico di Milano, Milano, Italy
2
European Commission, JRC, IPSC, Ispra, Italy
' Fraunhofer-Institute for Structural Durability LBF, Darmstadt, Germany

ABSTRACT
A new low-cycle multiaxial fatigue life prediction methodology based on the concept of an
effective shear strain is proposed. This effective shear strain is derived by averaging the total
shear strains acting on all planes passing through a material point. The proposed model, which
is formulated as a generalised equivalent strain, takes into account the effect of non-
symmetrical loading cycles. The main advantage of the model relies on the small number of
material parameters to be identified. The axial cyclic stress-strain curve, the basic strain-life
curve (Manson-Coffin) and an additional life curve obtained under zero to tension strain
controlled axial fatigue tests are sufficient to allow application of the proposed criterion in all
loading conditions. The experimentally observed fatigue lives of proportional and non-
proportional multiaxial strain controlled low-cycle fatigue tests from un-notched tubular
specimens, have been compared with the predicted lives of the proposed approach showing in
all cases a good agreement.

KEYWORDS
Multiaxial fatigue criteria, strain-controlled fatigue, mean strain, Inconel 7 18 alloy, steel.

INTRODUCTION

Since many mechanical components are subject to cyclic multiaxial loading, fatigue evaluation
is becoming one of the major issues in the lightweight design of structures. Many methods
have been proposed to reduce the complex multiaxial stresdstrain state to an equivalent
uniaxial condition, namely empirical formulas, stress or strain invariants, strain energy, critical
plane approaches and space average of stress or strain. Historically, the first multiaxial low-
cycle fatigue criteria have been based on the extension of static criteria, e.g. maximum
principal strain, maximum shear strain or maximum octahedral shear strain criteria: the main