Mechanics Aspects of Microcrack Growth in Inconel 718-Implications for Engine Retirement for Cause Analysis
Final rept. 1 Jul 1982-1 Jan 1984
BATTELLE COLUMBUS DIV OH
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Fatigue crack-growth rate predictions based on linear elastic fracture mechanics have been successfully applied in damage tolerance analyses for many years. These successes have prompted the Air Force Wright Aeronautical Laboratories to undertake the research needed to develop the technology to track the growth of defects in engine components. This application is not trivial because there is a growing body of experimental data that indicate that linear elastic fracture mechanics concepts do not always consolidate crack-growth-rate data. The lack of consolidation is particularly acute for physically short cracks and often the trend is toward much higher crack growth rates than expected. A literature review led to the conclusion that this short-crack effect arose primarily because of crack tip plasticity, transients from initiation to microcracking, and incorrect or incomplete implementation of LEFM. However, the data did not discriminate between these possible causes to that it was not clear which were significant. An experimental program was undertaken to isolate or emphasize the factors thought to promote the short-crack effect. The material used in the study was Inconel 718 in a heat-treatment condition found in current advanced engines. This report presents the results of those experiments.
- Metallurgy and Metallography