Shot Peening Success Stories for Aerospace Applications

Shot peening has proved its effectiveness in extending the service life and enhancing the performance of metal components by protecting them against fatigue, fretting fatigue, stress corrosion cracking and a variety of other failure mechanisms. Following are some of these success stories for aerospace applications:

• Auxiliary Power Unit (APU) Exhaust Ducts
• NASA Langley Crack Growth Study
• Turbine Engine Fan Disks
• Turbine Engine HP Compressor Rotors

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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).

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.)

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.

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:

• As Welded – 4,000 cycles*
• Welded and polished – 6,000 cycles
• Welded and peened – 16,000 cycles

* In aircraft engine terminology one cycle equals the ramp up required for one take-off of the aircraft for which the engine is configured.

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.

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