Toshiya Nakamura

Tensile and compressive tests of automated fiber placement (AFP) manufactured quasi-isotropic laminates with repeating embedded gaps or overlaps are conducted and reveal significantly lower tensile and compressive strengths compared to those of normal laminates in spite that the gaps and overlaps cut no fiber. The reduction in tensile strength is greater than that of the compressive strength. A two-dimensional mechanical model of the quasi-isotropic laminates with an arbitrary inclined gap is proposed and analytically solved using a super-position technique as the sum of a global uniform stress field that satisfies equilibrium and compatibility conditions by applying a virtual force at both ends of the gap and a local stress field caused by the virtual force. The local problem is approximately solved using a Green function technique. The stress and strain fields under a unidirectional load are obtained in a closed-form expression in which the effect of the tapered portions typically observed at both sides of the gap is incorporated. The present analytical solution agrees with the finite element solution. The derived expression shows that significant stress concentration occurs at the end portion of the gap.

The aim of this study is to establish a novel aircraft design approach replacing the conventional airframe by utilizing biomimetics. This design approach particularly focused on the dragonfly wing, whose reinforcement structures are composed of cross-veins and longitudinal veins. The cross-veins have been emulated by weighted Centroidal Voronoi Tessellation (WCVT) following the out-of-plane displacement on the skin, while the longitudinal veins have been emulated by extracting a centerline from the topology optimization result on the skin to be reinforced, through image analysis of binarization and skeletonization. The longitudinal layout can reduce the compliance distributing the inner load with only essential reinforcement on the skin without increasing the mass. The weighted CVT layout can improve the effectiveness of the reinforced skin against buckling drastically. Thus, the skin reinforced along the cross-longitudinal layout by the topology optimization and weighted CVT pattern increased buckling load 2.7 times higher even with less mass than the conventional layout.

The aim of this study is to establish a novel aircraft design approach combining structural optimized biomimetics. The intention of utilizing biomimetics lies in acquiring an efficient and novel composites airframe structural concept replacing the “black metal structure.” A typical macroscopic structural layout on a dragonfly wing, called Voronoi patterns, was applied to the reinforcement structures on a part of an aircraft wing. This design approach significantly contributed to structural weight reduction. The manufacturability of more complex structure is also confirmed by producing prototypes, evaluating quality of created models and identifying an effective sectional shape in In-Situ Consolidation process. Furthermore, this application has advanced to a novel design methodology defining reinforcement layout not as some feature extracted from dragonfly wings but as theoretical and mathematical disposition. This methodology allows composites cover to acquire excellent properties, namely lightness and stiffness combining themselves properly. Bionic structure with Voronoi patterns has potential for weight reduction replacing conventional composites aircraft structures.

The aim of present study is to establish the optimization design approach for light weight composites aircraft structure. A composite panel with a large cut-out representing a part of lower wing panel is optimally designed with the curved fiber orientation based on principal load path. The optimum curved fiber orientation is directly defined by the principal stress direction derived from finite element analysis result. Only the fiber orientation in 0 degree layers are steered based on principal load path. Three different types of the optimum panel with different staggering strategies are precisely manufactured by Automated Fiber Placement (AFP) with tow-steering layup technique. The optimum panels are evaluated by testing under tensile load. The results show that the initial failure load of the optimum panels with curved fiber orientation is 6% higher than that of the conventional panel with straight fiber orientation with the same weight, and the panel stiffness of the optimum panel is 3% higher than that of the conventional panel. The maximum strain level around the cut-out of the optimum panel is 8% smaller than that of the conventional panel. These results indicate that the optimum panel is capable of sustaining larger load than the conventional panel, and imply the potential of structural weight reduction for load path tailored composite panel. Those improvements in the testing correspond well with the numerical prediction which is used for the panel design.

Surface treatment is important to ensure adhesion properties of adhesive bonding or coating. Although laser irradiation or atmospheric plasma treatment has attracted as innovative surface treatment in aviation industry, silica deposition treatment could be another choice to improve adhesion properties of aircraft structural applications such as adhesive bonding joints or coating. The silica deposition treatment is a kind of flame treatment which uses liquid petroleum gas containing silane compounds as combusting feed, and could be quick, easy, clean and costless process. In this study, mechanical strength properties of the silica deposition treatment were investigated. Two kinds of testing were conducted. One is single lap tensile tests of adhesively-bonded joints using carbon/epoxy composites or titanium alloys. The results of the tests indicated lap shear strength increased as the number of passes increased. The 12 times passing coupons of both of composites and titanium alloys exhibited high strength comparable to the conventional treatment coupons. The other is coating adhesion pull-off tests using aluminum alloys. Just only one-time pass brought high adhesion comparable to the conventional treatment. This means, as for aluminum coating application, the silica deposition treatment might be beneficial treatment in actual production scene compared with conventional ways.

Out-of-plane impact loading causes both the internal damage including interlaminar delaminations and the visible deformation on CFRP laminates. The damaged specimens subjected to drop-weight impact were observed in detail by several inspection systems: UT, X-ray CT, and shape measurement systems with laser. The influence of laminate thicknesses, stacking sequences, impactor shapes were observed using cross-ply and quasi-isotropic CFRP laminates. The results showed the bulge with the fiber breakage affects the detectability of the impacted part. By quantitatively evaluating the dent depth and the projected dent area, it showed the possibility that more diverse discussions could be made about dent detectability.