New Tribological Aspects in the Micro-Areas of the Symmetric Rolling-Sliding Contact

: The study of wear that occurs during operation in the wheel–rail assembly is a difﬁcult process to analyze. The phenomena that accompany the wear process are extremely complex and involve many factors, which vary greatly over different periods of time and at different times of wheel–rail contact. Estimating the behavior of the system and its wear in operation is difﬁcult to obtain. However, for common engineering applications, for which the determining factors, such as road proﬁle, load, skid, speed and weather conditions, are known, useful results can be obtained by laboratory tests or by numerical simulation. The article aims to model the complex phenomena that take place in the rail wheel system, taking into account the impact that most essential operational factors have. For this, the Finite Element Method (FEM) is used, thus, trying to explain the wear mechanisms of the wheel–rail system. The obtained results are veriﬁed in the laboratory. The main observation in the paper refers to the fact that in the areas of maximum stress and deformation, cracks appear at the micro scale. FEM proved to be a method that can predict the appearance of these microcracks, the experimental results validating the numerical experiments. The research offers results that can prove to be of great importance in practice, for the analysis and improvement of railway safety.


Introduction
The system wheel-rail is a very complex system and the study of wear and the effect of different factors on this is a difficult process. Constant change and overlapping of various factors in a very short time represents reasons why the study of wear is hard to conduct [1,2]. However, thanks to the conducted simulation tests in the wheel-rail combination, it was possible to determine the wear mechanism depending on the limitations existing on the railroad track, e.g., load, slippage and speed, as well as atmospheric conditions, including the medium [3][4][5][6][7][8].

Influence of the Climate on Rolling-Sliding Contact
In the following, we try to offer a short synthesis on how climate particularities can influence the proposed phenomena, i.e., the rolling-sliding contact and, consequently, the wear magnitude.
Water presence in the wheel-rail contact is undoubtedly an important operating factor in the mechanism of wear. Thus, for the existence of water in the wheel-rail contact in Poland, according to the Central Statistical Office (GUS), from April to October, there are as many as 130 days of rainfall, i.e., the annual sum of rainfall is within the limits of Symmetry 2022, 14, 1523 2 of 13 500-700 mm, which may be calculated as approximately 500-700 L per m 2 . In most parts of Europe, the annual precipitation is 500-800 mm (500-800 L per m 2 ). In mountain areas, it is higher-usually 1000-2000 mm-also in the case of ranges and coasts particularly exposed to westerly winds from the ocean or seas 3000-4000 mm. About 500 mm and below falls on the south-eastern and eastern parts of the Balkan Peninsula, Apenian and Iberian Peninsula, the central and north-eastern parts of the Scandinavian Peninsula, in southern Ukraine and in the mountain-sheltered basins (in Old Castile even below 250 mm). In the European part of Russia, in the central belt, precipitation is mostly 400-600 mm and decreases toward the northeastern extremities to less than 300 mm, and in the Transcaspian Lowland, it falls even to 100-200 mm. In summer, in southern Europe, when it is overrun by tropical air masses, there is a dry season; precipitation occurs only in autumn and winter, and its maximum falls between September and December, only in northern Spain (in May). In the rest of Europe, precipitation is more evenly distributed over the year. In the west, cyclonic autumn-winter precipitation is more abundant, with the maximum between October and December. In Eastern Europe, Central Europe and the Scandinavian Peninsula, separated from the Atlantic by mountains, the summer convective precipitation is more effective. Their maximum falls in June (in the south of this part of Europe), in July (in the central belt) or in August (on the Scandinavian Peninsula) [9][10][11][12]. Attention should also be paid to the presence of water on the rolling surface of the railroad rails in the winter season, which is formed from standing snow. Thus, the influence of water has an important effect on the tribological behavior of the wheel-rail association, as it can penetrate into the resulting cracks and crevices in the surface layer of RCF (Figures 1 and 2) [13][14][15][16][17][18][19]. Poland, according to the Central Statistical Office (GUS), from April to October, there are as many as 130 days of rainfall, i.e., the annual sum of rainfall is within the limits of 500-700 mm, which may be calculated as approximately 500-700 L per m 2 . In most parts of Europe, the annual precipitation is 500-800 mm (500-800 L per m 2 ). In mountain areas, it is higher-usually 1000-2000 mm-also in the case of ranges and coasts particularly exposed to westerly winds from the ocean or seas 3000-4000 mm. About 500 mm and below falls on the south-eastern and eastern parts of the Balkan Peninsula, Apenian and Iberian Peninsula, the central and north-eastern parts of the Scandinavian Peninsula, in southern Ukraine and in the mountain-sheltered basins (in Old Castile even below 250 mm). In the European part of Russia, in the central belt, precipitation is mostly 400-600 mm and decreases toward the northeastern extremities to less than 300 mm, and in the Transcaspian Lowland, it falls even to 100-200 mm. In summer, in southern Europe, when it is overrun by tropical air masses, there is a dry season; precipitation occurs only in autumn and winter, and its maximum falls between September and December, only in northern Spain (in May). In the rest of Europe, precipitation is more evenly distributed over the year. In the west, cyclonic autumn-winter precipitation is more abundant, with the maximum between October and December. In Eastern Europe, Central Europe and the Scandinavian Peninsula, separated from the Atlantic by mountains, the summer convective precipitation is more effective. Their maximum falls in June (in the south of this part of Europe), in July (in the central belt) or in August (on the Scandinavian Peninsula) [9][10][11][12]. Attention should also be paid to the presence of water on the rolling surface of the railroad rails in the winter season, which is formed from standing snow. Thus, the influence of water has an important effect on the tribological behavior of the wheel-rail association, as it can penetrate into the resulting cracks and crevices in the surface layer of RCF (Figures 1 and  2) [13][14][15][16][17][18][19].   Some data give us a picture of the level of precipitation in different parts of th In the USA, the level of precipitation is 767 mm (based on data collected by weathe provided by NOAA National Climatic Data Center). In Asia, precipitation is 200 the South, between 2000 and 3000 mm in the Southeast and between 7600 and 12 in the East (Britanica). In South America, the precipitation varies from 610 to 1420 year and in Africa, less than 1000 mm per year (Britanica).
Of course, depending on the geographical area, connected with its local clim cific state, the magnitude of the water presence will be changed and, conseque wear process will be influenced. In the following, we will detail this very signif fluence of the water's presence in the contact area on the wear magnitude, as w the lifetime diminishing.

Wear Presence Particular Influence on the Contact Area
To analyze the wear of the wheel-rail system, it is necessary to know wh impact of the essential operational factors. Knowing these FEM factors become suitable method of analysis; within the FEM model, there is the possibility to in as input data, the factors that influence the wear of the system. It is, thus, possible t the stress field, which has a major influence on the wear phenomena. Thus, t where micro-cracks appear can be identified.
One of the many processes studied is the wear mechanism is the presence of cating medium. For such a contact, apart from the effect of pitting and other types there is a phenomenon of fatigue crack propagation due to the spreading actio fluid, which starts from the surface and leads inside to the material [20][21][22][23][24][25][26][27]. The of the material inside the rail is due to stress propagation. This process is closely to the so-called fluid confinement effect, whereby the fluid present in the crack is c as it passes through the contact area. The fluid closure occurs as a result of the s cuiting of the crack edges forced by the contact load when the walls inside the c bifurcated. It is assumed that at the moment of the crack edge closure ( Figure 3) fluid between its bifurcated walls below the edge, which entered the crack inter the external contact surface. When the crack is closed, there is a high fluid pressu the crack, which acts on the walls of the crack causing the crack to enlarge [28,29]  Some data give us a picture of the level of precipitation in different parts of the world. In the USA, the level of precipitation is 767 mm (based on data collected by weather station provided by NOAA National Climatic Data Center). In Asia, precipitation is 2000 mm in the South, between 2000 and 3000 mm in the Southeast and between 7600 and 12,700 mm in the East (Britanica). In South America, the precipitation varies from 610 to 1420 mm per year and in Africa, less than 1000 mm per year (Britanica).
Of course, depending on the geographical area, connected with its local climate specific state, the magnitude of the water presence will be changed and, consequently, the wear process will be influenced. In the following, we will detail this very significant influence of the water's presence in the contact area on the wear magnitude, as well as on the lifetime diminishing.

Wear Presence Particular Influence on the Contact Area
To analyze the wear of the wheel-rail system, it is necessary to know what is the impact of the essential operational factors. Knowing these FEM factors becomes a very suitable method of analysis; within the FEM model, there is the possibility to introduce, as input data, the factors that influence the wear of the system. It is, thus, possible to obtain the stress field, which has a major influence on the wear phenomena. Thus, the areas where micro-cracks appear can be identified.
One of the many processes studied is the wear mechanism is the presence of a lubricating medium. For such a contact, apart from the effect of pitting and other types of wear, there is a phenomenon of fatigue crack propagation due to the spreading action of the fluid, which starts from the surface and leads inside to the material [20][21][22][23][24][25][26][27]. The cracking of the material inside the rail is due to stress propagation. This process is closely related to the so-called fluid confinement effect, whereby the fluid present in the crack is confined as it passes through the contact area. The fluid closure occurs as a result of the short-circuiting of the crack edges forced by the contact load when the walls inside the crack are bifurcated. It is assumed that at the moment of the crack edge closure (Figure 3), there is fluid between its bifurcated walls below the edge, which entered the crack interior from the external contact surface. When the crack is closed, there is a high fluid pressure inside the crack, which acts on the walls of the crack causing the crack to enlarge [28,29].
Water can penetrate through cracks and pores and, as a result, will increase the length of the subsurface fracture (Figures 4 and 5).
In Figures 3 and 4, we observed the cross-section perpendicular to the rail surface using intrusion material.
Rolling/sliding phenomena have long been studied due to the importance of the phenomenon in engineering applications. Numerous results have been published in older studies by various researchers [30][31][32][33][34][35][36][37][38][39]. The present study provides an explanation of the wear mechanism using FEM simulation tests.  Water can penetrate through cracks and pores and, as a result, wi of the subsurface fracture (Figures 4 and 5). Water can penetrate through cracks and pores and, as a result, will increase the length of the subsurface fracture (Figures 4 and 5).   In Figures 3 and 4, we observed the cross-section perpendicular to the rail surface using intrusion material.
The present study provides an explanation of the wear mechanism using FEM simulation tests.

Materials and Methods
Numerical simulations using a computer microtomograph will allow locating the places most exposed to wear and damage depending on various operating conditions. The wear phenomenon is a complex phenomenon; the wear residues have size and shape determined by the essential factors that intervene in the contact between the rail and the wheel. They may depend on the condition and properties of the surface layer of the materials in contact or the type of heat treatment of the material. For a pertinent analysis, all these factors and their role in the interaction that takes place must be considered [40][41][42].
One of the tasks undertaken in this paper is an attempt to explain the cause-andeffect relationship of operating parameters to the wear of the wheel/rail contact zone. To obtain a faithful reflection of the friction surface, the following procedure was used.
Data imported from a computer microtomograph ( Figure 6) were processed using free software, FIJI. The images obtained under the microtomograph were saved in a lossless format (* .raw) in (* .stl) format. Using GMSH software, the geometric model allows you to generate a mesh and export to an MES solver (MSC.Marc). Figure 7 presents the stages of the procedure.

Materials and Methods
Numerical simulations using a computer microtomograph will allow locating the places most exposed to wear and damage depending on various operating conditions.
The wear phenomenon is a complex phenomenon; the wear residues have size and shape determined by the essential factors that intervene in the contact between the rail and the wheel. They may depend on the condition and properties of the surface layer of the materials in contact or the type of heat treatment of the material. For a pertinent analysis, all these factors and their role in the interaction that takes place must be considered [40][41][42].
One of the tasks undertaken in this paper is an attempt to explain the cause-and-effect relationship of operating parameters to the wear of the wheel/rail contact zone. To obtain a faithful reflection of the friction surface, the following procedure was used.
Data imported from a computer microtomograph ( Figure 6) were processed using free software, FIJI. The images obtained under the microtomograph were saved in a lossless format (*.raw) in (*.stl) format. Using GMSH software, the geometric model allows you to generate a mesh and export to an MES solver (MSC.Marc). Figure 7 presents the stages of the procedure.
The subject of numerical simulations is analysis in the field of stresses in the contact zone for the real contact surface in the following sets of tests: (a) Analysis of friction surface (deformable); (b) Analysis of friction surface (deformable) and rigid ideal surface (rigid).
The analyzed model was a section of the friction surface with a width of 1.9 mm and a height not exceeding 16 µm. The tests were performed for 2D and 3D models. The MSC.Software/Marc system was used for numerical analysis. Non-linear calculations were performed. Rail material with the following material coefficients was assumed: Young's modulus E = 210,000 MPa and Poisson's ratio ν = 0.3 and a density ρ = 7900 kg/m 3 . The detailed boundary conditions are illustrated in Figure 7. In the research, 453 (tri3) and 1942 (quad4) elements were used to build the model. In order to compare the obtained results, a model with a real contact surface was used (Figure 8).  The subject of numerical simulations is analysis in the field of stresses in the contact zone for the real contact surface in the following sets of tests: (a) Analysis of friction surface (deformable); (b) Analysis of friction surface (deformable) and rigid ideal surface (rigid).
The analyzed model was a section of the friction surface with a width of 1.9 mm and a height not exceeding 16 μm. The tests were performed for 2D and 3D models. The MSC.Software/Marc system was used for numerical analysis. Non-linear calculations were performed. Rail material with the following material coefficients was assumed: Young's modulus E = 210,000 MPa and Poisson's ratio ν = 0.3 and a density ρ = 7900 kg/m 3 . The detailed boundary conditions are illustrated in Figure 7. In the research, 453 (tri3) and 1942 (quad4) elements were used to build the model. In order to compare the obtained results, a model with a real contact surface was used (Figure 8). The subject of numerical simulations is analysis in the field of stresses in the zone for the real contact surface in the following sets of tests: (a) Analysis of friction surface (deformable); (b) Analysis of friction surface (deformable) and rigid ideal surface (rigid).
The analyzed model was a section of the friction surface with a width of 1.9 a height not exceeding 16 μm. The tests were performed for 2D and 3D mod MSC.Software/Marc system was used for numerical analysis. Non-linear calc were performed. Rail material with the following material coefficients was a Young's modulus E = 210,000 MPa and Poisson's ratio ν = 0.3 and a density ρ = 790 The detailed boundary conditions are illustrated in Figure 7. In the research, 453 ( 1942 (quad4) elements were used to build the model. In order to compare the results, a model with a real contact surface was used (Figure 8).

Results
From the experiments, we determined the contact area. As a result of the FEM analysis, the results of simulation tests are presented in Figure 9. Table 1 presents a summary of the obtained results.  a first stage, tribological tests were performed on the samples. The specimens were then analyzed microscopically to explain the main causes of the wear mechanism. Based on the microscopic analysis, discrepancies between dry and wet contact could be examined. Figure 10 shows the illustrative surfaces after performing the tribological test. During the tests, it was observed that there are discrepancies in the wear process on the  In a first stage, tribological tests were performed on the samples. The specimens were then analyzed microscopically to explain the main causes of the wear mechanism. Based on the microscopic analysis, discrepancies between dry and wet contact could be examined. Figure 10 shows the illustrative surfaces after performing the tribological test. During the tests, it was observed that there are discrepancies in the wear process on the friction surface. The cracks that are found to appear in the case of wet contact cause ragged flakes protruding upwards. At dry contact, the phenomenon of wear pitting does not occur (Figure 10a,b).  If the friction surface in a wet contact shows roughness and cracks, it leads to further propagation of cracks. If the water pressure is high, this can lead to the development of large craters on the surface. Figure 11 shows the wear mechanism in case of rolling/sliding contact between the wheel-rail. If the friction surface in a wet contact shows roughness and cracks, it leads to further propagation of cracks. If the water pressure is high, this can lead to the development of large craters on the surface. Figure 11 shows the wear mechanism in case of rolling/sliding contact between the wheel-rail. If the friction surface in a wet contact shows roughness and cracks, it leads to propagation of cracks. If the water pressure is high, this can lead to the developm large craters on the surface. Figure 11 shows the wear mechanism in case of rolling/ contact between the wheel-rail. The mechanism of wear and propagation of fatigue cracks is shown in Figure 8. The mechanism of wear and propagation of fatigue cracks is shown in Figure 8.

Discussion and Conclusions
Numerical analyses allow one to explain the mechanism of wear and determine local stress values. FEM proved to be a useful tool to identify the areas of special wear hazard. FEM can be used to explain the mechanism of wear in selected operational conditions.
In the paper, we modeled the real contact area and the relationship between the analyzed friction surface and its susceptibility to mechanical damage.
The following conclusions can be formulated: -Destruction of the surface layer in rolling-sliding contact takes the form of the delamination wear mechanism in the form of the flake wear debris, -Propagation of cracks is on the depth and from the surface, research has proven this, -Numerical analysis (FEM) allows one to explain the mechanism of wear and determine the local stress values, - The obtained results of operational investigations prove that cracks and spallings of the micro and macro scale appear in areas with maximum stress and deformation, -FEM is the right tool used to identify the areas of special wear hazard; in addition, this method also helps to explain the wear mechanisms and determine the characterization of wear, depending on the selected operational conditions, - The depth of residual stress can be determined and correlated with the thickness of the obtained flake wear debris appearing on the surface, -It is advisable to monitor the execution of the state of the mobile device surface layer for the diagnostic criteria proposed in this paper.