Advances in Experimental Mechanics VII

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Authors: Artur Galek, Harald Moser, Thomas Ring, Matthias Zeiml, Josef Eberhardsteiner, Roman Lackner
Abstract: When concrete structures are subjected to fire loading, temperature-dependent degradation of the material properties as well as spalling of near-surface concrete layers has a considerable effect on the load-carrying capacity and, hence, the safety of these structures. Spalling is caused by interacting thermo-hydro-chemo-mechanical processes with both mechanical and transport properties playing an important role. Within experimental research activities at the IMWS, these properties are subject of investigation, i.e., (i) the strain behavior of concrete under combined thermal and mechanical loading and (ii) the permeability increase of temperature-loaded concrete and cement paste.
Authors: Ole Thybo Thomsen, K.K. Kratmann
Abstract: The classical kink-band formation models predict that the compressive strength of UD carbon fibre reinforced composite materials (UD CFRP) is governed by fibre misalignment as well as of the mechanical shear properties. A new image analysis procedure for experimental determination of the fibre misalignment, the Fourier transform misalignment analysis (FTMA), has been developed. Moreover, a modified asymmetric Iosipescu test specimen geometry has been developed and validated for accurate measurement of the composite material shear properties without parasitic effects due to axial splitting. In the test procedure the shear strain distribution is measured using Digital Image Correlation (DIC) and the results calibrated based on FEA modelling results. Using the measured properties as input, the predictions of the classic compressive strength models have been compared with measured compressive strengths. Finally, an alternative approach to the classical kink band equilibrium has been proposed and demonstrated to provide more accurate predictions than the classical models.
Authors: Keith Worden, W.E. Becker, Manuela Battipede, Cecilia Surace
Abstract: This paper concerns the analysis of how uncertainty propagates through large computational models like finite element models. If a model is expensive to run, a Monte Carlo approach based on sampling over the possible model inputs will not be feasible, because the large number of model runs will be prohibitively expensive. Fortunately, an alternative to Monte Carlo is available in the form of the established Bayesian algorithm discussed here; this algorithm can provide information about uncertainty with many less model runs than Monte Carlo requires. The algorithm also provides information regarding sensitivity to the inputs i.e. the extent to which input uncertainties are responsible for output uncertainty. After describing the basic principles of the Bayesian approach, it is illustrated via two case studies: the first concerns a finite element model of a human heart valve and the second, an airship model incorporating fluid structure interaction.
Authors: Rhys Pullin, A. Clarke, Mark J. Eaton, Karen M. Holford, S.L. Evans, J.P. McCory
Abstract: The detection of damage in gear teeth is paramount to any condition monitoring or structural health monitoring (SHM) tool for aerospace power transmissions such as those used in helicopters. Current inspection techniques include vibration analysis and time-inefficient visual inspection. Acoustic Emission (AE) is a very sensitive detection tool that has been successfully used in many SHM systems. Successful application of AE for damage detection in gear teeth will enable the optimisation of gear box design (and hence weight saving) in addition to safety improvements. This paper details a small aspect of a larger project designed to demonstrate automatic detection and location of common gear tooth defects. A novel test rig was designed to allow the fatigue loading of an individual gear tooth which was monitored using AE. The gear tooth was static in order to exclude the detection of AE signals arising from rotation; this allows initial development of the methodology prior to investigating rotating gears. Digital Image Correlation was used to determine the onset of cracking for comparison with the detected AE. Preliminary results of the investigation show that the developed methodology is appropriate for developing an automated gear health monitoring system and that future work should concentrate on the development of sensors and data acquisition methods associated with obtaining signals from rotating machinery.
Authors: Pawel Malinowski, Tomasz Wandowski, Wiesław M. Ostachowicz
Abstract: In presented research a problem that belongs to the Structural Health Monitoring (SHM) topic was investigated. Special arrays of active sensors were used for damage detection. These sensors were piezoelectric transducers. They were attached to specimen under investigation and used to excite and sense guided elastic waves – Lamb waves. Each array comprised of uniquely placed transducers. The total number of transducers was the same for all considered arrays. This ensured that the same number of signals was used to obtain damage information. A numerical algorithm was proposed to process these signals. It was designed to be independent of sensor arrangement so it could be used for all considered arrays. The principal idea behind the algorithm is that obstacles on a wave path cause wave reflection. These reflections are represented in the time signals. The algorithm was used to associate energy of these reflections with a particular area of the investigated specimen. The value of the energy was extracted from all the signals and projected to coordinate system associated with the specimen edges. In order to test and compare proposed arrays artificial defects were introduced to the specimen to model damaged structure. Because the specimen with defect and signal processing algorithm were the same, the only variable that could influence damage detection was the type of the array. In the investigation damage detection results were obtained for considered arrays. Although the number of sensors were invariable, differences in damage indication exist. This suggest that the type of sensor array should be precisely chosen for a particular application. Even simplest linear array may be sufficient but it depends where we want to apply it.
Authors: R.K. Fruehmann, Janice M. Dulieu-Barton, S. Quinn
Abstract: The thermoelastic response obtained from an infra-red (IR) detector contains two components: the magnitude of the small stress induced temperature change caused by the thermoelastic effect and the phase angle of the temperature change relative to a reference signal generated by an application of a stress change. The phase angle is related to nonlinearity in the thermoelastic response and departures from the simple linear relationship that underpins thermoelastic stress analysis (TSA). The phase data could be used to make an assessment of temperature evolutions caused by viscoelastic behaviour resulting from damage and provide a basis for its evaluation. In the current paper the physics of other infra-red techniques used for non-destructive evaluation is used to better understand the nature of the thermoelastic response. The objective is to provide better exploitation of TSA by alternative processing of the IR measurements. Three case studies are presented that demonstrate the potential of the alternative processing for evaluating damage.
Authors: P. Harkness, Andrea Cardoni, J. Russell, Margaret Lucas
Abstract: A number of architectures for a hollow Langevin ultrasonic transducer are proposed and evaluated. One of these is optimised by finite element modelling and is then manufactured and analysed experimentally. The preload on the transducer ceramics is increased and the effect on the performance is measured. At maximum preload the results of an experimental modal analysis are used to determine the natural frequency and response of both the operating longitudinal mode and unwanted bending modes. The performance of the hollow transducer is compared to a solid commercial transducer containing the same volume of piezoceramic material. The efficiency is shown to be comparable. Higher ultrasonic displacement amplitudes are achieved with the hollow transducer although a lower Q-factor is found.

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