Cyclic Temperature Changes Research Articles

High-temperature oxidation and diffusion studies on selected Al–Cr–Fe–Ni high-entropy alloys for potential application in thermal barrier coatings

In this work, the properties of two high-entropy alloys (HEAs) Al25Cr20Fe20Ni35 and Al20Cr20Fe20Ni40 (at.%) were compared for potential application as bond coats in thermal barrier coatings (TBCs). For this reason, both their oxidation resistance under isothermal and thermal shock conditions, as well as diffusion phenomena occurring at the substrate-HEA diffusion couple interface, were investigated. It was determined that both alloys demonstrate good phase stability and oxidation resistance during prolonged exposure to air atmosphere at 1000°C. In the initial 100 h period, selective aluminum oxidation is responsible for the protective scale growth on both materials under both isothermal and thermal shock conditions. However, only the Al20Cr20Fe20Ni40 high entropy alloy maintains a single Al2O3 layer for over 1000 h of oxidation with cyclic temperature changes. On the other hand, diffusion studies indicate that mainly iron and chromium travel between the ferritic steel substrate and the HEAs. Aluminum diffusion is seemingly limited by the unexpected formation of a thin aluminum nitride layer at the interface between the materials. From all these results it can be concluded that the Al20Cr20Fe20Ni40 alloy shows more promise for application in TBCs based on high entropy materials than Al25Cr20Fe20Ni35.

Thermal performance evaluation of silica dip-coated phase change material beads for thermal energy storage in cement-based composites

The present study proposed a simple manufacturing method of silica dip-coated PCM beads based on the phase transition characteristics of PCM without using complex processes (e.g., centrifugal extrusion, interfacial polymerization, and suspension cross-link), reagent-grade chemicals (e.g., urea (CH4N2O), formaldehyde (CH2O), etc.), or any porous medium. In addition, the technique in this study incorporated markedly more PCM into the same volume of mixture than other existing PCM integration techniques. Mechanical properties and thermal performance of cementitious composites were tested in the presence of silica dip-coating and the substitutions of PCM beads and fly ash cenosphere (FAC). In the results, the silica dip-coating improved the strength (by pozzolanic reaction) and the latent heat by 10.9% and also reduced the thermal conductivity by 8.4% (by reducing PCM leakage). The silica dip-coated PCM beads in samples decreased the cooling rate and converged the temperature to ∼29.4 °C. In particular, the cementitious composite containing 20% silica dip-coated PCM effectively reduced indoor temperature fluctuation by lowering the internal peak temperature by up to 7.5 °C; it also delayed the peak time in an outdoor cyclic temperature change environment. The added FAC reduced thermal conductivity by 34% due to its hollow structure and accelerated the initial cooling rate of the sample containing PCM beads.

The Effect of Carbide Concentration at Grain Boundaries on Thermal Stresses in Castings for Heat Treatment Furnace Tooling

Austenitic Fe-Ni-Cr alloys are commonly used for the production of castings intended for high-temperature applications. One area where Fe-Ni-Cr castings are widely used is the equipment for heat treatment furnaces. Despite the good heat resistance properties of the materials used for the castings, they tend to develop cracks and deformations over time due to cyclic temperature changes experienced under high temperature operating conditions. In the case of carburizing furnace equipment, thermal stresses induced by the temperature gradient in each operating cycle on rapidly cooled elements have a significant influence on the progressive fatigue changes. In the carburized subsurface zone, also the different thermal expansion of the matrix and non-metallic precipitates plays a significant role in stress distribution. This article presents the results of analyses of thermal stresses in the surface and subsurface layer of carburized alloy during cooling, taking into account the simultaneous effect of both mentioned stress sources. The basis for the stress analyzes were the temperature distribution in the cross-section of the cooled element as a function cooling time, determined numerically using FEM. These distributions were taken as the thermal load of the element. The study presents the results of analyses on the influence of carbide concentration increase on stress distribution changes caused by the temperature gradient. The simultaneous consideration of both thermal stress sources, i.e. temperature gradient and different thermal expansions of phases, allowed for obtaining qualitatively closer results than analyzing the stress sources independently

Influence of ice structure on vitability of frozen sand-water and sand-clay mixtures

Purpose. To establish influence regularity of sand, water and clay preparation conditions on vitability of frozen mixtures made from combinations of these components and to increase the castings quality in foundries, as well as to improve technologies for artificial freezing of soils for underground constructions. Methodology. In this research, sand, clay, and water are used. Ice quality is estimated visually after water freezing at -15 °C in glass tubes. Frozen mixtures’ vitability at -15 °C is studied on beam-type samples. As indicators of survivability, the time to 1 mm bending of samples on supports and the time to their destruction are accepted. The time is recorded with a stopwatch, the temperature with an alcohol thermometer, the mass with electronic scales and the deflection arrow with a clock-type indicator. Findings. The presence and amount of water-soluble impurities in rare water significantly influence the nature, size and distribution of gas bubbles in ice, as well as frozen sand-water mixtures vitability. Frozen mixtures’ survivability increases with water content in them increasing, and, for sand  water mixtures, survivability is maximum if ice has a homogeneous structure. Among mixtures with clays, the mixture with non-swollen kaolin clay has the greatest vitability. Regarding survivability, recommendations for manufacturing products from frozen foundry mixtures have been developed. Originality. For the first time, deformation change kinetics (bending arrows) under the influence of beam-type samples’ self-mass from mixtures of quartz sand and water and quartz sand, clay and water frozen at -15 °C, which have been previously prepared in different ways, have been investigated. Insights into the influence of various factors and ice quality on the vitability of frozen mixtures have been further developed. Practical value. The obtained results can be useful for expanding ideas about natural frozen soils’ behavior during their cyclic temperature changes, soils artificially frozen during mine shafts elaboration, escalators’ and junctions’ tunnels, etc. when constructing subways. In foundries, the developed recommendations will reduce technological losses and will improve casting quality made using frozen casting molds and cores from sand-water or sand-clay-water mixtures, castings’ patterns and their pouring systems from sand-water mixtures.

Modeling damage caused by combined thermal and traffic loading using viscoelastic continuum damage theory

Various engineering models have been proposed to mitigate fatigue cracking associated with thermal and traffic loadings, which are the two primary fatigue cracking mechanisms that negatively affect the service life of asphalt pavement. Thermal fatigue cracking is induced by cycles of temperature changes with relatively long time periods, which can be regarded as slow frequency sinusoidal loading. Traffic fatigue cracking is induced by numerous vehicle load repetitions and is often characterized using fast frequency sinusoidal loading. These two loading types occur simultaneously in the field. In order to investigate how to properly model the damage due to these two loading types, a set of tests that vary the load, frequency, and mode of testing was designed to mimic the scenarios that asphalt pavements experience under thermal loading and traffic loading separately and under the combined and simultaneous effects of both thermal and traffic loading. The viscoelastic continuum damage (VECD) model was applied to simulate the changes in material integrity under each condition. After determining the appropriate configuration for the proposed prediction model, the material integrity predictions were found to be accurate for the fast frequency tests but less accurate for the slow frequency tests. This difference is attributable to plasticity being a much more significant factor under slow frequency conditions than under fast frequency conditions. For the tests combining both fast and slow frequency conditions, the material integrity results oscillated continuously. It was postulated that these oscillations were due to the recovery mechanisms of asphalt concrete such as nonlinearity and healing. When the simplified viscoelastic continuum damage (S-VECD) model was applied, which does not consider the material’s recovery characteristics, the predicted material integrity was able to match the overall trend observed in the tests. Ultimately, a new way to model combined damage simultaneously under two cracking mechanisms (thermal fatigue cracking and traffic fatigue cracking) is introduced in this paper.

Study of the Correlation between the Urban Wind–Heat Environment and Urban Development Elements in High-Density Urban Areas: A Case Study of Central Shanghai

To prevent damage to human life and health caused by high temperatures and extreme weather and to promote sustainable urban development, it is necessary to optimize the layout of urban development elements to improve the urban wind–heat environment. Taking the high-density urban area of Shanghai as an example, this study used wavelet analysis to investigate the cyclic changes of the average annual temperature of Shanghai from 1950 to 2022 and the average annual wind speed of Shanghai from 2000 to 2020. The correlation between the urban heat environment and the urban development elements was analyzed using geographically weighted regression. The correlation was then examined using BP neural network, and finally, the impacts of different urban spatial patterns on the wind environment were analyzed using CFD numerical simulation. The results show that the average annual temperature of Shanghai city has an overall upward trend, with significant cycles of 44 and 32 years that are shortening over time. The average annual wind speed has a downward trend, with a significant main cycle of 22 years. Greening and water coverage, as well as the floor area ratio, have a significant reducing effect on surface temperature, whereas building density is positively correlated with surface temperature. Building density has a significant reducing effect on wind speed, whereas the effect of floor area ratio is not significant. The effect of building density on wind speed is significantly weakened, whereas the effect of the floor area ratio is not significant. This study provides valuable references for Shanghai and other high-density cities to optimize urban spatial patterns in order to improve the safety and comfort of the urban wind–heat environments. This study is of significant importance and value in promoting sustainable urban development, protecting the health of urban residents, and advancing spatial justice and equal well-being.

Термические напряжения в изоляции тяговых электродвигателей локомотивов

Thermomechanical processes and cyclic temperature changes in the structure of insulation of traction electric motors lead to thermal gradient appearance. Increased temperature widens the elements of winding, damages insulation material (thermal fatigue, extreme deformation, creeping, etc.). Methods for the numerical study of stress-strain state of the material of the insulation of traction electric motors of diesel locomotives on different stages of static and cyclic loading have been developed. It is determined, that the main factor defining the intensiveness of insulation aging is the impact of alternating cyclic thermo-mechanical stresses, but not thermomechanical destruction of impregnating composition. Analytical expressions for the evaluation of stress-strain state of insulation of traction motor diesel locomotives are received and they are distinguished by the fact that they require a relatively small amount of basic data and let conduct accurate calculations taking into account the peculiarities of the electric drive system. An approximate predictive model for the evaluation of interphase thermal stresses in the assembly of the materials of winding slots of the traction electric motor of a diesel locomotive is developed. The insulation material is considered as linear elastic when the deformation level is less than the yield strength. The joining components can be considered from the view point of structural analyses as elongated rectangular plates suffering from linear elastic strains. The analyses results can be used for evaluation of thermomechanical stresses in the impregnating materials of traction electric motors of diesel locomotives and their analogues.

Numerical and Experimental Analysis of Strength Loss of 1.2709 Maraging Steel Produced by Selective Laser Melting (SLM) under Thermo-Mechanical Fatigue Conditions.

The result of the development of additive manufacturing (AM) methods is the increasing use of the selective laser melting (SLM) method as a technique for producing tooling for injection moulds and die casting pressure moulds from maraging steel powders. The mould components are subjected to varying thermo-mechanical loads during these operations. This paper presents a numerical model that is used to predict the fatigue life of a material that is loaded with a time-varying temperature field according to the classic and modified Coffin test. Using a computational model, the temperature changes in the resistance-heated specimen and the stress and strain fields that are caused by this phenomenon were determined. Using three different multiaxial fatigue criteria, the fatigue life of SLM steel was determined. Numerical calculations were verified using experimental thermal fatigue tests on 1.2709 SLM steel that was aged at 490 °C as well as via metallographic tests. The numerical model was used to predict the durability of the same steel aged at 540 °C. The effect of specimen clamping conditions on the fatigue life of SLM steel was determined numerically. The value of the decrease in strength of SLM steel as a result of the increasing number of cycles of temperature changes was determined experimentally; a great influence of ageing temperature on fatigue life was found. Changes in the structure of steel occurring during cyclic changes in temperature are presented.

Raman mapping microspectroscopy of the effects of cryogenic cycling on the interfacial micromechanics of carbon fiber-reinforced polyimide composites

Raman mapping microspectroscopy was then used to investigate the interfacial stress distributions of the films during different cryogenic-room temperature cycles (-198-25 °C, 0-300 cycles). It was found that the micro stress of CNT-PI films (around 175 MPa) had no significant changes even after 300 cycles. The cryogenic cycling had very little effect on the internal stress, indicating that PI had a good low temperature resistance. For the CF/CNT-PI films, the micro stress distributions of CFs, interface, and matrix regions were successfully obtained. It was found that the CFs bear a greater stress than the matrix, showing that CFs had always been the major stress bearer, confirming the strengthening effect of CFs. When the CF/CNT-PI films were cycled fewer than 250 times, the effect of cryogenic cycling on the micro stress was insignificant. But once the number of cycles reached 300, the compressive stresses on the fiber and interface increased by 21% and 12.9%, respectively, implying a deterioration of the mechanical properties. By Raman mapping, the micro-mechanical distributions of the reinforced material, matrix and interface of the composites under cyclic temperature changes were effectively quantified. This is therefore an effective method for evaluating the safety of composite materials.

Long-term thermal productivity of polystyrene concrete in a new composite wall in a fixed formwork

Purpose. Determination of the term of long-term thermal productivity of expanded polystyrene concrete in a new composite frame wall in a fixed formwork. Methodology. Methods of analysis and synthesis were used to conduct a patent search and develop a research methodology. The search was conducted using the patent database of Ukrainian Institute of Scientific and Technical Expertise and Information. Experimental studies were carried out in accordance with State Standard of Ukraine B V.2.7-38-95. The essence of the experiment consisted in reproducing the natural conditions of the “freeze-thaw-heat” cycle and measuring the thermal insulation characteristics of the main insulating material – polystyrene concrete – before and after the tests. Based on this, a conclusion was made about the long-term thermal productivity of the expanded polystyrene concrete in a new composite wall in the fixed formwork. The calculation of the dependence of thermal productivity on the number of cycles was carried out by arithmetic means using standard methods and programs such as Microsoft Excel. Findings. New technical solution of a composite wall made of expanded polystyrene concrete in a fixed formwork using light steel thin-walled structures has been created. A methodology for researching the long-term thermal productivity of this composite wall has been developed. An experimental study was conducted to research the influence of cyclic temperature changes (“freeze-thaw-heat”) on the long-term thermal productivity of the main heat-insulating element of the composite wall – expanded polystyrene concrete. An appropriate analytical model of the dependence of long-term thermal productivity of expanded polystyrene concrete samples on the number of “freeze-thaw-heat” cycles was calculated. The possibility of effective operation of the structure was confirmed by checking the normative values of the resource index and the factor of climatic destruction influence of materials during operation on their long-term thermal productivity. Implementation of a new composite wall solution in construction was conducted. Originality. For the first time, the dependence of cyclic temperature effects on the long-term thermal productivity of expanded polystyrene concrete in a new composite wall made in a fixed formwork using light steel thin-walled structures was determined, which made it possible to establish its effective operation life. This scientific result makes it possible to reduce material consumption, ensure economy, increase operational reliability and energy-efficient properties, and increase the service life of the composite wall. Practical value. New solution for installing a composite wall made of expanded polystyrene concrete in a fixed formwork using light steel thin-walled structures was developed and its effective thermal operation within the legally established term was substantiated. The period of effective exploitation of expanded polystyrene concrete as the least durable component of a composite wall is substantiated. Approbation of this design was carried out by installing it on a real construction site, which showed an increase in the manufacturability of construction processes compared to traditional enclosing structures.

Friction mechanisms of brake pads made of cast iron and composite material on the surface of railway wheels

The friction mechanisms of brake pads made of cast iron and composite material (rubber-asbestos or rubber-free mixtures, including those with cast-iron inserts) on the surface of wheels of railway rolling stock were analysed. Values of the friction coefficient are given. The main role in the mechanism of friction of cast iron pads on the surface of railway wheels belongs to the microstructure of cast iron, namely, the amount, shape, distribution in volume of graphite inclusions, dispersion of pearlite and cementite inclusions, and the construction of phosphide eutectics. The friction mechanism of pads made of composite material on the wheel surface is determined by the content and properties of the components from which the composite rubber mixture is formed, their chemical and granulometric composition, as well as the degree of burnout of the composite rubber mixture during the operation of the brake pads. It is shown that due to the lack of regulation in the standards and technical conditions for composite blocks of the percentage content of ingredients and their chemical composition in the composite material, its properties and the value of the coefficient of friction are uncontrolled. Cyclic temperature changes on the contact surface of the brake pads with the wheels during train braking and high peak temperatures (up to 800 °C) in individual periods of braking lead to changes in the structure and properties of the surface layer of the metal of the wheels and the material of the brake pads. This is reflected in the frictional characteristics of the friction process of brake pads with wheels. The disadvantage of brake pads made of composite material is their low thermal conductivity compared to cast iron pads. As a result, the temperature in the contact zone of composite pads with wheels increases more than when using pads made of cast iron. The hydrodynamic effects of moisture (water) in the contact zone of brake pads with the surface of vehicle wheels are considered from the standpoint of classical tribology approaches. The microrelief of the working surface of the brake pads was studied. The influence of various factors on the coefficient of friction of the brake pads on the surface of the wheels is shown. Recommendations are provided for improving standards and technical conditions for brake pads made of cast iron and other materials. Keywords: brake pads, wheels of railway transport, friction, cast iron, composite materials, surface microrelief.

Weathering Processes and Mechanisms of Low-Grade Metamorphic Rock following Freeze-Thaw Processes

A remarkable risk in the field of rock engineering is the natural exposure of low-grade metamorphic rock to the freeze-thaw process. Examples of such materials are slate and phyllite, which are employed approximately mountainous permafrost zones. In order to study the weathering processes and mechanisms in low-grade metamorphic rock following freeze-thaw weathering processes, the freeze-thaw cycles were applied. Accordingly to this process, low-grade metamorphic rock partially or completely deteriorates, and mechanical properties, microstructure, and mineral content changes are commonly associated with freeze-thaw processes. This study shows that, following numerous freeze-thaw cycles processes, the quality, wave velocity, and compressive strength of rock reduce; calcite, mica, feldspar, and pyrite contents decrease; and clay minerals and gypsum content increase. Furthermore, the study also demonstrated that the changes in quality, wave velocity, and compressive strength caused by weathering of the rock followed the sequence slate > phyllite. However, the mineral contents of slate and phyllite alter in the same way. Because of this increase in volume and subsequent F-T cycles, new cracks form on the surfaces of pores due to outward pressure, and a lot of water is absorbed by clay minerals. The amount of water increases once again due to freezing as the temperature falls below zero degrees Celsius. This course duplicates with cyclic temperature changes and causes deterioration of low-grade metamorphic rock materials. By analysis of an examination of the damage variables based on wave velocity, it is possible to conclude that mineral changes during the freeze-thaw effect may make for the formation of voids and the deterioration of metamorphic rock. This research contributes to a better understanding of weathering processes and mechanisms of low-grade metamorphic rock following freeze-thaw weathering processes.

ASR performance of concrete at external alkali supply – effects of aggregate mixtures and blended cement

ABSTRACT The long-term durability of concrete pavements in wet-freeze climate is influenced by deicing salts used for winter maintenance of roads. Alkaline deicers could also be an external source of alkalis, therefore, their role in respect to the development of deleterious alkali-silica reaction (ASR) should be considered. The performance of mineral aggregate mixtures in concrete was studied using RILEM test procedure ‘60°C concrete test with external alkali supply’. Air-entrained concrete specimens were exposed to cyclic temperature changes, wetting-drying and NaCl solution immersion. Tests revealed the development of concrete expansion in time and associated changes of its elastic modulus for 31 mixtures of fine and coarse aggregate and Portland and fly ash or slag blended cements. The expansive behaviour of air-entrained concrete was strongly influenced by the presence of micro- and cryptocrystalline silica in aggregates. A replacement of nonreactive limestone sand by moderately reactive quartz sand resulted in a substantial enhancement of concrete expansion by a factor of three. The observed reduction of elastic modulus was correlated with more abundant presence of alkali-silica reaction products in concrete. Effects of cement type (CEM I, CEM II/A-V, CEM II/B-S) on the ASR performance of concrete with crushed coarse aggregate and quartz sand are discussed.

Gas-saturated carbon dioxide hydrates above sub-seabed carbon sequestration site and the formation of self-sealing cap

Storing CO2 in sub-seabed sediments is a promising CO2 sequestration method to reduce atmospheric CO2 concentrations and mitigate climate change. Leaked CO2 from the sequestration site can form CO2 hydrates in the sediment and reduce the effective permeability of the sediment layer, developing a self-sealing cap above the storage site. Previous studies usually treat the CO2 hydrate layer similarly to a methane hydrate reservoir. However, unlike methane, CO2 has a larger solubility that increases significantly with depth, leading to high gas saturation levels in the sediment. Near the base of the CO2 hydrate stability zone, CO2 free gas, CO2 hydrate and dissolved CO2 coexist, and elevated surface energies of the curved surfaces of bubbles and hydrate crystals in pores shift the phase equilibria. The bubbles entrapped in the three-phase zone help to reduce the effective permeability besides accumulate CO2 hydrates. We simulate the three-phase zone in a shallow seabed using Monte Carlo methods in packed synthetic mono-dispersed spherical sediment grains, taking into account of the surface curvatures of the gas–liquid and hydrate–liquid interfaces in irregular pore spaces constrained by the sediment grains, and estimate the permeability reduction caused by entrapped CO2 bubbles. Finer sediment grains, larger geothermal gradients and shallower seawater all tend to broaden the zone, and the effective permeability in the three-phase zone can be further reduced by up to an order of magnitude than that with hydrates only. We also analyze the sensitivity of the three-phase zone due to temperature and pressure perturbations, and the results suggest possible thickness and depth variations of the zone due to cyclic temperature and sea level changes. This work demonstrates the difference between the CO2 hydrate-bearing sediment layer and the methane hydrate reservoir, and provides insight into the formation mechanism of the self-sealing cap above the sequestration sites.

Reinforcement Corrosion Testing in Concrete and Fiber Reinforced Concrete Specimens Exposed to Aggressive External Factors.

One of the leading causes of reinforced concrete degradation is chloride attack. It occurs due to the penetration of chlorides through pores and cracks into the concrete cover. This phenomenon becomes more dangerous if reinforced concrete elements are subjected to cyclic temperature changes. The concrete cover protects against corrosion. This paper presents research, the primary purpose of which was to determine the effect of the addition of steel fibers to concrete on the development of corrosion of the main reinforcement. The tests were carried out on three types of reinforced concrete specimens made of ordinary concrete and concrete with different amounts of steel fibers (0.25% and 0.50%). In order to initiate corrosion processes, specimens were partially submerged in a 3% sodium chloride solution and were subjected to freeze-thaw cycles. The electrochemical polarization galvanostatic pulse method was used for analyzing the reinforcement corrosion activity. Moreover, it was verified whether the corrosion of reinforced concrete elements affects the acoustic emission wave velocity. The addition of steel micro-reinforcement fibers increases the corrosion resistance of reinforced concrete. In addition, a strong linear correlation between the AE wave velocity and the values of the corrosion current density was revealed.

An analysis of interannual data on phytoplankton and zooplankton associations in fluctuating soda lakes

Aquatic ecosystems in arid regions are important and unique habitats for aquatic wildlife and support species biodiversity. Endorheic soda lakes in Central Asia are subject to great fluctuations in its size depending on climatic conditions. Here we present research results covering the last 20 years, including the period from the maximum filling of the Torey lakes (Transbaikalia, Russia) to their aridification and the initial phase of their refilling. Barun-Torey and Zun-Torey lakes are the largest soda lakes in the arid zone with a large water surface area. A unique feature of lakes is the unstable hydrological regime. The reason for the water level change is the periodic filling and drying of lakes due to cyclic climatic changes in humidity and temperature. We aim to determine the response of the plankton communities to environmental change in the Torey lakes over this period. We analysed the interannual variations in phytoplankton and zooplankton species richness, structure and density. We conducted our studies during various phases of the hydrological cycle, including three periods for Zun-Torey Lake (high (1999 and 2003), intermediate (2007, 2011) and low (2014 and 2016) lake levels) and two periods for Barun-Torey (high water level (1999 and 2003) and the initial filling phase (2014, 2016, 2018, 2020, 2021)). Barun-Torey Lake dried up completely in summer 2009 and since 2013 the initial refilling has been observed. Zun-Torey Lake dried up in fall 2017. Changes in water level and water chemistry determined plankton biodiversity transformation. Fluctuations of plankton communities were different for each lake and conformed the phases of the hydrological cycle. We identified four stage changes in plankton associations during the aridification of Zun-Torey Lake, and three stage changes in plankton associations as the water level fell and through the initial refilling phase for Barun-Torey Lake.

Effect of Heating and Cooling of Aluminium-Based Fibre-Metal Laminates on Their Tensile Strength

The widespread use of composite materials in the construction of machines encourages to better understand their properties and the impact of various external factors on these properties. Fibre metal laminates (FMLs) consist of alternating layers of metal and a polymer matrix laminate reinforced with continuous fibres. The aim of this work was to investigate the effect of cyclical temperature changes and thermal shocks (heating the sample to a high temperature in a short time) on the strength properties of FMLs from AW-1050A aluminium sheet, glass fibre fabric and carbon fibre fabric. The research concerns the determination of how cyclical temperature changes affect the tensile strengths properties of FMLs. The results indicate a small effect of the cycles on the tensile strength of the composites. The composites with glass fibre reinforced laminate also showed high resistance to delamination, moreover, the samples did not delaminate even after they were broken. The carbon fibre reinforced laminate composites showed a tendency to delaminate after heat treatment.

Long term coupling of AE sensors in high temperature environment

Acoustic emission monitoring took place in a long-term test in a coal fired power plant in Germany for 4 years. The test object was a steel pipe with a ceramic matrix composite (CMC) jacket, which was installed in a bypass of a live steam line. The peak operating temperature was approx. 530 °C. The aim of the AE monitoring was to investigate damage evolution in the CMC jacket. The test pipe was removed from the power plant in 2021. Due to several reasons, the jacket did not experience high loads during operation and therefore information about damage mechanisms in the AE data is rare. However, a lot could be learned from this project concerning long term stability of the equipment in rough environment, data storage and filtering and about sensor long term coupling of AE sensors at high temperatures. The high temperature AE sensors were applied directly onto the surface of the steel pipe and the CMC jacket using a ceramic glue. Additionally, the sensors were held by stainless steel bands which were acoustically decoupled by mineral wool. The application of the sensors took place at a temperature of about 30 °C. It was not possible to check on the quality of the coupling during operation as the whole pipe was covered by insulation, including all sensors. Also, the sensors did not allow pulsing as test for the coupling quality. Effects like thermal expansion of sensors, CMC jacket and ceramic glue had to be considered. Additionally, the power plant adepts its electrical output to the demands by flexible operation. This leads to cyclic changes in temperature and a high amount of heating and cooling cycles, which affects the coupling condition of the AE sensors. During dismantling of the test pipe from the power plant, a visual inspection of the sensor coupling was possible. The condition of the sensor coupling was documented before and after the sensors were removed. Pencil lead breaks were performed to quantify the quality of coupling as well. It could be shown that long term coupling of AE sensors is possible even in high temperature environment combined with cyclic temperature changes.

Дослідження залежності мікротвердості модифікованих поверхонь титанових сплавів від глибини насичення азотом при вакуумному іонному плазмовому азотуванні в термоциклічному режимі

A study of titanium alloys with a strengthened nitrided layer by the method of vacuum ion plasma nitriding in thermocyclic mode was carried out to determine the regularity of the effect of diffusion saturation parameters on microhardness. At the same time, the effect of anomalous mass transfer of nitrogen in the surface of the part being processed was used, by creating a field of thermal stresses in the surface layer due to the cyclic inclusion and exclusion of the glow discharge and cyclic temperature changes. Microhardness studies of strengthened surface layers were carried out on metallographic slides using a PMT-3M microhardness meter, the thickness of the nitride layer was controlled using microstructural analysis using a MIM-10 microscope, and the phase composition of the surface layer was monitored using a DRON-3M device. Vacuum ionic nitriding in the thermocyclic regime of titanium alloys allows changing the physical and mechanical characteristics within wide limits (diffusion layer depth up to 500 μm, microhardness up to 9600 MPa, phase composition of nitrided surfaces, etc.), obtaining surface layers with different phase composition (α , γ' and ε - phases) with and without the nitride zone, depending on the temperature, pressure, composition of the saturating medium, and the size of the temperature cycles. Nitriding in a glow discharge achieves a high hardness of the surface of titanium alloys – up to 10,000 MPa, but at the same time the plasticity of the nitrided layers is sharply reduced and the tensile strength of the material is reduced by 30%. Conclusions. 1. The amount of microhardness depends on the phase composition of the surface. Three phases TiN, Ti2N and Ti(N) are formed on the surface of the VT1-0 alloy after nitriding. The hardness of the surface layer of nitrided titanium depends on the ratio of these phases and is higher, the greater the amount of the TiN phase. The TiN phase has the highest hardness, the Ti2N phase has the lowest hardness. The hardness of the internal nitriding zone (Ti(N)) varies depending on the concentration of nitrogen in it. 2. By changing the parameters of the vacuum ion nitriding process in the pulse mode (temperature, pressure, composition of the saturating medium and nitriding time), it is possible to change the physical and mechanical characteristics (diffusion layer depth up to 300 μm, microhardness up to 9600 MPa, different hardness gradient along the depth , phase composition of nitrided surfaces, etc.), obtain surface layers with different phase composition (α, γ' and ε - phases) with and without a nitride zone, depending on temperature, pressure, composition of the saturating medium, and size temperature cycles, which allows to optimize the properties of the surface layer in specific operating conditions. 3. Increasing the nitriding time of titanium alloys contributes to increasing the thickness of the nitrided layer to 300 microns. Addition of inert helium and argon gases to the saturating medium helps to increase the plasticity and thickness of the nitrided layer.

Subrecent charophyte flora from Çeşmealti (Izmir Gulf, Western Turkey): Palaeoecological implications

A charophyte assemblage from lower Holocene sediments in Çeşmealtı (Izmir Gulf, Turkey) is described and illustrated for the first time here. This assemblage is composed of well-preserved gyrogonites and oospores of Lamprothamnium papulosum, which occur in association with seeds of Ruppia cf. maritima and up to eight ostracod species and 22 benthic foraminiferal taxa. The fossil assemblage was extracted from five clay intervals of the SK-2 borehole. The presence of monospecific assemblages of L. papulosum indicates that very shallow (up to 1 m deep), brackish, alkaline, and oligotrophic waters prevailed in the gulf under strong seasonality (marked cyclical changes in humidity and/or temperature). The dominance of germinated gyrogonites suggests that the water salinity of the lagoon decreased during the humid season, reaching at least 10‰. The associated ostracod fauna supports the palaeoenvironmental conditions inferred from the flora. Despite the gulf receiving considerable freshwater input, it remained semiconnected to the sea, as indicated by the diverse foraminifera assemblage. The availability of freshwater in the Izmir Gulf during the early Holocene Climatic Optimum, between ∼9000 and 5000 years BP, might have played an important role for the first human settlements in the area dating back 5500 years BP.