Faculty Profiles - OKUMURA, Dai

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

Web of Science

Scopus

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

CiNii Research

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

CiNii Research

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

CiNii Research

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

CiNii Research

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

CiNii Research

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

CiNii Research

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

CiNii Research

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

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