Faculty Profiles - OKUMURA, Dai

Evaluation of influence of crosslinking concentration and swelling time on tensile response of hydrogel

Sengoku Tatsuya, Uchida Makoto, Kaneko Yoshihisa, Okumura Dai, Tanaka Hiro, Ida Shohei

The Proceedings of Conference of Kansai Branch Vol. 2019.94 ( 0 ) page： P004 2019

Effects of loading conditions on linear elastic properties of swollen elastomers

KAWABATA Hironori, OKUMURA Dai

The Proceedings of The Computational Mechanics Conference Vol. 2019.32 ( 0 ) page： 237 2019

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

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

Effect of second strain invariant on linear elastic properties of swollen elastomers

KAWABATA Hironori, OKUMURA Dai

The Proceedings of Conference of Tokai Branch Vol. 2019.68 ( 0 ) page： 127 2019

Evolution from a hexagonal dimple pattern of a stiff gel membrane

MIYOSHI Hiroaki, OKUMURA Dai

The Proceedings of the Materials and Mechanics Conference Vol. 2019 ( 0 ) page： OS0234 2019

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

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

Bifurcation analysis of generation and evolution of dimple patterns in a stiff gel membrane

OKUMURA Dai, MIYOSHI Hiroaki

The Proceedings of The Computational Mechanics Conference Vol. 2019.32 ( 0 ) page： 232 2019

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

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

Measurement and numerical analysis with the crystal plasticity for strain in crystal grains of small Sn specimen

IKEDA Toru, SASAKI Takumi, KOGANEMARU Masaaki, YANASE Atsushi, OKUMURA Dai, KARIYA Yoshiharu

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2019 ( 0 ) page： J01102 2019

Analysis of linear elastic properties of gel membranes constrained on a substrate

KAWABATA Hironori, OKUMURA Dai, OHNO Nobutada

The Proceedings of the Materials and Mechanics Conference Vol. 2019 ( 0 ) page： OS1904 2019

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

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

Swelling amount and tensile behavior of hydrogels swollen under compression

SENGOKU Tatsuya, MORITA Syunpei, UCHIDA Makoto, KANEKO Yoshihisa, OKUMURA Dai, TANAKA Hiro, IDA Shohei

The Proceedings of the Materials and Mechanics Conference Vol. 2019 ( 0 ) page： OS1103 2019

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

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

Fundamental observation of growth-induced morphological formation using a hydrogel

TAKAMI Gakuto, HISHIDA Yushi, OKUMURA Dai, IDA Shohei, TANAKA Hiro, UCHIDA Makoto

The Proceedings of Conference of Tokai Branch Vol. 2019.68 ( 0 ) page： 617 2019

Formulation of rate-form constitutive equation for swelling of hydrogel

UCHIDA Makoto, SENGOKU Tatsuya, MORITA Syunpei, KANEKO Yoshihisa, OKUMURA Dai, TANAKA Hiro, IDA Syouhei

The Proceedings of The Computational Mechanics Conference Vol. 2019.32 ( 0 ) page： 226 2019

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

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 Reviewed

奥村大

計算数理工学論文集 Vol. 19 page： 31 - 36 2019

Effects of two scaling exponents on biaxial deformation and mass transport of swollen elastomers Invited Reviewed International journal

Dai Okumura, Masahiro Mizutani, Hiro Tanaka, Makoto Uchida

INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES Vol. 146 page： 507 - 516 2018.10

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

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. (C) 2017 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijmecsci.2017.08.042

Thermo-mechanical cyclic hardening behavior of 304 stainless steel at large temperature ranges: Experiments and simulations Invited Reviewed

Nobutada Ohno, Ryohei Yamamoto, Dai Okumura

INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES Vol. 146 page： 517 - 526 2018.10

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

Thermo-mechanical cyclic experiments on 304 stainless steel were performed at several temperature ranges with T-min (minimum temperature) of 150 degrees C and T-max (maximum temperature) ranging from 350 degrees C to 1000 degrees 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 T-max was around 600 degrees C. In contrast, almost no cyclic hardening occurred when T-max was 1000 degrees 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 T-max, 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 T-max in the saturated state of cyclic hardening under thermo-mechanical cyclic loading. (C) 2017 Elsevier Ltd. All rights reserved.

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 Reviewed

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

Smart Materials and Structures Vol. 27 ( 8 ) 2018.8

Ultimate swelling described by limiting chain extensibility of swollen elastomers Reviewed International coauthorship Open Access

Dai Okumura, Shawn A. Chester

INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES Vol. 144 page： 531 - 539 2018.8

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

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 Reviewed

Dai Okumura, Junya Sugiura, Hiro Tanaka, Yoji Shibutani

INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES Vol. 141 page： 78 - 88 2018.6

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

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 degrees C: EXPERIMENTS AND SIMULATIONS Invited Reviewed International journal Open Access

Nobutada Ohno, Tatsuya Sasaki, Takehiro Shimada, Kenji Tokuda, Kimiaki Yoshida, Dai Okumura

JOURNAL OF THEORETICAL AND APPLIED MECHANICS Vol. 56 ( 2 ) page： 497 - 510 2018.4

Language：English Publishing type：Research paper (scientific journal) Publisher：POLISH SOC THEORETICAL & APPLIED MECHANICS

Cyclic hardening and stress relaxation experiments of SUS3161HTP were performed under creep-fatigue loading with tensile strain holding at 700 degrees 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 Reviewed Open Access

Nobutada Ohno, Ryohei Yamamoto, Tatsuya Sasaki, Dai Okumura

ZAMM-ZEITSCHRIFT FUR ANGEWANDTE MATHEMATIK UND MECHANIK Vol. 98 ( 4 ) page： 518 - 531 2018.4

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY-V C H VERLAG GMBH

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 Reviewed

Dai Okumura, Masanari Otsuka, Yoji Shibutani

Zairyo/Journal of the Society of Materials Science, Japan Vol. 67 ( 2 ) page： 215 - 221 2018.2

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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