Propagation of short fatigue cracks from notches in a Ni base superalloy: experiments and modelling

A notched specimen containing a semicircular slot (0.1 mm deep) was designed to simulate the growth of three-dimensional short cracks under a stress concentration. Fatigue tests were performed on N18 superalloy at 650 °C with trapezoidal loading cycles. A high-resolution optical measurement technique proved to be capable of detecting half-surface crack increments as small as 10 μm, and the potential drop method was found to be inappropriate for very small crack lengths. The stress intensity factor, Δ K , was calculated using a weight functions method. Non-uniform stress fields were determined by FEM modelling using elasto-viscoplastic constitutive equations. The plasticity-induced crack closure effect was calculated within the specimen using viscoplastic FEM modelling. The prediction of crack aspect ratio was used to investigate differences of closure along the crack front. The role of notch plasticity on these differences is discussed. Using these calculations, it is shown that the apparent differences between the growth behaviour of short and long cracks can be largely accounted for.