• 한국재료학회지
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• 제22권1호
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• pp.42-45
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• 2012

The mechanical behavior and microstructural evolution during high temperature tensile deformation of recrystallized Ni3Al polycrystals doped with boron were investigated as functions of initial grain size, tensile strain rate and temperature. In order to obtain more precise information on the deformation mechanism, tensile specimens were rapidly quenched immediately after deformation at a cooling rate of more than $2000Ks^{-1}$, and were then observed by transmission electron microscopy (TEM). Mechanical tests in the range of 923 K to 1012 K were carried out in a vacuum of less than $3{\times}10^{-4}$ Pa using an Instron-type machine with various but constant cross head speeds corresponding to the initial strain rates from $1.0{\times}10^{-4}$ to $3.1{\times}10^{-5}s^{-1}$. After heating to deformation temperature, the specimen was kept for more than 1.8 ks before testing. The following results were obtained: (1) Flow behavior was affected by initial strain size; with decreasing initial grain size, the level of a stress peak in the true stress-true strain curve decreased, the steady state region was enlarged and elongation increased. (2) On the basis of TEM observation of rapidly quenched specimens, it was confirmed that dynamic recrystallization certainly occurred on deformation of fine-grained ($3.3{\mu}m$) and intermediate-grained ($5.0{\mu}m$) specimens at an initial strain rate of $3.1{\times}10^{-5}s^{-1}$ and at 973 K. (3) There were some dislocation-free grains among the new recrystallized grains. The obtained results suggest that both dynamic recrystallization and grain boundary sliding are operative during high temperature deformation.

• 한국분말재료학회지
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• 제22권1호
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• pp.6-9
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• 2015

In this study, the behavior of densification of copper powders during high-pressure torsion (HPT) at room temperature is investigated using the finite element method. The simulation results show that the center of the workpiece is the first to reach the true density of copper during the compressive stage because the pressure is higher at the center than the periphery. Subsequently, whole workpiece reaches true density after compression due to the high pressure. In addition, the effective strain is increased along the radius during torsional stage. After one rotation, the periphery shows that the effective strain is increased up to 25, which is extensive deformation. These high pressure and severe strain do not only play a key role in consolidation of copper powders but also make the matrix harder by grain refinement.

• Food Science and Biotechnology
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• 제18권4호
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• pp.1013-1018
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• 2009

Several rheological terms were introduced to estimate the properties of cooked noodles with different starch content using tensile tests. Ring-shaped specimens were prepared by connecting both ends of the noodle strip before cooking. Hencky strain and rate, as well as true stress were applied in constant deformation tests. The elastic region on the curves of strain vs. stress was not clearly identified. Strain hardening in the subsequent plastic region was more prominent in low-starch noodles. Elongational viscosities at lower strain rates were used to differentiate noodles with different starch content, representing the dominant effect of protein content in the range of lower strain rates. In stress relaxation tests, the reciprocal of Peleg's constant $K_1$ (initial decay rate) and $K_2$ (asymptotic level) increased and decreased respectively, with an increase in starch content. This indicated that addition of starch contributed to the noodles becoming viscous liquid rather than elastic solid.

• 대한기계학회논문집A
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• 제33권12호
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• pp.1357-1365
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• 2009

In this study, we enhance the numerical approach of Lee et al.$^{(1)}$ to spherical indentation technique for property evaluation of hyper-elastic rubber. We first determine the friction coefficient between rubber and indenter in a practical viewpoint. We perform finite element numerical simulations for deeper indentation depth. An optimal data acquisition spot is selected, which features sufficiently large strain energy density and negligible frictional effect. We then improve two normalized functions mapping an indentation load vs. deflection curve into a strain energy density vs. first invariant curve, the latter of which in turn gives the Yeoh-model constants. The enhanced spherical indentation approach produces the rubber material properties with an average error of less than 3%.

• 대한기계학회논문집
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• 제19권8호
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• pp.1799-1809
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• 1995

The applicability of the field-consistency paradigm, which was originally employed for analysis of locking due to constrained energy having the second power of a strain, is extended to the constrained energy having a quadratic form of strain. For the extension, nearly-incompressible plane strain problem is considered by introducing the concept of reduced minimization. The field-consistent analysis of the plane strain problem leads to a clear and systematic understanding on the relation amongst constraints imposed on element, spurious constraint -free optimal points, and integration order used.

• Nuclear Engineering and Technology
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• 제55권11호
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• pp.4134-4145
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• 2023

This paper proposes strain-based failure model of A533B1 pressure vessel steel to simulate failure, followed by application to OECD lower head failure (OLHF) test simulation for experimental validation. The proposed strain-based failure model uses simple constant and linear functions based on physical failure modes with the critical strain value determined either using the lower bound of true fracture strain or using the average value of total elongation depending on the temperature. Application to OECD Lower Head Failure (OLHF) tests shows that progressive deformation, failure time and failure location can be well predicted.

• Journal of Plant Biology
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• 제32권1호
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• pp.1-9
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• 1989

An endophyte was isolated from the root nodule of alnus hirsuta. The isolated endophyte was identified as a Frankia sp. through morphological characteristics. Their infectivity and effectivity were confirmed by nitrogen-fixing root nodules induced on inoculated Alnus seedlings. Reisolated endophyte from the induced nodule showed identical morphological characteristics as the first isolate, showing the nodule was induced by the first isolate. Consequently, the first isolate was confirmed as a true symbiont of Almus hirsuta root nodule. The isolate was designated as a Frankia SNU 014201 strain.

• Nuclear Engineering and Technology
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• 제38권7호
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• pp.619-638
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• 2006

Low temperature irradiation can significantly harden metallic materials and often lead to strain localization and ductility loss in deformation. This paper provides a review on the radiation effects on the deformation of metallic materials, focusing on microscopic and macroscopic strain localization phenomena. The types of microscopic strain localization often observed in irradiated materials are dislocation channeling and deformation twinning, in which dislocation glides are evenly distributed and well confined in the narrow bands, usually a fraction of a micron wide. Dislocation channeling is a common strain localization mechanism observed virtually in all irradiated metallic materials with ductility, while deformation twinning is an alternative localization mechanism occurring only in low stacking fault energy(SFE) materials. In some high stacking fault energy materials where cross slip is easy, curved and widening channels can be formed depending on dose and stress state. Irradiation also prompts macroscopic strain localization (or plastic instability). It is shown that the plastic instability stress and true fracture stress are nearly independent of irradiation dose if there is no radiation-induced phase change or embrittlement. A newly proposed plastic Instability criterion is that the metals after irradiation show necking at yield when the yield stress exceeds the dose-independent plastic instability stress. There is no evident relationship between the microscopic and macroscopic strain localizations; which is explained by the long-range back-stress hardening. It is proposed that the microscopic strain localization is a generalized phenomenon occurring at high stress.

• 한국자동차공학회논문집
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• 제13권5호
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• pp.171-180
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• 2005

Bending performances of aluminum square tube beams reinforced by aluminum plates under three point bending loads have been evaluated using experimental tests combined with theoretical and finite element analyses. A finite element simulation for the three-point bending test was performed. Basic properties of aluminum materials used for initial input data of the finite element simulation were obtained from the true stress-true strain curves of specimens which had been extracted from the Al tube beams. True stresses were determined from applied loads and cross-sectional area records of a tensile specimen with a rectangular cross-section by real-time photographing, and true strains were obtained from in-situ local elongation measurements of the specimen gage portion by the multi-point scanning laser extensometer. Six kinds of aluminum tube beam specimens adhered by aluminum plates were employed fur the bending test. The bending deformation behaviors up to the maximum load described by the numerical simulation were in good agreement with experimental ones. After passing the maximum load, reinforcing plate was debonded from the aluminum tube beam. An aluminum tube beam strengthened by aluminum plate on the upper web showed an excellent bending capability.

• 한국해양공학회지
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• 제27권2호
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• pp.19-26
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• 2013

Stress triaxiality is recognized as one of the most important factors for predicting the failure strain of ductile metals. This study dealt with the effect of the average stress triaxiality on the failure strain of a typical low-temperature high-strength marine structural steel, EH36. Tensile tests were carried out on flat specimens with different notches, from relatively smooth to very sharp levels. Numerical simulations of each specimen were performed by using ABAQUS. The failure initiation points in numerical simulations were identified from a comparison of the engineering stress vs. strain curves obtained from experiments with simulated ones. The failure strain curves for various dimensionless critical energy levels were established in the average stress triaxiality domain and compared with the identified failure strain points. It was observed that most of the failure initiation points were approximated with a 100% dimensionless critical energy curve. It was concluded that the failure strains were well expressed as a function of the average stress triaxiality.