PSI - Issue 64

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

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rate of 0.001 mm/s to maximum loads well below the load capacity (Heirani, 2021). Four-probe resistance measurements were made throughout the experiments using an Aglient E4980 LCR meter at 300 kHz frequency (Figure 3). Fractional changes in resistance ( ∆ / 0 ), load, and strain (using 30 mm foil strain gauges) data were recorded and analyzed.

Fig. 3. Sensor specimens were tested under cyclic compression.

3. Results 3.1. Fiber breakage

The initial length of CFs used in this study was 6 mm, but their brittle nature resulted in shorter lengths after mixing. The mixer type had a significant effect on both fiber breakage and dispersion quality. Figures 4A and 4B show representative optical microscope images of post-dispersion fibers for two otherwise identical samples (0.5% CF, GU cement, medium mixing speed, and mixing method A) mixed with the Hobart and Centrifugal planetary mixers, respectively. Figures 5A and 5B show the corresponding fiber length distributions for the same two representative samples. These results indicate that CF lengths after mixing with the Hobart mixer were mostly in the 0 - 2 mm range, consistent with results from (Li and Obla, 1994), who found the most probable CF length to be about 1.3 mm. When a centrifugal mixer was used, CF lengths remained larger on average and a had a more uniform distribution. Similar results were found for all mixes.

Fig. 4. Optical images of fibers after mixing with (A) Hobart mixer and (B) Centrifugal mixer

Fig. 5. Fiber length distribution after mixing with (A) Hobart mixer and (B) Centrifugal mixer