Papers by Author: Yasuhiro Yamazaki

Abstract: In this paper, the crack propagation behavior of naturally initiated small crack in in low-carbon/medium-nitrogen 316 stainless steel under thermo-mechanical fatigue loading was investigated. The experimental results indicated that the importance of the investigation of small crack propagation behavior because the information on the basis of physically long crack growth rate provides a dangerous evaluation on reliability to actual components. The small crack exhibits high growth rate under the In-phase TMF loading because of irreversible creep and plastic strains. However, the growth rates of small crack under the Out-of-phase TMF loading were lower because the effect of creep deformation became negligible in such condition.

Abstract: Collaborative research has been conducted by the Japan Thermal Spray Society (JTSS) to establish a standard test method for evaluating the interfacial fracture toughness of thermal sprayed coatings, including thermal barrier coatings. The test method is based upon the indentation test method utilizing a conventional Vickers hardness test machine. In this committee, round robin tests were performed to check differences in the evaluated results among collaborators. This paper reports on the progress of such activity in Japan.

Abstract: Thermal fatigue and isothermal oxidation tests of a plasma sprayed thermal barrier coatings have been carried out. The experimental results indicated that micro-cracking as the fatigue damage occurred in each cooling stage of thermal cycles. And the spallation fracture morphology was changed by the cycle wave form; the spallation failure occurred in the ceramic top-coating under the triangle cycle wave condition, on the other hand, it occurred near the top-coating/TGO interface under the trapezoidal wave condition. The finite element analysis indicated that the local residual stress, which is the driving force of interfacial damage, increases with the thickness of the thermal grown oxide. The degradation mechanism by thermal fatigue was discussed, in addition, a simple life prediction model for the thermal fatigue of TBC systems was proposed.

Abstract: Thermal barrier coatings (TBCs), that reduce the temperature in the underlying substrate material, are an essential requirement for the hot section components of industrial gas turbines. Recently, in order to take full advantage of the potential of the TBC systems, experimental and analytical investigations in TBC systems have been performed. However there is a little information on the deformation behavior of the top coating. In addition, the effects of the thermal exposure and the process parameters on the mechanical properties of the top coating have never been clarified. From these backgrounds, the effects of the process variables in APS and the thermal exposure on the mechanical properties were investigated in order to optimize the APS process of top coatings. The experimental results indicated that the mechanical properties of the APS-TBC, i.e. the tensile strength and the elastic modulus, were significantly changed by the process variables and the long term thermal exposure. The microstructural investigation was also carried out and the relationship between the mechanical properties and the porosity was discussed.

Abstract: Titanium alloy matrix composites (TMCs) have received considerable interest as structural materials for aeronautical applications, because of their higher specific strength and stiffness. When applying TMCs at elevated temperatures, high temperature isothermal low-cycle fatigue (LCF)failure is one of critical issues to be concerned. A unidirectionally reinforced SCS-6/ SP-700 composite is a tentative target in this work, where the matrix alloy, SP-700 is a new generation high strength Titanium alloy developed by NKK Inc., and the SCS-6 is a beta-SiC fiber developed by Textron Specially Materials, respectively. A merit to employ the SP-700 is that this alloy enables to reduce a fabrication temperature, because of its capability for superplasticity at relatively lower temperatures. The 7-plies composite specimen was produced by hot isostatic pressing (HIP) at 800°C for 0.5 hrs. in vacuum, alternating layers of thin-foils of the SP-700 and the green tapes of the SCS-6 fibers, so that the fibers were uniformly distributed as a hexagonal array in the matrix. The volume fraction of the fibers in the composite is about 28 %. In this work, the following articles in a unidirectionally reinforced SCS-6/SP-700 composite have been studied and evaluated: (i)mechanical properties of the SCS-6/SP-700 composite and the matrix alloy at temperatures ranged between room temperature and 450°C; (ii) LCF lives and the failure modes of the composite and the matrix alloy at room temperature and 450°C; (iii) fiber push-out tests at elevated temperatures ranged between room temperature and 600°C, to represent the fiber/matrix interfacial strength; and (iv) observation and the characterization of the interfacial reaction zone by means of a transmission electron microscope (TEM) and an energy dispersive X-ray spectrometer (EDS). Based on these experimental results, the effects of temperature and the loading frequency on LCF failure of the SCS-6/SP-700 composite were discussed.