Analytical and Numerical Investigation of Lacing Wire Damage Induced Mistuning in Turbine Blade Packet

Investigations of modal parameters for a mistuned packet of turbine blades due to lacing wire damage are reported using analytical and numerical studies with a simplified model. The turbine blade is assumed to be an Euler-Bernoulli beam connected with a lacing wire which is modeled as a mass less linear elastic spring. Thus, the blade is considered as a continuous system and lacing wire as a discrete system. The analytical results using Eigen value analysis are compared with numerical results obtained using commercial finite element package. In real life situation, though not reported in the literature, it is the failure of lacing wire that occurs quite often compared to the turbine blade and acts as precursor to the subsequent blade damage if it goes undetected. Therefore, studying the modal parameters of the grouped turbine blades in the context of lacing wire failure becomes important. The effect of variation of lacing wire location and stiffness indicative of damage resulting in the loss of stiffness on modal parameters is investigated. The study reveals a lot of fundamental understandings pertaining to dynamic behavior of grouped blades compared to the stand-alone blade under the influence of damaged lacing wire. 1. Introduction Turbine is the most widely used prime movers in power plants, turbo engines, and compressors in aircrafts and also in auxiliary turbo driven equipment such as turbo pumps. Turbine blade vibration and its failure under high cycle fatigue is an important area of research studies due to its critical applications. The blade failures are mainly due to resonant stresses when one of the natural frequencies of blade-disk system matches the nozzle passing frequency. Design of these turbomachine blades thus critically depends on accurate understanding of the blade vibration characteristics under varied operating conditions. However, modeling and analysis of turbine blade vibration becomes quite complex due to continuously tapered and twisted cross-section and blade to blade dynamic coupling through lacing wire or shroud rings. Almost 50% of low pressure steam turbine blade failure is due to fatigue caused by vibration [1]. A turbine blade is a complex geometry having aerofoil shape with varying width and thickness along its length. Most of the early research works have been based on simplified cantilever beam modeling where effects of root flexibility and crack in the stand-alone blade have been studied [2–9]. However, the dynamics of grouped blades is even more complex than a free stand-alone blade. Rao [10] has summarized the