L. Allegrucci, Italian Air Force, Pomezia, Italy; F. Bagnoli, Italian Air Force, Pomezia ROME, Italy

Summary:

Impact surface treatments have been shown to provide a highly effective method for fatigue limit increase. Shot peening and rolling are extensively used in many industrial applications as well as on aircraft components for the purpose of retarding or even preventing catastrophic failures from fatigue, fretting fatigue, corrosion fatigue and stress corrosion cracking.

The positive effects of the treatments have been attributed to surface work hardening and the development of surface and sub-surface layers compressive residual stresses. The combination of work-hardening and compressive residual stresses causes an impediment to the fatigue crack initiation stage improving the total fatigue life. These mechanical processes have been employed on different materials (such as aluminium alloys and steels) at the manufacturing stage or after maintenance operations. The choice of treatment depends on the geometric shape of the aircraft components and the right procedure of both applications is essential to avoid the opposite effect. Investigation of aeronautic particulars (break assembly and main landing gear retraction cylinder), where the mechanical processes are required in accordance with the specifications, are presented.

The failures have been featured by premature overcoming of fatigue life and they show the negative effect of the absence of the compressive residual stress on the surface.