In addition, fibers were evaluated untreated (FB), with alkali treatment, with silane treatment, and with silane treatment of the alkali-treated fibers ( Fig. 14.9). Composites were prepared by compression molding with fiber loadings up to 30 wt%. Both thermoplastic and thermosetting composites were prepared from polylactic acid (PLA) and an unsaturated polyester resin. Sawpan and coworkers conducted a detail investigation into the flexural properties of hemp fiber-reinforced composites. ![]() Similarly, several research groups studied the flexural properties of various biofiber-reinforced composites and surface treatments. It also shows that the chemical treatment on the fiber's surface increased the flexural strength and the modulus owing to better interaction between the surface-treated fibers and the polymer than the untreated fibers. It confirms that the flexural strength was higher than the tensile strength, as noted earlier. A comparison of the tensile and flexural properties of untreated kenaf fiber and treated (propionylated and succinylated) reinforced fiber is shown in Fig. 14.8. However, major defects on the surface will lead to different results. Therefore, it is usual for flexural strength to be higher than tensile strength for the same composites. If those fibers are free from defects, the flexural strength will be controlled by the strength of the “intact” fibers, whereas when the same fibers are subjected to tensile stress, all of the fibers are at the same stress and failure will occur when the weakest fiber reaches its limiting tensile stress. When the composite is bent, only the fibers at the extreme end of the composite are subjected to the largest stress. ![]() ![]() The flexural strength would be the same as the tensile strength if the biocomposites were homogeneous however, most biocomposites have small or large defects in them that act to stress concentration and cause local weaknesses. Flexural tests are performed primarily on composites and not fibers, unlike tensile tests. Flexural characterization is the second most performed mechanical test on biocomposites after tensile. It is based on two crucial properties: the elastic modulus of the material and the moment of inertia (function of geometry). Flexural stiffness is a standard for measuring deformability.
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