Issue 35

Š. Major et alii, Frattura ed Integrità Strutturale, 35 (2016) 379-388; DOI: 10.3221/IGF-ESIS.35.43 385 some multiaxial fatigue criteria, becose the pedicle screw is under multiaxial loading. If Fatemi-Socie criteria is used, so- called “initiation curves” can obtained [6, 7, 8], see Fig. 11. In the Fig. 11 initiations curves for length of crack a 1 and a 2 are displayed. These curves shows the number of cycles required to achieve a crack length a 1 , respectively a 2 , if Fatemi-Socie parameters FS are known. The damage parameter is given by the equation: max max 1 2 y FS k              (3) In this equation, Δγ max is the shear strain increment in the plane where it has maximum value, k is a constant that is obtained from the fatigue tests, σ max is the normal stress perpendicular to the plane where is the maximum shear strain, and σ y is the yield strength. The number of cycle can be calculated by equation     , f a i Total n a da N FS a N FS C K     (4) Figure 11 . Initiations curves for length of crack a 1 and a 2 Figure 12 . Application of the prediction model in the test with F = 400 N (Nitrided pedicle screw).