Issue 38

D. Marhabi et alii, Frattura ed Integrità Strutturale, 38 (2016) 36-46; DOI: 10.3221/IGF-ESIS.38.05 41 Eq Rb Eq Te Te Rb Rb Eq Te Te Rb Eq Rb Rb Eq Rb Rb Rb Rb Rb A B C , , , 1 , 1 , , 1 , 1 , , 6 2 2 2 4 *[(2 ) ( ) ( )] 2 2 2 2 2 [4 *[( ) (2 ) ]( )                            (10b) From the Eq. (10a) according to the postulate energy and (Eq. 9) we define: m Rb Rb Rb Rb Rb Rb Te Eq m Te Rb Te RB Database B B A C MPa A MPa MPa MPa , , , 1 , 1 2 2 4 427 * 2 290 658 560                                                                     (11) Prediction of the Critical Stress Our proposal consists to study an analytical model and predict the critical stress:     2 * * 2 * 2 ( ) Eq Rb Eq Rb Eq Rb A B C        (12) We use the fatigue database of 30NCD16 steel [3-13]. The endurance limit D D Te Fr , 1 , 1 ,     and various stresses values Rb m Rb and ,   identify the roots by (Eq.12). Figure 2 : Over-energy (D.R.B) for various stress values of 30NCD16 steel. The critical stresses of any curve in Tab. 1 are situated between σ* and σ -1 and designed respectively by the stress of small crack σ* min and the stress of the smallest crack σ* max . The over-energy allows for the critical stress and requires a vigilance on the fatigue design in engineering structures.