Experimental and analytical investigation on service life of film cooling structure for single crystal turbine blade

In this study, regarding the film cooling structure of single crystal (SC) turbine blade, the life tests simulating the service condition are conducted, and the life prediction modeling as well as damage mechanism analysis are carried out. In life test, the SC specimen with multiple film cooling holes is designed to simulate the geometrical characteristic of film cooling structure. The creep-fatigue loading spectrum characterizing a typical flight process is employed. The loading system is developed to simulate the cyclic service load in laboratory environment. Meanwhile, the strain distribution of film cooling structure is measured using digital image correlation (DIC) method. After that, the life tests are conducted, and eight groups of test lives are obtained. Taking the strain distribution measured by DIC as input, the model based on theory of critical distance (TCD) is improved to predict the service lives of film cooling structure. Furthermore, the crack nucleation mechanism of film cooling structure is discussed. Based on the microscopic observation, it can be found that the microcrack nucleation around hole edge is induced by persistent slip bands (PSBs). The matrix channel that significantly influences the dislocation accumulation rate and PSBs formation is chosen as the concerned microstructure. Under the effect of stress distribution of film cooling structure, the uneven spatial distributions of mean values and standard derivations for matrix channel width are quantificationally revealed. And the distribution of dislocation density on film cooling structure is measured using electron back-scatter diffraction (EBSD). It can be found that the concentration of dislocation density exists around the hole edge, and the distribution of dislocation density is quite unsmooth due to the scatter of matrix channel width. These findings can be employed in the life prediction and fault diagnosis for film cooling structure under service condition.