Heat transfer enhancement in gas tungsten arc welding using azimuthal magnetic fields generated by external current

Kim, Yiseul,Lee, Jaewook,Liu, Xiaolong,Lee, Boyoung,Chang, Yunlong 테크노프레스 2016 Multiscale and multiphysics mechanics Vol.1 No.1

This paper proposes the idea to enhance the heat transfer in Gas Tungsten Arc Welding (GTAW) by using the azimuthal magnetic field. The azimuthal magnetic field generated by the external currents makes the Lorentz force stronger, and consequently improves the heat transfer by the faster flow movement. The enhanced heat transfer might improve the welding performance by increasing the temperature at the workpiece. To validate the proposed idea, a two-dimensional axi-symmetric model of GTAW is built, and the multiphysics simulation of GTAW is carried out. As the analysis result, the distributions of electric current, electromagnetic fields, arc flow velocity, and temperature are investigated. Then, the proposed idea for heat transfer enhancement is validated by comparing the Lorentz force, flow velocity, and temperature distribution with and without azimuthal magnetic fields.

Prediction of the compressive and tensile strength of HPC concrete with fly ash and micro-silica using hybrid algorithms

Yin, Hang,Liu, Shuxian,Lu, Shasha,Nie, Wei,Jia, Baoxin 테크노프레스 2021 Advances in concrete construction Vol.12 No.4

Evaluating the impact of fly ash (FA) and micro-silica (MS) on the tensile (TS) and compressive strength (CS) of concrete in different ages provokes to find the effective parameters in predicting the CS and TS, which not only could be usable in the practical works but also is extensible in the future analysis. In this study, in order to evaluate the effective parameters in predicting the CS and TS of concrete containing admixtures and to present a fitted equation, the multivariate adaptive regression splines (MARS) method has been used, which could find a relationship between independent and dependent variables. Next, for optimizing the output equation, hybrid genetic algorithm (GA), particle swarm optimization (PSO), and grey wolf optimization (GWO) methods have been utilized to find the optimal conclusions. It could be concluded that for both predictions of CS and TS, all models have the coefficient of determination (R²) larger than 0.949 and 0.9138, respectively. Furthermore, between three hybrid algorithms, MARS-PSO could be proposed as the best model to obtain the most accuracy in the prediction of CS and TS. The usage of hybrid MARS-PSO techniques causes a noticeable improvement in the prediction procedure.

Heat jet approach for finite temperature atomic simulations of two-dimensional square lattice

Liu, Baiyili,Tang, Shaoqiang 테크노프레스 2016 Multiscale and multiphysics mechanics Vol.1 No.3

We propose a heat jet approach for a two-dimensional square lattice with nearest neighbouring harmonic interaction. First, we design a two-way matching boundary condition that linearly relates the displacement and velocity at atoms near the boundary, and a suitable input in terms of given incoming wave modes. Then a phonon representation for finite temperature lattice motion is adopted. The proposed approach is simple and compact. Numerical tests validate the effectiveness of the boundary condition in reflection suppression for outgoing waves. It maintains target temperature for the lattice, with expected kinetic energy distribution and heat flux. Moreover, its linear nature facilitates reliable finite temperature atomic simulations with a correct description for non-thermal motions.

Seismic risk assessment of staggered wall system structures

Kim, Jinkoo,Baek, Donggeol 테크노프레스 2013 Earthquakes and structures Vol.5 No.5

In this study the seismic risk assessments of six- and twelve-story staggered wall system structures with three different structural variations were performed. The performances of staggered wall structures with added columns along the central corridor and the structures with their first story walls replaced by beams and columns were compared with those of the regular staggered wall structures. To this end incremental dynamic analyses were carried out using twenty two pairs of earthquake records to obtain the failure probabilities for various intensity of seismic load. The seismic risk for each damage state was computed based on the fragility analysis results and the probability of occurrence of earthquake ground motions. According to the analysis results, it was observed that the structures with added columns along the central corridor showed lowest probability of failure and seismic risk. The structures with their first story walls replaced by beams and columns showed lowest margin for safety.

Cyclic testing of weak-axis column-tree connections with formation of plastic hinge at beam splice

Oh, Keunyeong,Chen, Liuyi,Hong, Sungbin,Yang, Yang,Lee, Kangmin 테크노프레스 2015 Earthquakes and structures Vol.8 No.5

The purpose of this study was to evaluate the seismic performance of weak-axis column-tree type connections used in steel moment frames. These connections are composed of a shop-welded and fieldbolted steel structure and can improve welding quality. On this basis, column-tree type connections are widely used in steel moment resisting frames in Korea and Japan. In this study, splices designed with a semirigid concept regarding the seismic performance of column-tree connections were experimentally evaluated. The structures can absorb energy in an inelastic state rather than the elastic state of the structures by the capacity design method. For this reason, the plastic hinge might be located at the splice connection at the weak-axis column-tree connection by reducing the splice plate thickness. The main variable was the distance from the edge of the column flange to the beam splice. CTY series specimens having column-tree connections with splice length of 600 mm and 900 mm were designed, respectively. For comparison with two specimens with the main variable, a base specimen with a weak-axis column-tree connection was fabricated and tested. The test results of three full-scale test specimens showed that the CTY series specimens successfully developed ductile behavior without brittle fracture until 5% story drift ratio. Although the base specimen reached a 5% story drift ratio, brittle fracture was detected at the backing bar near the beam-to-column connection. Comparing the energy dissipation capacity for each specimen, the CTY series specimens dissipated more energy than the base specimen.

A systematic approach for the development of porous concrete based on axiomatic design theory

Tran, X.H.,Tawie, R.,Lee, H.K. 테크노프레스 2009 COMPUTERS AND CONCRETE Vol.6 No.6

This paper presents a design framework developed using axiomatic design (AD) theory that can be applied in the design process of porous concrete. The main contribution of this paper is the definition of an AD framework based on the needs and functional requirements of porous concrete. The framework shows how AD theory can be used to provide guidelines for proportioning and manufacturing porous concrete. The advantage of the AD approach is that it systemizes the way to decouple design parameters and makes designers to think rationally between what we want to achieve and how we propose to satisfy the functional requirements of porous concrete. In this paper, test results of laboratory-size porous concrete specimens under compression were analyzed to evaluate the performance of the porous concrete based on the desired functional requirements.

Simple approach to calculate chloride diffusivity of concrete considering carbonation

Yoon, In-Seok 테크노프레스 2009 COMPUTERS AND CONCRETE Vol.6 No.1

Chloride diffusivity of concrete is a crucial material parameter for service life determination and durability designing of marine concrete. Many research works on this issue have been conducted, varying from empirical solutions obtained experimentally to image analysis, based on multi-scale modeling. One of the simple approaches is to express the chloride diffusivity of concrete by a multi-factor function, however, the influences of various factors on the chloride diffusivity are ambiguous. Furthermore, the majority of these research works have not dealt with the carbonation process of concrete, although this process affects the chloride diffusivity of concrete significantly. The purpose of this study is to establish a simple approach to calculate the chloride diffusivity of (non)carbonated concrete. The chloride diffusivity of concrete should be defined, based on engineering and scientific knowledge of cement and concrete materials. In this paper, a lot of parameters affecting the chloride diffusivity, such as the diffusivity in pore solution, tortuosity, micro-structural properties of hardened cement paste, volumetric portion of aggregate, are taken into consideration in the calculation of the chloride diffusivity of noncarbonated concrete. For carbonated concrete, reduced porosity due to carbonation is calculated and used for calculating the chloride diffusivity. The results are compared with experimental data and previous research works.

Prediction of ECC tensile stress-strain curves based on modified fiber bridging relations considering fiber distribution characteristics

Lee, Bang Yeon,Kim, Jin-Keun,Kim, Yun Yong 테크노프레스 2010 COMPUTERS AND CONCRETE Vol.7 No.5

This paper presents a prediction and simulation method of tensile stress-strain curves of Engineered Cementitious Composites (ECC). For this purpose, the bridging stress and crack opening relations were obtained by the fiber bridging constitutive law which is quantitatively able to consider the fiber distribution characteristics. And then, a multi-linear model is employed for a simplification of the bridging stress and crack opening relation. In addition, to account the variability of material properties, randomly distributed properties drawn from a normal distribution with 95% confidence are assigned to each element which is determined on the basis of crack spacing. To consider the variation of crack spacing, randomly distributed crack spacing is drawn from the probability density function of fiber inclined angle calculated based on sectional image analysis. An equation for calculation of the crack spacing that takes into quantitative consideration the dimensions and fiber distribution was also derived. Subsequently, a series of simulations of ECC tensile stress-strain curves was performed. The simulation results exhibit obvious strain hardening behavior associated with multiple cracking, which correspond well with test results.

Effect of fractional order on energy ratios at the boundary surface of elastic-piezothermoelastic media

Kumar, Rajneesh,Sharma, Poonam 테크노프레스 2016 Multiscale and multiphysics mechanics Vol.1 No.4

In the present investigation reflection and transmission of plane waves at an elastic half space and piezothermoelastic solid half space with fractional order derivative is discussed. The piezothermoelastic solid half space is assumed to have 6 mm type symmetry and assumed to be loaded with an elastic half space. It is found that the amplitude ratios of various reflected and refracted waves are functions of angle of incidence, frequency of incident wave and are influenced by the piezothermoelastic properties of media. The expressions of amplitude ratios and energy ratios are obtained in closed form. The energy ratios are computed numerically using amplitude ratios for a particular model of graphite and Cadmium Selenide (CdSe). The variations of energy ratios with angle of incidence are shown graphically. The conservation of energy across the interface is verified. Some cases of interest are also deduced from the present investigation.

Effects of thermal aging on mechanical properties of laminated lead and natural rubber bearing

Kim, Dookie,Oh, Ju,Do, Jeongyun,Park, Jinyoung 테크노프레스 2014 Earthquakes and structures Vol.6 No.2

Laminated rubber bearing is very popular base isolation of earthquake engineering pertaining to the passive structural vibration control technologies. Rubber used in fabricating NRB and LRB can be easily attacked by various environmental factors such as oxygen, heat, light, dynamic strain, and organic liquids. Among these factors, this study carried out thermal aging test to investigate the effect of thermal aging on the mechanical properties of laminated rubber bearings in accelerated exposure condition of $70^{\circ}C$ temperature for 168 hours. The compressive-shear test was carried out to identify the variation of compressive and shear properties of the rubber bearings before and after thermal aging. In contrast to tensile strength and elongation tests, the hardness of rubber materials showed the increasing tendency dependent on exposure temperature and period. Based on the test results, the property changes of rubber bearing mainly aged by heat are quantitatively presented.