Issue 43

F.Z. Seriari et alii, Frattura ed Integrità Strutturale, 43 (2018) 43-56; DOI: 10.3221/IGF-ESIS.43.03 46           max . 1 t t t p p r I I IP K BC a y K (6) where I, tp, tr and y max are respectively the I is the total moment of inertia of the plate repair, thickness of specimen, thickness of patch and y max is the distance of the outer sheet edge from neutral axis. Fatigue crack growth model The Forman/Mettu equation [36] also called Nasgro equation, used by several researchers [37-40] for prediction and investigation of fatigue crack growth of metals and alloys, is given by:                              ' ' Im 1 1 1 1 p th n I I q ax crit K K f da C K dN R K K (7) where C, n, p’, and q’ are empirically derived from experimental fatigue crack growth rate for specified material. The others parameters in equation 7 are detailed in Afgrow technical manual [37]. Beretta and Carboni [38] have used Nasgro equation in the investigation of the effect constant amplitude loading (R ratio) and spectrum loading on fatigue crack growth of 30NiCrMoV12 steel. In order to investigate the effect of variable amplitude loading on fatigue crack growth of patched specimens is necessary to account of the interaction of levels of applied load. Generalized Willenborg model [41] presents the most common load interaction models used in crack growth life prediction. The model is based on early fracture mechanics work performed at Wright-Patterson AFB, OH. The model uses an "effective" stress intensity factor based on the size of the yield zone in front of the crack tip. The formulation of the generalized Willenborg retardation model used in fatigue code and integrated in crack growth equation (3) is given below:             max( ) Im min( ) Im min( ) max( ) / r eff ax r eff in eff eff eff K K K K K K R K K (8) K r is the residual stress intensity factor due to overload, it is given by the following equation and R eff is the effective stress ratio.          Im ( ) Im ( ) 1 ol r ax ol ax y ol a a K K K R (9) Factor  expressed by equation 10, define the level of residual stress induced by application of overload.          Im 1 / / 1 th ax K K SORL (10) and the yield zone created by overload R y(ol) is expressed by the following equation:          2 Im ( ) ( ) 1 . 2 ax ol y ol e K R (11)