• Journal of Welding and Joining
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• v.12 no.2
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• pp.87-96
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• 1994

Correlation between microstructural features and segregation of elements (Si, Mn, P and S) near the mid of thickness in the base metal and the synthetic HAZ was investigated. Furthermore, the relationship between the degree of center segregation and weld cold cracking susceptibility in the thickness direction was also conducted by evaluating the effect of P concentration on the critical applied stress. The results obtained are as follows: 1) Pearlite band, containing the MnS type inclusion and a locally transformed structure with a higher hardness, was observed in the center segregation region. 2) By the weld thermal cycle, center segregation region was transformed to the white band which had a higher hardness than that of base metal due to a greater hardenability of concentrated Mn, P etc.. 3) Weld cold cracking susceptibility in the thickness direction was mainly dependent on the concentration of impurity elements rather than on the number of the segregated particles near the mid of thickness. 4) During welding, the higher concentrated region was easily changed into white band. Therefore, it could be predicted that the initiation and propagation of a cold crack would be promoted by increasing the restraint stress and hydrogen content.

• Bulletin of the Korean Chemical Society
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• v.35 no.4
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• pp.1043-1049
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• 2014

Density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) theoretical level were performed for two novel explosives (compounds B and C) based on the guanidine-fused bicyclic skeleton $C_4N_6H_8$ (A). The heats of formation (HOFs) were calculated via isodesmic reaction. The detonation properties were evaluated by using the Kamlet-Jacobs equations. The bond dissociation energies (BDEs) for the thermolysis initiation bond were also analyzed to investigate the thermal stability. The results show that the compounds have high positive HOF values (B, 1064.68 $kJ{\cdot}mol^{-1}$; C, 724.02 $kJ{\cdot}mol^{-1}$), high detonation properties (${\rho}$, D and P values of 2.04 $g{\cdot}cm^{-3}$ and 2.21 $g{\cdot}cm^{-3}$, 9.98 $km{\cdot}s^{-1}$ and 10.99 $km{\cdot}s^{-1}$, 46.44 GPa and 59.91 Gpa, respectively) and meet the basic stability requirement. Additionally, feasible synthetic routes of the these high energy density compounds (HEDCs) were also proposed via retrosynthetic analysis.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2010.05a
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• pp.129-132
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• 2010

The objective of IM rocket motor is to minimize the probability of inadvertent initiation and severity of subsequent collateral damage, hence it is important to define personnel and equipment survivability to a rocket motor accident. The violent response probability associated with shock, impact and thermal effects be minimized. And during production, transportation/storage and stack of rocket motor, sympathetic detonation, giving severe effects of the propagation of adverse reaction on its surroundings, be reduced. Hence Reaction type also based on reaction results of the overpressure, fragment throw and heat flux.

• Journal of Electrical Engineering and Technology
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• v.5 no.2
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• pp.329-336
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• 2010

The paper describes the electrical and optical properties of underwater discharges in highly inhomogeneous electric fields caused by 1.2/50 ${\mu}s$ impulse voltages as functions of the polarity and amplitude of the applied voltage, and various water conductivities. The electric fields are formed by a point-to-plane electrode system. The formation of air bubbles is associated with a thermal process of the water located at the tip of the needle electrode, and streamer coronas can be initiated in the air bubbles and propagated through the test gap with stepped leaders. The fastest streamer channel experiences the final jump across the test gap. The negative streamer channels not only have more branches but are also more widely spread out than the positive streamer channels. The propagation velocity of the positive streamer is much faster than that of the negative one and, in fact, both these velocities are independent of the water conductivity; in addition the time-lag to breakdown is insensitive to water conductivity. The higher the water conductivity the larger the pre-breakdown energy, therefore, the ionic currents do not contribute to the initiation and propagation of the underwater discharges in the test conditions considered.

• Nuclear Engineering and Technology
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• v.39 no.4
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• pp.257-272
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• 2007

Supercritical water-cooled reactors (SCWRs) are recognized as a Generation IV reactor concept. The Super LWR is a pressure-vessel type thermal spectrum SCWR with downward-flow water rods and is currently under study at the University of Tokyo. This paper reviews Super LWR safety. The fundamental requirement for the Super LWR, which has a once-through coolant cycle, is the core coolant flow rate rather than the coolant inventory. Key safety characteristics of the Super LWR inhere in the design features and have been identified through a series of safety analyses. Although loss-of-flow is the most important abnormality, fuel rod heat-up is mitigated by the "heat sink" and "water source" effects of the water rods. Response of the reactor power against pressurization events is mild due to a small change in the average coolant density and flow stagnation of the once-through coolant cycle. These mild responses against transients and also reactivity feedbacks provide good inherent safety against anticipated-transient-without-scram (ATWS) events without alternative actions. Initiation of an automatic depressurization system provides effective heat removal from the fuel rods. An "in-vessel accumulator" effect of the reactor vessel top dome enhances the fuel rod cooling. This effect enlarges the safety margin for large LOCA.

• Advances in materials Research
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• v.7 no.3
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• pp.195-201
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• 2018

High Strength low alloy steels containing various levels of C, Nb and Mn were used and for each of which, a simulated double thermal cycle was applied with the same first peak temperature and different second peak temperatures to produce HAZ microstructure corresponding to multi-pass weld. Effect of double cycle second temperature on the microstructure was observed and compared with single cycle results obtained from previous works, it was found that the percentage of martensite austenite constituent (MA) increases by Nb addition for all steels with the same Mn content and the increase in Mn content at the same Nb content shows an increase in MA area fraction as well. MA area fraction obtained for the double cycle is larger than that obtained for the single cycle for all steels used which imply that MA will have great role in the brittle fracture initiation for double cycle and the inter-pass temperature should be controlled for medium and high-carbon Mn steel to avoid large area fraction of MA. The beneficial effects of Niobium obtained in single pass weld were not observed for the double cycle or multi pass welds.

• Korean Journal of Metals and Materials
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• v.49 no.6
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• pp.462-467
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• 2011

In order to understand why annealing at $480^{\circ}C$ for several hour prevents the initiation of PWSCC, the residual stress variation with isothermal annealing at $480^{\circ}C$ and isochronal annealing between 480 and $800^{\circ}C$ in cold rolled Alloy 600 was investigated by the XRD method. The isothermal annealing decreased residual stress slightly in the rolling direction but not in the transverse direction, whereas the isochronal annealing for two hours increased residual stress. It seemed that the decrease in residual stress by isothermal annealing was due to lattice contraction by an ordering reaction because the isothermal annealing increased hardness. The effects of the isochronal annealing could be interpreted as the influence of thermal expansion and a disordering reaction.

• Steel and Composite Structures
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• v.42 no.2
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• pp.151-159
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• 2022

This study discusses a hypothetical method for tracking the propagation damage of Carbon Reinforced Fiber Plastic (CRFP) components underneath vibration fatigue. The High Cycle Fatigue (HCF) behavior of composite materials was generally not as severe as this of admixture alloys. Each fissure initiation in metal alloys may quickly lead to the opposite. The HCF behavior of composite materials is usually an extended state of continuous degradation between resin and fibers. The increase is that any layer-to-layer contact conditions during delamination opening will cause a dynamic complex response, which may be non-linear and dependent on temperature. Usually resulted from major deformations, it could be properly surveyed by a non-contact investigation system. Here, this article discusses the scanning laser application of that vibrometer to track the propagation damage of CRFP components underneath fatigue vibration loading. Thus, the study purpose is to demonstrate that the investigation method can implement systematically a series of hypothetical means and dynamic characteristics. The application of the relaxation method based on numerical simulation in the Artificial Intelligence (AI) Evolved Bat (EB) strategy to reduce the dynamic response is proved by numerical simulation. Thermal imaging cameras are also measurement parts of the chain and provide information in qualitative about the temperature location of the evolution and hot spots of damage.

• Steel and Composite Structures
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• v.44 no.4
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• pp.519-529
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• 2022

Sandwich structures with the superior mechanical properties such as high stiffness and strength-to-weight ratio, good thermal insulation, and high energy absorption capacity are used today in aerospace, automotive, marine, and civil engineering industries. These structures are composed of moderately stiff, thin face sheets that withstand the majority of transverse and in-plane loads, separated by a thick, lightweight core that resists shear forces. In this research, the finite element technique is used to simulate a sandwich panel with a truss core under axial compressive stress using ABAQUS software. A review of past experimental studies shows that the bondline between the core and face sheets plays a vital role in the critical failure load. Therefore, this modeling analyzes the damage initiation modes and debonding between face sheet and core by cohesive surface contact with traction-separation model. According to the results obtained from the modeling, it can be observed that the adhesive stiffness has a significant influence on the critical failure load of the specimens. To achieve the full strength of the structure as a continuum, a lower limit is obtained for the adhesive stiffness. By providing this limit stiffness between the core and the panel face sheets, sudden failure of the structure can be prevented.

• Korean Journal of Materials Research
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• v.10 no.2
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• pp.111-116
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• 2000

Fracture mode and carbide reactions of cast alloy 738LC during thermal exposure and creep at 816$^{\circ}C$/440MPa and 982$^{\circ}C$/152MPa were investigated. Crystallographic transgranular failure was observed in the specimen crept at 816$^{\circ}C$ due to shearing on the slip plane. Because selective oxidation at the grainboundaries which was exposed at the surface leads reduction in surface energy, however, early initiation of crack at the grainboundaries and intergranular failure were observed in the specimen crept at 982$^{\circ}C$/152MPa. As a result of decomposition of MC carbide at the tested temperatures, M(sub)23C(sub)6 carbide precipitated either on the grainboundaries or on the deformation band. The applied stress enhanced decomposition of MC. $\sigma$phase nucleated from Cr(sub)23C(sub)6 then grew to the ${\gamma}$+${\gamma}$\\` matrix. Precipitation of $\sigma$was accelerated by increasing temperature and applied stress.