5. Conclusions
In order to identify the surface integrity parameters that control the
fatigue strength of a 2024-T351 aluminium drilled part, fatigue tests
were carried out on open-hole specimens and the surface integrity of the
holes was characterized with a study scale and techniques suitable for
aluminium drilled surfaces. An innovative technique for the residual
stress evaluation of aluminium drilled parts, the HOCT, was implemented.
The main conclusions of the study are as follows:
- For hole roughness levels obtained with industrial drilling processes,
the hole roughness has no significant influence on the fatigue
strength
- The hole hardness has a significant positive influence on the fatigue
life and could be identified as a fatigue strength indicator for
2024-T351 aluminium aircraft drilled parts
- The increase in hole hardness for a 2024-T351 aluminium drilled part
is mainly due to the strain hardening of the subsurface material,
induced by severe mechanical loading during the machining process
- A high hole hardness is associated with a significant residual stress
state of the part in the hole edge area
The study focused on the fatigue life of open-hole specimens as a first
step, in order to avoid the phenomena associated with the introduction
of a fastener (impact of the tightening torque, impact of the assembly
clearance or interference level, etc.). In future works, it would be
interesting to study the fatigue life of assemblies of drilled parts
with fasteners, to assess the influence of the hole surface integrity in
this case.
Finally, in order to be able to improve the fatigue life of aircraft
structures, the influence of the drilling process (tool geometry,
cutting parameters, associated thermomechanical loading) will have to be
studied in more detail in order to develop a drilling process generating
optimum hole surface integrity.