Page 421 - Biaxial Multiaxial Fatigue and Fracture

Cyclic Behauiour of a Duplex Stainless Steel Under Multiaxial Loading: Experiments and Modelling 405

EXPERIMENTAL RESULTS

Monotonic behaviour

The forming process and the microstructure of the material studied seem to enable the
assumption that the mechanical behaviour is transverse isotropic. The first quarters of cycle of
cyclic tests in different loading directions were compared in order to verify this assumption.
The yield stress (at 0.02 %) was measured in four directions in the plane (c, &T): tension
(O"), tension-torsion at 45", torsion (90") and compression (180"). 25 tests were used. The
Young modulus E and the shear modulus G were measured on each elastic part. The mean
values of E and G are respectively 192 GPa and 66 GPa. The yield stress on each specimen
was calculated using an equivalent plastic strain threshold of 2 lo4. Figure 4 shows the set of
yield stresses obtained drawn in the plane (a, 7).
Yield stresses obtained are scattered close to the circle defined by von Mises criterion
minimizing the distance to the set of points. The standard deviation of these distances is 30
MPa. These results enable us to assume that the virgin material has an isotropic behaviour.

400 7(MPa)
1

r
-600

-400

Fig. 4. Yield stresses in tensiodtorsion calculated using an equivalent plastic strain threshold
of 2 lo4, represented in (c, 7) plane.

Cyclic hardening-softening behaviour under proportional loading

Proportional cyclic tests were conducted in three directions in the plane (a, 6 7): at 0", at 45"
and at 90" (see Fig. 3). The cyclic stress-strain responses obtained are shown in Fig. 5 with the
monotonic stress-strain curve. It is observed that the duplex stainless steel sofiens cyclically
whatever the loading path. Moreover, stabilized stress levels are similar for the three directions
tested, the duplex stainless steel shows an isotropic behaviour under cyclic loading. This
validates then the use of von Mises equivalents in the following non-proportional tests.