Key Engineering Materials Vols. 326-328

Abstract: The safety factor of hydraulic piston pumps and motors shows a tendency to decrease due to high pressurization, high speed and low weight/volume realization to enhance the output density. Therefore, more effective test models are necessary to predict the exact life. The sensitive parameters in the endurance life test are speed, pressure and temperature, and failure production increases in proportion to the operating time. In this research, the authors propose a combined accelerated life test model using the analysis method of the combined accelerated life test results of piston-shoe assemblies by simultaneously applying high speed, high pressure and high temperature in accordance with the variation in speed, pressure and temperature in order to reduce the life test time.

Abstract: This paper proposed a new life test method of pneumatic cylinders used in pneumatic system, whose loads consist of working pressure, piston velocity, and working temperature. It is expected to reduce accelerated life test time greatly in case of accelerated life test with creating combined accelerated model of these three factors. We can determine the maximum accelerated factor by calculating the combined accelerated factor of working pressure, piston velocity and working temperature for various cases.

Abstract: In the beginning of design, exact load data are actually necessary for the fatigue strength and life analysis to minimize the cost and time of designing. The procedure of practical load determination is developed by the combination of the principal stresses of F.E.Analysis and experiment. This paper conceives new procedure for the determination of load direction and magnitude applied on mechanical structures. New procedure is the combination of the analytical and empirical method with analyzed strain by F.E. Analysis under unit load and with measured principal stress by strain gages under driving load, respectively. In this paper, we theorize the procedure of practical load determination and make the validity and the practicality of the procedure with the application to T-shape jointed structure. F.E. Analysis is conducted to get the principal stress on arbitrary points in the F.E. model of T-shape joint under unit load. Then experiment is carried out to get the principal stress on the same points of F.E. model. To demonstrate the actual driving condition, the load conditions are bending and torsion. From these two data sets, the magnitude, the direction and the position of load can be obtained.

Abstract: This study is to estimate the feasibility of acoustic emission(AE) method for the internal leak from the valves. In this study, two types of valve(a 4 inch glove steam valve and 4 inch ball water valve) leak tests using three different leak path and various leak rates were performed in order to analyze AE properties when leaks arise in valve seat. As a result of leak test for specimens simulated valve seat, we conformed that leak sound amplitude increased in proportion to the increase of leak rate, and leak rates were plotted versus peak acoustic amplitudes recorded within those two narrow frequency bands on each spectrum plot. The resulting plots of leak rate versus peak AE amplitude were the primary basis for determining the feasibility of quantifying leak acoustically. The large amount of data attained also allowed a favorable investigation of the effects of different leak paths, leak rates, pressure differentials and AE sensors on the AE amplitude spectrum. From the experimental results, it was suggested that the AE method for monitoring of leak was feasible.

Abstract: Ultrasonic is one of the most wide use of nondestructive evaluation technique. Voids and cracks are the most common defects in a solid. In this investigation, the ultrasonic technique is used to distinguish the defects between the void and crack. As an incident wave impinges on the crack tip, the diffractive wave can be detected from various angles. The amplitude of the diffraction is dependent on the receiving angle. For a void, the incident wave is scattered, the amplitude of the scattering wave is also dependent on the receiving angle. By comparing the amplitudes between the diffractive wave and the scattering wave, one is able to identify the defect of a void or a crack from the other. In this work, 5MHz of longitudinal and shear transducers are used and placed in a variety of incident and receiving angles to examine the difference between the void and crack. The experimental results are validated by the theoretical calculation. In order to identify the void and crack, it is required to have significant difference between the diffractive and scattering waves. The range of the detecting angle, which possesses a meaningful difference between the diffractive and scattering waves, is proposed through the numerical and experimental study to help the identification of the void and crack.

Abstract: Laser-welded parts experience high local temperatures and severe heating-cooling cycles which lead to large local residual stresses. These stresses introduce unacceptable degradation of the mechanical properties of a weldment. Thermo-elasto plastic analyses with 3-D FE models, as well as experimental investigations were performed in order to predict temperature distribution and residual stresses of ND-YAG laser-welded joints with various gap widths between the dissimilar steel types of austenitic and precipitation-hardening stainless steel. The specimens have the shape of a pocket to optimize the weight of the structure, which consists of a thin skin (AISI304) and a thick skeleton (AISI630). The residual stresses at the surface of the weldments were measured using the instrumented indentation method. The residual stresses and melt-pool zone (MPZ) profiles show good agreement between the theoretical and experimental results. Considering the residual stresses, the allowable gap width range of the laser-welded joints for the pocket-shaped specimen was calculated. For a welding joint with gap widths, the longitudinal residual stress values at the yield stress level were observed. Melt-pool zone profiles described by the underfill and penetration depth also depend upon the gap size.

Abstract: This paper proposes an advanced signal processing technique for the precise estimation of a nonlinear ultrasonic parameter, based on power spectral and bispectral analysis. The power spectrum and bispectrum estimation of the pulse-like ultrasonic signal used in the commercial SAM (scanning acoustic microscopy) equipment is especially considered in this study. The usefulness of the proposed estimation is confirmed by experiments for a Newton ring with a continuous air gap and a real semiconductor sample with local delaminations. The results show that the nonlinear parameter obtained by the proposed method had a good correlation with the delamination.

Abstract: In the present work, the strain induced martensite in 316L stainless steel was quantitatively characterized by X-ray diffraction, the measurement of the magnetic coercivity and the AE technique during the monotonic tensile deformation of plate specimens. Plate specimens subjected to different heat treatments (i.e. having different initial microstructures) were tensile-deformed and the AE counts obtained during tensile deformation were correlated with the microstructural development. The AE count was observed to increase with increasing amount of strain induced martensite phase, as determined by X-ray diffraction analysis. The potential of the AE technique and the measurement of the magnetic coercivity to be used for the evaluation of the tensile deformation was discussed in relation to the existence of strain-induced martensite.

Abstract: This paper capitalizes on recent advances in the area of non-contact ultrasonic guided wave techniques. The present technique provides a decent method for nondestructive testing of defect thinning simulating a hidden corrosion or FAC(Flow Accelerated Corrosion) in a thin aluminum plate. The proposed approach is based on using EMAT(Electro-magnetic Acoustic Transducer) to generate guided waves and detect the wall thinning without any coupling. Interesting features in the dispersive behavior of selected guided modes are used for the detection of plate thinning. It is shown that mode cut-off measurement allows us to monitor a defect thinning level while a group velocity change can be used to quantify the thinning depth.

Abstract: The mechanical properties of in-service facilities under harsh environment a decrease as materials of the facilities degrade. This decrease of mechanical properties can affect the safety operation of the facilities. Therefore, the extent of degradation due to prolonged service exposure must be estimated. Nondestructive evaluation method is a good technique for monitoring the change of mechanical properties of in-service facilities. The most widely used nondestructive methods are the ultrasonic method and the indentation test, which is advantageous with respect of applicability to in-service facilities. The modified theoretical Vary's equation, considering nonlinear response due to material degradation, was proposed for obtaining the correlations between ultrasonic parameters and fracture toughness. Experimental results showed that ultrasonic attenuation, velocity, and nonlinear parameters have significant correlation with fracture toughness and yield strength. The nondestructive evaluation system can be used to obtain the yield strength and ultrasonic parameters simultaneously, and this information can be used to predict the fracture toughness. The predicted results produced good correlations with the experimental results, indicating that the nondestructive evaluation system can be effective in evaluating material properties and degradation, and the life time of facilities.