This paper presents the results of a conducted topology management optimization study based on the finite element analysis on a two-stage spur gear reducer housing body and cover using the SOLIDWORKS Simulation module. The main goal of the study is to optimize the overall weight of the reducer by thinning specific areas of the casted gearbox housing elements according to the calculated minimal strain energy. The topology optimization algorithm that is used in current research gives an optimal structural shape of the housing elements of the reducer with the largest stiffness, considering the given amount of mass that will be removed from the initial design space. The complete sequence of steps for conducting the topology management optimization study is shown, taking into account the constraints arising from the construction features and the method of manufacturing the housing elements of the gear reducer. Conclusions on the use of the topology optimization results are given and potential directions for further development of the approach are also identified.

The present work describes an experimental investigation of the fatigue durability of AISI 304 and AISI 316L austenitic stainless steels, which have regular reliefs (RR) of the IV-th type, formed by ball burnishing (BB) on flat surfaces, using a computer numerical control (CNC) milling center. The methodology and the equipment used for obtaining regular reliefs, along with a vibration-induced fatigue test setup, are presented and described. The results from the BB process and the fatigue life experiments of the tested austenitic stainless steels are gathered, using the approach of factorial design experiments. It was found that the presence of RR of the IV-th type do not worsen the fatigue strength of the studied steels. The Pareto, t-test and Bayesian rule techniques are used to determine the main effects and the interactions of significance between ball burnishing regime parameters. A stochastic model is derived and is used to find when the probability of obtaining the maximum fatigue life of parts made of AISI 304 or 316L reaches its maximum value. It was found that when the deforming force, the amplitude of the sinewaves and their wavenumber are set at high values, and the feed rate is set at its low value, the probability to reach maximum fatigue life for the parts made of AISI 304 or 316L is equal to 97%.

The present work focusses on the research of the plastic deformation accumulated effect obtained after two different plastic deformation treatments, over the fatigue life of AISI 304 austenitic stainless steel. The research is focused on ball burnishing as a finishing process to form specific, so-called “regular micro-reliefs” (RMRs) on a pre-rolled stainless-steel sheet. RMRs are formed using a CNC (Computerized Numerically Controlled) milling machine and toolpaths with the shortest unfolded length, generated by an improved algorithm, based on the Euclidean Distance calculation. The effect of the predominant tool trajectory direction during the ball burnishing process (which can be coinciding or transverse with the rolling direction), the magnitude of applied deforming force, and feed-rate is subjected to evaluation using Bayesian rule analyses of experimentally obtained results for the fatigue life of AISI 304 steel. The obtained results give us reason to conclude that the fatigue life of researched steel is increased when directions of pre-rolled plastic deformation and the tool movement during ball burnishing are coincident. It also been found that the magnitude of deforming force has a stronger impact over the fatigue life, than the feed-rate of the ball tool.

Microbiological activity in composting is a complex process that depends on many abiotic factors. The mathematical model presented in this article is based on collected empirical data, on the basis of which the problem of predicting the values of several independent variables is solved. A standard statistical procedure of correlation matrix calculation and regression analysis is used. Two compost variants, differing in the microbiological agent added in one of them, were set. The amount of microorganisms was higher in the variant with microbial addition, and in both variants the amount and activity of microorganisms decreased during the thermophilic phase of composting. Actinomycetes take the major part of the total microflora, followed by non-spore-forming bacteria, except in the thermophilic phase, where the proportion of spore-forming bacteria is higher and mould fungi completely die out. The results of the laboratory tests determining the microbiological activity and the results of the regression models for each stage of the two composting treatments under study were obtained. The presented mathematical model can be used to predict microbiological activity during composting under different abiotic conditions and to optimize the process.

Raw materials of plant origin can serve as an inexhaustible source of biologically active sub-stances, which in turn are used in various fields of medicine, especially in treatment of diseases that require an integrated approach. One of the contemporary trends is the search for herbal medicines that have a certain antimicrobial activity. The objects of our study were extracts and oils obtained from different plant fractions of oregano (roots, leaves, stems, whole plant) and using different solvents (water, ethanol, vinegar, wine, olive oil), grown in a vegetation cage and under greenhouse conditions. It was also determined that the antibacterial activity against Escherichia coli to the same plant fractions. The studied oregano extracts can be used for compost decontamination and other substrates in which Escherichia coli is found. For defining dependencies and trends among the experimental data were used mathematical statistics tools.

Common winter wheat (Triticum aestivum L.) makes up more than half of the cereals grown in the European Union and in Bulgaria. For two years (2021-2022) a field experiment was carried out at the experimental plain of the Department of Plant Production at the Technical University of Varna. Two varieties of winter common wheat were grown, a selection of the Dobrudja Agricultural Institute - General Toshevo. The trial pattern included a variant without fertilization (control - Fert 0), a variant with combined foliar feeding and soil fertilization (Fert SF) and a variant with soil fertilization alone (Fert S). The effect of the way of fertilizing on the productivity and quality of winter common wheat was investigated. Grain yield (GY) and its structural elements - spike length (LMS), number of grains per spike (NGS) and grain weight per spike (WGS), weight per 1000 grains (TGW) and hectoliter weight (TestW) were determined. Biochemical analysis of the wheat grain was performed and the content of protein (Pr), starch (St) and ash substances (Ash) was determined. Fertilization has a strong effect on productivity and grain quality in winter common wheat (both alone and in combination with growing conditions). The method of fertilization has an effect on the productivity of winter common wheat - feeding with foliar fertilizers during the spring vegetation (in the tillering and booting phases) increases grain yield as a result of increasing each of the productivity components. The method of fertilization does not have a significant impact on the quality indicators of winter common wheat, with the exception of the hectoliter mass, whose values are proven to be higher during soil nutrition. The two studied Bulgarian varieties, Sladuna and Nikodim, differ from each other in their efficiency in relation to the way of feeding in the spring.