Papers by Author: Yoji Shibutani

Abstract: In this study, we developed a new conceptual test method of adhesive joining using aluminum circular pipes with sloping bonded surfaces to evaluate the failure strength of the adhesive layer under a multiaxial stress state. In our experiment, we prepared several test specimens made of A5052 aluminum with cross-sectional surfaces possessing angles of inclination varying from 0–90 degrees. We then carried out uniaxial tensile tests, employing paired aluminum pipe halves joined by adhesives. From the load‒displacement curves, we estimated the first invariant of the stress tensor and the second invariant of the deviatoric stress tensor, and we applied two kinds of criteria, both of which effectively represent the failure stress of the adhesive joint. According to the failure criteria, the thickness-dependent strength of the adhesive layer and its power law are discussed.

Abstract: Dynamics and statics of defects interaction among crack, dislocations and twin boundary (TB) observed in magnesium were investigated using molecular dynamics and elasticity with the complex stress functions to clarify the effect of long-range elastic stress field. An atomic model containing a crack parallel to (10-11) TB was gradually elongated under K_I-mode tension by molecular dynamics simulations. Changing the distance between the crack and the TB, four kinds of crack propagation manners were observed, one of which showed the path transition from the crack to the TB itself by shielding effect of piled-up dislocations around the crack tip. The stress intensity factor of the nanosized crack in bulk is 0.28 MPam^1/2, which is smaller than that of crack on the TB. The shielding effect due to the piled-up dislocations drastically decreases stress concentration around the crack tip and the stress intensity factor diminishes down to the 0.22, and thus the crack nucleated from the void nucleation and coalescence on the TB was propagated instead. The elastic stress distributions obtained by the superposition of some complex stress functions suggest that the stress field around the crack tip is disturbed by the localized stress due to the TB in the case of crack closest to TB and also by the back stress due to the piled-up dislocations in the case of crack far from TB.

Abstract: To study the physical origin of the periodic arrangement of the quadrople solute-enriched layers in Mg-based LPSO structures, we carry out first-principles calculations of the formation energy of the L1₂ cluster and investigate effects of phonon on the inter-planer ordering of the solute-enriched layers using the 1-dimensional chain model with mass change. The formation energy of the L1₂ cluster increases as the period of the LPSO structure decreases. Thus, it is found that the electron-mediated interaction is short-range repulsive. On the other hand, in the case of considerably heavy mass change, the ordering of the mass changes is stabilized by phonons and the energy gain increases with the concentration of the mass changes, i.e., the short LPSO period is favorable. A promising mechanism of the inter-planer ordering of the LPSO structures is the phonon-mediated interaction of the quadrople layers where heavy solute atoms are enriched as the L1₂ clusters at SFs.

Abstract: The paper focuses on the characteristic local network structure of icosahedra observed in the relaxed amorphous metals and its contribution to their elastic properties. Using molecular dynamics method, CuxZr100-x binary amorphous models are constructed by rapidly quenching process from liquid. Voronoi polyhedron analyses expose the existence of the medium-range local network structure composed of inter-penetrating icosahedra. It is found that the globally averaged shear modulus obtained from the atomistically defined elastic moduli decreases with increasing free volume content in the condensed matter. The contribution of the densely packed icosahedral cluster with higher elastic rigidity to the global elastic modulus is discussed.

Abstract: Plastic deformability of the binary (copper and zirconium) amorphous alloy with embedded nanosized crystals under uniaxial tension and compression is analyzed using molecular dynamics simulations. The number and the size of the nanocrystals are taken as the study parameters. The number of nanocrystals affects the distribution of defects, that is, shear bands nucleation and thus changes the stress-strain curve, whereas the size of the nanocrystals does not significantly influence the response. As already reported in the experimental works, coalescent voids are found under tension in the shear bands or at the interface between crystalline and amorphous phases. This suggests that much attention should be paid to the interface strength around the particles.

Abstract: The present paper summarizes the crystallographic dependence of the displacement burst behavior observed in nanoindentation using two single crystalline aluminum (Al) materials and copper (Cu) with three kinds of surface indices, namely (001), (110) and (111). From the critical indent load at the first burst, the critical resolved shear stresses (CRSSs) of the collective dislocation nucleation were estimated in reference to molecular dynamics (MD) simulations. These are almost one-tenth of the shear modulus, which are close to the ideal values. We explain the nanoplastic mechanics by a comprehensive energy balance model to describe the linear relation between the indent load and the burst width of the first displacement burst and by the nucleation model consisting of three-dimensional discrete dislocations to evaluate the number of dislocations nucleating. The distance between the emitted dislocation loops of Al is found to be fairly large. Thus, Al is expected to exhibit a less tangled network of dislocations just below the indentation than Cu, which has a lower stacking fault energy.

Abstract: CuxZr1-x (x=0.30~0.85) binary amorphous alloy models were constructed using molecular dynamics simulations. In order to characterize the short-ranged local structures and the free volume of the whole cell, Voronoi polyhedral analyses were performed for their initial states and also for the deformed states. Both geometrical and physical factors are intimately related to each other and exhibit a large influence on mechanical properties. The elastic properties of shear modulus and bulk modulus tend to increase as the free volume decreases. The Poisson’s ratio, which is related to the glass fragility, increases as the free volume. However, the relation among three elastic properties obtained from the continuous mechanics of the isotropic body does not hold. Also, the maximum stress in the finite deformation drastically decreases with increasing free volume content.

Abstract: Process of a silica cluster on a polyimide surface was simulated by atomistic calculations combining the semi-empirical molecular orbital method and algorithms that can find the optimal path of a chemical bonding process. Specifically, we estimated the activation and sorption energies in the process. From the simulations, we found that chemical bonding occurs between a silica cluster and the polyimide substrate surface. In the adsorption processes, those reactions between the silica cluster and the polyimide surface which involve Si-O bonding between the cluster and the polyimide are the most probable.