Papers by Author: Seon Jin Kim

Abstract: The process of fatigue crack growth is generally random nature, due to many sources of variability, such as external loading, material properties and geometry conditions. Therefore is is important to investigate the statistical variability of fatigue crack growth in order to development the methodology for probabilistic fatigue crack growth life prediction. This paper is concerned with the randomness investigation of fatigue crack growth for the FSWed 7075-T651 aluminum alloys. In this investigation, the fatigue crack growth tests were conducted for five compact tension (CT) specimens of the FSWed 7075-T651 aluminum alloys, base metal (BM), heat affected zones (HAZ) and weld metal (WM), especially three different constant stress intensity factor range (SIFR) controls.

Abstract: This study aimed to assess creep crack growth rates on the base and welded metals of modified 9Cr-1Mo steel. For this purpose, welded specimens were prepared by the Shielded Metal Arc Weld method. To obtain mechanical properties concerning the base and welded metals, a series of creep and tensile tests were conducted at 600 °C, and creep crack growth tests were also performed under different applied loads using 1/2" compact tension specimens at 600 °C. Their creep crack growth rates were calculated using the empirical equation of the da/dt vs. C* parameter and compared. It appeared that, for a given value of C*, the rate of creep crack propagation was about 2.0 times faster in the welded metal than the base metal. This reason was that the welded metal was faster in the creep strain rate than the base metal.

Abstract: In this paper, a series of statistical studies were conducted on creep crack growth behavior of Grade 9Cr-1Mo steel for next generation reactor. Creep crack growth tests were performed on pre-cracked compact tension (CT) specimens under the applied load ranges from 3800 to 5000N at the identical temperature condition of 600oC. The creep crack growth behavior has been analyzed statistically using the empirical equation between crack growth rate da/dt and C* parameter, namely da/dt=B(C*)q. First, the determination methods of B and q obtained from experiments were investigated by the least square fitting method and the mean value method. The probability distribution functions of B and q have been investigated using the normal, log-normal and Weibull distribution. The constant B and q are followed well 2-parameter Weibull. Second, the creep crack growth rate data were generated by Monte-Carlo simulation method assuming the 2-parameter Weibull in B and q parameters. The probability distribution of creep crack growth rate for arbitrary C* parameter values seems to follow well Weibull distribution.

Abstract: An isochronous stress-strain curve (ISSC) needs to be generated for a creep design application for high-temperature materials. To generate the ISSC for type 316LN stainless steel (SS), a series of creep data, which was obtained from creep tests with different stress levels at 600oC, was used. Creep curves were modeled by means of a nonlinear least square fitting (NLSF) of the Garofalo model. In the fitting of the creep curve, a secondary creep region was separated into first and second phases, and its fitting range was suitable to use for the first phase. The Garofalo model revealed a good agreement with the experimental creep data, and its parameters, P1, P2 and P3 revealed a good linear relationship as a function of a stress. The ISSCs for type 316LN SS at 600oC were successfully generated up to 300,000 hours.

Abstract: In this study, finite element analyses for the mechanical behaviors of a power steering hose assembly during the swaging process were performed in order to investigate the stress level of hose components according to jaw strokes, and to examine the effect of friction coefficient on the contact force. The physical properties such as Mooney-Rivlin constants of rubber materials and the work-hardening data of the steel were obtained through the experiments, and the contact conditions identical to the swaging process were taken into account. Two reinforced layers included in the hose were modeled with axisymmetric rebar elements in the general-purpose program, MSC MARC2000. The updated Lagrange formulation and the large displacement option were employed in the analysis. The result interpretations were focused on the rubber components which are in contact with the metal components, because of their important roles in the hose efficiency.

Abstract: The purpose of this study is to investigate the high temperature creep life of Waspaloy using the Initial Strain Parameter Technique (ISPT). The creep tests were performed at the elevated temperatures from 550oC to 700 oC. Constant stress creep tests were carried out in the experiment. The initial strain was measured for one minute after loading. The creep life of Waspaloy was calculated using the creep life prediction equation of ISPT. The confidence level between the experimental rupture time and the calculated rupture time using the ISPT is within 95%. So, the results show that the creep life prediction by the ISPT was a good agreement with LMP method.

Abstract: In this paper, an analysis of fatigue crack growth (FCG) behavior from a statistical point of view has been carried out. Fatigue crack growth tests were conducted on sixteen pre-cracked compact tension (CT) specimens of the pressure vessel (SPV50) steel in controlled identical load and environmental conditions. The assessment of the statistical distribution of fatigue crack growth experimental data obtained from SPV50 steel was studied and also the correlation of the parameter C and m in the Paris-Erdogan law was discussed. The probability distribution function of FCG life and FCG rate seems to follow the 3-parameter Weibull.

Abstract: SiC particulates reinforced Al matrix composites were fabricated by thermal spray process, and the dry sliding wear behavior against four different counterparts was investigated under a varying of the sliding speed and the applied load conditions. Al/SiC composites were fabricated by flame spraying, and the dry sliding wear tests were performed using the sliding speed of 0.4 m/s and 0.8 m/s at the applied load of 3 N. Sliding distance was kept at 1000 m for all the tests. Al2O3, ZrO2, Si3N4 and AISI 52100 balls were used as the counterparts. Wear tracks on the Al/SiC composites were investigated using scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). It was revealed that wear behavior of Al/SiC composites was much influenced by counterpart materials.

Abstract: In this study, the creep rupture tests of STS304 stainless steels were investigated at three different elevated temperatures of 600
, 650
and 700
under the constant creep stresses. Creep rupture characteristics such as creep stress, creep rupture time, steady state creep rate and so on were evaluated. The behaviors of creep rate curve and initial strain are compared at three different elevated temperatures. The stress exponent (n) at 600
, 650
and 700
based on steady state creep rate showed 22.5, 20.6 and 11.4 respectively. By increasing the temperature, the stress exponent is decreased. At the temperature of 700
, the lowest stress exponents are shown and this behavior is also observed in the case of stress exponent based on rupture time. The creep life prediction by LMP method is presented and the equation of this result is as follows: T(logtr+20)=-0.005152-14.56+24126.

Abstract: This paper deals with the statistical properties of short time creep rupture characteristic values (for example, creep rupture time, steady state creep rate, total creep rate, initial strain, etc.) in STS304 stainless steels. From short time creep rupture tests performed by constant stresses at three different elevated temperatures 600, 650 and 700
, the scatter and probability distributions were investigated for rupture time, total creep rate, steady state creep rate, initial strain, and others. The effect of temperature on the statistical scatter of rupture time was the smallest at 700
. The effect of stress on the statistical scatter of rupture time was smaller with increasing stresses. The probability distributions of short time creep rupture data were well followed 2-parameter Weibull.