Effect of loading variables on overload induced delay cycle in fatigue

The introduction of single overload cycle superimposed on the constant ampl itude loading cycles causes retardation of the growing fatigue crack. Gener ally, the retardation is expressed by delay cycles N-D i.e. the total numbe r of cycles involved during retardation and the retarded crack length, a(D) The delay cycle N-D is considered to be an important parameter to characte rize crack growth retardation. The objective of the present investigation i s to investigate the effect of loading variables such as stress intensity r ange. DeltaK, stress ratio, R, and over loading ratio ( OLR) on delay cycle s, N-D. The study has been conducted on 2024 - T3 Aluminium alloy and 4130 Cr - Mo steel sheets with yield strength of 375 and 587 Mpa respectively. F atigue tests have been conducted on centrally cracked tension specimens emp loying an Electro - Magnetic Resonance machine at a frequency of similar to 100 Hz. The crack extension was measured using Electrical Potential Differ ence (EPD) technique as well as traveling microscope. An attempt has also b een made to study the effect of overload induced crack closure and plastic zone size on the retardation behavior. Further the delay cycle has also bee n correlated with the retarded crack length, a(D). The important findings o f the investigation have been highlighted.