Shigeyasu Amada

Fiber texture and mechanical graded structure of bamboo

Among plants, bamboo has an unique structure which resembles that of a unidirectional, fiber-reinforced composite with many nodes along its length. Furthermore, bamboos growth rate is very fast, producing an adult tree in only one year. This paper demonstrates that bamboo has a functionally graded and hierarchical structure. Bamboos diameter, thickness and internodal length have a macroscopically graded structure while the fiber distribution exhibits a microscopically graded architecture, which lead to smart properties of bamboo. The reinforcing fibers are oriented along the bamboos culm (trunk), whereas in the nodes the fibers become entangled in a complicated manner to produce nodes with isotropic properties that provide additional reinforcement for the culm.

THE MECHANICAL STRUCTURES OF BAMBOOS IN VIEWPOINT OF FUNCTIONALLY GRADIENT AND COMPOSITE MATERIALS

The structures of bamboos have been adapted to our natural environment over a long period of time. They are a composite material reinforced axially by fibers called bundle sheath. Furthermore, they have a hierarchical gradient structure, that is, a macroscopic gradient structure in culm (corresponding to trunk of wood) diameter, culm thickness and node length, and a microscopic one in the bundle sheath distribution. The macroscopic gradient structure leads to a constant surface stress at every height and the microscopic gradient structure provides a strength distribution in radial direction adapting to bending stress due to wind loads.

Influence of grit blasting pre-treatment on the adhesion strength of plasma sprayed coatings: fractal analysis of roughness

Abstract Since the adhesive strength of ceramic coatings by plasma spraying is fairly low, serious efforts have been made to improve it. In order to improve the adhesiveness of coatings, the substrate surfaces were roughened by blasting, sometimes by angled blasting as a pre-treatment. The measured adhesive strength showed the maximum value at blasting angle 75°. Average surface roughness of the substrates remains almost constant with respect to the blasting angle. Introducing fractal analysis to evaluating the surface topography of substrates, the fractal dimension was evaluated for the roughened surfaces. It showed a maximum fractal dimension at 75° where the adhesive strength reaches approximately its maximum value. It is concluded that fractal dimension is a better measure than average surface roughness for the evaluation of adhesive strength of ceramic coatings.

Introduction of fractal dimension to adhesive strength evaluation of plasma-sprayed coatings

The adhesive strength of ceramic coatings depends on the surface roughness of substrates. Taking account of the bonding mechanisms, however, surface roughness may not be a proper measure to evaluate the surface topography of substrates. Here, fractal dimension is proposed for the evaluation of surface topography. Physically the fractal dimension can include hook-shaped indentations which generate a mechanical interlocking force. By applying fractal dimension to the evaluation of the surface topography of substrates and relating it to the adhesive strength of alumina coatings, it was concluded that fractal dimension properly evaluates the adhesive strength as compared with surface roughness.

Quantitative evaluation of residual grits under angled blasting

Abstract Grit blasting can contribute greatly to an improvement in adhesiveness of ceramic coatings, but the residual grits on the substrate surfaces degrade adhesion. Thus, grit blasting produces a good effect in one way and a bad effect in another. Specimens were roughened by angled grit blasting, and the residual grits were evaluated after air-blowing off the roughened surfaces. To evaluate quantitatively the residual grits on those substrates, the images of residual elements from EPMA are analyzed by computer, and their areal fraction is derived with respect to the blasting angle. It was concluded that grit residues occur mostly at a blasting angle of 90° and decrease with decreasing angle.

Splat profile of impinging droplets on rough substrates: influence of surface roughness

Abstract A thermal spraying process contains various individual and fundamental processes. Among them, the flattening process is important and governs the quality of sprayed coatings. Therefore the flattening behaviors of spraying particles must be clearly understood. Ceramic and metal particles are generally sprayed onto the roughened substrates to enhance the adhesion strength of the coatings in plasma spraying technique. This report describes the flattening profile of the metal droplets impinging on the roughened substrates. Introducing the unevenness ratio, the droplet profile is evaluated with respect to surface roughness of the substrate surfaces, R e and W e numbers. The experimental results show that the unevenness ratio increases with surface roughness of the substrates. Under the droplets of almost the same size the unevenness ratio has a bilinear relationship with the impinging velocity, R e and W e numbers. Defining this corner point by a critical value of the droplets, the splat behaves the splash-like shape above the critical point. It was concluded that the critical value is influenced not only by the flattening parameters, but also by the surface roughness of the substrates.

Fractal analysis of surfaces roughened by grit blasting

Surfaces roughened by grit blasting influence the adhesion strength of plasma-sprayed ceramic coatings. The average surface roughness has been used to evaluate the surface topography of such surfaces. It is well known that the adhesion strength of ceramic coatings reaches a maximum value at a certain substrate surface roughness. However, this result cannot be understood based on only surface roughness. The blasted surface has fractal characteristics. There are two types of fractal surfaces, which are characterized by self-similarity and self-affinity. Using fractal analysis to evaluate the surface topography of substrates, the fractal dimension was measured for the roughened surfaces. The maximum fractal dimension was attained at a blasting angle of 75°, where the adhesion strength also reached approximately its maximum value. It is concluded that the fractal dimension is a more appropriate measure than the average surface roughness for evaluation of the adhesion strength of ceramic coatings.

Splat Formation of Molten Sn, Cu and Ni Droplets

Plasma spraying and other deposition or coating techniques cause splat formation of the molten droplets. This paper presents free falling and splat formation experiments in which molten droplets of Sn, Cu and Ni were impinged on the cooled substrate and solidified after the flattening process. The flattening ratio, defined as the flattening disc diameter to the droplet diameter, was measured under different conditions. The obtained results were different from the proposed theories, which neglect the solidifying effect. The flattening ratio experimentally obtained was proposed with respect to Re and We numbers, independently. The unevenness ratio was introduced and is defined by the contour length of the flattening disc to the circumferential length of an equivalent circle which has the same area as the flattening disc. This was measured with respect to Re and We numbers and represented an experimental formula of the unevenness ratio. It was concluded that the unevenness ratio was closely related to surface tension and impinging velocity.

Thermal shock strength of Al2O3 by laser irradiation method

Abstract Various structural ceramics have been developed as heat-resistant materials. It is very important to investigate their thermal shock characteristics. This report presents a newly developed, laser irradiation method to evaluate thermal shock strength and proposes that a critical power density can be a new measure to evaluate heat resistant materials. The temperature and thermal stress distributions of the cylindrical shaped Al 2 O 3 ceramics under irradiation by CO 2 laser beams are analyzed using FEM. The maximum tensile and compressive stresses are obtained with respect to beam diameters for various power densities. These relations lead to critical power densities at which materials are fractured. The relationships between the critical power density and beam diameters derive critical fracture curves which gives the fracture criterion of ceramics. Finally, the irradiation experiments were carried out and their results got a good agreement with the theoretical fracture criterion. It was concluded that the critical fracture curve can be a new measure to evaluate thermal shock resistance of ceramics.

Thermal shock resistance of carbon-carbon (C/C) composite by laser irradiation technique

Abstract Thermal shock strength of carbon–carbon composite has not been evaluated yet. This paper presents it by laser irradiation technique newly developed for ceramics. Traditionally adapted quenching test has a big difficulty in which heat transfer coefficient is very unstable under the quenching process. To avoid this problem, laser irradiation method was proposed and applied effectively to several ceramics. Thermal shock strength of carbon–carbon composite was evaluated by laser irradiation method and the critical laser power density which causes the material’s fracture was obtained. It was concluded that the critical power density can be a new measure to evaluate thermal shock strength.