PSI - Issue 64

Niloofar Heirani et al. / Procedia Structural Integrity 64 (2024) 6–13 Author name / Structural Integrity Procedia 00 (2019) 000–000

8

3

Table 2. Properties of K6371T coal tar pitch-based carbon fibers Length (mm) Diameter (µm) Carbon content Specific gravity Tensile strength (GPa)

2.3×10 −4

Electrical Resistivity (Ω -cm)

6

11

>99%

2.12

2.6

Fig. 2. Three mixing procedures were examined.

2.2. Microscopy analysis The conductivity and piezoresistive properties of CFCP sensors are highly influenced by the quality of CF dispersion and the post-dispersion lengths of the fibers. Fiber breakage was examined following the procedure described by (Li and Obla, 1994). A 0.2 gram sample of fresh mix was placed on a filter paper. Next, water was passed through the filter very slowly until most cement particles were washed off. The wet fibers were then allowed to dry on the filter paper. Images were taken using an optical microscope and the individual fiber lengths were analyzed using the open source software ImageJ. To evaluate fiber dispersion, images of thin sections, prepared from three perpendicular cut planes of cured samples, were used (Heirani, 2021). Backscattered images were taken using a scanning electron microscope (SEM) and thresholding was performed in ImageJ to detect the carbon fibers and measure their nearest distances. 2.3. Samples and experiments Eighteen sets of CFCP sensor samples (prismatic with dimensions: 40 mm × 40mm × 160 mm) were prepared and furnished with copper mesh electrodes. All specimens were de-molded after 24h and then cured for at least 2 months before testing. To evaluate their strain sensing properties, samples were subjected to cyclic compressive loading at a