## 3 Results and Discussion
### Mechanical Properties of Composites
The elongation at break of all the samples is displayed in Figure 12b, where it is obvious that a premature fracture of all the composites has taken place, since although HDPE's value is 716.7% \(\pm\) 24.8%, all the composites' values range from 1.7% to 5.5%. So, the mechanical behavior of the material has been totally altered, since fracturing has taken place without significant plastic deformation or elongation, known as brittle fracturing. The tensile stress at break of all composites (Figure 12c) is lower than the corresponding stress of neat HDPE due to the microstructure of the composites, since inadequate interfacial bonding is achieved [75], and structural irregularities may have been introduced during processing. More specifically, due to poor matrix-fiber adhesion, as indicated by the SEM microphotographs, the stress transfer at the polymer-fiber interphase is inefficient, while voids and fiber agglomerates promote the premature failure of composites when compared to the neat matrix. However, with the increase in fiber content, there are three different behaviors observed in the studied composites. Hemp-based composites become stronger,  
Figure 12: Mechanical properties of the prepared HDPE composites: (**a**) Young’s modulus, (**b**) elongation at break (%), (**c**) tensile stress at break (%), and (**d**) impact strength (J/m). -flax-based composites exhibit minute strength alteration, and ramie and kenaf composites become less strong. Generally, parameters affecting the strength of composites are related to filler-matrix interfacial adhesion, filler size, and filler loading, while the interplay between the abovementioned three factors can lead to various trends when referring to the effect of filler loading on the strength of composites [75]. At low-weight percentages of lignocellulosic fibers, the HDPE_10_hemp composite displays a low-tensile stress-at-break value, probably due to the fact that hemp fibers, with diameters of 24.2 \(\pm\) 11.1 \(\upmu\)m, probably are not well distributed and form agglomerations, while as the weight percentage of fibers increases, a better distribution of fibers may be achieved, with a more interconnected fiber network, leading to enhanced load-bearing behavior. Flux-based composites maintain approximately the same strength value upon filler increase, due to a combination of factors including the fibers' diameter, dispersion degree inside the matrix, and fiber-matrix interaction. Fibers' small diameters make a distinct contribution to the formation of a well-interconnected network at all filler contents, while the high impurity contents on the fibers' surface may reinforce fiber-matrix interfacial bonding and a 4.4% mass loss of fibers at processing temperatures, which may have caused the formation of voids in the composite. Furthermore, [PERSON] et al. [76] have proposed that during the evaluation of tensile strength at break, small differences in respective values for low filler contents are related to the small impact of fibers on the matrix due to the small interfacial area, while for higher filler contents, an enlargement of the interfacial area and increased interfacial interactions between the matrix-filler are caused. On the contrary, kenaf-based composite behavior may be associated with a variety of factors. The weak interfacial interaction of the matrix-fiber, as evidenced by the SEM microphotographs, leads to stress-concentration points or defects, reducing the strength. Furthermore, the elevated moisture content of fibers, as evidenced by TGA, may lead to the formation of big voids during processing, negatively affecting the mechanical properties of respective composites. The relatively low cellulose crystallinity, which is the component conferring strength to the fibers, and the greater diameter among the studied fibers, which has a negative effect on dispersion state, have resulted in composites where the reinforcing agents have difficulty in carrying loads as effectively as the rest of the composites.