PSI - Issue 64

Agostino Walter Bruno et al. / Procedia Structural Integrity 64 (2024) 1411–1418 Bruno et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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2.2. Thermal tests Thermal tests were performed on 40 mm × 40 mm × 160 mm prismatic samples according to the hot-disk transient plane source method as shown in Fig. 5a (Mankel et al., 2019b, Sam et al., 2022). The probe ( Fig. 5b ) consists of a 6.43 mm radius thin disk incorporating a double spiral of nickel enclosed in a Kapton capsule, which functions as both a heat source and a temperature sensor. A stepwise heat is generated by the probe sandwiched between two samples and this creates a dynamic temperature field within the material, whose temperature rise is measured as a function of time. The thermal conductivity and thermal diffusivity of the samples are subsequently evaluated by means of an analytical model depending on the type of probe and adopted boundary conditions. These measurements were taken after sample equalization at a temperature of 24 ± 1 °C and a relative humidity of 73 ± 5 %. The data were then processed using Hot Disk ® Software to evaluate the effect of earth type and fiber reinforcement on the thermal properties of the material.

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Fig. 5. (a) Schematic of hot-disk test set-up, (b) Hot-disk probe and (c) Hot-Disk device during measurements.

3. Results The mechanical and thermal proprieties of the different earth-flax fibers compositions were measured according to the previously described experimental procedures and are presented in the following sections. 3.1. Mechanical strength The tensile (bending) strength R f [MPa] of unreinforced and reinforced samples was measured from the three-point bending tests (EN 196-1, 2005) as: where b [mm] is the rib size of the beam cross section, F f [N] is the registered vertical load at failure (peak value), l [mm] is the distance between the beam supports. Fig. 6 reports the values of bending strength R f calculated by Eq. (1) for each earth-flax fibers composition. Growing fiber content results in an increase of the bending strength compared to the unreinforced material, which exhibits an elastic-fragile response. Unreinforced samples reach failure at values of bending strength equal to 6.39 MPa ( Bouisset ) and 11.30 MPa ( Nagen ), whereas reinforced samples with 0.50% and 1.0% fiber content attain noticeably higher values. The bending strength of Bouisset samples is consistently lower than that of Nagen samples because the illitic fraction of the Nagen earth develops stronger inter-particle capillary bonds than the kaolinitic fraction of the Bouisset earth. 3 1.5 f f F l  R b  =

(1)