Back stress in dislocation creep. Part 1: Basic concepts and measuring techniques

A theory is proposed whereby the plastic deformation of metal materials is determined by the difference between the applied stress and the back stress which characterizes the resistance of the material to plastic deformation. The back stress is usually equivalent to the internal stress or the friction stress and depends on the magnitude of the applied stress and temperature. The concept of back stress is applied to the case of the dislocation creep of precipitation-hardened or dispersion-strengthened metal materials. An additivity rule is formulated which can be useful in interpreting the creep behavior of such materials

Hedging Behaviour of Czech Exporting Firms

The hedging behaviour of Czech exporting firms is analysed using questionnaire information and interviews with banks. Approximately 60% of the 184 firms surveyed hedge their FX exposures, and about 88% of their exports are hedged. Most exporters use natural hedging, i.e. they balance incoming and outgoing payments in foreign currency as well as foreign currency assets and liabilities. Hedgers on financial markets prefer forwards and zero-cost option structures, as they are reluctant to pay option premiums. The typical maturity of financial instruments is three months to one year. More than one half of exporters hedge consistently, while around 60% hedge actively, taking advantage of currency moves. Our simple model of hedging behaviour for example suggests that trading within a group reduces the need for hedging.Exchange rate exposure, exchange rate risk, exports, hedging behaviour.

On Sp(2) and Sp(2)· Sp(1) -structures in 8-dimensional vector bundles

Let ξ be an oriented 8-dimensional vector bundle. We prove that the structure group SO(8) of ξ can be reduced to Sp(2) or Sp(2) · Sp(1) if and only if the vectorbundle associated to ξ via a certain outer automorphism of the group Spin(8) has 3 linearly independent sections or contains a 3-dimensional subbundle. Necessary and sufficient conditions for the existence of an Sp(2)-structure in ξ overa closed connected spin manifold of dimension 8 are also given in terms of characteristic classes

Radial profiles of temperature and viscosity in the Earth's mantle inferred from the geoid and lateral seismic structure

In the framework of dynamical modelling of the geoid, we have estimated basic features of the radial profile of temperature in the mantle. The applied parameterization of the geotherm directly characterizes thermal boundary layers and values of the thermal gradient in the upper and lower mantle. In the inverse modelling scheme these parameters are related to the observables (geoid and seismic structure of the mantle) through the viscosity profile which is parameterized as an exponential function of pressure and temperature. We have tested 104 model geotherms. For each of them we have found proper rheological parameters by fitting the geoid with the aid of a genetic algorithm. The geotherms which best fit the geoid show a significant increase of temperature (600-800ºC) close to the 660-km discontinuity. The value of the thermal gradient in the mid-mantle is found to be sub-adiabatic. Both a narrow thermal core-mantle boundary layer and a broad region with a superadiabatic regime can produce a satisfactory fit of the geoid. The corresponding viscosity profiles show similarities to previously presented models, in particular in the viscosity maximum occurring in the deep lower mantle. The best-fitting model predicts the values of activation volume V and energy E which are in a good agreement with the data from mineral physics, except for V in the lower mantle which is found somewhat lower than the estimate based on melting temperature analysis. An interesting feature of the viscosity profiles is a local decrease of viscosity somewhere between 500 and 1000 km depth which results from the steep increase of temperature in the vicinity of the 660-km discontinuity

Guest Artist Recital: Jerrie Cadek Lucktenberg, Violin; George Lucktenberg, Fortepiano/Harpsichord; March 25, 1976

Center for the Visual Arts110 GalleryThursday EveningMarch 25, 19768:15 p.m