Andreas J. Brunner

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

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Publications (31)26.28 Total impact

  • [Show abstract] [Hide abstract]
    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. · 1.88 Impact Factor
  • 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. 01/2014; 110:317–324.
  • Polymer. 01/2014;
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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 01/2014; · 3.33 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.
    Composite Structures. 01/2013; 99:375-379.
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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 01/2013; 32:1094–1098. · 1.65 Impact Factor
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    ABSTRACT: Two round robins on mode I fatigue delamination propagation organized by Technical Committee 4 of the European Structural Integrity Society compared three unidirectional carbon fiber reinforced composites, one with thermoplastic (poly-ether-ether-ketone) and two with thermoset (epoxy) matrix tested at five laboratories. Different approaches for data evaluation and their effect on the in- and inter-laboratory scatter are discussed and compared. Calculated delamination rates da/dN and applied GImax from displacement controlled tests are sensitive to small scatter in the load signal, and, therefore, a new route to evaluate the crack growth rate from pairs of load and displacement data is presented.
    Engineering Fracture Mechanics 01/2013; · 1.41 Impact Factor
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    ABSTRACT: Selected mode I fatigue data from five different types of fiber-reinforced, polymer–matrix composites tested in two round robins organized by the American Society for Testing and Materials subcommittee D30.06 and European Structural Integrity Society Technical Committee 4, respectively, are analyzed and discussed. The focus is on experimental scatter (in-laboratory and inter-laboratory) and on schemes for quantitative data analysis. It is shown that in spite of considerable scatter different composites can be distinguished and, under certain assumptions, a relative ranking be established. Further, effects from limited experimental measurement resolution are noted and implications for the test procedure and use of the test data in design of composite structures discussed. For comparative purposes, a rough ranking of different composites is feasible with test data generated within 24 h per specimen in an industrial test environment.
    Composites Science and Technology 06/2012; 72(10):1102–1107. · 3.33 Impact Factor
  • F. Ritschel, A.J. Brunner, P. Niemz
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    ABSTRACT: A methodological approach allowing insights into micromechanical phenomena during destructive testing of the macroscopic specimens is introduced for layered wood materials. Tensile tests of samples made from laminated veneer lumber (LVL) and from Plywood (PW) are performed with in-situ monitoring of acoustic emission (AE) and surface displacement with the application of digital image correlation (DIC). The mechanical behavior of the specimens is correlated with information from DIC and AE analysis. Due to the influence of the varying morphological structure of the samples, especially the number of adhesive layers and the fiber orientation of the wooden layers, different mechanisms dominate the damage evolution. These differences are reflected by the results of the first AE analysis dealing with AE activity and AE intensity, as well as by the varying formation of strain field distributions.
    17th International Nondestructive Testing and Evaluation of Wood Symposium, Sopron, Hungary; 09/2011
  • F. Ritschel, A.J. Brunner, P. Niemz
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    ABSTRACT: As a first step towards developing a test methodology for characterization of microscopic damage mechanisms in wood-based materials, quasi-static tensile tests were performed on simplified, laboratory-scale specimens of plywood or laminated veneer lumber and compared with solid wood. The tests were monitored by acoustic emission (AE) and digital image correlation (DIC). Evaluation of the AE and DIC data indicated distinct differences between the specimen types with respect to the time sequence of damage growth resulting in the macroscopic failure. Sufficient time resolution in data recording is crucial for the interpretation of the mechanical behavior and the damage accumulation. The behavior appears to be analogous to that observed in polymer-matrix laminates.
    16th International Conference on Composite Structures, ICCS-16, Porto; 06/2011
  • M Melnykowycz, A J Brunner
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    ABSTRACT: Piezoelectric elements integrated into fiber-reinforced polymer-matrix laminates can provide various functions in the resulting adaptive or smart composite. Active fiber composites (AFC) composed of lead zirconate titanate (PZT) fibers can be used as a component in a smart material system, and can be easily integrated into woven composites. However, the impact of integration on the device and its functionality has not been fully investigated. The current work focuses on the integration and performance of AFC integrated into carbon-fiber-reinforced plastic (CFRP) laminates, focusing on the strain sensor performance of the AFC–CFRP laminate under tensile loading conditions. AFC were integrated into cross-ply CFRP laminates using simple insertion and interlacing of the CFRP plies, with the AFC always placed in the 90° ply cutout area. Test specimens were strained to different strain levels and then cycled with a 0.01% strain amplitude, and the resulting signal from the AFC was monitored. Acoustic emission monitoring was performed during tensile testing to provide insight to the failure characteristics of the PZT fibers. The results were compared to those from past studies on AFC integration; the strain signal of AFC integrated into CFRP was much lower than that for AFC integrated into woven glass fiber laminates. However, the profiles of the degradations of the AFC signal resulting from the strain were nearly identical, showing that the PZT fibers fragmented in a similar manner for a given global strain. The sensor performance recovered upon unloading, which is attributed to the closure of cracks between PZT fiber fragments.
    Smart Materials and Structures 06/2011; 20(7):075007. · 2.02 Impact Factor
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    ABSTRACT: Active fiber composites (AFC) composed of lead zirconate titanate (PZT) fibers embedded in an epoxy matrix and sandwiched between two interdigitated electrodes provide a thin and flexible smart material device which can act as a sensor or actuator. The thin profiles of AFC make them ideal for integration in glass or carbon fiber composite laminates. However, due to the low tensile limit of the PZT fibers, AFC can fail at strains below the tensile limit of many composites. This makes their use as a component in an active laminate design somewhat undesirable. In the current work, tensile testing of smart laminates composed of AFC integrated in glass fiber laminates was conducted to assess the effectiveness of different packaging strategies for improving AFC sensor performance at high strains relative to the tensile limit of the AFC. AFC were encased in carbon fiber, silicon, and pre-stressed carbon fiber to improve the tensile limit of the AFC when integrated in glass fiber laminates. By laminating AFC with pre-stressed carbon fiber, the tensile limit and strain sensor ability of the AFC were significantly improved. Acoustic emission monitoring was used and the results show that PZT fiber breakage was reduced due to the pre-stressed packaging process.
    Smart Materials and Structures 11/2009; 19(1):015001. · 2.02 Impact Factor
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    ABSTRACT: Z-Pin reinforced carbon-fibre epoxy laminates were tested under Mode I and Mode II conditions, both quasi-statically and in fatigue. Test procedures were adapted from existing standard or pre-standard tests. Samples containing 2% and 4% areal densities of carbon-fibre Z-pins (0.28 mm diameter) were compared with unpinned laminates. Quasi-static tests under displacement control yielded a dramatic increase of the apparent delamination resistance. Specimens with 2% pin density failed in Mode I at loads 170 N, equivalent to an apparent GIC of 2 kJ/m2. Fatigue testing under load control showed that the presence of the through-thickness reinforcement slowed down fatigue delamination propagation.
    Engineering Fracture Mechanics. 01/2009;
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    A.J. Brunner, N. Murphy, G. Pinter
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    ABSTRACT: A round robin exercise on opening mode I fatigue delamination propagation has been performed with the aim of developing a standardized test procedure. The material chosen for the test was one type of carbon–fiber reinforced polymer–matrix laminate (IM7 fiber, 977-2 epoxy). The Double Cantilever Beam specimen from the quasi-static mode I delamination resistance test (ISO 15024) has been used for the fatigue test. Test set-up, measurements and data acquisition have been defined with an emphasis on applicability in an industrial test environment. Selected test parameters have been varied in order to investigate their effect on the results. Three different approaches for delamination length determination have been compared. Visual determination of delamination length, a compliance-based approach and an effective delamination length calculation based on a separate measurement of the modulus of elasticity yield reasonable agreement. This agreement suggests that further development of the test procedure to incorporate automated data acquisition and analysis may be worthwhile.
    Engineering Fracture Mechanics. 01/2009;
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    ABSTRACT: Piezoelectric Active Fiber Composites (AFC) and Macro Fiber Composites (MFC) have the potential to provide various sensor functions for nondestructive test methods. AFC have been integrated into fiber-reinforced laminates as a first step towards structures with sensing capability. These developments constitute initial stages for developing adaptive composite structures or structures with integrated health monitoring system. So far, the use of AFC and MFC has been explored in selected nondestructive tests for defect detection in model composite systems on laboratory scale with e.g., Acoustic Emission, Acousto-Ultrasonics, and Electromechanical Impedance testing. The present article will focus on limitations and current prospects for structural health monitoring with AFC or MFC and discuss selected concepts and approaches.
    Journal of Intelligent Material Systems and Structures 01/2009; 20(9):1045-1055. · 1.52 Impact Factor
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    ABSTRACT: Nano-sized, functionalized organo-silicate fillers dispersed in the epoxy matrix are one approach that is investigated for improving the delamination resistance of fiber-reinforced composites. The variety of nano-silicate fillers available on the market, of processing conditions and the related characterization effort make it desirable to have a simple, easily analyzed screening method for both, nano-modified resins and laminates. An impact test using hail simulation equipment and visual assessment on glass-fiber laminates with nano-modified epoxy matrix yields rough indications of the effect of nano-modification on the fracture behavior of the specimens that correlate with more sophisticated macroscopic and microscopic characterization.
    Journal of Composite Materials 01/2009; 43(9):959-976. · 0.94 Impact Factor
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    ABSTRACT: Active fiber composites (AFC) are thin and conformable transducer elements with orthotropic material properties, since they are made of one layer of piezoelectric ceramic fibers. They are suitable for applications in structural health monitoring systems (SHM) with acoustic non-destructive testing methods (NDT). In the presented work the transfer behavior of an AFC as an emitter of transient elastic waves in plate-like structures is investigated. The wave field emitted by an AFC surface bonded on an isotropic plate was simulated with the finite-difference method. The model includes the piezoelectric element and the plate and allows the simulation of the elastic wave propagation. For comparison with the model experiments using a laser interferometer for non-contact measurements of particle velocities at different points around the AFC on the surface of the plate were performed. Transfer functions defined as the ratio of the electric voltage excitation signal and the resulting surface velocity at a specific point are separately determined for the two fundamental Lamb wave modes. In order to take the orthotropic behavior of the AFC into account the transfer functions are determined for several points around the AFC. Results show that the AFC is capable to excite the fundamental symmetric and antisymmetric Lamb wave mode. The antisymmetric mode is mainly radiated in the direction of the piezoelectric fibers, while the symmetric mode is spread over a larger angle. The amplitudes of the emitted waves depend on the frequency of the excitation as well as on the geometric dimensions of the transducer.
    Ultrasonics 07/2008; 49(1):73-82. · 2.03 Impact Factor
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    Brunner A, Blackman B, Davies Peter
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    ABSTRACT: The development of fracture mechanics test methods for the determination of delamination resistance or fracture toughness of fiber-reinforced, polymer-matrix composites is an active area of research. The emphasis in this review is on standardisation of test methods. Recent developments leading towards new standardized test procedures will be presented, complementing and updating earlier reviews. (c) 2007 Elsevier Ltd. All rights reserved.
    Engineering Fracture Mechanics (0013-7944) (Elsevier), 2008-06 , Vol. 75 , N. 9 , P. 2779-2794. 01/2008;
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    ABSTRACT: Active fiber composites (AFC) made from piezoelectric fibers show promising potential, not only as actuators integrated into smart composites but also as sensors. Selected sensor properties of AFC elements are investigated with methods from acoustic emission sensor characterization and verification and from acousto-ultrasonic testing. Model experiments with AFC elements integrated in simple composite structures show that the various approaches for analysis of acousto-ultrasonic data yield indications of damage accumulation.
    Journal of Intelligent Material Systems and Structures 01/2007; 18(5):515-525. · 1.52 Impact Factor
  • Till Vallée, Andreas J. Brunner, Thomas Keller
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    ABSTRACT: Previous experiments related to the strength of adhesively bonded joints composed of glass fiber reinforced polymer (GFRP) pultruded adherends (Vallée et al. [1–3]) indicated that, even using enhanced mathematical methods on the basis of a fully linear mechanical model, a gap of around 10% between predicted and experimentally gathered joint strengths remain. One reason invoked in [3] were i) possible damages occurring by microscopic defect accumulation (micro-cracks or cavitation) and/or ii) a possible non-linear behaviour of the GFRP material at higher stresses, beyond those that could be reached by testing samples of a size of 40mmx40mm.
    12/2006: pages 141-142;