The effect of manufacturing variables on short fatigue crack growth in waspaloy.

The importance of short fatigue crack growth has been emphasized in a number of recent experimental works. Most have been concerned with the natural initiation of cracks from smooth surfaces, such as those produced by mechanical and electrolytical polishing, their early propagation and interaction with the material microstructure. It is, however, recognized that manufacturing operations, such as those involved in the production and finishing of engineering components, can have a strong effect on short fatigue crack growth. In many cases fatigue crack growth data generated by testing smooth surface laboratory specimens may not be directly applied to predict short crack growth in the surface of real components. In the present work the effect of manufacturing operations on short fatigue crack growth has been studied on a nickel base superalloy, Waspaloy, at room temperature, using four-point-bend square section beam specimens. Surface preparation procedures were systematically varied in order to obtain different grades of surface finish which ranged from polished (electropolished) to ground finish. Constant amplitude load controlled fatigue tests were performed using material in two different treatment conditions. Fatigue crack growth was monitored by a plastic replication technique, and surface crack measurements were performed by means of a computer image analysis system. Short fatigue crack growth was observed to be not only of intermittent nature but also to follow two distinct patterns of behaviour which appear to be related to the surface condition produced by the finishing operations. It was also observed that short crack growth behaviour in ground surfaces cannot be related with any conventional surface texture parameters, e.g. Ra or Rv, since crack growth in the early stages is affected not only by the depth of manufacturing defects, such as grinding marks, but also by their length and orientation. Moreover, residual stress patterns induced by grinding operations were also observed to play an important role in the early stages of fatigue crack growth in engineering surfaces. Based on the experimental observations and theoretical analyses carried out in the present study, an attempt was made to quantify the contribution of three inherent parameters, namely material microstructure, surface topography and residual stresses, at the early stages of fatigue crack growth in a mechanically prepared surface.