Andrea Riva

Analysis of the Scatter of the Elastic Properties in Conventionally Cast Nickel Base Superalloys: Quantification, Modeling and Evaluation of the Impact on Gas Turbine Blade Design

Cast nickel-base superalloys elastic properties have a very large scatter, mainly because of the coarse grain microstructure and in-grain anisotropy.This high dispersion must be taken into account in the design of gas turbine blades, in particular when evaluating phenomena directly linked to the elastic behavior, such as blades vibration. This source of elastic properties scatter becomes even more important on specimens for material characterization because of their inferior size, which entails a lesser number of grains (i.e. a larger scatter).In this paper a model aimed to quantify such scatter is proposed. The performances of the model in predicting the standard deviation of the Young’s modulus (and consequently of the eigenfrequencies) are also shown, both for tested specimens and blades excited on clamps.Finally, a sensitivity FEM modal analysis is performed in order to evaluate how the elastic property dispersion might affect the blade eigenfrequencies and the relative mode shapes, with particular emphasis on the case of a specific region of a geometrically complex component affected by an anomalous Young’s modulus. Besides, the influence of the blade mass is evaluated through both experimental clamp impact tests and FEM analyses. The effect on blades of such source of scatter is then compared to the effect of the elastic properties dispersion. ANSYS program has been used for the simulations.Copyright

Low Cycle Fatigue Model Selection by Performance Analysis

A reliable low cycle fatigue (LCF) model requires the selection of appropriate damage mechanism components necessary to accurately approximate experimental life data. In particular, two fundamental tasks have to be performed: firstly, identify the most appropriate model among those available in literature, and secondly, verify that the model is appropriate in relation to the operational conditions of the component whose life is under evaluation (e.g., check if the model accounts for all the relevant damage mechanisms and phenomena).The European Creep Collaborative Committee (ECCC) developed a procedure that supports the researcher in evaluating performances and reliability of creep models, known as Post Assessment Tests (PATs). At the moment, there is no equivalent procedure for low cycle fatigue and ECCC work may provide the LCF researcher with useful guidelines.This paper is intended to investigate, compare and suggest which kind of verification is appropriate to identify any possible misbehavior in a fatigue model or in the LCF tests that supported the model. This procedure involves an analysis of the model performances in terms of comparison with the experimental population, supported by a deep knowledge of the damage mechanisms of the given material. Particular attention will be paid to materials with a particularly high dispersion, such as cast nickel-based superalloys.This paper is also meant to stimulate the fatigue data user community to propose and share methodologies in the perspective of the creation of a recommendation code, similar to what has been done by ECCC for creep.Copyright

Stress Relaxation Modelling

Modern gas turbine bolts experience severe operational conditions due to high temperatures and elevated axial stresses, generated by the tightening couple applied during the turbine assembly. In such conditions the relaxation of the initial stress due to viscous phenomena has to be taken into account in order to guarantee the proper operation of the turbine.Relaxation modelling can either be based on strain controlled relaxation tests or load controlled creep tests. Both solutions present difficulties: relaxation tests entail critical experimental issues, whereas creep tests may not be significant for the given strain controlled operational condition of a gas turbine bolt. Some of these problems will be described in the paper and solutions will be provided.The performances of several models for stress relaxation quantification will be compared, highlighting advantages and disadvantages of each approach. In particular, great emphasis will be given to those aspects which are relevant for bolt design or tightening load calculation. For instance, some important requirements are: firstly, the possibility to implement the given model easily in finite element calculations; secondly, the possibility to accurately calculate the relaxation in the second life of a serviced bolt after re-tightening; lastly, the possibility to reduce as much as possible the time required for the experimental tests.In order to evaluate the coefficients of the different models considered in the study, creep tests were performed at 450°C and 475°C with applied stresses producing a strain e = 1% in a time range of 1000–10000h and stress relaxation tests were performed at the same temperatures with initial strain in the range of 0.2%. After some stress relaxation, the specimens were reloaded at the initial stress several times in order to simulate the aforesaid service conditions of bolts.In the paper it will be shown how a valid model, capable of predicting the stress relaxation with acceptable accuracy, can be fed either by creep or relaxation tests, provided that the experimental tests and the related data elaboration are conducted with the proper methodology. This scenario provides the engineer responsible for material model creation with a remarkable flexibility, essential to fulfill the requirements of modern GT design, in terms of accuracy, promptness of data collection and possibility of FEM implementation.Copyright