3. Results
3.1 Fatigue life
Fatigue test results are presented in Fig. 7-a and Fig. 7-b as semi-log
S-N curves (Wöhler curves), with S corresponding to the maximum stress
reached in the working section of the specimen during a fatigue cycle
and N corresponding to the number of cycles leading to failure. Power
trend curves are plotted on these graphs. For reasons of
confidentiality, the stress level S on the S-N graphs, as well as the
values of the surface integrity parameters presented in the following
sections (roughness, microhardness and nanohardness), are normalized by
their maximum value.
S-N curves obtained for D = 6.35 mm show no difference in fatigue
strength between the two drilling processes. In contrast, S-N curves
obtained for D = 9.53 mm show a significant increase in fatigue life for
the specimens drilled by the axial technique. This gain in fatigue life
can be observed for all stress levels but is greater for the lower ones.
In the aircraft industry, the fatigue performance of a structure is
commonly evaluated by the Airbus Fatigue Index (AFI), which corresponds
to the stress S associated with a forecast fatigue life of 100 000
cycles. Thus, AFI values were determined for all configurations from S-N
curves. They are shown in Fig. 7-c and Fig. 7-d (as values normalized by
the maximum AFI value obtained).
For D = 6.35 mm, similar AFI values can be observed for both drilling
processes whereas, for D = 9.53 mm, a gain in AFI of 15% is visible
with the axial drilling process.
The fracture surfaces were observed for all drilling configurations. No
difference in failure mode was noted between the different
configurations. For all specimens, the crack initiated in the hole edge
area and the fracture surface was divided into two distinct zones: the
crack propagation zone and the final fracture zone (Fig. 8).
Thus, the drilling configuration can have a significant influence on the
fatigue strength of the drilled part but it has no impact on the failure
mode of the specimen. The differences in fatigue life observed between
the drilling configurations were probably related to differences in the
hole surface integrity. Characterization test results of the hole
surface integrity are presented in the following sections.
3.2 Roughness measurements
Among all the roughness parameters measured, only the results of
arithmetic average height of profile (Ra), which gives an average
indication of the overall surface topological state, are presented here.
They are shown in Fig. 9.
For all drilling configurations, the measured Ra value complies with the
aeronautical specification. No correlation can be established between
the Ra results and the fatigue test results. A significant difference in
Ra is observed between the two drilling processes for D = 6.35 mm
whereas no difference in fatigue life is noticed between the two
drilling techniques for this hole diameter. This is probably related to
the relatively low Ra values measured compared to the aeronautical
specification. The Ra values are not high enough to have an impact on
the fatigue strength of the part.
The possible relationship between the hole roughness and the fatigue
life results was also studied for all the other 2D and 3D roughness
parameters measured. No obvious correlation between the roughness
parameters and the fatigue strength of the drilled part can be
highlighted from this analysis. As for Ra, this is explained by the low
roughness values measured.
Thus, in this study,
the hole roughness is not a major
factor influencing the fatigue life of the drilled part.
3.3 Hardness measurements
After analysis of the topographic aspects of the hole surface integrity,
the metallurgical and mechanical aspects were investigated. The Vickers
microhardness measurements carried out in this framework are presented
in Fig. 10.
For D = 6.35 mm, holes obtained by axial and orbital drilling
processes have similar microhardness levels whereas, for D = 9.53
mm, a significant difference in microhardness level is observed between
the two drilling techniques. For this diameter, a gain of 29% in
microhardness level can be seen for axial drilling.
These results show a correlation of fatigue life results with
microhardness measurements: the increase in hole microhardness for axial
drilling at D = 9.53 mm seems to explain the gain in fatigue life
observed for the same configuration (Fig. 7).
To study this result further, nanoindentation tests were performed. The
test results are presented in Fig. 11 as graphs showing the variation of
the nanohardness according to the distance from the hole edge.
For D = 6.35 mm, nanohardness results show a similar material
depth affected by hardness variation, around 15 µm for both drilling
processes. In contrast, for D = 9.53 mm, a significant difference
in material depth affected is observed between the two drilling
techniques. The material depth affected for orbital drilling seems to be
very small (less than 5µm) whereas it is around 40 µm for axial
drilling. Also, during indentation tests for the specimen corresponding
to axial drilling at D = 9.53 mm, the first indents column
induced quite small, irregular prints from which no hardness value could
be calculated. This indicates that the nanohardness value of the hole
subsurface obtained for this configuration seems to be high.
These results highlight the thinness of the subsurface material layer
affected by the drilling operation of a 2024-T351 aluminium part, in
agreement with the numerical model. The nanohardness results are in
accordance with the microhardness results. The hole hardness seems to be
a major factor influencing the fatigue behaviour of a drilled part with
a positive influence.
3.4 SEM-EBSD analysis
In order to identify the phenomena responsible for the difference in
hole surface hardness, a metallographic analysis was carried out. In
this framework, GROD maps of the hole edge area obtained through
SEM-EBSD analysis (Fig. 12) were analysed.
The GROD map obtained for axial
drilling at D = 9.53 mm shows a significant material depth affected by
grain misorientations, around 40 µm depth. This affected material layer
varies slightly according to the grains. In contrast, almost no grain
misorientation, or only some a few microns deep, is observed for the
other drilling configurations. A material layer affected by grain
misorientation corresponds to a strain hardened material layer. This
means that axial drilling at D = 9.53 mm induces a significant strain
hardening of the hole subsurface compared to other drilling
configurations.
These results explain the microhardness and nanohardness results. The
increase in hardness in the hole surface and subsurface, for axial
drilling at D = 9.53 mm, seems to be due to the significant strain
hardening of the material in the hole edge area. This assumption is
strengthened by the fact that the affected material depths identified
with the nanoindentation tests and with the SEM-EBSD tests are the same.
3.5 Residual stress
analysis
The residual stress state of the drilled part, which, according to the
literature, can also have a significant impact on the fatigue life, was
investigated through the novel HOCT technique. Specimen deformations
obtained with HOCT are presented in Fig. 13.
No deformation of the specimen is noticed after the material removal for
the case of axial and orbital drilling at D = 6.35 mm and the case of
orbital drilling at D = 9.53 mm. However, significant specimen
deformation is observed for axial drilling at D = 9.53 mm. This means
that this drilling configuration induces significant residual stress
fields in the hole edge area compared to the others.
These results show a correlation of the fatigue test results with the
residual stress state of the material. A high fatigue quality index is
associated with the presence of significant residual stresses in the
hole edge area. Furthermore, a high residual stress state is associated
with high hole hardness.