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Quantitative assessment of the surface relief in fatigue using AFM
Man, Jiří ; Obrtlík, Karel ; Polák, Jaroslav
Atomic force microscopy (AFM) was applied to study the early stages of fatigue damage in polycrystalline copper and stainless 316L steel. The analysis of the image formation using an AFM tip reveals the lateral distortion of the extrusions and intrusions in case of high extrusions and deep intrusions. The true extrusion height can be obtained in direct observation and the depth of intrusions using replica technique. The evolution of the shape and the height of extrusions allowed to judge the activity of the persistent slip bands (PSBs) during fatigue life.
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Fatigue crack initiation in fcc crystals
Lukáš, Petr ; Kunz, Ludvík
Initiation of fatigue cracks both in model fcc single crystals and in fcc single crystals used in engineering practice is discussed. The emphasis is placed on the role of different types of cyclic slip localisation. It is shown that the phenomenon of persistent slip bands (PSBs) is confined to single crystals of sufficiently high stacking fault energy cycled at a relatively narrow range of loading conditions. For single crystals of not suitable orientations and/or of low stacking fault energy cycled under stresses and/or strains outside the critical range and/or under high temperatures the cyclic plasticity manifests itself by other forms of slip activity leading to the formation of surface hill-valley topography.
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The effect of notches on high temperature creep behaviour of CMSX-4 superalloy single crystals
Preclík, Pavel ; Lukáš, Petr ; Čadek, Josef
This paper deals with the effect of notches on creep strength of the hi-tech high temperature material, CMSX-4 superalloy single crystals. Cylindrical bars of the orientation <001> with circumferential notches were tested at 850 °C under constant loads. Two types of notches with different notch geometry and therefore with different notch acuity and different degree of stress triaxiality were used. The notched specimens exhibit a longer creep lifetime than the smooth specimens for the same net-section stress. This can be attributed to the strain constraint caused by the stress triaxiality. Stress-strain analysis of the notched specimens was performed using the general purpose finite element system ANSYS. The creep data of smooth specimens were used as input data. An excellent correlation between the creep lifetime of the notched specimens and the average value of the calculated steady-state creep strain rate was found.
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Structure and magnetism of iron and iron overlayers from the first principles
Friák, Martin ; Šob, Mojmír ; Vitek, V.
A detailed theoretical study of magnetic behavior of iron along the bcc fcc (Bain's) transformation paths at various atomic volumes is presented. The total energies are calculated by spin polarized full potential LAPW method and are displayed in contour plots as functions of tetragonal distortion c/a and volume; borderlines between various magnetic phases are shown. Stability of tetragonal magnetic phases of fl Fe is discussed. The calculated phase boundaries are used to predict the lattice parameters and magnetic states of iron overlayers on various (001) substrates.
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Micromechanical aspects of transgranular and intergranular failure competition
Dlouhý, Ivo ; Němec, Ondřej
Main purpose of the work was to quantify causes and characteristics governing the intergranular fracture initiation and occurrence of this fracture micromechanism in competition with cleavage one. A CrNi steel of commercial quality and the same steel with increased content of impurity elements have been used for investigation. Step cooling treatment has been applied in order to induce intergranular embrittlement and brittle fracture initiation in both steel melts. Except for standard specimen geometry for three-point bending the pre-cracked Charpy type specimens were applied for determination of fracture mechanical properties. Fractal analysis was applied in order to characterise the quantitative morphological differences in fracture surfaces. Relation of cleavage fracture stress and critical stress for intergranular failure has been followed.
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Microstructure and Anisotropy of Creep in Ti-46Al-2W-0.5Si Castings
Dlouhý, Antonín ; Kuchařová, Květa ; Svoboda, Milan ; Válek, Robert ; Březina, Josef
The present study investigates microstructure and creep behaviour of the cast alloy Ti-46A1-2W-0.5Si (at-%) at temperatures 700, 750 and 800°C and applied stresses between 250-420 MPa. Solidification of the alloy during precision casting is associated with two processes: (i) the growth of the [0001].alpha. oriented columnar grains from the mould wall into the cast interior and (ii) the enrichment of the central part of the cast with aluminium. The first process results in a fully lamellar microstructure where lamellae are preferentially oriented parallel to the cast surface. The second process contributes to the development of the duplex microstructure in the central part of the cast. It is shown that distinct microstructural types in different regions of the cast result in different creep strengths. Specimens taken from inside of the cast and oriented parallel to the cast bar axis exhibited the lowest creep strength while similarly oriented specimens cut from the fully lamellar region near the cast surface were the strongest. The third type of the creep specimens cut out of the central region of the cast and oriented perpendicular to the cast bar axis showed the intermediate creep strength. The observed variations of the creep strength are associated with the preferential orientation of the lamellas and with the fraction of equiaxed .gamma.-grains in the duplex regions of the cast.
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Estimation of the Critical Stress for Failure of a Protective Layer
Náhlík, Luboš ; Knésl, Zdeněk
This contribution describes a study of the behaviour of a crack growing in a protective layer and penetrating through an interface into a substrate. Special attention is devoted to a crack touching the interface. Conditions for the stability of a crack terminating at the interface between two materials are analyzed. The problem is studied under the assumptions of linear-elastic fracture mechanics. A new tentative criterion of stability based on application of the strain energy density concept is formulated and applied to the problem. The suggested criterion is used to estimate the critical applied stress for the failure of a substrate caused by a crack growing through the protective layer. It is shown that the critical stress may strongly depend on the fracture toughness of the substrate and on the bi-material parameters.
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Two-Parameter Approach for Short Cracks at Notches
Seitl, Stanislav ; Hutař, Pavel ; Knésl, Zdeněk
A two parameter constraint-based fracture mechanics method is applied to the behavior of microstructurally and mechanically short fatigue cracks. A short crack in the vicinity of a grain boundary and a crack which is engulfed by the plastic strain field of a notch are studied as examples. The different levels of stress constraint are quantified by using the T-stress and the Q parameter. The corresponding calculations are performed by using a finite element method (ANSYS system). The influence of the T values on the propagation of short fatigue cracks is analyzed. The size of the plastic zone and the crack closure are significantly affected by the level of constraint. The influence of constraint on small fatigue crack growth kinetic is studied under the assumption that the controlling variable for the rate of propagation of the fatigue crack is the size of the plastic zone.
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