Motoki Sakaguchi

Degradation in Fatigue Crack Propagation Resistance of a Gas Turbine Vane after Long Term Service.

Degree of long term degradation damage of a retired gas turbine vane made of a Ni-based superalloy which had been operating for about 20000 hrs in a land-based gas turbine, was experimentally evaluated, through an assessment on the change in fatigue crack propagation resistance. For the purpose, a new testing apparatus has been developed to measure the local area fatigue crack propagation resistance, by extracting the miniature size compact tension specimens from the vane, where the specimens were extracted from the leading and the trailing edge, respectively. It was shown, the propagation rate of the crack that grew at the trailing edge part was significantly higher that that at the leading edge part. It was also shown that the crack propagation rates in the miniature specimen were generally higher than those in the standard size specimen. In order to explore an possibility to recover the degradation damage, the effect of a simple reheat treatment on the fatigue crack propagation resistance was also investigated

Thermal Fatigue Damage Evaluation of EB-PVD TBC Specimens - Its Potential and Problems

Thermal fatigue damage evolution behavior in thermal barrier coatings (TBCs) was studied, by employing the originally designed two dimensional ring-shape TBC specimen. The TBC specimen consisted of Ni-based superalloy IN738LC substrate, bond coat, and 8 wt.% Y2O3-stabilized ZrO2 (YSZ) top coat. The top coat was fabricated by electron-beam physical vapor deposition (EB-PVD) method with 250 micron-meters in thickness. Three kinds of MCrAlY bond coat alloys were specified as an experimental variable. Through the work, special attention was paid not only to the failure life of TBC specimen, but also to the underlying failure mechanisms. Some problems have been also pointed out, on feeding back these experimental findings to engineering applications.

Effect of Microstructure on Thermo-Mechanical and Isothermal Low Cycle Fatigue Failures of a Single Crystal Ni-Base Superalloy

Behavior of thermo-mechanical fatigue (TMF) failure of a single crystal Ni-base superalloy, CMSX-4, was studied and, compared with isothermal low-cycle fatigue (ILCF) of it. Strain-controlled TMF and ILCF tests of CMSX-4 were carried out under various test conditions, where the experimental variables were strain rates, strain ratio, test temperature and the range, and strain/temperature phase angle. At first it was shown from the experiments that the TMF and LCF failures associated with some noteworthy characteristics were rarely seen in the traditional pollycrystalline heat-resistant alloys. These phenomena could be explained inadequately, on the basis of the macroscopic parameters and the historical failure criteria; e.g., Manson-Coffin law and Ostergren approach. A new micromechanics model is proposed to predict the TMF and LCF lives, based on the Eshelbys theory.