Andreas J. Brunner

Empa - Swiss Federal Laboratories for Materials Science and Technology, Duebendorf, Zurich, Switzerland

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Publications (58)67.28 Total impact

  • Iurii Burda · Andreas J. Brunner · Michel Barbezat
    Engineering Fracture Mechanics 08/2015; DOI:10.1016/j.engfracmech.2015.08.009 · 1.66 Impact Factor
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    Dietmar Haba · Thomas Griesser · Ulrich Müller · Andreas J. Brunner
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    ABSTRACT: Inorganic fullerene-like tungsten disulfide (IF-WS2) nanoparticles are useful additives for polymers and lubricating agents, in particular when their surfaces are functionalized by silane modifiers. However, both the success of such a silanization reaction and its effect on the final dispersion quality are still doubtful. In this work, IF-WS2 are functionalized using three different silane modifiers and investigated with X-ray photoelectron spectrometry, infrared spectroscopy, titration, thermogravimetric analysis, and mass spectroscopy. Eventually, they are dispersed within ethanol by sonication to compare the dispersing behavior. The combination of the different analytical techniques revealed that the IF-WS2 surfaces can be functionalized with two of the used silane modifiers, while the third one was repeatedly unsuccessful. The amount of Si on the particles seems to be a fairly clear indication for the success of the functionalization reaction. The IF-WS2 seems to oxidize during the functionalization process, probably producing acidic SO2 or SO3, which can fully acidify a basic surface modifier. The executed treatment without any added silane modifier improved the dispersibility of the IF-WS2 within ethanol to some extent, but added modifiers deteriorated it significantly. TEM images indicate that IF-WS2 particles form aggregates, which might be the reason for the limited dispersibility.
    Journal of Materials Science 05/2015; 50(15). DOI:10.1007/s10853-015-9039-4 · 2.37 Impact Factor
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    Rhys Jones · A. Mutjaba · A. J. Kinloch · A. J. Brunner · S. Stelzer
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    ABSTRACT: Whilst composite materials are now widely used in aircraft structural components, for certification purposes current designs are such that any delamination will not grow. Despite this limit, fleet data and data obtained from full scale fatigue tests reveal that small sub mm initial delaminations can grow when subjected to operational flight loads. A possible alternative approach to certification was introduced by the FAA in 2011. This allows for a slow growth approach to the certification of composites and bonded structures. In this context recent studies have revealed that delamination growth in composites and disbond growth in adhesive bonds can be computed using a version of the NASGRO crack growth equation. This paper reveals the potential for this approach to capture the large scatter seen in fatigue tests associated with both delamination/disbond growth in both composite structures and adhesively bonded joints.
    16th Australian International Aerospace Conference, Melbourne; 02/2015
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    ABSTRACT: Whilst composite materials are now widely used in aircraft structural components, for certification purposes current designs are such that any delamination will not grow. Despite this limit, fleet data and data obtained from full scale fatigue tests reveal that small sub mm initial delaminations can grow when subjected to operational flight loads. A possible alternative approach to certification was introduced by the FAA in 2011. This allows for a slow growth approach to the certification of composites and bonded structures. In this context recent studies have revealed that delamination growth in composites and disbond growth in adhesive bonds can be computed using a version of the NASGRO crack growth equation. This paper reveals the potential for this approach to capture the large scatter seen in fatigue tests associated with both delamination/disbond growth in both composite structures and adhesively bonded joints.
    16th Australian Aerospace Congress, Melbourne, Australia; 02/2015
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    ABSTRACT: Tensile tests of miniature spruce wood specimens have been performed to investigate the damage evolution in wood at the microscopic scale. For this purpose, the samples were stepwise tensile loaded in the longitudinal (L) and radial (R) directions and the damage evolution was monitored in real-time by acoustic emission (AE) and synchrotron radiation micro-computed tomography (SRμCT). This combination is of outstanding benefit as SRμCT monitoring provides an insight on the crack evolution and the final fracture at microscopic scale, whereas AE permits the detection of the associated accumulation and interaction of single damage events on all length scales with high time resolution. A significant drawback of the AE testing of wood has been overcome by means of calibrating the AE amplitudes with the underlying crack length development. Thus, a setup-dependent and wood species-dependent calibration value was estimated, which associates 1 μm
    Holzforschung 01/2015; DOI:10.1515/hf-2014-0152 · 2.34 Impact Factor
  • L. Vergeynst · A. J. Brunner · Markus G. R. Sause
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    ABSTRACT: Brüche bzw. Versagen in Holzstrukturen resultieren aus der Häufung mikroskopischer Schädigung, die mit Schallemissionsmessungen nachgewiesen werden kann. Es konnte gezeigt werden, dass unterschiedliche Schädigungsmechanismen in Holz mittels Signalanalyse unterschieden werden können. Mustererkennung ergab zwei Signalklassen, die im Wesentlichen durch die gewichtete Spitzenfrequenz unterscheidbar waren. Die Signalklasse mit höherer Spitzenfrequenz konnte mit einer gewissen Wahrscheinlichkeit Zellwandbrüchen, jene mit niederer Spitzenfrequenz Zellablösungen zugeordnet werden. Es wurde daher vermutet, dass die unterschiedliche Risssausbreitungsgeschwindigkeit die beobachteten Frequenzspektren ergab. Zur Verifikation dieser Hypothese wurden in einem Finite Element Modell unterschiedliche Quellen-Anstiegszeiten, Quellenorte und Quellenorientierungen systematisch untersucht. Die Anstiegszeit konnte als wahrscheinlich wichtigster Parameter für das Auftreten unterschiedlicher Frequenzspektren identifiziert werden. Allerdings spielt die Sensorempfindlichkeit als Funktion der Frequenz bei diesen Messungen ebenfalls eine wichtige Rolle beim Nachweis der Schädigungsmechanismen. Der Beitrag diskutiert dies an ausgewählten Beispielen.
    20. Kolloquium Schallemission, Garmisch; 01/2015
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    ABSTRACT: Die Kombination von Mustererkennung zur Klassierung von experimentell ermittelten Wellenformen aus Schallemissionsüberwachung von Belastungsversuchen und von mittels Finite Element Methoden simulierten Signalen wurde ursprünglich zur Identifikation der Quellenmechanismen in Faserverbundwerkstoffen entwickelt. Aufgrund struktureller bzw. morphologischer Ähnlichkeit ist dieser Ansatz auf Holz und Holzwerkstoffe übertragen worden. Die Mustererkennung der experimentell gemessenen Schallemissionssignale hat für Zugversuche an Miniaturproben aus Fichtenholz zwei Signalklassen ergeben. Diese Klassen werden nun mittels Finite Element Simulationen der Wellenformen von Quellmechanismen mit unterschiedlicher Anstiegszeit vergleichend untersucht. Die Bestimmung des Einflusses der Messkette (Sensor bzw. Vorverstärker und Datenerfassung mit Frequenzfiltern) und die dafür verwendeten Ansätze werden diskutiert.
    DACH Jahrestagung, Graz; 01/2015
  • Franziska Baensch · Markus G.R. Sause · Andreas J. Brunner · Peter Niemz
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    ABSTRACT: Tensile tests on miniature spruce specimens have been performed by means of acoustic emission (AE) analysis. Stress was applied perpendicular (radial direction) and parallel to the grain. Nine features were selected from the AE frequency spectra. The signals were classified by means of an unsupervised pattern recognition approach, and natural classes of AE signals were identified based on the selected features. The algorithm calculates the numerically best partition based on subset combinations of the features provided for the analysis and leads to the most significant partition including the respective feature combination and the most probable number of clusters. For both specimen types investigated, the pattern recognition technique indicates two AE signal clusters. Cluster A comprises AE signals with a relatively high share of low-frequency components, and the opposite is true for cluster B. It is hypothesized that the signature of rapid and slow crack growths might be the origin for this cluster formation.
    Holzforschung 01/2015; 69(3). DOI:10.1515/hf-2014-0072 · 2.34 Impact Factor
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    F Baensch · M Zauner · M G R Sause · A J Brunner · P Niemz
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    ABSTRACT: Understanding of crack initiation and propagation in wooden structures is a difficult task because of the complexity of the cellular material. To enable real-time studies on the multi-scaled damage evolution, tensile tests on miniature specimens made of spruce wood were monitored by means of acoustic emission (AE) measurement and, simultaneously, by synchrotron radiation micro computed tomography (SRµCT). This combination is of outstanding benefit, since the SRµCT provides an insight on the microscopic structure and the damage development at different states as well as the ultimate failure, and additionally, the associated accumulation and interaction of single damage events is detectable as AE. The trials were carried out at the TOMCAT beamline (PSI Villingen, Switzerland) by using an especially designed loading device [1]. The AE monitoring was performed with digital AE equipment (AMSY-6, Vallen Systeme GmbH). Tensile stresses were applied in the radial and in the longitudinal direction; meanwhile the occurrence of microscopic cracks were precisely detected by two miniature piezoelectric sensors (type M31, Fuji Ceramics Corp.). Fig. 1 Averaged normalized AE frequency spectra of the AE signal classes a) A and b) B detected during tensile testing a spruce specimen in the longitudinal direction.
    Euromech; 01/2015
  • A. J. Brunner · F. Baensch · Markus G. R. Sause · M. Zauner · P. Niemz
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    ABSTRACT: Für ein vertieftes Verständnis des Schädigungsverlaufs von Lagenholz sind Zugversuche an Miniaturprüfkörpern aus verklebten Fichtenholz- Lamellen mit Schallemission und in-situ Synchrotron-basierter Röntgen-Computer- Mikrotomografie überwacht worden. Zu Vergleichszwecken wurden Zugversuche an Vollholz- und Lagenholz-Prüfkörpern aus Fichte im Labormassstab mit Schallemission überwacht. Die Schallemissionssignale aller Versuche wurden mit Mustererkennung klassifiziert. Dabei ergaben sich zwei Klassen von Signalen mit je zwei Frequenzspitzen. In einer Klasse war die niederfrequente, in der andern die höherfrequente Spitze von höherer Intensität, dies aber im Wesentlichen unabhängig vom Aufbau (Vollholz bzw. Lagenholz) und der Grössenskala der Prüfkörper. Der Einfluss der Klebschichten wurde an Lagenholz-Prüfkörpern im Labormassstab und an Miniaturprüfkörpern mit einer Klebschicht und ausgewählten Faser-Orientierungen untersucht. Dies ergab Hinweise, dass die Schallemissions-Signale aus dem Versagen der Klebschicht vermutlich der Klasse mit Signalen mit niederfrequenter Spitze im Leistungsspektrum zugeordnet werden können.
    20. Kolloquium Schallemission, Garmisch; 01/2015
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    Steffen Stelzer · Gerald Pinter · Andreas J. Brunner
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    ABSTRACT: Delamination resistance data from different carbon-fiber reinforced polymer-matrix (CFRP) composites are compared for different loading modes, i.e., quasi-static and cyclic fatigue, opening tensile mode I, in-plane shear mode II, and fixed-ratio mixed-mode I/II. For this, data from round robin tests conducted at the authors laboratories will be complemented by selected results from literature. Questions related to delamination resistance of CFRP composites with implications for composite structural design and testing include, e.g., the determination of threshold values in cyclic fatigue, the question of conservative mode (mode I versus mode II), approaches for data analysis, and possible analogies in short crack cyclic fatigue between fracture behavior of structural metal alloys and CFRP. The scatter in Paris-type law data analysis of cyclic fatigue tests and the resulting apparent threshold behavior that has implications for composite structural design will be presented. Load measurement resolution yields the major contribution to scatter in displacement controlled fatigue tests. The analogous displacement resolution for load controlled tests is discussed and limitations in test control and of power law displacement data fitting for analysis are pointed out.
    12/2014; 3:1087-1092. DOI:10.1016/j.mspro.2014.06.177
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    COST - Timber Bridges Conference 2014, Biel, Switzerland; 09/2014
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    Franziska Ritschel · Markus G R Sause · Andreas J Brunner · Peter Niemz
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    ABSTRACT: Even today, a detailed description of the damage progress in wood or wood materials under tensile loading is still a challenge. The complexity of the damage behaviour results from the various mechanisms occurring simultaneously at several length scales. So far, few studies focused on mechanical behaviour of wood or wood materials by analysing AE, although AE can provide information on multi-scale damage mechanisms and damage accumulation. The high time resolution of AE measurements is beneficial for detection of micro-mechanisms, their interactions and accumulation leading to macroscopic failure. For the AE analysis presented here, several types of industrial plywood and layered wood materials made from spruce were subjected to quasi-static tensile loads and simultaneously monitored by AE. Since polymer-composites and wood can be assumed to behave analogously, especially regarding their anisotropic properties, application of pattern recognition methods for fibre reinforced polymer-matrix composites are expected to have also high potential for AE signal classification of wood fracture. Such unsupervised pattern recognition, e.g., based on the frequency domain of the AE signals, are purely mathematical approaches to perform signal classification and to identify natural classes of AE signals, respectively. Within the present investigation, a signal classification approach originally developed for fibre-reinforced composite laminates is explored for plywood and layered wood materials. Problems and challenges are identified which have to be solved for a detailed understanding of their damage behaviour. The different layered structures of plywood all yield two AE signal clusters which can roughly be differentiated in signals of relatively high shares of low frequency and high frequency content, respectively. These occur essentially over the whole test duration and yield comparable AE signal amplitudes and energies. The challenge is to assign the features of the detected signals to their microscopic source mechanism.
    31st Conference of the European Working Group on Acoustic Emission (EWGAE); 09/2014
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    ABSTRACT: Observations of a nanometer-scale nodular morphology on differently prepared surfaces of thermosets have frequently been interpreted as a sign for an inhomogeneous molecular network, which would result in an inhomogeneous modulus distribution within those thermosets. In order to test this hypothesis, the Peak-Force Tapping atomic force microscopy (AFM) mode was used on fracture surfaces and ultramicrotome cuts of epoxy and other polymers using differently sharp AFM probes. The nodular morphology is quite likely caused by an AFM artifact, which also seems to cause an apparently inhomogeneous modulus distribution; a variation in the tip-sample contact area could explain this effect. Smooth surfaces are necessary in order to reduce the contribution from this artifact. Ultramicrotome cutting currently seems to be the most appropriate surface preparation technique for the measurement of modulus distribution at the nanometer scale. All investigated materials seem to be homogeneous on a scale on the order of 10 nm-1000 nm. If modulus inhomogeneities are present, their amount or their lateral size is too small to be unambiguously measurable with this technique. From this data, it seems unlikely that epoxy exhibits an inhomogeneous molecular network.
    Polymer 08/2014; 55(16). DOI:10.1016/j.polymer.2014.06.030 · 3.77 Impact Factor
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    ABSTRACT: Electrophoretic deposition (EPD) of carbon nanotubes (CNT) on carbon fibers has been implemented as a continuous process on laboratory-scale. The interfacial adhesion and fracture toughness of the carbon fibers in an epoxy composite is assessed by a modified single-fiber push-out test. A detailed energy analysis yields the different energy contributions in the push-out process. A comparison between CNT-deposited, as received and oxidized carbon fibers (passing through the EPD process without CNT) indicates that interfacial adhesion and fracture toughness are not affected by the different fiber treatments. Interfacial friction after fiber debonding, however, is significantly changed. This is confirmed by finite element simulation which has to include friction for reproducing the essential features of the load-displacement plots from fiber push-out. Scanning electron micrographs indicate little interaction between CNT and carbon fibers, but point to changes in surface roughness of CNT-deposited and oxidized fibers after push-out. Therefore, the cyclic loading-unloading fiber push-out test seems well suited to investigate the micromechanical behaviour of carbon fiber composites and to discriminate and quantify the different energy contributions to the total load-displacement curves.
    Composites Science and Technology 05/2014; 95. DOI:10.1016/j.compscitech.2014.02.017 · 3.63 Impact Factor
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    R. Jones · S. Stelzer · A.J. Brunner
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    ABSTRACT: This paper presents an approach for computing the growth of Mode I, II and Mixed Mode I/II delaminations in carbon fibre reinforced polymer composites (CFRP) using a modified Hartman–Schijve equation. Unlike other equations it does not involve splitting the energy release rate into its various components. One advantage of this formulation is that the exponent of the associated power law appears to be independent of the mode as is the constant of proportionality. This formulation is shown to accurately compute the delamination growth rates associated with a range of Mode I, II and Mixed Mode I/II data available in the open literature. The potential for this approach to be used to overcome the no growth philosophy associated with current composite designs is also discussed.
    Composite Structures 04/2014; 110(1):317–324. DOI:10.1016/j.compstruct.2013.12.009 · 3.32 Impact Factor
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    ABSTRACT: Several plywood materials made from spruce wood and, for comparison, solid spruce wood were investigated focusing on the sub-macroscopic damage evolution during tensile loading of the specimens. The destructive tests were simultaneously monitored by the acoustic emission (AE) method and strain field deformation measurement using digital image correlation (DIC). The bilinear interpretation of exponential defect growth identified the start of significant nonlinear behavior at 70 % of ultimate strength for all plywood materials. However, already the preceding and more stable damage evolution at lower stress levels has indicated a variation in intensity of the source mechanisms evaluated by AE energy of the detected events. Additional information on the formation of strain field concentration, which correlates with discrete accumulation in AE events and increased spreading in the distribution of AE energy, reveals the complexity of pre-damage due to the variation in cracks’ magnitude and timescales involved. The correlation between ultimate tensile strength and damage accumulation below 70 % of ultimate strength is determined, as well as the influence of layered structures on damage size shown by the percentage distribution of AE energy.
    Wood Science and Technology 03/2014; 48(3):611–631. DOI:10.1007/s00226-014-0628-1 · 1.87 Impact Factor
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    ABSTRACT: The processing of thermoplastics can induce a wide range of defects such as stress whit- ening, cavitation and porosity, which can adversely affect the reliability of the final products. Hence, fast and effective non-destructive detection methods for such defects are highly important for quality assurance on production lines. In this paper, X-ray dark field imaging is presented as a new non-destructive testing method that allows the visualiza- tion of stress whitening or cavitation efficiently. The performance of the method is demonstrated for the case of an injection-moulded polyvinylidene fluoride part that ex- hibits stress whitening. Whereas the stress whitening could not be detected by conven- tional X-ray imaging, it was localized by an X-ray dark field image acquired within a few minutes. Once the precise location of the stress whitening was known, it was possible to verify the result by local micro X-ray computed tomography and by a micro section image.
    Polymer Testing 09/2013; 32:1094–1098. DOI:10.1016/j.polymertesting.2013.06.008 · 1.82 Impact Factor
  • A.J. Brunner · S. Stelzer · G. Pinter · G. P. Terrasi
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    ABSTRACT: Delamination resistance testing of fiber-reinforced polymer–matrix laminates under fatigue loads is important for materials development and structural design. Mode II in-plane shear fatigue test development using three-point bending end-notched flexure (3-ENF) and two-point bending end-loaded split (ELS) set-ups is performed in a round robin. Effects of specimen restraint observed earlier in ENF tests are confirmed and preliminary data indicate differences between the two set-ups, if simple beam theory or experimental compliance analyses are applied. Possible reasons for the observed disagreement are discussed.
    International Journal of Fatigue 05/2013; 50:57-62. DOI:10.1016/j.ijfatigue.2012.02.021 · 1.69 Impact Factor
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    R. Jones · S. Pitt · D. Hui · A. Brunner
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    ABSTRACT: This paper discusses the potential of a variant of the Hartman–Schijve equation to represent fatigue crack growth in a range of nano-composites. It is found that when expressed in this form the exponent of this variant is approximately 2 and, as such, is considerably lower than the exponent in ‘‘Paris like’’ power law representations for delamination growth in composites. As such we see that, in these examples, the present variant of the Hartman–Schijve representation of delamination growth in nano-composites is similar to that seen for crack growth in metals, delamination growth in composites and the environmental degradation of adhesive bonds. This suggests that the present formulation may be useful for the damage tolerant assessment of small naturally occurring defects in nano-composite structures and for the ranking of nano-composites.