PSI - Issue 64

Chiara Pepi et al. / Procedia Structural Integrity 64 (2024) 1896–1903 Maria Eleonora Pipistrelli / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Materials and methods 2.1. Vegetal fibers and treatments

Spanish Broom and Hemp fibers samples of 15 mm are used in this experimental investigation. Hemp fibers are manually extracted from hemp ropes supplied by the French based company Naturellement Chanvre . Spanish Broom fibers are supplied by Fabbrica Tessile Bossio, based in Italy. The fibers undergo an alkali treatment using a 250 mL sodium hydroxide (NaOH) solution with a concentration of 1.25 mol cm -3 . Both hemp and Spanish broom fibers are soaked in the NaOH solution for different durations: 30 minutes, 1 hour, 3 hours, 6 hours, 18 hours and 24 hours, keeping the same solution concentration throughout the treatments. Sample are then allowed to air dry. For each treatment time, 3 samples of each fiber type are tested to have some information on variability.

Fig. 1: (a) The optical microscope used; (b) example of a Spanish broom fiber observed with optical microscope. Subsequently, effects of graphene coating are explored. Graphene is initially dispersed in 150 mL of distilled water to create the solution, with a concentration of 25 mg/mL. Post-alkali treatment, fibers are soaked into 20 mL of graphene solution until the solvent is completely evaporated. For each treatment duration, 3 samples of both hemp and Spanish broom fibers are tested. Additional tests are performed on rough fibers and on graphene coated hemp and Spanish broom fibers without prior alkali treatment to have reference data. 2.2. Characterization and experimental tests Prior to experimental tests, f ibers’ cross-section areas are estimated measuring average apparent diameters and assuming circular shape. For each fiber, the apparent diameter is evaluated as the mean values of three different measurements taken along the fiber using an optical microscope (see Figure 1-a,b). Apparent diameters of selected fibers range from about 50 μ m to 105 μ m, Scanning electron microscopy (SEM) analysis is carried out using a Gemini LEO 1525 SEM. SEM images captured before experimental tests are shown in Figure 2-a,b,c,d. These are captured after performing metallization procedure with chrome. Tensile tests are performed on samples of hemp and Spanish broom fibers according to ASTM D 3379-75 standards, using a Lloyds universal testing machine equipped with a 20 N load cell and connected to a Nexygen software for data acquisition. According to the standard protocol, fibers are placed on a self-adhesive paper frame (Figure 3-a), securely clamped to the testing machine, and cut before the experimental tests (Figure 3-b,c). Each experimental group consists of three samples of: untreated fibers, fibers subjected to varying durations of alkali treatment, fibers coated with graphene after the alkali treatment and fibers coated with graphene without any prior treatment.