Isamu Riku

Effect of Size-Dependent Cavitation on Micro- to Macroscopic Mechanical Behavior of Rubber-Blended Polymer

In rubber-blended polymer, the onset of cavitation in the particles relaxes the high triaxiality stress state and suppresses the onset of crazing in the polymer. As a result, large plastic deformation is substantially promoted compared with single-phase polymer. On the other hand, it is also well known that the onset of cavitation depends on the size of particle. To investigate the size dependence of cavitation behavior in the particle, a theoretical analysis is done employing a void model under plane strain condition, which takes into account the surface tension and the limiting stretch of the void. Continuously, to study the effect of the size-dependent cavitation on the micro- to macroscopic mechanical behavior of the blend, a computational model is proposed for the blend consisting of irregularly distributed heterogeneous particles containing the void with surface force. The results indicate that when the size of the particle decreases to a critical value that depends on both the initial shear modulus of particle and the surface tension on the surface of void, the increase of the critical stress for the onset of cavitation becomes remarkable and consequently, the onset of cavitation is eliminated. When the particle is embedded in polymer, the relation between average normal stress, which is acting on the interface of particle and matrix, and volumetric strain of particle shows dependence on the size of particle but no dependence on the triaxiality of macroscopic loading condition. For the blend consisting of particles smaller than the critical value, the onset of cavitation is eliminated in particles and as a result, the conformation of the shape of particle to the localized shear band in matrix becomes difficult and the shear deformation behavior tends to occur all over the matrix. Furthermore, in this case, the area of the maximum mean stress is confined to the area adjacent to the particle and the value of it increases almost linearly throughout the whole deformation process, which would lead to the onset of crazing in matrix. On the other hand, it is clarified that the onset of cavitation is predominant in the localized microscopic region containing heterogeneous particles and therefore, the plastic deformation is promoted in this region.

Experimental Evaluation of Dynamic Buckling Load of GFRP Rod for Gas Circuit Breaker

Dynamic elastic buckling behaviors of GFRP and Aluminum rods were experimentally investigated. In a gas circuit breaker of an electric power substation, an operating rod consisting of insulating materials is connected to an interrupter. The rod securely insulate between the interrupter and an operating mechanism. The rod is configured with a slender rod made of Glass Fiber Reinforced Plastics (GFRP). When the gas circuit breaker ends the opening operation, impulsive compression load acts on the GFRP rod. To develop a smaller and lighter GFRP rod, dynamic buckling loads of the rod must be studied. In this study, dynamic elastic buckling loads for slender GFRP and aluminum rods were investigated by an experimental method. The drop weight impact tests of the GFRP rods and aluminum rods were employed. In the testing device, a special load cell called the “Load Sensing Block” was used to measure the dynamic load of long time duration. Slender GFRP rods with various lengths were axially loaded at the impact velocities ranging from 0.75m/s to 4.5m/s. From the experimental results, an empirical criterion for the dynamic buckling load of the GFRP rod was proposed in terms of the impact velocity and the slenderness ratio. Furthermore, results showed the proposed criterion could successfully describe the buckling behavior of not only the GFRP rod but also the aluminum rod.

Studies on Micro- to Macroscopic Mechanical Behavior of Porous Polymer under Compaction

The 2D extended homogenization model based on molecular chain network theory is employed to investigate the micro-to macroscopic mechanical behavior of polymer with randomly distributed voids under macroscopic compaction. A parametric study is performed to quantify the effect of the volume fraction of voids and the macroscopic stress triaxiality of loading conditions on the compaction behavior of porous polymer. The results suggest that the onset of localized shear band at the ligament between voids leads to the macroscopic yield of porous polymer. Furthermore, the microscopic localized shear deformation behavior is promoted in the polymer with high-volume fraction of voids under high macroscopic stress triaxiality loading condition, which results in the early appearance of the macroscopic yield. After the macroscopic yield, a remarkable strain hardening is shown in the macroscopic response of porous polymer under high macroscopic stress triaxiality loading condition, which is due to the onset and propagation of a large number of shear bands at the ligaments between voids. On the other hand, microscopic buckling develops at the narrowest ligament in the polymer with high-volume fraction of voids under high macroscopic stress triaxiality loading condition, which leads to the relative low macroscopic deformation resistance. Furthermore, to develop the macroscopic constitutive model of polymer with high-volume fraction of voids under high macroscopic stress triaxiality loading condition, a modified Gurson model is proposed which takes account of microscopic buckling and gives a good agreement with the unit cell computational results.

Development of 3-Dimensional Compression Device and its Application to Clarification of Densification Behavior of Urethane-Foam

This paper deals with the results of three dimensional compression tests carried out for high stiffness urethane foams (Penguin-foam, Sunstar Engineering Ltd.), and also deals with the constitutive modelling base on Shima-Oyane’s consolidation condition for the tested foamed urethane. Three kinds of urethane foams, relative densities of which were 0.1, 0.2 and 0.33, were employed in the experiments. Like metallic porous materials, the tested urethane foams show the strong plastic-compressibility. On the other hand, in modelling, unlike metallic porous materials, the identified material constants for different density foams do not take the same (or unified) values but take the different values when Shima-Oyane’s constitutive model is assumed. Furthermore, the experimentally derived stress-relative density curves could not be satisfactorily described by Shima-Oyane’s original constitutive model; the experimental stress-relative density curves show stronger work hardening as compared with the simulated ones especially in the large deformation stage. To avoid those inconvenience, in this paper, a modified Shima-Oyane type constitutive equation was also proposed, and it was shown that the proposed model could well express both the low work hardening area of the stress-relative density curves at the initial deformation stage and the strong work hardening area at the final deformation stage by supposing the stress restriction at initial deformation stage due to the buckling of cell walls of each foam, and the rapid stress increase at the large deformation stage caused by the successive contact and the friction between the bent cellular walls, respectively.

A Nonaffine Molecular Chain Network Model for Elastomeric Gel

Submerged in a solvent-containing environment and subject to applied forces, a nonaffine molecular chain network absorbs the solvent and deforms, forming an elastomeric gel. The Helmholtz free energy of the gel is separated into the contribution due to stretching the nonaffine molecular chain network and that due to mixing the polymer and the solvent. The effect of the nonaffine movement of the molecular chain on the deformation behavior of the gel is discussed.

MD Analysis of Effect of Relative Humidity on Molecular Chain’s Network Structure of PEM

To clarify the effect of relative humidity on molecular chain’s network structure, we at first employ Molecular Dynamics (MD) method to constitute the computational model for Nafion membrane, in which the water channel is artificially reproduced with an aggregation of water molecules. And then, relaxation calculation is performed and a relatively stable microstructure of Nafion membrane is derived. It is found that the regions of relatively low density of molecular chain’s network appear interchangeably together with those of relatively high density of water molecules.

Dynamic Buckling Behavior of Long Column made of Grass Fiber Reinforced Plastics for Gas Circuit Breaker

In a gas circuit breaker of an electric power substation, a long operating rod made of an insulating material such as grass fiber reinforced plastics is employed, and when the breaker is operated, the rod was subjected to a strong impulsive compression load. In designing of the operating rods, therefore, the precise knowledge of their dynamic buckling loads is required. In this study, the dynamic elastic buckling behavior of GFRP rods the ends of which were tightly clamped was experimentally investigated. In the experiments, elastic buckling loads of slender GFRP rods in a wide range of loading velocities were measured by using the three different type testing machines: universal testing machine, hydraulic servo system and drop weight type impact loading machine. From the obtained experimental results, an empirical criterion for the dynamic buckling load of the GFRP rod was proposed in terms of the impact velocity and the slenderness ratio. Furthermore, we may show possibility that the proposed criterion can successfully describe buckling behaviors of long rods of many sorts of engineering materials with the same material parameters.

Computational Characterization of Micro-To Macroscopic Deformation Behavior of Double Network Hydrogel

To take advantage of the toughness mechanism of DN gels and explore the possibility for engineering application as the structural member, the information on the mechanical behaviour of DN gels under various loading conditions is indispensable. Therefore, in this paper, we at first constitute a model of DN gel by paralleling a slider element with a nonlinear rubber elasticity spring element based on the nonaffine molecular chain network model, where each element represents the first and the second network of DN gel respectively. The theoretical stress-strain relation of this model shows a strain softening and subsequent strain hardening response, which has been considered as an agent of the propagation of the necking during the simple tension of glassy polymer. Continuously, based on this model, we propose a constitutive equation for DN gel and a three-dimensional simple tension simulation is performed. The computational results show that the propagation of the necking together with the macroscopic mechanical response of DN gel can be reproduced by the proposed model very well.

Numerical Evaluation of Micro- to Macroscopic Mechanical Behavior of Plastic Foam

In this study, we employ the two-dimensional homogenization model based on molecular chain network theory to investigate the micro- to macroscopic mechanical behavior of plastic foam under macroscopic uniform compression. A parametric study is performed to quantify the effect of a characteristic value of matrix, distribution and initial volume fraction of voids, and the macroscopic triaxiality of loading condition on the deformation behavior of the foam. The results suggest that the onset of localized shear band at the ligament between voids together with the microscopic buckling of the ligament leads to the macroscopic yield of the foam. The initial modulus and the macroscopic yield stress of the foam have no dependence on the characteristic value of matrix. Furthermore, as the microscopic buckling of the ligament is promoted in case of high initial volume fraction of voids and high triaxiality loading condition, the macroscopic yield point appears at early deformation stage. After the macroscopic yield, macroscopic strain hardening appears in the macroscopic response and a remarkable strain hardening is shown in case of high initial volume fraction of voids and high triaxiality loading condition due to the considerable increase of the density of the foam in these cases.