Page 340 - Biaxial Multiaxial Fatigue and Fracture

324 S. POMMIER

orientation towards the load direction. In a polycrystal, the crystalline orientation varies
significantly and suddenly at grain boundaries. This effect could lead to the formation, in the
polycrystal, of a load percolation network analogous to that observed in a granular material. In
such a case, the intrinsic scale of the material would be larger than the grain size, since it would
be related to the scale of this load percolation network. Experiments and FEM analyses have
been conducted in order to check if this scale exists in elastic polycrystals and what would be
its importance in fatigue.

EXPERIMENTS
A few experiments have been conducted using the photostress technique. When a
photoelastically coated sample is subjected to loads, the resulting stresses cause strains to exist
over its surface. Because the photoelastic coating is bonded to the surface of the sample, the
strains in the sample are transmitted to the coating. The strains in the coating produce
proportional optical effects, which appear as isochromatic fringes when viewed with a
reflection polariscope.

10 mm

4

356 MPa 800 MPa 850 MPa

Fig. 2. Observations of the strain at the surface during a tensile test using the photostress
technique on a TA6V titanium alloy.

The experiments were conducted on a duplex TA6V, for which sets of a nodules were
shown to display the same crystallographic orientation over areas, so-called “macrozones”
[ 13,141, whose diameter is close to one millimetre. The conventional yield stress of this alloy is
850 MPa. In Fig. 2 are displayed pictures taken during a tensile test at various stress levels. At
356 MPa the sample is fully elastic, however it is observed that inclined bands are crossing the
specimen. It was checked using micro-strain gauges that these bands do not correspond to
plastic strain localisation bands. Moreover, in the points indicated as black dots in Fig. 2., the
principal strain directions were determined by photostress analysis. It was found that the local
principal strain direction in the inclined bands is aligned with the load axis, showing that the
observed inclined bands are not shear bands. At 800 MPa the bands are obvious and located at