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Changes in dislocation substructure of S235JR steel during fatigue loading
Jandová, D. ; Gajdoš, Lubomír ; Šperl, Martin ; Kaiser, J.
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. The results concerning the dislocation density were compared with changes in the microplastic limit (MPL) which were determined by the measurement of the inductance of the “specimen - coil” system. They consisted in a rapid initial decrease at the first stage of the fatigue process and in a gradual increase during the major part of the life. These changes can be interpreted on the basis of changes in dislocation density as was verified independently by X-ray diffraction and nanoindentation tests. On the basis of the measurement of dislocation density by TEM it appears that changes in MPL can be connected with the dislocation density in slip bands rather than with the total dislocation density.
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Micro-plastic limit of low-carbon steel CSN 41 1375 as determined by the measurement of electrical impedance
Gajdoš, Lubomír ; Šperl, Martin ; Kaiser, J. ; Mentl, V.
It is generally accepted that not all cyclic stresses below the fatigue limit are non-damaging. Particularly in spectrum loading, the inclusion of some cyclic stresses below the fatigue limi can reduce the fatigue life of a component. It is believed that the boundary between damaging and non-damaging stresses is the so-called micro-plastic limit (MPL) defined as a macro-stress at which dislocation pile-up stresses begin to obstruct the magnetic domains in rotation to the direction of the tensile stress. This paper shows that MPL can be determined from changes in magnetic permeability during tensile loading. In our work, changes in magnetic permeability were measured indirectly - by measuring changes in electrical impedance (a.c. resistence and induktance). Measurements were performed on normalized low-C steel CSN 411375, and the microplastic limit was determined by evaluating
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Vliv opakovaného přetížení plynovodních potrubí na jejich integritu
Gajdoš, Lubomír ; Šperl, Martin
Při realizaci některých tlakových zkoušek plynovodních potrubí jsou zkoušené plynovodní úseky podrobeny i opakovanému přetížení, a to v závislosti na druhu a účelu tlakové zkoušky. Právě opakovaná přetížení starších provozovaných potrubí, v jejichž plášti nelze vyloučit výskyt trhlin, jsou předmětem určitých nejasností, pokud jde o zachování či porušení jejich integrity. Na základě experimentálních výsledků, získaných na plynovodní trubce s uměle vygenerovanými únavovými trhlinami, jsou z pozic soudobé lomové mechaniky analyzovány podmínky, vedoucí k protržení ligamentu před čelem trhliny při opakovaném přetížení a jsou diskutovány možné mechanismy sníženého lomového tlaku při opakování přetížení trubky.
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