Dokument-ID Dokumenttyp Autoren/innen Persönliche Herausgeber/innen Haupttitel Abstract Auflage Verlagsort Verlag Herausgeber (Institution) Erscheinungsjahr Titel des übergeordneten Werkes Jahrgang/Band ISBN Veranstaltung Veranstaltungsort Beginndatum der Veranstaltung Enddatum der Veranstaltung Ausgabe/Heft Erste Seite Letzte Seite URN DOI Lizenz Datum der Freischaltung OPUS4-24359 Zeitschriftenartikel Altenkirch, J.; Gibmeier, J.; Kromm, Arne; Kannengießer, Thomas; Nitschke-Pagel, T.; Hofmann, M. In situ study of structural integrity of low transformation temperature (LTT)-welds We discuss the stability of weld residual strain under static and quasi cyclic transverse tensile loading in the elastic and elastic-plastic region. The test welds were joined with low transformation temperature weld filler materials with 10 wt% Cr and varying Ni-content from 8 to 12 wt%. Using neutron diffraction the residual lattice strain in the martensitic α'- and austenitic γ-phase in the fusion zone as well as the ferritic α-phase in the heat affected zone and base metal as induced by welding, superimposed by stepwise tensile loading and after unloading was measured. The amount of retained austenite in the fusion zone increases with increasing Ni-content, but it decreases with increasing load level due to stress induced martensite formation. In the as-welded condition the transverse macroscopic residual lattice strain was found to be in low compression in the fusion zone in each weld, while the heat affected zone was in tension. Local plastic deformation of the γ-phase as a result of yielding during tensile loading in combination with the change in phase fraction resulted in increased macroscopic compression in the fusion zone. The reduced yield strength in the heat affected zone resulted in plastic deformation and a considerable shift into compression. Comparison with the cross weld distribution of the hardness and FWHM of the neutron diffraction interference lines supported the assumption of plastic deformation of the γ- and α-phase in the fusion and heat affected zone, respectively, while the α'-phase in the fusion zone was stressed within the elastic regime only. Microstructural observations as well as measurement of the local γ-phase fraction by means of laboratory X-ray diffraction in the fusion zone strengthen these observations. Amsterdam Elsevier 2011 Materials science and engineering A 528 16-17 5566 5575 10.1016/j.msea.2011.03.091 2016-02-19 OPUS4-56317 Zeitschriftenartikel Lay, Vera; Effner, Ute; Niederleithinger, Ernst; Arendt, J.; Hofmann, M.; Kudla, W. Ultrasonic quality assurance at magnesia shotcrete sealing structures Engineered barriers are a key element to enable safe nuclear waste disposal. One method currently under research for their construction is magnesia concrete applied in a shotcrete procedure. In this study, the ultrasonic echo method is evaluated as a means for quality assurance. Imaging of internal structures (backwall, boreholes) and defects, such as delamination, has successfully been achieved in the shotcrete. Additionally, detailed information about the potential cause of selected reflectors are obtained by phase analysis. In several test blocks of various sizes, no consistent concrete section boundaries have been found by ultrasonic imaging, which was verified by subsequent drilling and complementary tests. An experiment with artificial defects imitating cracks, air-filled voids, and material with lower density has been challenging and shows the limitations of the current methods. Although significant defects, such as a large delamination, are reliably identified, several smaller defects are not identified. Generally, ultrasonic imaging provides a suitable base as a mean for quality assurance during and after the construction of sealing structures. However, further developments are required to enhance the reliability of the method and a full validation is still pending. Still, the method has potential to increase the safety of nuclear waste repositories. Basel MDPI 2022 Sensors 22 22 1 16 urn:nbn:de:kobv:b43-563170 10.3390/s22228717 https://creativecommons.org/licenses/by/4.0/deed.de 2022-11-21 OPUS4-52636 Zeitschriftenartikel Serrano Munoz, Itziar; Ulbricht, Alexander; Fritsch, Tobias; Mishurova, Tatiana; Kromm, Arne; Hofmann, M.; Wimpory, R. C.; Evans, Alexander; Bruno, Giovanni Scanning Manufacturing Parameters Determining the Residual Stress State in LPBF IN718 Small Parts The influence of scan strategy on the residual stress (RS) state of an as-built IN718 alloy produced by means of laser powder bed fusion (LPBF) is investigated. Two scan vector rotations (90°-alternation and 67°-rotation), each produced following two different scan vector lengths (long and short), are used to manufacture four rectangular prisms. Neutron diffraction (ND) and laboratory X-ray diffraction (XRD) techniques are used to map the bulk and surface RS state, respectively. The distortion induced upon removal from the baseplate is measured via profilometry. XRD measurements show that the two long scan vector strategies lead to higher RS when compared with the equivalent short scan vector strategies. Also, the 67°-rotation strategies generate lower RS than their 90°-alternation counterparts. Due to the lack of reliable stress-free d0 references, the ND results are analyzed using von Mises stress. In general, ND results show significant RS spatial non-uniformity. A comparison between ND and distortion results indicates that the RS component parallel to the building direction (Z-axis) has a predominant role in the Z-displacement. The use of a stress balance scheme allows to discuss the d0 variability along the length of the specimens, as well as examine the absolute RS state. Wiley 2021 Advanced engineering materials 23 7 158 urn:nbn:de:kobv:b43-526360 10.1002/adem.202100158 https://creativecommons.org/licenses/by/4.0/deed.de 2021-05-12 OPUS4-51290 Zeitschriftenartikel Ulbricht, Alexander; Altenburg, Simon; Sprengel, Maximilian; Sommer, Konstantin; Mohr, Gunther; Fritsch, Tobias; Mishurova, Tatiana; Serrano Munoz, Itziar; Evans, Alexander; Hofmann, M.; Bruno, Giovanni Separation of the Formation Mechanisms of Residual Stresses in LPBF 316L Rapid cooling rates and steep temperature gradients are characteristic of additively manufactured parts and important factors for the residual stress formation. This study examined the influence of heat accumulation on the distribution of residual stress in two prisms produced by Laser Powder Bed Fusion (LPBF) of austenitic stainless steel 316L. The layers of the prisms were exposed using two different border fill scan strategies: one scanned from the centre to the perimeter and the other from the perimeter to the centre. The goal was to reveal the effect of different heat inputs on samples featuring the same solidification shrinkage. Residual stress was characterised in one plane perpendicular to the building direction at the mid height using Neutron and Lab X-ray diffraction. Thermography data obtained during the build process were analysed in order to correlate the cooling rates and apparent surface temperatures with the residual stress results. Optical microscopy and micro computed tomography were used to correlate defect populations with the residual stress distribution. The two scanning strategies led to residual stress distributions that were typical for additively manufactured components: compressive stresses in the bulk and tensile stresses at the surface. However, due to the different heat accumulation, the maximum residual stress levels differed. We concluded that solidification shrinkage plays a major role in determining the shape of the residual stress distribution, while the temperature gradient mechanism appears to determine the magnitude of peak residual stresses. Basel MDPI 2020 Metals 10 9 urn:nbn:de:kobv:b43-512903 10.3390/met10091234 https://creativecommons.org/licenses/by/4.0/deed.de 2020-09-23 OPUS4-50647 Zeitschriftenartikel Evsevleev, Sergei; Sevostianov, I.; Mishurova, Tatiana; Hofmann, M.; Garcés, G.; Bruno, Giovanni Explaining Deviatoric Residual Stresses in Aluminum Matrix Composites with Complex Microstructure The residual stresses in multiphase metal Matrix composites with both random planar-oriented short fibers and particles were studied by neutron diffraction and by a model based on the reformulation of classic Maxwell's homogenization method. Contrary to common understanding and state-of-the-art models, we experimentally observed that randomly oriented phases possess non-hydrostatic residual stress. The recently developed modeling Approach allows calculating the residual stress in all phases of the composites. It rationalizes the presence of deviatoric stresses Accounting for the interaction of random oriented phases with fibers having preferential orientation. Springer 2020 Metallurgical and Materials Transactions A 51 6 3104 3113 urn:nbn:de:kobv:b43-506472 10.1007/s11661-020-05697-1 https://creativecommons.org/licenses/by/4.0/deed.de 2020-04-09 OPUS4-49460 Zeitschriftenartikel Evsevleev, Sergei; Cabeza, S.; Mishurova, Tatiana; Garcés, G.; Sevostianov, I.; Requena, G.; Boin, M.; Hofmann, M.; Bruno, Giovanni Stress-induced damage evolution in cast AlSi12CuMgNi alloy with one and two ceramic reinforcements. Part II: Effect of reinforcement orientation While there is a large body of literature on the micro-mechanical behavior of metal matrix composites (MMCs) under uniaxial applied stress, very little is available on multi-phase MMCs. In order to cast light on the reinforcement mechanisms and damage processes in such multi-phase composites, materials made by an Al-based piston alloy and containing one and two ceramic reinforcements (planar-random oriented alumina fibers and SiC particles) were studied. In-situ compression tests during neutron diffraction experiments were used to track the load transfer among phases, while X-ray computed tomography on pre-strained samples was used to monitor and quantify damage. We found that damage progresses differently in composites with different orientations of the fiber mat. Because of the presence of intermetallic network, it was observed that the second ceramic reinforcement changed the load transfer scenario only at very high applied load, when also intermetallic particles break. We rationalized the present results combining them with previous investigations and using a micromechanical model. Springer 2020 Journal of Materials Science 55 3 1049 1068 10.1007/s10853-019-04069-4 2019-10-31 OPUS4-52066 Zeitschriftenartikel Fritsch, Tobias; Sprengel, Maximilian; Evans, Alexander; Farahbod-Sternahl, L.; Saliwan Neumann, Romeo; Hofmann, M.; Bruno, Giovanni On the determination of residual stresses in additively manufactured lattice structures The determination of residual stresses becomes more complicated with increasing complexity of the structures investigated. Additive manufacturing techniques generally allow the production of 'lattice structures' without any additional manufacturing step. These lattice structures consist of thin struts and are thus susceptible to internal stress-induced distortion and even cracks. In most cases, internal stresses remain locked in the structures as residual stress. The determination of the residual stress in lattice structures through nondestructive neutron diffraction is described in this work. It is shown how two difficulties can be overcome: (a) the correct alignment of the lattice structures within the neutron beam and (b) the correct determination of the residual stress field in a representative part of the structure. The magnitude and the direction of residual stress are discussed. The residual stress in the strut was found to be uniaxial and to follow the orientation of the strut, while the residual stress in the knots was more hydrostatic. Additionally, it is shown that strain measurements in at least seven independent directions are necessary for the estimation of the principal stress directions. The measurement directions should be chosen according to the sample geometry and an informed choice on the possible strain field. If the most prominent direction is not measured, the error in the calculated stress magnitude increases considerably. 2021 Journal of Applied Crystallography 54 228 236 urn:nbn:de:kobv:b43-520663 10.1107/S1600576720015344 https://creativecommons.org/licenses/by/4.0/deed.de 2021-02-03 OPUS4-41008 Zeitschriftenartikel Przondziono, R.; Timothy, J. J.; Weise, Frank; Krütt, Enno; Breitenbücher, R.; Meschke, G.; Hofmann, M. Degradation in concrete structures due to cyclic loading and its effect on transport processes - Experiments and modeling According to the objectives of the research group 1498, this paper deals with degradation effects in concrete structures that are caused by cyclic flexural loading. The goal is to determine their influence on the fluid transport processes within the material on the basis of experimental results and numerical simulations. The overall question was, to which extent the ingress of externally supplied alkalis and subsequently an alkali-silica reaction are affected by such modifications in the microstructure. Degradation in the concrete microstructure is characterized by ultrasonic wave measurements as well as by microscopic crack analysis. Furthermore, experiments on the penetration behavior of water into the investigated materials were performed. The penetration behavior into predamaged concrete microstructures was examined by the classical Karsten tube experiment, nuclear magnetic resonance method, and time domain reflectometry techniques. In order to create an appropriate model of the material's degradation on the water transport, the Darcy law was applied to describe the flow in partially saturated concrete. Material degradation is taken into account by an effective permeability that is dependent on the state of degradation. This effective permeability is obtained by the micromechanical homogenisation of the flow in an Representative Elementary Volume (REV) with distributed ellipsoidal microcracks embedded in a porous medium. The data gained in the microscopic crack analysis is used as input for the micromechanical model. Finite element simulations for unsaturated flow using the micromechanical model were compared with the experimental results showing good qualitative and quantitative agreement. Ernst & Sohn 2017 Structural Concrete 18 4 519 527 10.1002/suco.201600180 2017-07-24 OPUS4-58477 Zeitschriftenartikel Schröder, Jakob; Heldmann, A.; Hofmann, M.; Evans, Alexander; Petry, W.; Bruno, Giovanni Determination of diffraction and single-crystal elastic constants of laser powder bed fused Inconel 718 High energy X-ray synchrotron diffraction is used to investigate the elastic anisotropy of the nickel-based superalloy IN718 produced by laser powder bed fusion (PBF-LB). This material is characterized by a columnar grain morphology with some crystallographic texture. The material is subjected to elastic loading to determine the diffraction elastic constants (DECs). Furthermore, the single-crystal elastic constants (SCEC) are refined from these experiments using different micromechanical models. The results show that each micromechanical model predicts a specific set of SCEC that well describes the elastic anisotropy of PBF-LB/IN718. Elsevier B.V. 2023 Materials Letters 353 1 5 10.1016/j.matlet.2023.135305 2023-10-05 OPUS4-58496 Corrigendum Lay, Vera; Effner, Ute; Niederleithinger, Ernst; Arendt, J.; Hofmann, M.; Kudla, W. Correction: Lay et al. Ultrasonic quality assurance at magnesia shotcrete sealing structures. Sensors 2022, 22, 8717 The authors wish to correct the following errors in the original paper Lay, V.; Effner, U.; Niederleithinger, E.; Arendt, J.; Hofmann, M.; Kudla, W. Ultrasonic Quality Assurance at Magnesia Shotcrete Sealing Structures. Sensors 2022, 22, 8717, https://doi.org/10.3390/s22228717. Basel MDPI 2023 Sensors 23 18 1 3 urn:nbn:de:kobv:b43-584969 10.3390/s23187966 https://creativecommons.org/licenses/by/4.0/deed.de 2023-09-19