Papers by Author: Ludvík Kunz

Abstract: The use of S355 high strength steel in civil engineering to design bridges, its elements or simple engineering parts allows material and economical savings meeting the strict construction requirements. The knowledge of the fatigue resistance of material plays the key role during design and maintenance of the bridge structures. This contribution brings a comparison of the fatigue crack growth resistance of two standard S355 J0 and S355 J2 steel grades. Differences in chemical composition and the texture of material structure could generally play a role in the fatigue crack growth. This study shows that in the case of studied steels the chemical composition has an influence on material fatigue behaviour, whereas the texture of material structure is irrelevant.

Abstract: Growth of long fatigue cracks in Ti6Al4V alloy manufactured by direct metal laser sintering (DMLS) was investigated. Two DMLS systems, EOSINT M270 and EOSINT M290, with different process parameters were used for production of CT specimens having three different orientations of crack propagation with respect to the DMLS build direction. The as-built specimens were stress relieved at 740 °C. The fatigue crack growth curve and the threshold values of the stress intensity factor for crack propagation were experimentally determined. It has been found that the chosen DMLS processing parameters and the used stress relieving procedure results in material exhibiting isotropic crack growth behavior, i.e. the crack growth was found to be independent of the DMLS build direction. The fatigue crack growth rates and the threshold values for the crack growth were compared with published results characterizing the as-built material and material after different post processing heat treatments.

Abstract: Direct Metal Laser Sintering (DMLS) is a complex process where a part is build-up by localized melting of gas atomized powder layers by a concentrated laser beam followed rapid solidification. The microstructure of DMLS produced material is substantially different from that of conventionally manufactured materials, although the ultimate strength is similar. However, yield strength and elongation and especially fatigue behavior may vary considerably according to the process parameters and post fabrication heat treatment because they affect structural heterogeneity, porosity content, residual stresses, and surface conditions. Fatigue tests of DMLS Ti6Al4V alloy are interpreted in the light of a thorough metallographic and fractographic investigation. The fatigue crack initiation for three different cyclic stress directions with respect to build direction is determined by fractography.

Abstract: 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.

Abstract: 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 μm 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.

Abstract: Microstructure, mechanical properties, cyclic plastic deformation behavior and fatigue strength of ultrafine-grained (UFG) magnesium alloy AZ91 processed by equal channel angular pressing (ECAP) were investigated. ECAP of originally cast alloy results in development of bimodal structure, improved yield stress, tensile strength and ductility when compared to the as-cast state. Endurance limit based on 10⁷ cycles is also improved, however exhibits large scatter. Initiation of fatigue cracks takes place in regions of large grains in the bimodal structure, where the content of Mg₁₇Al₁₂ particles is low.

Abstract: Fatigue strength, crack initiation and microstructure were experimentally investigated in an as-cast AZ91 alloy and in ultrafine-grained (UFG) AZ91 alloy processed by equal channel angular pressing (ECAP). The microstructure after ECAP is bimodal, consisting of fine-grained regions and clusters of larger grains with lower density of intermetallic particles. It has been found that the ECAP substantially increases the tensile strength (factor of two), improves ductility (factor of five) and improves the fatigue strength in low-cycle fatigue region. The improvement of the endurance limit based on 10⁷ cycles is weak. The cyclic slip bands, as sites of the fatigue crack initiation on material surface, were investigated. Focussed ion beam technique (FIB) was applied to reveal the surface relief and the microstructure in the vicinity of early fatigue cracks. No grain coarsening was observed in the close vicinity of the initiated cracks. Fatigue cracks in ultrafine-grained structure develop both in the regions of larger grains and also in the fine grained areas. Two types of crack initiation were observed.

Abstract: The fatigue life, cyclic deformation behavior and microstructure of ultra-fine grained (UFG) copper produced by equal-channel angular pressing (ECAP) were tested and observed in present work. Development of surface relief and fracture surfaces were observed by means of scanning electron microscopy. The results show that the higher tensile strength UFG copper has lower ductility and its strain controlled cyclic properties are worse while comparing with those of lower strength CG counterpart. All the ECAP specimens display obvious cyclic softening behavior under the applied strain range. The cyclic softening ratio increases with increasing strain amplitude. The cyclic hysteresis loops of UFG Cu shows the tension and compression peak stresses decreased gradually with number of cycles. The formation of SBs and grain boundary sliding is the main factor causing crack nucleation as well as premature failure.

Abstract: Fatigue life of a cast Ni-base superalloy IN 713LC under combined cycle loading consisting of a superposition of low- and high-cycle fatigue at 800 °C was experimentally determined. No measurable effect of combined cycle was found for studied loading conditions. High scatter of fatigue life related to the initiation of cracks on casting defects was observed. Size of the largest defect in a specimen was predicted by the largest extreme value distribution method. The predicted size was compared with fractographic observation of defects resulting in final fatigue failure.

Abstract: The lifetime of superalloy single crystals CMSX-4 and CM186LC subjected to tensile mean stress with vibrations at high temperatures has been experimentally studied. Both beneficial and detrimental effect of cyclic stress component has been observed. An increase of lifetime due to superposition of cyclic component on the mean stress is explained by reduction of creep rate due to vibrations. The onset of decrease of lifetime with increasing stress amplitude is observed when the fatigue damage due to initiation and propagation of fatigue crack overbalances the beneficial influence of high frequency cycling.