Kyohei Kondo

Moisture Diffusivity of Unidirectional Composites

The moisture diffusion in unidirectional fiber-reinforced composite materials is formulated in terms of relative moisture concentration which cor responds to the temperature for the heat conduction problem. And the effec tive transverse moisture diffusivity of the composite materials with random array packing of fibers is predicted based on the properties of the constituent matrix and fibers by establishing a random array model. It is found that if the degree of randomness increases, the transverse diffusivity decreases remarkably. The theoretical prediction is in good agreement with the ex perimental data obtained by the authors and other researchers. And the analogy between the moisture diffusion and the thermal conduction or the longitudinal shear loading is discussed, pointing out some error in similarity used to predict the moisture diffusivity of composite materials in the literature.

Analytical solution of spatial elastica and its application to kinking problem

Abstract An analytical solution is presented for the spatially large deformation of a thin elastic rod (spatial elastica) which is naturally straight and uniform with equal principal stiffnesses and is subjected to terminal loads. The elastica can suffer not only flexure and torsion as in the classical Kirchhoff theory, but also extension and shear. The present solution is expressed in integral form and described in terms of only four parameters. This solution clears the difficulty with the polar singularity in the use of Euler angles. Hence, the numerical analysis is possible for various boundary value problems with no limitation. In this paper we study the post-buckling behavior of an elastica under the terminal twist and uniaxial end-shortening, and give a theoretical explanation to commonly observed phenomena such as secondary bifurcation, formation of a kink, snap-through behavior. The contact problem is analyzed in the case where the elastica contacts with itself and forms a kink. These results are available for other analysis, e.g., based on finite element approximations.

Analysis of double cantilever beam specimen

A double cantilever beam (DCB) specimen is analyzed by utilizing a Timoshenko beam supported by an elastic foundation with an extensional stiffness. The deflections and stresses of the DCB specimen are compared with the results based on the other beam models as well as the potential energy release rates. It is found that the present results for graphite-epoxy composites are as accurate as those by Whitney using a higher order beam theory.

An Energy Release Rate Approach for Free-edge Delamination Problem in Composite Laminates

Location where delamination occurs in a balanced, symmetric laminate with free edge under uniaxial mechanical loading and/or hygrothermal expansion is predicted by comparing the energy release rates due to delamination growth from the free edge on various interfaces. The change of release rate with the delamination crack length is obtained by utilizing the conventional finite element method. And a new method is proposed to evaluate a saturated value of energy release rate in the delamination crack extension. The saturated value of energy release rate is used as a measure to predict the occurrence of delamination which has yielded good agreement with observations on tension—tension and compression—compression fatigue tests conducted on carbon—epoxy composite laminates.

Large deformations of rigid-plastic shallow spherical shells

Abstract The axisymmetric large deformations of rigid-plastic shallow spherical shells are analysed by using the generalized yield line method which takes into account the changes in geometry of the structures. The deflection of a shell is assumed to be of the shape of a number of right circular cones separated by concentric hinge circles with no meridian strain in each cone. Then, the general equation to obtain the load-deflection relation is derived by making use of the principle of virtual velocity. Simply supported shallow spherical shells under circular loading are investigated, the boundary of which is restrained against inward movement. The load-deflection curves obtained are asymmetric with respect to the origin, and have a minimum value after collapsing. A method to account for the elastic deformations is discussed.

Large Deformations of Rigid-Plastic Beams

ABSTRACT The large deflections of rigid-plastic shallow curved beams are analyzed by making use of the generalized plastic hinge method which takes into account the geometrical change. The beams are assumed to deform into a number of rigid regions separated by plastic hinges. Then, using the modified principle of virtual velocity, general equations to get the load-deflection relations are obtained. Simply supported and rotationally fixed shallow curved beams which are subjected to a concentrated load and elastically constrained against longitudinal movement at the ends are analyzed. The effects of curvature and end constraints on the behavior of beams are clarified. The method to include the elastic deformation in the rigid-plastic analyses is discussed.

Delamination energy release rates under thermal loading in fiber-reinforced composite laminates

Abstract Free-edge delaminations of fiber-reinforced composite laminates under thermal loading are theoretically investigated. The weak form equilibrium equations are derived for a quasi-three-dimensional problem of laminates and an associated finite element method is used. The strain energy release rate due to the delamination growth is calculated from the finite element method and a newly developed simplified method which is an extension of the method formerly developed by the authors for laminates under mechanical loading. Attention is paid to the longitudinal constraint conditions of laminates, which have significant effects on the energy release rate. Numerical examples are shown for ( 90° 0° )s cross-ply and ( 45° -45° )s angle-ply laminates.

Large Deformations of Rigid-Plastic Polygonal Plates

ABSTRACT Large deformations of rigid-plastic polygonal plates are analyzed by making use of the generalized yield line method which takes into account the changes in geometry. The plates are assumed to deform into a number of rigid regions separated by straight-line plastic hinges. Then the general equation to obtain the load-deflection relation is derived by the principle of virtual velocity. Simply supported or clamped regular polygonal plates are investigated, the sides of which are either restrained against inward movement or free to move inward. The influence of rotational and in-plane boundary conditions on the behavior of the plate is clarified. The method to account for the elastic deformations is discussed.

Analysis of Potential Energy Release Rate of Composite Laminate Based on Timoshenko Beam Theory

The Timoshenko beam theory is used to model each part of cracked beam and to calculate the potential energy release rate. Calculations are given for the double cantilever beam specimen, which is simulated as two separate beams connected elastically along the uncracked interface.

Delamination originating from transverse crack tips in laminates under mechanical loadings

A singular finite element method is formulated utilizing the asymptotic solutions for displacements and stresses near the tip of the interface crack between dissimilar anisotropic materials and the variational principle of a hybrid functional. The interfacial crack problem can be analyzed by the singular hybrid element at the interfacial crack tip and the conventional displacement-based elements surrounding the crack element. Analyzing a small central interfacial crack between two large anisotropic composite layers with different fiber orientations subjected to uniform tensile load, we demonstrate that the present numerical solutions are in good agreement with the analytical ones for an interfacial crack between two semi-infinite anisotropic solids. Then, we apply the present method for the analysis of delamination originating from the transverse crack tip in laminates under plane strain extension, antiplane shear and plane strain bending.