Issue 41

A. Cernescu, Frattura ed Integrità Strutturale, 41 (2017) 307-313; DOI: 10.3221/IGF-ESIS.41.41 313 a) b) Figure 8 : The fatigue crack propagation in sample overloaded with 82%P max,CA : a) before overloading; b) after overloading C ONCLUSION his paper presents an experimental study on fatigue crack propagation in elastic-plastic material. Is pursued in particular the character of crack propagation in terms of crack tip shielding mechanisms occurring or may occur under certain conditions of crack growth. It is obvious that a change in material behavior (introducing a plastic zone) it falls upon the crack propagation by changing the crack tip shielding mechanisms. The results also indicate that in the plastic zone dominant mechanism is dislocation crack tip shielding. In plane strain condition this mechanism is characterized in particular by interlocking effects of the crack. For plane stress the dislocation crack tip shielding mechanism is manifested by branching and interlocks giving more tortuous character of crack path. In relation to plasticity induced crack closure this mechanism can be considered as an active process that could help to crack extension even if the crack tip is shielded. Returning to description of Elber about the loading effect on fatigue crack growth and which say that the crack is fully open only for a part of the loading cycle characterized by Δ K eff , than what might happen on the portion between the minimum and opening load? Could this variation to trigger crack tip shielding mechanisms, considering that the crack is surrounded by a plastic deformation field? The results of this study indicate that the variation P op – P min could determine crack tip shielding mechanisms if P op is high enough. This can meet in area with severe plastic deformations such as those introduced by overloadings. R EFERENCES [1] James, M.N., Some unresolved issues with fatigue crack closure – measurement, mechanism and interpretation problems, https://www.gruppofrattura.it/ocs/index.php/ICF/ICF9/paper/viewFile/4331/1491 ; [2] Christensen, R.H., Fatigue crack growth affected by metal fragments wedged between opening-closing crack surfaces, Appl. Mater. Res., 2(1963) 207-210; [3] Elber, W., Fatigue crack closure under cyclic tension, Engng. Fract. Mech., 2(1970) 37-45; [4] Elber, W., The significance of fatigue crack closure, Damage Tolerance in Aircraft Structures ASTM STP 486 (1971) 230-242; [5] Ritchie, R.O., Mechanisms of fatigue crack propagation in metals, ceramics and composites: role of crack tip shielding, Materials Science and Engineering, A103 (1988) 15-28. [6] Pippan, R., Hohenwarter, A., Fatigue crack closure: a review of the physical phenomena, FFEMS, 00 (2017) 1-25. doi: 10.1111/ffe.12578; [7] Mutoh, Y., Korda, A.A., Miyashita, Y., Sadasue, T., Stress shielding and fatigue crack growth resistance in ferritic- pearlitic steel, Materials Science and Engineering A, 468-470 (2007) 114-119. doi: 10.1016/j.msea.2006.07.171; [8] Weertman, J., Dislocation crack tip shielding and the Paris exponent, Materials Science and Engineering A, 468-470 (2007) 59-63. doi: 10.1016/j.msea.2006.08.128; [9] ASTM E 647, Standard test method for measurement of fatigue crack growth rates, ASTM, USA. T