Issue 41

A. Cernescu, Frattura ed Integrità Strutturale, 41 (2017) 307-313; DOI: 10.3221/IGF-ESIS.41.41 312 a) b) Figure 7 : The variation curves of load vs. COD for: a) 26%P max,CA overloading; b) 82%P max,CA overloading. Specimen type Thickness [mm] Overloading Overloading crack opening load, P op,OL [N] CT_overload26% 2.42 26%P max,CA 1950 CT_overload82% 2.43 82%P max,CA 2400 Table 4 : The crack opening load corresponding to overloading cycles. In the first case of overloading, the value of P op,OL is less than the maximum force of constant amplitude loading. This made possible tracking by compliance technique the fatigue crack propagation in the plastic zone and respectively was able to determine the fatigue crack growth rate on the retardation zone given by the overloading cycle, fig. 6.a. Instead, in the second case of overloading the value of P op,OL is greater than the maximum force of constant amplitude loading and therefore the compliance technique could not detect the crack extension in the plastic zone, fig. 6.b. Meanwhile, in both cases the fatigue crack continued to propagate in the plastic zone but with a propagation character totally changed from the propagation mode in front of the overloading, fig. 8-9. a) b) Figure 8 : The fatigue crack propagation in sample overloaded with 26%P max,CA : a) before overloading; b) after overloading. The fatigue crack propagation in plastic zone was guided by a dislocation crack tip shielding mechanism characterized by branching and interlocking of the crack tip.