Short fatigue crack propagation from notches in N18NI based superalloy

Fatigue crack growth rates were measured on notched specimens with a tiny crack (congruent to 0.1 mm deep) located at the minimum section. These tests were performed on a powder metallurgy Ni base superalloy, N18. The material was tested at 650 degrees C under trapezoidal load cycles. Two experimental techniques were used to measure the length of small semielliptical cracks initiated from the EDM slot. The first technique which is the conventional potential drop method is shown to be inappropriate to measure crack lengths less than 400 mu m in depth. On the other hand, the second technique based on high resolution optical observations proved to be efficient to detect crack growth increments as small as 10 pm from the EDM slot. These crack growth rates were correlated with Delta K calculated from 2D elasto-viscoplastic FEM calculations. Crack closure effects were also modelled using FEM node release scheme. It is shown that a proper account of these effects partly rationalizes the differences observed in growth rates between small and long cracks.