• Steel and Composite Structures
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• v.38 no.1
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• pp.103-120
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• 2021

This research examines the eccentric compression performance of composite columns composed of recycled aggregate concrete (RAC)-filled square steel tube and profile steel. A total of 17 specimens on the composite columns with different recycled coarse aggregate (RCA) replacement percentage, RAC strength, width to thickness ratio of square steel tube, profile steel ratio, eccentricity and slenderness ratio were subjected to eccentric compression tests. The failure process and characteristic of specimens under eccentric compression loading were observed in detail. The load-lateral deflection curves, load-train curves and strain distribution on the cross section of the composite columns were also obtained and described on the basis of test data. Results corroborate that the failure characteristics and modes of the specimens with different design parameters were basically similar under eccentric compression loads. The compression side of square steel tube yields first, followed by the compression side of profile steel. Finally, the RAC in the columns was crushed and the apparent local bulging of square steel tube was also observed, which meant that the composite column was damaged and failed. The composite columns under eccentric compression loading suffered from typical bending failure. Moreover, the eccentric bearing capacity and deformation of the specimens decreased as the RCA replacement percentage and width to thickness ratio of square steel tube increased, respectively. Slenderness ratio and eccentricity had a significantly adverse effect on the eccentric compression performance of composite columns. But overall, the composite columns generally had high-bearing capacity and good deformation. Meanwhile, the mechanism of the composite columns under eccentric compression loads was also analysed in detail, and the calculation formulas on the eccentric compression capacity of composite columns were proposed via the limit equilibrium analysis method. The calculation results of the eccentric compression capacity of columns are consistent with the test results, which verify the validity of the formulas, and the conclusions can serve as references for the engineering application of this kind of composite columns.

• International Journal of Naval Architecture and Ocean Engineering
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• v.13 no.1
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• pp.493-510
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• 2021

Mineral wool is an insulation material commonly used in passive fire protection (PFP) systems on offshore installations. Insulation materials have only been considered functional materials for thermal analysis in the conventional offshore PFP system design method. Hence, the structural performance of insulation has yet to be considered in the design of PFP systems. However, the structural elements of offshore PFP systems are often designed with excessive dimensions to satisfy structural requirements under external loads such as wind, fire and explosive pressure. To verify the structural contribution of insulation material, it was considered a structural material in this study. A series of material tensile tests was undertaken with two types of mineral wool at room temperature and at elevated temperatures for fire conditions. The mechanical properties were then verified with modified methods, and a database was constructed for application in a series of nonlinear structural and thermal finite-element analyses of an offshore bulkhead-type PFP system. Numerical analyses were performed with a conventional model without insulation and with a new suggested model with insulation. These analyses showed the structural contribution of the insulation in the structural behaviour of the PFP panel. The results suggest the need to consider the structural strength of the insulation material in PFP systems during the structural design step for offshore installations.

• Steel and Composite Structures
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• v.39 no.4
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• pp.401-417
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• 2021

This study experimentally reveals the influence of welding on grade S690Q high strength steel (HSS) butt joints from both micro and macro levels. Total eight butt joints, taking plate thickness and welding heat input as principal factors, were welded by shielded metal arc welding. In micro level, the microstructure transformations of the coarse grain heat affected zone (CGHAZ), the fine grain heat affected zone (FGHAZ) and the tempering zone occurred during welding were observed under light optical microscopy, and the corresponding mechanical performance of those areas were explored by micro-hardness tests. In macro level, standard tensile tests were conducted to investigate the impacts of welding on tensile behaviour of S690Q HSS butt joints. The test results showed that the main microstructure of S690Q HSS before welding was tempered martensite. After welding, the original microstructure was transformed to granular bainite in the CGHAZ, and to ferrite and cementite in the FGHAZ. For the tempering zone, some temper martensite decomposed to ferrite. The performed micro-hardness tests revealed that an obvious "soft layer" occurred in HAZ, and the HAZ size increased as the heat input increased. However, under the same level of heat input, the HAZ size decreased as the plate thickness increased. Subsequent coupon tensile tests found that all joints eventually failed within the HAZ with reduced tensile strength when compared with the base material. Similar to the size of the HAZ, the reduction of tensile strength increased as the welding heat input increased but decreased as the thickness of the plate increased.

• Structural Engineering and Mechanics
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• v.86 no.3
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• pp.291-309
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• 2023

This paper addresses the finite element modeling of functionally graded porous (FGP) beams for free vibration and buckling behaviour cases. The formulated finite element is based on simple and efficient higher order shear deformation theory. The key feature of this formulation is that it deals with Euler-Bernoulli beam theory with only three unknowns without requiring any shear correction factor. In fact, the presented two-noded beam element has three degrees of freedom per node, and the discrete model guarantees the interelement continuity by using both C⁰ and C¹ continuities for the displacement field and its first derivative shape functions, respectively. The weak form of the governing equations is obtained from the Hamilton principle of FGP beams to generate the elementary stiffness, geometric, and mass matrices. By deploying the isoparametric coordinate system, the derived elementary matrices are computed using the Gauss quadrature rule. To overcome the shear-locking phenomenon, the reduced integration technique is used for the shear strain energy. Furthermore, the effect of porosity distribution patterns on the free vibration and buckling behaviours of porous functionally graded beams in various parameters is investigated. The obtained results extend and improve those predicted previously by alternative existing theories, in which significant parameters such as material distribution, geometrical configuration, boundary conditions, and porosity distributions are considered and discussed in detailed numerical comparisons. Determining the impacts of these parameters on natural frequencies and critical buckling loads play an essential role in the manufacturing process of such materials and their related mechanical modeling in aerospace, nuclear, civil, and other structures.

• Advances in materials Research
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• v.11 no.2
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• pp.121-146
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• 2022

In the recent years, the use of agricultural waste in green cement mortar and concrete production has attracted considerable attention because of potential saving in the large areas of landfills and potential enhancement on the performance of mortar. In this research, microparticles of palm oil fuel ash (POFA) obtained from a multistage thermal and mechanical treatment processes of raw POFA originating from palm oil mill was utilized as a pozzolanic material to produce high-performance cement mortar (HPCM). POFA was used as a partial replacement material to ordinary Portland cement (OPC) at replacement levels of 0, 5, 10, 15, 20, 25, 30, 35, 40% by volume. Sand with particle size smaller than 300 ㎛ was used to enhance the performance of the HPCM. The HPCM mixes were tested for workability, compressive strength, ultrasonic pulse velocity (UPV), porosity and absorption. The results portray that the incorporation of micro POFA in HPCMs led to a slight reduction in the compressive strength. At 40% replacement level, the compressive strength was 87.4 MPa at 28 days which is suitable for many high strength applications. Although adding POFA to the cement mixtures harmed the absorption and porosity, those properties were very low at 3.4% and 11.5% respectively at a 40% POFA replacement ratio and after 28 days of curing. The HPCM mixtures containing POFA exhibited greater increase in strength and UPV as well as greater reduction in absorption and porosity than the control OPC mortar from 7 to 28 days of curing age, as a result of the pozzolanic reaction of POFA. Micro POFA with finely graded sand resulted in a dense and high strength cement mortar due to the pozzolanic reaction and increased packing effect. Therefore, it is demonstrated that the POFA could be used with high replacement ratios as a pozzolanic material to produce HPCM.

• Journal of Korean Society of Steel Construction
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• v.10 no.3 s.36
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• pp.423-435
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• 1998

In this paper fatigue strength reduction of butt weld with penetration defect, which can be seen frequently in the steel bridge, was assessed quantitatively. S-N curves were derived and investigated through the constant amplitude fatigue test of fully or partially penetrated welded specimen made of SWS490 steel. The fracture mechanical method was applied in order to calculate the remaining fatigue life of the partially penetrated butt welds. The fatigue limit of the fully penetrated butt welds was higher than that of category A in AASHTO's fatigue design curves, and the slope of S-N curves with 5.57 was stiffer than that of other result for welded part generally accepted as 3. The fatigue strength of the partially Penetrated butt weld was strongly influenced by the size of lack of penetration, D. It decreased drastically with increasing D from 3.9 to 14.7mm. Fracture behaviour of the partially penetrated butt weld is able to be explained obviously from the beach mark test that a semi-elliptical surface crack with small a/c ratio initiates at a internal weld root and propagates through the weld metal. To estimate the fatigue life of the partially penetrated butt weld with fracture mechanics, stress intensity factors K of 3-dimensional semi-elliptical crack were calculated by appling finite elements method and fracture mechanics parameters such as C and m were derived through the fatigue test of CT-specimen. As a result, the fatigue lives obtained by using the fracture mechanical method agreed well with the experimental results. The results were applied to Sung-Su bridge collapsed due to penetration defects in butt weld of vertical member.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.11
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• pp.2150-2158
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• 1992

This study was carried out to investigate the flow structure and mixing process of a cross mixing flow formed by two round jets with respect to the ratio of mass flow rate. This flow configuration is of great practical relevance in a variety of combustion systems, and the flow behaviour of a cross jet defends critically on the ratio of mass flow rate and the cross angle. The mass flow rate ratios of two different jets were controlled as 1.0, 0.8, 0.6, and 0.4, and the crossing angle of two round jets was fixed at 45 degree. The velocities issuing from jet nozzle with an exit diameter of 20mm were adjusted to 40m/s, 32m/s, 24m/s, and 16m/s, and the measurements have been conducted in the streamwise range of $1.1X_0$to $2.5X_0$ by an on-line measurement system consisted of a constant temperature type two channel hot-wire anemometry connected to a computer analyzing system. The original air flow was generated by a subsonic wind tunnel with reliable stabilities and uniform flows in the test section. For the analysis of the cross mixing flow structure in the downstream region after the cross point, the mean velocity profiles, the resultant velocity contours, and the three-dimensional profiles depending upon the mass flow rate ratio have been concentrately studied.

• Journal of the Korean Society for Nondestructive Testing
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• v.21 no.1
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• pp.62-68
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• 2001

In order to characterize the microscopic fracture behaviour of the weldment din stress corrosion cracking(SCC) phenomena, SCC and acoustic emission(AE) tests were carried out simultaneously and the correlation between mechanical paramenters obtained from SCC and AE tests was investigated. In the case of base metal, much more AE events were produced at -0.5V than at -0.8V because of the dissolution mechanism before the maximum load. Regardless of the applied voltages to the specimens, however, AE events decreased after the maximum load. In the case of weldment, lots of AE events with larger amplitude $range(40{\sim}100dB)$ were produced because of the singularities of weld HAZ in comparision to the base metal and post-weld heat-treated(PWHT) specimens. Numerous and larger cracks for the weldment were observed on the fractured surfaces by SEM examination. From these results, it was concluded that SCC for the weldment appeared most severely in synthetic seawater. Weld HAZ was softened by PWHT which also contributed to the reduced susceptibility to corrosive environment in comparison to the weldment.

• Korean Journal of Materials Research
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• v.10 no.8
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• pp.569-574
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• 2000

To improve the ductility of $Al_3Hf$ and $Al_3Ta$ intermetallic compounds, which are the potential temperature structural materials, the mechanical alloying behaviour and the effect of ternary additions on the $Ll_2$ phase formation were investigated. During the mechanical alloying by the SPEX mill, the $Ll_2$ $Al_3Hf$ intermetallic compound was formed after 6 hours of milling in AL-25%Hf system. In AL-25%Ta system, however, only the $D0_{22}$ $Al_3Ta$ intermetallic compound was formed until 30 hours of milling and the $Ll_2$ phase was not observed. In AL-12.5%M-25%Ta(M=Cu, Zn, Mn, Fe, Ni) systems, the additions of Cu and Zn had no effect on the $D0_{22}$ structure of the binary $Al_3Hf$ and the additions of Mn, Fe and Ni produced the amorphous phase. Therefore it was considered that these ternary additions could not overcome the energy difference between $Ll_2$ and $D0_{22}$ structures in the $Al_3Hf$ intermetallic compound. In AL-12.5%M-25%Hf(M=Cu, Zn, Mn, Fe, Ni)systems, the additions of Cu and Zn did not affect the $Ll_2$ structure of the binary $Al_3Hf$ but the additions of oMn, Fe and Ni produced the amorphous phase as they did in AL-12.5%M-25%Ta systems. Therefore, it was considered that the Ni, Mn and Fe additions promote the formation of amorphous phase in $Al_3X$ intermetallic compounds.

• Journal of the Korean Society for Nondestructive Testing
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• v.29 no.5
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• pp.473-478
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• 2009

The purpose of this study is to evaluate the behaviour of the plastic deformed zone at crack tip on the standard Charpy specimens which were taken from the multi-passed weld block of the pressure vessel steel. Notch was machined on the standard Charpy test specimens and pre-crack which was located around the fusion line was made under the repeat load. Four point bend and acoustic emission tests were carried out simultaneously. The size of plastic region at crack tip was calculated using stress intensity factor. Relationships between characteristics of acoustic emission and plastic zone size at crack tip were discussed through the cumulative AE energy. Regardless of the specimens, AE signals were absent within the elastic region almost and most of AE signals were produced at the plastic deformation region from yield point to the mid-point between yield and maximum load. More AE signals for the weldment were produced compared with the base-metal and PWHT specimen. Relations between plastic deformed zones at crack tip and cumulative AE energy for the weldment and PWHT specimen were different quietly from the base-metal. Besides, number of AE counts for the weldment was the larger than those of the base-metal and PWHT specimen.