For quite ten years, it's been usually established that fine grain microstructure is
relared to higher LCF resistance whether or not this trend is related to a large scatter of LCF
results particularly once the grain size decreases.
In the gift study , we are going to propose a replacement model supported an oversized range of information : a lot of
than 2750 specimens are investigated when strain controlled check, within the vary of
temperature from 200°C to 650°C and of strain vary between zero.48% and 1.24%. All the
fatigue tests are performed on machined and polished cylindrical specimens. The
applied strain controlled fatigue cycle is curved with a frequency of one to a pair of Hz. The loading
ratio is zero ANd solely an axial load is applied.
On figure a pair of, we are able to notice that a grain size of ASTM ten microstructures lead from many
thousands to 3 million cycles. From this thought, Snecma Moteurs has developed
some numerical models which permit to predict LCF resistance ensuing from the
microstructural parameters and in commission conditions. Moreover, this microstructure depends
on the shaping conditions in itself.
Sometimes. this trend will be delayed considering very low results. These abnormal
points were known to flow from to the LCF initiation section and additional exactly to initiation
sites on carbides once placed on the sample surface.
In the studied vary of temperature and deformation, test-pieces exhibit 2 completely different rupture
mechanisms. On one hand, initiation on carbides consistently makes rock bottom fatigue life
when they are on the surface of the sample and once they become as giant because the grain
structure (i.e. : grain size of regarding ASTM 7-8). Considering that the common size of a inorganic compound
is measured between twenty and forty microns, we will guess that in step with the thermomechanical
solicitation conditions, competition will occur between carbides and grains in
regards to fissure initiation once the microstructure become the identical order of
magnitude than the inorganic compound size
On the opposite hand, initiations on grains induce higher fatigue lives particularly as
grain size is finer and as they're located removed from the test-piece edge. Some failures on
carbides are ascertained close to the surface or within the test-piece, however they're associated
with longer lives. This behavior are often explained by completely different initiation mechanisms like
carbide cleavage, "intrusion-extrusion" on the surface grains, ... By characteristic the fatigue initiation part result and therefore the discrepancy between 2
microstructure measurements, the scatter in low cycle fatigue results is reduced. during this paper,
the inorganic compound rupture mode won't be thought-about additional on.