In this study the discrete element method (DEM) is developed in the framework of the constitutive law of elastic isotropic materials for two-dimensional plane stress analysis of solids. Contact stiffnesses (normal and tangential spring constants) are theoretically derived for hexagonal elements in an arbitrary arrangement as a function of thickness and material parameters including Young's modulus and Poisson's ratio. Moreover, a new method is presented to calculate stress components within an element. To validate the accuracy and efficiency of our discrete model several test problems are given. At first, uniaxial tension test using a bar is performed and the convergence of the solution to a definite value in the limit of mesh refinement is validated. In addition, two examples of stress wave propagation problems are given. Compressive wave speed is calculated and through comparing the numerical results with the other discrete element models and also analytic solution, the accuracy of the present DEM model is then discussed.

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The guided wave effect resembling that of annual rings found in woods and the cortical region of bones is believed to be observable in vascular tissues of herbaceous plants. The properties of acoustic waves traveling through the vascular tissue in the stem of a miniature tomato were measured using a piezoelectric pulser and receiver. The thickness of the vascular tissues and the stem's water content were measured. The detected acoustic waves showed a guided wave effect. The apparent sound velocity, v_a, was related to the vascular tissue thickness, t_v. These results reveal that the detected acoustic waves traveled along the vascular tissues in stems. The maximum peak intensity of the detected acoustic waves, I_max was also related to t. Furthermore, wilting of the examined plants decreased the I_max, although v_a was not changed. The decrease in I_max might result from cavitations and embolisms with a subsequent increase in air pores in xylem tissues. These results demonstrate that the measurement of acoustic waves traveling through vascular tissue is a useful tool for the non-destructive evaluation of vascular tissue development and embolism density in xylem tissues.

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This paper describes motion of 748 grains during superplastic deformation of TZP (Tetragonal Zirconia Polycrystal) ceramics. In the same way as the latest paper of the present authors, the same TZP specimen was 4 times further deformed at 1673K (=1400°C) in air under tensile loading. The increment of true plastic strain was set to be about 2%, and the final true plastic strain reached 25.3%. After each deformation, a position vector of the same grain was read out from a FE-SEM photograph, and thereby its displacement vector was determined. By changing the increment of true plastic strain in calculation, various displacement vector maps were obtained in consideration of the previous data also. At an increment of about 25% of true plastic strain; most of the grains seem to move along stationary streamlines similar to laminar flow. When we look at the grain motion at increments of about 5%, several tens of grains move to the same direction instantaneously, and therefore a kind of domain is formed at the first loading. In a subsequent deformation, however, the domains gradually disappear and each grain seems to move randomly, indicating that grain motion becomes spatially uniform. At increments of about 2%, convergence and divergence in grain motion gradually diminish with increasing the true plastic strain and this tendency corresponds to the observation at increments of about 5%, although each grain moves in a zigzag way gradually turning to the loading direction. From these results, the grain motion has a similarity to diffusion phenomenon because the observed time period makes a profound effect on the motion. As for time evolution of grain structure, by the first 15% of true plastic strain, local deformation resistance (viz. constraint by neighbor grains) is gradually homogenized, and consequently spatially uniform grain motion is attained leading to superplasticity in TZP ceramics.

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The high temperature strengths of the dissimilar friction welded superalloys joint between the cast polycrystalline Mar-M247 and the forged IN718 alloys have been investigated under low cycle and thermo-mechanical fatigue loadings, in comparison with those of the base metals. The experiments showed that the lives of the dissimilar joints were significantly influenced by the test conditions and loading modes. Not only the lives themselves but also the failure positions and mechanisms were sensitive to the loading mode. The fracture behaviors depending on the loading modes and test conditions were discussed, based on the macroscopic elastic follow-up mechanism and the microstructural inhomogeneity in the friction weld joint.

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This study focuses on understanding the stiffness and damping characteristics of carbon nanocoil (CNC)/polymer composites. To determine the effective elastic and viscoelastic properties of a polymer reinforced with aligned CNCs, the finite element analysis was performed using a three-dimensional unit cell model. The coil morphology of CNCs was examined using a scanning electron microscope. The effects of coil diameter, tube diameter, coil pitch and number of coil turns on the effective properties are discussed. The effective properties of a polymer reinforced with randomly oriented CNCs were also predicted by the results for the unit cell model. The tradeoffs between stiffness and damping characteristics are assessed by changing the volume fraction and geometrical parameters of CNCs. Additionally, experimental measurements of the elastic modulus and the loss factor of the manufactured CNC/polymer composites were carried out by using dynamic mechanical analysis. The predictions were then compared with the experimental results. Furthermore, scanning electron microscopy was performed to observe the fracture surface of CNC/polymer composites.

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The purpose of this paper is to develop a non-destructive test method for evaluating the bond of reinforcing bars in concrete structure which was damaged by earthquake while still cast in the form. In the experimental design, the specimens containing a steel reinforcing bar with one end extruded outside were constructed. Different degrees of damage on the rebar-concrete interfaces were successfully created by the resonant vibration of exposed steel bar induced by the cyclic motion of the specimen. The local bond-loss of the steel bars was evaluated by both non-destructive impact-echo tests and the destructive pullout tests. To create different kinds of failure mode in the pullout test, some of the specimens contain stirrups surrounding reinforcing bar. Poor-bond was quantitatively evaluated before the pullout test by the amplitude of the peak corresponding to the multiple reflections from the steel bar in the normalized spectra derived from the impact-echo tests. As a result, two empirical formulas displaying the proportional relations between the percentage of loss of local bond-stress and loss of steel-amplitude for specimens failed by split failure and pull-out failure modes were established.

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Using a fatigue testing method by which fatigue cracks can be initiated and propagated in a film adhered to cover an elliptical through-hole in a base plate subjected to push-pull cyclic loads, annealed pure titanium films with the thickness of 90µm were fatigued under a constant stress amplitude with a stress ratio of -1. The crystal rotation behavior with the tensile/compressive loading at the fatigue crack tip was evaluated from the crystal orientation measured by EBSD (Electron Back-scatter Diffraction) method. The titanium films under the tensile /compressive stress condition were prepared as follows; the films were adhered to the base plates loaded by a servo hydraulic testing machine, then the base plates were detached from the testing machine. From the crystal rotation behavior at the fatigue crack tip, it was found that the crystal rotation angle was relatively small and the rotation axis tended to different directions between the upper side and lower side of the grain in front of the crack tip. The fatigue crack propagated along the borderline between two regions in the crystal grain where the crystal rotation axes with different directions were obtained before the fatigue crack propagation.

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The impact damage threshold to cause internal damage in cross-ply CFRP laminates has been determined, by monitoring dynamic surface strain and Lamb-wave AE (Acoustic Emission) along with the impact force history in SACMA-type impact tests. Four types of cross-ply CFRP plates ([0^o _n /90^o _n]_sym., n=4,6,8,10) are impacted with a spherical steel tup, 16 mm diameter, at 0.8 and 1 m/s. Surface strain of a test laminate is measured using a strain gauge and Lamb-wave AE signals are monitored by small AE sensors on both surfaces. The time history of surface strain matches with the impact force history when no damage occurs as predicted by theory of impact dynamics. Here, only impact-induced AE (or Impact AE) is obtained when the impact tup contacts the specimen. When internal fracture occurs, measured strain history deviated from that expected from the dynamic force and both Impact AE and fracture-induced AE (Fracture AE) are detected. Fracture AE is detected when the dynamic force and strain history indicates the initiation of internal CFRP damage. Impact damage threshold values for the four types of cross-ply CFRPs are measured by comparing the impact force, strain and Fracture AE. Initial AE signals monitored simultaneously on both sample surfaces are symmetric Lamb waves. Wave simulation analysis has revealed the nature of the onset of fracture to be the matrix cracking of the center plies. Complex AE signals following the initial fracture for a few ms are expected to be from delamination damage, which was confirmed with an ultrasonic C-scan study.

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Laser cutting is one of methods for breaking a brittle material by using local thermal stress due to laser irradiation without melting and vaporization of the material. In this study, a method for controlling crack nucleation and propagation behavior was studied experimentally as well as numerically. In case of a specimen with a starter notch, crack propagated by following a laser spot. However, crack did not follow the laser spot trace when the laser scanning direction changed. It was found from the result of FEM analysis that crack propagation behavior was controlled by a stress intensity factor for the maximum tangential stress, K_θmax ahead of crack tip. Twin beam is considered as an effective method to control crack propagation direction for the laser cutting. Crack nucleation behavior was studied with a glass specimen without a starter notch. A crack could nucleate from an edge for staring of laser irradiation in case of the specimen with defects induced by polishing with abrasive papers. However, crack nucleation and propagation behavior was unstable in case of the specimen with mirror-like smooth surface. Effect of laser spot radius on crack nucleation behavior was also studied by FEM analysis.

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The plastic behaviour in the crack tip region has a strong influence on the fatigue life of engineering components. In general, residual stresses developed as a consequence of the plasticity being constrained around the crack tip have a significant role on both the direction of crack propagation and the propagation rate. Finite element methods (FEM) are commonly employed in order to model plasticity. However, if millions of cycles need to be modelled to predict the fatigue behaviour of a component, the method becomes computationally too expensive. By employing a multiscale approach, very precise analyses computed by FEM can be brought to a global scale. The data generated using the FEM enables us to identify a global cyclic elastic-plastic model for the crack tip region. Once this model is identified, it can be employed directly, with no need of additional FEM computations, resulting in fast computations. This is done by partitioning local displacement fields computed by FEM into intensity factors (global data) and spatial fields. A Karhunen-Loeve algorithm developed for image processing was employed for this purpose. In addition, the partitioning is done such as to distinguish into elastic and plastic components. Each of them is further divided into opening mode and shear mode parts. The plastic flow direction was determined with the above approach on a centre cracked panel subjected to a wide range of mixed-mode loading conditions. It was found to agree well with the maximum tangential stress criterion developed by Erdogan and Sih, provided that the loading direction is corrected for residual stresses. In this approach, residual stresses are measured at the global scale through internal intensity factors.

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