Original paper(Vol.49 No.1 pp.61)
Fatigue Properties of Shot-Peened Notched Components in Ti-6Al-4V Alloy at Elevated Temperatures
Abstract:The effect of shot peening on the fatigue strength of Ti-6AL-4V alloy was investigated in the temperature range of 20 degree C to 450 degree C. Specimens with a semicircular and a 60 degree V-shaped notches were used.
The increase in fatigue strength of semicircular specimens was 34.3%, 27.6%, -7.4% and -37.5%, and that of V-shaped specimens was 26.5%, 20.9%, 0% and 0% at 20 degree C, 250 degree C, 350 degree C and 450 degree C, respectively. The shot effect decreased with increasing temperature and became null or negative at 450 degree C. The behavior of the semicircular notched specimens was similar to that of unnotched specimens. The fatigue strength of semicircular specimens as well as unnotched specimens was decreased drastically by shot-peening at 450 degree C.
The decrease in the shot effect with increasing temperature could be explained as a consequence of the relief of compressive residual stress originating in cyclic loading at elevated temperatures. The fatigue strength of both the semicircular and the unnotched specimens was mainly controlled by the development of microcracks in the surface layer of specimens, and therefore the strength was sensitive to the surface roughness. At 450 degree C, the surface roughness remained and compressive residual stress became null during cyclic loading, so a severe drop in strength resulted in. At 450 degree C, the shot peened, and then electropolished specimens showed fatigue strength comparable to that of the unpeened and surface polished specimens.
The fatigue strength of V-shaped notched specimens is controlled by the nonpropagating behavior of a crack developed from a sharp notch root. The fatigue behavior of this specimens is insensitive to surface roughness. Therefore, even at 450 degree C, where the compressive residual stress could become null, there is no negative effect as found in semicircular notched specimens.
Key Words:Fatigue strength, Ti-6AL-4V alloy, Shot peening, Elevated temperatures, Notched specimens