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Effect of the load eccentricity on fracture behaviour of cementitious materials subjected to the modified compact tension test
Seitl, Stanislav ; Ríos, J. D. ; Cifuentes, H. ; Veselý, V.
Fracture properties of quasi-brittle cementitious composites are typically determined from the load–displacement response recorded during a fracture test by using the work-of-fracture method or possibly other relevant fracture models. Our contribution is focused on a set of experimental tests which are used to study the fracture behaviour on notched dog-bone-shaped specimens made of cementitious materials. These specimens are subjected to modified compact tension (ModCT) test under a specific range of eccentricity of the tensile load. This type of test generates a stress state in the specimen ligament which combines a direct tension with a defined level of bending due to eccentricity of the tensile load. Several values of relative notch length are also considered. While the crack propagates, a variety of stress states, resulting in variations in the crack-tip stress and deformation constraint, appears in the ligament zone because of the changes in the eccentricity of the applied load, which influences the fracture behaviour of the investigated specimens. The K-calibration, T-stress, CMOD and COD curves for ModCT specimens are introduced and variations of these curves with varying load eccentricity are discussed.
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Prediction of the Traction Separation Law of Ceramics Using Iterative Finite Element Modelling
Kozák, Vladislav ; Chlup, Zdeněk ; Padělek, P. ; Dlouhý, Ivo
Specific silicon nitride ceramics, the influence of the grain size and orientation on the bridging mechanisms was found. In ceramic matrix composites, crack-bridging mechanisms can provide substantial toughness enhancement coupled with the same and/or increased strength. The prediction of the crack propagation through interface elements based on the fracture mechanics approach and cohesive zone model is investigated. From a number of damage concepts the cohesive models seem to be especially attractive for the practical applications. Within the standard finite element package Abaqus a new finite element has been developed; it is written via the UEL (user’s element) procedure. Its shape can be modified according to the experimental data for the set of ceramics and composites. The element seems to be very stable from the numerical point a view. The shape of the traction separation law for four experimental materials is estimated via the iterative procedure based on the FEM modeling and experimentally determined displacement in indentation experiments, J-R curve is predicted and stability of the bridging law is tested.
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High cycle fatigue life of Ti6Al4V alloy produced by direct metal laser sintering
Konečná, R. ; Nicoletto, G. ; Bača, A. ; Kunz, Ludvík
High cycle fatigue life of Ti6Al4V alloy specimens manufactured by Direct Metal Laser Sintering (DMLS) was experimentally determined. The DMLS fabrication process was characterized by a 400 W laser power and 50 micrometer layer melted thickness. Post-fabrication heat treatment consisted in stress relieving for 3 h at 720 °C in vacuum with subsequent cooling in argon atmosphere. Fatigue testing of specimens oriented in three different directions with respect to the material build direction was performed with the aim to examine the influence of the layered microstructure on the fatigue behavior. Results of measurement of surface roughness, metallographic examinations of the layered material and fractographic investigation of the fatigue fracture surfaces were employed in the discussion of fatigue crack initiation in DMLS fabricated Ti6Al4V alloy.
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Short-term stable crack propagation through polyolefin singleand bilayered structures - Influence of welding, composition and direction of crack propagation
Lach, R. ; Krolopp, T. ; Hutař, Pavel ; Nezbedová, E. ; Grellmann, W.
The overall stable crack initiation and propagation behaviour of specimens cut from plastic pipes that were composed of different polyolefin materials were investigated using concepts of elastic–plastic fracture mechanics including the crack propagation kinetics. The effect of specimen shape, orientation, welding, lading rate, composition/microstructure and direction of crack propagation on the crack resistance (R) behaviour of these materials has been thereby assessed.
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Influence of Extension of Load Spectrum on Estimation of Residual\nFatigue Lifetime of Railway Axle
Pokorný, Pavel ; Hutař, Pavel ; Náhlík, Luboš
Railway axles are subjected to cyclic amplitude loading which can lead to fatigue failure.\nFor safe operation of railway axles a damage tolerance approach taking into account a possible\ndefect in railway axle is often required. Because of different operation regimes of trains (fast/slow\nride, ride on straight track, on curved track, over switches etc.) the load amplitude of axle is not\nconstant. The variability of load is defined by a load spectrum, which is determined experimentally\nby measuring of load in service conditions. Even though the load spectrum is measured on several\nhundreds or thousands of operation kilometres, the railway axles are in operation much longer time\n(often tens of years). Therefore, some load amplitudes higher than ones measured in the test can\noccur during a long-term axle service. The contribution presented deals with the effect of extension\nof load spectrum by rare high load amplitudes, which can occur during long-term operation, on\nresidual fatigue lifetime of railway axles.
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Interaction of Creep and High Cycle Fatigue of IN 713LC Superalloy
Horník, V. ; Šmíd, Miroslav ; Hutař, Pavel ; Kunz, Ludvík ; Hrbáček, K.
The study deals with the interaction of creep and high cycle fatigue of cast polycrystalline nickel-based superalloy IN 713LC at high temperatures. Previous works indicated that creep lifetime of superalloy structures was un-affected or even slightly increased in the cases with superimposed vibrations. The reason for this behaviour was not well described up to now. Therefore, set of fatigue tests was conducted at high mean stresses level to observe this phenomenon. The mean stress was kept constant while the stress amplitudes were selected in order to measure wide range of conditions from pure creep to pure fatigue. Fractographic analysis by scanning electron microscopy (SEM) was done with the aim to identify governing damage mechanisms for particular test conditions as a preliminary evaluation of conducted tests.
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Crack initiation in austenitic stainless steel sanicro 25 subjected to thermomechanical fatigue
Petráš, Roman ; Škorík, Viktor ; Polák, Jaroslav
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.
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Effect of alloying and thermal processing on mechanical properties of tial alloys
Chlupová, Alice ; Heczko, Milan ; Obrtlík, Karel ; Beran, Přemysl ; Kruml, Tomáš
Two -based TiAl alloys with 7 at.% of Nb, alloyed with 2 at.% Mo and 0.5 at.% C, were studied. A heat treatment leading to very fine lamellar microstructure was applied on both alloys. Microstructure after the heat treatment was described and mechanical properties including fatigue behaviour were measured. The as-received material alloyed with C possesses high strength and very limited ductility, especially at RT. After application of selected heat treatment it becomes even more brittle; therefore, this process could be considered as not appropriate for this alloy. On the contrary, in the case of Mo alloyed material, both strength and ductility are improved by the heat treatment at RT and usual working temperature (~750 °C). Presence of the phase is responsible for this effect. The selected heat treatment thus can be an alternative for this alloy to other thermomechanical treatments as high temperature forging.
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