There has been a flurry of actlvity lately to revlse the specifications that control the processlnq of parts subject to fatigue and corrosion fatigue failures. Our aginq Commercial, Mllitary and Civll airplane fleets, coupled wlth the costs of replacement, have caused great pressures to extend component operating life. Stress cycling of both englne and airframe hardware often is sufficient to cause fatigue damage. Shot peening has been proven to increase the ability of these parts to withstand significant increases in stress cycling. Other factors (corrosion, fretting and foreign object damage, to name a few) cause stress concentrations which are often initiation points for fatigue damage. Shot peening is used to encapsulate those stress concentrations in layers of residual compression thereby offsetting the conditions for fatigue damage.
It has become very obvious to those responsible for the safe operation of aircraft that the residual compressive stress from shot peening is a critical factor that must be controlled. AMS 2430 and MIL-S-13165 are the most notable specifications to be updated. New specifications have been written to control the quality of peening media and now include ceramic and cut wire steel shot in addition to the more usual cast steel shot and glass beads. More importantly, a new AMS specification for Computer Monitored Shot Peening has been written and is now referenced by the Military Spec. Quality auditors from the FAA, the military, and the various engine and airframe manufactures are now maklng sure processors have certified shot; they are also checking details such as anqle of impingement and particularly that parts are not peened manually.
Because of the critlcal environment in which aerospace structures and power plants operate and the need to extend the useful lives of these components, specifications are required that guarantee that all processes used after initial qualification are equivalent or superior to those used originally. Shot peening equipment that is capable of reproducing precise results is now mandatory. The ability to monltor all of the processing variables to insure repeatability is now being required. This paper addresses present requirements as well as what to expect in the future.