For a new insight on the mechanism of hysteresis of polymers, several molecular dynamics simulations are conducted on an amorphous polyethylene under cyclic deformation. Each role of the bond stretch, bending, torsion and van der Waals is investigated in detail, revealing that the bond stretch and van der Waals dominate the hysteresis in the cyclic deformation. In the 1st cycle, or the “pre-stretching”, the trans nodes increase to orient the molecular chains in loading direction. However, the trans⇒gauche transition takes place during the unloading, resulting in the “curled” chain structure which has more gauche nodes than the initial configuration. In the 2nd cycle up to the maximum strain of the 1st cycle, the gauche⇔ trans change occurs in the “pseudo reversible” manner among the gauche nodes introduced in the 1st cycle. When the polyethylene is elongated in the 2nd cycle beyond the maximum strain, the dihedral nodes with high torsional energy, which are not in the trans nor gauche angles, decrease to lead the increase of trans nodes. This suggests that the overloading orients the entangled chains in loading axis, resulting in the dissociation of the chain entanglements.

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One of the most frequently encountered problems in bimaterial media is interfacial cracking, sometimes also known as delamination. The stress intensity factors of a crack between dissimilar materials are important parameters for evaluating delamination strength. A new method is proposed for the stress intensity factor analysis- of a crack between dissimilar anisotropic materials under thermal stress. The virtual crack extension method (VCEM), which is used with the finite element method (FEM), is reliable methods for estimating the energy release rate. Energy release rate obtained by the VCEM is separated into individual stress intensity factors, K_I, K_II and K_III using the principle of superposition. The present method was applied to interface cracks between jointed dissimilar plates under thermal stress. The distribution of stress and the crack opening displacement obtained by the asymptotic solution with the stress intensity factors are compared with those of obtained by the FEM with a fine mesh. They are almost identical each other.

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Avaraging the anisotropy of each crystal, the macroscopic behavior of polycrystalline materials is isotropic and homogenous in terms of elastic deformation. However, the anisotropic and/or inhomogeneous property influences on the stress field ahead of a crack if the crack size is small in comparison with the grain. This bring about the change in the stress intensity factor (SIF). In present study, in order to invistigate the cause and magnitude of the change in the SIF, the finite element analysis is performed. The calculations are carried out for the polycrystal cracked plate model, which consists of hexagonal pillar shape of crystal grains with random orientation. The results implied that the SIF of microstructurally small crack is greatly affected by the deformation constraint caused by the difference in elastic modulus of grains near the crack. The statistical scatter of SIF due to the random orientation of crystal orientation in a polycrystal is examined by a Monte Carlo simulation. The variation in the SIF becomes large as the crack size decreases and anisotropy of the material increases. We propose an equation for estimating the variation in the SIF of the microstructurally small crack.

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A simple method of evaluating the crack propagation rate of a specimen with sharp notch by using an effective stress intensity factor range was proposed. This study also had a special purpose of evaluating the crack propagation after overload. A 6 mm long U-shaped notch was cut in the center section of the specimen. The notch root raidus ρ was chosen as 0.1 mm and 1 mm. These were regarded as center notched specimens. The crack propagation behavior of these specimens was compared with a center cracked specimen. Push-pull fatigue tests were carried out under stress ratios R=0, -1, and -1.5. After overload, retardation of crack growth was observed under R=0 and -1, in the cases of the notched specimen and center cracked specimen. However, where R= -1.5, the acceleration of crack growth was observed after overload in both types of specimens. Even though crack growth was accelerated or decelerated, the rate was able to be evaluated by effective stress intensity factor range. The effective stress intensity factor range of the center cracked specimen could also be applied to that of the notched specimen with ρ=0.1 mm and 1mm.

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This study investigates the thermoelectromechanical fracture behavior of a parallel crack in a piezoelectric strip under thermoelectric loading. The crack faces are supposed to be insulated thermally and electrically. By using the Fourier transform, the thermal and electromechanical problems are reduced to systems of singular integral equations, respectively. The singular integral equations and solved by using the Gauss-Jacobi integration formula. Numerical calculations are carried out, and the stress and electric displacement intensity factors are presented for various values of demensionless parameters representing the crack size, the crack location and the magnitude of the electric loading.

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The ultrasonic fatigue tests (test frequency : 20 kHz) and the conventional tension compression fatigue tests (10 Hz) have been conducted using the annealed and 10% pre-strained specimens of 0.13% carbon steel. Two small artificial holes (diameter and depth : 100μm) were introduced onto the specimen surface to invistigate the effect of test frequency on the crack initiation and growth behavior. In order to suppress the temperature rise of the ultrasonic fatigue specimens, the intermittent ultrasonic loading method and the air cooling were used. The dynamic stress concentration factor and the stress intensity factor under the ultrasonic fatigue tests were calculated. It has been revealed that the dynamic stress concentration factors for a small circular hole and a small spherical cavity and the dynamic stress intensity factor for a penny-shaped crack are almost the same as those at low strain rates from the conventional tension compression fatigue tests. On the other hand, the fatigue properties were dependent on the test frequency. The ultrasonic fatigue tests showed longer fatigue life and lower fatigue crack growth rate for the annealed and 10% pre-strained specimens. The evident slip bands were not observed in the neighborhood of the crack in the annealed specimen under the ultrasonic fatigue tests, while the slip bands were observed in the wide area around the crack under the conventional fatigue tests. Slip bands were observed slightly in the 10% pre-strained specimen under the ultrasonic fatigue tests. In order to determine the stress-strain curve for 0.13% carbon steel under the high strain rates, the Split Hopkinson Bar tests were carried out. The yield stress and deformation stress were increased with the strain rate. Thus, the effect of test frequency on fatigue strength can be explained from the viewpoint of the effect of test frequency on fatigue crack growth for 0.13% carbon steel.

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By applying strain at high temperature to polyurethane-shape memory polymer film, followed by holding the strain under various thermomechanical conditions, the behavior of strain recovery by heating after these processes was investigated. In the experiments, the influence of the strain-holding conditions on shape recovery and secondary-shape forming was discussed. The results obtained are summarized as follows. (1) If strain is held at high temperature for short time, irrecoverable strain starts to appear at the holding time t_h =0.5 h and strain is not recovered at all at t_h =8 h in the care of holding temperature T_h = T_g + 20 K. In the case of T_h = T_g + 10 K, irrecoverable strain appears for short holding time if initial strain is large and the rate of secondary-shape forming S is 42% at t_h =8 h. (2) If initial strain of 50% is held at low temperature for long time, strain becomes not to be recovered and S is 93% at t_h=12 h in the care of T_h = T_g + 10 K. In the care of T_h = T_g, the increasing rate of S increases if t_h becomes longer than 40 h. If T_h is lower than T_g - 10 K for t_h=72 h, strain is recovered perfectly by heating and secondary-shape forming does not appear.

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A method for identification of a crack on the surface of the material by means of direct-current electrical potential difference method of multiple-point measurement type was proposed and related experiments were carried out to verify the proposed method. The geometry of the crack was given by the two-dimensional location of the crack center, the surface and inward angles of the crack, the length and depth of the crack. The experiments were carried out for metal plates with various surface cracks made by electric discharge machining. When most of the area of potential disturbance by a crack was included in the measurement area, the geometry of the crack was successfully identified by the proposed method based on the potential differences measured at multiple locations around the crack.

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In the several engineering fields, biological, sports, aeronautical, aerospace engineering, and so on, measurements of pressure distribution applied on the surface of the structure are very important issues. In the previous paper and the present paper, identification of pressure distribution applied on the structure employing strain data measured on the area apart from contact zone was investigated. In the previous paper, the authors suggested the basic theory and numerical treatment for identification of the pressure based on inverse analysis, and examined validity of the method by a numerical demonstration for the estimation of foot pressure. In the present paper, an experimental verification of the identification technique suggested in the previous paper is demonstrated. Outline of the load cell to measure the pressure distribution based on the theory and actual process of the identification using this load cell is described. Furthermore, Tiknonov's method and truncation of singular value decomposition (TSVD) has been employed to stabilize the numerical solution obtained by inverse analysis, and the usefulness of the optimization techniques is demonstrated.

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Crack Energy Density (CED) is the parameter that enables a unified description of crack behavior without any restriction on constitutive equation and its applicability has been shown for ordinary materials. In this paper, the concept of CED is extended to piezoelectric body and its fundamental matters are studied. The definitions of CED and its mechanical and electrical parts are given first and their path independent expressions are derived through the energy conservation law. Subsequently, the loading path dependence of mechanical and electrical parts even for reversible process is made clear. Moreover, some quantities related to CED are defined and their path independent expressions are given.

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In this study, an eptitaxial fabrication technique of a newly designed biocompatible piezoelectric material MgSiO₃ is developed, which has a tetragonal perovskite lattice structure. This crystal structure was designed by using HSAB rule, the geometrical compatibility assessment, and the first principles DFT analyses. Here, the helicon wave plasma sputter (HWPS) deposition method is adapted to deposit an epitaxial thin film of MgSiO₃ tetragonal perovskite on a biocompatible substrate Ir/Ti/Si. Ir/Ti/Si substrate has better compatibility with MgSiO₃ (111) plane, because of its close lattice constant. An optimum condition of HWPS for depositing MgSiO₃ tetragonal perovskite was sought by using the design of experiments method. As the result, the target composition ratio and the surface temperature were found as the dominant fabrication factor, and precise optimum condition search was carried out. Finally, MgSiO₃ thin film was successfully fabricated and the piezoelectric and ferroelectric properties were measured. It shows a piezoelectric constant d₃₁ is -1.74×10^-12 m/V.

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