Papers by Author: Pavel Šandera

Abstract: The absorbed impact energy KV and the dynamic fracture toughness K_Id of the low-alloyed steel OCHN3MFA were measured in the respective temperature ranges 〈–40, 90〉 and 〈–60, –20〉 °C. The values of impact energy in the range of 〈14.1, 21.3〉 J were obtained using Charpy V-notch samples after subtracting parasitic energies (friction in bearings, etc.). All these values corresponded to an extended transition region since the temperature –40 °C was still higher than the lower shelf impact-energy (shear lips present) and the temperature 90 °C was lower than the upper shelf (only 20 % of ductile morphology). Dynamic fracture toughness was determined using Charpy V‑notch samples with fatigue pre-cracks. The related K_Id-values were obtained in the range 〈55.8, 77.5〉 MPa.m^1/2. They were found to be valid linear-elastic fracture toughness and were somewhat higher than the previously reported static K_Ic-values. The study revealed that OCHN3MFA steel has sufficient resistance to dynamic fracture, particularly at low temperatures.

Abstract: The effect of surface roughness of virgin specimens (prior roughness) made of low-alloyed high strength steel on their fatigue life after the case-hardening treatment was studied by rotating bending tests of virgin and nitrided samples. In a whole range of S-N curves, the fatigue strength of virgin samples after lathe-turning machining (high roughness) was naturally much higher than that of samples after grinding (low roughness). As expected, the fatigue strength of nitrided specimens was higher than that of virgin samples when averaged through the whole fatigue life range. When distinguishing the rough and smooth nitrided samples, the low-cycle fatigue strength of rough samples revealed to be lower than that of the smooth ones and vice a versa in the high-cycle region. This could be explained by the fact that, in the low cycle region, the cracks in the rough samples nucleated from deep surface defects while, in the smooth specimens, they nucleated from shallow defects (or as fish eyes) which prolonged their initiation stage. Almost all cracks in the high cycle region initiated as fish-eyes but the extent of nitrided layers in the rough specimens was slightly higher than that in the smooth specimens. Therefore, the fish-eye centers were shifted further to the interior of the rough specimens which increased their fatigue life.

Abstract: The critical length a_Ic of a mode I crack that corresponds to a vanishing of the influence of the notch stress concentration can be approximately expressed by a formula reported by Lefort. This study aimed to generalize his approach to shear mode cracks by finding a criterion for a statistical compatibility of formulae for critical lengths of cracks. It revealed that the Lefort ́s formula describes the critical crack length a_Ic at which the relative level of the notch effect on the mode I SIF is less than 1%. Based on this criterion, a mathematically similar formula for the critical length a_IIIc was found. A part of this study was also a development of a transformation procedure suitable for fitting the obtained SIF data by simple analytical formulae and for clear related illustrative plots of results.

Abstract: The influence of the notch geometry on the stress intensity factor at the front of the emanating cracks is well known for the opening loading mode. The critical length of the crack corresponding to a vanishing of the influence of the notch stress concentration can be approximately expressed by the formula a_I,c = 0.5ρ(d/ρ)^1/3, where d and ρ are the depth and radius of the notch, respectively. The aim of the paper was to find out if this formula could be, at least nearly, applicable also to the case of shear mode loading. The related numerical calculations for mode II and III loading were performed using the ANSYS code for various combinations of notch depths and crack lengths in a cylindrical specimen with a circumferential U-notch. The results revealed that, for mode II loading, the critical length was much higher than that predicted by the formula for mode I loading. On the other hand, the critical lengths for mode I and mode III were found to be nearly equal.

Abstract: The paper is related to experiments on near-threshold fatigue cracks under shear modes II, III and II+III in bcc metals. Cylindrical bars with circumferential cracked notch were loaded by shear force. In-plane precracks with microtortuous geometry were created by compressive cyclic loading in mode I to measure the effective values of the remote crack driving force. Fatigue cracks in bcc metals loaded under remote shear modes II, III and II+III always grew by creation of local tongues loaded in mode II and their coalescence. Therefore, serrated precrack fronts of a linear roughness identical to those of the real fronts were modeled and the related local stress intensity factors k₂ were calculated. Since such FEM calculation for various values of roughness were time consuming, a further task was to identify a lowest number of isolated teeth that produces k₂ components identical with those created by the continuously serrated crack front. The results reported in this article reveal that this condition is fulfilled by only two isolated teeth.

Abstract: In spite of the fact that the US Food and Drug Administration (FDA) has approved Intracoil and s.m.a.r.t Nitinol stents for superficial femoral artery (SFA), some alternative designs of Nitinol stents are being implanted today, representing the off-label use of the devices. Among the currently stents used for the SFA, s.m.a.r.t and Intracoil stents show the most desirable long-term results but it is not understood why. In the present work, delivery of the s.m.a.r.t and Intracoil Nitinol stents and their release inside a stenotic point of the artery was simulated by FE implemented SMA model using a creative manual controlled method. The influence of the stent design on the stent–vessel interactions and stress state within the stent material after completion of the deployment was revealed. It was found that the Intracoil stent shall be more successful in eliminating the stenosis and less prone to fatigue failure, even though it had less thickness and less mean coil diameter.

Abstract: This paper deals with a testings of the Selective Laser Melting (SLM) process parameters on mechanical properties, material behavior, surface quality and on machinability of austenitic steel 316L. The SLM process conditions were changed to assess their effect on mechanical properties of the sintered material measured in tensile testing. The laser power, the scanning velocity of laser beam, the layer thickness and the laser focus plane were set to find the most appropriate set of process parameters to obtain the mechanical properties of parts.

Abstract: Most fracture mechanical models do not take the materials microstructure into account and use averaged material properties although the real crack flanks and fronts exhibit microstructurally induced tortuous shapes. These influences the stress-strain field at the crack front as well as the related stress intensity factors. This phenomenon was investigated by some authors only for remote mode I loading and was named geometrically induced shielding. This study is focused on the analysis of the stress intensity factors for rough cracks loaded in the shear mode II by modelling the Compact-Tension-Shear (CTS) specimen containing cracks of various roughness. The study revealed that, in accordance with our previous study, already small changes in the crack roughness have a rather high influence on the local field of stress intensity factors. The local stress intensity factor K_II decreases with increasing roughness and, for its higher levels, it converges to a constant value that is significantly lower than that of the remote mode II loading associated with the planar crack of the same length.

Abstract: The aim of the contribution is to assess the influence of the microstructure of materials on the effective values of stress intensity factors. In this paper some results of 3D-finite element analyses of a CTS-specimen with a tortuous crack are presented. The specimen is subject to an in-plane shear remote loading (mode II) and tortuous crack flanks simulate rough cracks in polycrystalline materials. Finite element calculation by using the commercially available FE-code ANSYS has been carried out to determine stress/strain distribution in the vicinity of crack front and the local values of stress intensity factors are evaluated along the crack front. The existence of friction forces generated by sliding of crack wake asperities is included into calculations. Respective effective values are determined in dependence of the roughness of crack flanks. Results achieved allows to characterize the influence of microstructure to crack growth.

Abstract: Determination of fatigue crack growth characteristics under shear-mode loading is a rather complicated problem. To increase an efficiency and precision of such testing, special specimens enabling simultaneous propagation of shear cracks under II, III and II+III loading modes started to be used rather recently. However, a description of crack growth rate in terms of appropriate fracture mechanics quantities demands a precise assessment of plastic zone size under various shear-mode loading levels. This contribution is focused on the numerical elasto-plastic analysis of stress-strain field at the crack tip in specimens made of a pure polycrystalline (ARMCO) iron loaded by mixed mode II+III. The dependence of plastic zone size on the J-integral value described the wide region of loading. The results reveal that formixed mode II+III the small scale yielding conditions are fulfilled in the region where plastic zone size is smaller than 1/10 of the total crack length.