Case Studies of Fatigue Life Improvement Using Low Plasticity Burnishing in Gas Turbine Engine Applications

Surface enhancement technologies such as shot
peening, laser shock peening (LSP), and low plasticity
burnishing (LPB) can provide substantial fatigue life
improvement. However, to be effective, the
compressive residual stresses that increase fatigue
strength must be retained in service. For successful
integration into turbine design, the process must be
affordable and compatible with the manufacturing
environment. LPB provides thermally stable
compression of comparable magnitude and even
greater depth than other methods, and can be
performed in conventional machine shop environments
on CNC machine tools. LPB provides a means to
extend the fatigue lives of both new and legacy aircraft
engines and ground-based turbines. Improving fatigue
performance by introducing deep stable layers of
compressive residual stress avoids the generally cost
prohibitive alternative of modifying either material or
design.
The x-ray diffraction based background studies of
thermal and mechanical stability of surface
enhancement techniques are briefly reviewed,
demonstrating the importance of minimizing cold work.
The LPB process, tooling, and control systems are
described. An overview of current research programs
conducted for engine OEMs and the military to apply
LPB to a variety of engine and aging aircraft
components are presented. Fatigue performance and
residual stress data developed to date for several case
studies are presented including:
· The effect of LPB on the fatigue performance of
the nickel based super alloy IN718, showing the
fatigue benefit of thermal stability at engine
temperatures.
· An order of magnitude improvement in damage
tolerance of LPB processed Ti-6-4 fan blade
leading edges.
· Elimination of the fretting fatigue debit for Ti-6-4
with prior LPB.
· Corrosion fatigue mitigation with LPB in Carpenter
450 steel.
· Damage tolerance improvement in 17-4PH steel.
Where appropriate, the performance of LPB is
compared to conventional shot peening after exposure
to engine operating temperatures.