Advanced Materials Research Vols. 47-50

Abstract: With increasing attention being devoted to the problem of reducing service costs for a wide range of turbines and compressors, an attractive option is repair of damaged blades, instead of replacing them by new ones. A lot of different methods are used to repair blades and other parts of turbines and compressors. The processes of laser metal deposition (LMD) and direct laser forming (DLF) are modern methods used to repair blades made of titanium alloys. In the present study, the finite element analysis (FEM) has been applied to determine the stress-strain state of the repaired blades in service conditions. Different forms and sizes of the damaged zones have been analyzed. Several kinds of alloys, such as Incoloy 903, Carlson C800 and Inconel 718, appropriate for the LMD and DLF methods, have been utilized in repairing the blades. Two kinds of blades, with and without cooling hollows, have been examined. Lifetime of the repaired turbine has been evaluated by comparing the results obtained from the modal and stress-strain state analyses of the repaired and original parts. The results allow evaluating influence of sizes and forms of the damaged zones, as well as choice of the alloys applied, on lifetime of the repaired blades.

Abstract: A systematic comparison of single and binary metal oxide TiO2, TiO2-Ga2O3, TiO2-Er2O3 and TiO2-Ta2O5 gas sensors with nanocrystalline and mesoporous microstructure, prepared by solgel route, was conducted. The gas sensitivity was increased by secondary phase introduction into TiO2 film via two mechanisms, firstly through the inhibition of anatase-to-rutile transformation, since the anatase phase accommodates larger amounts of adsorbed oxygen, and secondly through the retardation of grain growth, since the higher surface area provides more active sites for gas molecule adsorption. The binary metal oxides exhibited a remarkable response towards low concentrations of CO and NO2 gases at low operating temperature of 200°C, resulting in increasing thermal stability of sensing films as well as decreasing their power consumption. The calibration curves revealed that all sensors followed the power law ( B gas A S ] [ = ) (where S is sensor response, coefficients A and B are constants and [gas] is gas concentration). The response magnitude of the sensors obtained in this work is superior to TiO2-based sensors reported in previous studies.

Abstract: A new finish process that uses a rotational magnetic-assistance with high efficiency to assist discharging dregs out of the electrodes’ gap during the ultrasonic electrochemical finishing on the freeform-surfaces is investigated in the current study. The factors affecting the effects of the magnetic-assistance in ultrasonic electrochemical finishing are primarily discussed. The mainly experimental parameters are rotational speed of the magnetic-assistance mechanism, magnetic strength, distance between the two magnets, current density, and frequency and power level of ultrasonics. A large rotational speed of the magnetic-assistance mechanism produces a better finish. Providing large magnetic field intensity or using a small distance between the two magnets produces a larger magnetic force and discharge ability, and a better finish. A higher current density with magnetic-assistance reduces the finish time and avoids the difficulty of dreg discharge. The rotational magnetic-assistance during the ultrasonic electrochemical finishing process makes a great contribution in a short time by making the freeform-surfaces of workpiece smooth and bright.

Abstract: In general, structures in service gradually lose their original performance-level over time due to initial defects in design and construction, or exposure to unfavorable external conditions such as repeated loading or deteriorating environment, and in extreme cases, may collapse. So, in order to maintain the serviceability of structures at optimal level, advanced structure measuring system which can inform optimal time point and method of maintenance is required in addition to accurate prediction of residual life of the structure by periodic inspection. In Korea, the large-scale project for the safety network integration for long-term smart monitoring systems for bridge structures started in 2007, and this is the second year of the project. In this system, various structure types such as bridge, tunnels and cut slopes were considered with an emphasis on safety. The purpose of this system is to integrate the operation centers for these structure types in order to control and evaluate the structure’s real-time safety level through measured behavioral data. In this paper, the progress methodology for this network system and the smart monitoring system installation techniques for each structure types are briefly suggested.

Abstract: The first long-term bridge monitoring system in Korea was installed in 1995, and many bridges have been maintained by long-term monitoring systems. Recently, reliability of data and cost effectiveness have been increased by advanced sensor technology (fiber optic sensor, RFID, USN etc) and measuring equipment. In Korea, the large-scale project for the safety network integration for long-term smart monitoring systems for bridge structures started in 2007, and this is the second year of the project. In this system, various innovative sensor types are considered. To increase the effectiveness of this network system, an analysis of the problems with the conventional long-term bridge monitoring system and solution investigations are needed. The biggest problems are low durability and data reliability because of noise, and the lack of data applications techniques. Therefore, in this paper, a brief summary of the projects is presented and the state of bridge monitoring systems in Korea is investigated, and various problems and solutions for these problems are briefly suggested.

Abstract: The transmission of sound through all-metallic sandwich panels with corrugated cores is investigated using the space-harmonic method. The sandwich panel is modeled as two parallel panels connected by uniformly distributed translational springs and rotational springs, with the mass of the core sheets taken as lumped mass. Based on the periodicity of the panel structure, a unit cell model is developed to provide the effective translational and rotational stiffness of the core. The model is used to investigate the influence of sound incidence angle and the inclination angle between facesheet and core sheet on the sound transmission loss (STL) of the sandwich structure. The results show that the inclination angle has a significant effect on STL, and sandwich panels with corrugated cores are more suitable for the insulation of sound having small incidence angle.

Abstract: Most of the ultrasonic surgical or therapeutic devices apply the outside body designs with only the vibration tip contacting and delivering energy to the tissue. For inside body applications, a miniaturized 421 kHz piezoelectric transducer with OD 2 mm and length 10.35 mm based on longitudinal vibration mode was developed in this paper. The transducer is a one-wavelength design with a stepped horn to focus the energy. The theoretical analyses on mechanical, electrical and acoustic properties were performed using finite element analysis (FEA) and equivalent circuit (EC) model, which showed consistent results. The Qm and keff were found to be 393 and 0.21, respectively. The maximum acoustic output power was observed to be 27 mW at 10 V input when the depth of immersion is 1/4 wavelength. Ultrasonic streaming was also observed when the whole transducer was immersed in the water and the input power level was in the order of 0.6 W.

Abstract: In the current paper, a macroscopic differential model is constructed on the basis of the Landau theory of the first order phase transformation. Hysteresis loops and butterfly-shaped behaviors are modeled as a consequence of polarizations and orientation switchings. A non-convex free energy function is constructed to characterize different polarization orientations in the materials. Polarizations and orientation switchings are modeled by formulating the system state switching from one equilibrium state to another, as differential equations. The hysteresis loops and butterfly-shaped behaviors are successfully modeled. Comparison of the model results with the experimental counterpart is also presented.

Abstract: In the current paper, a macroscopic differential model for the hysteretic dynamics in shape memory alloy actuators is constructed by using the modified Landau theory of the first order phase transformation. An intrinsic thermo-mechanical coupling is achieved by constructing the free energy as a function depends on both mechanical deformation and the material temperature. Both shape memory and pseudoelastic effects are modeled. The hysteretic dynamics is linearized by introducing another hysteresis loop via nonlinear feedback strategy, which cancels the original one.

Abstract: Thin films of lead-free piezoelectric ceramics (Bi1/2Na1/2)TiO3 (abbreviated as BNT) were prepared on pure titanium substrates by a hydrothermal method. Several properties of BNT films synthesized in various Bi3+ and Ti4+ concentrations of starting materials were investigated using SEM, EDX, XRD and other instruments. Moreover, the effects of ion concentrations of starting materials on permittivity and piezoelectric effect of BNT films were discussed. The Bi2O3 crystals were more deposited on the surface of films with the increase of the concentration of Bi3+. The relationship between the deflection and applied electric field was measured on unimorph cantilever type actuators made from three samples which had different XRD patterns. The results showed that the piezoelectric effect of BNT films was dependent on the crystallization level of BNT.