Issue 41

A. Cernescu, Frattura ed Integrità Strutturale, 41 (2017) 307-313; DOI: 10.3221/IGF-ESIS.41.41 310 Figure 3 : The fatigue crack growth rate function of stress intensity factor range for R = 0.1 Based on the curves in Fig. 3 have been determined the propagation constants corresponding to considered loading. The values are given in Tab. 3. Specimen type Thickness [mm] R-ratio Paris constants Δ K th,app [MPa √ m] Δ K th,eff [MPa √ m] C [m/cycle MPa √ m] n CT 10 0.1 4 ∙ 10 -11 2.51 22.5 10.5 Table 3 : The fatigue crack growth rate constants for constant amplitude loading with R = 0.1 In case of the second sample, after the application of overloading cycle the testing was continued with constant amplitude loading for a total number of 409768 cycles. Throughout the test the compliance technique could not detect an extension of the crack in the plastic deformation zone created by the overload, fig. 4. However, the microscopic analysis revealed that the crack propagated in the plastic deformation zone, fig. 5. Moreover, the character of fatigue crack growth in the plastic zone is totally changed in comparison with the propagation character before applying the overload. This indicates a change of the crack tip shielding mechanisms that govern the fatigue crack propagation. From a combined zone and contact shielding mechanisms was switched to a dislocation crack tip shielding mechanism. Accepting the crack closure function defined by Elber in the relations (1) and (2), it was made an estimation of the crack opening force corresponding to overloading cycle with maximum force of 20 kN and minimum force of 0.8 kN.    eff app P U P (1) and    0.5 0.4 U R (2) For the overloading cycle results:      max, , max, min, 0.5 0.4 OL op OL OL OL OL P P R P P (3) where R OL is the stress ratio of the overloading cycle. Solving the Eq. (3) results an estimation value for the crack opening force corresponding to overloading cycle of P op,OL ≈ 10.092 kN.