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International audience; The stinging nettle Urtica dioica L. is a perennial crop with low fertilizer and pesticide requirements, well adapted to a wide range of environmental conditions. It has been successfully grown in most European climatic zones while also promoting local flora and fauna diversity. The cultivation of nettle could help meet the strong increase in demand for raw materials based on plant fibers as a substitute for artificial fibers in sectors as diverse as the textile and automotive industries. In the present review, we present a historical perspective of selection, harvest, and fiber processing features where the state of the art of nettle varietal selection is detailed. A synthesis of the general knowledge about its biology, adaptability, and genetics constituents, highlighting gaps in our current knowledge on interactions with other organisms, is provided. We further addressed cultivation and processing features, putting a special emphasis on harvesting systems and fiber extraction processes to improve fiber yield and quality. Various uses in industrial processes and notably for the restoration of marginal lands and avenues of future research on this high-value multi-use plant for the global fiber market are described.

The development of sustainable sandwich materials is needed in the transportation sector to address environmental concerns related to the production and operation of vehicles. In addition to biobased composite skins, alternatives to classic synthetic core materials must be found to reduce the ecological footprint of whole sandwich-structured composites. This study focused on three eco-friendly lightweight core materials: balsa wood, paper honeycomb, and recycled PET foam. The effect of the hygrothermal ageing on their shear creep/recovery behaviour has been here investigated. Two different environmental conditions were tested: 23 °C-50% RH and 70 °C-65% RH. The results indicate that the maximum shear strain, the time-delayed strain and the residual strain increase for the three core materials with the severity of the hygrothermal conditions. This was attributed to the softening of the constitutive polymeric materials of the cell walls at temperatures close to 70 °C. The balsa wood exhibits the best creep resistance under the two environmental conditions. The identification of the viscoelastic properties highlights that the release times and the shear viscous parameter of the balsa wood and the PET foam depended on the stress level and the hygrothermal conditions.

This work investigates the technical requirement for the actuation of the bi-directional rotational motion (BRM) of engineering-material-based non-rigid origami robots. While the vast majority of previously published results have focused on paper-based origami structures driven by translation-motion, polypropylene (PP) is implemented in this research to investigate its ability to respond to engineering requirements according to BRM. Following this objective, three experiments are proposed to identify the technical performances of PP-based origami and kirigami robots based on Kresling pattern. First, the stabilization test shows that two hundred full folding cycles are required to reach a repeatable mechanical response. Second, the BRM test characterizes the various mechanical performances of both origami and kirigami structure: the PP-based origami outperforms existing structures in the literature. Third, the actuation test shows that the actuation mechanical requirements can be described using three key parameters: the required torque for folding, the shape-blocking stiffness, and the bistable portion. Finally, in order to support the development of PP-based origami/kirigami robots, a ‘Bar and Hinge’ reduced-order model is implemented for the description of the nonlinear hysteretic behavior and bistability. This method constitutes a useful tool for the design of highly nonlinear/bistable engineering structures based on PP origami and kirigami.

This work describes the physical and mechanical characterization of unidirectional [0]12 and crossply [(0/90)3/ 0 ‾ ]S flax fibre reinforced composites fabricated in autoclave using a prepreg flax tape impregnated with fire retardant epoxy polymer. Tensile, bending and impact properties are evaluated along the longitudinal and transverse fibre directions. The tensile-tensile fatigue behaviour is characterised along the fibre direction. Physical and specific properties are also assessed to identify the potential characteristics of these bio-based composites for lightweight and secondary loadbearing applications. The robust manufacturing process described in this work, coupled with precision laser cutting, makes this type of composite a promising sustainable material for aircraft, transport and lightweight construction designs.

International audience; Acoustic emission (AE) is a passive monitoring technique used for learning about the behavior of an engineered system. The streaming obtained by continuously recording AE transient signals is treated by a four steps procedure : 1) The detection of salient AE signals by distinguishing noise against non-noise signals using wavelet denoising, 2) the statistical representation of randomly selected AE signals using Autoregressive Weakly Hidden Markov Models, 3) an inference phase by applying those models to unknown AE signals and generating a set of novelty scores reflecting differences between signals, 4) the clustering of novelty scores using constraint-based consensus clustering. Compared to the standard way relying on the transformation of all AE signals by manual feature engineering (MFE) before clustering, the main breaktrough proposed in this paper holds in the use of the raw AE signals, with different lengths and various scales, to build high level information and organise the low level streaming data. Validated first on simulated data, we show the potential of this methodology forinterpreting acoustic emission streaming originating from composite materials.

International audience; This work assesses the potential of nettle (Urtica dioica L.) fibers produced on contaminated lands for composite applications. The nettles studied in this work grew spontaneously and in a prevalent manner in poplar short rotation coppice planted for the phytomanagement of a land contaminated by traces of metals. Results show that the contaminant contents in nettle bast fibers are low: only traces were measured. It makes it possible to consider this biomass for material use. The measured matter yield is lower than those obtained with traditional fiber crops cultivated in Europe on agricultural lands but the tensile properties of the bast fibers are equal to or better than those of hemp and flax, making spontaneous nettle an interesting supplement to traditional European fiber crops for composite applications

International audience; Vibro-impact (VI) based control strategies exploit non-linear phenomena occurring during impacts between oscillating masses and vibrating structures to mitigate vibrations. They are known to be effective on a wide frequency range, in harsh environnement, and they are eco-friendly in comparison to some visco-elastic polymers used for the same application. Different assumptions emerge to explain energy dissipation such as material and contact effects, or dynamic energy transfers between the structure and the oscillating masses acting as non-linear absorbers. As classical vibratory measurement tools do not allow to accurately and easily measure what happens in the contact area and the motion of the absorber, it is difficult to validate and quantify the phenomenon. The purpose of the study is to take advantage of full field measurements of the vibro-impact phenomenon to characterise a vibro-impact absorber and to better understand the interactions occurring during impact. To reach this objective, a dedicated experimental set-up has been designed and a method involving Digital Image Correlation (DIC) and a high-speed camera is used to capture and reconstruct the motion of the oscillating mass. The characterisation allows to estimate the Coefficient Of Restitution (COR) and friction forces, and provides data that are used in two different contact modelling strategies based on the COR and on the Hertz contact theory. The quantities identified in this paper enable to better understand the physical

International audience; The sensitivity of AE sensors makes the AE technique very interesting for detecting damages at various scales as encountered in complex heterogeneous materials made of various constituents with different damage kinetics. However, there are unknowns behind the generation process of AE signals, as well as behind the modification of those signals along the propagation path until the sensors. Those make the interpretation of AE signals a difficult task in terms of pattern recognition. Even though some damage families are expected to occur for a given material under specific loading, there is a lack of knowledge to bridge the gap between AE signals collected on sensors with the related AE source. This accounts for the use of unsupervised learning when one is interested to discover relationships between AE data.When using clustering for unsupervised learning, the output is a set of clusters representing a set of labels assigned to each datum and forming a partition. In AE work, clusters are estimated using features, computed from AE signals. Feature extraction has become a must-to-do step due to the use of particular clustering methods and to the fact that AE streaming has not been particularly analysed for damage sources identification. The contribution of this paper is a new methodology for AE signal clustering which does not require feature extraction as usually done. It works directly on raw AE signals derives from several tools of the literature, namely unsupervised shapelets, anomalydetection and statistical modelling by Autoregressive Weakly Hidden Markov Models. When used in inference on unknown signals or streaming, those models allow to generate a set of novelty scores which are then processed by a chronology-based clusteringalgorithm to get a partition, accompanied by the uncertainty around clusters and a quantification of the robustness of the results obtained.

International audience; Viscoelastic materials are widely used to control vibrations. However, their mechanical properties are known to be frequency and temperature-dependent. Thus, in a narrow frequency bandwidth, there is an optimal temperature that corresponds to a maximum loss factor and it is tricky to get a high damping level over a wide frequency range. Furthermore, an optimal temperature for a maximum structural damping leads to a low static stiffness because the peak of the loss factor is obtained during the glass transition when the storage modulus is decreasing. In order to obtain a compromise between stiffness and damping it is suggested to use a viscoelastic material which properties are functionally graded thanks to a non-uniform temperature field over the structure. In this work, a composite structure has been designed integrating a viscoelastic core and a heat control device. The optimal temperature field has been obtained through the minimization of a cost function that reflects the compromise between structural damping over a wide frequency band and high static rigidity. The experimental validation has been performed on a reduced scale airplane model: the composite wings are sandwich structures made of aluminum skins and a viscoelastic core in tBA/PEGDMA with a non-uniform temperature field and skins are in an aluminum and FR-4. A broadband excitation is produced with a shaker and the measurements are performed with a set of accelerometers. Several temperature fields are tested. The frequency response functions show the compromise obtained between static and dynamic behaviors when using the optimal temperature field determined by numerical simulation.

International audience; This work assesses the potential of stinging nettle (Urtica dioica L.) growing on trace element contaminated soils to produce fibres for material applications. The nettles studied in this work grew spontaneously and dominated the vegetation cover in poplar short rotation coppices planted for the phytomanagement of lands contaminated by trace elements. Two sites were studied, contaminated by Hg for the first one and a mix of As, Cd, Pb and Zn for the second one. Results show that, for the considered soils, the contaminant contents in nettle bast fibres were at low levels, comparable to those collected at unpolluted control areas. It makes it possible to consider this biomass for material use. The measured matter yield was lower than those obtained with traditional fibre crops cultivated in Europe on agricultural lands. However, the tensile properties of the bast fibres mechanically extracted without field retting or prior alkaline treatment were equal to or better than those of industrial hemp and flax, making spontaneous nettles an interesting supplement to traditional European fibre crops for material applications.

Although the research and development in plant fibre composites is growing rapidly, these materials still require specific considerations before being adopted by industry in structural applications. One of the main issue is related to their durability. The objective of this study is to investigate the influence of hydrothermal ageing on the fatigue behaviour of an unidirectional flax-epoxy laminated composite by implementing the fatigue tests in a water bath. Results show that while the quasi-static strength and rigidity are significantly affected by the ageing, the fatigue strength is however improved. The fatigue strength coefficient is more than two times lower after ageing, indicating a lower decrease of the maximum stress with the increasing number of cycles. After ageing, the maximum stress level for an expected lifetime of 5.3 106 cycles is approximately 90 MPa, which is comparable to the one of unaged specimens. It is also pointed out that the remaining quasi-static tensile properties and behaviour of the aged specimen are not significantly affected after 1.3 106 cycles at a stress level of 90 MPa.

International audience; Even if the knowledge on the fatigue behaviour of plant fibre composites has increased steadily in the last few years, some issues still remain open at the present time. Such is the case, for instance, of the high-cycle fatigue strength. Actually, most of the fatigue studies available in the open literature to date are limited to a maximum of 1-2 million cycles. All available stress-life plots exhibit linear trends with constant slope and does not reveal any fatigue limit for these given cycle numbers. So, this paper proposes to investigate the High-Cycle Fatigue behaviour of a flax-epoxy laminated composite. The effect of loading frequency is firstly evaluated on the Low-Cycle Fatigue behaviour using a multi-instrumented analysis including infrared thermography and acoustic emission. Results show that high frequency could be a suitable method to shorten the fatigue tests and study the High Cycle Fatigue behaviour of this type of composite material. Based on this result, high-frequency (30 Hz) is used to investigate the behaviour of the flax-epoxy composite on a range of 106- 108 cycles. Results show that fatigue damage continues to evolve and the maximum stress continues to decrease as a function of increasing number of cycles, following a power-law trend. This result suggests that, if a fatigue limit does exist for unidirectional flax-epoxy composite laminates, it is so low that it cannot observed in tests up to 108 cycles. It is also recommended to take it into consideration when designing plant fibre composite structures.

International audience; Are nettle fibres produced on metal-contaminated lands suitable for composite applications ?