Soo-Chang Kang

The morphology of Al−Ti−O complex oxide inclusions formed in an ultra low-carbon steel melt during the RH process

Internal morphologies of bulk shape oxides in a melt of Ti bearing low-carbon steel were analyzed. The outer shapes of nearly spherical inclusions containing Ti were quite different from the shape of alumina inclusions that typically have a cluster form. A cross-section investigation using a polishing method and the FIB method revealed that the TiOx−Al2O3 complex oxide covers alumina clusters and that the interfaces of the alumina and complex oxide are modified as time passes.

Reliability-based design optimisation combining performance measure approach and response surface method

The necessity of assessing uncertainties to guarantee safety in structural design arises the potential of reliability-based design optimisation (RBDO), which is a methodology based on reliability analysis and design optimisation through probabilistic models. RBDO differs from conventional design optimisation by the presence of probabilistic constraints, which are evaluated using reliability index approach (RIA) or performance measure approach (PMA). It is generally known that PMA is more stable and efficient than RIA. Despite the advantages brought by PMA, RBDO still requires excessive computational cost for large scale structures involving complex finite element analysis. Accordingly, this paper presents a new scheme for RBDO achieving improved stability and efficiency by combining response surface method with PMA. The moving least squares (MLS) method is used to approximate the limit state function. Applications to a mathematical example, the 10-bar truss problem and the vehicle side impact problem verify that the proposed method shows better convergence and efficiency than other approaches.

Fatigue Reliability Evaluation of Steel-Composite High-Speed Railway Bridge with Tuned Mass Damper

This study proposes a fatigue reliability evaluation procedure for steel-composite high-speed railway bridge based on dynamic analysis and investigates the effectiveness of Tuned Mass Damper(TMD) in terms of the extension of fatigue life of the bridge. For the fatigue reliability evaluation, the limit state is determined using S-N curve and linear fatigue-damage accumulation. Dynamic analyses are peformed repeatedly to consider the uncertainties of train-velocity and damping ratio of the bridge. The distribution of random variables related to fatigue damage for the intended service life is then statistically estimated from analytical results. Finally, the fatigue reliability indices are obtained by means of the Advanced First-Order Second-Moment (AFOSM) method. Through numerical simulation of a steel-composite bridge of 40m span, the effectiveness of TMD on fatigue life of the bridge is examined and the results are presented.

An Optimal Design of TMD for the Improvement of Fatigue Reliability of Steel-composite High-speed Railway Bridges using Target Performance Approach

This paper will discuss how many efforts strive to use Tuned Mass Damper (TMD) for the mitigation of bridge dynamic responses. Previous research investigated the control effectiveness of TMD on high-speed railway bridges. Previous research also applied TMD to reduce vibrations of cable stayed bridges crossed by high-speed trains. Recently, a quantitative evaluation method was also developed to investigate the effects of TMD on the fatigue reliability of a bridge by combining dynamic analysis of the bridge subjected to the high-speed trains with the S-N curve based approach. The S-N curve-based approach is a useful method to visualize time to failure in terms of stress ranges versus cycles to failure. Its application for fatigue reliability analysis requires the number of cycles at stress-range levels endured throughout the intended service life of the bridge. Therefore, time domain dynamic analysis is performed for the bridge crossed by high-speed trains running at various velocities in order to obtain stress time histories at critical structural components. In the dynamic analysis, the bridge structure is modeled as an assembly of grillage members of which flexural and torsional stiffnesses approximate the behavior of the slab, girders and cross-beams. The high-speed train used in this study is the Korea Train eXpress (KTX), which is actually operating in Korea. Simple moving forces without consideration of the train-bridge interaction are employed to model the high-speed train for simplicity. This choice has been conducted because of the tremendous amount of repetition of the dynamic analysis required for fatigue reliability analysis required and the extreme time-consumption brought by the train-bridge interaction consideration. The numerical solution of the dynamic equilibrium equation of the bridge subjected to the high-speed trains is obtained using Newmarks method.