Rupture de fibre : comparaison entre un modèle écrit a l'échelle microstructural et une détection du phénomène par tomographie très haute résolution

International audienceOn présente dans cette étude la discrimination des phénomènes de ruptures de fibre, de microfissuration intralaminaire, de macrofissuration intralaminaire ainsi que du microdélaminage au sein des composites stratifiés d'unidirectionnels orientés, dans le cadre d'une méthode de détection des endommagements basée sur la technique de l'Emission Acoustique. Ces résultats expérimentaux sont confrontés aux prévisions issues d'un modèle de comportement de matériaux composites unidirectionnels, basés sur la physique des phénomènes à l'échelle des constituants

Detailed experimental validation and benchmarking of six models for longitudinal tensile failure of unidirectional composites

Longitudinal tensile failure of unidirectional fibre-reinforced composites remains difficult to predict accurately. The key underlying mechanism is the tensile failure of individual fibres. This paper objectively measured the relevant input data and performed a detailed experimental validation of blind predictions of six state-of-the-art models using high-resolution in-situ synchrotron radiation computed tomography (SRCT) measurements on two carbon fibre/epoxy composites. Models without major conservative assumptions regarding stress redistributions around fibre breaks significantly overpredicted failure strains and strengths, but predictions of models with at least one such assumption were in better agreement for those properties. Moreover, all models failed to predict fibre break (and cluster) development accurately, suggesting that it is vital to improve experimental methods to characterise accurately the in-situ strength distribution of fibres within the composites. As a result of detailed measurements of all required input parameters and advanced SRCT experiments, this paper establishes a benchmark for future research on longitudinal tensile failure

The Importance of the Heel Effect in X-Ray Ct Imaging of Soils

Non-destructive and non-invasive X-ray computed tomography (CT) is increasingly used in environmental geotechnics research. As a result of recent advances in technology and image processing techniques, CT with rapid scanning now has the potential to track changes in soil structure or soil water conditions as they happen, rather than as previously on a specimen in (temporary) stasis. Gathering meaningful data in a short scan time requires compromises to be made on parameters such as exposure time, and / or the use of higher X-ray intensities and energies. Data processing and imaging processing - including the removal of any artefacts, which can cause errors in interpretation of soil structure or phase proportions - then become especially important. One such artefact is the heel effect. It has been recognised in medical imaging, owing to its association with high scan energies. However, it has not previously been identified in soil imaging, despite the trend towards using higher energies. This paper presents an investigation into the potential for the heel effect to affect the soil property determination. It is shown for the first time that a noticeable heel effect will be present in CT images of soils and derived phase proportion data, when certain types of X-ray reflection targets are used. A correction method for the heel effect is presented, use of which will prevent significant errors in derived soil parameters such as water content

Computed tomography reconstructions of burrow networks for the Opheliid polychaete, Armandia cirrhosa

The morphology and architecture of structures formed by sediment-dwelling invertebrates, such as excavations or burrows, are often assumed to be characteristic of a given species, consistent across a range of environmental conditions, and used to categorise species contributions to ecosystem functioning. However, very few investigations use non-invasive high-resolution techniques capable of determining fine scale variations in burrow form and complexity, or consider whether or not the form of the burrow is context dependent. Here, we provide replicate high-resolution micro-focus computed tomography data for the complete burrow systems of the Opheliid polychaete, Armandia cirrhosa, across a range of salinity and habitat conditions. These data provide reference models which can be used by ecologists investigating intraspecific variation in species traits and organism-sediment interactions and, more generally, by those tasked with pattern and shape recognition of objects that are morphologically highly variable and which adjust their architecture with changing circumstance or context

In situ fibre fracture measurement in carbon-epoxy laminates using high resolution computed tomography

High resolution Synchrotron Radiation Computed Tomography (SRCT) has been used to capture fibre damage progression in a carbon-epoxy notched [90/0]s laminate loaded to failure. To the authors knowledge this provides the first direct in situ measurement of the accumulation of fibre fractures for a high performance material under structurally relevant load conditions (i.e. fractures within the bulk of an essentially conventional engineering laminate). A high level of confidence is placed in the measurements, as the failure processes are viewed internally at the relevant micromechanical length-scales, as opposed to previous indirect and/or surface-based methods. Whilst fibre breaks are the dominant composite damage mechanism considered in the present work, matrix damage, such as transverse ply cracks, 0° splits and delaminations, were also seen to occur in advance of extensive fibre breaks. At loads where fibre break density levels were significant, splitting and delamination were seen to separate the central 0° ply in the near notch region from the 90° plies. Fibre breaks were initially observed in isolated locations, consistent with the stochastic nature of fibre strengths. The formation of clusters of broken fibres was observed at higher loads. The largest clusters observed consisted of a group of eleven breaks and a group of fourteen breaks. The large clusters were observed at the highest load, at sites with no prior breaks, indicating they occurred within a relatively narrow load range. No strong correlation was found between the location of matrix damage and fibre breaks. The data achieved has been made available online at www.materialsdatacentre.com for ongoing model development and validation

The application of digital volume correlation (DVC) to study the microstructural behaviour of trabecular bone during compression

Digital Volume Correlation (DVC) has emerged recently as an innovative approach to full volume (i.e. internal) displacement and strain field measurement in materials and structures, particularly in conjunction with high resolution X-ray computed tomography (CT). As a relatively novel technique certain aspects of precision, accuracy and the breadth of application are yet to be fully established. This study has applied DVC to volume images of porcine trabecular bone assessing the effect of noise and sub-volume size on strain measurement. Strain resolutions ranging between 70 to 800 ?? were obtained for the optimum sub-volume size of 64 voxels with a 50 % overlap for metrological studies conducted. These values allowed the mechanical behaviour of porcine trabecular bone during compression to be investigated. During compression a crushed layer formed adjacent to the boundary plate which increased in thickness as the specimen was further deformed. The structure of the crushed layer was altered to such an extent that it confounded the correlation method. While investigating this factor, it was found that for reliable strain calculations a correlation coefficient of 0.90 or above was required between the sub-volumes in the reference and the deformed volumes. Good agreements between the results and published bone strain failures were obtained. Using the full field strain measurements, the Poisson's ratio was identified for each compression step using a dedicated inverse method called the virtual fields method (VFM). It was found that for a given region outside of the crushed zone the Poisson's ratio decreased from 0.32 to 0.21 between the first and thefinal compression steps, which was hypothesised to be due to the bone geometry and its resulting deformation behaviour.This study demonstrates that volumetric strain measurement can be obtained successfully using DVC, making it a useful tool for quantitatively investigating the micro-mechanical behaviour of macroscale bone specimens.<br/

Three-dimensional characterization of fibroblast foci in idiopathic pulmonary fibrosis

In idiopathic pulmonary fibrosis (IPF), the fibroblast focus is a key histological feature representing active fibroproliferation. On standard 2D pathologic examination, fibroblast foci are considered small, distinct lesions, although they have been proposed to form a highly interconnected reticulum as the leading edge of a wave of fibrosis. Here, we characterized fibroblast focus morphology and interrelationships in 3D using an integrated micro-CT and histological methodology. In 3D, fibroblast foci were morphologically complex structures, with large variations in shape and volume (range, 1.3 × 10(4) to 9.9 × 10(7) μm(3)). Within each tissue sample numerous multiform foci were present, ranging from a minimum of 0.9 per mm(3) of lung tissue to a maximum of 11.1 per mm(3) of lung tissue. Each focus was an independent structure, and no interconnections were observed. Together, our data indicate that in 3D fibroblast foci form a constellation of heterogeneous structures with large variations in shape and volume, suggesting previously unrecognized plasticity. No evidence of interconnectivity was identified, consistent with the concept that foci represent discrete sites of lung injury and repair