Issue34

A. Shanyavskiy, Frattura ed Integrità Strutturale, 34 (2015) 199-207; DOI: 10.3221/IGF-ESIS.34.21 205 a) b) Figure 7 . S-N curves: (а) for notched specimens subjected to stress ratio R=-1.0; (b) for smooth specimens at R=0.1. ▲ – TSP; □ – Т; ■ – specimens failed under tension. a) b) Figure 8 : View of : (a) fatigue striations in fracture surface of one of the fatigued specimens; (b) typical dependence fatigue striation spacing  and number of load cycles Np on the crack length a . Relationship N p /N f between crack growth period N p and durability N f has been estimated based on the above result, Fig.9. The evaluated dependence has the same tendency for all tested specimens without significant difference in the range of durability 10 6 – 2x10 7 load cycles. TP – specimens have lower crack growth period for the same durability in accordance with the T – specimens. In notched specimens, crack growth period is more than that for smooth specimens at the same durability, but this difference has significance only for durability lower than 5x10 5 load cycles. It can be summarized the above tendency by the following sequence: more crack growth period (on the stage of fatigue striation formation) for notched specimens without shoot peening; then lower growth period for notched and peened specimens; then smooth specimens without peening; and finally smooth-peened specimens. Therefore, crack origination subsurface and on the surface with fatigue striations formation has approximately the same dependence relationship N p /N f against durability N f . It can be used for aircraft structures to estimate durability by the results of fatigue striation spacing measurement or, based on knowledge about in-service durability, we have possibility to estimate crack growth period. This result agrees with earlier discovered possibility to estimate durability in low-cycle- fatigue for subsurface crack origination by the crack growth period based on the relation N p /N f against durability N f , found for crack origination on the surface [12].