Identification of fatigue crack propagation modes by means of roughness measurements

This paper is a study of the applicability of fracture surface roughness measurements to identify the crack propagation mode in nickel base superalloys (transgranular, intergranular or mixed). The results obtained indicated that this technique can be used as an alternative or as a complement to standard fractography. The best roughness parameters for identifying the crack propagation mode are average roughness, mean roughness depth and mean height profile peak (Ra, Rz and Rpm). Analysis of roughness spectra in terms of the frequency range showed that the amplitude values of the profiles with wavelengths identical to the grain size are significant when propagation is intergranular. The roughness measurements were used to study the influence of loading frequency and stress state on the fatigue crack propagation mode occurring in Inconel 718 tested at 600°C.