Keishiro Yoshida

Feasibility Study of Triaxially-Woven Fabric Composite for Deployable Structures

Triaxially woven fabric (TWF) composite is the form of composite system made up of three sets of interwoven tows placed in three equally placed in-plane directions combined with the matrix resin impregnated in the tows. The bending characteristics of carbon fiber/epoxy TWF composite are analytically and experimentally investigated in this study. The stiffness isotropy observed under tensile loading is shown to hold as well under the flexural loading based on the linear analysis combined with the assumption of infinite size specimens. The effect of specimen width shows up as well under bending analysis. The anisotropy that emerges at high loading levels under tensile load is predicted to be not as salient under the flexural loading. To verify the analytical predictions, three point bending and pure bending tests are conducted. The experimental results support the predicted isotropy for infinite plates, though the actual specimens are of finite width. Thermal expansion test is also conducted to experimentally obtain the thermal expansion coefficients, which are found to be larger than that predicted previously through the analysis. The overall isotropic nature as well as the excellent flexural strength of the TWF indicates the expected applications as various components of light-weight deployable structures.

Mechanical and Thermal Behaviors of Triaxially-Woven Carbon/Epoxy Fabric Composite

Triaxially woven fabric (TWF) constitutes of three sets of inter-woven yarns oriented in 0 o -, +60 o - and -60 o - angles. This fabric is impregnated with resin to constitute single-ply TWF composite. The fundamental mechanical and thermal properties of TWF composite made from carbon fibers and epoxy resin are thoroughly investigated both theoretically and experimentally. Homogenization method, in which the unit cell is analyzed by the finite element method, is first employed to verify that the composite exhibits isotropy both for inplane and flexural stiffness, and the coefficient of thermal expansion as well. Moreover the composite has tension/torsion and bending/shear couplings due to the existence of B16 and B26 components of the stiffness matrix. The beam network model is also used to simulate the nonlinear tensile and flexural behaviors of the finite size coupon specimens with free boundaries. The stiffening effect observed in the tensile test in yarn direction (0 o -direction) is well simulated by the analytical calculation, which is attributed to the existence of initial yarn waviness. The tensile tests conducted with specimens of different width exhibited tendencies of increasing stiffness with increasing width for both 0 o - and 90 o -direction specimens, which eventually converged to the identical value. This converged value is equal to that obtained from the homogenization method representing the tensile stiffness of the infinite plate. The effect of the finite size of the frequently used coupon specimens should be well taken into account for experimentally evaluating the mechanical properties of the current type of material systems.

Numerical analysis of bending and transverse shear properties of plain-weave fabric composite laminates considering intralaminar inhomogeneity

In this study, the bending and transverse shear properties of plain-weave fabric composite laminates are investigated by considering the intralaminar inhomogeneity through finite element analysis. Using homogenization procedures, the effective Reissner–Mindlin plate bending and transverse shear properties of composite laminates are calculated. Assuming the number of plies to be infinite, the effective three-dimensional (3D) continuum properties (elastic properties of the equivalent 3D bulk material) of the composite are also calculated. Then, the effect of the number of plies on the bending stiffness and transverse shear stiffness of the laminate, in other words, the effect of the intralaminar inhomogeneity on the laminate stiffness, is investigated. Through the numerical investigation, it is found that if the number of plies of the laminate is very small, the bending stiffness and transverse shear stiffness of the laminate are significantly lower than those evaluated using effective 3D continuum properties.

Suppression of initial failure at tapered end-closure sandwich panel joint by taper angle change

To examine the configuration of CFRP face plates/foamed plastic core sandwich panel joints, a tapered end-closure-type joint is selected and studied. In this type of joint, the sandwich panel is tapered to form a solid laminate comprising two face plates near the joint, and the two panels to be joined are mechanically fastened at the solid laminate with a splice plate. This type of joint may be suitable for aircraft panels because a flat surface can be obtained at the joint, which is advantageous from an aerodynamic viewpoint. However, in a previous study on such a joint, it was found that a delamination crack initiated from the tapered core end and propagated through the interface between the two face plates as an initial failure mode at a much lower tensile load than the final failure load in a tensile strength test. In this study, the angle of the tapered panel was focused on and the effect of changing the taper angle on suppressing the initial failure was investigated through experiments and numerical analysis. It was found that a smaller taper angle is more effective for suppressing the initial failure.

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.