Ludvík Kunz - Academia.edu (original) (raw)
Papers by Ludvík Kunz
Procedia Structural Integrity
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Deterioration of fatigue properties of weathering steels is a serious problem of operating life o... more Deterioration of fatigue properties of weathering steels is a serious problem of operating life of engineering components subjected to corrosion and cyclic loads. There are two possible mechanisms of damage, which are discussed in literature. There is an effect of surface roughening, which develops during corrosion, and an effect of corrosion cracks, which can influence the fatigue crack initiation. The paper contains results of an experimental determination of fatigue life of ATMOFIX 52A grade steel with a corroded surface and comparison with the fatigue strength of basic smooth material. The notch effect of corrosion dimples, which initiated fatigue cracks, was determined on the basis of fractographic observation and numerical evaluation of stress concentration. It has been found that the reduction of the fatigue life can be quantitatively well predicted by means of the fatigue notch effect of corrosion dimples. No risky corrosion cracks were found in the studied corroded materia
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Kovove Materialy, 1991
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Evaluation de l'acier moule, de la fonte a graphite nodulaire et de la fonte grise au point d... more Evaluation de l'acier moule, de la fonte a graphite nodulaire et de la fonte grise au point de vue leur resistance a la propagation des criques de fatigue
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High-cycle fatigue strength of Ni-base superalloy IN 713LC was experimentally determined for symm... more High-cycle fatigue strength of Ni-base superalloy IN 713LC was experimentally determined for symmetrical loading and for loading with tensile mean stresses at a temperature of 800 °C in air. An analysis of the influence of microstructure on fatigue crack initiation and related fatigue strength was performed. Statistical method of the largest extreme value has been applied to characterize the casting defect distribution.
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MATERIALS TRANSACTIONS, 2010
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Materials Science and Engineering: A, 1994
Abstract The cyclic stress-strain response and cyclic creep behaviour of polycrystalline copper w... more Abstract The cyclic stress-strain response and cyclic creep behaviour of polycrystalline copper were shown to depend not only on the length but also on the mode of the start-up ramp in stress-controlled tests at zero mean stress. In tests with a non-zero mean stress, these phenomena are additionally affected by the magnitude of the mean stress. There is a strong correlation between the cyclic creep strain and the saturated plastic strain amplitude. In the studied region of small amplitude cycling, the explanation lies in the response of the one-phase matrix dislocation structure.
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WIT transactions on engineering sciences, 1970
A notch effect under creep/high-cycle-fatigue loading conditions was studied both theoretically a... more A notch effect under creep/high-cycle-fatigue loading conditions was studied both theoretically and experimentally in notched specimens of an advanced 9%Cr steel of the type P91 at 600°C. Experiments were performed under condition of the constant maximum net stress. The stress ratio R, i.e., the cyclic stress component of creep/fatigue loading was varied. The dependence of time to fracture on R exhibits nonmonotonous behaviour with a pronounced maximum coinciding with the fracture mode transition from pure creep to fatigue. The notched specimens show increased rupture strength when compared with smooth bars. The strengthening due to the notch depends on the notch geometry. It is shown that the notch tip stress triaxiality parameter is not sufficient to describe this phenomenon.
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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.
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Materials Science Forum, 2017
Direct Metal Laser Sintering (DMLS) is a complex process where a part is build-up by localized me... more Direct Metal Laser Sintering (DMLS) is a complex process where a part is build-up by localized melting of gas atomized powder layers by a concentrated laser beam followed rapid solidification. The microstructure of DMLS produced material is substantially different from that of conventionally manufactured materials, although the ultimate strength is similar. However, yield strength and elongation and especially fatigue behavior may vary considerably according to the process parameters and post fabrication heat treatment because they affect structural heterogeneity, porosity content, residual stresses, and surface conditions. Fatigue tests of DMLS Ti6Al4V alloy are interpreted in the light of a thorough metallographic and fractographic investigation. The fatigue crack initiation for three different cyclic stress directions with respect to build direction is determined by fractography.
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Solid State Phenomena, 2016
High cycle fatigue life of Ti6Al4V alloy specimens manufactured by Direct Metal Laser Sintering (... more High cycle fatigue life of Ti6Al4V alloy specimens manufactured by Direct Metal Laser Sintering (DMLS) was experimentally determined. The DMLS fabrication process was characterized by a 400 W laser power and 50 μm layer melted thickness. Post-fabrication heat treatment consisted in stress relieving for 3 h at 720 °C in vacuum with subsequent cooling in argon atmosphere. Fatigue testing of specimens oriented in three different directions with respect to the material build direction was performed with the aim to examine the influence of the layered microstructure on the fatigue behavior. Results of measurement of surface roughness, metallographic examinations of the layered material and fractographic investigation of the fatigue fracture surfaces were employed in the discussion of fatigue crack initiation in DMLS fabricated Ti6Al4V alloy.
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Procedia Engineering, 2016
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Fracture of Nano and Engineering Materials and Structures
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Strain, 2010
Abstract: Effect of cyclic loading and thermal exposition on microstructure of bulk ultrafine‐gr... more Abstract: Effect of cyclic loading and thermal exposition on microstructure of bulk ultrafine‐grained Cu prepared by equal channel angular pressing was experimentally investigated by means of electron backscattering diffraction and by transmission electron microscopy. Stability of the microstructure under stress‐controlled cyclic loading with a tensile mean stress of 200 MPa was shown to be high. Neither dynamic grain coarsening nor development of bimodal microstructure was observed. However, annealing at 250 °C for 30 min resulted in formation of bimodal microstructure. Consequently, fatigue strength of annealed specimens was low.
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Materials Science and Engineering: A, 1991
ABSTRACT
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Materials Science and Engineering: A, 2001
Abstract Cyclic plastic behavior of a ferritic martensite 9%Cr–1%Mo steel was studied under load ... more Abstract Cyclic plastic behavior of a ferritic martensite 9%Cr–1%Mo steel was studied under load controlled cycling with positive mean stresses. The results were compared with data determined under symmetrical cycling. Cyclic hardening followed by softening was found to be inherent to cycling at positive mean stress contrary to the symmetrical loading where only continuous softening was observed. For a given stress amplitude, higher mean stress results in higher plastic strain amplitude. The onset of cyclic softening and the onset of cyclic creep acceleration occur simultaneously. In the region of cyclic softening a unique relation between the instantaneous cyclic creep rate and the instantaneous plastic strain amplitude was found for cycling at constant mean stress.
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Copper Alloys - Early Applications and Current Performance - Enhancing Processes, 2012
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S-N curve of 14109 bearing steel was experimentally determined in symmetrical tension-compression... more S-N curve of 14109 bearing steel was experimentally determined in symmetrical tension-compression in the interval of 10 to 5 to 10 to 8 cycles to fracture. Continuous increase of the number of cycles to fracture with decreasing stress amplitude was observed. Large scatter of experimental data is related to the initiation of fatigue cracks on internal defects of Al2O3 and TiN type
Bookmarks Related papers MentionsView impact
Procedia Structural Integrity, 2019
Bookmarks Related papers MentionsView impact
Procedia Structural Integrity
Bookmarks Related papers MentionsView impact
Deterioration of fatigue properties of weathering steels is a serious problem of operating life o... more Deterioration of fatigue properties of weathering steels is a serious problem of operating life of engineering components subjected to corrosion and cyclic loads. There are two possible mechanisms of damage, which are discussed in literature. There is an effect of surface roughening, which develops during corrosion, and an effect of corrosion cracks, which can influence the fatigue crack initiation. The paper contains results of an experimental determination of fatigue life of ATMOFIX 52A grade steel with a corroded surface and comparison with the fatigue strength of basic smooth material. The notch effect of corrosion dimples, which initiated fatigue cracks, was determined on the basis of fractographic observation and numerical evaluation of stress concentration. It has been found that the reduction of the fatigue life can be quantitatively well predicted by means of the fatigue notch effect of corrosion dimples. No risky corrosion cracks were found in the studied corroded materia
Bookmarks Related papers MentionsView impact
Kovove Materialy, 1991
Bookmarks Related papers MentionsView impact
Evaluation de l'acier moule, de la fonte a graphite nodulaire et de la fonte grise au point d... more Evaluation de l'acier moule, de la fonte a graphite nodulaire et de la fonte grise au point de vue leur resistance a la propagation des criques de fatigue
Bookmarks Related papers MentionsView impact
High-cycle fatigue strength of Ni-base superalloy IN 713LC was experimentally determined for symm... more High-cycle fatigue strength of Ni-base superalloy IN 713LC was experimentally determined for symmetrical loading and for loading with tensile mean stresses at a temperature of 800 °C in air. An analysis of the influence of microstructure on fatigue crack initiation and related fatigue strength was performed. Statistical method of the largest extreme value has been applied to characterize the casting defect distribution.
Bookmarks Related papers MentionsView impact
MATERIALS TRANSACTIONS, 2010
Bookmarks Related papers MentionsView impact
Materials Science and Engineering: A, 1994
Abstract The cyclic stress-strain response and cyclic creep behaviour of polycrystalline copper w... more Abstract The cyclic stress-strain response and cyclic creep behaviour of polycrystalline copper were shown to depend not only on the length but also on the mode of the start-up ramp in stress-controlled tests at zero mean stress. In tests with a non-zero mean stress, these phenomena are additionally affected by the magnitude of the mean stress. There is a strong correlation between the cyclic creep strain and the saturated plastic strain amplitude. In the studied region of small amplitude cycling, the explanation lies in the response of the one-phase matrix dislocation structure.
Bookmarks Related papers MentionsView impact
WIT transactions on engineering sciences, 1970
A notch effect under creep/high-cycle-fatigue loading conditions was studied both theoretically a... more A notch effect under creep/high-cycle-fatigue loading conditions was studied both theoretically and experimentally in notched specimens of an advanced 9%Cr steel of the type P91 at 600°C. Experiments were performed under condition of the constant maximum net stress. The stress ratio R, i.e., the cyclic stress component of creep/fatigue loading was varied. The dependence of time to fracture on R exhibits nonmonotonous behaviour with a pronounced maximum coinciding with the fracture mode transition from pure creep to fatigue. The notched specimens show increased rupture strength when compared with smooth bars. The strengthening due to the notch depends on the notch geometry. It is shown that the notch tip stress triaxiality parameter is not sufficient to describe this phenomenon.
Bookmarks Related papers MentionsView impact
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.
Bookmarks Related papers MentionsView impact
Materials Science Forum, 2017
Direct Metal Laser Sintering (DMLS) is a complex process where a part is build-up by localized me... more Direct Metal Laser Sintering (DMLS) is a complex process where a part is build-up by localized melting of gas atomized powder layers by a concentrated laser beam followed rapid solidification. The microstructure of DMLS produced material is substantially different from that of conventionally manufactured materials, although the ultimate strength is similar. However, yield strength and elongation and especially fatigue behavior may vary considerably according to the process parameters and post fabrication heat treatment because they affect structural heterogeneity, porosity content, residual stresses, and surface conditions. Fatigue tests of DMLS Ti6Al4V alloy are interpreted in the light of a thorough metallographic and fractographic investigation. The fatigue crack initiation for three different cyclic stress directions with respect to build direction is determined by fractography.
Bookmarks Related papers MentionsView impact
Solid State Phenomena, 2016
High cycle fatigue life of Ti6Al4V alloy specimens manufactured by Direct Metal Laser Sintering (... more High cycle fatigue life of Ti6Al4V alloy specimens manufactured by Direct Metal Laser Sintering (DMLS) was experimentally determined. The DMLS fabrication process was characterized by a 400 W laser power and 50 μm layer melted thickness. Post-fabrication heat treatment consisted in stress relieving for 3 h at 720 °C in vacuum with subsequent cooling in argon atmosphere. Fatigue testing of specimens oriented in three different directions with respect to the material build direction was performed with the aim to examine the influence of the layered microstructure on the fatigue behavior. Results of measurement of surface roughness, metallographic examinations of the layered material and fractographic investigation of the fatigue fracture surfaces were employed in the discussion of fatigue crack initiation in DMLS fabricated Ti6Al4V alloy.
Bookmarks Related papers MentionsView impact
Procedia Engineering, 2016
Bookmarks Related papers MentionsView impact
Bookmarks Related papers MentionsView impact
Fracture of Nano and Engineering Materials and Structures
Bookmarks Related papers MentionsView impact
Strain, 2010
Abstract: Effect of cyclic loading and thermal exposition on microstructure of bulk ultrafine‐gr... more Abstract: Effect of cyclic loading and thermal exposition on microstructure of bulk ultrafine‐grained Cu prepared by equal channel angular pressing was experimentally investigated by means of electron backscattering diffraction and by transmission electron microscopy. Stability of the microstructure under stress‐controlled cyclic loading with a tensile mean stress of 200 MPa was shown to be high. Neither dynamic grain coarsening nor development of bimodal microstructure was observed. However, annealing at 250 °C for 30 min resulted in formation of bimodal microstructure. Consequently, fatigue strength of annealed specimens was low.
Bookmarks Related papers MentionsView impact
Materials Science and Engineering: A, 1991
ABSTRACT
Bookmarks Related papers MentionsView impact
Materials Science and Engineering: A, 2001
Abstract Cyclic plastic behavior of a ferritic martensite 9%Cr–1%Mo steel was studied under load ... more Abstract Cyclic plastic behavior of a ferritic martensite 9%Cr–1%Mo steel was studied under load controlled cycling with positive mean stresses. The results were compared with data determined under symmetrical cycling. Cyclic hardening followed by softening was found to be inherent to cycling at positive mean stress contrary to the symmetrical loading where only continuous softening was observed. For a given stress amplitude, higher mean stress results in higher plastic strain amplitude. The onset of cyclic softening and the onset of cyclic creep acceleration occur simultaneously. In the region of cyclic softening a unique relation between the instantaneous cyclic creep rate and the instantaneous plastic strain amplitude was found for cycling at constant mean stress.
Bookmarks Related papers MentionsView impact
Copper Alloys - Early Applications and Current Performance - Enhancing Processes, 2012
Bookmarks Related papers MentionsView impact
S-N curve of 14109 bearing steel was experimentally determined in symmetrical tension-compression... more S-N curve of 14109 bearing steel was experimentally determined in symmetrical tension-compression in the interval of 10 to 5 to 10 to 8 cycles to fracture. Continuous increase of the number of cycles to fracture with decreasing stress amplitude was observed. Large scatter of experimental data is related to the initiation of fatigue cracks on internal defects of Al2O3 and TiN type
Bookmarks Related papers MentionsView impact
Procedia Structural Integrity, 2019
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