Umwandlungsverstärkte Keramische Werkstoffe

Transformation-Toughened Ceramics A summary is given on the toughening of ceramics by utilizing the tetragonal-to-monoclinic phase transformation of ZrO₂ particles dispersed in the ceramic matrix. Various parameters controlling this martensitic transformation (especially M_s) are discussed together with in situ-TEM observations. The possible toughening mechanisms that allow for additional energy dissipation at crack tips are stress-induced transformation and nucleation of microcracks. Further strengthening is achieved by introducing steady-state surface compressive stresses. The type and magnitude of toughening is controlled by the microstructural design parameters such as volume fraction, chemical composition, size and size distribution of ZrO₂ particles which again determine the M_s temperature. The homogeneous dispersion of ZrO₂ is the most important step in the technology of transformation-toughened ceramics. Experimental examples for the toughening effect are given for the ceramic matrices Al₂O₃, ZnO, spinel, mullite and Si₃N₄.     

Grain-Size Dependence of Fracture Energy in Ceramics

A simple model incorporating thermal elastic anisotropy stresses is used to calculate the microcrack zone size around cracks in Al₂O₃. It is found that the ratio of microcrack zone size to grain size is almost constant for notched beam tests, but increases with grain size for double cantilever beam data. It is suggested that notched-beam ratios of fracture toughness are related to crack initiation, whereas double cantilever beam values are related to propagation and reflect R-curve behavior of the material.     

Theoretical analyses of some simple wave power devices

The problem of how to extract energy from ocean waves is discussed by analysing theoretically the hydrodynamic properties of a few ideal devices, involving vertical energy absorbing plates. the aim is to find a mechanism with high ideal efficiency, but still simple enough, such that its realization in practice can be economically feasible. The most promising device involves one or more vertical elastic plates situated in the water perpendicular to the incoming waves. About a quarter of a wavelength after the (first) plate there is a stiff vertical construction extending sufficiently deep in the water such that it is rather immobile. It serves as a reflector of the waves. the elastic plate is set in oscillation by the wave motion and this motion is used to extract the energy by applying a braking force. the optimal force field is discussed in some detail, and it is shown that not much of the optimal efficiency is lost by letting the force field simply be proportional to the velocity. the device is also shown to be sensitive to a fairly broad spectrum of wavelengths, and this spectrum can be broadened by adding more swinging plates. In this way efficiencies of 80-100 per cent are attained in theory over a wavelength spectrum extending over an order of magnitude.     

On “type l” multivariable linear systems

The purpose of this paper is to summarize results for linear multivariable systems that are analogous to those single input single output systems which are “type l”, i.e. that contain l integrators. Both frequency domain and time-domain properties are given. Appropriate conditions that guarantee zero steady state error in tracking vector polynomial inputs are presented.     

Large-area Fabry-Perot modulator based on electro-optic polymers

We present a large-area electro-optic Fabry-Perot modulator utilizing a photoaddressable bis-azo polymer placed between two dielectric mirrors with an open aperture of 2 cm. A modulation efficiency of 1% at an effective modulation voltage of 20 V for a wavelength of 1.55 microm is demonstrated. By comparing distance tuning of the cavity with wavelength tuning, an effective electro-optic coefficient of -7 pm/V is measured.     

On the reliability of standard wave spectra in structural response analysis

The present study is intended to elucidate the reliability of the most common standard models of one-dimensional wave spectral densities applied in the design of fixed offshore structures. The results presented refer especially to flexible offshore platforms of the gravity type. The study is based on a comparative analysis of typical structural response quantities derived, by applying wave spectra obtained from wave measurements and standard wave spectra. The analysis includes1008 empirical wave spectra estimated from wave measurements at three different locations on the Norwegian Continental Shelf.     

Dynamic simulation of grain boundary migration

Dynamic simulation of grain boundary migration in two dimensions requires the combined movement of grain boundaries and three-junctions. In this work, a method of dynamic simulation is described where the migration is broken down into two steps: I. The grain boundaries migrate with frozen threejunctions. II. The three-junctions are released in order to obtain equilibrium. In principle, the development of any initial two-dimensional grain-structure can be studied. The CPU-time and memory requirements increase with increasing number of grains.