## 1 Introduction  
The use of composite materials is becoming more and more widespread each year in various sectors of the industry [1], due to the development of technical applications with ever-increasing performance requirements and ever-stronger constraints on raw material consumption. Constraints to which composite materials are particularly well adapted, given their combination of reinforcements of different natures and a matrix allowing fine-tuning for each specific application. In the frame of agro-composites, many types of fibers can be considered usable [1, 2, 3, 4, 5], but the present study will focus on the nettle fibers only [6] (Figure 1). Nettle is part of the Urticaceae family and belongs to the genus Urtica [7], which comprises more than 70 species. In Europe, 11 species of nettles are listed, 5 of which are common in France: it is, therefore, a locally available resource, although little exploited at present. Long used for its medicinal properties [8; 9], it was gradually forgotten during the 19 th century before a revival of interest during the First World War in the textile industry for the manufacturing of tents, bags, and clothing due to supply shortages of conventional fibers such as cotton, hemp, and linen [10; 11]. Nevertheless, the increasing industrialization of agricultural processes that followed the war did not apply to nettle, and labor costs increased sharply. Consequently, the plant fell into oblivion and only recently came back into the limelight due to current environmental and societal concerns. The quality of natural nettle fibers depends on the growing conditions, extraction, treatment, and processing, with high variability in mechanical properties due to the chemical composition of the fibers [12; 13; 14] and the difficulty in obtaining an accurate estimation of the cross-section of these fibers [15]. It varies from one species to another and even often from one plant to another within the same species and the same crop [10; 16; 17]. The main constituents of lignocellulosic fibers are cellulose, hemicellulose, pectin, and lignin [18]. Cellulose is a natural polymer consisting of long glucose chains in crystalline (80%) or amorphous (20%) form [19]. Hemicellulose and pectins are the second major polysaccharide constituents of natural fibers and constitute the interface between cellulose fibers. Lignin is a phenyl propane-based polymer combined with polysaccharides. More resilient, it is one of the most abundant biopolymer constituents on Earth after polysaccharides; it provides structural rigidity to the cell walls and limits microbiological attacks [20]. Although these different chemical elements are the base components of natural fiber, the overall organization of the fiber also plays a major role in its macroscopic properties [10; 21; 22; 23] and explains the craze for nettle fiber. Our choice to use PLA (poly-lactic acid) as the matrix for this agro-composite is explained by the desire to achieve an entirely natural and therefore completely biodegradable material. Among all thermoplastics, PLA has undoubtedly experienced the fastest growth in its use in recent years, not to mention that it is also one of the latest to be widely commercially available, in 2002 [24]. Its main advantage is that it is completely biobased and biodegradable (or at least compostable under industrial conditions). This category only comprises a few other materials, such as starch, PHA (poly-hydroxy-alkanoates), cellulose acetate, and regenerated cellulose [25]. PLA is obtained from the extraction of starch from potatoes, corn, or beets. After a fermentation step, lactic acid is obtained to form lactides, which can then be distilled and polymerized to form PLA. One of the major properties of PLA is its molecular weight, the higher the molecular weight, the longer the polymer chains and the better its temperature and mechanical resistance. However, the temperature remains a parameter to be monitored when using PLA.