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Šíření dlouhých únavových trhlin v austenitické oceli při smykových módech II a III
Holáň, Libor ; Man, Jiří (oponent) ; Polák, Jaroslav (oponent) ; Pokluda, Jaroslav (vedoucí práce)
Práce je zaměřena na realizaci experimentu umožňujícího současné zatěžování vzorku kruhového průřezu ve smykových módech II a III. Postup experimentu byl navržen tak, aby byl co nejvíce potlačen vliv zavíraní trhliny při cyklickém zatěžování a získané prahové hodnoty rozkmitu faktoru intenzity napětí tak odpovídaly efektivním prahovým hodnotám. Toho bylo dosaženo za pomocí unikátních experimentálních přípravků a postupů přípravy iniciační trhliny u vzorků s~obvodovým vrubem, vyrobených z korozivzdorné austenitické oceli. Takto získané hodnoty je poté možno aplikovat na teoretické modely s podporou molekulární dynamiky a ab inicio. Na základě získaných poznatků bylo zjištěno, že šíření trhliny v austenitické oceli probíhá dle zákonitostí dekohezního modelu. Pro kvantitativní analýzu lomových ploch vytvořených ve smykových módech II a III bylo využito metod optické chromatografie a 3D stereofotogrametrie, které umožnily srovnání morfologií, vytvořených jednotlivými smykovými módy. Pro vyhodnocení morfologie lomových ploch vytvořených (staticky, resp. cyklicky) iniciační trhlinou byly použity vybrané drsnostní parametry. V rámci experimentálních prací byl rovněž navržen mechanismus umožňující předpovídat směr vyhnutí trhliny v módu II.
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Propagation of Fatigue Cracks under Shear Loading Modes II, III and II+III in the Near-Threshold Region
Vojtek, Tomáš ; Man, Jiří (oponent) ; Polák, Jaroslav (oponent) ; Pokluda, Jaroslav (vedoucí práce)
The work is focused on fatigue crack propagation under modes II, III and II + III in the near-threshold region in metallic materials. Experiments were conducted for ARMCO iron, titanium, nickel and stainless steel on three types of specimens for shear-mode crack loading. A special technique of precrack generation was used which enabled a closure-free loading at the beginning of the shear-mode experiments. For all investigated materials the effective thresholds under the remote mode II loading were found to be about 1.7 times lower than those under the remote mode III. Tendency to local mode I branching was assessed by a measurement of local deflection and twist angles of fracture facets using stereophoto-grammetry in SEM. The lowest angles of both mode II and mode III cracks were identified for the ARMCO iron, the intermediate ones in titanium and nickel and the highest ones (pure mode I branches) were determined for the stainless steel. These differences can be explained by different numbers of available slip systems in crystal lattices of investigated materials. Ratios of measured effective thresholds in modes I, II and III were compared with results predicted by theoretical models. An analytical formula for effective mode-II thresholds was proposed and found in a good agreement with experimental results. The mode I branching criterion in terms of effective thresholds led to a critical deflection angle of 40° related to a transition from local shear to opening mode.
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MICROSTRUCTURAL INVESTIGATION AND MECHANICAL TESTING OF AN ULTRAFINE-GRAINED AUSTENITIC STAINLESS STEEL
Chlupová, Alice ; Man, Jiří ; Polák, Jaroslav ; Karjalainen, L. P.
Special thermomechanical treatment based on high degree deformation followed by reversion annealing was applied to 301LN austenitic stainless steel to achieve ultrafine-grained (UFG) structure with considerably enhanced mechanical properties. Two different conditions of the thermomechanical treatment were adopted and resulting microstructures with different grain sizes were characterised by optical and high resolution scanning electron microscopy (FEG SEM). Hardness measurements and tensile tests were performed to characterize mechanical properties. To reveal structural changes induced during thermomechanical treatment and during tensile tests a magnetic induction method was additionally applied. Experimental study validated the ability of the treatment to produce an austenitic stainless steel with the grain size of about 1.4 mu m which exhibits tensile strength of around 1000 MPa while ductility remains close to 60 %. The results obtained for both thermomechanical conditions are compared and the relationship between microstructure refinement, phase content and mechanical properties is discussed.
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MICROSTRUCTURAL INVESTIGATION AND MECHANICAL TESTING OF AN ULTRAFINE-GRAINED AUSTENITIC STAINLESS STEEL
Chlupová, Alice ; Man, Jiří ; Polák, Jaroslav ; Karjalainen, L. P.
Special thermomechanical treatment based on high degree deformation followed by reversion annealing was applied to 301LN austenitic stainless steel to achieve ultrafine-grained (UFG) structure with considerably enhanced mechanical properties. Two different conditions of the thermomechanical treatment were adopted and resulting microstructures with different grain sizes were characterised by optical and high resolution scanning electron microscopy (FEG SEM). Hardness measurements and tensile tests were performed to characterize mechanical properties. To reveal structural changes induced during thermomechanical treatment and during tensile tests a magnetic induction method was additionally applied. Experimental study validated the ability of the treatment to produce an austenitic stainless steel with the grain size of about 1.4 mu m which exhibits tensile strength of around 1000 MPa while ductility remains close to 60 %. The results obtained for both thermomechanical conditions are compared and the relationship between microstructure refinement, phase content and mechanical properties is discussed.
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LCF behaviour of 301LN steel: coarse-grained vs. UFG-bimodal structure
Man, Jiří ; Chlupová, Alice ; Kuběna, Ivo ; Kruml, Tomáš ; Man, O. ; Järvenpää, A. ; Karjalainen, L. P. ; Polák, Jaroslav
Low-cycle fatigue (LCF) behaviour of metastable austenitic 301LN steel with different grain sizes – coarse-grained (14 µm) and UFG (1.4 µm) with a grain bimodality – produced by reversion annealing (RA) was investigated. Symmetrical push-pull LCF tests were conducted on flat sheet specimens at room temperature with constant strain rate of 2×10–3 s–1 and constant total strain amplitude ranging from 0.4% to 0.8%. After completion of fatigue tests a ferritescope was adopted for quantitative assessment of volume fraction of deformation induced martensite (DIM). Microstructural changes, distribution and morphology of DIM in the volume of material were characterized at different scales by colour etching, TEM and EBSD techniques. Experimental data on microstructural changes are confronted with the stress-strain response and with the chemical heterogeneity present in the material.
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Influence of surface morphology on fatigue behavior of metastable austenitic stainless steel AISI 347 at ambient temperature and 300°C
Smaga, M. ; Skorupski, R. ; Mayer, P. ; Kirsch, B. ; Aurich, J. C. ; Raid, I. ; Seewig, J. ; Man, Jiří ; Eifler, D. ; Beck, T.
The effect of surface modification by cryogenic turning on fatigue behavior of metastable austenitic stainless steel AISI 347 was investigated in stress-controlled fatigue tests at ambient temperature (AT) and 300 °C in air. Five different surface morphologies were manufactured by the variation of turning parameters. Surface and near surface morphology were comprehensively characterized by various experimental techniques. The experimental data on the cyclic deformation behavior, stress-strain response and fatigue life for all surface morphologies are reported.
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Propagation of Fatigue Cracks under Shear Loading Modes II, III and II+III in the Near-Threshold Region
Vojtek, Tomáš ; Man, Jiří (oponent) ; Polák, Jaroslav (oponent) ; Pokluda, Jaroslav (vedoucí práce)
The work is focused on fatigue crack propagation under modes II, III and II + III in the near-threshold region in metallic materials. Experiments were conducted for ARMCO iron, titanium, nickel and stainless steel on three types of specimens for shear-mode crack loading. A special technique of precrack generation was used which enabled a closure-free loading at the beginning of the shear-mode experiments. For all investigated materials the effective thresholds under the remote mode II loading were found to be about 1.7 times lower than those under the remote mode III. Tendency to local mode I branching was assessed by a measurement of local deflection and twist angles of fracture facets using stereophoto-grammetry in SEM. The lowest angles of both mode II and mode III cracks were identified for the ARMCO iron, the intermediate ones in titanium and nickel and the highest ones (pure mode I branches) were determined for the stainless steel. These differences can be explained by different numbers of available slip systems in crystal lattices of investigated materials. Ratios of measured effective thresholds in modes I, II and III were compared with results predicted by theoretical models. An analytical formula for effective mode-II thresholds was proposed and found in a good agreement with experimental results. The mode I branching criterion in terms of effective thresholds led to a critical deflection angle of 40° related to a transition from local shear to opening mode.
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Šíření dlouhých únavových trhlin v austenitické oceli při smykových módech II a III
Holáň, Libor ; Man, Jiří (oponent) ; Polák, Jaroslav (oponent) ; Pokluda, Jaroslav (vedoucí práce)
Práce je zaměřena na realizaci experimentu umožňujícího současné zatěžování vzorku kruhového průřezu ve smykových módech II a III. Postup experimentu byl navržen tak, aby byl co nejvíce potlačen vliv zavíraní trhliny při cyklickém zatěžování a získané prahové hodnoty rozkmitu faktoru intenzity napětí tak odpovídaly efektivním prahovým hodnotám. Toho bylo dosaženo za pomocí unikátních experimentálních přípravků a postupů přípravy iniciační trhliny u vzorků s~obvodovým vrubem, vyrobených z korozivzdorné austenitické oceli. Takto získané hodnoty je poté možno aplikovat na teoretické modely s podporou molekulární dynamiky a ab inicio. Na základě získaných poznatků bylo zjištěno, že šíření trhliny v austenitické oceli probíhá dle zákonitostí dekohezního modelu. Pro kvantitativní analýzu lomových ploch vytvořených ve smykových módech II a III bylo využito metod optické chromatografie a 3D stereofotogrametrie, které umožnily srovnání morfologií, vytvořených jednotlivými smykovými módy. Pro vyhodnocení morfologie lomových ploch vytvořených (staticky, resp. cyklicky) iniciační trhlinou byly použity vybrané drsnostní parametry. V rámci experimentálních prací byl rovněž navržen mechanismus umožňující předpovídat směr vyhnutí trhliny v módu II.
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