Life assessment for a gas turbine blade under creep conditions based on continuum fracture mechanics

Abstract

Based on the semi-analytic finite element method and relationships of the continuum fracture mechanics, a numerical investigation on the creep and extension of the continuum fracture zone in a gas turbine blade is performed. The value of life prior to the formation of a crack-like defect and applicability limits of the relationships of continuum fracture mechanics are determined.Ensuring the safety of gas turbine units (GTU) during long periods of service usage requires the availability of data on the variation in the load-carrying capacity of their critical parts and assemblies. Under loading after exposure to elevated temperatures, the life of GTU blades is largely determined by the accumulation of creep strains, which necessitates the long-term strength calculation. The problems concerning the determination of the stress-strain state of GTU blades taking into account the creep strains were considered by the authors of works [1-3] and others. In this connection, it is noted in [4] that the simulation of the deformation processes in elements of stationary GTU whose design life is about 50 to 100 thousand years requires the use of physical equations that take into account the material damage accumulation accompanying the creep. The necessity of predicting the occurrence of defects in a material at the initial stage of their development is also noted in [5].In work [6], the life assessment prior to the beginning of fracture and determination of the location of a macroscopic defect (initial zone of fracture) in a GTU blade was made using the procedure of numerical simulation of the stress-strain state, creep and continuum fracture of three-dimensional bodies developed on the basis of the semi-analytic finite element method (SAFEM). It was shown that the life assessment problem should be solved in three-dimensional formulation taking into account the stress redistribution in the process of creep and the temperature dependence of the material physicomechanical properties. However, the question as to the value of the additional life related to the extension of the zone of continuum fracture is of importance.The goal of the present work is to develop the procedure for simulating the extension of the zones of fracture in three-dimensional bodies, to estimate the value of the additional life related to the extension of the fracture zone, and to determine the applicability limits of the relationships of continuum fracture mechanics (CFM) for the type of objects under consideration. For solving this problem, it is required to develop the finite element base that enables simulating both the variable geometry of the blade along the vertical axis and the presence of the zones of fracture, to derive efficient algorithms for simulating the extension of fracture zones and solving sets of nonlinear equations of the SAFEM.The creep process description is made on the basis of the strengthening theory using the Kachanov-Rabotnov damage accumulation parameter.A blade is a three-dimensional body of a complex shape due to the variable ...

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