Byeong Soo Lim

Fatigue crack growth behavior of heat affected zone in P92 steel weldment

In this study, the effect of microstructure on fatigue crack growth behavior of HAZ was investigated using newly developed P92 steel weldment. Fatigue crack growth rates of the fusion line and fusion line+2 mm in HAZ were found to be faster than that of welds or base metal, while fatigue crack growth rate of the fusion line+1 mm was the slowest. The fracture appearance of the fusion line and fusion line+2 mm revealed mainly transgranular, cleavage-like faceted fracture surfaces and the fusion line+1 mm showed quasi-cleavage fractures with scattered dimples, which increased resistance to fatigue crack growth.

A mass weighted chemical elastic network model elucidates closed form domain motions in proteins

An elastic network model (ENM), usually Cα coarse‐grained one, has been widely used to study protein dynamics as an alternative to classical molecular dynamics simulation. This simple approach dramatically saves the computational cost, but sometimes fails to describe a feasible conformational change due to unrealistically excessive spring connections. To overcome this limitation, we propose a mass‐weighted chemical elastic network model (MWCENM) in which the total mass of each residue is assumed to be concentrated on the representative alpha carbon atom and various stiffness values are precisely assigned according to the types of chemical interactions. We test MWCENM on several well‐known proteins of which both closed and open conformations are available as well as three α‐helix rich proteins. Their normal mode analysis reveals that MWCENM not only generates more plausible conformational changes, especially for closed forms of proteins, but also preserves protein secondary structures thus distinguishing MWCENM from traditional ENMs. In addition, MWCENM also reduces computational burden by using a more sparse stiffness matrix.

Effect of temperature on fatigue crack growth in P92 steel

Fracture at high temperature has become a critical problem for such high temperature components as those used in power plants or oil refinery plants, because both high operating temperature and pressure are required for better thermal efficiency. Therefore, it is very important to approach such problems from the viewpoint of high temperature material properties. Since fatigue and creep are closely related to such components failures, the fracture behavior in high temperature components must be evaluated through fatigue and creep crack growth tests, and based on these results, better operating conditions can be determined. In this study, recently developed P92 (9Cr-2W) alloy steel, which is a high strength material for high temperature use, is investigated and its fatigue crack growth has been characterized by Paris law. A series of high temperature fatigue tests were carried out at 400, 500, 550, 600, 625, 650, and 700°C to verify the temperature effect. The results indicated that the Paris exponent remained at approximately the same value up to a certain temperature. From 600 to 700°C, creep rupture tests were conducted in order to investigate the creep behavior with temperature. Further analysis has also been carried out to investigate the effect of temperature on fracture mode shift, dimple formation, and its role in crack growth rate and deformability at high temperature.

DNA nanotube formation based on normal mode analysis

Ever since its inception, a popular DNA motif called the cross tile has been recognized to self-assemble into addressable 2D templates consisting of periodic square cavities. Although this may be conceptually correct, in reality certain types of cross tiles can only form planar lattices if adjacent tiles are designed to bind in a corrugated manner, in the absence of which they roll up to form 3D nanotube structures. Here we present a theoretical study on why uncorrugated cross tiles self-assemble into counterintuitive 3D nanotube structures and not planar 2D lattices. Coarse-grained normal mode analysis of single and multiple cross tiles within the elastic network model was carried out to expound the vibration modes of the systems. While both single and multiple cross tile simulations produce results conducive to tube formations, the dominant modes of a unit of four cross tiles (one square cavity), termed a quadruplet, fully reflect the symmetries of the actual nanotubes found in experiments and firmly endorse circularization of an array of cross tiles.

Efficient prediction of protein conformational pathways based on the hybrid elastic network model.

Various computational models have gained immense attention by analyzing the dynamic characteristics of proteins. Several models have achieved recognition by fulfilling either theoretical or experimental predictions. Nonetheless, each method possesses limitations, mostly in computational outlay and physical reality. These limitations remind us that a new model or paradigm should advance theoretical principles to elucidate more precisely the biological functions of a protein and should increase computational efficiency. With these critical caveats, we have developed a new computational tool that satisfies both physical reality and computational efficiency. In the proposed hybrid elastic network model (HENM), a protein structure is represented as a mixture of rigid clusters and point masses that are connected with linear springs. Harmonic analyses based on the HENM have been performed to generate normal modes and conformational pathways. The results of the hybrid normal mode analyses give new physical insight to the 70S ribosome. The feasibility of the conformational pathways of hybrid elastic network interpolation (HENI) was quantitatively evaluated by comparing three different overlap values proposed in this paper. A remarkable observation is that the obtained mode shapes and conformational pathways are consistent with each other. Our timing results show that HENM has some advantage in computational efficiency over a coarse-grained model, especially for large proteins, even though it takes longer to construct the HENM. Consequently, the proposed HENM will be one of the best alternatives to the conventional coarse-grained ENMs and all-atom based methods (such as molecular dynamics) without loss of physical reality.

A study of creep-fatigue life prediction using an artificial neural network

In this study, using AISI 316 stainless steel, creep-fatigue tests were carried out under various test conditions (different total strain ranges and hold times) to verify the applicability of the artificial neural network method to creep-fatigue life prediction. Life prediction was also made by the modified Coffin-Manson method and the modified Ostegren method using 21 data points out of a total 27 experimental data points. The six verification data points were carefully chosen for the purpose of evaluating the predictability of each method. The predicted lives were compared with the experimental results and the following conclusions were obtained within the scope of this study. While the creep-fatigue life prediction by the modified Coffin-Manson method and the modified Ostegren method had average errors of 35.8% and 47.7% respectively, the artificial neural network method had only 15.6%. As a result, the artificial neural network method with the adaptive learning rate was found to be far more accurate and effective than any of the others. The validity of the artificial neural network method for life prediction checked with the six verification data points also proved to be very satisfactory.

Mechanical and Electrical Properties of Carbon Nanotubes in Copper-Matrix Nanocomposites

Carbon nanotubes have received considerable attention because of their excellent electrical and mechanical properties. In this study, carbon nanotube - copper nanocomposites with homogeneously dispersed nanotubes within the copper matrix have been fabricated by two different methods; a mechanical mixing process and a molecular-level mixing process, which consists of mixing copper ions with functionalized nanotubes in a solvent. Small punch creep tests showed significantly improved mechanical properties of the nanocomposites. The electrical resistance of the nanocomposites also decreased.

Effect of Creep Holding Time on the Fatigue Behavior in P92 Steel Weldment at High Temperature

In this study, the crack growth behavior in P92 steel (9%Cr-2%W) weldment was investigated at 600ı under the load of trapezoidal wave shape with various holding times. The relationship between the crack growth behavior and holding time was studied and it was characterized using the ΔK and (Ct)avg parameters. The number of micro-voids/cavities at the crack tip and fracture modes were examined and the relationship between crack growth rate and holding time was investigated.

Analysis of the small punch creep test results according to the normalized lifetime fraction

The small punch creep test method was developed to complement the demerits of the existing uniaxial creep test method. A 10×10×0.5mm thin-plate specimen was used in the test, which could help save specimens and testing time. However, theories on the mechanism and significance of the small punch creep test have not been established clearly yet, and further research should be conducted. In this study, to analyze the behaviors of the small punch creep test, experiments were conducted using the interrupted small punch creep test method for five different lifetime fractions under the designed load and temperature using the Ni-base alloy, which is one of the very attractive candidate materials for the future advanced fossil power plant. The stress concentration part of the specimen, which is critical in analyzing the failure characteristics, was determined via FEM, and its effect on the failure mechanism was also studied through a microstructural analysis.

Relationship Between Small-punch Creep Test Data and Uniaxial Creep Test Data based on the Monkman–Grant Relation

The relationship between the small-punch creep test and the conventional creep test was investigated experimentally using a method similar to that of the Monkman?Grant relationship. Uniaxial and small-punch creep rupture tests were carried out on 9Cr-2W ferritic steel(Commercial Grade 92 steel: X10CrWMoVNb 9-2) at elevated temperatures. From the relation derived in the same manner as the Monkman?Grant relation, a correlation between the displacement rate in response to the small-punch creep test and the strain rate in the uniaxial creep test was found, and the creep life was calculated using this relation. Furthermore, the failure modes of the small punch creep test specimens were investigated to show that the fracture was caused by creep.