Key Engineering Materials Vol. 827

Abstract: Thermal barrier coating (TBC) is deposited onto the gas turbine blade surface in order to protect the substrate from high-temperature combustion gas. Cracks and delamination of the ceramic coating which come from high heat flux loading are serious problem in TBC. In this study, the rapid thermal cycling device utilizing laser irradiation was developed. It was then investigated how the damage progresses in the ceramic coating exposed to cyclic rapid thermal loading. As a result, a sintering layer was formed in the surface of the ceramic coating, although such phenomenon was not recognized in TBC sample tested by the conventional thermal cycling test using an electric furnace. It was also revealed from the cross-sectional observation that the vertical crack was initiated at the surface of TBC and propagated into sintering layer. Finally, mechanical factors of those damages from finite element analysis using the TBC model including sintering progress was discussed

Abstract: CMAS attack is known to occur owing to the deposition of volcanic ash onto thermal barrier coating (TBC) surface at a high-temperature environment. The serious problem is TBC spallation resulting from the infiltration of molten volcanic ash into the porous microstructure of TBC. The infiltration induces inner stress and phase transformation, which directly results in those serious problems. In this study, the diffusional equation for expressing the infiltrating process of the molten ash into the porous structure of TBC and the associated constitutive equation considered regarding phase transformation are formulated. The equations are installed into commercial finite element (FE) code (MARC) using the user subroutine. The numerical simulation results are compared with the cross-sectional SEM observation for the volcanic-ash-deposited TBC sample exposed at a high-temperature to confirm verification of the model proposed herein.

Abstract: Ni-based single crystal (SC) superalloys are widely applied in the turbine blades of advanced aero-engine due to their excellent heat resistance. Rafting is a major degradation mode of SC under creep damage. In this study, the morphology evolution and probability characteristic of shape parameters are investigated for cuboidal γ´ phase during creep rafting. Three groups of SC creep tests are carried out at 850 °C and 600 Mpa along [001] direction. Scanning electron microscope (SEM) is used to observe the microstructure of SC at four different creep times including 0 h, 8 h, 16 h and 64 h. In addition, an image processing program is developed to identify the length (normal to uniaxial stress) and width (parallel to the stress) of each γ´ phase. Two interesting findings of this investigation can be summarized as: 1) During the N-type rafting of SC, the vertical matrix channel widths are gradually decreased, and the γ phases near the vertex of the cuboidal γ´ phases disappear 2) The length, width of γ´ phases both obey lognormal distributions during creep rafting, and the standard deviations change slightly for the width, but increase significantly for the length. These above findings are of significance to multi-scale probability modeling for creep deformation and fracture.

Abstract: The paper extends Orowan’s method to the prediction of ductile fracture in plane strain rolling. In general, any uncoupled ductile fracture criterion can be used in conjunction with the solution found. However, the present paper focuses on a ductile fracture criterion based on the workability diagram. It is assumed that the initiation of fracture occurs at the axis of symmetry.

Abstract: The chopped aramid fiber reinforced rubber composite (AFRC) has been widely used in the tire treads for its excellent characteristics. The viscoelasticity which is an important mechanical property for rubber matrix would be influenced by the adding of aramid fiber. In this study, the dynamic and quasi-static viscoelasticity were investigated via the dynamic thermal analysis experiment and the mullins experiment with multi-step relaxation, respectively. The frequency and temperature scanning were employed for AFRC with different fiber volume fractions, fiber aspect ratios and fiber orientation distributions. The effects of constituent parameters on the dynamic viscoelasticity were studied as well as the general rule of reinforcing effect of aramid fiber on rubber materials was presented. The stress relaxtion for AFRC were analysed basing on the experimental results. In addition, The ability of the viscoelastic constitutive model to describe the quasi-static viscoelastic behavior of AFRC were explored.

Abstract: Cracks are induced during hydrogen charging in Nickel. We have reported that the cracks initiate near the boundary between nickel matrix and nickel hydride formed near the surface by hydrogen charging. The driving force is considered to be residual stress caused by hydride formation. In the present paper, the stress distribution has been calculated near the boundary using Finite element method to clarify the crack initiation mechanism. As a result, the high-stress area corresponded to the area where the cracks were observed in the experiment.

Abstract: Based on the principle of a Body Force Method (BFM), any inclusion problem can besolved only by using a Kelvin solution which corresponds to a stress field caused by a point forceacting in a homogeneous infinite plate, regardless of the mechanical properties of the inclusion. Thischaracteristic is true even for an anisotropic inclusion in which the number of independent elasticconstants are larger than that of a homogeneous material. In the present study, some problems among anisotropic inclusions were analyzed numerically to demonstrate the validity.

Abstract: While mixed-mode loading conditions became hot topic recently, it is still quite unexplored area, even in case of metals. In case of polymers, this is even more complex problem, thanks to their higher sensitivity on higher temperatures connected with friction. Since polymer materials are very popular, this topic is of high interest. One of the best experimental specimens for achieving mixed mode loading conditions is the CRB specimen, as it provides clear mixed-mode I + III conditions under tension + torsion. Furthermore, for lifetime predictions it is necessary to calculate some material constants based on measured crack growth rates. However, these crack growth rates are not as easily and accurately measured on CRB specimens, as on CT specimens. Therefore, the main focus of this study is to observe difference between lifetime predictions based on CT and CRB data and possible application of CT data for CRB specimens.

Abstract: In this study we compare the effectiveness of the Normalised Smoothed Envelope (NSET) method for Artificial Neural Network (ANN) based impact localisation under simulated environmental and operational conditions with respect to other ANN based localisation methods developed by other studies. It is shown that when the testing and training impact case is the same, most studies give comparably good accuracy of localisation irrespective of feature extraction method or structure geometry. However, when the testing and training impact cases are not the same, only the NSET method is able to negate the variations caused by various impact cases and provide good localisation accuracy for an ANN trained using only a single impact case thus allowing for smaller training data set size requirements and increasing feasibility for real life application.

Abstract: A method of analysis for calculating a precise distribution of a stress intensity factor alonga front of 3D crack was improved by introducing a closed-form integral. In the present study, the crackface is discretized with number of triangular boundary elements on which the weighting function ofbody force doublet varies linearly with coordinate variables. A closed form solution of a resultant forceover an arbitrary planar-triangular area due to a presence of an isolated point force was derived andused to satisfy a stress boundary condition of a creased crack problem. In principle, arbitrary shaped3D cracks which may contain asperities and multiple creased lines in its face can be solved by presentapproach.