Pavel Pokorný - Academia.edu (original) (raw)

Papers by Pavel Pokorný

Materials

Fatigue crack propagation in near-threshold regime was studied in the 304L austenitic stainless s... more Fatigue crack propagation in near-threshold regime was studied in the 304L austenitic stainless steel in two microstructural states: as-received (AR) with finer microstructure and low susceptibility to the transformation-induced plasticity (TRIP) effect, and solution-annealed (SA) with coarser microstructure and higher susceptibility to TRIP. At the load ratio R = 0.1 the threshold was higher in the SA state than in the AR state due to coarser grains and possibly the TRIP effect. In order to clarify the role of crack closure, experiments at R = 0.7 were done. The threshold in the SA state was still higher by 1 MPa·m0.5. This effect was identified as crack tip shielding induced by phase transformation, an example of a non-closure shielding effect. Higher resistance to crack growth in the SA state was attributed to promoted martensitic transformation in non-favorable oriented grain families rather than thicker martensite layers in the crack path area. The conclusions were verified by ...

Materials, 2021

Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominant... more Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominantly considered as insignificant cycles without any influence on fatigue lifespan of engineering structural components. However, this paper shows that in some cases these underload cycles can retard the consequent crack propagation quite significantly. This phenomenon is a consequence of oxide-induced crack closure development during cyclic loading below the threshold. The experimentally described effect of fatigue crack growth retardation was supported by measurement of the width and the thickness of the oxide debris layer using the EDS technique and localized FIB cuts, respectively. Both the retardation effect and the amount of oxide debris were larger for higher number and larger amplitudes of the applied underload cycles. Crack closure measurement revealed a gradual increase of the closure level during underload cycling. Specimens tested in low air humidity, as well as specimens left w...

Solid State Phenomena, 2017

Growth of long fatigue cracks in Ti6Al4V alloy manufactured by direct metal laser sintering (DMLS... more Growth of long fatigue cracks in Ti6Al4V alloy manufactured by direct metal laser sintering (DMLS) was investigated. Two DMLS systems, EOSINT M270 and EOSINT M290, with different process parameters were used for production of CT specimens having three different orientations of crack propagation with respect to the DMLS build direction. The as-built specimens were stress relieved at 740 °C. The fatigue crack growth curve and the threshold values of the stress intensity factor for crack propagation were experimentally determined. It has been found that the chosen DMLS processing parameters and the used stress relieving procedure results in material exhibiting isotropic crack growth behavior, i.e. the crack growth was found to be independent of the DMLS build direction. The fatigue crack growth rates and the threshold values for the crack growth were compared with published results characterizing the as-built material and material after different post processing heat treatments.

International Journal of Fatigue, 2021

Abstract The experimental determination of the resistance to fatigue crack propagation in steel, ... more Abstract The experimental determination of the resistance to fatigue crack propagation in steel, particularly in the so-called near-threshold regime, is a crucial issue for safety-relevant components which are designed to operate for a large number of loading cycles before undergoing periodic inspections. This work summarizes the most relevant results of extensive experimental campaigns conducted at BAM and IPM over the last years, which have been devoted to the determination of fatigue crack propagation characteristics on medium and high strength steels. The influence of environment, oxide-induced crack closure and test methodologies in the near-threshold regime are discussed.

Theoretical and Applied Fracture Mechanics, 2020

Decomposition of the resistance to fatigue crack growth into the intrinsic and extrinsic componen... more Decomposition of the resistance to fatigue crack growth into the intrinsic and extrinsic component is very important for understanding of fatigue failure mechanisms, relation to microstructure and modelling of residual fatigue life. Crack closure for four grades of steel were estimated by the difference between K max values and the effective ΔK eff values (measured at the load ratio R = 0.8) corresponding to the same crack growth rate. The results showed that crack closure values obtained by the difference K max-ΔK eff were not in agreement with the available crack closure models, both the Newman's model of plasticity-induced closure and the results from finite element analysis. The discrepancies could not be explained by the effect of mean stress, specimen thickness, loading amplitude or T-stress. Therefore, the application of fracture mechanics to fatigue cracks should be revisited. It was pointed out that ΔK eff may not be a good parameter for quantification of the crack driving force, since the relationship between K max-K cl and the cyclic plastic deformation at the crack tip might not be linear.

Theoretical and Applied Fracture Mechanics, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Engineering Failure Analysis, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The Journal of Strain Analysis for Engineering Design, 2019

Experimental measurements of fatigue properties of old steel used for a crane runway were perform... more Experimental measurements of fatigue properties of old steel used for a crane runway were performed to capture the influence of corrosion on fatigue life of the material. Basquin’s law was used to quantify the fatigue properties of old steel with different cross sections and with different surface of specimens (polished and corroded). The finite element model was prepared to assess and quantify the various stress distribution in specimens with circular and rectangular cross sections. Fracture surfaces of the three kinds of specimens (circular polished, rectangular polished and rectangular corroded) were studied and they showed the surface crack initiation. The following fatigue cracks developed from the surface and expanded into specimen with radiation pattern. Observed crack initiation areas confirmed the ones expected according to stress concentrations.

Strojnícky casopis – Journal of Mechanical Engineering, 2017

Description of stress singularity in the vicinity of a free surface is presented. Its presence ca... more Description of stress singularity in the vicinity of a free surface is presented. Its presence causes the retardation of the fatigue crack growth in that region and fatigue crack is being curved. Numerical model is used to study dependence of the stress singularity exponent on Poisson’s ratio. Estimated values are compared to those already published. Experimentally measured angles of fatigue crack on SENB specimens confirm the relation between Poisson’s ratio and the angle between crack front and free surface.

Materials Science and Engineering: A, 2020

The hybrid laser-tungsten inert gas welding technology was applied to butt weld 3-mm-thick S460MC... more The hybrid laser-tungsten inert gas welding technology was applied to butt weld 3-mm-thick S460MC and S700MC high-strength low-alloy steel sheets. The intent of low-current arc addition was to preheat the material to reduce extremely fast cooling rate accompanying laser welding. High-cycle fatigue tests were performed to evaluate the effect of welding conditions on fatigue behavior of S460MC and S700MC laser and hybrid welds. Both base materials exhibit approximately the same fatigue limit. However, corresponding samples with the weld behaved significantly different under cyclic loading. We demonstrated that the drop of their fatigue lifetime is caused by the combination of microstructural changes and especially by the presence of surface notches resulting from the welding process. The two series of both steels were tested to reveal the contribution of induced surface roughness and microstructural changes itself. The first one was in the as-welded condition. The second one had a smooth surface resulting from surface layer grinding after the welding. It was found, that the geometric notches created by the welding process are the determining factor in case of both tested steels. Testing of the ground samples showed, that in case of S460MC steel, the application of pre-heat did not improve the fatigue properties. On the other hand, the performed fatigue tests showed improved fatigue limit of ground series samples of the preheated S700MC steel, when compared with the conventional laser welding.

Procedia Structural Integrity, 2019

The growth of long fatigue cracks in Ti6Al4V alloy manufactured by direct metal laser sintering w... more The growth of long fatigue cracks in Ti6Al4V alloy manufactured by direct metal laser sintering was experimentally determined on CT specimens. The crack propagation was investigated in three different orientations with regard to the building direction. The as-built CT specimens were heat treated at temperatures 380, 740 and 900 °C. The crack growth curves with the threshold values of the stress intensity factor Kth were determined. The measurement was carried out in laboratory air at room temperature and stress ratios R = 0.1 and 0.8. It was found that the crack growth rate in the Paris region is identical for all three heat treatments for the same R ratio. For the particular stress intensity factor range K the crack growth rates at R = 0.1 and 0.8 differs by factor of two. Contrary to the Paris region there is a clear effect of the heat treatment in the near threshold region. For the material heat treated at 380 °C and loaded at R = 0.1 the threshold is 3.7 MPam 1/2 , for 740 °C 2.7 MPam 1/2 and for 900 °C 3.5 MPam 1/2. The explanation of the observed dependence of the threshold on the post processing heat treatment is discussed in terms of microstructure and residual stresses.

Theoretical and Applied Fracture Mechanics, 2019

A propagating fatigue crack may be partly retarded thanks to a phenomenon called fatigue crack cl... more A propagating fatigue crack may be partly retarded thanks to a phenomenon called fatigue crack closure. The ability to accurately describe this phenomenon is of interest for the scientific and engineering community, because of its significant impact on the fatigue crack propagation rate. A strategy for numerical modelling of the most common closure mechanism the plasticity induced crack closureis presented in this paper. It was observed that the generally adopted suggestions for this type of simulations, such as the length of the crack growth or the number of substeps, are not necessarily valid in general, but require to be individually specified for particular conditions. The size of the elements in the vicinity of the crack front is also a widely discussed issue and it is shown here that even without convergence, the element size may be chosen as a fixed parameter leading to very reasonable closure values with low computational costs. A method of closure level determination based on change in specimen stiffness is described here and its performance is compared to the traditional first node displacement method with Load-Debond-Unload (LDU) and Load-Debond-Unload-Load-Unload (LDULU) loading schemes.

Engineering Fracture Mechanics, 2017

Engineering Failure Analysis, 2017

The railway axles are subjected to cyclic loading during their operation. Their load is of long-t... more The railway axles are subjected to cyclic loading during their operation. Their load is of long-term nature, therefore a real risk of fatigue failure exists. This failure could lead to derailment of the whole train with serious consequences. To prevent such scenario, the railway axles have to be safely removed from operation before their final failure occurs. This paper presents methodology for the residual fatigue lifetime prediction of the railway axle based on the linear elastic fracture mechanics concept. The methodology contains estimation of the critical position of initial crack, prediction of the fatigue crack front shape development during crack propagation, separation of the bending and press-fitting contributions to the axle load, experimental measurement of the crack growth kinetics of EA4T steel and subsequent estimation of the residual fatigue lifetime of railway axle. Part of the presented study is also devoted to the probability aspects of determination of material characteristics describing fatigue crack propagation and retardation effects caused by existence of plastic zone ahead of propagating fatigue crack. Described methodology is already applied in the design process of new railway axles in Bonatrans company.

Solid State Phenomena, 2016

Railway axles are subjected to cyclic amplitude loading which can lead to fatigue failure. For sa... more Railway axles are subjected to cyclic amplitude loading which can lead to fatigue failure. For safe operation of railway axles a damage tolerance approach taking into account a possible defect in railway axle is often required. Because of different operation regimes of trains (fast/slow ride, ride on straight track, on curved track, over switches etc.) the load amplitude of axle is not constant. The variability of load is defined by a load spectrum, which is determined experimentally by measuring of load in service conditions. Even though the load spectrum is measured on several hundreds or thousands of operation kilometres, the railway axles are in operation much longer time (often tens of years). Therefore, some load amplitudes higher than ones measured in the test can occur during a long-term axle service. The contribution presented deals with the effect of extension of load spectrum by rare high load amplitudes, which can occur during long-term operation, on residual fatigue lif...

Journal of Multiscale Modelling, 2016

Railway axles are subjected to cyclic loading which can lead to fatigue failure. For safe operati... more Railway axles are subjected to cyclic loading which can lead to fatigue failure. For safe operation of railway axles a damage tolerance approach taking into account a possible defect on railway axle surface is often required. The contribution deals with an estimation of residual fatigue lifetime of railway axle with initial inclined surface crack. 3D numerical model of inclined semi-elliptical surface crack in railway axle was developed and its curved propagation through the axle was simulated by finite element method. Presence of press-fitted wheel in the vicinity of initial crack was taken into account. A typical loading spectrum of railway axle was considered and residual fatigue lifetime was estimated by NASGRO approach. Material properties of typical axle steel EA4T were considered in numerical calculations and lifetime estimation.

Key Engineering Materials, 2016

The contribution deals with the issue of mechanical response of the particulate ceramic composite... more The contribution deals with the issue of mechanical response of the particulate ceramic composites used in microelectronic. Mechanical properties and behaviour of composites are highly influenced by residual stresses which are developed in material during cooling in manufacturing process due to the different coefficients of thermal expansions of individual constituents. The main aim of this paper is to estimate the elastic constants and strength of the selected particulate ceramic composites with considering the residual stresses. Three dimensional models and finite element method are used for numerical simulations. Results contribute to determination and better understanding of mechanical behaviour of the particulate ceramic composites.

Procedia Engineering, 2015

The paper deals with an estimation of residual fatigue lifetime of railway axles under real loadi... more The paper deals with an estimation of residual fatigue lifetime of railway axles under real loading spectrum. The residual fatigue lifetime is given by magnitude of fatigue crack propagation rate. This rate depends predominantly on load, geometry and material of the axle. Standard steel EA4T for manufacturing of railway axles is considered in this paper. The scatter of data in v-K curve could be caused by inaccuracy of experimental measurement or by local change of material properties. The paper shows important differences between obtained residual fatigue lifetime estimations considering scatter in measured material data, especially near the threshold region.

Procedia Engineering, 2014

The railway axles are subjected to variable amplitude loading. The variability is caused by many ... more The railway axles are subjected to variable amplitude loading. The variability is caused by many different regimes of train operation. The dominant load is caused by the weight of the vehicle, which generates rotary bending during train movement. Nevertheless, there are additional forces, which are generated when train goes through curved track, over crossovers, switches, rail joints etc. Present non-destructive defectoscopy can reliably detect only relatively long cracks (approximately 2 mm long or longer). Hence there is a risk that the existing crack is not detected. Therefore, for conservative estimation of the residual fatigue lifetime the railway axle with a crack should be considered. The behaviour of the crack depends on load spectrum of railway axle. Hence for accurate determination of residual fatigue lifetime of the railway axle is necessary to know representative load spectrum. This paper compares several measured load spectra of railway axles, which are available in the literature. The effect of different load spectra is shown on two widely used materials for railway axles: EA1N and EA4T steels. Obtained results could be used for safer operation of railway axles.

Materials

Fatigue crack propagation in near-threshold regime was studied in the 304L austenitic stainless s... more Fatigue crack propagation in near-threshold regime was studied in the 304L austenitic stainless steel in two microstructural states: as-received (AR) with finer microstructure and low susceptibility to the transformation-induced plasticity (TRIP) effect, and solution-annealed (SA) with coarser microstructure and higher susceptibility to TRIP. At the load ratio R = 0.1 the threshold was higher in the SA state than in the AR state due to coarser grains and possibly the TRIP effect. In order to clarify the role of crack closure, experiments at R = 0.7 were done. The threshold in the SA state was still higher by 1 MPa·m0.5. This effect was identified as crack tip shielding induced by phase transformation, an example of a non-closure shielding effect. Higher resistance to crack growth in the SA state was attributed to promoted martensitic transformation in non-favorable oriented grain families rather than thicker martensite layers in the crack path area. The conclusions were verified by ...

Materials, 2021

Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominant... more Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominantly considered as insignificant cycles without any influence on fatigue lifespan of engineering structural components. However, this paper shows that in some cases these underload cycles can retard the consequent crack propagation quite significantly. This phenomenon is a consequence of oxide-induced crack closure development during cyclic loading below the threshold. The experimentally described effect of fatigue crack growth retardation was supported by measurement of the width and the thickness of the oxide debris layer using the EDS technique and localized FIB cuts, respectively. Both the retardation effect and the amount of oxide debris were larger for higher number and larger amplitudes of the applied underload cycles. Crack closure measurement revealed a gradual increase of the closure level during underload cycling. Specimens tested in low air humidity, as well as specimens left w...

Solid State Phenomena, 2017

Growth of long fatigue cracks in Ti6Al4V alloy manufactured by direct metal laser sintering (DMLS... more Growth of long fatigue cracks in Ti6Al4V alloy manufactured by direct metal laser sintering (DMLS) was investigated. Two DMLS systems, EOSINT M270 and EOSINT M290, with different process parameters were used for production of CT specimens having three different orientations of crack propagation with respect to the DMLS build direction. The as-built specimens were stress relieved at 740 °C. The fatigue crack growth curve and the threshold values of the stress intensity factor for crack propagation were experimentally determined. It has been found that the chosen DMLS processing parameters and the used stress relieving procedure results in material exhibiting isotropic crack growth behavior, i.e. the crack growth was found to be independent of the DMLS build direction. The fatigue crack growth rates and the threshold values for the crack growth were compared with published results characterizing the as-built material and material after different post processing heat treatments.

International Journal of Fatigue, 2021

Abstract The experimental determination of the resistance to fatigue crack propagation in steel, ... more Abstract The experimental determination of the resistance to fatigue crack propagation in steel, particularly in the so-called near-threshold regime, is a crucial issue for safety-relevant components which are designed to operate for a large number of loading cycles before undergoing periodic inspections. This work summarizes the most relevant results of extensive experimental campaigns conducted at BAM and IPM over the last years, which have been devoted to the determination of fatigue crack propagation characteristics on medium and high strength steels. The influence of environment, oxide-induced crack closure and test methodologies in the near-threshold regime are discussed.

Theoretical and Applied Fracture Mechanics, 2020

Decomposition of the resistance to fatigue crack growth into the intrinsic and extrinsic componen... more Decomposition of the resistance to fatigue crack growth into the intrinsic and extrinsic component is very important for understanding of fatigue failure mechanisms, relation to microstructure and modelling of residual fatigue life. Crack closure for four grades of steel were estimated by the difference between K max values and the effective ΔK eff values (measured at the load ratio R = 0.8) corresponding to the same crack growth rate. The results showed that crack closure values obtained by the difference K max-ΔK eff were not in agreement with the available crack closure models, both the Newman's model of plasticity-induced closure and the results from finite element analysis. The discrepancies could not be explained by the effect of mean stress, specimen thickness, loading amplitude or T-stress. Therefore, the application of fracture mechanics to fatigue cracks should be revisited. It was pointed out that ΔK eff may not be a good parameter for quantification of the crack driving force, since the relationship between K max-K cl and the cyclic plastic deformation at the crack tip might not be linear.

Theoretical and Applied Fracture Mechanics, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Engineering Failure Analysis, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The Journal of Strain Analysis for Engineering Design, 2019

Experimental measurements of fatigue properties of old steel used for a crane runway were perform... more Experimental measurements of fatigue properties of old steel used for a crane runway were performed to capture the influence of corrosion on fatigue life of the material. Basquin’s law was used to quantify the fatigue properties of old steel with different cross sections and with different surface of specimens (polished and corroded). The finite element model was prepared to assess and quantify the various stress distribution in specimens with circular and rectangular cross sections. Fracture surfaces of the three kinds of specimens (circular polished, rectangular polished and rectangular corroded) were studied and they showed the surface crack initiation. The following fatigue cracks developed from the surface and expanded into specimen with radiation pattern. Observed crack initiation areas confirmed the ones expected according to stress concentrations.

Strojnícky casopis – Journal of Mechanical Engineering, 2017

Description of stress singularity in the vicinity of a free surface is presented. Its presence ca... more Description of stress singularity in the vicinity of a free surface is presented. Its presence causes the retardation of the fatigue crack growth in that region and fatigue crack is being curved. Numerical model is used to study dependence of the stress singularity exponent on Poisson’s ratio. Estimated values are compared to those already published. Experimentally measured angles of fatigue crack on SENB specimens confirm the relation between Poisson’s ratio and the angle between crack front and free surface.

Materials Science and Engineering: A, 2020

The hybrid laser-tungsten inert gas welding technology was applied to butt weld 3-mm-thick S460MC... more The hybrid laser-tungsten inert gas welding technology was applied to butt weld 3-mm-thick S460MC and S700MC high-strength low-alloy steel sheets. The intent of low-current arc addition was to preheat the material to reduce extremely fast cooling rate accompanying laser welding. High-cycle fatigue tests were performed to evaluate the effect of welding conditions on fatigue behavior of S460MC and S700MC laser and hybrid welds. Both base materials exhibit approximately the same fatigue limit. However, corresponding samples with the weld behaved significantly different under cyclic loading. We demonstrated that the drop of their fatigue lifetime is caused by the combination of microstructural changes and especially by the presence of surface notches resulting from the welding process. The two series of both steels were tested to reveal the contribution of induced surface roughness and microstructural changes itself. The first one was in the as-welded condition. The second one had a smooth surface resulting from surface layer grinding after the welding. It was found, that the geometric notches created by the welding process are the determining factor in case of both tested steels. Testing of the ground samples showed, that in case of S460MC steel, the application of pre-heat did not improve the fatigue properties. On the other hand, the performed fatigue tests showed improved fatigue limit of ground series samples of the preheated S700MC steel, when compared with the conventional laser welding.

Procedia Structural Integrity, 2019

The growth of long fatigue cracks in Ti6Al4V alloy manufactured by direct metal laser sintering w... more The growth of long fatigue cracks in Ti6Al4V alloy manufactured by direct metal laser sintering was experimentally determined on CT specimens. The crack propagation was investigated in three different orientations with regard to the building direction. The as-built CT specimens were heat treated at temperatures 380, 740 and 900 °C. The crack growth curves with the threshold values of the stress intensity factor Kth were determined. The measurement was carried out in laboratory air at room temperature and stress ratios R = 0.1 and 0.8. It was found that the crack growth rate in the Paris region is identical for all three heat treatments for the same R ratio. For the particular stress intensity factor range K the crack growth rates at R = 0.1 and 0.8 differs by factor of two. Contrary to the Paris region there is a clear effect of the heat treatment in the near threshold region. For the material heat treated at 380 °C and loaded at R = 0.1 the threshold is 3.7 MPam 1/2 , for 740 °C 2.7 MPam 1/2 and for 900 °C 3.5 MPam 1/2. The explanation of the observed dependence of the threshold on the post processing heat treatment is discussed in terms of microstructure and residual stresses.

Theoretical and Applied Fracture Mechanics, 2019

A propagating fatigue crack may be partly retarded thanks to a phenomenon called fatigue crack cl... more A propagating fatigue crack may be partly retarded thanks to a phenomenon called fatigue crack closure. The ability to accurately describe this phenomenon is of interest for the scientific and engineering community, because of its significant impact on the fatigue crack propagation rate. A strategy for numerical modelling of the most common closure mechanism the plasticity induced crack closureis presented in this paper. It was observed that the generally adopted suggestions for this type of simulations, such as the length of the crack growth or the number of substeps, are not necessarily valid in general, but require to be individually specified for particular conditions. The size of the elements in the vicinity of the crack front is also a widely discussed issue and it is shown here that even without convergence, the element size may be chosen as a fixed parameter leading to very reasonable closure values with low computational costs. A method of closure level determination based on change in specimen stiffness is described here and its performance is compared to the traditional first node displacement method with Load-Debond-Unload (LDU) and Load-Debond-Unload-Load-Unload (LDULU) loading schemes.

Engineering Fracture Mechanics, 2017

Engineering Failure Analysis, 2017

The railway axles are subjected to cyclic loading during their operation. Their load is of long-t... more The railway axles are subjected to cyclic loading during their operation. Their load is of long-term nature, therefore a real risk of fatigue failure exists. This failure could lead to derailment of the whole train with serious consequences. To prevent such scenario, the railway axles have to be safely removed from operation before their final failure occurs. This paper presents methodology for the residual fatigue lifetime prediction of the railway axle based on the linear elastic fracture mechanics concept. The methodology contains estimation of the critical position of initial crack, prediction of the fatigue crack front shape development during crack propagation, separation of the bending and press-fitting contributions to the axle load, experimental measurement of the crack growth kinetics of EA4T steel and subsequent estimation of the residual fatigue lifetime of railway axle. Part of the presented study is also devoted to the probability aspects of determination of material characteristics describing fatigue crack propagation and retardation effects caused by existence of plastic zone ahead of propagating fatigue crack. Described methodology is already applied in the design process of new railway axles in Bonatrans company.

Solid State Phenomena, 2016

Railway axles are subjected to cyclic amplitude loading which can lead to fatigue failure. For sa... more Railway axles are subjected to cyclic amplitude loading which can lead to fatigue failure. For safe operation of railway axles a damage tolerance approach taking into account a possible defect in railway axle is often required. Because of different operation regimes of trains (fast/slow ride, ride on straight track, on curved track, over switches etc.) the load amplitude of axle is not constant. The variability of load is defined by a load spectrum, which is determined experimentally by measuring of load in service conditions. Even though the load spectrum is measured on several hundreds or thousands of operation kilometres, the railway axles are in operation much longer time (often tens of years). Therefore, some load amplitudes higher than ones measured in the test can occur during a long-term axle service. The contribution presented deals with the effect of extension of load spectrum by rare high load amplitudes, which can occur during long-term operation, on residual fatigue lif...

Journal of Multiscale Modelling, 2016

Railway axles are subjected to cyclic loading which can lead to fatigue failure. For safe operati... more Railway axles are subjected to cyclic loading which can lead to fatigue failure. For safe operation of railway axles a damage tolerance approach taking into account a possible defect on railway axle surface is often required. The contribution deals with an estimation of residual fatigue lifetime of railway axle with initial inclined surface crack. 3D numerical model of inclined semi-elliptical surface crack in railway axle was developed and its curved propagation through the axle was simulated by finite element method. Presence of press-fitted wheel in the vicinity of initial crack was taken into account. A typical loading spectrum of railway axle was considered and residual fatigue lifetime was estimated by NASGRO approach. Material properties of typical axle steel EA4T were considered in numerical calculations and lifetime estimation.

Key Engineering Materials, 2016

The contribution deals with the issue of mechanical response of the particulate ceramic composite... more The contribution deals with the issue of mechanical response of the particulate ceramic composites used in microelectronic. Mechanical properties and behaviour of composites are highly influenced by residual stresses which are developed in material during cooling in manufacturing process due to the different coefficients of thermal expansions of individual constituents. The main aim of this paper is to estimate the elastic constants and strength of the selected particulate ceramic composites with considering the residual stresses. Three dimensional models and finite element method are used for numerical simulations. Results contribute to determination and better understanding of mechanical behaviour of the particulate ceramic composites.

Procedia Engineering, 2015

The paper deals with an estimation of residual fatigue lifetime of railway axles under real loadi... more The paper deals with an estimation of residual fatigue lifetime of railway axles under real loading spectrum. The residual fatigue lifetime is given by magnitude of fatigue crack propagation rate. This rate depends predominantly on load, geometry and material of the axle. Standard steel EA4T for manufacturing of railway axles is considered in this paper. The scatter of data in v-K curve could be caused by inaccuracy of experimental measurement or by local change of material properties. The paper shows important differences between obtained residual fatigue lifetime estimations considering scatter in measured material data, especially near the threshold region.

Procedia Engineering, 2014

The railway axles are subjected to variable amplitude loading. The variability is caused by many ... more The railway axles are subjected to variable amplitude loading. The variability is caused by many different regimes of train operation. The dominant load is caused by the weight of the vehicle, which generates rotary bending during train movement. Nevertheless, there are additional forces, which are generated when train goes through curved track, over crossovers, switches, rail joints etc. Present non-destructive defectoscopy can reliably detect only relatively long cracks (approximately 2 mm long or longer). Hence there is a risk that the existing crack is not detected. Therefore, for conservative estimation of the residual fatigue lifetime the railway axle with a crack should be considered. The behaviour of the crack depends on load spectrum of railway axle. Hence for accurate determination of residual fatigue lifetime of the railway axle is necessary to know representative load spectrum. This paper compares several measured load spectra of railway axles, which are available in the literature. The effect of different load spectra is shown on two widely used materials for railway axles: EA1N and EA4T steels. Obtained results could be used for safer operation of railway axles.