EFFECT OF CRACK SURFACE GEOMETRY ON FATIGUE-CRACK CLOSURE

The geometry of crack faces often plays a critical role in reducing cr ack extension forces when crack closure occurs during fatigue crack gr owth. Most previous studies of fatigue crack closure are concerned wit h mechanical measures of closure as related to the crack growth rate; very little attention has been given to the geometry of the crack surf aces. Our objective is to identify those aspects of crack surface geom etry that are important in the closure process, to develop quantitativ e fractographic techniques to estimate such attributes in a statistica lly significant and robust manner, and to correlate them to the physic al process of crack closure. For this purpose, fatigue crack propagati on experiments were performed on a Ni-base superalloy and crack growth rates and crack closure loads were measured. Digital image profilomet ry and software-based analysis techniques were used for statistically reliable and detailed quantitative characterization of fatigue crack p rofiles. It is shown that the dimensionless, scale-independent attribu tes, such as height-to-width ratio of asperities, fractal dimensions, dimensionless roughness parameters, etc., do not represent the aspects of crack geometry that are of primary importance in the crack closure phenomena. Furthermore, it is shown that the scale-dependent characte ristics, such as average asperity height, do represent the aspects of crack geometry that play an interactive role in the closure process. T hese observations have implications concerning the validity of geometr y-dependent, closure-based models for fatigue crack growth.