Faculty Profiles - OKUMURA, Dai

Experiments and modeling of the viscoelastic behavior of polymeric gels Reviewed International coauthorship Open Access

Nikola Bosnjak, Siva Nadimpalli, Dai Okumura, Shawn A. Chester

JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS Vol. 137 2020.4

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

A polymeric gel is a cross-linked polymer network swollen with a solvent. Also, as is well known many polymeric materials exhibit viscoelastic behavior in their mechanical response, In this work, we report on our recent experimental observations and theoretical modeling involving the change in the viscoelastic response of the polymeric material VHB caused by equilibrium swelling in multiple solvents. To characterize this change, we have developed an experimental procedure that includes mechanical testing when virgin, free swelling, mechanical testing when swollen, and mechanical testing after drying. A model is used to help interpret the experimental results. The key finding of this work is that our results indicate that the viscoelastic behavior of initially dry VHB, reversibly, becomes essentially hyperelastic when swollen to equilibrium in multiple solvents. (C) 2019 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jmps.2019.103829

Inhomogeneous instability patterns in polyacrylamide hydrogel bilayers

Computational and physical aspects for the occurrence of crease in an elastomer under general loading conditions Reviewed International journal

Seishiro Matsubara, Atsuya Ogino, So Nagashima, Dai Okumura

International Journal of Solids and Structures Vol. 288 page： 112610 - 112610 2024.2

NAGASHIMA So, UCHIYAMA Wataru, HAYASHI Shunsuke, MATSUBARA Seishiro, OKUMURA Dai

Mechanical Engineering Journal Vol. 11 ( 6 ) page： 24-00350 - 24-00350 2024

Language：English Publisher：The Japan Society of Mechanical Engineers

Instability-induced patterns with spatially varying dimensions and geometric shapes, or inhomogeneous instability patterns, are ubiquitous in biological structures and have attracted much interest in both fundamental research and engineering applications. However, it remains a challenge to create such patterns using materials with stiffnesses and scales comparable to biological structures. Here, we demonstrate the creation of inhomogeneous instability patterns using film–substrate bilayers of polyacrylamide hydrogels. Swelling of thickness-gradient films on the substrates allows for the coexistence of different instability patterns, including creases and wrinkles, on the surface. The film thickness and the stiffness ratio between the film and substrate are the factors that control the characteristic length of the patterns on the millimeter scale, as well as the switching of the patterns between creases and wrinkles. Furthermore, these patterns can be generated at prescribed local areas on the surface. Our approach to creating inhomogeneous instability patterns on a single surface would help to gain insight into the morphological development of biological patterns and to fabricate bio-inspired functional surfaces.

DOI： 10.1299/mej.24-00350

ノンレシプロカルゲルの円筒押込み大変形解析 Reviewed

野々垣翔真，松原成志朗，永島壮，奥村大

計算数理工学論文集 Vol. 23 page： 123 - 128 2023.11

クリースの発達過程に対する影響因子の調査 Reviewed

石田竜也，永島壮，松原成志朗，奥村大

計算数理工学論文集 Vol. 23 page： 143 - 148 2023.11

均質化解析によるノンレシプロカルゲルの引張・圧縮非対称性評価 Reviewed

布施卓馬，松原成志朗，永島壮，奥村大

計算数理工学論文集 Vol. 23 page： 137 - 142 2023.11

LARGE DEFORMATION ANALYSIS OF A NONRECIPROCAL GEL UNDER CYLINDRICAL INDENTATION Reviewed

NONOGAKI Shoma, MATSUBARA Seishiro, Nagashima So, OKUMURA Dai

Transactions of the Japan Society for Computational Methods in Engineering Vol. 23 ( 0 ) page： 123 - 128 2023.11

Authorship：Last author Language：Japanese Publisher：Japan Society for Computational Methods in Engineering

In this study, we investigate the large deformation analysis of a nonreciprocal gel under cylindrical indentation. The nonreciprocity of the gel is modeled by extending the framework of anisotropic linear elasticity [Wang et al., 2023. Mechanical nonreciprocity in a uniform composite material. Science 380, 192–198]. Plane-strain finite-element analysis is performed by assuming the frictionless between the gel and the rigid indenter, leading to reproducing the asymmetric response of the nonreciprocal gel. It is found that severe large deformations cause non-convergence of incremental calculations, which is resolved by introducing artificial viscosity and hourglass control. The combination prevents the occurrence of hourglass deformation modes around the area directly below the indenter as well as obtains converged solutions in efficient incremental calculations. The use of larger values of the two parameters causes the increase of computational efficiency and the decrease of computational accuracy. Parametric studies elucidate the existence of the proper region of the two parameters.
Nonreciprocal gel, Tension compression asymmetry, Large deformations, FEA

DOI： 10.60443/jascome.23.0_123

TENSION-COMPRESSION ASYMMETRY EVALUATION OF A NONRECIPROCAL GEL BY HOMOGENIZATION ANALYSIS Reviewed

FUSE Takuma, MATSUBARA Seishiro, NAGASHIMA So, OKUMURA Dai

Transactions of the Japan Society for Computational Methods in Engineering Vol. 23 ( 0 ) page： 137 - 142 2023.11

Authorship：Last author Language：Japanese Publisher：Japan Society for Computational Methods in Engineering

A nonreciprocal gel consisting of hydrogel and nanosheet exhibits mechanical nonreciprocity, which has potential applications in mechanical engineering. An earlier study has revealed that this mechanical nonreciprocity is triggered by the tension-compression asymmetry resulting from the microscopic buckling behavior of nanosheets during compressive deformation, but the relevant influencing factors remain unknown. In this study, we investigate the microscopic buckling behavior and the resultant tension-compression asymmetry in a nonreciprocal gel subjected to uniaxial conditions. Eigenvalue buckling and post-buckling analyses equipped with computational homogenization are performed on a unit cell modeled as an elastic bilayer for which ratios of Young’s modulus and thickness are parameterized. The results confirm that selecting a dilute microscopic buckling with the characteristic wavelength or a non-dilute microscopic buckling with the infinite wavelength hinges on the ratios of Young’s modulus and thickness, which is consistent with the theoretical solution for the buckling behavior of a layered composite. We also elucidate that the tension-compression asymmetry is more pronounced as the Young’s modulus ratio increases or the thickness ratio decreases.

DOI： 10.60443/jascome.23.0_137

Evaluation of the Effects of Cross-Link Density and Swelling Ratio on Loading-Unloading Response in Large Strain Range of Hydrogels Reviewed

Time-swelling Superposition Principle for the Linear Viscoelastic Properties of Polyacrylamide Hydrogels Reviewed International journal

S. Matsubara, A. Takashima, S. Nagashima, S. Ida, H. Tanaka, M. Uchida, D. Okumura

Advanced Structured Materials: Creep in Structures VI, IUTAM Symposium Proceedings Vol. 194 page： 175 - 204 2023.8

Self-Wrinkling in Polyacrylamide Hydrogel Bilayers Reviewed

So Nagashima, Naoki Akamatsu, Xiangfu Cheng, Seishiro Matsubara, Shohei Ida, Hiro Tanaka, Makoto Uchida, Dai Okumura

Langmuir Vol. 39 ( 11 ) page： 3942 - 3950 2023.3

Self-Wrinkling in Polyacrylamide Hydrogel Bilayers Reviewed

So Nagashima, Naoki Akamatsu, Xiangfu Cheng, Seishiro Matsubara, Shohei Ida, Hiro Tanaka, Makoto Uchida, Dai Okumura

Langmuir 2023.3

クリースの発達過程に対する影響因子の調査 Reviewed

永島壮, 松原成志朗, 奥村大

計算数理工学論文集 Vol. 23 page： 143 - 148 2023

Syunpei Morita, Makoto Uchida, Yoshihisa Kaneko, Dai Okumura, Hiro Tanaka, Shohei Ida

Advanced Experimental Mechanics Vol. 7 ( 0 ) page： 90 - 96 2022.8

Language：English Publishing type：Research paper (scientific journal) Publisher：The Japanese Society for Experimental Mechanics

Hydrogel contains a large amount of solvent such as water inside a three-dimensional molecular chain network structure. The content of the solvent changes during the swelling and drying processes of the hydrogel. The mechanical properties of the gel significantly change depending on the solvent content. In this study, the effects of the cross-link density and swelling ratio on the nonlinear mechanical responses of hydrogel were investigated by the cyclic tensile test. To evaluate the irreversible deformation of the hydrogel, we performed the one-time and stepwise loading-unloading tests. The stress-strain curves for the loading and unloading process obtained from the crosshead displacement were different owing to the sliding between chuck and specimen. In contrast, they were consistent for all the experimental conditions when the strain was evaluated by the digital image correlation method. Therefore, the hydrogel showed the less-rate-dependent reversible deformation from small to large strain range. Furthermore, obtained results were compared with the theoretical model based on the molecular chain network model with swelling effect. Although the model could represent the nonlinear increase in the stress during the tensile test and the decrease in the stiffness owing to the swelling, the model results were not consistent with the experimental results. Experimental and numerical results indicate that the improvement of the mechanical model is necessary based on the less-rate-dependent reversible deformation mechanisms of the hydrogel.

DOI： 10.11395/aem.7.0_90

Surface Instabilities of Soft Materials Invited

OKUMURA Dai

Journal of the Society of Materials Science, Japan Vol. 71 ( 6 ) page： 555 2022.6

Finite element analysis for nucleation and propagation of crease in an elastic solid

Matsubara Seishiro, Hoshi Ryogo, Nagashima So, Okumura Dai

NCTAM papers, National Congress of Theoretical and Applied Mechanics, Japan Vol. 66 ( 0 ) page： 5 2022

Language：Japanese Publisher：National Committee for IUTAM

This study investigates the nucleation and propagation of crease from a V-shaped notch in an elastic solid. Finite element analyses are conducted on an incompressible Neo-Hookean elastic solid. The first crease is induced by a V-shaped notch, whereas the subsequent sequential creases occur as the self-organization induced by previous creases. The crease propagation is regularized at the threshold compressive strain of 0.323 in the case of the global compressive strain εG <=0.323, while an elastic solid is supercritical in the case of εG>0.323. The nonlinear perturbation approach reveals that the periodic distance between creases minimizing total elastic energy is larger than that predicted by self-organization and increases in response to εG approaching to 0.323.

DOI： 10.11345/japannctam.66.0_5

Modeling of cyclic hardening and evaluation of plastic strain range in the presence of pre-loading and ratcheting Reviewed

リッジパターン変態のプレストレッチ依存性と分岐座屈特性 Reviewed

松原成志朗, 永島壮, 奥村大

計算数理工学論文集 Vol. 22 page： 117 - 121 2022

Nobutada Ohno, Hisashi Nakamoto, Yusuke Morimatsu, Dai Okumura

INTERNATIONAL JOURNAL OF PLASTICITY Vol. 145 2021.10

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

An extended evolution equation of cyclic hardening is proposed to include the effect of maximum plastic strain due to pre-loading and ratcheting. A set of extended evolution equations is also proposed for a plastic strain surface to properly evaluate the cyclic plastic strain range in the presence of pre-loading and ratcheting. These extensions are verified by performing uniaxial cyclic experiments of 316 stainless steel at 600 degrees C. The experiments are accurately simulated using a constitutive model with the extensions in addition to the resetting scheme of the plastic strain surface recently proposed by the authors. The extension for the plastic strain surface allows fairly good simulation of the experiments without recourse to the resetting scheme if the strain range does not markedly decrease with the number of cycles. The extended equations are further used to discuss the effect of maximum plastic strain considering previous experiments.

DOI： 10.1016/j.ijplas.2021.103074

Implicit rule on the elastic function of a swollen polyacrylamide hydrogel Reviewed

Ryota Kawai, Hiro Tanaka, Seishiro Matsubara, Shohei Ida, Makoto Uchida, Dai Okumura

SOFT MATTER Vol. 17 ( 19 ) page： 4979 - 4988 2021.5

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

A full understanding of the elastic properties of hydrogels under swelling is required for their practical application in the chemical and biomedical engineering fields. This is because hydrogels are expected to retain water during mechanical use in moist atmospheres. In the present study, we investigated the relationship between the elastic modulus and the swelling ratio in a specific type of hydrogel (a polyacrylamide gel). The elasticity and swelling data revealed that these two parameters are proportionally related in hydrogels comprising adequate amounts of monomers and crosslinkers. We also demonstrated that this proportional relationship inherently conforms to the linear elastic behaviour predicted by the Flory-Rehner free energy function (the F-R model). The implicit rule is established by the extended F-R model with two scaling exponents. The extended model is capable of representing the irregular elasticity of swollen gels formed from low- or high-molecular-weight polymers.

DOI： 10.1039/d1sm00346a

Effects of initial imperfection and mesh resolution on wrinkle and crease analyses Reviewed

PubMed

HOSHI Ryogo, MIYOSHI Hiroaki, MATSUBARA Seishiro, OKUMURA Dai

Transactions of the JSME (in Japanese) Vol. 87 ( 898 ) page： 21 - 00045-21-00045 2021.5

Authorship：Last author Language：Japanese Publisher：The Japan Society of Mechanical Engineers

This study investigates the effects of initial imperfection and mesh resolution on the wrinkling and creasing. Finite element analyses are conducted to simulate compression behaviors of an incompressible Neo-Hookean elastic plane with a V-shaped notch as initial imperfection, so that the occurrences and evolutions of wrinkle and crease are evaluated quantitatively. As a result, it is demonstrated that the origins of wrinkle and crease can be determined by the overall and local strains even for the introduction of a V-shaped notch. Further, we illustrate that the converged specific local strain with respect to the increase of mesh resolution almost coincides with the condition of the occurrence of wrinkle (i.e., εw = 0.456) when the depth of a V-shaped notch is sufficiently small. We also show that the sizes of wrinkle and crease do not converge due to strain localization around a V-shaped notch. In the investigation of pattern evolutions about initial imperfection, we illustrate that two kinds of pattern evolution can be comprehensively discussed by varying the depth of a V-shaped notch, so that the condition of the occurrence of crease can be organized by the sharpness of a V-shaped notch.

DOI： 10.1299/transjsme.21-00045

ヘリンボン及びトリプルジャンクションパターンの発達に対する影響因子解析 Reviewed

奥村大

計算数理工学論文集 Vol. 21 page： 51 - 56 2021

A general expression for linearized properties of swollen elastomers undergoing large deformations Reviewed International coauthorship Open Access

Dai Okumura, Hironori Kawabata, Shawn A. Chester

JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS Vol. 135 2020.2

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

In this study, we develop a general expression for the linearized properties of swollen elastomers undergoing large deformations. The free energy function of swollen elastomers is assumed to obey the Frenkel-Flory-Rehner hypothesis, i.e., the elastic and mixing contributions are additive. The elastic strain energy is not assumed to have a particular form but is assumed only to be a function of a set of strain-invariants. A linearization procedure is used to obtain the general expression for the Young's modulus and Poisson's ratio under an arbitrary base state. The derived expression includes a characteristic term, which has the ability to describe a transient state between the extreme states prescribed by two distinct conditions. The verification is performed by estimating the shear modulus and considering the original Flory-Rehner framework. In addition, to show the usefulness, an extended Gent model is examined to elucidate the interactions between limiting chain extensibility and the second strain-invariant. (C) 2019 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jmps.2019.103805

The effect of water content on viscoelastic properties of polyacrylamide hydrogels

TAKASHIMA Akira, MATSUBARA Seishiro, IDA Shohei, TANAKA Minoru, UCHIDA Makoto, OKUMURA Dai

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2020 ( 0 ) page： J03127 2020

Mechanism analysis of morphology formation from a gel membrane to a three dimensional structure

HOSHI Ryogo, MIYOSHI Hiroaki, OKUMURA Dai

The Proceedings of Conference of Tokai Branch Vol. 2020.69 ( 0 ) page： 121 2020

Analysis of influential factors in 3D structure formation of an anionic polyacrylamide (copolymer) gel film on a glass substrate

KAWAI Ryota, MATSUBARA Seishiro, IDA Shohei, TANAKA Hiro, UCHIDA Makoto, OKUMURA Dai

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2020 ( 0 ) page： J02102 2020

Finite element analysis of influential factors in a three dimensional structure assembled from a gel film

HOSHI Ryogo, MIYOSHI Hiroaki, MATSUBARA Seishiro, OKUMURA Dai

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2020 ( 0 ) page： J03140 2020

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

We perform a finite element analysis of gel membrane constrained on a grass substrate to investigate influence factors in a three dimensional structure assembled from a gel film. In-plane compressive stresses cause a three dimensional deformation due to swelling of a gel film. Such a deformation is influenced by attractive force, friction, imperfection and so on. In this study, we, particular, investigate the influence of attractive force by conducting finite element analysis for the Neo-Hookean body. The obtained results are shown to discuss deformation patterns caused in response to different attractive forces.

DOI： 10.1299/jsmemecj.2020.j03140

Finite Element Implementation of Gent-Gent Hyperelastic Model for Swollen Elastomers

KIKUCHI Shotaro, MIYOSHI Hiroaki, MATSUBARA Seishiro, OKUMURA Dai

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2020 ( 0 ) page： J03139 2020

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

In this study, we conduct finite element implementation of a Gent-Gent hyperelastic model for swollen elastomers. To this end, an inhomogeneous field theory for swollen elastomers is extended using the Gent-Gent model, which includes the effects of limiting chain extensibility and the second strain-invariant. The extended theory is implemented into a commercial finite element package using a user-defined material subroutine. The inflation of a spherical balloon made of swollen elastomers is computed and the results are verified with an analytical prediction.

DOI： 10.1299/jsmemecj.2020.j03139

Finite element implementation of Gent-Gent hyperelastic model for swollen elastomers

KIKUCHI Shotaro, MIYOSHI Hiroaki, KAWABATA Hironori, OKUMURA Dai

The Proceedings of Conference of Tokai Branch Vol. 2020.69 ( 0 ) page： 120 2020

Finite element implementation of Gent-Gent hyperelastic model for swollen elastomers Reviewed

KIKUCHI Shotaro, MIYOSHI Hiroaki, MATSUBARA Seishiro, OKUMURA Dai

Transactions of the JSME (in Japanese) Vol. 86 ( 890 ) page： 20 - 00233-20-00233 2020

Authorship：Last author Language：Japanese Publisher：The Japan Society of Mechanical Engineers

In this study, we conduct finite element implementation of a Gent-Gent hyperelastic model for swollen elastomers. To this end, an inhomogeneous field theory for swollen elastomers is extended using the Gent-Gent model, which includes the effects of limiting chain extensibility and the second strain-invariant. The extended theory is implemented into a commercial finite element package using a user-defined material subroutine. The accuracy of the implementation is verified by analyzing Young's modulus and Poisson's ratio of swollen elastomers undergoing large deformations. Further, the inflation of a spherical balloon made of swollen elastomers is computed as an example. The results are verified with an analytical prediction and demonstrate the inflation behavior of swollen elastomers.

DOI： 10.1299/transjsme.20-00233

Bifurcation analysis of pattern evolution in a stiff gel film

MIYOSHI Hiroaki, MATUBARA Seishiro, OKUMURA Dai

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2020 ( 0 ) page： J02103 2020

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

In this study, we perform buckling and postbuckling analysis to invesigate the pattern evolution in a sttiff gel film bonded 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 increases in the film, which causes buckling, i.e., patterns 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 bifurcations. The employment allows us to sequentially trace the first, second and further bifurcations on the bifurcated paths. The obtained results are shown and discussed.

DOI： 10.1299/jsmemecj.2020.j02103

Resetting scheme for plastic strain range evaluation in cyclic plasticity: Experimental verification Reviewed

Nobutada Ohno, Hisashi Nakamoto, Yusuke Morimatsu, Takeshi Hamada, Dai Okumura

INTERNATIONAL JOURNAL OF PLASTICITY Vol. 123 page： 56 - 69 2019.12

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

It has been proposed that a plastic strain surface is reset to a point and re-evolves every cycle to correctly evaluate the plastic strain range under cyclic loading (Ohno et al., 2018b). This resetting scheme has the advantage of providing a definite value for the evolution parameter irrespective of the amounts of cyclic hardening, pre-straining, and ratcheting. In the present study, the resetting scheme was experimentally verified by performing the following three types of uniaxial cyclic tests of 316 stainless steel at 600 degrees C: constant strain range, gradual decrease, and step decrease tests. The gradual decrease tests, a new type of cyclic tests, had gradual strain range reduction with increasing number of cycles in the absence and presence of ratcheting such as found at notch roots. Thus, the effect of the strain range history on cyclic hardening was observed to be weaker than that due to stepwise decreasing the strain range. Using a constitutive model with the resetting scheme enabled good simulation of the cyclic hardening behavior in the gradual decrease tests. Cyclic softening after stepwise decreasing the strain range was also simulated well by extending the evolution equation of cyclic hardening.

DOI： 10.1016/j.ijplas.2019.07.008

Evaluation of the effects of cross-linking and swelling on the mechanical behaviors of hydrogels using the digital image correlation method Reviewed Open Access

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

SOFT MATTER Vol. 15 ( 16 ) page： 3389 - 3396 2019.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

Experimental evaluation and modeling are important steps in the investigation of the mechanical behaviors of hydrogels in the small- to large-strain range. In this study, the effects of cross-linking and swelling on the true stress-strain response of a specific type of hydrogel (polyacrylamide) were evaluated using a uniaxial tensile test. The development of true strain on the surface of the hydrogel was measured using the digital image correlation method. The specimens with higher cross-link density exhibited a higher initial elastic modulus and earlier orientation hardening. The initial elastic modulus was reduced by the swelling, whereas the orientation hardening occurred in an earlier strain range in the swollen hydrogel. The mechanical responses of the as-prepared and swollen hydrogels with different cross-linker contents were fitted using a non-Gaussian statistical model. The conventional model underestimated the decrease in the elasticity owing to the swelling effect and overestimated the increase in the stress in the large-strain range. The mechanical model was suitably modified to yield an accurate reproduction of the mechanical responses. The proposed model, which was characterized by five material parameters, was found to reproduce the characteristics of the mechanical responses of the as-prepared and swollen hydrogels with different cross-linker contents.

DOI： 10.1039/c9sm00049f

Measurements and FEM Analyses of Strain Distribution in Small Sn Specimens with Few Crystal Grains Reviewed International journal Open Access

Takumi Sasaki, Atsushi Yanase, Dai Okumura, Yoshiharu Kariya, Masaaki Koganemaru, Toru Ikeda

MATERIALS TRANSACTIONS Vol. 60 ( 6 ) page： 868 - 875 2019

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN INST METALS & MATERIALS

Soldering is used to bond a semiconductor chip to a print circuit board (PCB). It is known that Sn, which is the base metal of Pb-free solder, shows remarkable crystal anisotropy. Clarifying the effect of Sn anisotropy on strain distribution is important for lifetime evaluation. The strain distribution in a micro specimen was measured under a tensile test by a digital image correlation method (DICM) with a microscope. Strain distributions were also analyzed by the finite element method with Hill's anisotropic yield criterion and the crystal plasticity finite element analysis (CPFEA) with considering the critical resolves shear stress (CRSS) of each slip system. The deformation of the crystal structure of beta-Sn depends on the size, number, and orientation of crystal grains. The CRSS was noticeably different for each slip system, and the yield stress varied with the orientation of crystal grains. Although the CPFEA without considering strain hardening was effective for predicting deformation within crystal grains, it is necessary to consider the strain hardening of crystals to predict the stress-strain curve of a micro specimen.

DOI： 10.2320/matertrans.MH201808

Refinement of Flory-Rehner model and application to dimple pattern transformation analysis

MIYOSHI Hiroaki, KAWABATA Hironori, OKUMURA Dai

The Proceedings of Conference of Tokai Branch Vol. 2019.68 ( 0 ) page： 616 2019

Affector analysis in 3D structure development induced by surface instability in gel films

HISHIDA Yushi, KAWAI Ryota, OKUMURA Dai, IDA Shohei, TANAKA Hiro, UCHIDA Makoto

The Proceedings of The Computational Mechanics Conference Vol. 2019.32 ( 0 ) page： 230 2019

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

In this study, we observe three-dimensional (3D) structure formation induced by the surface instability of gel films generated on petri dishes. As the volume swelling ratio of the films increases, surface instability induces wrinkling and creasing, which result in causing the development of the unique 3D structure. The effective factors are discussed.

DOI： 10.1299/jsmecmd.2019.32.230

Fundamental observation of swelling-induced morphological formation in gel films on a glass substrate

KAWAI Ryota, HISHIDA Yushi, OKUMURA Dai, IDA Shohei, TANAKA Hiro, UCHIDA Makoto

The Proceedings of the Materials and Mechanics Conference Vol. 2019 ( 0 ) page： OS1104 2019

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

In this study, we conducted the fundamental observation of swelling-induced morphological formation in gel films on a glass substrate. As the swelling ratio increases, pattern transformation occurs in incremental steps. It was found that surface creasing caused a unique pattern transformation, which generated a 3D structure from a 2D gel film.

DOI： 10.1299/jsmemm.2019.os1104

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

The effect of cyclic hardening on stress relaxation behavior in creep-fatigue test of SUS 316 at 700°C

SASAKI Tatsuya, OHNO Nobutada, SHIMADA Takehiro, TOKUDA Kenji, YOSHIDA Kimiaki, OKUMURA Dai

The Proceedings of Conference of Tokai Branch Vol. 2018.67 ( 0 ) page： 511 2018

Swelling dependence analysis of an extended Flory-Rehner model using linear perturbation method

MANO Takafumi, OKUMURA Dai

The Proceedings of Conference of Tokai Branch Vol. 2018.67 ( 0 ) page： 202 2018

Analysis of wiggling buckling induced by etching process for bilayer structure under intrinsic compressive stress

SUGIURA Junya, OKUMURA Dai, TANAKA Hiro

The Proceedings of Conference of Tokai Branch Vol. 2018.67 ( 0 ) page： 509 2018

Eigenvalue buckling analysis of long wave-length deformation in porous gel films

KASUGAI Akishi, OKUMURA Dai

The Proceedings of Conference of Tokai Branch Vol. 2018.67 ( 0 ) page： 508 2018

Uniaxial tensile tests of polyacrylamide hydrogels and its estimation.

The Proceedings of Conference of Tokai Branch Vol. 2018.67 ( 0 ) page： 212 2018

Validation of Crush Energy Calculation Methods for Use in Accident Reconstructions by Finite Element Analysis

Shusuke Numata, Koji Mizuno, Daisuke Ito, Dai Okumura, Hisashi Kinoshita

SAE INTERNATIONAL JOURNAL OF TRANSPORTATION SAFETY Vol. 6 ( 2 ) page： 133 - 145 2018

Language：English Publishing type：Research paper (scientific journal) Publisher：SAE INT

The crush energy is a key parameter to determine the delta-V in accident reconstructions. Since an accurate car crush profile can be obtained from 3D scanners, this research aims at validating the methods currently used in calculating crush energy from a crush profile. For this validation, a finite element (FE) car model was analyzed using various types of impact conditions to investigate the theory of energy-based accident reconstruction.Two methods exist to calculate the crush energy: the work based on the barrier force and the work based on force calculated by the vehicle acceleration times the vehicle mass. We show that the crush energy calculated from the barrier force was substantially larger than the internal energy calculated from the FE model. Whereas the crush energy calculated from the vehicle acceleration was comparable to the internal energy of the FE model.In full frontal impact simulations, the energy of approach factor (EAF) has a linear relation with the residual crush, which had been validated in previous experimental studies. In our study using FE analysis, we found that the slope of EAF versus the residual crush was comparable with that of the dynamic crush energy versus the dynamic crush for crashes at 55 km/h. Using this slope and the residual crush from a 55 km/h impact test, the slope and the intercept of the EAF vs. residual crush can be determined using only one crash test. A database of the slopes and the intercepts was made using Japan New Car Assessment Program (JNCAP) tests. In offset impact simulations, the crush energy calculated from the crush profile agreed with the internal energy of the car FE model when at least one front rail was involved. In oblique impacts, the correction factor for crush energy is not necessary within 20 degrees of principal direction of force of the car's longitudinal axis.

DOI： 10.4271/09-06-02-0009

Effect of biaxial deformation on ulimate swelling of elastomers International coauthorship

OKUMURA Dai, CHESTER U Shain A.

The Proceedings of The Computational Mechanics Conference Vol. 2018.31 ( 0 ) page： 217 2018

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

In this study, ie investigate the effect of biaxial deformation on ultimate sielling characterized by limiting chain extensibility of elastomers. Limiting chain extensibility is introduced into the Flory–Rehner theory. To obtain a single unified inequality, stretches are normalized using the material constant that accounts for limiting chain extensibility. Using the derived inequality, systematic analysis provides the ultimate values of sielling ratio under biaxial loading.

DOI： 10.1299/jsmecmd.2018.31.217

Experimental evaluation of interactions between swelling and elasticity of a hydrogel

HISHIDA Yushi, OKUMURA Dai, IDA Syohei, UCHIDA Makoto, TANAKA Hiro

The Proceedings of the Materials and Mechanics Conference Vol. 2018 ( 0 ) page： OS0715 2018

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

In this study, we performed the equilibrium free swelling and subsequent uniaxial tensile tests of polyacrylamide hydrogels to investigate the interactions between swelling and elasticity. The experimental data were reproduced by adjusting two scaling exponents of the extended Frory-Rehner model. We discussed the tendency of a set of the obtained material parameters.

DOI： 10.1299/jsmemm.2018.os0715

Derivation of closed-form solution for linear elastic properties of swollen elastomers

KAWABATA Hironori, OKUMURA Dai, OHNO Nobutada

The Proceedings of the Materials and Mechanics Conference Vol. 2018 ( 0 ) page： OS0305 2018

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

In this study, we derive a closed-form solution for linear elastic properties of swollen elastomers. To this end, linear perturbation analysis is applied to the Flory-Rehner theory. Instead of the Neo-Hookean solid model in the theory, the strain energy function is assumed to consist of three invariants to obtain the most general expression.

DOI： 10.1299/jsmemm.2018.os0305

Relations between limiting chain extensibility and ultimate swelling of swollen elastomers

OKUMURA Dai, A. CHESTER Shawn

The Proceedings of the Materials and Mechanics Conference Vol. 2018 ( 0 ) page： OS0609 2018

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

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. The derived inequality 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.

DOI： 10.1299/jsmemm.2018.os0609

Strain distribution in small Sn specimens with few crystal grains obtained by the measurements and the crystal plasticity finite element analysis

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

The Proceedings of The Computational Mechanics Conference Vol. 2018.31 ( 0 ) page： 084 2018

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

Soldering is used for bonding a print circuit board (PCB) and a semiconductor chip. It is known that Sn, which is the base metal of Pb-free solder, shows remarkable crystal anisotropy. To clarify the effect of anisotropy of Sn on strain distribution is important for lifetime evaluation. The strain distribution in a micro specimen under a tensile test was measured by a digital image correlation method (DICM) with a microscope. The strain distribution was also analyzed by the crystal plasticity finite element analysis (CPFEA) considering the critical resolves shear stress (CRSS) of each slip system. The deformation of the crystal structure of β-Sn depends on the size, number and orientation of crystal grains. The CRSS was noticeably different for each slip system, and the yield stress varied with the orientations of crystal grains. Although the CPFEA without considering strain hardening was effective for predicting deformation within crystal grains, it is necessary to consider the strain hardening of crystals to predict the stress-strain curve of a micro-specimen.

DOI： 10.1299/jsmecmd.2018.31.084

Analysis of linear elastic properties of swollen elastomers using elastic strain energies

KAWABATA Hironori, OKUMURA Dai

The Proceedings of The Computational Mechanics Conference Vol. 2018.31 ( 0 ) page： 314 2018

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

In this study, we perform analysis of linear elastic properties considering the influence of elastic strain energies of swollen elastomers. A closed form solution derived in the most general expression is used to introduce the Gent model in the Flory-Rehner theory. The effects of limiting chain extensibility are discussed.

DOI： 10.1299/jsmecmd.2018.31.314

Modeling of the out-of-plane mechanical response of a laminated cellular structure showing negative Poisson’s ratio under uniaxial tensile deformation

TANAKA Hiro, SUGA Kaito, OKUMURA Dai, SHIBUTANI Yoji

The Proceedings of The Computational Mechanics Conference Vol. 2018.31 ( 0 ) page： 250 2018

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

In this study, we analyze the mechanical responses of the two types of laminated cellular structures using finite element models based on a homogenization method. In the numerical simulations, we obtain the homogenized elastic modulus tensors for the two structural units and 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 then assess the sensitivity of the linear and nonlinear out-of-plane response to changes in the component geometry. According to the analytical results, we propose a simple periodic framework consisting polyhedron blocks and spherical joints, which can represent the similar auxetic behavior the cellular structure exhibits in the in-plane and out-of-plane directions.

DOI： 10.1299/jsmecmd.2018.31.250

アクリルアミドハイドロゲルを用いた拡張Flory-Rehnerモデルの基礎的検証 Reviewed

エラストマーの極限膨潤に及ぼす分子鎖伸び切り限界の二軸変形依存性 Reviewed International coauthorship

奥村大

計算数理工学論文集 Vol. 18 page： 7 - 18 2018

奥村大, 清水章司, 伊田翔平, 内田真, 田中展

計算数理工学論文集 Vol. 17 page： 53 - 58 2017.12

アクリルアミドハイドロゲルを用いた拡張Flory-Rehnerモデルの基礎的検証 Reviewed

奥村大, 清水章司, 伊田翔平, 内田真, 田中展

計算数理工学論文集 Vol. 17 page： 53 - 58 2017.12

Evaluation of Deformation Behavior of Hydrogel on Uniaxial Tensile Test using Digital Image Correlation Reviewed

Keita Suzuki, Makoto Uchida, Yoshihisa Kaneko, Dai Okumura, Hiro Tanaka, Shohei Ida

Proceedings of the 12th International Symposium on Advanced Science and Technology in Experimental Mechanics page： 035 2017.11

Finite element analysis of swelling-induced pattern transformation in gel membranes

TERAI Yoshihiko, OKUMURA Dai

The Proceedings of Conference of Tokai Branch Vol. 2017.66 ( 0 ) page： 521 2017

Study on implementation of the extended Flory-Rehner model and its incremental stability

SHIMIZU Shoji, OKUMURA Dai

The Proceedings of Conference of Tokai Branch Vol. 2017.66 ( 0 ) page： 522 2017

Influence of plate thickness on out-of-plane undulating buckling analysis of microscopic patterned structure

SUGIURA Junya, OKUMURA Dai, TANAKA Hiro

The Proceedings of Conference of Tokai Branch Vol. 2017.66 ( 0 ) page： 518 2017

Influence of plate thickness and intrinsic stress on wiggling buckling of a microscopic patterned structure

Sugiura Junya, Okumura Dai, Tanaka Hiro

The Proceedings of The Computational Mechanics Conference Vol. 2017.30 ( 0 ) page： 205 2017

Microscopic plastic deformation analysis of β-Sn single crystal using molecular dynamics

MATSUNO Natsuki, KINOSHITA Yusuke, OKUMURA Dai

The Proceedings of Conference of Tokai Branch Vol. 2017.66 ( 0 ) page： 523 2017

Analysis of elastic properties of swollen elastomers under prestrain

OKUMURA Dai, MANO Takafumi

The Proceedings of The Computational Mechanics Conference Vol. 2017.30 ( 0 ) page： 212 2017

Analysis of domain wall formation mechanism in swelling-induced pattern transformation of porous gel membranes

OKUMURA Dai, KASUGAI Akishi

The Proceedings of Conference of Tokai Branch Vol. 2017.66 ( 0 ) page： 520 2017

Buckling Eigenvalue Analysis of Swelling-induced Instability on Porous Gel Films

ISOMURA Taiji, Okumura Dai

The Proceedings of Conference of Tokai Branch Vol. 2017.66 ( 0 ) page： 519 2017

Dependence of finite elements on swelling-induced instability in porous gel films

KASUGAI Akishi, ISOMURA Taiji, OKUMURA Dai

The Proceedings of The Computational Mechanics Conference Vol. 2017.30 ( 0 ) page： 204 2017

Eigenvalue buckling analysis of domain wall defect in porous gel films

KASUGAI Akishi, OKUMURA Dai, OHNO Nobutada

The Proceedings of the Materials and Mechanics Conference Vol. 2017 ( 0 ) page： OS1604 2017

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Mechanical Engineers

In this paper, we perform eigenvalue buckling analysis to investigate a defect of domain walls (DWs) in a diamond plate pattern (DPP), which is caused by swelling-induced pattern transformation in porous gel films. It is found that DWs in a DPP can be captured as long wavelength buckling. The critical stress of DWs in a DPP is higher than that of a uniform DDP, but as periodic units increase, both are identical. This means that this defect, i.e., DW, is hardly eliminated in DPPs. Further, the pattern of DWs consists of 4 basic buckling modes, which are obtained from extending the buckling mode of the DPP using the Bloch wave.

DOI： 10.1299/jsmemm.2017.os1604

Experimental verification of extended Flory-Rehner model using polyacrylamide hydrogels

SHIMIZU Shoji, OKUMURA Dai, IDA Shohei, TANAKA Hiro, UCHIDA Makoto

The Proceedings of the Materials and Mechanics Conference Vol. 2017 ( 0 ) page： OS0306 2017

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Mechanical Engineers

An extended F-R model is experimentally verified using polyacrylamide hydrogels. Stress-stretch responses of the extended F-R model are compared with those of the original F-R model and the Ogden model. As a result, the extended F-R model is superior to the original F-R model because the extended F-R model can adjust swelling dependence of Young's modulus. Further, the extended F-R model is comparable to the Ogden model for small deformations because the extended F-R model does not include the effect of limiting chain extensibility. The results of the extended F-R model imply that polyacrylamide hydrogels can cause swelling-induced strain softening.

DOI： 10.1299/jsmemm.2017.os0306

Comparative analysis of extended Flory-Rehner model and Ogden model based on uniaxial tensile tests of polyacrylamide hydrogels

SHIMIZU Shoji, OKUMURA Dai, IDA Shohei, TANAKA Hiro, UCHIDA Makoto

The Proceedings of The Computational Mechanics Conference Vol. 2017.30 ( 0 ) page： 241 2017

Effects of two scaling exponents on swelling-induced softening of elastomers under equibiaxial and planar extensions

Okumura D., Mizutani M.

Key Engineering Materials Vol. 725 KEM page： 427 - 432 2017

Language：Japanese Publisher：Key Engineering Materials

We study the effects of two scaling exponents on the mechanical properties of swollen elastomers under equibiaxial and planar extensions. Two scaling exponents are introduced to extend the Flory-Rehner free energy function, and are adjusted based on the previous study. Results show that swelling-induced strain softening is apt to occur under equibiaxial extension compared to uniaxial extension. The additional tensile stress in a lateral direction enables it to occur in relatively poor solvents, and accelerates the onset point. Planar extension shows more complicated responses because the stress in the constrained direction changes dramatically depending on the combination of two scaling exponents and the Flory-Huggins interaction parameter.

DOI： 10.4028/www.scientific.net/KEM.725.427

Thermo-mechanical cyclic plastic behavior of 304 stainless steel at large temperature ranges

Ohno N., Yamamoto R., Okumura D.

Key Engineering Materials Vol. 725 KEM page： 275 - 280 2017

Language：Japanese Publisher：Key Engineering Materials

Thermo-mechanical cyclic experiments on 304 stainless steel were performed at several temperature ranges which had maximum temperatures ranging from 350°C to 1000°C and a minimum temperature of 150°C. Related isothermal cyclic experiments were also performed. Temperature-history dependent cyclic hardening significantly occurred under thermo-mechanical cyclic loading with maximum temperatures around 600°C, whereas almost no cyclic hardening was observed when the maximum temperature was 1000°C. The observed thermo-mechanical cyclic plastic behavior in the saturated state of cyclic hardening was then simulated using a cyclic viscoplastic constitutive model, leading to the following findings. It was difficult to predict the saturated thermo-mechanical cyclic behavior using only the isothermal cyclic experimental data. The saturated thermo-mechanical cyclic behavior was simulated well by introducing a cyclic hardening parameter depending on the maximum temperature. This means that the cyclic hardening parameter should not change with temperature but depend on the maximum temperature in the saturated state of cyclic hardening under thermo-mechanical cyclic loading.

DOI： 10.4028/www.scientific.net/KEM.725.275