研究者詳細 - 奥村　大

ハイドロゲルの引張応答に及ぼす架橋濃度および膨潤時間の影響の評価

仙石達也, 内田真, 兼子佳久, 奥村大, 田中展, 伊田翔平

関西支部講演会講演論文集 2019.94 巻 ( 0 ) 頁： P004 2019年

膨潤エラストマーの線形弾性特性に及ぼす負荷条件の影響

川端宏典, 奥村大

計算力学講演会講演論文集 2019.32 巻 ( 0 ) 頁： 237 2019年

記述言語：日本語出版者・発行元：一般社団法人日本機械学会

This study investigates the effect of loading conditions on the linear elastic properties of swollen elastomers. We employ the Flory-Rehner theory with the extended Gent model as the elastic strain energy. The interactions of limiting chain extensibility, the second strain-invariant, solvent migration and the loading conditions are analyzed via the Young's modulus and Poisson's ratio.

DOI： 10.1299/jsmecmd.2019.32.237

膨潤エラストマーの線形弾性特性に及ぼすひずみの第二不変量の影響

川端宏典, 奥村大

東海支部総会講演会講演論文集 2019.68 巻 ( 0 ) 頁： 127 2019年

硬質ゲル膜の六方ディンプルパターンから派生する形態

三好宏明, 奥村大

M&M材料力学カンファレンス 2019 巻 ( 0 ) 頁： OS0234 2019年

記述言語：日本語出版者・発行元：一般社団法人日本機械学会

In this study, buckling and postbuckling analysis is performed to investigate the evolution of dimple patterns in a stiff gel membrane constrained on a soft substrate. The film and substrate are modeled as the Neo-Hookean solid model. The film is also assumed to obey the Flory–Rehner framework for polymeric gels. Since the film swells, the in-plane compressive stresses increase in the film, which causes buckling, i.e., pattern transformation occurs. In finite element analysis, a step-by-step eigenvalue buckling analysis is employed to analyze the onset points and modes of buckling. The employment allows us to sequentially trace the first, second and further bifurcations on the bifurcated paths. The obtained results show the pattern transformation process from hexagonal dimples to labyrinths.

DOI： 10.1299/jsmemm.2019.os0234

硬質ゲル膜に生じるディンプルパターンの発生と発達の分岐座屈解析

奥村大, 三好宏明

計算力学講演会講演論文集 2019.32 巻 ( 0 ) 頁： 232 2019年

記述言語：日本語出版者・発行元：一般社団法人日本機械学会

In this study, we perform the bifurcation analysis of the generation and evolution of of dimple patterns in a stiff gel membrane constrained on a soft substrate. The film and substrate are modeled as the Neo-Hookean solid model. Further, the film is assumed to obey the Flory–Rehner framework for polymeric gels. Since the film swells, the in-plane compressive stresses increase in the film, which causes buckling, i.e., pattern transformation occurs. Finite element analysis is conducted by introducing the Flory–Rehner framework into a user-defined material subroutine (Hong et al., 2009). The step-by-step eigenvalue buckling analysis, which was developed by Okumura et al. (2018), is employed to investigate the onset points and modes of bifurcation. The employment allows us to sequentially trace the first, second and further bifurcations under the bifurcated paths. The obtained results are shown and discussed.

DOI： 10.1299/jsmecmd.2019.32.232

微小スズ試験片の結晶粒中のひずみ計測と結晶塑性理論による解析

池田徹, 佐々木拓海, 小金丸正明, 柳瀬篤志, 奥村大, 苅谷義治

年次大会 2019 巻 ( 0 ) 頁： J01102 2019年

基盤に拘束されたゲル膜の線形弾性特性解析

川端宏典, 奥村大, 大野信忠

M&M材料力学カンファレンス 2019 巻 ( 0 ) 頁： OS1904 2019年

記述言語：日本語出版者・発行元：一般社団法人日本機械学会

In this study, we investigate the linear elastic properties of gel membranes constrained on a rigid substrate. To this end, the Flory–Rehner framework for polymeric gels is considered, which is extended by introducing an extended Gent model. The linear perturbation analysis demonstrates the dependence of limiting chain extensibility and the second strain invariant on the in-plane stiffness of gel membranes constrained on a rigid substrate.

DOI： 10.1299/jsmemm.2019.os1904

圧縮環境下で膨潤させたハイドロゲルの膨潤量および引張挙動

仙石達也, 森田隼平, 内田真, 兼子佳久, 奥村大, 田中展, 伊田翔平

M&M材料力学カンファレンス 2019 巻 ( 0 ) 頁： OS1103 2019年

記述言語：日本語出版者・発行元：一般社団法人日本機械学会

The effect of swelling on the true stress-strain response of a hydrogel was evaluated using a uniaxial tensile test, and the response was represented by the constitutive equation based on the molecular chain network theory. The experimental results clarified that the initial elastic modulus was reduced by an increase in the degree of swelling. Two different constitutive equations, where the swelling stretch was introduced in different forms, were proposed to predict the effect of swelling. The obtained results of two equations underestimated and overestimated the effect of the swelling on the true stress-strain curve. This result indicates that the mechanical response of the swollen hydrogel can accurately be predicted using the parameter related to the swelling stretch.

DOI： 10.1299/jsmemm.2019.os1103

ハイドロゲルを用いた成長誘起形態形成の基礎的観察

鷹見岳杜, 菱田湧士, 奥村大, 伊田翔平, 田中展, 内田真

東海支部総会講演会講演論文集 2019.68 巻 ( 0 ) 頁： 617 2019年

ハイドロゲルの膨潤に関する速度形構成式の定式化

内田真, 仙石達也, 森田隼平, 兼子佳久, 奥村大, 田中展, 伊田翔平

計算力学講演会講演論文集 2019.32 巻 ( 0 ) 頁： 226 2019年

記述言語：日本語出版者・発行元：一般社団法人日本機械学会

Mechanical and chemical coupling behavior of hydrogel was modeled by formulating the rate-form constitutive equation in which the elastic part of the deformation was introduced into the molecular chain network theory. The time-dependent development of the swelling was represented by the simple power-law with osmotic pressure between the polymer and solvent. The material parameters employed in the proposed model were given by comparing the decrease in the tensile load applied to the hydrogel specimen in the solvent. Finally, the FEM simulation of the buckling behavior owing to the swelling of the hydrogel in the container was performed.

DOI： 10.1299/jsmecmd.2019.32.226

ゲル膜のリンクル解析に及ぼす軟質基盤厚さの影響査読有り

奥村大

計算数理工学論文集 19 巻頁： 31 - 36 2019年

Effects of two scaling exponents on biaxial deformation and mass transport of swollen elastomers 招待有り査読有り国際誌

Okumura, D; Mizutani, M; Tanaka, H; Uchida, M

INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES 146 巻頁： 507 - 516 2018年10月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：International Journal of Mechanical Sciences

In this study, we investigate the effects of two scaling exponents on the biaxial deformation and mass transport of swollen elastomers. Two scaling exponents are included in an extended version of the Flory–Rehner model (Okumura et al., J. Mech. Phys. Solids, 2016); two scaling exponents are used to adjust the swelling effects on the Young's modulus and osmotic pressure of swollen elastomers, resulting in the ability to predict swelling-induced strain softening under uniaxial tensile loading. It is found that when biaxial tensile loading is given under stress control, strain softening is accelerated by increasing the biaxial stress ratio, while under strain control, the responses become more complicated, which can be interpreted by considering that Poisson's ratio at equilibrium free swelling can take negative values depending on the two scaling exponents. Further, the effect of the two scaling exponents on mass transport is discussed by analyzing the swelling kinetics of a gel layer constrained on a rigid substrate.

DOI： 10.1016/j.ijmecsci.2017.08.042

Thermo-mechanical cyclic hardening behavior of 304 stainless steel at large temperature ranges: Experiments and simulations 招待有り査読有り

Ohno, N; Yamamoto, R; Okumura, D

INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES 146 巻頁： 517 - 526 2018年10月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：International Journal of Mechanical Sciences

Thermo-mechanical cyclic experiments on 304 stainless steel were performed at several temperature ranges with Tmin (minimum temperature) of 150 °C and Tmax (maximum temperature) ranging from 350 °C to 1000 °C. Corresponding isothermal cyclic experiments were also performed at several temperatures. Temperature-history dependent cyclic hardening was thus observed to significantly occur under thermo-mechanical cyclic loading when Tmax was around 600 °C. In contrast, almost no cyclic hardening occurred when Tmax was 1000 °C. The observed, thermo-mechanical cyclic hardening behavior was then simulated using a cyclic viscoplastic constitutive model with a cyclic hardening parameter. The simulation focused on the saturated state of cyclic hardening, leading to the following findings. The saturated thermo-mechanical cyclic hysteresis loops were not predicted well by simply taking into account temperature dependence in the cyclic hardening parameter. Then, by assuming the cyclic hardening parameter to be dependent on Tmax, the saturated thermo-mechanical hysteresis loops were simulated well. These mean that the cyclic hardening parameter of 304 stainless steel should not change with temperature but depend on Tmax in the saturated state of cyclic hardening under thermo-mechanical cyclic loading.

DOI： 10.1016/j.ijmecsci.2017.06.018

Macroscopic out-of-plane auxetic features of a laminated open-cell structure with in-plane negative Poisson's ratios induced by bridging beam components 査読有り

Suga, K; Tanaka, H; Okumura, D; Shibutani, Y

SMART MATERIALS AND STRUCTURES 27 巻 ( 8 ) 2018年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Smart Materials and Structures

This paper presents the out-of-plane mechanism of a laminated open-cell structure that is exerted by a simple control of the geometric configuration. We propose two types of structural units that are assembled with beam and column members in a tetragonal system, and with the geometric difference between them being the presence of a reinforced beam. For the two unit cells, we develop finite element models according to a homogenization method and analyze the macroscopic linear and nonlinear elastic properties. The numerical results show that the cellular structure, which is bridged by additional beams, exhibits strong auxeticity with high negative values of Poisson's ratio in the lamination direction, while retaining the in-plane negative Poisson's ratios. We assess the sensitivity of the linear and nonlinear out-of-plane response to changes in the component geometry. Finally, we discuss the large deformation behaviors of the out-of-plane mechanisms to quantify the amounts of three-dimensional rotations of the internal components.

DOI： 10.1088/1361-665X/aacd31

Ultimate swelling described by limiting chain extensibility of swollen elastomers 査読有り国際共著 Open Access

Okumura, D; Chester, SA

INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES 144 巻頁： 531 - 539 2018年8月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：International Journal of Mechanical Sciences

In this study, we study ultimate swelling characterized by limiting chain extensibility of swollen elastomers. Limiting chain extensibility is introduced into the Flory–Rehner theory using the Arruda–Boyce eight chain model and the Gent phenomenological model. The difference between these models is unified by defining a single scalar function. The inequality derived from this function allows for analysis to provide an ultimate value of swelling ratio. This ultimate value is not exceeded at equilibrium swelling regardless of the set of material constants. Under uniaxial loading at equilibrium swelling, deswelling can occur even in tension. Further, the very large swelling behavior of pH sensitive hydrogels is found to originate from the resistance generated by approaching the ultimate value of swelling ratio.

DOI： 10.1016/j.ijmecsci.2018.06.011

Open Access

Buckling and postbuckling of etching-induced wiggling in a bilayer structure with intrinsic compressive stress 査読有り

Okumura, D; Sugiura, J; Tanaka, H; Shibutani, Y

INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES 141 巻頁： 78 - 88 2018年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：International Journal of Mechanical Sciences

In this study, we investigate buckling and postbuckling of etching-induced wiggling in a bilayer structure consisting of mask and masked layers. To show effects of explicit modeling of etching process, two mask–masked ridge models with and without etching (Models w/E and w/oE) are analyzed using finite element analysis. The etching process is explicitly introduced via step-by-step eigenvalue buckling analysis. Although Model w/oE predicts a constant value of the critical wavelength of wiggling regardless of the change in ridge width, Model w/E predicts a shorter wavelength depending on the decrease in ridge width and the increase in intrinsic compressive stress in the mask layer. In postbuckling analysis, Model w/oE predicts a monotonic increase in the wiggling amplitude with the constant wavelength, whereas Model w/E predicts saturation of the wiggling amplitude owing to the decreasing wavelength. In the explicit modeling of etching process, the wiggling behavior shows completely opposite tendencies. Dimensional analysis is performed to obtain empirical equations, which are compared with an experiment.

DOI： 10.1016/j.ijmecsci.2018.03.032

EFFECT OF CYCLIC HARDENING ON STRESS RELAXATION IN SUS316HTP UNDER CREEP-FATIGUE LOADING AT 700°C: EXPERIMENTS AND SIMULATIONS 招待有り査読有り国際誌 Open Access

Ohno, N; Sasaki, T; Shimada, T; Tokuda, K; Yoshida, K; Okumura, D

JOURNAL OF THEORETICAL AND APPLIED MECHANICS 56 巻 ( 2 ) 頁： 497 - 510 2018年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Theoretical and Applied Mechanics (Poland)

Cyclic hardening and stress relaxation experiments of SUS316HTP were performed under creep-fatigue loading with tensile strain holding at 700◦C. Experiments revealed that under strain holding, the slow stress-relaxation stage satisfying Norton’s law with slight cyclic hardening followed a rapid stress-relaxation stage that was noticeably affected by cyclic hardening. This suggests that in the slow stress-relaxation stage, inelastic deformation mechanisms different from that of viscoplasticity occurred. Experiments were simulated using a cyclic viscoplastic-creep model in which the inelastic strain-rate was decomposed into viscoplastic and creep components that were affected differently by cyclic hardening. The simulation accurately reproduced the experiments.

DOI： 10.15632/jtam-pl.56.2.497

Open Access

Resetting scheme for plastic strain surface in constitutive modeling of cyclic plasticity 査読有り Open Access

Ohno, N; Yamamoto, R; Sasaki, T; Okumura, D

ZAMM-ZEITSCHRIFT FUR ANGEWANDTE MATHEMATIK UND MECHANIK 98 巻 ( 4 ) 頁： 518 - 531 2018年4月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ZAMM Zeitschrift fur Angewandte Mathematik und Mechanik

A resetting scheme is proposed for correctly evaluating the plastic strain range to analyze structural components subjected to regular cyclic loading following pre-loading. In this scheme, a plastic strain surface, referred to as the plastic-strain-range surface in the present paper, is reset to a point and re-evolves every cycle, with its size at the end of the last cycle considered to be the plastic strain range of the current cycle. This resetting scheme provides a definite value for the evolution parameter of the plastic-strain-range surface irrespective of the amounts of cyclic hardening, pre-straining, and ratcheting. The resetting scheme is implemented in a cyclic viscoplastic model in finite element methods, and is applied to analyze a notched bar subjected to cyclic loading at its ends. Furthermore, the validity of the resetting scheme for non-proportional cyclic loading is examined by analyzing the cyclic behavior along elliptic and rhombic strain paths. It is shown that the resetting scheme allows the use of a definite value for the evolution parameter of the plastic-strain-range surface in the presence of any cyclic hardening and/or ratcheting. Moreover, it is shown that the resetting scheme can be valid for cyclic loading with a certain degree of non-proportionality.

DOI： 10.1002/zamm.201700298

Mechanism of plastic deformation in a magnesium nanotwinned structure by molecular dynamics simulations 査読有り

Dai Okumura, Masanari Otsuka, Yoji Shibutani

Zairyo/Journal of the Society of Materials Science, Japan 67 巻 ( 2 ) 頁： 215 - 221 2018年2月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Society of Materials Science Japan

In this study, we perform molecular dynamics simulations to investigate plastic deformation modes of a magnesium (Mg) nanotwinned structure. Periodic units including (10 11) twin boundaries (TBs) are analyzed using an embedded atom method potential. Equal spaces between the TBs are assumed at the initial state and the space is parametrized in the range between about 5 nm and 30 nm. It is found that plastic deformation is triggered by the slip along a (10 11) twinning plane near a TB, and that this event induces two different deformation modes depending on the space, i.e., the migration of the TBs and the evolution of double twinning, which leads to void nucleation and polycrystallization. The plastic deformation provided by the two different modes is quantitatively verified from geometric analysis. As the space decreases, the migration of the TBs is superior to the evolution of double twinning.

DOI： 10.2472/jsms.67.215