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Structural analysis of a scissor structure

Ario I., Yamashita T., Chikahiro Y., Nakazawa M., Fedor K., Graczykowski C., Pawłowski P.

Bulletin of the Polish Academy of Sciences. Technical Sciences

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2020

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Vol. 68, nr 6

1319--1332

EN

This paper presents equilibrium mechanics and a finite element model for analysing a scissor structure that contains pivots with zero bending stiffness representing structural instability. The pivot at the centre of each structural unit, which is a feature of scissor structures, can be used to transfer the displacement between the units. It cannot, however, transfer the rotation between these units, and the angular stiffness must be considered independently for each unit. To construct a general model of the scissor structure, a scissor unit was developed using the left and right boundary connections of adjacent units to simulate a periodically symmetric structure. The proposed method allows us to obtain an accurate distribution of the internal forces and deflections without the use of special elements to account for central pivots.

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Optimization of differentiation time of mesenchymal-stem-cell to tenocyte under a cyclic stretching with a microgrooved culture membrane and selected measurement cells

Morita Y., Yamashita T., Toku Y., Yu Y.

Acta of Bioengineering and Biomechanics

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2018

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Vol. 20, no. 1

3--10

EN

There is a need for efficient stem cell-to-tenocyte differentiation techniques for tendon tissue engineering. More than 1 week is required for tenogenic differentiation with chemical stimuli, including co-culturing. Research has begun to examine the utility of mechanical stimuli, which reduces the differentiation time to several days. However, the precise length of time required to differentiate human bone marrow-derived mesenchymal stem cells (hBMSCs) into tenocytes has not been clarified. Understanding the precise time required is important for future tissue engineering projects. Therefore, in this study, a method was developed to more precisely determine the length of time required to differentiate hBMSCs into tenocytes with cyclic stretching stimulus. Methods: First, it had to be determined how stretching stimulation affected the cells. Microgrooved culture membranes were used to suppress cell orientation behavior. Then, only cells oriented parallel to the microgrooves were selected and evaluated for protein synthesis levels for differentiation. Results: The results revealed that growing cells on the microgrooved membrane and selecting optimally-oriented cells for measurement improved the accuracy of the differentiation evaluation, and that hBMSCs differentiated into tenocytes in approximately 10 h. Conclusions: The differentiation time corresponded to the time required for cellular cytoskeleton reorganization and cellular morphology alterations. This suggests that cells, when subjected to mechanical stimulus, secrete mRNAs and proteins for both cytoskeleton reorganization and differentiation.

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Constrains on stresses in isotropic homogeneous infinite half-spaces being in welded contact: 2D anti-plane and in-plane cases

Rybicki K. R., Yamashita T.

Acta Geophysica

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2008

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Vol. 56, no. 2

286-292

EN

Formulas are derived for two-dimensional problems relating stresses across a plane boundary that divides infinite homogeneous half-spaces being in welded con-tact. The calculations are made for both anti-plane and in-plane stress cases. The results obtained for the former case that involve only two stress components are useful in the analysis of fracture of strike-slip type. For the in-plane case, the relations that link stresses in one half-space with the corresponding homogeneous stresses in the other half-space are presented for arbitrarily oriented shear and normal stresses and for the center of compression (dilatation). The above relations provide a compete set of expressions that, among other things, make it possible to analyze stresses involved in faulting of deep-slip type in an inhomogeneous medium. The quantitative preliminary evaluations based on the results obtained demonstrate the great role of low rigidity media in fracture processes of all kinds within the Earth's crust.

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Stress and dislocation field evolution and prediction problems of numerically simulated events

Teisseyre R., Yamashita T.

Acta Geophysica Polonica

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2000

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Vol. 48, nr 1

43-56

EN

Numerical simulations of the dislocation density evolution exhibit some perturbations which may be identified as seismic events. The influences of tidal stresses and random disturbances related to the material heterogeneities are analyzed. In relation to this analysis some problems of earthquake prediction and earthquake precursors are discussed. Occurrences of seismic events are very sensitive to small disturbances, while the great disturbances in tidal influence or in material properties cause the disappearance of the events.

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Splitting stress motion equation into seismic wave and fault-related fields

Teisseyre R., Yamashita T.

Acta Geophysica Polonica

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1999

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Vol. 47, nr 2

135-137

EN

Considering elastic continuum with defects (strictly speaking: continuum with a continuous distribution of the self-strain nuclei, like in thermo-elastic or elasto-plastic bodies), we have to consider the elastic and self parts of the total strain and stress fields. Accordingly, we can split the motion equations (as expressed for stresses) into a wave field and a fault-related field, assuming that the self stresses play a decisive role on a fault. We show that the fault constitutive equation is a special case of the motion equation. The fault-related equation can be transformed into equation for dislocation density, identical with the 1D equation for evolution of dislocation field equivalent to the commonly considered fault slip constitutive equation. We discuss the fault equation as expressed in terms of dislocation field and we consider the static and dynamic approximations. Static approximation relates to the early phases: the infaltion phase from dislocation stress resistance to friction (phase characterized by an exponential decrease of stress resistance which corresponds to dislocation velocity increase in the expotential creep) and the fracture slip nucleation phase (governedby slip weakening law). Dynamic approximation relates to the slip propagation phase (governed by slipp velocity weakening law). Instead of the instability source introduced by the friction weakening laws for all these phases, we might introduce the source/sink function; such a function should first of all describe the coalescence processes between the dislocations of opposite sings (such an elementary process preserves material continuity) and also between the dislocation arrays of opposite sings (such a coalescence of the arrays can well describe a nucleation process of a crack-material fracturing, or/and a coalescence of two cracs).When the put this function proportional to a stress surface curvature, or equivalently a gradient of dislocation density on a fault, it will represent an inverse of a mean distance between the opposite dislocation groups; we image, in this way, and ability to create a coalescence process - and instability. Atraction forces between the opposite dislocations can effectively produce a weaking effect, making it easier for a dislocation (or crack) to propagate. Some exaples of the numerical solutions for stresses on a fault with the spontaneously simulated seismic events are discussed. From the fault solution we obtain an evolution pattern of a dislocation field; the respective values on a fault plane may then serve as the boundary conditions to slove the wave part equation for stresses in the 2D space and time domain.