Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
86 Chapter III. HOT CRACKING RESISTANCE
III.6.3.2 How to answer this question?
III.6.3.2.1 Procedure
For dimensional reasons, the work of necking per unit area scales with the plate thickness while the
work of diffuse plasticity scales with the ligament length. The total work, Wtot thus may be written
W
� ��
2
2
tot � t0l0
w0
� t0
l0wn
t0l0
wp
, eq III-21
where t0 is the initial plate thickness, l0 is the initial ligament length, α and β are shape factors, w0 is
the average work spent per unit area for growing voids to final fracture, wn is the average work per unit
volume dissipated in the neck in front of the crack tip and wp is the average work per unit volume spent
in plasticity in the diffuse plastic zone. By measuring the total energy for separating a DENT specimen,
Wtot, for different thicknesses and ligament lengths and dividing this energy by the ligament area (wtot
= Wtot / l0t0), it is thus possible to separate w0, wn and wp (see Figure III.46):
wtot � w0
��t
0 wn
��l
0 wp
. eq III-22
w n
w tot= W tot/l 0t 0
t 0
w 0
Figure III.46. Procedure followed to separate the three contributions to the total energy: the work of
diffuse plasticity, wp (per unit volume), the work of localized plasticity wn (per unit volume), and the fracture
work w0 (per unit surface) [76].
III.6.3.2.2 Qualitative answer
In this work, the total energy for breaking a DENT specimen was not determined for different thick-
nesses, as a consequence it is not possible to separate the three contributions. However, Pardoen et
al. [76] have demonstrated that the average work per unit volume spent in the neck, wn is a function of
the rheology of the investigated material and of the final fracture strain.
As the equivalent strain at fracture exhibit similar values when comparing the D2_E microstructure
with the D2_W microstructure (Table III.10), it remains to check if both morphologies present a differ-
ent rheology.
Normalizing the force-displacement curves by dividing the force by the area of the ligament and by
dividing the displacement by the final displacement gives a good idea of the rheology (stress-strain
curve) of the material. As shown in Figure III.47, the curves belonging respectively to D2_E and D2_W
are superimposed. In others words, it means that there is no difference in terms of macroscopic rheo-
logical behaviour between D2_E and D2_W.
w p
l 0