• KSCE Journal of Civil and Environmental Engineering Research
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• v.33 no.4
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• pp.1281-1293
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• 2013

A sophisticated finite element model is illustrated to analyze the behavior of Prefabricated Parallel Wire Strand(PPWS) sockets for main cables of suspension bridges. An orthotropic model is proposed for the casting material by considering both effects of individual wires and a casting alloy, and the contact between surfaces of a socket and a casting alloy is idealized by using the Coulomb friction and the surface-based contact model. The proposed FE model is verified by comparing the strain distributions obtained from the tensile test and FE analysis. The mechanical behavior of a socket is investigated with respect to the variation of the frictional coefficient. The result shows that the friction between surfaces significantly diminishes the stress concentration of a socket and a casting alloy, and the normal stress from the design equation represents the averaged value of the upper and lower quartiles in the distribution of contact stresses between a socket and a casting alloy.

• Journal of the Korean Society for Nondestructive Testing
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• v.26 no.5
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• pp.285-290
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• 2006

The evaluation of clamping condition has been regarded as the main issue in the safety-maintenance of the clamped high-tension bolts. For this, this paper proposes a method to estimate the axial stress by measuring the TOF (Time-Of-Flight) of ultrasonic wave, which is based on the acoustoelasticity or the dependency of sound speed on the stress. In this method, however, the variation of sound speed within the range of stress induced under the field condition is very small, and thus the accuracy of the TOF measurement is important. We adopted the phase detection method using tone-burst ultrasonic wave to measure the precise TOF. In order to verify the usefulness of the proposed method experiments are carried out and the results were compared with the stress measured by the strain gage. The results show good agreement with each other, and from these we can conclude that the proposed method is highly useful fnr the evaluation of clamping condition in the clamped high-tension bolts.

• Structural Engineering and Mechanics
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• v.50 no.6
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• pp.755-772
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• 2014

A simplistic approach towards evaluation of complete load deflection response of Reinforced Concrete (RC) flexural members under post fire (residual) scenario is presented in this paper. The cross-section of the RC flexural member is divided into a number of sectors. Thermal analysis is performed to determine the temperature distribution across the section, for given fire duration. Temperature-dependent stress-strain curves for concrete and steel are then utilized to perform a moment-curvature analysis. The moment-curvature relationships are obtained for beams exposed to different fire durations. These are then utilized to obtain the load-deflection plots following pushover analysis. Moreover one of the important issues of modeling the initial stiffness giving due consideration to stiffness degradation due to material degradation and thermal cracking has also been addressed in a rational manner. The approach is straightforward and can be easily programmed in spreadsheets. The presented approach has been validated against the experiments, available in literature, on RC beam subjected to different fire durations viz. 1hr, 1.5hrs and 2hrs. Complete load-deflection curves have been obtained and compared with experimentally reported counterparts. The results also show a good match with the results obtained using more complicated approaches such as those involving Finite element (FE) modeling and conducting a transient thermal stress analysis. Further evaluation of the beams during fire (at elevated temperatures) was performed and a comparison of the mechanical behavior of RC beams under post fire and during fire scenarios is made. Detailed formulations, assumptions and step by step approach are reported in the paper. Due to the simplicity and ease of implementation, this approach can be used for evaluation of global performance of fire affected structures.

• Journal of the Korean Society for Heat Treatment
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• v.18 no.1
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• pp.3-11
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• 2005

Using various thermo-mechanical schedules characterized by varying reheating temperature, deformation temperature and strain, the austenite recrystallization and ferrite refinement of a Nb bearing low carbon steel(0.15C-0.25Si-1.11Mn-0.04Nb) were investigated. For single pass heavy deformations at $800^{\circ}C$, the 40% deformed austenite was not recrystallized while the 80% deformed one was fully recrystallized. Ferrite grains formed in the 80% deformed specimen was not very small compared with those in the 40% deformed specimen, which implied the recrystallized austenite was not more beneficial to ferrite refinement than the non-recrystallized one. In case of deformation in low temperature austenite region, a multi-pass deformation made finer ferrites than a single-pass deformation, as the total reduction was the same, due to more ferrite nucleation sites in the non-recrystallization of austenite for multi-pass deformation. When specimen was deformed at $775^{\circ}C$ that was $10^{\circ}C$ higher than $Ar_3$, the ferrite of about $1{\mu}m$ was formed through deformation induced ferrite transformation(DIFT), and the amount of ferrite was increased with increasing reduction. Dislocation density was very high and no carbides were observed in DIFT ferrites, presumably due to supersaturated carbon solution. By deformation in two phase(50% austenite+50% ferrite) region the very refined ferrite grains of less than $1{\mu}m$ were formed certainly by recovery and recrystallization of deformed ferrites and, a large portion of ferrites were divided by subgrain boundaries with misorientation angles smaller than 10 degrees.

• Explosives and Blasting
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• v.23 no.1
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• pp.9-17
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• 2005

This study is to clarify the comparative relationship and mechanical anisotropy of granite distributed in the Nam-weon on the subject of weathered rock mass sea water surroundings. Artificial weathering test is defined as a test, which controls the weathering rate and agents by controlling the weathering rate and agents by artificial environmental of salt water. Increased weathering degree is large indicated by weathering salt water, such as apparent specific gravity, absorption, porosity, uniaxial compression strength, P-wave velocity, slake durability, shore hardness, indirect tensile strength(brazilian test) and cohesion were measured. As the Weathering salt water proceeds, cracks develope increasingly. A number the cracks affect the rock deformation. Therefore, stress-strain curve of weathered salt water rock in one confined state are quite differ from weathered fresh water rock those. A reason of their deformation type is the formation of micro-cracks and potential porosity caused by artificial weathering test.

• Tunnel and Underground Space
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• v.23 no.4
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• pp.280-287
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• 2013

Finite element or difference methods are applied to the analysis of the tunnel stability and they provide detailed behaviour of analyzed tunnel sections but it is rather inefficient to analyze all the section of tunnel by using these methods. In this study, the authors suggest a new stability analysis method for whole tunnel to provide an efficient and easy way to understand the behaviour of whole tunnel by using an analytical solution with the assumption of equivalent circular tunnel. The mechanical behaviour, radial strain and plastic zone radius of whole tunnel were analyzed and appropriate support pressure to maintain the displacement within the allowable limit was suggested after the application of this method to the tunnel. Consequently, it was confirmed that this method can provide quick analysis of the whole tunnel stability and the quantitative information for subsequent measures such as selection of tunnel sections for detailed numerical analysis, set up of the monitoring plan, and so on.

• Transactions of Materials Processing
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• v.6 no.3
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• pp.239-249
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• 1997

The behaviour of alloys in the semi-solid state strongly depends on the imposed stress state and on the morphology of the phase which can vary from dendritic to globular. To optimal net shape forging of semi-solid materials, it is important to investigate for filling phenomena in forging process of arbitrarily shaped dies. To produce a automotive part which has good mechanical property, the filling pattern according to die velocity and solid fraction distribution has to be estimated for arbitrarily shaped dies. Therefore, the estimation of filling characteristic in the forging simulation with arbitrarily shaped dies of semi-solid materials are calculated by finite element method with proposed algorithm. The proposed theoretical model and a various boundary conditions for arbitrarily shaped dies is investigated with the coupling calculation between the liquid phase flow and the solid phase deformation. The simulation process with arbitrarily shaped dies is performed to the isothermal conditions of two dimensional problems. To analysis of forging process by using semi-solid materials, a new stress-strain relationship is described, and forging analysis is performed by viscoelastic model for the solid phase and the Darcy's law for the liquid flow. The calculated results for forging force and filling limitations will be compared to experimental data. The filling simulation of simple products performed with the uniform billet temperature(584$^{\circ}C$) from the induction heating by the commercial package MAGMAsoft. The initial step of computation is the touching of semi-solid material with the end of die gate and the initial concept of proposed system just fit with the capability of MAGMAsoft.

• Journal of Korean Society of Steel Construction
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• v.14 no.1
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• pp.79-85
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• 2002

Recently, prestressed H-Beam bridges with external unbonded Tendons are increasingly built. The mechanical behavior of prestressed steel H-beams is different from that of normal bonded PSC beams in a point of the slip of tendons at deviators and the change of tendon eccentricity that occurs, when service load are applied in external unbonded steel H-beams. The concept of prestressing steel structures has been widely considered, in spite of long and successful history of prestressing concrete members. In the study, The flexural test on prestressed steel H-beams has been performed in the various aspects of prestressed H-beam including the tendon type and profile. The load was plotted against the deflection and the strain respectively in the steel beam and prestressing bars. The value expected with the equation of internal force equilibrium and compatibility between the deflection of the bars and the H-beam was found to correlate well with the measured data.

• KCI Concrete Journal
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• v.13 no.1
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• pp.43-51
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• 2001

Although portland cement concrete is one of the most universal construction materials, it has some disadvantage such as shrinkage, which is an inherent characteristic. Because of this shrinkage, combined with the low tensile strength of the material, cracks of varying sizes can be found in every reinforced concrete. To prevent this cracking, keeping the concrete in compression by mechanical prestress has been used. This study discusses application of expansive additives for concrete to improve the serviceability of precast concrete box culvert by inducing chemical prestress. For this purpose, both expansive concrete slabs and normal concrete slabs are tested to verify the effect of expansive additives. Then the failure tests of the fullscale precast box culverts were carried out and the critical aspects of the structural behavior were investigated. The result of the material testis shows that the optimal proportion of expansive additives is 13 percent of cement weight, and the properties of expansive concrete are the same as those of normal concrete in that proportion. Both the experimental cracking load and service load of the expansive concrete members are increased in comparison with those of the normal concrete, but the ultimate load is decreased slightly. In addition to the above results, the deformation of expansive concrete member is lets than that of normal concrete member, and permanent strain which results from cyclic load is decreased. It can be concluded that the use of expansive additives to induce chemical prestress in precast concrete box culvert greatly improves the serviceability.

• Journal of Ocean Engineering and Technology
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• v.22 no.6
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• pp.75-82
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• 2008

Flame bending has been extensively used in the shipbuilding industry for hull plate forming In flame bending it is difficult to obtain the desired shape because the residual deformation dependson the complex temperature distribution and the thermal plastic strain. Mechanical bending such as reconfigurable press forming multi-point press forming or die-less forming has been found to improve the automation of hull plateforming because it can more accurately control the desired shape than line heating. Multi-point forming is a process in which external forces are used to form metal work-pieces. Therefore it can be a flexible and efficient forming technique. This paper presents an optimal approach to determining the press-stroke for multi-point press forming of curved shapes. An integrated configuration of Finite element analysis (FEA) and spring-back compensation algorithm is developed to calculate the strokes of the multi-point press. Not only spring-back is modeled by elastic plastic shell elements but also an iterative algorithm to compensate the spring-back is applied to adjust the amount of pressing stroke. An iterative displacement adjustment (IDA) method is applied by integration of the FEA procedure and the spring-back compensation work. Shape deviation between the desired surface and deform￡d plate is minimized by the IDA algorithm.