1 | INTRODUCTION
The turbine blades are the indispensable components of aero-engines, which usually suffer from various loads (such as thermal load, centrifugal load, aerodynamic load and vibration load[1-4]. The effect of thermal load and centrifugal load is considered to be the low cycle fatigue (LCF) due to the large stress during the flight cycles. Besides, the blades may suffer from the aerodynamic load and vibration load causing the supernumerary stress which is recognized to be the high cycle fatigue (HCF)[5]. During the actual operation, turbine blades are subject to the combined HCF and LCF loads simultaneously so that the fatigue damage of the turbine blades increases largely [6-9]. Therefore, it is necessary to investigate how to extend the CCF life of the turbine blades.
The PSP is an effective surface treatment method to improve the surface properties of metal. High-speed particles are sprayed onto the metal surface in order to produce the plastic deformation and compressive residual stress which can improve the fatigue strength and life of metal. Currently, PSP treatment has been used in the manufacture of fans, turbine blades and discs to improve the fatigue life [10]. However, the effect of PSP treatment on the CCF life of turbine blades has rarely been reported before.
Large number of researches about the effect of PSP on the fatigue life of the metal are carried out [11-23]. Gao et al. [11] investigate the effect of PSP on the small crack growth rate and fatigue life of 7475-T7351 aluminum alloy. The results indicate that fatigue crack growth rate greatly decreases after the PSP treatment. And the fatigue life extension introduced by PSP could be attributed to the beneficial compressive residual stress in the surface layer. Takeda et al. [12] study the influence of the PSP treatment on the fatigue life of TiNi shape memory alloy. They find that the fatigue life increases significantly due to the residual compressive stress. Benedetti et al. [13] study the effect of PSP on the high and very-high cycle plain fatigue resistance of the Al-7075-T651 aluminum alloy. It is found that the PSP treatment could retard the initiation and subsequent the propagation of cracks.
At present, the effect of PSP on material fatigue properties is usually studied with the smooth specimens. However, the differences between the full-scale turbine blades and smooth specimens, such as the geometry structure, heat treatment, manufacturing process and microstructure, may cause the discrepancy between their mechanical performance and life distribution [16]. And these factors may further affect the improvement of PSP on the fatigue performance. Therefore, it is necessary to investigate the effect of PSP on the full-scale turbine blades.
Accordingly, this investigation aims to study the effect of PSP treatment on the CCF behavior of full-scale turbine blades. One CCF test system is designed, which can apply the high and low cycle loads simultaneously without interfering with each other. And the CCF tests based on untreated and PSP treated turbine blades are carried out to study the effect of PSP treatment on the CCF life of the full-scale turbine blades.