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Vliv plasticky indukovaného zavírání trhliny a bi-materiálového rozhraní na prahové hodnoty
Náhlík, Luboš ; Hutař, Pavel
The paper is devoted to fatigue crack propagation in layered materials. The influence of plasticity induced crack closure on threshold values for fatigue crack propagation through interfaces between different materials layers is studied. The main aim was to estimate the influence of the loading ratio R on threshold values for crack propagation through a bi-material interface as a function of the elastic mismatch of both materials. The finite element method (FEM) is used for numerical calculations. Results obtained for different loading ratios, materials, boundary conditions and magnitude of applied loading can be generalized and used for the design of composite bodies with different material layers.
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Comparison of low cycle fatigue characteristics of cast nickel base superalloys Inconel 713 LC a Inconel 792-5A
Petrenec, Martin ; Obrtlík, Karel ; Polák, Jaroslav
Cylindrical specimens of cast polycrystalline nickel base superalloys Inconel 713 LC and Inconel 792-5A were cyclically strained under total strain control at room temperature and at 800 °C. Structural characteristics of both materials are documented. Metallographic observations revealed coarse grains with dendrites, shrinkage pores and carbide inclusions. TEM observation shows differences in the morphology of ordered γ´ precipitates in both materials. Cyclic hardening/softening curves, cyclic stress-strain curves, and fatigue life curves were obtained for both temperatures. The cyclic hardening/softening curves depend both on temperature and plastic strain amplitude. The cyclic stress-strain curves can be approximated by power law. Experimental points of fatigue life curves satisfy the Manson-Coffin and Basquin law. Stress-strain response and fatigue life characteristics are compared at both temperatures and discussed in relation to structural parameters of materials studied.
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Fatigue behaviour of nickel base superalloy Inconel 713LC at high temperature
Juliš, M. ; Obrtlík, Karel ; Podrábský, T. ; Petrenec, Martin
Total strain controlled tests have been performed on cylindrical specimens of Inconel 713 LC polycrystals at 800 °C. Cyclic hardening-softening, cyclic stress-strain and fatigue life curves were obtained. Fracture surface was studied using scanning electron microscopy. High amplitude cycling results in early pronounced hardening followed by softening. Low amplitude cycling is characterized by the stable stress response. Manson-Coffin and Basquin laws can reasonably approximate the fatigue life data. Fatigue crack initiation at casting defects is documented.
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Material analysis of magnesium alloy AZ 91 with various amount of lithium
Man, O. ; Petrenec, Martin ; Ptáček, L.
he contribution deals with magnesium alloy of commercial grade AZ 91 with various amount of lithium addition (5.5, 8 and 12 wt.%). The alloy was observed in the as-cast state and after annealing. Various types of analysis were done, such as metallographic one using light microscopy, scanning electron microscopy and transmision electron microscopy, in order to gain knowledge about the phase composition of the alloy and its development. Several phases and morphological features were identified, but the rest was not, due to the properties of lithium-rich areas.
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Použití lokálního přístupu při řešení transferability lomově mechanických parametrů
Dlouhý, Ivo ; Hadraba, Hynek ; Jurášek, L.
In some applications the pre-cracked Charpy specimens (PC) can be, after meeting the validity conditions applied for specification of fracture toughness characteristics corresponding to standard specimens. Typically this is the case of pearlitic steel R7T (used for railway wheelset production) with Lüders’ flow and higher strength properties. It has been found that there is a gap in fracture toughness data of PC specimens between the 60 and 90 MPam1/2 due to plastic hinge in net section of PC specimens. The distribution of maximum principal stress ahead of the crack tip in PC specimen loaded into fully plastic regime was found to be identical with that of reference stress-strain field in plane-strain SSY condition. 3D FEM simulations also shown that there is no constraint loss in PC specimens.
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