Examination of Successful Modal Analysis Techniques Used for Bladed-Disk Assemblies

摘要

Modal testing of bladed-disk assemblies in turbomachines is used to identify the critical natural frequencies and mode shape information used for avoiding the per-rev resonant conditions that cause high cycle fatigue (HCF) leading to premature blade and disk failures. In order to obtain the high quality modal data necessary for accurate modal identification, experience plays a major role in understanding the strengths and weaknesses associated with the variety of testing techniques. Application-specific concerns such as the blade- root disk interface connectivity, tiewire looseness and cover band design must be understood prior to test. Choices such as pre-test bladed-disk preparation, modal excitation driving point location, hammer versus shaker force excitation methods, shaker driving signal approaches, accelerometer type and location, and windowing are all important aspects that must be considered when testing specific bladed-disk configurations. Turbomachinery-specific modal analysis techniques including extraction of harmonic content and use of interference diagrams for identification of resonance conditions are also presented. The mentioned concepts are described in detail with reference to examples, which highlight the importance of the modal testing techniques implemented for a variety of applications.