Faculty Profiles - OKUMURA, Dai

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