Papers by Author: Jaroslav Polák

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Authors: Martin Petrenec, Jaroslav Polák, Tomáš Šamořil, Jiří Dluhoš, Karel Obrtlík
Abstract: In-situ Low Cycle Fatigue test (LCF) at temperature 635 °C have been performed in SEM on flat specimen ofcast Inconel 713LC superalloy. The aim of the investigation was to studymechanisms of the fatigue damage during elastic-plastic cycling by theobservations of the characteristic surface relief evolution and theaccompanying internal dislocation structures. The selected locations on thesurface were systematically studied in-situ and documented by SEM and usingAFM. The surface relief in the first tensile half-cycle was formed by numerousslip steps on the primary slip planes (111). In the following compressionhalf-cycle additional opposite slip were formed. The relief was modified in thenext cycles but without forming additionally new slip traces in the primarysystem. The reorientation of two grains in the gauge area was measured usingEBSD. At the end of cyclic loading the relation between surface persistent slipmarkings and persistent slip bands in the interior of the material wasdocumented by TEM on lamella prepared by FIB. The early stages of extrusion andintrusion formation were documented. The damage mechanism evolution is closelyconnected with the cyclic strain localization to the persistent slip bands thatare also places of fatigue crack initiation.
Authors: Jiří Man, Miroslav Valtr, Ivo Kuběna, Martin Petrenec, Karel Obrtlík, Jaroslav Polák
Abstract: Atomic force microscopy (AFM) and focused ion beam technique (FIB) were adopted to study the early stages of surface relief evolution in 316L steel and polycrystalline copper fatigued with constant plastic strain amplitudes at different temperatures (316L steel at 93, 173 and 573 K; copper at 83, 173 and 295 K). Qualitative and quantitative data on the morphology and shape of persistent slip markings (PSMs), occurrence of extrusions and intrusions and the kinetics of extrusion growth are reported. They are discussed in relation with recent physically based theories of surface relief formation leading to fatigue crack initiation.
Authors: Miroslav Šmíd, Martin Petrenec, Jaroslav Polák, Karel Obrtlík, Alice Chlupová
Abstract: Cyclic multiple step test in strain control have been performed on cylindrical specimens of cast polycrystalline Inconel 738LC and 792-5A superalloys at 800 °C in laboratory atmosphere. Hysteresis loops were analyzed according to the statistical theory of hysteresis loop. The effective and internal stress components were evaluated. The effective stress of γ´ precipitate has significant influence on the stress-strain response both materials. The stress amplitude in IN 792-5A is higher than in IN 738LC at approximately same total strain amplitude due to significantly higher effective stress of γ´ phase. Cyclic hardening/softening curves and cyclic stress-strain curves using short-cut procedure were obtained. Cyclic hardening/softening behavior depends both on temperature and strain amplitude. Low amplitude straining is characterized by the saturation of the stress amplitude. In high amplitude straining slight softening was found. The cyclic stress-strain curves for both materials can be fitted by power law. Cyclic stress-strain response in terms of internal and effective stress components is discussed in relation to microstructural parameters of the materials. The observation of surface relief revealed the presence of persistent slip markings.
Authors: Jaroslav Polák, Jiří Man, Karel Obrtlík
Abstract: The possibilities of atomic force microscopy in studying surface features and early fatigue damage in materials are reviewed. Examples of a true relief arising on the surface of cyclically strained materials as recorded by atomic force microscopy are given. Characteristic features of the surface relief are well-defined persistent slip markings consisting of extrusions and intrusions. The shape of extrusions and intrusions can be obtained by observation of metal surface and replicas and in combination with high resolution scanning electron microscopy. The growth of extrusions during fatigue life is reported for austenitic and ferritic stainless steels. The experimental data are compared with models of the localized cyclic plastic straining and predictions of the fatigue crack nucleation models.
Authors: Roman Petráš, Viktor Škorík, Jaroslav Polák
Abstract: Thermomechanical fatigue experiments were performed with austenitic stainless Sanicro 25 steel. Several amplitudes of mechanical strain in a wide temperature interval (250-700 °C) were applied to the specimens. Mechanical response was recorded and fatigue lives were obtained. Scanning electron microscopy combined with FIB technique was used to study the mechanism of crack initiation in in-phase and in out-of-phase thermomechanical cycling. Different mechanisms of the crack initiation were found in these two types of loading. During in-phase loading fatigue cracks start in grain boundaries by cracking of the oxide. Cracks grew preferentially along grain boundaries which resulted in rapid crack initiation and low fatigue life. In out-of-phase loading multiple cracks perpendicular to the stress axis developed only after sufficiently thick oxide layer was formed and cracked in low temperature loading half-cycle. The cracks in oxide allowed localized repeated oxidation and finally also cracking. The cracks grow transgranularly and result in longer fatigue life.
Authors: Jaroslav Polák
Abstract: Recently the decisive role of plastic strain amplitude for the initiation and the growth rate of short cracks has been demonstrated. The plastic strain amplitude can be related to the rate of short crack growth and also to the fatigue life. Since the cyclic stress-strain response of a material determines the plastic strain amplitude it influences basically its fatigue life. The experiments in stress and plastic strain controlled loading and short crack growth are presented and used to demonstrate the importance of the cyclic plastic response for the evaluation of the fatigue life.
Authors: Jaroslav Polák, Martin Petrenec, Jiří Man, Tomáš Kruml
Abstract: Smooth specimens made from austenitic-ferritic duplex steel were subjected to constant stress amplitude loading with positive mean stresses. Hysteresis loops were recorded during the fatigue life and plastic strain amplitude and cyclic creep rate were determined. Fatigue hardening/softening curves, cyclic creep curves and cyclic stress-strain curves for different positive mean stresses were evaluated. Typical dislocation structures developed in both phases of the duplex steel were identified using TEM, compared with the saturated plastic strain amplitude and correlated with the decrease of the cyclic creep rate during cycling and the slope of the cyclic stress-strain curve.
Authors: Tomáš Kruml, Karel Obrtlík, Martin Petrenec, Jaroslav Polák
Abstract: Fatigue properties of the new generation of TiAl alloys with 8at.% of Nb were studied and compared with those reported in the literature for previous generation with 2at.% of Nb. The high Nb content improves substantially the fatigue life time in the Basquin representation. Nevertheless, the fracture behaviour is still rather brittle even at 750°C. TEM study of the material after the fatigue testing was performed. The dislocation activity is concentrated in the  phase. The properties and behaviour of individual dislocations seem to be similar as those observed in single -phase TiAl alloys.
Authors: Jaroslav Polák, Jiří Man
Abstract: Cyclic plastic straining in crystalline materials is localized to persistent slip bands (PSBs) and results in formation of persistent slip markings (PSMs) consisting of extrusions and intrusions. Intensive plastic strain in PSBs results in dislocation interactions and formation of point defects. The extended model based on point defect formation, migration and annihilation is presented describing surface relief formation in the form of extrusion-intrusion pairs. Point defect migration and resulting mass transfer is the principle source of cyclic slip irreversibility leading to crack-like defects - intrusions. Fatigue cracks start in the tip of sharp intrusions.
Authors: Jaroslav Polák, Martin Petrenec, Jiří Man
Abstract: Austenitic stainless steel was cycled at a series of temperatures in the interval from 296 K to 113 K. Constant plastic strain amplitude loading at different levels of plastic strain amplitude and testing similar to multiple step test method were applied at different temperatures. The stress amplitude was continually recorded and selected hysteresis loops were stored and later analyzed using statistical theory of the hysteresis loop. Effective stress component and probability density function as a function of temperature were evaluated. The results were discussed in terms of the temperature dependence of the cyclic yield stress and its sources.
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