University of Niš: Facta Universitatis (E-Journals) / Универзитет у Нишу
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DYNAMIC STABILITY OF BEAMS ON PASTERNAK FOUNDATION UNDER TIME-VARYING AXIAL LOADS
Full text linkThis paper investigates the dynamic stability of structures subjected to periodic loads, modeled as beams on a Pasternak foundation experiencing time-varying compressive forces. The stability analysis is conducted using the Euler-Bernoulli beam theory, the Mathieu-Hill equations, and the Floquet theory. The results indicate that variations in the foundation’s stiffness and shear modulus significantly influence stability regions, especially at higher frequencies. Stiffness has a more pronounced effect, reducing the unstable region, while changes in both parameters affect the minimum excitation intensity required to induce instability. These findings highlight complex interactions between stiffness and shear properties, suggesting the need for further investigation
POROSITY EFFECTS ON NONLINEAR STATIC PERFORMANCES OF FUNCTIONALLY GRADED SHELLS CONSIDERING THICKNESS STRETCHING
Full text linkThis study delves into the intriguing realm of nonlinear responses exhibited by porous functionally graded (FG) 3D shell structure. A power law approach is formulated to simulate the mechanical behavior of FG materials, considering two distinct porosity distributions. This approach provides a comprehensive exploration of porosity phenomena in FG materials. The finite element (FE) formulation is based on an improved first-order shear deformation theory (FSDT) with the inclusion of thickness stretching parameters. The theoretical framework improved in this manner provides a more accurate representation of the transverse shear stress distribution within the structure, capturing the complexities of its behavior under loading conditions. This research enriches understanding of FG materials by integrating porosity that may be distributed evenly or unevenly, thus contributing to the advancements in the field
MAKING MORE INFORMED DECISIONS IN MULTI-CRITERIA PROBLEMS UNDER UNCERTAIN CONDITIONS OF CRITERIA WEIGHTS: DRONE SELECTION STUDY CASE
Full text linkThe growing demand for robust Decision Support Systems (DSS) highlights the need for methods that enhance decision-making under uncertainty, particularly when criteria weights in multi-criteria evaluations are unknown. This study presents a novel approach for addressing Multi-Criteria Decision-Making (MCDM) problems where precise criteria weights are uncertain, focusing on drone selection for challenging, remote locations. Using the MultiAttributive Ideal-Real Comparative Analysis (MAIRCA) method, the research explores a range of criteria weight scenarios with various distributions, enabling a more comprehensive evaluation. A key innovation is the integration of fuzzy ranking techniques to aggregate results from multiple assessments, enhancing the robustness of decision outcomes. This approach overcomes the limitations of traditional ranking aggregation methods, providing a more reliable understanding of the stability and reliability of recommendations. By offering a more adaptive and uncertainty-resilient framework, this study advances multi-criteria decision analysis and improves reliability of recommendations provided by decision support systems in problems in which criteria weights remain unknown. Furthermore, it equips decision-makers with deeper insights into the reliability of recommendations, empowering more confident decision-making in complex, uncertain environments
SRPSKA VERZIJA REVIDIRANE SKALE ZAVISNOSTI OD VEŽBANJA: PSIHOMETRIJSKA ANALIZA
No full textThe aim of the study was to evaluate the factorial validity and reliability of the Serbian version of the Exercise Dependence Scale-Revised (EDS-R). The sample of participants consisted of 222 students from the Faculty of Sport and Physical Education, including 164 male participants and 58 female participants. All participants were over 18 years of age and reported engaging in exercise with a frequency ranging from 3 to 7 sessions per week. The main statistical analysis for this study was conducted using confirmatory factor analysis (CFA). The results indicate good overall reliability of the complete inventory (α > .82), while the reliability of individual subscales ranges from .56 to .84. The results of the confirmatory factor analysis demonstrate that the model nearly aligns with the established fit indices (χ² = 600.299; df = 168; p .82), dok se pouzdanost pojedinačnih podskala kreće od .56 do .84. Rezultati CFA pokazuju da se model približno uklapa u utvrđene indekse dobrog uklapanja (χ² = 600.299; df = 168; p < 0.001; CFI = .891; NFI = .935; RMSEA = .093). Sedam faktora ukupno objašnjava 73,1% varijanse, pri čemu prvi faktor objašnjava 16,4%, drugi 12,7%, treći 11,1%, četvrti 10,9%, peti 8,3%, šesti 8,0%, a sedmi faktor 5,7% varijanse. Srpska verzija Revidirane skale zavisnosti od vežbanja pokazuje solidna psihometrijska svojstva, sa prihvatljivim indeksima uklapanja koji ukazuju na njenu relevantnost za procenu zavisnosti od vežbanja u ovoj populaciji
TECHNO-COMMERCIAL VIABILITY OF HYBRID SOLAR-BATTERY PROJECTS FOR DISTRIBUTED GRIDS WITH TIME-OF-DAY TARIFFS
Full text linkIn India, the central Government, as well as the respective states, have laid out policies to encourage the adoption of solar energy. For many types of consumers, especially residential and commercial, the Levelized Cost of Energy (LCOE) of solar energy is lesser than the cost of energy imported from the grid. However, solar energy is an intermittent and variable resource, which cannot fulfil the load at all times. Addition of battery storage energy systems (BESS) to solar PV projects can help the consumer better meet their load profile through renewable energy, while improving grid resilience. In many states, time-of-day (ToD) tariffs are offered, which provide incentives for exporting energy beyond sunlight hours. It has been postulated that with ToD tariffs, and falling battery prices, addition of storage can provide benefits to the customer. However, it needs to be analyzed whether hybrid solar-battery storage configurations would make financial sense. In this study, we compare the performance of two popular battery types, Lead Acid and Lithium Ferro Phosphate (LFP), to see which battery type would provide better economic returns for the given customer. The impact of parameters like round trip efficiency, replacement State of Health (SOH), maximum State of Charge (SOC) and cycle time have been considered. It is shown that for current market tariffs and prevalent battery costs, addition of batteries does not improve the Net Present Value (NPV) for residential or commercial customers. The best returns are obtained when the system has only solar, with no batteries attached, while commercial systems achieve better NPV as compared to residential customers. The highest NPV for a residential customer is 4165 for a 7.7 kW project. It is seen that accelerated depreciation benefits play a major role in improved returns from commercial installations. It is also seen that retail rate dispatch provides higher returns as compared with the peak shaving and self-consumption dispatch algorithms, since it reduces both energy as well as demand charges. It is demonstrated how the retail rate dispatch modulates the battery discharge & charge configuration every hour considering the prevalent tariff at that time and the energy left in the battery
PRIMJENA GEOELEKTRIČNE TOMOGRAFIJE U KARAKTERIZACIJI PODZEMNI PROSTOR ZA POTREBE IZGRADNJE OBJEKATA
No full textThis paper presents the application of geoelectrical tomography with a Wenner electrode array to characterize the subsurface conditions at the site of a planned commercial building in Kakanj. The geoelectrical surveys provided insight into the distribution of soil electrical resistivity, enabling the identification of layers with differing physical and mechanical properties. Most electrical resistivity values were below 700 Ωm, suggesting the presence of saturated fine sediments and clays. However, in the central part of the profile, at a depth of 10–15 meters, an anomaly exceeding 10,000 Ωm was detected, potentially indicating solid rock masses, concrete structures, or unconsolidated material. The results provide a valuable basis for foundation design. Supplementary investigations, including exploratory drilling and laboratory testing, are recommended to better define foundation conditions and reduce geotechnical risks.U ovom radu prikazana je primjena geoelektrične tomografije s Wennerovim rasporedom elektroda u svrhu inženjerskogeološke karakterizacije podzemnog prostora na lokaciji planirane izgradnje poslovnog objekta u Kaknju. Geoelektrična ispitivanja omogućila su uvid u raspodjelu električne otpornosti tla, čime su identifikovani slojevi s različitim fizičko-mehaničkim karakteristikama. Većina vrijednosti otpora kretala se ispod 700 Ωm, što ukazuje na prisutnost zasićenih finih sedimenata i glina, dok je u središnjem dijelu profila, na dubini od 10–15 metara, detektovana anomalija s otporom većim od 10.000 Ωm, koja može ukazivati na prisustvo čvrstih stijenskih masa, betonskih konstrukcija ili nevezanog materijala. Dobijeni rezultati predstavljaju važnu osnovu za projektovanje temelja, a preporučuje se dopunska istraživanja, uključujući istražno bušenje i laboratorijska ispitivanja, radi preciznijeg definisanja uslova temeljenja i smanjenja geotehničkih rizika
NIGERIJSKI FINANSIJSKI SISTEM: VEZA IZMEĐU CEDIRANJA REOSIGURANJA I FINANSIRANJA KAPITALOM U SEKTORU NEŽIVOTNOG OSIGURANJA
No full textIn an era marked by increasing regulatory scrutiny and market volatility, insurers’ decisions regarding capital acquisition and reinsurance utilization are paramount to their ability to thrive, adapt, safeguard policyholders’ interests and maintain a margin of profit. This study examines the relationship between reinsurance utilization and equity financing in the Nigerian insurance industry as part of the Nigerian financial system. It hypothesizes that there is no significant relationship between reinsurance ceded and equity in non-life insurance companies in Nigeria. The study hinges on the bankruptcy cost theory and the renting capital hypothesis. The ex-post facto research design is adopted for the study and the population comprises all the registered insurance companies in Nigeria undertaking non-life insurance from which a sample of thirty-six (36) companies are selected using the purposive sampling technique. Data is drawn from the annual reports of the selected companies and Nigeria Insurers’ Digest over a period of 23 years and analyzed using the pooled ordinary least squares regression, fixed effect, random effect regression and generalized method of moments. The results reveal a significant relationship between equity and reinsurance ceded. It is concluded that reinsurance utilization impinges on equity financing in non-life insurance companies. It is recommended that low-capitalized insurers should cede more risks as a buffer to their capital, guard against bankruptcy, and enhance profitability and solvency.U eri koju obeležava sve veći regulatorni nadzor i nestabilnost tržišta, odluke osiguravača u vezi sa sticanjem kapitala i korišćenjem reosiguranja su od najveće važnosti za njihovu sposobnost da napreduju, prilagođavaju se, štite interese osiguranika i održavaju maržu profita. Ova studija ispituje vezu između korišćenja reosiguranja i finansiranja kapitalom u nigerijskoj osiguravajućoj industriji kao delu nigerijskog finansijskog sistema. Pretpostavlja se da ne postoji značajna veza između cediranog reosiguranja i kapitala u kompanijama za neživotno osiguranje u Nigeriji. Studija se zasniva na teoriji troškova bankrota i hipotezi o iznajmljivanju kapitala. Za studiju je usvojen dizajn istraživanja ex-post facto, a populacija obuhvata sve registrovane osiguravajuće kompanije u Nigeriji koje se bave neživotnim osiguranjem, iz kojih je uzorak od trideset šest (36) kompanija izabran tehnikom namernog uzorkovanja. Podaci su prikupljeni iz godišnjih izveštaja odabranih kompanija i publikacije Nigeria Insurers’ Digest tokom 23 godine, a analiza je sprovedena korišćenjem metodologija: pooled ordinary least squares (POLS), regresije sa fiksnim efektom, regresije sa slučajnim efektom i generalized method of moments (GMM). Rezultati otkrivaju značajnu vezu između kapitala i cediranog reosiguranja. Zaključeno je da korišćenje reosiguranja utiče na finansiranje kapitalom u kompanijama za neživotno osiguranje. Preporučuje se da osiguravači sa niskom kapitalizacijom cediraju više rizika kao zaštitu svog kapitala, zaštite od bankrota i poboljšaju profitabilnost i solventnost
REGULISATI ILI NE REGULISATI: OCENA POTREBE ZA ZAKONSKIM UREĐENJEM SISTEMA VEŠTAČKE INTELIGENCIJE U POSTUPKU DONOŠENJA SUDSKIH ODLUKA U SVETLU ČLANA 6 (1) EVROPSKE KONVENCIJE O LJUDSKIM PRAVIMA
No full textThis article assesses the necessity of regulating artificial intelligence (AI) systems in judicial decision-making through the lens of the fair trial guarantees of Article 6(1) of the European Convention on Human Rights (ECHR). Using a doctrinal legal methodology, the study examines how different AI deployment models, namely AI-assisted adjudication (‘e-assistants’) and fully autonomous AI judges (‘e-judges’), interact with three crucial procedural safeguards: equality of arms, public hearing, and reasoned decisions. Drawing on jurisprudence of the European Court of Human Rights and the Council of Europe soft law instruments, the analysis demonstrates that AI systems, as currently developed, run the risks associated with opacity, imbalance, and normative ambiguity incompatible with fair trial standards. The article concludes that robust national and regional regulatory frameworks are needed to mitigate these risks, especially in light of the growing use of AI in courts in Europe and beyond. This research contributes to the broader debate on algorithmic governance in democratic societies and calls for regulatory foresight to preserve judicial integrity in the digital age.U članku se razmatra potreba za zakonskim uređenjem sistema veštačke inteligencije u postupku donošenja sudskih odluka kroz prizmu prava na pravično suđenje koje je garantovano članom 6(1) Evropske konvencije o ljudskim pravima. Koristeći doktrinarnu pravnu metodologiju, rad istražuje kako različiti modeli primene veštačke inteligencije, tačnije sudski postupci asistirani veštačkom inteligencijom (e-asistent) i potpuno autonomni modeli (e-sudije), deluju u interakciji sa ključnim procesnim garancijama: jednakost stranaka, načelo javnosti, i obrazloženje sudskih odluka. Na osnovu analize relevantne sudske prakse Evropskog suda za ljudska prava i instrumenata “mekog” prava Saveta Evrope, rezultati istraživanja ukazuju da postojeći sistemi veštačke inteligencije donose brojne rizike koji su nespojivi sa standardima pravičnog suđenja, poput netransparentnog, nekonsistentnog i nesrazmernog pristupa odlučivanju, normativne dvosmislenosti sudskih odluka, itd. Kako bi se predupredili ili ublažili ovi rizici, autor u zaključku ističe potrebu za uspostavljanjem robusnih nacionalnih i regionalnih regulatornih okvira, posebno u svetlu sve veće upotrebe sistema veštačke inteligencije u Evropi i širom sveta. Ovo istraživanje predstavlja doprinos široj debati o algoritamskom upravljanju u demokratskim društvima, i poziva na regulatornu dalekovidost kako bi se očuvao integritet sudstva u digitalnom dobu
ON GENERAL SPHERICALLY SYMMETRIC FINSLER METRICS
Full text linkIn this paper, we study some non-Riemannian curvature properties of general spherically symmetric Finsler metrics. First, we prove that every general spherically symmetric Finsler metric is semi-C-reducible. Then, we find the necessary and sufficient condition under which a general spherically symmetric Finsler metric has vanishing weakly stretch curvature
FAULT-TOLERANT CONTROL OF LOW-ALTITUDE UNMANNED AERIAL VEHICLE SWARMS USING A REDUCED-ORDER SLIDING MODE OBSERVER
Full text linkThe operational demands placed on unmanned aerial vehicles (UAVs) have increased significantly, rendering single low-altitude UAVs insufficient for addressing complex missions and navigating dynamically evolving information environments. To overcome these limitations, the deployment of low-altitude UAV swarms was adopted, offering enhanced agility, adaptability, and scalability. However, the expansion in swarm size leads to a higher probability of actuator faults and increased complexity in fault diagnosis and mitigation. In this study, a fault-tolerant control framework was proposed for low-altitude UAV swarms, incorporating a reduced-order sliding mode observer for real-time fault detection and isolation. A dynamic model of the UAV swarm was first established based on Newtonian mechanics and coordinate transformation techniques, accurately capturing inter-agent dynamics and external disturbances. Building upon this model, a reduced-order sliding mode observer was constructed to enable prompt estimation of actuator faults while ensuring robustness against uncertainties and model inaccuracies. Fault estimates obtained in real time were subsequently utilized in the design of a fault compensation controller to maintain swarm stability and performance in the presence of partial actuator failures. The stability and reachability of the proposed observer were rigorously validated through Lyapunov-based analysis, ensuring convergence of the fault estimation error. To evaluate the proposed methodology, an experimental UAV swarm platform operating at low altitude was developed. Experimental results demonstrate the high accuracy, resilience, and computational efficiency of the fault-tolerant control system under various fault scenarios, highlighting its practical applicability to real-world UAV swarm deployments. The proposed framework offers a scalable and robust solution for maintaining mission continuity in safety-critical UAV swarm operations