To
go back to the main page of the Success Stories,
click
here
Auxiliary
Power Unit (APU) Exhaust Ducts
One
particular type of Auxiliary Power Unit APU is used
to provide power to an aircraft when it is on the ground
with the main engines turned off. The tubular exhaust
ducts are a high temperature 8009 aluminum alloy welded
in an end-to-end design.
Tension-tension
fatigue tests measured fatigue strength of 23 ksi (156
MPa) at 3,000 cycles in the as-welded condition. Glass
bead peening of the welds resulted in a 13% fatigue
strength improvement to 26 ksi (180 MPa).
top
of page
NASA
Langley Crack Growth Study
Engineers
at NASA performed a study on crack growth rates of 2024-T3
aluminum with and without shot peening. The samples
were tested with an initial crack of 0.050" (1.27
mm) and then cycle tested to failure. (It should be
noted that the United States Air Force damage tolerance
rogue flaw is 0.050" (1.27 mm)).
It
was found that crack growth was significantly delayed
when shot peening was included. As the following results
demonstrate, at a 15 ksi (104 MPa) net stress condition
the remaining life increased by 237%. At a 20 ksi (138
MPa) net stress condition the remaining life increased
by 81%.
This
test reflects conditions that are harsher than real
world conditions. Real world conditions would generally
not have initial flaws and should respond with better
fatigue life improvements at these stress levels.
(Note
on sample preparation: A notch was placed in the surface
via the Electro-Discharge Machining (EDM) process. The
samples were loaded in fatigue until the crack grew
to ~ 0.050" (1.27 mm). If samples were shot peened,
they were peened after the initial crack of 0.050"
(1.27 mm) was generated. This was the starting point
for the above results.)
top of page
Turbine
Engine Fan Disks
In
1991, the Federal Aviation Authority (FAA) issued an
airworthiness directive that required inspection for
cracks in the low pressure fan disk. At the time, over
5,000 engines were in use on business jets in the United
States and Europe. The FAA required that engines that
did not have lance (shot) peening following machining
in the fan blade dovetail slot be inspected. Those engines
having fan disks without lance peening were required
to reduce service life from 10,000 to 4,100 cycles (takeoffs
and landings). Disks that were reworked with lance peening
per AMS 2432 (Computer Monitored Shot Peening) prior
to 4,100 cycles were granted a 3,000 cycle extension.
top
of page
Turbine
Engine HP Compressor Rotors
Two
leading companies in the manufacture of jet turbine
engines jointly manufacture high pressure compressor
rotors. Separate pieces are machined from forged titanium
(Ti 4Al-6V) and then welded together. Testing produced
the following results: Initially, shot peening was used
as additional "insurance" from failure. After
many years of failure free service, coupled with innovations
in shot peening controls, shot peening has been incorporated
as a full manufacturing process in engine upgrades.
*
In aircraft engine terminology one cycle equals
the ramp up required for one take-off of the aircraft
for which the engine is configured.
top
of page
