• 제목/요약/키워드: Thermal Transformation

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N.M.for the Effect of P.T. on Resicual Stress Relaxation (잔류응력 완화에 미치는 상변태의 수치적 모델링)

  • 장경복;손금렬;강성수
    • Journal of Welding and Joining
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    • v.17 no.6
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    • pp.84-89
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    • 1999
  • Most of ferrous b.c.c weld materials may experience martensitic transformation during rapid cooling after welding. It is well known that volume expansion due to the phase transformation could influence on the relaxation of welding residual stress. To apply this effect practically, it is a prerequisite to establish a numerical model which is able to estimate the effect of phase transformation on residual stress relaxation quantitatively. For this purpose, the analysis is carried out in two regions. i.e., heating and cooling, because the variation of material properties following a phase transformation in cooling is different in comparison with the case in heating, even at the same temperature. The variation of material properties following phase transformation is considered by the adjustment of specific heat and thermal expansion coefficient, and the distribution of residual stress in analysis is compared with that of experiment by previous study. consequently, in this study, simplified numerical procedures considering phase transformation, which based on a commercial finite element package was established through comparing with the experimental data of residual stress distribution by other researcher. To consider the phase transformation effect on residual stress relaxation, the transition of mechanical and thermal property such as thermal expansion coefficient and specific heat capacity was found by try and error method in this analysis.

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Numerical Modeling of the Transformation Temperature Effect on the Relaxation of Welding Residual Stress (용접 잔류응력 완화에 미치는 변태 온도의 영향에 관한 수치적 모델링)

  • Jang, Gyoung-Bok;Kang, Sung-Soo
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.24 no.10 s.181
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    • pp.2552-2559
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    • 2000
  • Most of ferrous b.c.c weld materials have martensitic transformation during rapid cooling after welding. It is well known that volume expansion due to the phase transformation could influence on the relaxation of welding residual stress. To apply this effect practically, it is necessary to establish a numerical model which is able to estimate the effect of phase transformation on residual stress relaxation quantitatively. For this purpose, the analysis is carried out in two regions, i.e., heating and cooling, because the variation of material properties following a phase transformation in cooling is different in comparison with the case in heating, even at the same temperature. The variation of material properties following phase transformation is considered by the adjustment of specific heat and thermal expansion coefficient, and the distribution of residual stress in analysis is compared with that of experiment by previous study. In this study, simplified numerical procedures considering phase transformation, which based on a commercial finite element package was established through comparing with the experimental data of residual stress distribution by other researcher. To consider the phase transformation effect on residual stress relaxation, the transition of mechanical and thermal property such as thermal expansion coefficient and specific heat capacity was found by try and error method in this analysis. In addition to, since the transformation temperature changes by the kind and control of alloying elements, the steel with many kinds of transformation temperature were selected and the effect of transformation on stress releasement was investigated by the numerical procedures considering phase transformation.

Change of phase transformation and bond strength of Y-TZP with various hydrofluoric acid etching

  • Mi-Kyung Yu;Eun-Jin Oh;Myung-Jin Lim;Kwang-Won Lee
    • Restorative Dentistry and Endodontics
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    • v.46 no.4
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    • pp.54.1-54.10
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    • 2021
  • Objectives: The purpose of this study was to quantify phase transformation after hydrofluoric acid (HF) etching at various concentrations on the surface of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP), and to evaluate changes in bonding strength before and after thermal cycling. Materials and Methods: A group whose Y-TZP surface was treated with tribochemical silica abrasion (TS) was used as the control. Y-TZP specimens from each experimental group were etched with 5%, 10%, 20%, and 40% HF solutions at room temperature for 10 minutes. First, to quantify the phase transformation, Y-TZP specimens (n = 5) treated with TS, 5%, 10%, 20% and 40% HF solutions were subjected to X-ray diffraction. Second, to evaluate the change in bond strength before and after thermal cycling, zirconia primer and MDP-containing resin cement were sequentially applied to the Y-TZP specimen. After 5,000 thermal cycles for half of the Y-TZP specimens, shear bond strength was measured for all experimental groups (n = 10). Results: The monoclinic phase content in the 40% HF-treated group was higher than that of the 5%, 10%, and 20% HF-treated groups, but lower than that of TS-treated group (p < 0.05). The 40% HF-treated group showed significantly higher bonding strength than the TS, 5%, and 10% HF-treated groups, even after thermal cycling (p < 0.05). Conclusions: Through this experiment, the group treated with SiO2 containing air-borne abrasion on the Y-TZP surface showed higher phase transformation and higher reduction in bonding strength after thermal cycling compared to the group treated with high concentration HF.

Analysis on Thermomechanical Response to Tensile Deformation of GaN Nanowires (GaN 나노와이어의 인장 변형에 의한 열기계적 거동 해석)

  • Jung, Kwangsub;Zhou, Min;Cho, Maenghyo
    • Journal of the Computational Structural Engineering Institute of Korea
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    • v.25 no.4
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    • pp.301-305
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    • 2012
  • In this work the mechanical behaviors of GaN nanowires are analyzed during tension, compression, and unloading deformations. The thermal conductivity of the nanowires at each deformed state is evaluated using an equilibrium Green-Kubo approach. Under tensile loading, the [0001]-oriented nanowires with hexagonal cross-sections undergo a phase transformation from wurtzite to a tetragonal structure. The phase transformation is not observed under compressive loading. The thermal conductivity decreases on going from compressive strains to tensile strains. The strain dependence of the thermal conductivity results from the relaxation time of phonon. A reverse transformation from the tetragonal structure to the wurtzite structure is observed during unloading. The thermal conductivities in the intermediate states are lower than the conductivity in the wurtzite structure at same strain. Such differences in the thermal conductivity between different atomic structures are mainly due to changes in the group velocity of phonon.

An Alternative One-Step Computation Approach for Computing Thermal Stress of Asphalt Mixture: the Laplace Transformation (새로운 아스팔트 혼합물의 저온응력 계산 기법에 대한 고찰: 라플라스 변환)

  • Moon, Ki Hoon;Kwon, Oh Sun;Cho, Mun Jin;Cannone, Falchetto Augusto
    • KSCE Journal of Civil and Environmental Engineering Research
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    • v.39 no.1
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    • pp.219-225
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    • 2019
  • Computing low temperature performance of asphalt mixture is one of the important tasks especially for cold regions. It is well known that experimental creep testing work is needed for computation of thermal stress and critical cracking temperature of given asphalt mixture. Thermal stress is conventionally computed through two steps of computation. First, the relaxation modulus is generated thorough the inter-conversion of the experimental creep stiffness data through the application of Hopkins and Hamming's algorithm. Secondly, thermal stress is numerically estimated solving the convolution integral. In this paper, one-step thermal stress computation methodology based on the Laplace transformation is introduced. After the extensive experimental works and comparisons of two different computation approaches, it is found that Laplace transformation application provides reliable computation results compared to the conventional approach: using two step computation with Hopkins and Hamming's algorithm.

Developed Inherent Strain Method Considering Phase Transformation of Mild Steel in Line Heating (선상가열시 강의 상변태를 고려한 개선된 고유변형도 기반의 등가하중법)

  • Ha, Yun-Sok;Jang, Chang-Doo
    • Journal of the Society of Naval Architects of Korea
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    • v.41 no.6
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    • pp.65-74
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    • 2004
  • The inherent strain method is known to be very efficient in predicting the deformation of steel plate by line heating. However, in the actual line heating process in shipyard, the rapid quenching changes the phase of steel. In this study, In order to consider additional effects under phase transformation, inherent strain regions were assumed to expand. Also, when calculating inherent strain, material properties of steel in heating and cooling are applied differently considering phase transformation. In this process, a new method which can reflect thermal volume expansion of martensite is suggested.8y the suggested method, it was possible to predict the plate deformations by line heating more precisely.

Effect of Thermal Cycle and Aging Heat Treatment on Transformation Characteristics of Cu-Zn-Al Shape Memory Alloys (Cu-Zn-Al계 형상기억합금의 변태특성에 미치는 열 Cycle 및 시효열처리의 영향)

  • Park, Pyeongyeol;Kim, Ickjun;Park, Seyun;Kim, Inbae;Park, Ikmin
    • Journal of the Korean Society for Heat Treatment
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    • v.2 no.4
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    • pp.47-55
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    • 1989
  • The effects of thermal cycle, aging heat treatment and Boron addition on the phase transformation characteristics and mechanical properties of the shape memory alloys of Cu-Zn-Al system, which was designed to operate about $80^{\circ}C$ by this research group, were studied. From the view point of the effects of thermal cycle on the phase transformation temperature change, it was found that up to 100 cycles Ms and Af points increased by $3-7^{\circ}C$ and Mf decreased a little bit and after that all of them were remain constant, and As point was not affected. All of the phase transformation temperatures were decreased $5-7^{\circ}C$ by aging heat treatment, at $140^{\circ}C$ for 24h however the effects of thermal cycle on aged alloys were same as on unaged alloys. As the thermal cycle increased the shape memory ability decreased a little up to 20 cycles, but above that it kept almost same ability. By Boron addition, grain size was refined from $1500{\mu}m$ to about $330{\mu}m$ and the hardness, fatigue property were improved but shape memory ability was lowered.

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On the distribution of temperature and metallic structures in quenching process considering latent heat of phase transformation (변태잠열을 고려한 담금시편의 온도 및 조직분포에 대하여)

  • 민수홍;구본권
    • Journal of the korean Society of Automotive Engineers
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    • v.6 no.4
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    • pp.46-53
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    • 1984
  • The analysis of temperature distribution and change of metallic structures during water quench were presented by finite element method. In temperature calculation the equation of unsteady state hear conduction problem considering latent heat due to phase transformation was applied to finite solid cylinder, SM 45C of 40mm diameter and 40mm height. In metallic structure analysis iso-thermal transformation curve and the equations of evolution in pearlite-martensite transformation were applied. The calculated results upon temperature and metallic structures were agreed with those of experimental observations.

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The Thermal Behavior of Transformation by Simultaneous $\alpha$-$Al_2O_3$ Seed Addition on the Al-Sec-Butoxide Hydrolysis (Al-Sec-Butoxide의 가수분해시에 있어서 $\alpha$-$Al_2O_3$종의 동시첨가에 의한 열적 전이거동)

  • 김창은;이해욱;최진관;김배연
    • Journal of the Korean Ceramic Society
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    • v.27 no.6
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    • pp.808-816
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    • 1990
  • The thermal behavior of transformation and characteristics of seeded sample powders prepared by simultaneous $\alpha$-Al2O3 seed addition with water on the Al-sec-butoxide hydrolysis were studied. $\alpha$-Al2O3 seed particles are shown to act as nuclei for transformation of $\theta$-to $\alpha$-Al2O3 and to result in an increase in thetransformation kinetics and lowering of the transformation temperature by as much as 143$^{\circ}C$. Simultaneous seed addition on the hydrolysis resulted in uniform dispersin and creation of nucleation site on seed surface and only 0.1wt% seeding lowered the transformation temperature by as much as 115$^{\circ}C$. For 3wt% seed addition, $\alpha$-Al2O3 single phase was obtained at 95$0^{\circ}C$ for 100 minutes and the specific surface area of products were lowered to 11.9$m^2$/g as compared with that of $\alpha$-Al2O3 powder prepared without seed at 115$0^{\circ}C$ ; 15.1$m^2$/g due to depression of vermicular structure growth.

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Thermal Stability of the R Phase of a Rapidly Solidified Ti-47.3Ni (at%) Alloy

  • Moon, Hyo-Jung;Chun, Su-Jin;Nam, Tae-Hyun;Liu, Yinong;Yang, Hong;Kim, Yeon-Wook
    • Transactions on Electrical and Electronic Materials
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    • v.13 no.1
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    • pp.19-22
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    • 2012
  • Transformation behavior of rapidly solidified Ti-47.3Ni (at%) alloy ribbons and thermal stability of the R phase in the ribbons were investigated by means of differential scanning calorimetry (DSC), X-ray diffraction, and transmission electron microscopy. Rapidly solidified Ti-47.3Ni alloy ribbons showed the two-stage B2-R-B19' martensitic transformation behavior. The B2-R transformation in the ribbons was observed even after annealing at 1,223 K, which was attributed to the fact that a specific orientation relationship between $Ti_2Ni$ and matrix in the ribbons is maintained after annealing at 1,223 K. The DSC peak temperature of the B2-R transformation ($T_R^*$) decreased with raising annealing temperature, which was attributed to the increased volume fraction of $Ti_2Ni$, thus causing an increased Ni content in the matrix.