Radosław Nowak

This study investigates the nonlinear dynamics and stability of a two-degree-of-freedom Ziegler column subjected to a non-conservative follower force, with a primary focus on the influence of Coulomb (dry) friction. The column's interaction with a rough surface is modeled using a continuously regularized dry friction law integrated into Lagrange's equations. Through linear eigenvalue evaluation and comprehensive nonlinear bifurcation analysis, the system's behavior is mapped across a continuous parameter space of loads and friction coefficients. The results provide a clear manifestation of Ziegler's paradox: infinitesimally small dry friction destabilizes the system at loads significantly below the theoretical frictionless critical threshold, inducing subcritical vibrations. Conversely, higher friction levels stabilize the pre-critical state, delaying the onset of flutter to align with a classical supercritical Hopf bifurcation. Furthermore, global parameter analysis reveals that post-critical vibration amplitudes exhibit a highly non-monotonic sensitivity to friction; intermediate damping values can paradoxically amplify oscillations due to complex phase shift changes. These findings emphasize the critical necessity of incorporating accurate dry friction models when predicting the stability and post-bifurcation dynamics of non-conservative systems.

Helicopter pilots operate in a vibratory environment that substantially exceeds that of fixed-wing aircraft, creating stringent demands for the durability and dynamic stability of head-supported equipment. This study presents an experimental investigation of the vibration behaviour of prototype helicopter pilot goggles tested on an electrodynamic exciter over a frequency range exceeding applicable defence standards. The aim was to verify assembly integrity, assess the short-term strength of load-bearing components, identify natural frequencies of the goggle system, and generate reference data for future finite-element model. Acceleration and displacement responses were recorded using accelerometers and processed through custom software to obtain amplitude–frequency characteristics. The results indicate significant amplification of vibration in selected structural regions, eg. 50–250 Hz and 450–500 Hz, corresponding to natural modes of both the holder and the goggles. These findings confirm the sensitivity of the system to vibratory inputs comparable to those in helicopter cockpits

Three-wheeled vehicles are gaining popularity in European and Asian cities due to their low cost, stability, maneuverability, and compact size. Among these, tilting vehicles facilitate cornering, maintain stability, and reduce centrifugal forces. This study investigates a delta-configured, three-wheeled tilting vehicle designed for people with reduced mobility. Vehicle dynamics were analyzed using ADAMS/Car simulations, including steady-state cornering and single-lane change tests, focusing on body motion and forces in suspension and steering systems. Results show that tilting of the body significantly enhances cornering safety compared to non-tilting three-wheelers, providing insights for designing efficient urban vehicles for diverse user groups.

This article presents the concept, research, and modeling of a sandwich composite made from ULTEM 9085 and polycarbonate (PC). ULTEM 9085 is relatively expensive compared to polycarbonate, and the composite structure made of these two materials allows for maintaining the physical properties of ULTEM while reducing the overall costs. The composite consisted of outer layers made of ULTEM 9085 and a core made of polycarbonate. Each layer was 3D-printed using the fused filament fabrication (FFF) technology, which enables nearly unlimited design flexibility. The geometry of the test specimens corresponds to the ISO 527-4 standard. Tensile and three-point bending tests were conducted. The structure was modeled in a simplified manner using averaged stiffness values, and with the classical laminate theory (CLT). The models were calibrated through tensile and bending tests on ULTEM and polycarbonate prints. The simulation results were compared with experimental data, demonstrating good accuracy. The 3D-printed ULTEM-PC-ULTEM composite exhibits favorable mechanical properties, making it a promising material for cost-effective engineering applications.

Research conducted around the world shows that energy harvesting (EH) systems can be used in modern vehicles powered by combustion, hybrid or electric engines. The efficiency of modern combustion engines is about 40%, the rest of the energy is lost and can be recovered to some extent. Therefore,...

Modern city buses are made of various construction materials and the share of material groups has changed over the decades. By replacing heavy materials for structural elements or bus components with their lighter counterparts, the kerb weight of the bus can be reduced by up to several hundred kilograms. This article presents the issues of the development of city bus design in terms of passenger space comfort and bus structure in the context of reducing the vehicle's own weight since the 1970s. The main changes in vehicle design allowing for reducing the weight of structural elements of bodies and chassis as well as the main assemblies in city buses are presented as well as research on body types in terms of aerodynamics, safety and travel comfort. It has been shown that reducing the weight of the bus does not negatively affect its load capacity and the new bus designs are equipped with safety and comfort systems, including ABS (Anti-lock Braking System), ASR (Automatic Stability Regulation), ESP (Electronic Stability Program) and air conditioning in the passenger space. Thanks to modern light construction materials, we gain the opportunity to improve safety and comfort without losing the transport capabilities developed as a result of the development of city buses over the years.

Road transport is one of the most important factors for the national economy due to its universality and comprehensive possibilities of transporting people and goods. Unfortunately, from the energy point of view, it is also the most cost-intensive and has a negative influence on the natural environment. For these reasons, issues related to limiting the use of conventional fuels are very important, which results in reducing emissions from this sector, as well as reducing transport costs. This article presents currently used energy sources for propulsion of road vehicles, including fossil and alternative fuels, gaseous fuels and other energy sources such as fuel cells. The following section presents technologies that allow to recover some of the energy lost in motor vehicles and internal combustion engines used for their propulsion. The principle of operation of these solutions, their structure and their main features are presented. The last part focuses on discussing and identifying the most universal technologies for energy harvesting in vehicles and showing further directions of energy development in the automotive sector.

The main aim of the study includes research concerning the strength of samples printed out of 316L steel in the form of laminates and the creation of reflective simulation models with regard to the results obtained during the research. In addition, the tests addressed the effect of the arrangement of the printed layers on the final strength of the object. Static tensile tests allowed the material constants of 316L steel in the form of dimensionally printed laminate to be determined. Tests were conducted on samples with different printed angles. The tests also covered the impact of the printing envelope on samples with the printing angles. Based on the determined material constants, simulation models for calculations using the finite element method were created. Furthermore, the study includes analytical and simulation calculations of plain laminate in order to verify the accuracy of the Composite Layup module in Abaqus CAE software. The study was summarized by compiling and commenting on the results obtained from the conducted research. Tests showed that there is a possibility of simulating the strength of the printouts from 316L steel using the FEM calculations. It was shown that the FEM model results are similar to those obtained in the tests. The calculated errors were from 3.6 to 14.4%. The linear model describes well the first part of the stress–strain curve, but in further research, it is strongly recommended that a proper and checked nonlinear anisotropic one is presented.

Closed-cell expanded polypropylene (EPP) foam is commonly used in car bumpers for the purpose of absorbing energy impacts. Characterization of the foam’s mechanical properties at varying strain rates is essential for selecting the proper material used as a protective structure in dynamic loading application. The aim of the study was to investigate the influence of loading strain rate, material density, and microstructure on compressive strength and energy absorption capacity for closed-cell polymeric foams. We performed quasi-static compressive strength tests with strain rates in the range of 0.2 to 25 mm/s, using a hydraulically controlled material testing system (MTS) for different foam densities in the range 20 g/dm³ to 220 g/dm³. The above tests were carried out as numerical simulation using ABAQUS software. The verification of the properties was carried out on the basis of experimental tests and simulations performed using the finite element method. The method of modelling the structure of the tested sample has an impact on the stress values. Experimental tests were performed for various loads and at various initial temperatures of the tested sample. We found that increasing both the strain rate of loading and foam density raised the compressive strength and energy absorption capacity. Increasing the ambient and tested sample temperature caused a decrease in compressive strength and energy absorption capacity. For the same foam density, differences in foam microstructures were causing differences in strength and energy absorption capacity when testing at the same loading strain rate. To sum up, tuning the microstructure of foams could be used to acquire desired global materials properties. Precise material description extends the possibility of using EPP foams in various applications.