Faculty of Mechanical Engineering and Naval Architecture in Zagreb

Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.

OBJECTIVES: to determine the mechanical properties of hybrid and high-viscosity glass ionomer cements. Compressive strength and hardness of three glass ionomer cements (GIC) were measured: Ketac ™ Universal Aplicap ™, EQUIA Fil® and EQUIA FORTE Fil®, and the SEM sample analysis were performed. MATERIALS AND METHODS: The samples for measuring the compressive strength were prepared using silicone molds with standard dimensions of 6 mm x 4 mm and stored in deionized water for five days, while the samples for hardness measurement were prepared using Teflon molds with a cylindrical opening in the middle, dimensions 2 mm in height and 5 mm in width. For each material, one sample was made (n = 1) and stored in deionized water at 37ºC for 25 days. A representative sample of each material was analyzed using SEM. For the comparison of obtained values, the ANOVA test was used, while Tukey test was used for the multiple comparison. RESULTS: There were no significant differences between the compressive strength of the three tested materials (p <0.05). The hardness values were: 157 HV0,2 for Ketac ™ Universal Aplicap ™, 47 HV0,2 for EQUIA Fil® and 39 HV0,2 for EQUIA FORTE Fil®, respectively, and were significantly different, implying that Ketac ™ Universal Aplicap ™ has much higher hardness values than the other materials tested. SEM sample analysis revealed similar fracture modes of the tested materials. CONCLUSION: It was concluded that there were no statistically significant differences in compressive strength and fracture modes between the tested materials, while Ketac ™ Universal Aplicap ™ hardness results were significantly higher than the ones measured for EQUIA Fil® and EQUIA FORTE Fil®.

Although the countries of the Western Balkans are mostly electrified, there are still regions which do not have access to the electricity network or where the network capacity is insufficient. For the most part such areas are under special care of the state (i. e. underdeveloped, devastated by war, depopulated), on islands or in mountainous regions. Since the decentralized energy generation covers a broad range of technologies, including many renewable energy technologies that provide small-scale power at sites close to the users, such concept could be of interest for these locations. This paper identifies the areas in Western Balkans where such systems could be applied. Consideration is given to geographical locations as well as possible applications. Wind, hydro, solar photovoltaic, and biomass conversion systems were taken into consideration. Since the renewable energy sources data for Western Balkans region are rather scarce, the intention was to give a survey of the present situation and an estimate of future potential for decentralized energy generation based on renewable energy sources. The decentralized energy generation based on renewable energy sources in Western Balkans will find its niche easier for the users that will produce electricity for their own needs and for the users located in remote rural areas (off-grid applications).

In this project the influence of dilution of synthetic and mineral engine oils with two commercial fuelsgasoline (type Eurosuper 95) and diesel fuel (type eurodiesel) was studied.The situation that could really occur in the automotive engine was simulated -a breakthrough of unburned fuel to engine oil and its mixing and dissolution.Viscosity, fire point and flash point were chosen as the most important parameters to follow the changes in oil-fuel mixtures.The dilution range was between 0 and 10 wt% of fuel in the oil-fuel mixture.The samples used in this investigation were bought directly from the two producers present on the Croatian market, without further modification.The samples were multigrade mineral and synthetic oils -mineral oil with SAE gradation 15W-40 and synthetic oil with SAE gradation 5W-30.The changes of oil properties were too high for assumed safe use in engines in the case of oil dilution with 10 wt% of gasoline.The dilution of oil with 5 wt% of gasoline was maximal level of dilution that could be expected to be still adequate for use in engines, because the flash points of these mixtures were mostly near 150 C, which is assumed as maximal allowed working temperature of engine oil.The properties of mixtures of the same oils with diesel fuel showed smaller changes than mixtures with gasoline.

Limitations of traditional first-law analysis, based upon thermodynamic performance of process unit coupled with mass and energy balances, are not a serious limitation when dealing with familiar systems. However, when dealing with more uncongenial, complex ones, it provides incomplete insight for such evaluation. These limitations came from the fact that first-law analysis does not indicate the sources or magnitudes of entropy production, which is, by the second law, essential criterion for scaling losses. An evaluation of plant performance will usually require a comparison of the thermodynamic performance of process units with available data from existing plants. Therefore, exergy analysis is more than useful, providing information about magnitudes of losses and their distribution throughout the system as well. Such analysis is very thankful at the level of process units but applied on higher system levels e.g. the comparison of overall plant performance (total system) or the performance of subsystems, represents the valuable method for indicating where research resources can be directed to best advantage.

The goal of this study was to determine corrosion behavior of three orthodontic wires based on nickel-titanium alloy (NiTi) in artificial saliva at temperature of 37 °C as function of immersion time. Following orthodontic wires were used: uncoated (NiTi), rhodium coated (Rh NiTi) and nitrified (N NiTi) orthodontic wires. Corrosion of investigated orthodontic wires were monitored by measuring of Ni2+ and Ti4+ ions released in artificial saliva by inductively coupled plasma-optical emission spectroscopy (ICP-OES) after 3, 7, 14, 21 and 28 days of immersion. Obtained results indicate that corrosion reaction of the NiTi wires in artificial saliva follows the parabolic rate law. According to the obtained values of parabolic corrosion rate constants, corrosion susceptibility of orthodontic wires decreases in the following order: Rh NiTi wire (Kp = 2.48 μg2/cm4 h) > NiTi wire (Kp = 1.6 × 10–3 μg2/cm4 h) > N NiTi wire (Kp = 6.0 × 10–4 μg2/cm4 h). These results indicate that in comparison with uncoated NiTi wire, rhodium coating significantly increases corrosion susceptibility, while nitrification effectively suppresses the release of Ni2+ and Ti4+ ions.

Hourly resolution is essential to realistically address the matching of supply and demand for fluctuating energy sources like solar and wind. This work introduces a novel method to model energy variability in an Integrated Assessment Model building upon a previous work, where regression analysis was utilized to extract hourly-level information from an energy system model. The enhancements include: (1) improved experimental design and more efficient computing, and (2) modelling the management of variability in an integrated assessment model by (i) incorporating a portfolio of flexibility options, and (ii) offering the ability to regulate system curtailment by limiting the expansion of renewables. The scenarios focus on the electricity sector, mirroring current Europe-27’s policies that aim for higher renewable energy and electrification contributions by 2050. Without any variability control measures, significant curtailment (up to 60%) is observed, the introduction of flexibility options reducing it to half (30%). Controlling the capacity expansion of renewables is introduced to avoid these unrealistically high curtailment, allowing the model to achieve a penetration of renewables in electricity of 80% and a 53% reduction in greenhouse gas emissions compared to 2015 levels in the electricity system. In conclusion, the methodology employed yields consistent outcomes.

A novel approach for the solution of Stefan problem within the framework of the multi fluid model supplemented with Volume of Fluid (VOF) method, i.e. two-fluid VOF, is presented in this paper. The governing equation set is comprised of mass, momentum and energy conservation equations, written on a per phase basis and supplemented with closure models via the source terms. In our method, the heat and mass transfer is calculated from the heat transfer coefficient, which has a fictitious function and depends on the local cell size and the thermal conductivity, and the implementation is straightforward because of the usage of the local value instead of a global parameter. The interface sharpness is ensured by the application of the geometrical reconstruction scheme implemented in VOF. The model is verified for three types of computational meshes including triangular cells, and good agreement was obtained for the interface position and the temperature field. Although the developed method was validated only for Stefan problem, the application of the method to engineering problems is considered to be straightforward since it is implemented to a commercial CFD code only using a local value; especially in the field of naval hydrodynamics wherein the reduction of ship resistance using boiling flow can be computed efficiently since the method handles phase change processes using low resolution meshes.

Original scientific

Abstract Deep cryogenic treatment in combination with classic heat treatment shows a significant improvement in wear resistance of high speed steel tools. The aim of this research was to investigate how the microstructure of the substrate tool steel material, which was altered by deep cryogenic treatment and plasma nitriding, influences the properties of TiAlN coating. The microstructure, topography and composition of the TiAlN coating were investigated using field‐emission scanning electron microscope, atomic force microscopy, XRD, and glow discharge optical emission spectroscopy. The coating adhesion was measured using the scratch test. The sliding wear resistance and the force required to break the coating were determined with the ball‐on‐flat method. Resistance to microabrasion was measured by free ball abrasion test. The results show that deep cryogenic treatment combined with plasma nitriding influence the adhesion of the TiAlN coating to the high speed steel substrate. Wear resistance tests show better wear resistance of deep cryogenic treated samples in comparison with conventionally heat treated ones.

This work presents experimental results for heat transfer caused by convection of air in tubes with wire coil and twisted tape inserts. The characteristic parameters of the inserted tubes under investigation are length and position in the tube of the tested inserts. All tests are performed in a turbulent flow regime. Significantly higher inside heat transfer and friction factor coefficients than those of the smooth tube under same operating conditions have been observed. The performance of the wire coil and twisted tape inserts has been evaluated by three different criteria: maximizing heat transfer rate, minimizing heat exchanger size, and minimizing pumping power.

A high level of autonomy is a prerequisite for achieving robotic presence in a broad spectrum of work environments. If there is more than one robot in a given environment and the workspaces of robots are shared, then the robots present a dynamic obstacle to each other, which is a potentially dangerous situation. This paper deals with the problem of motion planning for two six-degrees-of-freedom (DOF) industrial robots whose workspaces overlap. The planning is based on a novel hall of fame - Pareto-based co-evolutionary algorithm. The modification of the algorithm is directed towards speeding-up co-evolution, to achieve real-time implementation in an industrial robotic system composed of two FANUC LrMate 200 iC robots. The results of the simulation and implementation show the great potential of the method in terms of convergence, robustness and time.

The paper presents the study of weldability of micro-alloyed high-strength S960 QL steel with the application of experimental methods and mathematical models from the aspect of cold cracks occurrence.In the experimental part of the research RD (Research Department) test was applied to estimate the steels susceptibility to the occurrence of cold cracks.For the analysis of the impact of heat input on the mechanical properties of the welded joint the test steels were welded with one, two or more runs, out of which samples were made for the production of specimens in order to measure the hardness and impact strength in HAZ (heat affected zone).Based on the obtained results an estimate was provided regarding the weldability of S 960 QL steel as well as the proposal for further research.

As part of the FP6 Program, a project named WEB-ENV 'Development of environ- mental guidelines for the region of Western Balkans' dealt with the state of the environment in the region of the West Balkan countries: Albania, Bosnia and Herzegovina, Croatia, FYR Macedonia, Montenegro, and Serbia. The parameters which were observed were air and water quality, solid waste pollution, soil degradation, and state of biodiversity. The results are presented as an extended mapping exercise identifying major environmental problems in each country. This paper outlines the comparative analysis of the parameters observed in the project and their respective resources. This paper also presents the impact of some polluters on the environmental condition of the countries surrounding Western Balkans. The analysis of data proves that certain areas have insufficient parameters especially in the field of ground water quality and soil degradation. Another problem in the comparative analysis was discrepancy of year of data collection. In order to get a better understanding of the environmental issue in the region it has been determined that it is necessary to provide comprehensive data monitoring via regional projects on the territory of the Western Balkans.

Mileage data collected via surveys based on self-estimation, reports from garages and other sources which use estimations are rough estimates and differ greatly from the actual mileage. Vehicle mileage is a major factor in emission calculations and needs to be as accurate as possible to obtain reliable emission models. Odometer readings are collected annually at the periodic technical inspection in Croatia. Average mileage data were analyzed for vehicles up to 20 years of age in 2017. Vehicles were classified by curb weight and fuel type. Such classification proved to follow driver behavior and the intended purpose of the vehicle. For each vehicle class the model was applied using vehicle age and population size as inputs for calculating average mileage. Real data show that vehicles in Croatia considerably exceed the estimated mileage in the years following the first registration of the vehicle and that they cannot be compared to data collected in other studies based on estimations. The difference lies in the covered mileage after vehicles reach ten years of age. The outcome of this study has resulted in a model for calculating average vehicle mileage. The model is suitable for use in various analyses for Croatia or for countries with similar driving habits and economic status now and for years to come.

OBJECTIVES: To examine the effect of two aging protocols on the chemical and phase composition as well as the surface state of monolithic translucent zirconia ceramics. MATERIAL AND METHODS: Translucent zirconia ceramics KATANA-Zirconia STML with different surface treatments (no treatment, K1, K2; glazed, G1-G8; polished, P1-P8) underwent testing in order to examine how the two aging protocols (three-hour hydrothermal degradation in an autoclave at 134 °C and 2 bars: G1-G4, P1-P4, and sixteen-hour chemical degradation in four-percent acetic acid at 80 °C (ISO 6872): G5-G8, P5-P8) affect chemical composition, particularly the share of stabilizing yttrium oxide (Energy Dispersive X-Ray Fluorescence - EDXRF), phase composition (X-ray diffraction - XRD) and surface state in terms of roughness and gloss. RESULTS: Aging protocols did not affect the tested chemical composition stability of specimens and a high share of stabilizing yttrium-oxide (≥10% of total content), which correlates with the absence of monoclinic phase. A decrease in gloss on all specimens is statistically significant. Chemical degradation substantially increased the surface roughness of tested specimens. CONCLUSIONS: Translucent monolithic zirconia demonstrated a stable chemical composition and resistance to tetragonal-to-monoclinic transformation. Surface gloss was significantly reduced, especially in polished specimens. Contrary to glazed specimens, the tested polished specimens manifested an increase in surface roughness. Glazing the surface of translucent monolithic zirconia produces better esthetic, tribological and hygienic effects than polishing.

This paper presents the analysis of the influence that black surface paint layers have on the differences in the results obtained from numerical modelling and those obtained experimentally. Surface paints are commonly used for the purposes of pulse experiments in order to increase the sample surface emissivity and help enhance the signal obtained. The paper argues that it is important to include these paint layers in the numerical analysis either directly, as additional material layer, or alternatively, to estimate their influence and take it into account when comparing the corresponding results.

A formulation for the kinematics of multibody systems is presented, that uses Lie group concepts. With line coordinates the kinematics is parameterized in terms of the screw coordinates of the joints. Thereupon, the Lagrangian motion equations are derived, and explicit expressions are given for the objects therein. It is shown how the kinematics and thus the motion equations can be expressed without the introduction of body-fixed reference frames. This admits the processing of CAD data, which refers to a single (world) frame. For constrained multibody systems, the Lagrangian motion equations are projected to the constraint manifold, which yields the equations of Woronetz. The mathematical models for numerical integration routines of MBS are surveyed and constraint gradient projective method for stabilization of constraint violation is presented.

Compressed natural gas is in automotive industry recognized as one of the ?cleanest? fossil fuels which can be used in internal combustion engines with a number of benefits. Since natural gas has much higher octane rating than gasoline it is expected that higher compression ratios can be used. The goal of the research is to determine the change of performance of spark ignited engine with the increase of compression ratio to values similar to compression ignited engine while keeping the exhaust emissions on the acceptable level and avoiding knock combustion. Measurements are performed at compression ratios 12, 16, and 18 at three different values of air excess ratio. Methane with known composition from a pressure cylinder is used instead of natural gas and the results are comprised of indicating results (in-cylinder and intake pressure in a crank angle space), emissions, temperatures, and mass-flows on various intake and exhaust positions. Analysis of results shows high influence of compression ratio and excess air ratio on combustion, performance, and exhaust gas emissions.

Uvod: Cirkonij-oksidna (Y-TZP) keramika zbog specifinih svojstava esto se primjenjuje u stomatologiji. Gradivni materijal moe korodirati