Advanced Experimental and Analytical Investigations on Combined Cycle Fatigue (CCF) of Conventional Cast and Single-Crystal Gas Turbine Blades

Abstract

Rotor blades are the highest thermal-mechanical loaded components of gas turbines. Their service life is limited by interaction of creep, low cycle fatigue (LCF), high cycle fatigue (HCF) and surface attack. Because assurance of adequate HCF strength of the rotor blade is an important issue of the blade design the European project PREMECCY has been started by the European aircraft engine manufacturers and research institutes to enhance the predictive methods for combined cycle fatigue (CCF), as a superposition of HCF and LCF. Although today’s predictive methods ensure safe blade design, there are certain shortcomings of assessing fatigue life with Haigh or “modified Goodman diagrams”, such as isolated HCF assessment as well as uni-axial and off-resonant testing. HCF and LCF are considered without taking into account their interaction. PREMECCY is aimed to deliver new and improved CCF prediction methods for exploitation in the industrial design process. Beside development of predictive methods the authors are involved in the design and testing of advanced specimens representing rotor blade features. In this connection the paper presents a novel test specimen type and a unique hot gas rig for CCF feature test at mechanical and ambient representative conditions.

Experimental investigation of fatigue crack growth behavior of GH2036 under combined high and low cycle fatigue

“…To minimize the effects of pre-cracking at room temperature on the test at 550°C, we ensure that the plastic zone sizes at the end of the precracking stage were the same as the start of the crack growth rate testing [7]. The size and shape of the developed crack with this specimen are more representative of turbine disc's operation conditions than those reproducted in a compact tension or bend specimen [6,8,[19][20]. All the test pieces were cut from a forged turbine disc of GH2036 given in Fig.…”

Experimental investigation on creep‐fatigue behaviours of as‐received and service‐exposed turbine blades: Mechanism and life evaluation

“…Subsequently, the idea of blade-like specimens was put forward to represent the local feature of turbine blade. 4 This idea provided a new way to simulate the failure mode and the fatigue behaviour. However, the geometry and manufacture process of blade-like specimen still differed from the full-scale turbine blade, leading to the different fatigue behaviours.…”

“…These material properties are insufficient to fully represent the durability of turbine blade due to the differences in heat treatment, processing, surface finish and geometrical feature. Subsequently, the idea of blade‐like specimens was put forward to represent the local feature of turbine blade 4 . This idea provided a new way to simulate the failure mode and the fatigue behaviour.…”

Investigation on Material's Fatigue Property Variation Among Different Regions of Directional Solidification Turbine Blades—Part I: Fatigue Tests on Full Scale Blades

“…In most cases, tested life data of smooth specimens in the laboratory environment are used to predict components behavior [7], However, the differences between blades and smooth specimens, such as heat treatment, ' processing, microstructure, surface finish, and specimen size, may cause discrepancy between their mechanical performance and life distribution, and will definitely affect the accuracy of above tradi tional life assessment methods [8]. To decrease this discrepancy, some researchers design bladelike specimens [9][10][11], or even directly use full scale blades to conduct high temperature fatigue tests [8,[12][13][14][15][16][17] in a laboratory/bench environment. Compared with smooth or bladelike specimens, full scale blades own the same manufacture process with real operating blades, therefore there do not exist mechanical behavior discrepancy between the test specimens and flight ones.…”

“…There also exist couple effects between LCF and HCF loads. NASA has developed a multi-axial servohydraulic testing system to simulate the loading which a gas turbine engine blade experiences during operation [16], and recently Europe also initiated a program PREMECCY (Predictive methods for the combined cycle fatigue in gas turbines) to develop test system and theoretical models to predict turbine blade CCF life [9].…”