The effect of nitrided layer at notch surface on initiation and early growth of fatigue crack in A.I.S.I. 4140 steel was studied. The crack initiation period was defined as the number of elapsed cycles until a crack extends over the full lateral dimension of the notch surface, i.e. 5mm thickness of the specimen.
The cycles to fatigue crack initiation, $N_i$ was plotted as a function of $({\Delta K}/\surd{\rho})$for specimens with different nitriding time and for specimens without nitriding, from which the fatigue crack initiation threshod, $({\Delta K}/\surd{\rho})_{th}$ was determined. The result shows that $({\Delta K}/\surd{\rho})_{th}$ increases as the nitriding time increases. The period until the initiated crack propagated through the distance of 0.4mm in the nitrided layer for specimens of 2 hour nitriding time. The result shows that the propagation is more rapid in nitrided layer than in the un-nitrided specimen. However, the sum of fatigue crack initiation period and propagation period in the nitrided layer is much longer than that in the un-nitrided specimen near threshold value of $({\Delta K}/\surd{\rho})$.
Crack growth rate, da/dn was measured from the moment that the crack completely propagated through the nitrided layer in the front of the notch. The growth rate, as expected, was depending, on the thickness of the nitrided layer on both sides of the specimen.
