Turbine Blade Fatigue Crack Growth

This paper presents an approach that has not previously been applied to predict power plant component fatigue failure. While many computer tools and procedures do exist for life prediction, recent experience shows that those methods often lack needed precision. In addition detailed knowledge of the material fracture mechanics properties and of the component load history, including the sequence of events. For complex load applications the sequence of events has been shown to be significant. For these applications Linear Damage Models (Minors Rule) are inadequate. A case where the accuracy of existing methods was found to be insufficient involved a steam power plant. Fatigue cracks were observed in the blade root area of 10% of the L-0 row turbine blades. The cracked blades were removed and replaced with new blades. The question to be answered was how safe are the reinstalled blades that had no visible damage. A successful solution was obtained by integrating NASA technology with STI experience in the analysis of steam and gas turbine blades. This led to the development of a proprietary, advanced life prediction computer code that: • incorporates contributions from both HCF and LCF to calculate crack growth, • treats the actual sequence in which all HCF and LCF loading has been applied, and • initiates the crack growth process from the microscopic inclusions and flaws. Complete analysis included comprehensive material tests, performed at a qualified material-testing laboratory. Test specimens were created from actual components, which had been subjected to over 30 years of service. Realizing that fatigue testing data variations can be relatively large, results from Life Cycle showed solid agreement with both short-term (10⁵ to 10⁶ cycles) material tests and with long-term (∼160×10⁹ cycles) operation under high strain steady load and low strain cyclic load conditions.