Tomohiro Yokozeki

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.

Development of Variable Camber Morphing Airfoil Using Corrugated Structure

This paper describes the development and the wind tunnel test of a variable geometry morphing airfoil using corrugated structures. Proof-of-concept study of a morphing wing with corrugated flexible seamless flap-like structure is verified by finite element analysis, and a prototype is manufactured using carbon fiber reinforced plastics. For the actuation system, two servomotors are installed inside the prototype wing to control the airfoil shape by the chordwise tension of the connected wires. Successful actuation of the prototype wing is demonstrated under the air speed up to 30  m/s in the wind tunnel test. Basic aerodynamic properties are also evaluated in comparison to traditional airfoil with a hinged control surface. Lift increase of variable corrugated wing is recognized compared to the traditional wing when the aileron angle increases.

Mechanical and electrical properties of PANI-based conductive thermosetting composites

Polyaniline (PANI) is one of the most studied intrinsic electrically conductive polymer in recent years. In the present work, PANI-based conductive composites have been prepared using dodecylbenzenesulphonic acid (DBSA) as a dopant and divinylbenzene (DVB) as a cross-linking polymer to enhance the rigidity. In this mixture, DBSA acts as the dopant of PANI as well as the curing agent of DVB, and this means that doping and curing of the composite occur simultaneously. PANI–DBSA mixture has been prepared by physical mixing using centrifugal mixer. The ratio of PANI and DBSA has been kept constant in weight ratio 30:70. Furthermore, DVB content has been varied to prepare the different samples. It has been found that with the increase in DVB content, a good dispersion of PANI–DBSA/DVB solution is achieved. However, significant improvement in the electrical conductivity of the samples is observed with the increase in PANI content. Mechanical flexural test has been performed on the prepared samples using a three-point bending configuration. It has been found that the flexural modulus and brittleness of the prepared samples increase with the increase in the content of DVB in the composite. Morphology of the composite surfaces has been also studied and it has been found that low concentration of DVB results into a poor dispersion and hence agglomeration of PANI–DBSA in composite can be seen clearly in the images.

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.

Excellent mechanical properties of carbon fiber semi-aligned electrospun carbon nanofiber hybrid polymer composites

The present investigation reports the excellent mechanical properties of carbon fiber fabric hybrid polymer composites obtained by integrating continuous semi-aligned carbon nanofiber layers in the interlaminar region between carbon fiber fabric plies. Different weight fractions (0 to 2.0 wt%) of continuous nanofiber layers were sandwiched with carbon fiber fabric preform to prepare polymer matrix hybrid nanocomposites by a compression molding technique. It was found that layers with 1.1 wt% content of semi-aligned nanofibers significantly enhance the bending strength by 175% and the modulus by 200%. The greater interfacial adhesion of the high surface area of the nanofibers significantly increases the interlaminar shear strength by 190%. The improvement in the mechanical properties of the polymer composites is due to the modification of the matrix properties, crack deflection and debonding. The high surface area and voids that form around the carbon nanofibers can absorb higher fracture energy. However, nanofiber content greater than 1.1 wt% at the interlaminar region of the composites has negative effects on the overall mechanical properties of the hybrid composites. The thick nanofiber layers are believed to be acting as micron size carbon fibers, which results in a downward trend in the bending properties. The present strategy for the preparation of high performance carbon fiber hybrid polymer composites utilizing electrospun continuous carbon nanofibers at the interlaminar region provides a promising approach for improving the properties of composites for different applications.

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.

Development of variable camber wing with morphing leading and trailing sections using corrugated structures

This article describes the development of variable camber morphing wing, which is mainly composed of corrugated structures. The morphing wing with both leading edge and trailing edge morphing sections is proposed and the prototype model is designed by consideration of finite element structural analysis with actuation mechanisms and aerodynamic analysis. Through wind tunnel experiment with the manufactured prototype model, smooth actuation without harmful deformation under 20 m/s airflow is demonstrated. The observed deformation shape is well correlated with simulated shape by analysis. Thereby, the feasibility of the present morphing wing mechanism and design process are verified.

Evaluation of Compressive Nonlinear Response of Unidirectional Carbon Fiber Reinforced Composites using a Modified Sandwich Beam Specimen in Flexure

Nonlinear mechanical behaviors of unidirectional composites were evaluated under compressive loading up to high-strain ranges using a sandwich beam specimen with the emphasis on the elastic material nonlinearity in the fiber direction. Specimen configuration and evaluation method, including material nonlinear effects, were proposed, and the compressive nonlinear behavior and strength of unidirectional carbon fiber reinforced composites were experimentally obtained. The measured compressive behaviors were compared with those from coupon-type compression tests of unidirectional and quasi-isotropic laminates. The compressive strength of unidirectional laminates evaluated, using the present sandwich method, was much higher than that of coupon specimens, whereas the failure strain of the former was almost identical to that of coupon-type quasi-isotropic laminates. The proposed method is expected to contribute to the accurate evaluation technology of compression behaviors of unidirectional laminates up to high-strain ranges.