Hiro Tanaka

Bi-stiffness property of motion structures transformed into square cells

Cellular solids with internal microstructures enable the reduction in some environmental loads because of their lightweight bodies, and deliver unique elastic, electromagnetic and thermal properties. In particular, their large deformability without topological change is one of their most interesting solid properties. In this study, we propose a bar-and-joint framework assembled with a basic unit of motion structure, which has eightfold rotational symmetry (MS-8). The MS-8 is made of tetragons, arranged in a concentric fashion, which are transformed into either one of two different aligned patterns of square cells according to the coordinated rotations of the inside squares. Square cells are extremely anisotropic, which is why the stiffness of the MS-8 changes dramatically in the transformation process. Thus, the MS-8 exhibits bi-stiffness according to the two different motions. Taking advantage of the bi-stiffness property, the possibilities of deformation behaviours for repetitive structures of MS-8s are discussed.

Nonlinear large deflection of nanopillars fabricated by focused ion-beam induced chemical vapor deposition using double-cantilever testing

Nanopillars with nanosized diameter and microsized length can be constructed by chemical vapor deposition using a focused ion beam. For a pillar consisting of an outer amorphous carbon ring and an inner gallium core, we performed the bending tests using the unique double-cantilever specimen joining two pillars together by an electron-beam deposition technique in a scanning electron microscope. The precise load-deflection curves indicate that the pillars have a nonlinear softening region after the linear response as the diameter increases. However, pillars finally become extremely hardened at the large deformation. Thus, the pillar intrinsically possesses much more flexibility and stable deflection for bending than expected, in contrast to tensile deformation. The bending rigidity obtained by the infinitesimal deflection corresponds well to that by the resonance vibration tests reported so far. It also certifies that the proposed double-cantilever bending method can maintain high accuracy for the nanoscale...

Orthotropic Laminated Open-cell Frameworks Retaining Strong Auxeticity under Large Uniaxial Loading

Anisotropic materials form inside living tissue and are widely applied in engineered structures, where sophisticated structural and functional design principles are essential to employing these materials. This paper presents a candidate laminated open-cell framework, which is an anisotropic material that shows remarkable mechanical performance. Using additive manufacturing, artificial frameworks are fabricated by lamination of in-plane orthotropic microstructures made of elbowed beam and column members; this fabricated structure features orthogonal anisotropy in three-dimensional space. Uniaxial loading tests reveal strong auxeticity (high negative Poisson’s ratios) in the out-of-plane direction, which is retained reproducibly up to the nonlinear elastic region, and is equal under tensile and compressive loading. Finite element simulations support the observed auxetic behaviors for a unit cell in the periodic framework, which preserve the theoretical elastic properties of an orthogonal solid. These findings open the possibility of conceptual materials design based on geometry.

Transition mechanism for a periodic bar-and-joint framework with limited degrees of freedom controlled by uniaxial load and internal stiffness

A specific periodic bar-and-joint framework with limited degrees of freedom is shown to have a transition mechanism when subjected to an external force. The static nonlinear elasticity of this framework under a uniaxial load is modelled with the two angular variables specifying the rotation and distortion of the linked square components. Numerically exploring the equilibrium paths then reveals a transition state of the structure at a critical value of the internal stiffness. A simplified formulation of the model with weak nonlinear terms yields an exact solution of its transition state. Load–displacement behaviour and stability for the two systems with or without approximation are analysed and compared.

Failure Criteria of Adhesive Joints between Aluminum Circular Pipes Under Multiaxial Stress State

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.

Size effect of large deformable nanopillar by focused-ion-beam chemical vapor deposition

Nanoscopic fabrication technique has been achieved by the direct deposition methods using focused-ion-beam chemical vapor deposition (FIB-CVD). The nanopillar fabricated by FIB-CVD consists of an outer amorphous carbon ring and a inner gallium core. We developed the original double-cantilever (DC) bending test using two pillars rigidly connected by the exposure of a focused electron beam in a scanning electron microscope. The obtained deflection curves suggest that nanopillars have the size dependence to the mechanical response. The pillar with the diameter over 180 nm exhibits a wide region of stiffness weakening after linear response and then becomes extremely hardened at a large deflection. Thus, the pillar intrinsically possesses much more flexibility for bending without any fracturing. The accuracy of a DC testing is also discussed by estimating the bending rigidities of nanopillars, comparing to those obtained by resonance frequency tests.