No. 3, 2007


Jan Siegl, Jan Bensch, Jan Cizek, Khiam Aik Khor : Influence of spraying conditions on fatigue behaviour of hydroxyapatite coatings, pp. 205-215

Plasma spray deposition of low carbon steel specimens was carried out under three different conditions (denoted HA-1, HA-2, HA-3). To determine the influence of hydroxyapatite (HA) coatings on fatigue lives of machined low carbon steel specimens, fatigue experiments using a symmetrical cyclical bending load were carried out. It was found out, all HA coatings led to prolongation of the fatigue lives as compared to specimens without coatings (by 43 %, 23 %, and 38 %, respectively). Subsequent fractographic analysis confirmed that the fracture micromorphology of individual HA coatings is influenced by different spraying conditions, and the fatigue cracks in coated bodies initiate underneath the coatings, i.e., on the steel substrate surface.

Frantisek Chvala : Estimates of the Boltzmann gain collision operators, pp. 217-224

Boundedness of the bilinear Boltzmann gain collision operators is established in weighted spaces of essentially bounded functions for collision models generalizing rigid spheres and a class of interaction potentials with angular cut-off.

Jan Spetlik, Josef Tlusty : Stability analysis of distributed generation systems, pp. 225-259

Today's trends are tending to environmental friendly sources utilization. This reality is affecting electrical energy transmission and distribution systems, means new technical solutions and brings new tasks to be solved. Many of new source types in power generation technologies need to be connected to distribution network, in which they are referred to as distributed generation.

The stability properties of a hypothetic distribution system with regard to growing distributed generation penetration level has been analyzed. Dynamic stability analysis is based on system eigenvalues evaluation in various operation states and gives results concerning system stability after next source connections. The transient stability on the other hand provides additional information of the system stability in the case of subjected to a reference fault.

Viktor Pekarek : Molar heat increment under impact of transverse contraction in metals, pp. 261-268

The increase of the molar heat capacity enforced through the interatomic transverse contraction inside expanding structure in metals is analysed using the methods of the classical statistics. Applying the Maxwell three-dimensional velocity distribution, the molar heat increments were found to alternate in the proximity of the gas constant R throughout the domain of metals. A considerable exception with approx. 0.1 R represents Beryllium thanks to its extremely low contraction coefficient s = 0.03.

Miros Pirner, Shota Urushadze : Pedestrian dynamics-footbridge loads, pp. 269-283

Experimental investigation of a dynamic pedestrian-structure interaction are of great importance in footbridge design because such bridges tend to be light and lively; the dynamics of these structures generally only affects the user's comfort.

The ITAM tested many footbridges and have revealed (as the earlier experimenters) that these lightweight constructions, particularly in the form of a freely suspended catenary, can be very sensitive to the human actions because their natural frequency is very low and very close to the resonance with pacing rate. Therefore we started the new series of experiments. All new experiments were designed for dynamic factor evaluating of one up to small group of goers with or without correlation of pacing rate.

Ayhan Albostan : Direct methods for transient stability studies in power system analysis, pp. 285-309

The Lyapunov direct method is used for the stability analysis of power systems. The transfer conductances are incorporated in the model.

The transient stability of a multimachine power system, which consists of ng synchronous generators and ny loads is investigated using the Lyapunov direct method. Two different state-space models, namely the ``equivalent admittance model'' and the ``structure preserving model'' are considered.

Although the structure preserving model has a higher dimension than the equivalent admittance model (2ng+ny-1 versus 2ng-1), it provides a more accurate representation of the system with transfer conductances incorporated in the model and results in estimating a large stability region. The paper also provides a concise review of Lyapunov's direct method application to transient stability problem, including a comparison of the two models.

Vlastimil Brozek, Pavel Ctibor, Jiri Matejicek, Pavel Rohan, Jan Janca : Consolidation of tungsten disilicide by plasma spraying, pp. 311-320

Wear-resistant and heat-resistant coatings as well as free-standing parts of tungsten disilicide were manufactured by plasma spraying. Tungsten disilicide powder was processed by water stabilized plasma, generated by IPP's WSP plasma torch. Oxidation of the molten silicide was prevented by an argon protective atmosphere and by ``auto-shrouding'' effect. Their mechanical and oxidation behavior was compared with samples prepared by powder metallurgy techniques, sintered in the ASEA apparatus and also in the HP-type apparatus. Heat-resistant coatings exhibited high resistance against oxidation similarly to MoSi2 produced by the same way. Tungsten disilicide feedstock was prepared by direct heating of the powder mixture of tungsten and silicon at 1350-1400 C and secondly by plasma spheroidization of granules (20-63 mm in size) of powder mixture of tungsten and silicon with 3 % of a polyvinylalcohol binder by the technique known as freeze granulation. Coarse-grained feedstock is necessary for plasma spraying by water-stabilized plasma gun WSPhaving at the exit temperatures about 28-30000 K. Analysis of both deposits and solidified free-flight particles determined the WSi2 and W5Si3 phases and their mechanical properties, especially HV, E-modulus, density and porosity. Plasma treatment also enabled obtaining spheroidal powder product with large content (over 30 %) of well-spheroidized particles, useful for e.g. high temperature filters production.