Papers by Author: Joon Soo Park

Abstract: In this study, the expanded multi-Gaussian beam model is adopted to develop a model to calculate the ultrasonic beam fields radiated from an ultrasonic phased array transducer. Combining this beam model with three other components including time delays, a far-field scattering model and a system efficiency factor, we develop a complete ultrasonic measurement model for predicting the phased array ultrasonic signals that can be captured from a flat-bottom hole in a steel specimen in a pulse-echo set-up using an array transducer mounted in a solid wedge. This paper describes the complete model developed with its key ingredients.

Abstract: For the reliable application of ultrasonic phased array technique, it is necessary to understand the characteristics of the radiation beam field from the array transducers quantitatively. Very recently, it has been developed the expanded multi-Gaussian beam model that can calculate the radiation beam field from a single, rectangular transducer with great computational efficiency. In this paper, this model is adopted to calculate the radiation beam field from array transducers with various time delays to achieve steering and/or focusing. The calculation results are compared to those obtained by well known Rayleigh-Sommerfeld integral that provides the exact solution in order to explore the validity of the expanded multi-Gaussian beam model in this task.

Abstract: Ultrasonic guided waves have been widely employed for the long range inspection of structures such as plates and pipes. In ultrasonic guided waves, however, there are numerous modes with different wave velocities, so that the generation and detection of the appropriate wave mode of the guided wave is one of key techniques in the application of guided waves. In the present work, mode tuning using an array transducer was investigated with hardware implements. For this purpose, 8-channel ultrasonic pulser and their controller which enables sequential activation of each channels with given time delay were developed. Transmitter tuning, group velocity measurements, reciver tuning, mode identification and long range transmission testing were carried out. As a result, the selective tuning of wave mode can be achieved by changing the time interval between adjacent elements of an array transducer.

Abstract: In this paper, the dispersion curves of a bent cylindrical pipe are obtained by using the 3-dimensional finite element modeling and 2-dimenstional Fourier transform. The transient responses of the bent pipe are calculated by using a general-purpose finite element program, and the displacements are extracted at a series of sequential points as a function of spatial position and time, u(x,t). Then 2-dimentional FFT of u (x,t) offers U (k,ω), the relation between wave number and angular frequency so that the phase velocity and group velocity can be calculated. In addition, verification of the result is made by the mode identification using wavelet transform. The modes invoked by both methods agree very well.

Abstract: SiC has been extensively studied for high temperature components in advanced energy system and gas turbine because of its excellent high temperature mechanical properties and good thermal-chemical stability etc. However, the brittle characteristics of SiC such as low fracture toughness and low strain-to fracture still impose a severe limitation on practical applications of SiC materials. For these reasons, SiCf/SiC composites can be considered as a promising for various structural materials, because of their good fracture toughness compared with monolithic SiC. But, high temperature and pressure lead to the degradation of the reinforcing fiber during the hot pressing. Therefore, reduction of sintering temperature and pressure is key requirements for the fabrication of SiCf/SiC composites by hot pressing method. In the present work, monolithic Liquid Phase Sintered SiC (LPS-SiC) was fabricated by hot pressing method in Ar atmosphere at 1800oC under 20MPa using Al2O3, Y2O3 and SiO2 as sintering additives. The starting powder was high purity β-SiC nano-powder with an average particle size of 30nm. The characterization of LPS-SiC was investigated by means of SEM and three point bending test. Base on the composition of sintering additives-, microstructure- and mechanical property correlation, the compositions of sintering additives are discussed.

Abstract: SiC materials have excellent high temperature strength, low coefficient of thermal expansion, good resistance to oxidation and good thermal and chemical stability etc. However, the brittle characteristics of SiC such as low fracture toughness and low strain-to fracture still impose a severe limitation on practical applications of SiC materials. Therefore, in the interests of safety, we are required to measure fracture toughness of materials. In the present work, monolithic Liquid Phase Sintered SiC (LPS-SiC) was fabricated by hot pressing method under 20MPa using sintering additives at different temperature such as 1760oC, 1780oC, 1800oC and 1820oC. The starting powder was high purity β-SiC nano-powder with an average particle size of 30nm. Compositions of sintering additives were Al2O3 / Y2O3 = 0.7 and 1.5 (wt. %). Monolithic LPS-SiC was evaluated in terms of sintering density, hardness and fracture toughness through indentation fracture method by the Vickers hardness tester. Sintered density, hardness and fracture toughness of fabricated LPS-SiC increased with the increase of sintering temperature. They are higher than those of fabricated SiC by the chemical vapor deposition method.