Tadashi Shioya

Fundamental solution for a penny-shaped crack in apiezoelectric medium

Abstract This paper intends to present a general three-dimensional analysis of a penny-shapedcrack subjected to normal mechanical loads and free surface electric charges symmetricallyapplied on its upper and lower surfaces. To this end, the potential theory method is employed andgeneralized to analyze the piezoelectric crack problem under consideration. In particular, anotherpotential of a simple layer, corresponding to the electric effect, is introduced. As a typicalexample, a closed-form solution is first obtained for a penny-shaped crack subjected to a pair ofconcentrated forces acting in opposite directions and a pair of point charges on crack surfaces.Exact expressions for stress and electric displacement intensity factors are also presented.

Elastic-plastic analysis of the yield process in mild steel

Abstract A theoretical analysis of the particular yield-zone pattern in mild steel is presented. The constitutive relation adopted utilizes work of W.G. Johnston and J.J. Gilman (1959) and G.T. H ahn (1962) in conjunction with a newly-proposed criterion for the production of the initial, mobile dislocations. Numerical analysis was undertaken for the solution of an initial-value problem using the continuously distributed dislocations model. The patterns obtained for twisted bars of circular and square cross-sections closely resemble the well-known strain figures. It was also found that (i) the particular patterns come from the negative-slope characteristic of the stress-strain relation, (ii) the yield drop takes place when a small yield-zone, already formed around a defect at a lower nominal stress, begins to spread, and (iii) a scale-effect due to a characteristic length introduced in the proposed criterion exists. Besides, two experiments were conducted which gave support to the constitutive equation and the newly-proposed criterion.

Degradation of carbon-based materials due to impact of high-energy atomic oxygen

Spacecraft structures in low earth orbit encounter a degradation problem caused by the impact of atomic oxygen (AO) in the space environment. This paper presents an experiment of impact degradation on the ground. The experiment was carried out in a vacuum space chamber. AO produced at a plasma torch was accelerated fluid-dynamically with argon (Ar) working gas. As the target materials, several carbon-based materials were employed; i.e., graphite, carbon fiber/carbon composite (C/C composite) and silicon impregnated C/C composite (Si-C.C.). The degradation was analyzed in the aspect of macroscopic mass loss per unit area and the microscopic degradation modes. The temperature effect and the exposure time effect are investigated and the comparison among the target materials is developed. The mass loss per unit area is approximately proportional to the exposure time and depends on the temperature of material surface. The eroded surfaces of materials were observed with a scanning electron microscope and an electron probe micro-analyzer. In case of C/C composite, the matrix region erodes more than carbon fiber, however, this effect is more pronounced in the normal oxidation at the ambient atmosphere. The mass loss of Si-C.C. is less than that of the corresponding C/C composite and only the C/C composite region erodes deeply with almost no erosion in Si and SiC regions.

Approximate analysis of the stress state near the fibre ends of short fibre-reinforced composites and the consequent microfracture mechanisms

An approximate stress analysis is performed to evaluate the microfracture behaviour near a fibre end in short fibre-reinforced thermoplastic composites. The model presented here predicts the axisymmetric stress distribution in a concentric cylinder assemblage containing the fibre, the matrix and the interphase, taking into account the stress transfer across the fibre end. Then, the calculated stresses are used to explain the damage initiation and growth behaviour of injection-moulded short fibre-reinforced thermoplastic composites with different fibre surface treatments.

Dynamic Fracture Toughness and Crack Propagation in Brittle Material

Experiments of crack propagation in PMMA plates are performed from which the dynamic fracture toughness G c of the material is obtained. The relationship between G c , and crack velocity v 0 is associated to the characteristic appearance on the fracture surface. Periodic patterns are observed on the fracture surface which suggest local oscillation of crack velocity. A model for analyzing crack propagation by global energy equilibrium concept is proposed, from which crack motion equation is deduced. Unstable crack propagation and local velocity oscillation is explained using this equation and the particular G c (v 0 ) relationship.

Mechanical properties of silicon impregnated C/C composite material at elevated temperature

Fracture tests were conducted for silicon impregnated C/C composite material at elevated temperature. Two kinds of carbon fiber orientations were used; one was (0/90) cross-ply and the other was (+45/-45) cross-ply. The fracture mode of this material is discussed compared with that of the conventional C/C composite material. The roughness of the fracture surface of silicon impregnated C/C composite material is much smaller than that of C/C composite material. The delamination between layers is not so remarkable in the fracture of silicon impregnated C/C composite material while it is predominant on the fracture surface of C/C composite material. The anisotropy of the tensile strength of silicon impregnated C/C composite material is small compared with that of conventional C/C composite material.

BRITTLE-DUCTILE TRANSITION AND SCALE DEPENDENCE: FRACTAL DIMENSION OF FRACTURE SURFACE OF MATERIALS

The fractal dimension of fracture surfaces obtained within brittle-ductile transition regime is evaluated at various observation scales. Fracture surfaces are generated by the tensile fracture test. The brittle-ductile transition is accomplished by using the round-notched bar specimens with various notch radii, which cause the variation in stress triaxiality. The specimens are manufactured from mild steel, steel and cast-iron bar. The fracture model is identified according to the observation through scanning electron micrographs. The fractal dimension for ductile fracture surfaces is almost constant despite variations in observing scale and changes in stress triaxiality. Meanwhile, the fractal dimension on brittle fracture surfaces shows the different values for macroscopic and microscopic observing scales. This transition-like scale dependence of fractal dimension for brittle fracture surfaces is considered to reflect such a characteristic of the fracture i.e. its specific length in microscopic fracture mechanism. The existence of transition in fractal dimension with observing scale is considered to be an index, used to distinguish the ductile fracture surface from the brittle fracture one.

Conditions of micro-cracking and their applications to ceramic coatings of carbon–carbon composites

Abstract Based upon the theoretical solutions of flat and wavy coatings bonded to an infinite substrate, conditions have been put forward for the micro-cracking normal and parallel to the coating–substrate interface in tension and compression. In this paper, these conditions have been used to investigate the micro-cracking in the ceramic coatings of carbon–carbon composites. Furthermore, from the solutions for plane strain, plane stress and three dimensional cases, the experimental phenomenon that cracks tend to occur at the center of a coating has been explained as a result of larger stress due to its plane strain-like condition compared with the stress near edges.

Computational Analysis of the Stress Distribution along the Bimaterial Interface near a Crack by the Singular Integral Equations Method

There are a great many analytical or computational studies related to the interaction between a bimaterial interface and a crack in an elastic body composed of two or more kinds of materials. Most of these studies are intended to obtain the stress intensity factors or the singularities at the tips of the crack. However, the stress distribution along the bimaterial interface has been scarcely focused on in spite of its importance from the viewpoint of the debonding at the interface.

A theoretical study of micro-structural size effect on crack extension in composite materials

Effect of micro-structural size on the crack extension in composites is theoretically studied by the use of a simplified two-dimensional crack model in uni-directionally reinforced composites. The model assumes that the plastic deformation occurs in the matrix layer ahead of the main crack and that micro-crack initiation in the plastic region leads to the global fracture of the composites. The continuous dislocations method is introduced, and used for the analysis of the crack and plastic fields in composites. The micro-structural size effect is discussed in terms of the ratio of reinforcement thickness to the possible in situ crack size.