Narrow Results

Nanoindentation is an (almost) non-invasive method for obtaining material properties of different types of materials from the interpretation of experimental data related to indenter load (P) and penetration depth (h). In most cases, the material properties that are obtained by nanoindentation are elastic modulus (E), shear modulus (G) and hardness (H). The main advantages of this method are that no extensive preparation of the test specimen is required and that the mechanical properties can be probed at small scales. Moreover, nanoindentation test procedure is automated and the test equipment is easy to use. In this paper, we review different analytical methods that could be used for obtaining the mechanical properties of biomaterials based on the force-displacement curves generated by nanoindentation machines. Some practical issues including different types of machines and tips, calibration of nano-indentation machines, sources of error and specimen preparation are also briefly discussed. The main interest of this paper is the elastic behavior of biological tissues and biomaterials. Nevertheless, there is one section on elasto-plasticity, because purely elastic deformation of linearly elastic materials is difficult to achieve. The analytical solutions found in the literature for different material models are presented including the relationships found for linear elastic, elasto-plastic, hyperelastic, viscoelastic and poroelastic materials. These material models are relevant material models for studies of biological tissues and biomaterials.

This paper uses estimates of total factor productivity of small enterprises to identify the reasons underlying idiosyncratic variation. Empirical analysis is used to segregate internal and external determinants of productivity using a novel dataset. For reliable estimation, the baseline estimates are corrected for simultaneity bias using instrumental variables and selectivity bias through Heckman correction. Results identify significance of factors operating within firms; educational qualification and professional training of entrepreneurs for higher levels of productivity and the external drivers of productivity differences; sources of energy, selective access to credit and agglomeration economies. The research has important implications for entrepreneurs and policy intervention.

When the size of structures approaches to the sub-micron scale, physical responses of such systems become size-dependent, hence, classic theories may not be able to predict the behavior of the miniature structures. In the present article, the modified couple stress theory (MCST) is employed to account for the effect of the size-dependency on the dynamic instability of torsional nano-electromechanical systems (NEMS) varactor. By incorporating the Coulomb, Casimir and damping forces, the dimensionless governing equations are derived. The influences of Casimir force, applied voltage and length scale parameter on the dynamic behavior and stability of fixed points are investigated by plotting the phase portrait and bifurcation diagrams. It is found that the Casimir force reduces the instability threshold of the systems and the small-scale parameter enhances the torsional stability. The pull-in instability phenomenon shows the saddle-node bifurcation for torsional nano-varactor.

By now there doesn't exist any exact algorithm for the thermoacoustic tomography (TAT) in the limited-view case. Here an approximate algorithm, called the deconvolution reconstruction (DR), is proposed for the limited-view TAT. In this algorithm, a new function is firstly constructed from detected thermoacoustic signals. Then the electromagnetic absorption distribution of the object is reconstructed from this function based on the deconvolution method. The discrete deconvolution is the key step of this algorithm and is solvable either in the full-view or in the limited-view. With simulation studies, this algorithm is proved to have a good imaging precision for small-scale objects even in the limited-view case. It is shown that detected signals over 90 degrees in the two-dimensional case are enough to reconstruct the image of the scanned object. Simulation results of the DR algorithm are also compared with those of two popular algorithms: the time-domain reconstruction (TDR) and the filtered back projection (FBP). It is also shown that results of the DR algorithm are much better than those of the TDR and the FBP for small-scale objects in the limited-view case. Therefore, the DR algorithm is an efficient one for the limited-view TAT of small-scale objects.

Two types of in-situ triaxial testing apparatus to measure stress-strain relationship of rock mass (i.e. large-scale apparatus and small-scale apparatus) are introduced. The large-scale apparatus comprises of inner and outer cells to apply confining pressure to the specimen, axial loading system which is generally installed in the ground surface, special instrumentation devices and data recording unit. The small-scale apparatus consists of a triaxial cell to measure deformations of the specimen under confining pressures and axial load and a separate axial loading system, which enables to apply axial load to the specimen in any depths. Axial and radial deformations of the specimen can be measured inside the triaxial cell and recorded with a data recording unit. For drilling the borehole, preparation of the specimen and performing the in-situ triaxial test, a special procedure is introduced. Some typical results of proof tests were presented. The results, which were similar to conventional laboratory triaxial tests, showed that the proposed test method was successful to measure average stress-strain relationships of a rock mass.