Page 418 - Biaxial Multiaxial Fatigue and Fracture

402 J! AUBIN. I? QUAEGEBEUR AND S. DEGALLAIX

to loading history, in terms of loading amplitude and loading path [13-141. On the other hand,
the individual ferritic phase (BCC) shows a low sensitivity to non-proportional loadings and to
loading history [15-161.
The purpose of the present work is firstly to establish the mechanical behaviour of a forged
duplex stainless steel under uniaxial and biaxial cyclic loadings and secondly to test the ability
of a class of constitutive models to account for the behaviour observed. A large number of
phenomenological models have been developed during the last two decades to describe the
extra-hardening under non-proportional cyclic loadings [ 17-27, 121. These models consist
more often in modifylng the isotropic andor the kinematic rule through the introduction of a
non-proportionality parameter defined as a relationship between stress, plastic strain or back-
stress, or through the introduction of a structural tensor. The base model studied in the present
work is a cyclic plasticity model with non-linear isotropic and kinematic rules of the type
initially proposed by Armstrong and Frederick [28]. Three modifications of this base model are
tested to improve the description of the extra-hardening under non-proportional cyclic loading.
The first model was proposed by Benallal and Marquis [20] and modified by Calloch [lo, 121,
the second model is a modification of the first one proposed by Abdul-Latif et al. [25]. The
third model was developed by Tanaka [26].

MATERIAL

The material studied is a X2 Cr Ni Mo N 25-07 duplex stainless steel. This steel contains
approximately 60 % ferrite and 40 % austenite. The composition is given in Table 1. It was
supplied in rolled bars of 70 mm diameter, solution treated for an hour at 1060°C and then
water-quenched before machining the specimens. The resulting microstructure consists of long
austenitic rods (0 10 pm x 1 mm) in a ferritic matrix (Fig. 1). The microstructure seems to be
transverse isotropic.

Table 1. Chemical composition of the duplex stainless steel studied (in wt YO).

C Cr Ni Mo Mn Si N Cu P S Fe

0,024 24,68 6,54 2,84 0,79 0,62 0,17 0,07 0,021 <0,003 Bal.

Fig. 1. Microstructure in sections perpendicular (a) and parallel (b) to the bar axis. The
austenite is in white, the ferrite in grey.