Ľubomír Gajdoš - Academia.edu (original) (raw)
Papers by Ľubomír Gajdoš
Journal of Pressure Vessel Technology, 2015
An investigation was made into the fatigue properties of L485MB (X70) pipeline steel after 13 yr ... more An investigation was made into the fatigue properties of L485MB (X70) pipeline steel after 13 yr of exploitation as a crude oil pipeline material. Fatigue tests in zero-to-tension loading were carried out (i) in air, (ii) in crude oil, (iii) in a mixture of crude oil with rainwater, and (iv) in water separated from the crude oil phase. The aim of the investigation was to assess the degree of degradation of the fatigue properties of this steel due to the action of environments typical for crude oil processing and transport. The results are not directly comparable to any corrosion fatigue experienced by the crude oil pipeline, since the strain rate for the tests was higher by an order of five than the strain rate observed in typical loading cycles. The results showed that crude oil and a mixture of crude oil with rainwater had no aggressive effect on the steel in the sense of reducing its fatigue characteristics, while the separated water had an aggressive effect.
Metals, 2022
The aim of this article is to quantify the loads exerted by heavy-duty vehicles when crossing ove... more The aim of this article is to quantify the loads exerted by heavy-duty vehicles when crossing over buried pipeline. This problem arises in connection to the question pertaining to the use of protective sleeves (casings) applied to gas pipelines in regions with increased demands on pipeline operation safety. An experiment was conducted on a test pipe section made from L360NE pipeline steel equipped with strain gauges along the pipe perimeter, measuring strains in the axial and circumferential directions. Strain measurements were taken after back-filling the pipe trench, then during vehicle crossings over the empty pipe, and again after pressurizing the test pipe with air. Strain-based hoop stresses at the surface of the empty test pipe were found to exceed 30 MPa after back-filling the trench and increased to more than 40 MPa during the vehicle crossings. Similarly, axial stresses reached extremes of around 17 MPa in compression and 12 MPa in tension. Applying internal air pressure t...
MMM offers a very interesting alternative to traditional defectoscopy methods such as ultrasound ... more MMM offers a very interesting alternative to traditional defectoscopy methods such as ultrasound or magnetic powder method. Like any method, it has its advantages and limitations. Among the most interesting advantages is the possibility of detecting imperfections in the material without cleaning or even pre-treatment of the test surface. This is a major advantage especially when large and complicated parts are inspected on which a continuous layer of corrosive product is formed or other leachate. The disadvantage is the inability to use this method on non-magnetic materials (Al alloys) and the fact that only places with potentially high internal stresses are identified here, which does not necessarily mean a problem. This method was used to identify defects in two cases:
Changes in dislocation substructure were studied in normalized and annealed S235JR steel in relat... more Changes in dislocation substructure were studied in normalized and annealed S235JR steel in relation to the number of symmetrical reversed stress cycles with the amplitude σa = 242 MPa and corresponding mean life Nf = 17,950 cycles. The microstructure of the steel consisted of ferrite with a small amount of pearlite. The substructure was observed in as received condition and after application of various relative numbers of cycles n/Nf, these being 0.25, 0.50 and 0.75. An irregular dislocation net occurred in the virgin specimen, however, during cyclic loading the dislocations started to accumulate gradually in slip bands and to form a cell substructure in grains of favourable crystallographic orientations. Total dislocation density, dislocation density in slip bands and inter-band distance were measured using transmission electron microscopy (TEM). The total dislocation density was found to slightly decrease and the density in slip bands to increase with increasing number of cycles....
Materials, 2021
The purpose of this article is to compare two predictive methods of J integral assessments for ce... more The purpose of this article is to compare two predictive methods of J integral assessments for center-cracked plates, single-edge cracked plates and double-edge cracked plates produced from X52 and X70 steels, and a longitudinally cracked pipe produced from X70 steel. The two methods examined are: the GSM method and the Js procedure of the French RCC-MR construction code, designated here as the FC method. The accuracy of J integral predictions by these methods is visualized by comparing the results obtained with the “reference” values calculated by the EPRI method. The main results showed that both methods yielded similar J integral values, although in most cases, the GSM predictions were slightly more conservative than the FC predictions. In comparison with the “reference” values of the J integral, both methods provided conservative results for most crack configurations, although the estimates for cracks of a relative length smaller than 1/8 were not found to be so conservative. Th...
Materials Today: Proceedings, 2016
For standard fracture mechanical tests flat specimens (principally CT or SENB) are required. When... more For standard fracture mechanical tests flat specimens (principally CT or SENB) are required. When investigating fracture mechanical properties of thin-walled pipes this brings about a problem because it is necessary to straighten pipe bands. This operation causes internal stresses to be induced not only in the semi-product subjected to straightening but also in finished specimens. A question therefore arises to what extent are then the magnitudes of the fracture toughness determined representative for the actual cylindrical wall. To solve this problem fracture mechanics tests were carried out on flat (straightened) CT specimens as well as on curved CT specimens with the natural curvature. The R-curves as well as the resulting parameters of the fracture toughness, obtained for both types of CT specimens, were compared and it was concluded that the fracture toughness of the pipe material determined on straightened CT specimens was practically the same as that obtained on curved CT specimens.
Materials Today: Proceedings, 2017
Applied Mechanics and Materials, 2016
For standard fracture mechanical tests flat specimens (principally CT or SENB) are required. When... more For standard fracture mechanical tests flat specimens (principally CT or SENB) are required. When investigating fracture mechanical properties of thin – walled pipes this brings about a problem because it is necessary to straighten pipe bands. However, this operation causes internal stresses to be induced not only in the semi-product subjected to straightening but also in finished specimens. A question therefore arises to what extent are then the magnitudes of the fracture toughness determined representative for the actual cylindrical wall. To solve this problem fracture mechanics tests were caried out on flat (straightened) CT specimens as well as on curved CT specimens with the natural curvature. The R – curves as well as the resulting parameters of the fracture toughness, obtained for both types of CT specimens, were compared and it was concluded that the fracture toughness of the pipe material determined on straightened CT specimens was practically the same as that obtained on curved CT specimens.
Key Engineering Materials, 2014
The paper presents experimental results concerning the variation of microplastic limit (MPL) duri... more The paper presents experimental results concerning the variation of microplastic limit (MPL) during fatigue as determined by the measurement of changes in inductance L of specimens made from normalized low-C steel CSN 411375. Some indications have been obtained that MPL varies during fatigue with dislocation density. This was proved by nanoindentation, X-ray diffraction and transmision electron microscopy.
Procedia Engineering, 2013
The experimental determination of fatigue limit in case of steels represents an expensive and tim... more The experimental determination of fatigue limit in case of steels represents an expensive and time consuming process not speaking about the necessity of a sufficient volume of representative material necessary for the specimens manufacturing, manufacturing of numerous test specimens, the need of an expensive (electrohydraulic) test equipment etc. Microplastic strains occurring at stresses below the yield point play a significant role at cyclic loading of metals within high cycle fatigue region. The microplastic strains cannot be measured by common methods due to the discontinuity of their distribution and a varying character of their level in the metal structure, but however, owing to the magnetoelastic effect the threshold stress at which dislocation pileup stresses begin to obstruct the magnetic domains in rotation to the direction of the tensile stress can be determined by means of changes in magnetic permeability during tensile loading. In the paper, the changes of the magnetic permeability were measured indirectly-by means of the changes of the electrical impedance (a.c. resistance). The measurement was performed on the normalized low-carbon structural steel CSN 41 1375. The microplastic limit was determined by evaluation the ΔR-σ records. A comparison with the fatigue limit in a reversed fatigue loading showed that the MPL was below the fatigue limit. The microplastic limit can thus be considered to separate nondamaging and damaging cyclic stresses. The paper presents the results of the abovementioned characteristics what may shorten and make cheeper the determination of the fatigue limit and non-damaging cyclic stresses of structural materials.
This study offers a new simple method for assessing an axial part-through crack in a pipe wall. T... more This study offers a new simple method for assessing an axial part-through crack in a pipe wall. The method utilizes simple approximate expressions for determining the fracture parameters K, J, and employs these parameters to determine critical dimensions of a crack on the basis of equality between the J-integral and the J-based fracture toughness of the pipe steel. The crack tip constraint is taken into account by the so-called plastic constraint factor C, by which the uniaxial yield stress in the J-integral equation is multiplied. The results of the prediction of the fracture condition are verified by burst tests on test pipes.
Journal of Pressure Vessel Technology, 2015
An investigation was made into the fatigue properties of L485MB (X70) pipeline steel after 13 yr ... more An investigation was made into the fatigue properties of L485MB (X70) pipeline steel after 13 yr of exploitation as a crude oil pipeline material. Fatigue tests in zero-to-tension loading were carried out (i) in air, (ii) in crude oil, (iii) in a mixture of crude oil with rainwater, and (iv) in water separated from the crude oil phase. The aim of the investigation was to assess the degree of degradation of the fatigue properties of this steel due to the action of environments typical for crude oil processing and transport. The results are not directly comparable to any corrosion fatigue experienced by the crude oil pipeline, since the strain rate for the tests was higher by an order of five than the strain rate observed in typical loading cycles. The results showed that crude oil and a mixture of crude oil with rainwater had no aggressive effect on the steel in the sense of reducing its fatigue characteristics, while the separated water had an aggressive effect.
Metals, 2022
The aim of this article is to quantify the loads exerted by heavy-duty vehicles when crossing ove... more The aim of this article is to quantify the loads exerted by heavy-duty vehicles when crossing over buried pipeline. This problem arises in connection to the question pertaining to the use of protective sleeves (casings) applied to gas pipelines in regions with increased demands on pipeline operation safety. An experiment was conducted on a test pipe section made from L360NE pipeline steel equipped with strain gauges along the pipe perimeter, measuring strains in the axial and circumferential directions. Strain measurements were taken after back-filling the pipe trench, then during vehicle crossings over the empty pipe, and again after pressurizing the test pipe with air. Strain-based hoop stresses at the surface of the empty test pipe were found to exceed 30 MPa after back-filling the trench and increased to more than 40 MPa during the vehicle crossings. Similarly, axial stresses reached extremes of around 17 MPa in compression and 12 MPa in tension. Applying internal air pressure t...
MMM offers a very interesting alternative to traditional defectoscopy methods such as ultrasound ... more MMM offers a very interesting alternative to traditional defectoscopy methods such as ultrasound or magnetic powder method. Like any method, it has its advantages and limitations. Among the most interesting advantages is the possibility of detecting imperfections in the material without cleaning or even pre-treatment of the test surface. This is a major advantage especially when large and complicated parts are inspected on which a continuous layer of corrosive product is formed or other leachate. The disadvantage is the inability to use this method on non-magnetic materials (Al alloys) and the fact that only places with potentially high internal stresses are identified here, which does not necessarily mean a problem. This method was used to identify defects in two cases:
Changes in dislocation substructure were studied in normalized and annealed S235JR steel in relat... more Changes in dislocation substructure were studied in normalized and annealed S235JR steel in relation to the number of symmetrical reversed stress cycles with the amplitude σa = 242 MPa and corresponding mean life Nf = 17,950 cycles. The microstructure of the steel consisted of ferrite with a small amount of pearlite. The substructure was observed in as received condition and after application of various relative numbers of cycles n/Nf, these being 0.25, 0.50 and 0.75. An irregular dislocation net occurred in the virgin specimen, however, during cyclic loading the dislocations started to accumulate gradually in slip bands and to form a cell substructure in grains of favourable crystallographic orientations. Total dislocation density, dislocation density in slip bands and inter-band distance were measured using transmission electron microscopy (TEM). The total dislocation density was found to slightly decrease and the density in slip bands to increase with increasing number of cycles....
Materials, 2021
The purpose of this article is to compare two predictive methods of J integral assessments for ce... more The purpose of this article is to compare two predictive methods of J integral assessments for center-cracked plates, single-edge cracked plates and double-edge cracked plates produced from X52 and X70 steels, and a longitudinally cracked pipe produced from X70 steel. The two methods examined are: the GSM method and the Js procedure of the French RCC-MR construction code, designated here as the FC method. The accuracy of J integral predictions by these methods is visualized by comparing the results obtained with the “reference” values calculated by the EPRI method. The main results showed that both methods yielded similar J integral values, although in most cases, the GSM predictions were slightly more conservative than the FC predictions. In comparison with the “reference” values of the J integral, both methods provided conservative results for most crack configurations, although the estimates for cracks of a relative length smaller than 1/8 were not found to be so conservative. Th...
Materials Today: Proceedings, 2016
For standard fracture mechanical tests flat specimens (principally CT or SENB) are required. When... more For standard fracture mechanical tests flat specimens (principally CT or SENB) are required. When investigating fracture mechanical properties of thin-walled pipes this brings about a problem because it is necessary to straighten pipe bands. This operation causes internal stresses to be induced not only in the semi-product subjected to straightening but also in finished specimens. A question therefore arises to what extent are then the magnitudes of the fracture toughness determined representative for the actual cylindrical wall. To solve this problem fracture mechanics tests were carried out on flat (straightened) CT specimens as well as on curved CT specimens with the natural curvature. The R-curves as well as the resulting parameters of the fracture toughness, obtained for both types of CT specimens, were compared and it was concluded that the fracture toughness of the pipe material determined on straightened CT specimens was practically the same as that obtained on curved CT specimens.
Materials Today: Proceedings, 2017
Applied Mechanics and Materials, 2016
For standard fracture mechanical tests flat specimens (principally CT or SENB) are required. When... more For standard fracture mechanical tests flat specimens (principally CT or SENB) are required. When investigating fracture mechanical properties of thin – walled pipes this brings about a problem because it is necessary to straighten pipe bands. However, this operation causes internal stresses to be induced not only in the semi-product subjected to straightening but also in finished specimens. A question therefore arises to what extent are then the magnitudes of the fracture toughness determined representative for the actual cylindrical wall. To solve this problem fracture mechanics tests were caried out on flat (straightened) CT specimens as well as on curved CT specimens with the natural curvature. The R – curves as well as the resulting parameters of the fracture toughness, obtained for both types of CT specimens, were compared and it was concluded that the fracture toughness of the pipe material determined on straightened CT specimens was practically the same as that obtained on curved CT specimens.
Key Engineering Materials, 2014
The paper presents experimental results concerning the variation of microplastic limit (MPL) duri... more The paper presents experimental results concerning the variation of microplastic limit (MPL) during fatigue as determined by the measurement of changes in inductance L of specimens made from normalized low-C steel CSN 411375. Some indications have been obtained that MPL varies during fatigue with dislocation density. This was proved by nanoindentation, X-ray diffraction and transmision electron microscopy.
Procedia Engineering, 2013
The experimental determination of fatigue limit in case of steels represents an expensive and tim... more The experimental determination of fatigue limit in case of steels represents an expensive and time consuming process not speaking about the necessity of a sufficient volume of representative material necessary for the specimens manufacturing, manufacturing of numerous test specimens, the need of an expensive (electrohydraulic) test equipment etc. Microplastic strains occurring at stresses below the yield point play a significant role at cyclic loading of metals within high cycle fatigue region. The microplastic strains cannot be measured by common methods due to the discontinuity of their distribution and a varying character of their level in the metal structure, but however, owing to the magnetoelastic effect the threshold stress at which dislocation pileup stresses begin to obstruct the magnetic domains in rotation to the direction of the tensile stress can be determined by means of changes in magnetic permeability during tensile loading. In the paper, the changes of the magnetic permeability were measured indirectly-by means of the changes of the electrical impedance (a.c. resistance). The measurement was performed on the normalized low-carbon structural steel CSN 41 1375. The microplastic limit was determined by evaluation the ΔR-σ records. A comparison with the fatigue limit in a reversed fatigue loading showed that the MPL was below the fatigue limit. The microplastic limit can thus be considered to separate nondamaging and damaging cyclic stresses. The paper presents the results of the abovementioned characteristics what may shorten and make cheeper the determination of the fatigue limit and non-damaging cyclic stresses of structural materials.
This study offers a new simple method for assessing an axial part-through crack in a pipe wall. T... more This study offers a new simple method for assessing an axial part-through crack in a pipe wall. The method utilizes simple approximate expressions for determining the fracture parameters K, J, and employs these parameters to determine critical dimensions of a crack on the basis of equality between the J-integral and the J-based fracture toughness of the pipe steel. The crack tip constraint is taken into account by the so-called plastic constraint factor C, by which the uniaxial yield stress in the J-integral equation is multiplied. The results of the prediction of the fracture condition are verified by burst tests on test pipes.