Issue 35

Š. Major et alii, Frattura ed Integrità Strutturale, 35 (2016) 379-388; DOI: 10.3221/IGF-ESIS.35.43 384 beginning of process, see Fig. 10. The stress intensity factor grows much faster at the surface of screw. Therefore, it is necessary to implement a plurality of simulations for the initial phase of the process. Figure 8 : Crack in the thread, growing plane. Figure 9 : Relations between ratio a/b and the length of crack. Figure 10 : Relations between g radient of stress-intensity factor and the length of crack. T HEORETICAL MODEL or theoretical analysis of fatigue process was used new method proposed by Navarro [7, 8] and multiaxial fatigue criteria modified for notched specimens were tested [9]. Model proposed by Navarro combined initiation and propagation phase, without the need to define boundary length of crack, to differentiate between the two phases of crack growth. It is necessary, To calculate the number of cycles needed to reach the length a (curve a - N i , is necessary to know fatigue curve σ-N . The fatigue curve σ-N is in Fig. 1. This curve can be expressed by parameters obtained from F