Page 417 - Biaxial Multiaxial Fatigue and Fracture

BiaxiaVMultiaxial Fatigue and Fracture
Andrea Carpinteri, Manuel de Freitas and Andrea Spagnoli (Eds.)
0 Elsevier Science Ltd. and ESIS. All rights reserved. 40 I

CYCLIC BEHAVIOUR OF A DUPLEX STAINLESS STEEL
UNDER MULTIAXIAL LOADING: EXPERIMENTS AND MODELLING

Vkronique AUBIN, Philippe QUAEGEBEUR and Suzanne DEGALLAIX
Laboratoire de Mbcanique de Lille, Ecole Centrale de Lille,
Citk scientifique, BP 48, 59651 Yilleneuve d 'Ascq Cedex, France

ABSTRACT
The low-cycle fatigue behaviour of a duplex stainless steel, 60 % ferrite - 40 % austenite, is
studied under tension-compressiodtorsion loading at room temperature. It is shown that the
duplex stainless steel has an isotropic behaviour under cyclic proportional loading. The non-
proportional loading paths induce an extra-hardening, but lower on duplex stainless steel than
on austenitic stainless steels. Three models able to account for the extra-hardening are
identified and tested on the experimental data base. Two of them give accurate predictions.

KEYWORDS

Duplex stainless steel, cyclic plasticity, biaxial loading, extra-hardening, experimental study,
low-cycle fatigue, constitutive modelling

INTRODUCTION
The use of austeno-ferritic stainless steels (duplex stainless steels) in branch of industry with
severe conditions in terms of corrosion and mechanical resistance widely developed for about
thirty years. Duplex stainless steels are notably used for applications in the power, offshore,
petrochemical and paper industries. The combination of their austenitic and ferritic phases
bring them an excellent resistance to corrosion, particularly to intergranular and chloride
corrosion, and very high mechanical properties, in terms of Yield Stress and Ultimate Tension
Strength as well as in terms of ductility. These properties result from their "composite" nature
and from their very small grain size (10 pm). A nitrogen addition, essentially concentrated in
the austenitic phase, is nowadays a usual practice. It enables to improve the corrosion
resistance and to increase the Yield Stress [ 1-31.
The properties of duplex stainless steels are closely linked to the two-phase nature of these
materials, in terms of crystallographic structure (FCC for austenite and BCC for femte),
volume fraction and morphology of each phase, interactions between phases. These various
parameters influence the cyclic mechanical behaviour of that composite material and modify its
stress response to variable loading, with variations of amplitude and/or direction in time and in
space. Especially, the austenitic phase (FCC) of duplex stainless steels has a low stacking fault
energy which favours the planar slip of dislocations. This phase is consequently very sensitive
to non-proportional cyclic loadings, the obtained extra-hardening is clearly observed in
austenitic stainless steels such as AIS1 304L or 316L [4-121. Moreover, this phase is sensitive