Kozo Kawata

On Young's modulus of short fibre composites

Abstract In this paper we firstly describe a mechanism of load transfer from matrix to fibre about single short fibre models. Next, applying the above results, we predict Youngs modulus of short fibre-reinforced materials provided that the orientation of the fibres obeys a certain rule. Finally, we compare the numerical results with experimental data. Youngs modulus derived by our calculation seems to explain the experimental results fairly well.

On the stress concentration factor in fibrous composites

Abstract In order to presume a probabilistic strength of a composite material, it is necessary to know the stress states in a composite plate. In the present paper, the stress states in a model, unidirectionally reinforced with continuous fibres, except for some discontinuous fibres, and subjected to a tensile load parallel to the fibre direction, are considered. A stress concentration factor and an ineffective length in the model are calculated using a new method proposed previously by the authors. Comparison is made with experimental values and results by means of shear-lag theory.

On the strength distribution of unidirectional fibre composites

Abstract The statistical strength distribution of composite materials reinforced with unidirectional fibres is predicted using the Monte Carro method. The effects of the fibre-matrix modulus ratio, Ef/Em, the volume fraction of the fibres, Vf, and the specimen length on the strength of composites are discussed. It has also been made clear that the ‘rule of mixture’ overestimates the strength of composites.

Constitutive Relation in High/Very High Strain Rates

Presenting a general survey of recent developments in the research of constitutive relations in high and very high strain rates and related problems in high-velocity solid mechanics, this work focuses on: material characterization of solids in high-velocity deformation; experimental techniques, modelling and constitutive relations; strain rate-dependent elasto-visco-plastic stress waves; crack initiation, propogation and dynamic fracture toughness; dynamic stress concentration; structural dynamics in impact and constitutive relations of solids; and impact-related problems.

Mechanical characterization of unidirectional CFRP thin strip and CFRP cables under quasi-static and dynamic tension

Under uniaxial tension, full stress-strain curves up to fracture and mechanical properties of a variety of CFRP cables having different structures are evaluated experimentally at three strain rates ranging from quasi-static (10-5 s-1) to dynamic (102 s-1). Such properties as tensile strength, chord or tangent modulus, strain at tensile strength and absorbed energy per unit volume are obtained. Experimental difficulties on stress and strain measurements and specimen fastening are overcome by introducing special instrumental arrangements and an expansive-cement fastening technique. For all cables, at quasi-static (10-5 s-1) and intermediate (10-2 s-1) strain rates, the stress-strain relation is linear. On the other hand, at a dynamic (102 s-1) strain rate, the stress-strain relation is non-linear and all cables tend to increase their elastic modulus and tensile strength. This obviously leads to increase of the absorbed energy per unit volume under dynamic tension. However, the strain at tensile strength remains almost the same at all strain rates. Thus, from a consideration based on the fracture strain criterion, it is concluded that strain rate insensitivity is assured for all cables.

Macro- and Micro-Mechanics of High-Velocity Brittleness and High-Velocity Ductility of Solids

An extensive study of high velocity brittleness and ductility of solids, hitherto little known systematically is reported. As this study needs complete acquisition of dynamic stress-strain relation in uniaxial tension up to breaking, newly developed “one bar method” for the characterization of solids that enables the acquisition of full tensile stress-strain diagram up to breaking, with its macromechanical analysis formulae, is introduced. Stress-strain relations in dynamic tension in the strain rate range up to 103/sec, for (1) metallic materials of bcc, fcc and hcp crystal lattice systems; pure irons, structural steels, Ni alloys, Al alloys, and Ti alloy, (2) composite materials: GFRP, CFRP, CF/GF hybrid FRP, and (3) inorganic non-ductile material: Pyrex glass, are obtained and their dynamic macromechanical characteristics are discussed, from the standpoint of high velocity brittleness or ductility. The mechanisms of these various phenomena are discussed micromechan-ically, based upon corresponding models.

Stress distribution of laminates including discontinuous layers

Abstract Discontinuity of fibres plays a critical role in the strength of fibre-reinforced composite materials. In this paper, the stress distribution of laminated plates of finite layers with some discontinuous layers is analysed using shear-lag theory. The strength of such plates is also predicted and the theoretical value is compared with experimental data.

Stress and strain fields in short fibre-reinforced composites

Abstract The axial stress of a fibre, the shear stress at the fibre-matrix interface and the stress in the matrix of short fibre-reinforced materials under tension are calculated using a new model proposed by the present authors. In the present model, the fibre and matrix do not overlap at the region corresponding to the fibre while they have been reported as overlapping in most of the former reports published by other authors. Both single short fibre models and models of regular fibre array are considered. Effects of the aspect ratio of the fibre and the modulus ratio of the fibre and matrix are clarified in the former case and effects of the spacing of fibres in the latter case. The numerical results are found to coincide with the experimental data fairly well.

Photoelastic-coating analysis of dynamic stress concentration in composite strips

Dynamic stress concentration in high-velocity tension is investigated for fiber-reinforced composite strips with a central circular hole. The photoelastic-coating technique is utilized with a newly constructed nine-frame Cranz-Schardin high-speed reflection-type camera. The excellent reflective plane is the key to obtaining clear isochromaticfringe patterns. Four different kinds of composites in a very wide range of anisotropy are tested to study the effects of anisotropy, loading direction and the hole-diameter/specimenwidth ratio on the dynamic-stress distribution. Dynamic-stress and strain distribution around a hole is found to vary remarkably with the change in anisotropy of each composite. In some tests, dynamic fractures caused by stress concentration are also detected in high-speed photos. Photoelastic-coating results are then compared with the results obtained by straingage experiments and finite-element numerical analysis in order to verify the validity of the present dynamic photoelasticoating analysis.

Sharpening diamond tools having an apex angle of less than 60° with a low energy ion beam

Abstract A simulation method has been developed for predicting the profile changes of diamond tools which are uniformly rotated at a fixed tilt angle during ion beam machining. The method can treat a three dimensional object such as a diamond knife with hemicylindrical tip and a diamond stylus with hemispherical tip in a two dimensional manner. Diamond knives and styli having an apex angle less than 60° are machined with Ar ions of 1.0 keV using an ion beam machining apparatus with Kaufman type ion source. By comparing the profile changes obtained by computer simulation with that obtained by experiment, it is concluded that the method is useful for the prediction of profile changes of hemicylindrical and hemispherical objects having an apex angle of less than 60° during ion beam machining with Ar ions of 1.0 keV.