EndoFEM Intergrated Methodology of Fatique Crack Propagation With Overloaded Delay Retardation

In this paper, EndoFEM integrated methodology developed for fatigue crack growth under constant amplitude load, was extended successfully to investigate the Kumar's experimental observations, especially in the increasing severity of delayed retardation due to increasing overload ratio. This phenomenon was accompanied with an increasing max. crack closure load (P cl ) within 0.5mm crack extension after overload, and then followed by a longer recovery stage with lower fatigue crack propagation (FCP) rate. In the methodology, the EndoFEM with r c controlled node-released strategy was employed to produce a 2mm fatigue crack length from 6% prestrained SEN specimen. To simulate the delayed retardation of overload with/without instant crack extension, five strategies of nodal release were proposed, and then the CMOD method was employed to determine P cl . The resulting variations of P cl during crack extension had the same tendency as those of experimental findings. These results allowed one to propose a quadratic relationship between P cl and K max under a fixed overload ratio. As a consequence, a conventional formula of crack closure model could be preserved with a modified coefficient, i.e. , da/dN = C (UΔK) m . The formula proposed could predict both the a vs . N crack growth data and the da/dN vs . ΔK data under fixed overload ratios quite well.