Pokročilé kovové materiály a kompozity na bázi kovů

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Advantageous Description of Short Fatigue Crack Growth Rates in Austenitic Stainless Steels with Distinct Properties
(MDPI, 2021-03-01) Trávníček, Lukáš; Kuběna, Ivo; Mazánová, Veronika; Vojtek, Tomáš; Polák, Jaroslav; Hutař, Pavel; Šmíd, Miroslav
In this work two approaches to the description of short fatigue crack growth rate under large-scale yielding condition were comprehensively tested: (i) plastic component of the J-integral and (ii) Polak model of crack propagation. The ability to predict residual fatigue life of bodies with short initial cracks was studied for stainless steels Sanicro 25 and 304L. Despite their coarse microstructure and very different cyclic stress-strain response, the employed continuum mechanics models were found to give satisfactory results. Finite element modeling was used to determine the J-integrals and to simulate the evolution of crack front shapes, which corresponded to the real cracks observed on the fracture surfaces of the specimens. Residual fatigue lives estimated by these models were in good agreement with the number of cycles to failure of individual test specimens strained at various total strain amplitudes. Moreover, the crack growth rates of both investigated materials fell onto the same curve that was previously obtained for other steels with different properties. Such a "master curve" was achieved using the plastic part of J-integral and it has the potential of being an advantageous tool to model the fatigue crack propagation under large-scale yielding regime without a need of any additional experimental data.
Entropy-Driven Grain Boundary Segregation: Prediction of the Phenomenon
(MDPI, 2021-08-01) Lejček, Pavel; Hofmann, Siegfried
The question is formulated as to whether entropy-driven grain boundary segregation can exist. Such a phenomenon would be based on the assumption that a solute can segregate at the grain boundary sites that exhibit positive segregation energy (enthalpy) if the product of segregation entropy and temperature is larger than this energy (enthalpy). The possibility of entropy-driven grain boundary segregation is discussed for several model examples in iron-based systems, which can serve as indirect evidence of the phenomenon. It is shown that entropy-driven grain boundary segregation would be a further step beyond the recently proposed entropy-dominated grain boundary segregation as it represents solute segregation at "anti-segregation" sites.
Effect of Underload Cycles on Oxide-Induced Crack Closure Development in Cr-Mo Low-Alloy Steel
(MDPI, 2021-05-01) Pokorný, Pavel; Vojtek, Tomáš; Jambor, Michal; Náhlík, Luboš; Hutař, Pavel
Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominantly considered as insignificant cycles without any influence on fatigue lifespan of engineering structural components. However, this paper shows that in some cases these underload cycles can retard the consequent crack propagation quite significantly. This phenomenon is a consequence of oxide-induced crack closure development during cyclic loading below the threshold. The experimentally described effect of fatigue crack growth retardation was supported by measurement of the width and the thickness of the oxide debris layer using the EDS technique and localized FIB cuts, respectively. Both the retardation effect and the amount of oxide debris were larger for higher number and larger amplitudes of the applied underload cycles. Crack closure measurement revealed a gradual increase of the closure level during underload cycling. Specimens tested in low air humidity, as well as specimens left with the crack open for the same time as that needed for application of the underload cycles, revealed no retardation effect. The results can improve our understanding of environmental effects on fatigue crack propagation and understanding the differences between the results of laboratory testing and the fatigue lives of components in service.
Fatigue characterization of polyethylene under mixed mode I/III conditions
(Elsevier, 2021-04-01) Gosch, Anja; Arbeiter, Florian; Berer, Michael; Vojtek, Tomáš; Hutař, Pavel; Pinter, Gerald
Mixed mode I/III fatigue fracture behaviour of bulk polymers is scarcely investigated. Thus, the aim of this work is to focus on the comparison of pure mode I and mixed mode I/III fatigue loading of polyethylene. During mixed mode I/III testing, both increases and decreases of lifetime were observed, depending on the levels of applied mode I and mode III loading. Additionally, the critical influence of friction and the accompanying increase of local temperature between the shear loaded crack flanks in mode III, as well as the dominant fracture mechanisms were discussed based on the fracture surface morphology.
Effect of Solution Annealing on Fatigue Crack Propagation in the AISI 304L TRIP Steel
(MDPI, 2021-03-01) Jambor, Michal; Vojtek, Tomáš; Pokorný, Pavel; Šmíd, Miroslav
Fatigue crack propagation in near-threshold regime was studied in the 304L austenitic stainless steel in two microstructural states: as-received (AR) with finer microstructure and low susceptibility to the transformation-induced plasticity (TRIP) effect, and solution-annealed (SA) with coarser microstructure and higher susceptibility to TRIP. At the load ratio R = 0.1 the threshold was higher in the SA state than in the AR state due to coarser grains and possibly the TRIP effect. In order to clarify the role of crack closure, experiments at R = 0.7 were done. The threshold in the SA state was still higher by 1 MPa center dot m(0.5). This effect was identified as crack tip shielding induced by phase transformation, an example of a non-closure shielding effect. Higher resistance to crack growth in the SA state was attributed to promoted martensitic transformation in non-favorable oriented grain families rather than thicker martensite layers in the crack path area. The conclusions were verified by experiments at R = 0.7 and temperature 150 degrees C > M-s which did not reveal any notable difference in thresholds. However, the threshold values were affected by the load-shedding gradient C = -d Delta K/da, which had to be equalized in both experimental setups inside and outside the furnace.

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