Papers by Author: Byeong Soo Lim

Abstract: There exists strong environmental and economic pressure to increase the thermal efficiency of fossil fuel power stations and this has led to a steady increase in operating temperature and pressure resulting in the world wide construction plans for ultra super-critical power plants. Consequently, in order to improve the thermal efficiency of power plant, there has been a strong drive to develop more advanced heat resistant steels with excellent creep, high temperature fatigue and thermal fatigue resistant properties as well as superior oxidation and corrosion resistant properties. In this study, the test material was P122 alloy which was developed for ultra super-critical power plant. To measure the fatigue crack growth rate in low #K range, fatigue tests were performed on the P122 alloy welds by #K decreasing method at three different microstructure (Base metal, HAZ, Weld metal) regions. Microstructure observation and micro-hardness tests performed for all three regions to find the relationship among the crack growth rate, microstructure and hardness. Fatigue tests were performed with compact tension specimens at 600°C, 650°C and 700°C at the loading frequency of 20Hz.

Abstract: Resistance spot welds (RSW) have been widely used in the sheet metal joining process due to high productivity, low cost and convenience. The application of aluminum alloys in automobile manufacturing has resulted in the reduction of weight in vehicle, fuel economy and improvement of drivability and driving performance. Therefore, the resistance spot weld (RSW) process has attracted attention to fabrication of aluminum structures. However, applications of Al RSW are limited due to the unreliable durability of the weld-bonded joints. This study presents an experimental investigation on the use of a post-weld cold working process to improve the fatigue strength of Al RSW. The post-weld cold working process includes a pair of uniquely shaped indenters that are pressed or driven into the structure in order to induce the compressive residual stresses around the RSW. The mechanical properties of the post-weld cold worked Al RSW were investigated, including the experimental results of fatigue. The effect of the post-weld cold working process parameters on the fatigue strength of the Al RSW was also investigated. Comparisons of the mechanical properties and qualitative results between the as-welded RSW specimens and the post-weld cold worked RSW specimens of each post-cold working parameter have been carried out. Also, discussions on the experimental results using the finite element method (FEM) for the post-weld cold working parameters are performed.

Abstract: Carbon nanotubes have received considerable attention because of their excellent electrical and mechanical properties. In this study, carbon nanotube - copper nanocomposites with homogeneously dispersed nanotubes within the copper matrix have been fabricated by two different methods; a mechanical mixing process and a molecular-level mixing process, which consists of mixing copper ions with functionalized nanotubes in a solvent. Small punch creep tests showed significantly improved mechanical properties of the nanocomposites. The electrical resistance of the nanocomposites also decreased.

Abstract: Various hold periods in a cyclic wave of fatigue load were introduced to investigate loading frequency effects on crack growth behavior and microstructural damage. The crack growth path and microstructural damage characteristics at 600°C in tempered martensitic 9Cr-2W (P92) HAZ of welded steel were studied. Generally, low frequency effect with increasing hold periods affects microstructural damage with microvoids/cavities nucleation due to the effect of creep. Results showed that the fatigue crack growth behavior was sensitive to the loading frequency. As frequency decreased, the fatigue crack growth rate increased and the crack path mode changed from transgranular to intergranular in terms of microstructural damage. As the loading frequency decreased, it was found that the microvoids /cavities and microcracks that formed along the prior austenite grain boundaries ahead of the main crack contributed to the intergranular crack growth.

Abstract: The initiation and growth of micro-defects such as micro cracks and voids usually causes the failure of long term operated structural components at high temperature. In this study, the creep characteristics and void nucleation and growth characteristics of P92 steel which is used as main steam pipe material in power plant were investigated at several temperatures and loading conditions. The area fraction of void increased with increase of test temperature, stress, and load holding time. In case of internal defect presence, micro-voids initiated in the early stage of loading period and resulted in the increased load line displacement and crack growth rate. The microvoids were found to form along the prior austenite grain boundaries and at the martensite packet boundaries.

Abstract: The results of cavity inspection in uniaxial creep tests and creep crack growth tests were compared to determine the cavity effect. The quantity of the cavities increased with increasing strain and crack growth rate as stress and temperature were increased. This resulted in the change of da/dt vs Ct. The increased rate in the number and size of the cavities in P92 steel with temperature was relatively fast, resulting in the high rate of da/dt against Ct. In this study, to determine the characteristics of defects in structure, the sub surface defects of P92 and P122 steels were investigated by an intelligent phased array ultrasonic inspection system. The system was found to be capable of detecting the creep crack length and the formation of cavities at the crack tip.

Abstract: This work investigates the relationship between the creep-fatigue life and ultrasonic test of creep-fatigue damage. Under the creep-fatigue interaction, the main cause of life reduction is the initiation and growth of microvoid with increasing hold time. The number/size of microvoid/cavity, the fraction of cavity area varied with the hold time. Therefore, the life evaluation using the microvoid with the variation of hold time is very informative for safety of components in power plants. In this study, using the heat resisting alloy, P122 steel for USC (ultra super critical) power plant, the creep-fatigue tests with various hold times and their ultrasonic inspection were carried out for the purpose of evaluation for creep-fatigue life. The results obtained by Rayleigh surface wave of backscattered ultrasound were compared and analyzed with the experimental parameters. The good agreement between the experimental life and the predicted life was obtained.

Abstract: Generally, as the hold time of static load increases, the degradation of material becomes more rapid and the creep-fatigue life decreases. Under the creep-fatigue interaction, the cause of life decrease was mainly the initiation and growth of microvoid with increasing hold time. In this study, using the newly developed alloy of P92, the artificial creep-fatigue degradation tests and its ultrasonic inspection were carried out to evaluate creep-fatigue life. From the relations between the creep-fatigue life and the nondestructive degradation assessment by backscattered ultrasound inspection, the new nondestructive life evaluation parameter, SDA (Slope of Decaying Amplitude) was proposed. Also, to verify the nondestructive life evaluation ability, the life evaluation results by SDA were compared with those of the destructive life evaluation obtained through the fraction of cavity area. From the test result of backscattering Rayleigh surface wave, the linear relationship between SDA and experimental life could be obtained. The good agreement between the experimental life and the predicted life by SDA was also obtained. Finally, it can be stated that the new life evaluation/degradation parameter, SDA (Slope of Decaying Amplitude) could be utilized for the evaluation of the material degradation under creep-fatigue interaction.

Abstract: A nondestructive inspection is required to check for defects inside of the actually used components and structures and to confirm their generation and growth rate. Using the backward radiated ultrasonic inspection system, we performed nondestructive inspection of cracks and micro cavities resulting from creep. From the ultrasound test results, the generation and growth of cracks and micro-cavities were confirmed, and the fracture life of the components and structures could be predicted. We confirmed degraded region by crack size and cavity based on the amplitude of the backward radiated ultrasound. The size of degraded region at 600oC was about 10mm, while that at 650oC was about 15mm. The size of crack and the cavity area fraction confirmed by ultrasound were very close to the actual size and cavity area fraction, indicating the validity of the predicted creep crack growth rate and creep.