Issue 41

A. Cernescu, Frattura ed Integrità Strutturale, 41 (2017) 307-313; DOI: 10.3221/IGF-ESIS.41.41 308 crack tip. Due to direct implications in fatigue crack growth retardation, the crack closure phenomenon has been extensively analyzed both experimentally and numerically. In 1988, R.O. Ritchie, [5], brings into discussion the idea that fatigue crack growth retardation can be achieved by reducing the applied load or by toughening the material. He presents two classes of toughening mechanisms: intrinsic, by increasing the inherent microstructural resistance to crack advance and extrinsic, where the toughness arises from mechanisms of crack tip shielding . This term was first introduced and covers four categories: crack deflection and meandering, zone shielding (transformation toughening, microcrack toughening, crack wake plasticity, crack field void formation, residual stress fields, crack tip dislocation shielding), contact shielding (wedging, bridging, sliding, wedging + bridging), combined zone and contact shielding (plasticity-induced crack closure, phase transformation-induced closure). In the same manner Pippan and Hohenwarter, [6], describes the role of intrinsic and respectively extrinsic mechanisms in fatigue crack propagation, where the intrinsic mechanisms are responsible for the formation of new fracture surfaces at the crack tip by cyclic deformation. In a different study, Mutoh et al., [7], show that in steels with ferritic-pearlitic microstructure (networked or distributed pearlite), besides the crack closure phenomenon there are also other mechanisms of crack-tip stress-shielding phenomena, respectively branching and interlocking. They define an effective crack tip stress intensity factor range, Δ K eff,tip , that manages to better correlation of fatigue crack propagation data in a single curve, compared to the well-known Δ K eff . In Reference [8] it is described a model of fatigue crack propagation based on dislocation emission at the crack tip and respectively the dislocations in the plastic zone and plastic wake. The fatigue crack growth rate into a plastic zone can be reduced through a dislocation crack tip shielding mechanism. However, this mechanism can stop the advancing of the crack when the crack tip is fully shielded, but does not guarantee that do not form another tip that lead to an extension of the crack in another direction. Given the above, one can say that fatigue crack propagation is guided by the crack tip shielding mechanisms that acts actively by forming new crack tips and respectively passive crack tip shielding mechanisms. The passive crack tip shielding mechanisms cannot create crack tips if the applied loading does not exceed a certain value. From this point of view plasticity-induced closure, as extrinsic mechanism of combined zone and contact shielding, can be considered passive. If the applied loading does not exceed P op , the crack tip remains protected by the plastic field and the crack does not extend. In the following sections it is presented an analysis of fatigue crack growth into a plastic deformation zone given by an overloading cycle. In the case of constant amplitude loading the introduction of an overloading cycle leads to increasing of the crack opening load corresponding to that cycle. If the crack opening force associated to the overloading cycle, P op,OL is greater than the maximum force of the constant amplitude loading, then the crack tip is shielded, fig. 1. Figure 1 : The constant amplitude loading with an overloading cycle. However, it is known that the fatigue crack propagates within the plastic zone created by the overloading which indicates that once the material is strengthening there are active crack tip shielding mechanisms that helps the crack to growth through the retardation period. M ATERIALS AND METHODS or this analysis, fatigue crack growth rate tests were carried out in elastic-plastic steel. The chemical composition and mechanical properties of the material are given in Tabs. 1 and 2. The fatigue crack growth rate tests were conducted on CT samples, according to ASTM E 647, [9]. Also, tests have been performed corresponding to plane F