Jiří Man

Academy of Sciences of the Czech Republic, Praha, Hlavni mesto Praha, Czech Republic

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Publications (34)18.29 Total impact

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    Jaroslav Polák, Jiří Man
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    ABSTRACT: The role of point defects in the formation of surface relief and in the initiation of a fatigue crack in crystalline materials is analyzed. The dislocation interactions in the bands of intensive cyclic slip (persistent slip bands – PSBs) are specified and relations describing the formation and annihilation of interstitial and vacancy type defects in the channels of the ladder-like PSB are derived. The continuous formation, annihilation and primarily the migration of point defects are proposed to be responsible for the mass redistribution within PSB and between PSB and the PSB/matrix boundary. The redistribution of the matter results in local tensile and compressive stresses that are the sources of the principal irreversibility of slip within PSB. Local tensile and compressive stresses are relaxed by dislocation movement within PSB in the direction of the active Burgers vector and lead to the formation of characteristic surface relief in the form of extrusions and intrusions. The intrusions represent crack-like defects and fatigue cracks initiate in the tip of intrusions.
    International Journal of Fatigue 08/2014; 65:18–27. · 1.98 Impact Factor
  • J. Polák, J. Man
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    ABSTRACT: Experimental data on the formation of persistent slip markings (PSMs) at the egressing persistent slip bands (PSBs) and physically based models of surface relief formation and fatigue crack initiation are surveyed. The original Polák's model of fatigue crack initiation based on the formation, migration and annihilation of vacancy-type defects in PSBs is further refined and extended by taking into consideration the annihilation of vacancies in the matrix and the formation of internal stresses. The proposed mechanisms of surface relief formation are based on the plastic relaxation of internal compression stresses in PSBs and internal tensile stresses in the matrix. By solving the migration of vacancies from the PSB and their annihilation in the matrix, the shapes of extrusion and parallel intrusions are derived analytically under some simplifying assumptions.
    Materials Science and Engineering A 02/2014; 596:15–24. · 2.41 Impact Factor
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    ABSTRACT: Cyclic strain behaviour of metastable austenitic 301LN steel with different grain sizes – coarse (14 μm) and ultrafine (1.4 μm) grained – produced by the special thermo-mechanical treatment based on the martensite-to-austenite reversion was investigated. Low-cycle fatigue (LCF) tests were conducted on flat specimens at room temperature in fully push-pull mode under strain control with constant strain rate of 2×10–3 s–1 and constant total strain amplitude ranging from 0.4% to 0.9%. LCF behaviour was characterised by cyclic hardening/softening curves and fatigue life curves. After completion of fatigue tests surface fatigue damage was studied using scanning electron microscopy (SEM). Ferritoscope was adopted to reveal the destabilization of austenitic structure and evaluate the extent of deformation induced martensitic transformation.
    Procedia Engineering 01/2014; 74:147–150.
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    ABSTRACT: Focused ion beam (FIB) technique together with other advanced microscopic techniques was applied to study the early microstructural changes leading to fatigue crack initiation in cyclically strained polycrys-tals (nickel, 316L steel). Dislocation structures of persistent slip bands (PSBs) and surrounding matrix were investigated in detail using electron channeling contrast imaging (ECCI) technique (concurrently in the FIB cross-section and on the specimen surface) and simultaneously with the surface relief topog-raphy using transmission electron microscopy (TEM) of thin surface foils prepared by in situ lift-out tech-nique. True shape of extrusions and intrusions and the path of initiated fatigue cracks were assessed in three dimensions by FIB micro-tomography. The role of twin boundary in cyclic strain localization and fatigue crack initiation is discussed. Principal advantages and some limitations of FIB technique in fatigue crack initiation studies in polycrystals are highlighted.
    International Journal of Fatigue 04/2012; 39. · 1.98 Impact Factor
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    ABSTRACT: The effect of positive mean stress on the fatigue behavior of ferritic–pearlitic–bainitic steel has been studied. Specimens, produced from a massive forging, were cycled with two constant stress amplitudes and various positive mean stresses. Plastic strain amplitude and cyclic creep rate were measured during cyclic loading and the effect of the mean stress on saturated plastic strain amplitude and mean strain at half-life was established. Plastic strain amplitude is weakly dependent but creep strain increases with the mean stress exponentially. Fatigue life decreases with the mean stress for both stress amplitudes. The contributions of cyclic plastic strain and cyclic creep to the fatigue damage were evaluated and discussed in relation with the Manson-Coffin curve.
    International Journal of Fatigue - INT J FATIGUE. 01/2012;
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    ABSTRACT: The effect of the mean stress on the crack initiation and short crack growth of austenitic–ferritic duplex steel has been studied. High mean stresses and stress amplitudes result in appreciable mean strain relaxation and long-term hardening. Mean stress produces unidirectional slip bands and slip steps that serve as nuclei for persistent slip bands and persistent slip markings. It leads to the acceleration of the crack initiation and production of a high density of cracks. Crack linkage contributes to the growth of short cracks. The concept of equivalent crack was used to describe the crack growth. The kinetics of short crack growth with positive mean stress is similar to that in symmetric loading, that is, exponential growth is observed. Positive mean stress results in earlier crack initiation and in the acceleration of the crack growth rate. Both factors contribute to the decrease of the fatigue life.
    Fatigue & Fracture of Engineering Materials & Structures 08/2011; 35(3):257 - 268. · 0.86 Impact Factor
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    ABSTRACT: Solution-annealed AISI 316L steel was fatigued with constant plastic strain amplitudes at room temperature and under various conditions at depressed temperatures down to 113 K to reveal its stability against deformation-induced martensite formation. Microstructural changes induced by fatigue were characterized by transmission electron microscopy (TEM), electron channeling contrast imaging (ECCI) and electron backscattering diffraction (EBSD) techniques. Neutron diffraction and magnetic induction method were adopted for quantification of martensite content. Deformation-induced martensite formation in the bulk of material was evidenced for low temperature cyclic straining under various conditions. Room temperature cycling, even with high plastic strain amplitudes, results in a local very limited martensite formation in areas closely linked with the long fatigue crack growth.
    Procedia Engineering 01/2011;
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    ABSTRACT: Focused ion beam (FIB) technique together with other advanced microscopic techniques were applied to study early microstructural changes leading to crack initiation in fatigued polycrystals. Dislocation structures of persistent slip bands (PSBs) and surrounding matrix were revealed in the bulk of surface grains by electron channelling contrast imaging (ECCI) technique on the FIB cross-sections. True shape of extrusions, intrusions and the path of initiated fatigue cracks were assessed in three dimensions by serial FIB cross-sectioning (FIB tomography). Advantageous potential of FIB technique and its other possible utilization in fatigue crack initiation studies in polycrystals are highlighted.
    Journal of Physics Conference Series 08/2010; 240(1):012058.
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    ABSTRACT: Surface relief of austenitic stainless 316L steel cycled with constant plastic strain amplitude at 93, 173 and 573 K to different early stages of fatigue life was studied using atomic force microscopy (AFM) and high resolution scanning electron microscopy (SEM–FEG). Characteristic features of surface relief topography and the evolution of persistent slip markings (PSMs) were documented for all temperatures. At 173 and 573 K PSMs consist of growing extrusions accompanied later by parallel intrusions while at the lowest temperature only static extrusions were found. In addition to distinct true PSMs fine slip markings with peak-to-valley topography covering homogeneously different parts of grains and deformation induced martensite were detected in 316L steel fatigued at 93 K. Experimental results on the PSM topography and the kinetics of extrusion growth are discussed in relation to point defect models of surface relief formation leading to fatigue crack initiation.
    Procedia Engineering 01/2010;
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    ABSTRACT: The early stages of fatigue damage in cyclic loading in several polycrystalline materials were studied. The initiation of fatigue cracks is influenced by a number of factors like the presence of inclusions, grain size and cyclic creep strain. The growth of short cracks was measured in symmetrical loading and in cycling with positive mean stress. The kinetics of short crack growth is affected by the presence of secondary cracks and can be approximated by an exponential law. The integrated crack growth law is equivalent to Coffin-Manson law allowing the interpretation of this law in terms of short crack growth.
    Procedia Engineering. 01/2010; 2(1):883-892.
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    ABSTRACT: Surface relief within persistent slip markings (PSMs) was studied using confocal scanning laser microscopy (CSLM) and atomic force microscopy (AFM) in cast nickel base superalloy Inconel 738LC cyclically strained in strain control at room temperature. Extrusion and intrusion topography and kinetics are documented. The dependence of extrusion height on the number of cycles is obtained. Two regimes of extrusion growth are identified. Average intrusion growth rate is assessed.
    Journal of Physics Conference Series 01/2010; 240(1).
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    ABSTRACT: Early stages of surface relief evolution of persistent slip markings (PSMs), formed in areas where persistent slip bands (PSBs) intersect the free surface, in polycrystalline 316L stainless steel cycled with constant plastic strain amplitude were studied using atomic force microscopy (AFM) and electron backscattering diffraction (EBSD). Focused ion beam (FIB) technique was employed to obtain additional, more detailed information on the shape of PSMs. To reveal true qualitative and quantitative information on the simultaneous growth of intrusions and extrusions within individual PSMs, identical areas both on the specimen surface and on its inverse copy obtained via plastic replica were studied using AFM. Intrusions are preceded by extrusions regardless of orientation of individual grains of the polycrystal. The first intrusions appear largely around 1% of fatigue life at the moment when 'static' extrusions are developed. They appear predominantly but not exclusively at the side of extrusions where the emerging active slip plane is inclined to the surface at an acute angle. They grow faster than the stage of stable extrusion growth. Typical morphology of mature PSMs developed at 15% of fatigue life consists of ribbon-like extrusions accompanied by two thin parallel intrusions along both PSB– matrix interfaces. Experimental data on the morphology and growth of extrusions and intrusions are discussed in relation to the theoretical models and computer simulations of surface relief evolution leading to fatigue crack initiation.
    Philosophical Magazine A 07/2009; 89(16):1337-1372.
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    ABSTRACT: Total strain-controlled tests have been performed on cylindrical specimens of polycrystalline Inconel 792-5A at 23 and 900°C to study the effect of temperature on low cycle fatigue characteristics and cyclic strain localization. Hardening/softening curves, cyclic stress–strain curves, and fatigue life curves are presented. Two linear dependencies are used to approximate the room temperature data in Manson–Coffin plot. Technique of oriented foils observed in transmission electron microscope is used to study dislocation structure. Effect of temperature on surface relief topography and fracture surface is documented using scanning electron microscopy and atomic force microscopy. High-amplitude straining is characterized by slight initial hardening followed by saturation at room temperature and sustained weak softening at 900°C. Low-amplitude cycling results in the stable stress response. Plastic strain localization into persistent slip bands lying along {111} slip planes was observed at both temperatures.
    Journal of Materials Science 01/2009; 44(12):3305-3314. · 2.31 Impact Factor
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    ABSTRACT: Scanning electron microscopy (SEM) combined with local milling by focused ion beam (FIB) was used to study the true shape of the relief developed on the surface of polycrystalline 316L steel cycled with constant plastic strain amplitude. Localization of the cyclic strain to persistent slip bands results in early formation of persistent slip markings. The evolution of the early relief formation is reported using the observation on two perpendicular surfaces. The observed configurations of extrusions and intrusions are discussed and compared with predictions of the latest models of fatigue crack initiation. The prominent role of intrusions in fatigue crack initiation is emphasized.
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing - MATER SCI ENG A-STRUCT MATER. 01/2009; 517(1):204-211.
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    J. Man, K. Obrtlik, J. Polak
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    ABSTRACT: Current state and historical progress in experimental and theoretical studies of surface relief appertaining to persistent slip bands (PSBs) and leading to fatigue crack initiation in cyclically deformed metals is presented as a thorough critical overview. A comprehensive inventory of microscopic techniques used for this study is tabulated chronologically with emphasis to their applicability to polycrystals. The most relevant experimental characteristics concerning surface relief evolution, namely the form of extrusions and intrusions in single- and polycrystalline materials, are surveyed. Theoretical models and computational simulations of extrusion and intrusion formation and fatigue crack initiation are critically reviewed. †In 2003, it was 100 years since J.A. Ewing and J.C.W. Humfrey first documented the nature of early surface fatigue damage in Swedish iron in their paper The Fracture of Metals under Repeated Alternations of Stress published in the journal Philosophical Transactions of the Royal Society of London, Series A [1]. Let this work (Part 1 and 2) commemorate their pioneering effort to understand the physical mechanisms of early fatigue damage.
    Philosophical Magazine A 01/2009; 89(16):1295-1336.
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    ABSTRACT: The slip activity of persistent slip bands (PSBs) in polycrystalline nickel was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The half-cycle slip activity as well as the local shear strain amplitudes was investigated after half-cycle deformation at different numbers of cycles in the domain of stress saturation. Moreover, the fraction of grains containing cumulated PSBs and the accumulated volume fraction of PSBs was estimated depending on the number of cycles during fatigue life. The volume fraction of active PSBs during half-cycle deformation is significantly lower than the cumulated PSB volume and decreases with increasing number of cycles. Additionally, an increasing localization of cyclic plastic strain within the PSBs was observed. However, with increasing number of cycles the average local shear strain amplitude remains almost unchanged. Thus, PSBs in polycrystals are subjected to a life history which is characterized by active and inactive periods of their half-cycle slip activity during cyclic deformation at different stages of the saturation state.
    Materials Science and Engineering A 01/2008; 492:118-127. · 2.41 Impact Factor
  • Materials Science Forum - MATER SCI FORUM. 01/2008;
  • Materials Science Forum - MATER SCI FORUM. 01/2008;
  • Key Engineering Materials - KEY ENG MAT. 01/2007;
  • Key Engineering Materials - KEY ENG MAT. 01/2007;