[Show abstract][Hide abstract] ABSTRACT: Early fatigue damage has been studied in a number of model and structural polycrystalline materials using modern experimental techniques. Early initiation of fatigue cracks was found to be related to the localization of the cyclic plastic strain into persistent slip bands during cyclic loading. Internal dislocation structure of the persistent slip bands and also the evolution of the surface relief as surface persistent slip markings are documented. The model of the surface relief formation describing the formation of extrusions and intrusions is briefly described. The intrusions representing sharp surface crack-like defects play the principal role in the initiation of fatigue cracks.
[Show abstract][Hide abstract] ABSTRACT: Early stages of surface relief evolution of persistent slip markings (PSMs) in polycrystalline 316L austenitic stainless steel cycled with constant plastic strain amplitude at 93, 173 and 573 K were studied using atomic force microscopy (AFM) and high-resolution scanning electron microscopy (SEM-FEG). Qualitative and quantitative data on the morphology of PSMs, occurrence of extrusions and intrusions and the kinetics of extrusion growth are reported for all temperatures. PSMs start in all cases as surface extrusions which are later accompanied by formation of intrusions. This finding is discussed with respect to the point defect formation within areas of localized cyclic slip and primarily to their mobility at different temperatures. Consequences of migration of respective point defects for surface relief formation and the conditions for creation of fatigue crack embryos, i.e. sharp intrusions are highlighted.
International Journal of Fatigue 07/2015; 76. DOI:10.1016/j.ijfatigue.2014.09.019 · 1.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The application of electron channeling contrast imaging (ECCI) technique to the study of polycrystalline austenitic steel is presented. Specifics of the ECCI technique for near surface defects observation are explained. Practical examples of the use of ECCI for study of dislocation structure of fatigue test samples are shown. Usage of a "rocking beam" technique for electron channeling pattern (ECP) and selected area channeling pattern (SACP) acquisition on polycrystalline material is described. Advantage of combination of SACP and ECCI technique is shown.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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. DOI:10.1016/j.ijfatigue.2013.10.016 · 1.69 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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. DOI:10.1016/j.msea.2013.12.005 · 2.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The mechanisms of the fatigue damage in nickel superalloys and TiAl intermetallics used for the design of components working at elevated temperatures are studied. The characteristic surface relief and the accompanying internal dislocation structures of these materials subjected to strain controlled cycling are documented using high resolution SEM, AFM and TEM. Typical structures corresponding to cyclic strain localization are shown in each class of materials and they are discussed in relation to the characteristic internal structure of a particular material and the types of dislocations participating in cyclic plastic deformation. The relation between surface persistent slip markings (extrusions and intrusions) and persistent slip bands in the interior of the material is presented. The mechanisms leading to fatigue crack initiation and early crystallographic crack growth are discussed.
[Show abstract][Hide abstract] ABSTRACT: Persistent slip markings (PSMs) were experimentally studied in 316L steel fatigued to early stages of the fatigue life. High resolution SEM, combined with focused ion beam (FIB) technique and atomic force microscopy (AFM) were used to assess the true shape of PSMs in their early stage of development. General features of PSMs in fatigued metals are extrusions and intrusions. Their characteristic features were determined. They were discussed in relation with the theories of surface relief formation and fatigue crack initiation based on the formation, migration and annihilation of point defects in the bands of intensive cyclic slip - persistent slip bands (PSBs)
[Show abstract][Hide abstract] ABSTRACT: Polycrystalline austenitic 316L steel specimens were polished and cyclically deformed with constant strain amplitude into the early stage of fatigue life (N/Nf=0.006). Surface relief consisting of persistent slip markings (PSMs) has been studied using high resolution SEM and FIB cuts. Mostly extrusions were produced in this early stage of fatigue damage. Only in a few well developed PSMs the intrusions were detected lying parallel to an extrusion. The findings were discussed in relation to fatigue crack initiation models and it was concluded that fatigue cracks start at the tip of crack-like intrusions in the central part of the PSMs where corresponding persistent slip band (PSB) depth is most pronounced.
Materials Science and Engineering A 03/2013; 564:8–12. DOI:10.1016/j.msea.2012.11.086 · 2.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Early stages of damage evolution in cyclic loading are described and discussed. The importance of the role of cyclic plastic strain in damage evolution is emphasized and the relation between stress and strain in cyclic straining is clarified. The principal stages of damage evolution in fatigued crystalline structural material are identified. The basic characteristic and theories of fatigue crack initiation are sketched and confronted with experimental observations. Early fatigue crack growth is characterized and quantitatively described. The relation between the growth of short cracks and fatigue life in the form of Manson-Coffin law is established.
[Show abstract][Hide abstract] 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 06/2012; 39. DOI:10.1016/j.ijfatigue.2011.02.012 · 1.69 Impact Factor
[Show abstract][Hide abstract] 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. DOI:10.1016/j.ijfatigue.2011.05.002 · 1.69 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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. DOI:10.1088/1742-6596/240/1/012058
[Show abstract][Hide abstract] 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 04/2010; 240(1). DOI:10.1088/1742-6596/240/1/012054
[Show abstract][Hide abstract] 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.