Identification of fatigue crack propagation modes by means of roughness measurements

This paper is a study of the applicability of fracture surface roughness me asurements to identify the crack propagation mode in nickel base superalloy s (transgranular, intergranular or mixed). The results obtained indicated t hat this technique can be used as an alternative or as a complement to stan dard fractography. The best roughness parameters for identifying the crack propagation mode are average roughness, mean roughness depth and mean heigh t profile peak (R-a, R-z and R-pm). Analysis of roughness spectra in terms of the frequency range showed that the amplitude values of the profiles wit h wavelengths identical to the grain size are significant when propagation is intergranular. The roughness measurements were used to study the influen ce of loading frequency and stress state on the fatigue crack propagation m ode occurring in Inconel 718 tested at 600 degreesC. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.