Please see seminar notice below (note location outside of ARMS). MATERIALS SCIENCE AND ENGINEERING SEMINAR Effect of Machining Damage on Low Cycle Fatigue Crack Initiation Life in Drilled Holes in Udimet(r) 720 by Christine Magadanz Master of Science Final Examination Advisor: Prof. K. Trumble ABSTRACT White layer is a generic term for a light etching surface layer on metal alloys that can result under extreme deformation conditions in wear, sliding or machining. While there has been some characterization of white layer due to abusive machining, the specific effect on fatigue crack initiation life has not been well documented. This study aimed to establish a relationship between the presence of white layer due to abusive machining and fatigue crack initiation life in a wrought nickel based superalloy (Udimet(r) 720). Low cycle fatigue testing was conducted on large specimens containing through holes drilled with parameters aimed at creating holes with and without white layer. Initially, Acoustic Emission monitoring technologies were used to monitor for acoustic events associated with crack initiation, however, this technology was deemed unreliable for this testing. Instead, cycles to crack initiation was determined using striation density measurements on each fracture surface to estimate the number of cycles of crack propagation, which was subtracted from the total number of cycles for the specimen. A total of sixteen specimens were tested in this manner. The results suggested that the crack initiation lives of holes machined with good machining parameters were statistically longer than crack initiation lives of holes machined with poor machining parameters. The mean initiation life of the poorly machined specimens was a factor of approximately 2 times shorter than the mean initiation life of the well machined specimens. The holes machined with good machining parameters exhibited subsurface initiations which suggested that no anomalies affected crack initiation for these specimens. It was also shown that some of the poorly machined holes exhibited subsurface initiations rather than initiations at white layer damage. These holes had better surface finish than the poorly machined specimens that did fail at white layer. The mean initiation life of the subsurface initiation holes was 10 times longer than the mean initiation life of the holes that cracked at white layer. Lastly, no apparent correlation between white layer thickness and initiation life was demonstrated in this study. Date: Wednesday, October 31, 2012 Time: 10:30 A.M. Place: HAMP 3153 PURDUE MSE [cid:image003.png@01CDB2A0.580CFC90] Lisa Stacey Secretary/Development Assistant Purdue University School of Materials Engineering 765/494-4100
