• Journal of the Korea institute for structural maintenance and inspection
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• v.14 no.2
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• pp.176-183
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• 2010

In the steel deck box girder bridges, the deck is composed of deck plate, longitudinal and lateral direction ribs. The bridge, that is considered in this study, has been used for about 40 years and, recently, several cracks were found in the connection area of U-ribs. Further, additional cracks were occurred after some lateral rib plates and longitudinal frames were attached for the purpose of reinforcement. Therefore, the connection method in the U-ribs reinforcement was changed from the bolting to the weldment to get rid of stress concentration and further cracking. In this study, the stress in the U-ribs connection was analysed numerically and variable amplitude stress for the real traffic loads was measured experimentally before and after the frame reinforcement. Finally, the effects of reinforcement method were investigated and discussed.

• Journal of Welding and Joining
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• v.27 no.6
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• pp.31-35
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• 2009

When conducting CTOD test, especially in thick welded steel plate, fatigue pre-cracking occasionally failed to satisfy the requirements of standards thus making the test result invalid. Internally accumulated residual stress of test piece has been thought as one of the main reasons. The propagation of fatigue crack, started from the tip of machined notch, which might have propagated irregularly due to residual stress field. To overcome this kind of difficulty three methods to modify the residual stress are suggested in standard i.e. local compression, reverse bending and stepwise high-R ratio method. In this paper not only multi pass welding but also local pre-compressing process of thick steel plate has been simulated using finite element method for clarifying variation of internal welding residual stress. The simulated results show that welding residual stress is compressive in the middle section of the model and it is predominantly increased after machining the specimen. Comparing as-welded state all component of the welding residual stress changing to compressive in the tip of machine notch whereas residual stress of the outer area remain as tensile condition relatively. Analysis results also show that this irregular residual stress distribution is improved to be more uniformly by applying local compression.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.30 no.3D
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• pp.271-278
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• 2010

Semi rigid pavement is a pavement type using advantages of both flexibility of asphalt pavement and rigidity of concrete pavement by infiltrating cement paste into voids of open graded asphalt mixtures. The semi rigid pavement has better smoothness and smaller driving vibration or noise comparing to the concrete pavement, and has smaller permanent deformation and has temperature falling effect comparing to the asphalt pavement. The temperature falling effect were investigated at a semi rigid overlay pavement test section, and the temperature falling and water retaining effects were verified by measuring the temperature and weight of specimens at a housetop. Horizontal and vertical stresses and strains were compared by structural analysis of the semi rigid pavement and asphalt pavement using the Abaquser o, a commercial 3D finite element analysis program. The results were verified by Bisar 3.0, a multi-layered elastic analysis program. Performance of the semi rigid pavement and asphalt pavement were compared by predicting fatigue cracking based on the structural analysis results.

• Structural Engineering and Mechanics
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• v.47 no.4
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• pp.531-544
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• 2013

Compared with pavement structures of ordinary road sections, pavement structures in the intersection are exposed to more complex traffic characteristics which may exacerbates pavement distresses such as fatigue-cracking, shoving, shear deformation and rutting. Based on a field survey about traffic characteristics in the intersection conducted in Shanghai China, a three dimensional dynamic finite-element model was developed for evaluating the mechanistic responses in the pavement structures under different traffic characteristics, namely uniform speed, acceleration and deceleration. The results from this study indicated that : (1) traffic characteristics have significant effects on the distributions of the maximum principal strain (MPS) and the maximum shear stress (MSS) at the pavement surface; (2) vehicle acceleration or deceleration substantially impact the MPS and MSS at pavement surface and could increase the magnitude of them by 20 percent to 260 percent; (3) in the vertical direction, with the increase of vehicle deceleration rate, the location of the MPS peak value and the MSS peak value changes from the sub-surface layer to the pavement surface.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.10a
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• pp.228-232
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• 2001

Fatigue strength, electrical conductivity and stress-corrosion-cracking resistance are considered as important factors at aircraft Al alloys, therefore Al7050 alloy has been developed to improve such properties. However, hammer-forged Al7050 parts showed the undesirable structures such as severe local grain coarsening and inhomogeneous material flow, resulted in the degraded mechanical properties. In this paper, process conditions are investigated for elimination of the grain coarsening and improved material flow during forging process by both of experiments and FEM analysis. Particular interest has been given to understand role of preform shape on the grain coarsening behavior and magnitude of the hammer forging load The use of preform has been beneficial for reduction of the forging load and elimination of the grain coarsening. However, in the cases of as received bar and the round bar, which was machined to 2.5mm thickness in surface layer, some degree of local grain coarsening behavior has been observed. The optimized preform shape could be properly designed by applying the FEM simulation.

• Journal of Power System Engineering
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• v.8 no.3
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• pp.18-22
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• 2004

Boiler is one of the most important utilities providing steam to turbine in order to supply mechanical energy in thermal power plant. It is composed of thousands of tubes for high efficient heat transfer. The material for boiler tubes is used in such high temperature and pressure condition as $540^{\circ}C$, 22MPa. The boiler tube material is required to resist creep damage, fatigue cracking, and corrosion damages. 2.25%Cr-1Mo steel is used for conventional boiler tubes, and austenitenite stainless steel is used for higher temperature boiler tubes. But the temperature and pressure of steam in power plant became higher for high plant efficiency. So, the property of boiler tube material must be upgaded to fit the plant property. Several boiler tube material was developed to fit such conditions. X20CrMoV12.1 steel is also developed in 1980's and used for superheater and reheater tubes in supercritical boilers. The material has martensite microstructures which is difficult to evaluate the degradation. In this thesis, degrade the X20CrMoV12.1 steel at high temperatures in electric furnace, and evaluate hardness with Vickers hardness tester and strengths with Indentation tester.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.325-330
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• 2004

Recently, the request for the high strength of material is more and more increased in the area of industrial environment and machinery. To accomplish the high strength of materials, carbonizing treatment, nitrifying treatment, shot-peening method are representatively applied, however, shot-peening method is generally used among the surface processes. Shot peening is a cold working process used to impact Compressive residual stressed in the exposed surface layers. Benefits due to shot peening are increase in resistance to fatigue, stress corrosion cracking, fretting, galling, erosion and closing of pores. In this study, the influence of shot peening on the corrosion was investigated on spring steel immersed in 3.5% NaCl. The immersion test as performed on the two kinds of specimens. Corrsion potential, polarization curve, residual stress and etc, were investigated from experiment results. From test result the effect of shot peening on the corrosion was evaluated. The important results of the experimental study on the effects of shot peened on the environment corrosion of spring steels are as follows; In case of corrosion potential, shot peened specimen shows more activated negative direction as compared with parent mental. Surface of specimen, which is treated with the shot peened is placed as more activated state against inner base metal. It can cause t도 anti-corrosion effect on the base metal.

• Korean Journal of Materials Research
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• v.14 no.9
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• pp.642-648
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• 2004

The microstructure and crack tip opening displacement (CTOD) of deposit weld metal were investigated for a 30 mm- thick plate welded with flux cored arc welding (FCAW) and submerged arc welding (SAW) processes. The CTOD test was carried out both as welded condition and as stress-relieved specimen by local compression. The crack growth rates in FCAW were faster than those in a SAW, and the acicular ferrite content by the SAW process was increased relatively more than that by the FCAW process. The fatigue crack growth rate in a welded specimen was faster than that in locally compressed specimen. The CTOD value of locally compressed specimens was lower than that of as welded specimen. Furthermore, the CTOD value tested with the SAW process was higher than that tested with the FCAW process.

• Journal of Ocean Engineering and Technology
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• v.21 no.2 s.75
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• pp.50-59
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• 2007

Steam generator tubes are degraded from wear, stress corrosion cracking, rupture and fatigue and so on. Therefore, the failure assessment of steam generator tube is very important for the integrity of energy plants. In the steam generator tubes, sometimes, the local wall thinning may result from severe degradations such as erosion-corrosion damage and wear due to vibration. In this paper, the elasto-plastic analysis was performed by FE code ANSYS on steam generator tubes with wall thinning. Also, the four-point bending tests were performed on the wall thinned specimens, and then it was compared with the analysis results. We evaluated the failure mode, fracture strength and fracture behavior from the experiment and FE analysis. Also, it was possible to predict the crack initiation point by estimating true fracture ductility under multi-axial stress conditions at the center of the thinned area from FE analysis.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.23 no.11 s.170
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• pp.2087-2095
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• 1999

One of the main causes of unwanted dimensional changes in precision metal mold casting parts is excessive and irregular residual stresses induced by temperature gradients and plastic deformation in the solidifying shell. Residual stresses can also cause stress cracking, and lower the fatigue life and fracture strength of the casting parts. In the present study, aluminum alloy casting system with metal mold equipped with electrical heating elements and water cooling units was designed and the casting specimens were produced to quantify the effects of different cooling conditions on the development of residual stresses. The layer removal method was used to measure the biaxial residual stresses in casting specimens produced from the experiments. The experimental results agreed with Tien-Richmond's theoretical model for thermal stress development for the solidifying metal plate.