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Investigation of Functional Properties of Thin NiTi Filaments for Applications in Smart Structures and Hybrid Textiles
Pilch, Jan ; Dlouhý, Antonín (referee) ; Heczko,, Oleg (referee) ; Pokluda, Jaroslav (advisor)
PhD thesis focuses the field of textile application of modern functional materials, namely metallic shape memory alloys with unique thermomechanical properties deriving from martensitic transformation in solid state. Particularly, it deals with the development of a nonconventional thermomechanical treatment of thin NiTi filaments via Joule heating by electric current and related basic research involving thermomechanical testing and modeling of functional properties of the filaments, investigation of martensitic transformations and deformation processes in NiTi and investigation of the fast recovery and recrystallization processes in metals heated by short pulses of controlled electric power. The method was developed and called FTMT-EC. In contrast to conventional heat treatment of metallic filaments in environmental furnaces, this method allows for precise control of the raise of the filament temperature and filament stress during the fast heating (rate ~50 000 °C/s). As a consequence, it is possible to precisely control the progress of the fast recovery and recrystallization processes in heat treated filaments. In this way it is possible to prepare filaments with desired nanostructured microstructure and related functional properties. A prototype equipment for application of the method for heat treatment of continuous SMA filaments during respooling in textile processing was designed and built. Comparing to the conventional heat treatment of SMA filaments in tubular environmental furnaces, this approach is faster, saves energy and allows for preparation of filaments with special functional properties. International patent application was filed on the method. It is currently utilized in the research and development of smart textiles for medical applications.
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Computer modeling of twin-boundaries in shape memory alloys
Heczko, Martin ; Pokluda, Jaroslav (referee) ; Zelený, Martin (advisor)
This Master‘s thesis is focused on theoretical study of twinning in magnetic shape memory alloys based on Ni2MnGa using ab initio calculations of electronic structure within the projector augmented wave method. In particular, the effect of increasing concentration of manganese at the expense of gallium was studied on total energy and stress profiles along different deformation paths in the (10-1)[101] shear system of non-modulated martensite. Further, this work deals with the effect of the concentration of manganese on the energy of planar fault caused by presence of partial dislocation due to motion of twin boundary. The results show that the shear modulus in studied shear system increases with the increasing concentration of manganese as well as energy barrier and deformation characteristics along shear deformation paths increases, which makes the shear more difficult in Mn-rich alloys. Increasing concentration of manganese also leads to rising the planar fault energy. All these effects can be responsible for lower mobility of twin boundaries in alloys with higher concentration of manganese.
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PROPAGATION OF LONG FATIGUE CRACKS IN AUSTENITIC STEEL UNDER SHEAR MODES II AND III
Holáň, Libor ; Man, Jiří (referee) ; Polák, Jaroslav (referee) ; Pokluda, Jaroslav (advisor)
This work is focused on the realization of experiment allowing simultaneous loading under mode II and III in a single circular specimen. Proposed experiment allowed to minimize crack closure during the cyclic loading and obtained values of thresholds of stress intensity range can be considered to be very close to effective values. This was attained by means of an unique experimental devices and procedure of preparation of pre-crack of specimen with circumferential notch, which was made of stainless austenitic steel. The obtained values were compared with theoretical models with the support of molecular dynamics and ab-anitio calculation. Based on observation was found out, that fatigue crack propagation is controlled by decohesion model in austenitic steel. The morphology of fracture surfaces was studied by means of optical chromatographie and 3D stereophotogrammetry, which allowed a comparison of created morphology under shear modes II and III. Morphology of fracture surface formed (static and cyclic loading) by pre-crack was also studied by means of selected roughness parameters. The mechanism of deflection (kink) of crack growth under mode II was defined.
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Influence of Plasma-nitriding on Fatigue Life of High-strength Steels under Biaxial Loading
Major, Štěpán ; Kohout, Jan (referee) ; Růžička,, Milan (referee) ; Obrtlík, Karel (referee) ; Pokluda, Jaroslav (advisor)
This work deals with fatigue life and fractographycal analysis of plasma-nitrided and virgin specimen made of the low-alloy high-strength steel. Specimens were subjected to in-phase combined bending-torsion loading. The work is focused on three groups of problems. The first group of problems was associated with the fatigue life and its prediction. The objective was to examine a relationship between the presence of the nitrided layer and fatigue life. The application of surface layers has significantly improved the fatigue life in the high-cycle region. An extended comparison between classical and advanced multiaxial criteria was performed. Criterions proposed by Marin and Matake were found to be most precise. The second objective of this work was to examine a relationship between the loading ratio ( is the bending amplitude and is the torsion amplitude ) and the fracture surface topography in the highcycle fatigue. Properties of fracture surfaces were quantified by many roughness and fractal parameters. Many roughness parameters start to incrise rapidly above a critical value of the loading ratio zc = 0,5. The last part of this work deals with the fractographical analysis of fish-eye cracks that were formed in plasma-nitrided steel specimens under symetrical bending, symetrical torsion and biaxial in-phase bending-torsion combinations.
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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 (referee) ; Polák, Jaroslav (referee) ; Pokluda, Jaroslav (advisor)
Práce je zaměřena na šíření únavových trhlin v módech II, III a II + III v prahové oblasti v kovových materiálech. Byly provedeny experimenty pro ARMCO železo, titan, nikl a austenitickou ocel na třech druzích vzorků pro zatěžování trhliny ve smykových módech. Byl použit zvláštní způsob vytváření iniciační trhliny, který eliminoval efekt zavírání trhliny na začátku smykového zatěžování. Ve všech zkoumaných materiálech byly efektivní prahové hodnoty v módu II přibližně 1,7 krát nižší než v módu III. Měřením úhlů lokálních odklonů a natočení lomových faset s použitím stereofotogrammetrie v SEM bylo usuzováno na tendenci trhliny k vyhýbání do lokálního módu I. Nejmenší úhly byly zjištěny u ARMCO železa pro trhliny v módech II i III, středně velké úhly u titanu a niklu a největší úhly (vyhnutí do čistého módu I) byly pozorovány u austenitické oceli. Tyto rozdíly lze vysvětlit různým počtem dostupných skluzových systémů v krystalových mřížkách daných materiálů. Poměry mezi naměřenými prahovými hodnotami v módu I, II a III byly srovnány s předpověďmi teoretických modelů. Byl navržen analytický vztah pro efektivní prahové hodnoty v módu II, který byl v dobré shodě s experimentálními výsledky. Pomocí efektivních prahových hodnot bylo vyčísleno kritérium pro vyhýbání do módu I, které vedlo ke kritickému úhlu odklonu 40° souvisejícímu s přechodem z lokálního smykového módu do otevíracího módu.
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Apartment House
Pokluda, Jiří ; Havířová, Zdeňka (referee) ; Lavický, Miloš (advisor)
The aim of this bachelor's thesis is the elaboration of project documentation for the construction of a new apartment building in the southwestern part of the village Holubice. The building is designed as a detached building with four above-ground and one underground floor partly below ground level. In the apartment building with a single-layered flat roof, there are 8 residential units, technical facilities and house equipment. There is also a designed elevator in the building. The drawing part is processed using the AutoCad program, the visualization is processed in the ArchiCad and Lumion program.
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Apartment House
Pokluda, Jiří ; Havířová, Zdeňka (referee) ; Lavický, Miloš (advisor)
The aim of this bachelor's thesis is the elaboration of project documentation for the construction of a new apartment building in the southwestern part of the village Holubice. The building is designed as a detached building with four above-ground and one underground floor partly below ground level. In the apartment building with a single-layered flat roof, there are 8 residential units, technical facilities and house equipment. There is also a designed elevator in the building. The drawing part is processed using the AutoCad program, the visualization is processed in the ArchiCad and Lumion program.
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