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Sandwich panels are structures employed in many different applications. Here, sandwich panels with the core made of Arundo donax rings and the skins made of flax fibre reinforced epoxy composites are proposed. The influence of the surface roughness of the rings on the mechanical properties is investigated. Pull-off tests between the ring and the skin were carried out to understand how sandpaper polishing can affect the adhesion between the skin and the core. The sandwich panel is characterized by three points bending tests. The results show a significant influence of the polishing treatment on the ring lateral surfaces.

International audience

International audience; The Acoustic Emission (AE) technique is widely used in Structural Health Monitoring (SHM) in order to detect, in real-time, small-scale damages and to evaluate their kinetics. The piezoelectric transduction is generally favored for detecting the subnanometric displacements of the surface of a material induced by the propagation of the elastic waves generated by the sudden release of energy during damage. Yet highly sensitive, this type of sensor has a great impact on the signals collected introducing interpretation biases when assigning damage families to AE signals. In this paper, new Capacitive Micromachined Ultrasonic Transducers (CMUT) have been designed and tested with the objective to decrease the footprint of AE sensors compared to conventional piezoelectric sensor. CMUT allow getting AE sensors with wide-band and flat response making them of great interest for the collection of AE signals in SHM solutions.

In order to develop 100% bio-based high-grade epoxy composites, in this study, bio-based epoxy thermosets and hemp slivers are processed and characterized by different technologies. Epoxy resins are synthesized from the diglycidylether of Eugenol, extracted from cloves. They are cured with bio-based acid anhydrides. The physicochemical properties of the resulting epoxy resins are characterized using thermogravimetric analyses (TGA), differential scanning calorimetry (DSC), and nanoindentation. The mechanical properties of hemp fibres extracted from the slivers are also determined using tensile tests. After their processing and characterization, these bio-based constituents are then used to manufacture unidirectional composites by thermocompression. Composite specimens are tested under 3-points bending. Preliminary results show a stiffness of about 9 GPa and a strength of approximately 170 MPa for a fibres volume fraction of 22%. These mechanical properties are promising since they make this material competitive to substitute petroleum-based composites in secondary structural applications.

International audience; This study proposes to investigate the effect of moisture uptake on a wide spectrum of mechanical behaviours of unidirectional flax fibre-reinforced epoxy laminates. It includes tensile monotonic and fatigue testing. Results show that UD flax-epoxy composites, whenexposed to hygrothermal conditioning at 70°C and 85% RH, exhibit a diffusion kinetic which follows a one dimensional Fickian behaviour. The mass uptake at equilibrium is approximately 3.3 % and the diffusion coefficient 6.5 10-6 m2.s-1. Water vapour sorption is shown to induce a significant change in the shape of the tensile stress-strain curve. Contrary to all expectations, water saturation does not degrade the monotonic tensile strength of such a flax-epoxy composites and induces an increase in the fatigue strength for a high number of cycles

International audience; In Acoustic Emission (AE) applications, the processing of continuous signals, which results from high AE rates or superimposition of transients emitted from different emission sources, is a major problem. In complex systems like Organic Matrix Composites (OMC) fatigue tests, a high AE activity is produced due to the emergence of several emission sources. Such a kind of tests involves often a lot of noise that corrupts the original signal. Conventional threshold-based techniques are highly influenced by the noise level leading to erroneous hits detection. A suitable denoising method is thus necessary to process the signal before performing the hits detection and separation. In this work, continuous AE signals obtained from experimental tests carried out on Carbon Fiber Reinforced Plastics specimens were processed. A numerical routine was implemented allowing the treatment of these signals. As the size of each acquisition is large due to the sampling rate (generally from 2 to 5 MS/s), the signal was divided into short segments. The Discrete Wavelet Transform (DWT) was then used for signal denoising. Adjustment of their input parameters is achieved in order to improve the denoising process. Hits determination was thereafter performed in order to localize potential hits contained in each signal segment. By comparing obtained results to a usual threshold-based technique, we remark that the problem of erroneous hits is overcome. The performance of the proposed approach as well as the sensitivity to the denoising parameters were evaluated by studying the impact of errors in hit detection on feature extraction and on damage assessment based on pattern recognition algorithms. The proposed approach leads to a better identification of natural clusters in AEs and improves the interpretation of damage mechanisms.

National audience; Relations entre les angles microfibrillaires, les propriétés mécaniques et la composition biochimique de fibres de lin

International audience; The health assessment of composite structures from acoustic emission data is generally tackled by the use of clustering techniques. In this paper, the K-means clustering and the newly proposed Partially-Hidden Markov Model (PHMM) are exploited to analyse the data collected during mechanical tests on composite structures. The health assessment considered in this paper is made difficult by working in unconstrained environments. The presence of the noise is illustrated in several examples and is shown to distort strongly the results of clustering. A solution is proposed to filter out the noisy partition provided by the clustering methods. After filtering, the PHMM provides results which appeared closer to the expectations than the K-means. The PHMM offers the possibility to use uncertain and imprecise labels on the possible states, and thus covers supervised and unsupervised learning as special cases which makes it suitable for real applications.

International audience; Clustering of acoustic emission signals aims at interpreting the dynamical materials behaviour through solicitations. It is usually posed as problem of finding clusters with good shape and well separated in the feature space, where features are extracted from AE signals. We here propose an alternative which does not use such features. The methodology relies on anomaly detection and includes a criterion that optimises the spread of onsets of clusters in place of shape.

Retting of fibrous plants such as flax is an essential step in the extraction of fibre bundles and their transformation into textiles and reinforcement fabrics for use in garments and composites. Dew retting is traditionally performed from Northwest France to the Netherlands, as the climate is highly favourable for this process. Hemp is a plant that can be grown almost all over Europe with a low environmental impact. A retting step is also required to facilitate the separation of the hemp fibres before their transformation into textiles for garments or for 1D to 3D reinforcement composites, which requires thoroughly separated fibres. Dew-retting is currently used in flax production zones. The present work demonstrates that dew retting can be conducted under different climates on different soils, from dry Mediterranean environments up to the cooler regions of eastern France. If the ternary combination of moisture, temperature and solar radiation is appropriate, field retting (dew-retting) can be as short as about three weeks. In less favourable conditions, such as in dryer areas or when retting is performed late in the season after seed maturity (cooler temperatures), it lasts longer, but it can reach suitable levels. When conducted with care and with proper monitoring of the retting level, the dew-retting process does impact neither the tensile properties of elementary hemp fibres (by degrading crystalline cellulose I) nor the tensile properties of unidirectional and injected composite materials. Consequently, if extracted with a suitable process such as scutching and hackling, fibres suitable for load-bearing composites can be produced from dew-retted hemp stems produced in a wide range of climates and locations, therefore not limited to the conventional “dew retting zone” of flax production areas.

International audience