Institute of Physics and Materials Science

Vienna, Austria

Institute of Physics and Materials Science

Vienna, Austria

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Fitzka M.,Institute of Physics and Materials Science | Mayer H.,Institute of Physics and Materials Science | Schuller R.,Institute of Physics and Materials Science | Stanzl-Tschegg S.E.,Institute of Physics and Materials Science | And 2 more authors.
Fatigue and Fracture of Engineering Materials and Structures | Year: 2014

Spray-formed hypereutectic aluminium silicon alloy DISPAL® S232-T6x is cycled with variable amplitude at ultrasonic frequency up to the very high cycle fatigue (VHCF) regime under fully reversed tension-compression loading. The Powder Metallurgy alloy is tested using a Gaussian cumulative frequency distribution of load cycles, and lifetimes are compared with constant amplitude data. Miner calculation delivers mean damage sums between 0.5 and 0.9 for mean lifetimes between 8-×-107 and 1.6-×-10 10-cycles, respectively. Cracks are initiated at voids, at inclusions or at distributed inhomogeneities (porous areas or oxides) at the surface or in the interior. In situ analysis of vibration properties indicates that cracks are formed and start growing from the beginning of fatigue cycling, even if failure occurs in the very high cycle fatigue regime. Crack initiation stage is negligible. Lifetime prediction calculation is performed using an adapted Paris-law and considering lifetime as cycles necessary to propagate an initial crack to failure. Measured and predicted mean lifetimes differ by factor 0.4-1.0. Large crack-initiating defects strongly reduce the fatigue lifetimes, which is successfully covered in the crack propagation model. © 2014 Wiley Publishing Ltd.


Lassenberger A.,University of Natural Resources and Life Sciences, Vienna | Bixner O.,University of Natural Resources and Life Sciences, Vienna | Gruenewald T.,Institute of Physics and Materials Science | Lichtenegger H.,Institute of Physics and Materials Science | And 2 more authors.
Langmuir | Year: 2016

Fundamental research on nanoparticle (NP) interactions and development of next-generation biomedical NP applications relies on synthesis of monodisperse, functional, core-shell nanoparticles free of residual dispersants with truly homogeneous and controlled physical properties. Still, synthesis and purification of e.g. such superparamagnetic iron oxide NPs remain a challenge. Comparing the success of different methods is marred by the sensitivity of analysis methods to the purity of the product. We synthesize monodisperse, oleic acid (OA)-capped, Fe3O4 NPs in the superparamagnetic size range (3-10 nm). Ligand exchange of OA for poly(ethylene glycol) (PEG) was performed with the PEG irreversibly grafted to the NP surface by a nitrodopamine (NDA) anchor. Four different methods were investigated to remove excess ligands and residual OA: membrane centrifugation, dialysis, size exclusion chromatography, and precipitation combined with magnetic decantation. Infrared spectroscopy and thermogravimetric analysis were used to determine the purity of samples after each purification step. Importantly, only magnetic decantation yielded pure NPs at high yields with sufficient grafting density for biomedical applications (?1 NDA-PEG(5 kDa)/nm2, irrespective of size). The purified NPs withstand challenging tests such as temperature cycling in serum and long-term storage in biological buffers. Dynamic light scattering, transmission electron microscopy, and small-angle X-ray scattering show stability over at least 4 months also in serum. The successful synthesis and purification route is compatible with any conceivable functionalization for biomedical or biomaterial applications of PEGylated Fe3O4 NPs. © 2016 American Chemical Society.


Schuller R.,Institute of Physics and Materials Science | Fitzka M.,Institute of Physics and Materials Science | Irrasch D.,Institute of Physics and Materials Science | Tran D.,Robert Bosch GmbH | And 2 more authors.
Fatigue and Fracture of Engineering Materials and Structures | Year: 2015

High cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of two 18Ni maraging steels with different cobalt and titanium content and similar static strength are investigated. Ultrasonic fatigue tests are performed with thin sheets with nitrided surfaces at load ratio R = 0.1. The specimens are mounted on a carrier and are forced to joint vibrations at approximately 20 kHz. The increase of Co content and the elimination of Ti improved the HCF and VHCF strength of 18Ni maraging steel. TiN inclusions if Ti is present and Al2O3 inclusions in the Ti free material with sizes (areaINC)1/2 smaller than 10 μm were preferential crack initiation locations. Considering inclusions as initial cracks, the minimum stress intensity range for VHCF failure is 1.2 MPam1/2 for TiN inclusions and 1.8 MPam1/2 for Al2O3 inclusions. Data scatter may be slightly reduced if lifetimes are presented versus stress amplitudes multiplied by (areaINC)1/12 rather than in an S-N diagram. © 2014 Wiley Publishing Ltd.


Mayer H.,Institute of Physics and Materials Science | Schuller R.,Institute of Physics and Materials Science | Fitzka M.,Institute of Physics and Materials Science | Tran D.,Robert Bosch GmbH | Pennings B.,Robert Bosch GmbH
International Journal of Fatigue | Year: 2014

Thin sheets of nitrided 18Ni maraging steel are tested under cyclic tension (load ratio R = 0.1) in the very high cycle fatigue (VHCF) regime. The ultrasonic fatigue testing method with a cycling frequency of about 20 kHz has been further developed for these experiments. Sheet specimens with 0.35 mm thickness are mounted on a carrier specimen, they are pre-stressed and are forced to vibrate jointly. Between 107 and 109 cycles, fatigue cracks are initiated exclusively at internal TiN inclusions. The areas of the crack initiating inclusions projected perpendicular to the applied tensile stress are evaluated. The square root of inclusion areas, (area INC)1/2 lies between 2.5 μm and 5.3 μm. Considering inclusions as cracks, their stress intensity range is between ΔK INC = 1.3 MPa m1/2 and 2.4 MPa m1/2. The sizes of crack initiating inclusions influence fatigue lifetimes. This is considered in a crack propagation model and by presenting lifetimes versus the stress amplitudes multiplied by (areaINC)1/12. A mean lifetime of 109 cycles is found at a stress amplitude of 22% of the tensile strength, which is comparable to other high strength steels tested under cyclic tension. © 2013 Elsevier B.V. All rights reserved.


Mayer H.,Institute of Physics and Materials Science | Schuller R.,Institute of Physics and Materials Science | Karr U.,Institute of Physics and Materials Science | Irrasch D.,Institute of Physics and Materials Science | And 3 more authors.
International Journal of Fatigue | Year: 2014

Cyclic torsion fatigue tests with superimposed static torsion loads are performed with VDSiCr spring steel with shot-peened surface in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. Fatigue properties are investigated at load ratios R = 0.1, R = 0.35 and R = 0.5 up to limiting lifetimes of 5 × 109 cycles with a newly developed ultrasonic torsion testing method. Increasing the load ratio reduces the shear stress amplitude that the material can withstand without failure. Fatigue cracks are initiated at the surface in the HCF regime. In the VHCF regime, cracks are preferentially initiated internally in the matrix, below the surface layer with compression residual stresses, and less frequently at the surface. Cyclic and mean shear stresses with 50% survival probability in the VHCF regime are presented in a Haigh diagram. Linear line approximation delivers a mean stress sensitivity of M = 0.33 for load ratios between R = -1 and R = 0.5. © 2014 Elsevier Ltd. All rights reserved.


Mayer H.,Institute of Physics and Materials Science | Schuller R.,Institute of Physics and Materials Science | Karr U.,Institute of Physics and Materials Science | Fitzka M.,Institute of Physics and Materials Science | And 3 more authors.
International Journal of Fatigue | Year: 2016

The very high cycle fatigue (VHCF) properties of shot-peened VDSiCr spring steel have been investigated with the ultrasonic fatigue testing method. Fatigue behaviour under cyclic torsion and cyclic tension loading at load ratios between R = −1 and R = 0.5 is compared. For 90% of the VHCF failures under axial loading, fractured grain boundaries or inclusions in the interior of the material act as crack starters. In contrast, the initial crack is produced by cyclic shear in the interior or, less frequently, at the surface for more than 90% of the VHCF failures under torsional loading. The crack path deflects from mode II/mode III to mode I at the border of the initiating shear area. The change of crack path correlates to a stress intensity factor range for a crack loaded in shear mode of ΔKτ,ISA = 5.6 ± 0.5 MPam1/2 for load ratio R = 0.1 and ΔKτ,ISA = 3.8 ± 0.4 MPam1/2 for R = 0.35, respectively. Residual stresses due to shot-peening are stable during VHCF axial loading while they are reduced during VHCF torsional loading. © 2016 Elsevier Ltd


Fitzka M.,Institute of Physics and Materials Science | Mayer H.,Institute of Physics and Materials Science
International Journal of Fatigue | Year: 2015

Constant amplitude (CA) and variable amplitude (VA) fatigue lifetimes of the aluminum alloy 2024-T351 were measured with servo-hydraulic (8-70. Hz) and ultrasonic testing equipment (20. kHz) at positive load ratios. Experiments in the high cycle fatigue regime served to identify influences of frequency and testing method on lifetimes. CA tests showed similar numbers of cycles to failure for both methods. Ultrasonic tests were performed in pulsed mode. In ultrasonic VA tests vibration amplitude of successive pulses of 2000 cycles length is varied. Servo-hydraulic VA tests are performed by varying the load of successive blocks. Servo-hydraulic VA tests with block length 2000 cycles delivered lifetimes similar to the ultrasonic tests. No frequency effect is found in CA and VA tests. Cracks are preferentially initiated at secondary phase particles at both frequencies. Lifetimes in servo-hydraulic VA tests are reduced when block length is decreased from 2000 to 200, 20 and single load cycles. Varying the load for each successive cycle at 50. Hz is realized with a feed-forward optimization of control parameters. Lifetimes differ by a factor 6 for different block lengths indicating a strong load sequence effect. © 2015 Elsevier Ltd.


Mayer H.,Institute of Physics and Materials Science | Fitzka M.,Institute of Physics and Materials Science | Schuller R.,Institute of Physics and Materials Science
Ultrasonics | Year: 2013

Ultrasonic fatigue testing equipment is presented that is capable of performing constant amplitude (CA) and variable amplitude (VA) experiments at different constant load ratios. This equipment is used to study cyclic properties of aluminium alloy 2024-T351 in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime at load ratios R = -1 and R = 0.5. CA loading does not reveal a fatigue limit below 1010 cycles. Cracks leading to VHCF failure start at broken constituent particles. Specimens that survived more than 1010 cycles at R = -1 contain non-propagating cracks of lengths below grain size. Resonance frequency and nonlinearity parameter βrel show changes of vibration properties of specimens at low fractions of their VHCF lifetime. VA lifetimes are measured in the HCF and VHCF regime and compared with Miner calculations. Damage sums decrease with decreasing load (and increasing mean lifetimes) and are lower for R = 0.5 than R = -1. © 2013 Elsevier B.V. All rights reserved.


Mayer H.,Institute of Physics and Materials Science | Fitzka M.,Institute of Physics and Materials Science | Schuller R.,Institute of Physics and Materials Science
International Journal of Fatigue | Year: 2014

Ultrasonic fatigue tests with variable amplitude are performed for the first time at positive load ratios. The method utilising ultrasonic equipment, a servo-hydraulic or electromechanical load frame and computer control is described. Aluminium alloy 2024-T351 is tested at approximately 20 kHz with variable amplitude in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime at load ratios R = -1, R = 0.1 and R = 0.5. Constant amplitude data are available and are used for Miner damage accumulation calculations. For load ratio R = -1, the mean damage sum decreases from 0.5 to 0.3 when lifetimes increase from the HCF regime to 109 cycles, and increases at higher numbers of cycles. For load ratio R = 0.1, the mean damage sum decreases from 1.1 to 0.2 when lifetimes increase from HCF to VHCF. At load ratio R = 0.5, the mean damage sum is approximately 0.1 in the VHCF regime. Cracks leading to VHCF failures start at broken constituent particles, probably Al7Cu 2(Fe, Mn), or at agglomerations of fractured particles. Crack initiation occurs at the surface or less frequently in the interior. © 2013 Elsevier Ltd. All rights reserved.


Mayer H.,Institute of Physics and Materials Science | Schuller R.,Institute of Physics and Materials Science | Fitzka M.,Institute of Physics and Materials Science
International Journal of Fatigue | Year: 2013

Fatigue properties of 2024-T351 aluminium alloy are investigated in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. Endurance tests are performed with ultrasonic equipment at 20 kHz cycling frequency at load ratios of R = -1, R = 0.1 and R = 0.5 up to 1010 cycles. Additional servo-hydraulic tests between 8 and 10 Hz at R = 0.1 show no frequency influence on fatigue lifetimes. Linear lines in double logarithmic S-N plots are used to approximate data. Slope exponents of approximation lines increase with increasing numbers of cycles for all load ratios. Failures above 5 × 109 cycles (R = -1 and R = 0.1) or 1010 cycles (R = 0.5) occur, and no fatigue limit is found. Fatigue cracks leading to failures above 109 cycles are initiated at the surface or slightly below at broken constituent particles or at agglomerations of fractured particles, which are probably Al7Cu2(Fe, Mn). Specimens stressed with more than 1010 cycles at R = -1 without failure show several cracks starting at constituent particles. Maximum crack lengths are 30 μm, which is considerably below grain size. © 2012 Elsevier B.V. All rights reserved.

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