Takahira Aoki

The pipette aspiration applied to the local stiffness measurement of soft tissues

A simple method of identifying the initial slope of the stress-strain curve (i.e., Youngs modulus of the soft tissue) by introducing the pipette aspiration technique is presented. The tissue was assumed to be isotropic and macroscopically homogeneous. Numerical simulations by the linear finite element analysis were performed for the axisymmetric model to survey the effects of friction at the tissue-pipette contact boundary, pipette cross-sectional geometry, relative size of the specimen to the pipette, and the layered inhomogeneity of the specimen tissue. The friction at the contact region had little effect on the measurement of Youngs modulus. The configuration of the pipette was shown to affect the measurement for small pipette wall thickness. The measurement also depended on the relative size of the specimen to the pipette for relatively small specimens. The extent of the region contributing to the measurement was roughly twice the inside radius of the pipette. In this region, the maximum stress did not exceed the level of the aspiration pressure, with only minor exceptional locations. Calculation of strain energy components indicated that the major contributions to the deformation under pipette aspiration were by the normal extension and shear deformation in pipette axial direction. Experimental verification of the present method for the isotropic, homogeneous artificial material is also presented.

Analysis and Experiment of Gas Leakage Through Composite Laminates for Propellant Tanks

The mechanism of the through-thickness gas leakage of carbon fiber-reinforced plastics (CFRP) laminates is investigated in view of propellant leaks for composite tanks of reusable launch vehicles. In this study analysis of leakage caused by the existence of matrix cracks acting as the chain of leakage paths is developed under the simple assumption that conductance for leakage is a function of crack-opening displacements. The analytical results in consideration of mechanical and thermal loads are compared with experimental results, which are measured as helium gas leaks through carbon fiber-reinforced plastics laminates containing matrix cracks at room temperature. Good agreement between the analytical and experimental data is confirmed. Numerical analysis based on the proposed method can be used to evaluate the influence of mechanical loads on propellant leak through CFRP cross-ply laminates. The analytical calculations show that the increase of propellant leakage can be caused by the enlargement of crack-opening displacements caused by mechanical and thermal loads, increase of crack density, and decrease of temperature.

Fatigue growth of matrix cracks in the transverse direction of CFRP laminates

The damage process of matrix cracks in cross-ply and quasi-isotropic toughened CFRP laminates under cyclic loading is studied. Detailed observations of both crack density and crack length in coupon specimens suggest that there is a three-stage process comprising of edge crack initiation, edge crack growth, and crack propagation across the width in quasi-isotropic laminates, whereas simultaneous transverse crack initiation and propagation in the width direction takes place in cross-ply laminates. Energy release rates associated with crack propagation in the width direction are derived using the two-dimensional analytical model. It is shown that transverse crack propagation behavior can be estimated using the power law of the associated energy release rates.

“Furoshiki satellite” - a large membrane structure as a novel space system

Abstract We have been studying a large membrane space structure named “Furoshiki Satellite,” as a promising candidate of a future space system for those missions requiring large area in space such as solar power generation, a large communication antenna, or a large radiator. This membrane is folded in a very small volume during launch and is deployed and controlled by a set of several satellites at its corners or using centrifugal force generated by rotating the central satellite. It is expected that such a structure will reduce the weight per area of the space structure and, if the control technology is properly applied, it can be efficiently folded during launch and easily deployed after release. This paper shows the concept of Furoshiki Satellite, its applications, and its dynamics on orbit and how to control it. A nano-satellite project on demonstrate the concept of Furoshiki Satellite will also be described briefly.

Cryogenic mechanical properties of CF/polymer composites for tanks of reusable rockets

Cryogenic properties of different types of CFRPs are experimentally evaluated to survey the basic applicability of different material systems to the cryogenic propellant tanks for future reusable launch vehicles. Temperature dependent material constants, tensile strength and interlaminar fracture toughness are experimentally obtained, together with detailed observations of matrix cracks and delaminations. Up to about 20% reduction in cryogenic static tensile strength is observed for most of the material systems tested. The damage initiation stresses also decreased under cryogenic conditions. The results indicate that matrix cracks may be one of the major critical issues when current material systems are applied to cryogenic propellant tanks. Numerical predictions of the delaminations and matrix cracks are conducted to theoretically support the experimental consequences.

Transverse crack propagation in the specimen width direction of CFRP laminates under static tensile loadings

The damage process of transverse cracks in cross-ply and quasiisotropic toughened CFRP laminates under static loading is studied. To investigate specimen configuration effect on width-direction propagation of transverse cracks, coupon specimens with ranges of specimen width and transverse ply thickness are tested. Detailed observations of transverse cracks indicate that there are consecutive three-stage processes of edge crack initiation, edge crack increase without inward propagation, and crack propagation across the width in quasiisotropic laminates, whereas simultaneous transverse crack propagation takes place across the width in conjunction with new edge crack formation in cross-ply laminates. The detailed process of transverse cracking is investigated using three-dimensional FEA in order to clarify the difference of damage process between quasiisotropic and cross-ply laminates. It is shown that damage mode transition from edge cracking to widthwise propagation can be characterized with energy release rates near free-edges and inner regions. Finally, an average propagation model based on the conventional two-dimensional models is presented and verified with experimental results and FEA. Useful information about experimental results of transverse crack propagation and the characterization of edge cracking and inward propagation isprovided.

Stress analysis of symmetric laminates with obliquely-crossed matrix cracks

Simple methodology for stress analysis of laminates with obliquely-crossed matrix cracks is presented. An oblique coordinate system along the matrix cracks is introduced in conjunction with the derivation of oblique coordinate tensor components. Using displacement/strain covariant components and force/stress contravariant components, it is shown that the obliquely crossed crack problem with any oblique angle can be treated as the orthogonally-crossed crack problem, which ensures that obliquely-crossed cracked laminates can be regarded as orthogonally-crossed cracked laminates with appropriate transformation of properties. This approach is combined with twodimensional shear-lag analysis and analytical solutions of [ m/90n]s and [S/ m/90n]s laminates with matrix cracks in both- and 90-plies under general in-plane loadings are obtained in terms of oblique coordinate components. Calculated stress distributions are compared with 3D FEM results and the effectiveness of the present analysis is verified.

The effect of matrix cracks on gas permeability through CFRP laminates

Diffusion-controlled gas permeability through CFRP laminates was experimentally investigated as fundamental research on the feasibility of composite propellant tanks. Using helium gas and a helium leak detector, through-the-thickness gas permeability in CFRP laminated tubes with or without matrix cracks was measured at room temperature. The effect of loadings on the in situ gas permeability was also clarified. It is shown that, although gas permeability through CFRP laminates increases on account of the existence of matrix cracks and tensile loadings, these effects turned out not to be crucial in comparison to the leakage through multi laminar matrix cracks, which is three or four orders higher than the diffusion-controlled permeation. These results suggest that the existence of no less than one intact layer is important for the feasibility of composite propellant tanks. Finally, a diffusion model including the combined effects of damages and loads is applied to the experimental results and a successful characterization of gas permeability is presented.

VaRTM process of composites using porous mold

This paper introduces Porous Mold Process (PMP) as a reliable low-cost manufacturing process of fiber-reinforced composites. PMP using porous aluminum is applied to the vacuum-assisted resin transfer molding (VaRTM) process of carbon fiber reinforced plastics (CFRPs). Experimental evaluation on resin infusion behavior and quality and mechanical properties of the cured plates is performed with varying the dimensions of the plates. The results show that quality and mechanical properties of the cured plates using PMP are satisfactory, stable, and almost independent of plate dimensions. It is concluded that PMP provides a reliable and knowhow-less resin infusion process of composite materials.

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.