**1. Introduction**

In the 1830s, the use of plant resources such as flax, hemp and others was widespread as their fibers were in high demand by the textile, the paper and sailing industries. These plants were grown over large areas for exploitation. However, with the progress of science and technology (loom, steam engine, development of cotton harvesting and processing technique and others), materials such as metals, ceramics, glass, polymers, stones and concrete were preferred to plant resources. In 1900, fiber plants experienced their lowest implantation in terms of surface area. Indeed, the rise of new materials has greatly contributed to the improvement of human living conditions through the construction of more robust and sophisticated habitats, the development of the automotive, railway, aeronautics, textile industries, etc. Subsequently, in a concern for economy, lightness and performance, the development of composite materials was born during the 1930s.

The industrial use of plant fibers began in the early twentieth century with the manufacturing of aircraft seats, fuel tanks or other electronic boxes in plant fibers reinforced polymer materials. The need for securing constructions or structures that are made up of these materials inevitably arose. From an engineering viewpoint, this is taken into consideration during the design, due to a good knowledge of the material characteristics. Plant fibers have specific properties that make them good candidate reinforcing materials for high-performance composites and other applications [1]. However, the mechanical properties of PFs vary considerably both within the same species and from one species to another. Humidity variation, for example, leads to shrinkage or swelling that changes mechanical properties [2]. Similarly, their thermal properties are by far very different from those of synthetic fibers.

Various studies also indicate that plant fibers exhibit, for example, a very complex anisotropic behavior [3–5]. This anisotropy must be accounted for if a reliable design is to be achieved. Close collaboration between scientific disciplines such as botany, chemistry, biochemistry, molecular structural biology, plant genetics, physics and mechanics allows each of them to make a constructive and complementary contribution. PFs must withstand stresses of all kinds when they are associated with their deriving plants. They are loaded when it comes to supporting the weight of the plant or when it comes to resisting the winds, storms and hurricanes so common in their environment. PFs are diverse, and can all be studied for their use as engineering materials, in order to take benefit of the particular advantages offered by each of them. Meanwhile, their mechanical, physical and chemical characterization can differ between members of the same species and from one species to another. They are most often in the form of bundles (technical fibers) comprising one to twenty elementary fibers. They have a complex hierarchical structure inducing anisotropy and, have great geometric and mechanical variability. Humidity variation, for example, leads to shrinkage or swelling and changes in mechanical properties.

The characterization of material generally involves so-called monotonous tests (tensile, compression, torsion, bending or a combination) according to the load's direction (uniaxial or multiaxial), cyclic tests, hardness and resilience tests. Tensile test is undoubtedly the most common test applied to PFs [6–12] because it allows obtaining Young's modulus, strength and elongation at break. Recent works show that PFs exhibit a delayed behavior over time and temperature [13, 14] highlighting their viscoelastic nature. A tensile test alone is therefore not sufficient to characterize these materials.

This chapter is structured in four sections. Following this introduction, Section 2 will give an overview of some essential applications, the supply chain and the techniques of separating fibers from their plant. In Section 3, we will describe the experimental characterization methods generally used to derive their structure morphology and their elastic, viscoelastic and thermomechanical properties. Section 4 is the conclusion.
