The present work focuses on the study of the coupling behavior between electrical and mechanical fields around sharp notches in piezoelectric materials. In this report, the singularities of the electro-mechanical field at a notch tip ae obtained analytically with the aid of eigenfunction method, and it is shown that four singularities exist at a notch chip generally.

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This work focuses on the study of the coupling behavior between electrical and mechanical fields around sharp notches in piezoelectric materials. The theoretical basis of the singularities of the electro-mechanical field at a notch tip were shown with the aid of eigenfunction method in the former report. The numerical analysis corresponding to the piezoelectric medium with sharp notch is carried out and the singularities around the notch-tip are evaluated. The results are compared with the theoretical ones.

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The behavior of the interface and the lateral to longitudinal strain ratio due to the stress-induced martensitic transformation (SIMT) were investigated in the regions of superelasticity (SE) and shape memory effect (SME) in a TiNi shape-memory alloy wire through the tensile test. The interface between a parent phase and a martensitic phase which appears due to SIMT during the loading process and its reverse transformation during unloading starts at both ends in the gripping parts of a wire and propagates towards the central part. When the two interfaces meet, SIMT completes.

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The thermomechanical properties of polyurethane-shape memory polymer foam were investigated. The main results are summarized as follows. (1) Strain is recovered at temperatures around the glass transition temperature during heating. The temperature at which strain is recovered decreases with an increase in maximum strain. (2) The rate of strain recovery is 99% and does not change under cyclic deformation. (3) The rate of strain fixity is 100%. (4) Stress diminished during cooling. Stress decreases markedly at temperatures below the glass transition temperature. (5) Because large deformation and large variation in volume are obtained in shape-memory polymer foam, shape-memory polymer foam has excellent functional performance for large elements of mechanics and structures.

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A shape-memory polymer of polyurethane series can be designed for its glass-transition temperature T_g, beyond which a remarkable change in stiffness can occur, within the range from -30℃ to 70℃. A material containing such shape-memory polymer particles, hence, may be also designed for its macroscopic stiffness by adjusting the T_g distribution of particles. If it is so, this composite material then can be regarded as an intelligent material to macroscopic stiffness. In order to examine the possibility, micromechanical analysis on the macroscopic stiffness of such an intelligent material will be performed by taking into consideration of the T_g distribution of particles. Effects of temperature on the macroscopic stiffness of the material will be calculated with respect to various T_g distributions of particles. As a consequence, we can see that the macroscopic stiffness of the material is closely related with the T_g distribution of particles and, especially for the case of bimodal type of distribution, it decreases as a flight of three stairs with increase in temperature.

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A constitutive model to describe the rate-dependent behavior of unidirectional fiber-reinforced composites is developed from phenomenological and continuum mechanics points of view. A particular emphasis is placed on a unified modeling of elastic, viscoelastic and viscoplastic deformation. Composites are assumed to be homogeneous transversely isotropic media. The constitutive equations are systematically derived from thermodynamic potentials, i.e., free energy and dissipation energy functions. A capability to describe the viscoelastic nature is incorporated into the present model through the Ziegler's thermodynamic reconstruction of the classical rheology models. The present model enables us to describe the rate-dependence of the initial tangent modulus and of the flow stress in the subsequent nonlinear regime. The present model may be interpreted as an extension of the Gates-Sun model for the elastic and viscoplastic behavior of unidirectional composites.

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A methodology to constitute a finite element model of a wove fabric composite is investigated on the basis of its X-ray CT images. The composite is discretized into fixed grid/mesh elements. Assuming the material property of the element is to be related with average pixel value of the area corresponding with the element in the X-ray CT image, we derive a formulation of the inverse problem to identify Young's modulus of the element. The relationship between the pixel value and Young's modulus is described by means of the sigmoid function, the parameters of which are determined so as to make the static behavior analyzed by the finite element model identical with that observed by experiments. A numerical example follows the formulation to prompt readers' understanding.

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Short-term stress relaxation behavior of angle-plied T800H/Epoxy [±θ] has been studied at high temperature. Stress relaxation tests were performed at 100℃ using plain coupon specimens with various fiber orientation angles : θ=0,30,45,60,and 90°. For each fiber orientation angle, the stress relaxation tests were carried out at three different total strain levels. The stress relaxation phenomenon was clearly observed for all kinds of angle-plied laminate, including θ=0°. Just after the total strain was kept constant, the stress rapidly relaxed with time in a short period. Then, the relaxation rate of the composite stress apparently vanished. The extent of the transient stress relaxation was significantly dependent on the total strain kept constant and the fiber orientation angle. Normalized relaxation curves eventually fell on a single relationship.

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Interlaminar fracture behavior of hybrid composites with non-woven carbon tissue has been investigated under mode I (DCB) and mode II (ENF) loading condition. Hybrid composites are made by inserting non-woven tissue between prepreg layers. Two kinds of specimens were prepared from [0]_<24> and [0_<12>/0_<12>]. Where, the symbol "/" means that non-woven carbon tissue is located at 0/0 mid-plane of the specimen. The initial interlaminar crack was located to mid-plane of the specimen. Interlaminar fracture toughness (G_<IC>, G_<IIC>) of hybrid composites is compared with those of CFRP. The fracture surfaces of the specimens were observed by SEM, and the fracture mechanism was discussed. The unidirectional laminates, which are made by inserting non-woven carbon tissue between layers, seem to be effective to improve the interlaminar fracture toughness.

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Structural health monitoring is a noticeable technology for aged civil structures. The present paper proposes a new diagnostic tool for the structural health monitoring that employs a statistical diagnosis of self-learning method. Most of the structural health monitoring systems adopt parametric method based on modeling or non-parametric method such as artificial neural networks. The new statistic diagnosis method does not require the complicated modeling and a large number of data for the training of the artificial neural networks. The present study deals monitoring of delamination of composite beam using change of strain judged by the statistical tools such as Response Surface and F-Statistics. The new method successfully diagnoses the damage without using modeling and a large number of data for training.

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The influences of stress waveform and water absorption on the tension-tension fatigue fracture behavior were investigated using aramid fiber/epoxy matrix unidirectional composite (Kevlar49/#2500). The fatigue tests were performed for specimens preconditioned in dry air (dry specimen) and the ones that were immersed in deionized water at 80℃ (wet specimen). Negative and positive pulse waveforms were used for fatigue tests. For dry specimens, the fatigue strength under negative pulse waveform was lower than that under positive pulse waveform. At a high stress level, the fatigue strength under positive pulse waveform became hardly independent of applied stress, and therefore the S-N curve became flat. The fatigue strength of wet specimens was lower than that of dry specimens at high stress level. The fracture surface was closely examined using a scanning electron microscope (SEM).

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We investigate the mechanical behavior of short carbon fiber reinforced thermoplastic composites in high vacuum and temperature condition. The material used in this study is short carbon fiber reinforced thermoplastic polyimide composites (SCFR-PI) with various fiber contents. Tension tests are carried out with the aid of AE method. From the experimental results, the SCFR-PI composite at temperature 298K in high vacuum condition has higher tensile strength and higher equivalent Young's modulus than than in the room condition. Furthermore, it is seen that the fracture surface of the specimen in high vacuum conditions is not flat. The strength of interfacial bonding between carbon fibers and the thermoplastic polyimide resin matrix becomes better in high vacuum conditions. It is found that there exist the degas and/or moisture effects on the strength of SCFR-PI composites in the high vacuum condition and their microfractures.

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