Papers by Author: Ouk Sub Lee

Abstract: The difference in the Coefficient of Thermal Expansion (CTE) between the chip and the substrate generate thermal displacement in the solder joint. It seems to be a main cause of failure in the solder joint when the chip and the substrate are heated repeatedly. The failure of the solder joints by thermal fatigue is known to be influenced by varying boundary conditions such as the difference in CTE, the height of solder, the Distance of the solder joint from the Neutral Point (DNP) and the temperature variation. In this paper, the effects of varying boundary conditions on the failure probability of the solder joint are studied by using the probabilistic approach methods such as the First Order Reliability Method (FORM) and the Second Order Reliability Method (SORM).

Abstract: The reliability estimation of pipeline is performed in accordance with the probabilistic methods such as the FORM (first order reliability method) and the SORM (second order reliability method). A limit state function has been formulated with help of the FAD (failure assessment diagram). Various types of distribution of random variables are assumed to investigate its effect on the failure probability. It is noted that the failure probability increases with the increase of the dent depth, the operating pressure and the outside radius, and the decrease of the wall thickness. Furthermore it is found that the failure probability for the random variables having the Weibull distribution is larger than those of the normal and the lognormal distributions.

Abstract: The Split Hopkinson Pressure Bar (SHPB) technique with some special experimental apparatus can be used to obtain the dynamic material behavior under high strain rate loading conditions. An experimental technique that modifies the conventional SHPB has been developed for measuring the compressive stress strain responses of materials with low mechanical impedance and low compressive strengths such as rubber. This paper uses PEEK (Poly-ether-ether-ketone plastic) bars to achieve a closer impedance match between the pressure bar and the specimen materials. In addition, a pulse shaper is utilized to lengthen the rise time of the incident pulse to ensure stress equilibrium and homogeneous deformation of the rubber specimen. It is confirmed that the modified technique is useful to record the dynamic deformation behavior of rubbers under various conditions such as high strain rate with various temperature effect. Furthermore, the dynamic deformation behaviors of heat-aged rubber material under compressive high strain rate are evaluated using the modified SHPB technique.

Abstract: It is well-known that the vibration significantly affect the life of solder joint. In this paper, the effects of the vibration on the failure probability of the solder joint are studied by using the failure probability models such as the First Order Reliability Method (FORM) and the Second Order Reliability Method (SORM). The accuracies of the results are estimated by a help of the Monte Carlo Simulation (MCS). The reliability of the lead and the lead-free solder joint was also evaluated. The reliability of lead-free solder joint is found to be higher than that of lead solder joint.

Abstract: In these days, most urban railway vehicles have been serviced under the random load application. However, it is considered to be a major factor of safety to predict the fatigue life for structures. It is thus required that fatigue assessment method for cumulative damage approach while Korea domestic regulations practices only the only has endurance limit approach. With this endurance limit approach, fatigue life prediction is impossible. In this research, the fatigue assessment for urban transit structure by using of cumulative damage approach method and related theories are presented.

Abstract: In this paper, transparent dynamic photoelastic experimental hybrid method for propagating cracks in orthotropic material was developed. Using transparent dynamic photoelastic experimental hybrid method, we can obtain stress intensity factor and separate the stress components from only isochromatic fringe patterns without using isoclinics. When crack is propagated with constant velocity, the contours of stresses components in the vicinity of crack tip in orthotropic material are similar to those of isotropic material or orthotropic material with stationary crack under the static load. Dynamic stress intensity factors are decreased as crack growth. It was certified that the dynamic photoelastic experimental hybrid method was very useful for the analysis of the dynamic fracture mechanics.

Abstract: The differences in the coefficient of thermal expansion (CTE) between the chip and the FR-4 board generate the shear strains and the bending moment in the solder joint. It seems to be a main cause of failure in the solder joint when the chip and the FR-4 board are heated repeatedly. Thus, the fatigue loading induced by thermal cycling is a major concern in the reliability of the solder joint. The magnitude of shear strain and the final failure are known to be influenced by varying boundary conditions such as the difference of CTE, the height of solder, the distance of the solder joint from the neutral point (DNP) and the temperature variation. In this paper, the effects of boundary conditions on the failure probability of the solder joint are studied by using the failure probability models such as the First Order Reliability Method (FORM) and the Monte Carlo Simulation (MCS). Furthermore, the stiffness of the solder joint is considered to investigate the influence at the failure probability.

Abstract: In this paper, the failure probability is estimated by using the FORM (first order reliability method), the SORM (second order reliability method) and the Monte Carlo simulation to evaluate the reliability of the corroded pipeline. It is found that the FORM technique is more effective in estimating the failure probability than the SORM technique for B31G and MB31G models with three different corrosion models. Furthermore, it is noted that the difference between the results of the FORM, the SORM and the Monte Carlo simulation decreases with the increase of the exposure time.

Abstract: This paper presents a modified Split Hopkinson Pressure Bar(SHPB) technique to obtain compressive stress-strain data for a rubber material. An experimental technique with a modified the conventional SHPB has been developed for measuring the compressive stress strain responses of materials with low mechanical impedance and low compressive strengths such as a rubber. This paper uses an aluminum pressure bar to achieve a closer impedance match between the pressure bar and the specimen materials. In addition, a pulse shaper is utilized to lengthen the rising time of the incident pulse to ensure stress equilibrium and homogeneous deformation of a rubber. It is found that the modified technique can determine the dynamic deformation behavior of rubbers under various conditions such as high strain rate and low temperature effects.

Abstract: The reliability estimation of pipelines is performed with help of the probabilistic method which considers the uncertainties included in the load and resistance parameters. The FORM (first order reliability method) is carried out to calculate the failure probability of corroded pipeline for the classified six distribution types of random variables. And the reliability of pipelines is assessed by using the failure probability. Furthermore, the MCS (Monte Carlo Simulation) is used to verify the results of the FORM. It is noted that the failure probabilities of the linear and the power models for surface corrosion are similar and those of the two-phase model for surface corrosion are somewhat different for six cases of two failure pressure models such as ASME B31G and MB31G models. And it is also found that the FORM and the MCS give similar results for varying exposure time.