70 J. Gao et al. Fig. 11.1 Schematic of the experimental setup In this work, we transversely loaded a pre-notched S-2 glass/SC-15 composite beam at a speed of ~6 m/s on a modified Kolsky compression bar. We visualized in real time the damage evolution inside the composite in front of the notch by virtue of high-speed X-ray phase contrast imaging technique. The experimental results were then compared with the damage progression captured by the optical imaging technique. 11.2 Materials and Methods S-2 glass fiber roving was provided by AGY World Headquarters and manufactured into unidirectional fabric. Each fabric was then brushed with SC-15 matrix (Applied Poleramic, Benicia, CA, USA) and then laid up to 24 layers. Composites were cured in a vacuum bag at room temperature for 24 h and given another 24 h for additional cure. Post-curing was conducted at 120 ◦C for 2 h without vacuum. The large panel was then cut by a diamond saw into smaller beams with a span (l) of ~18 mm and a width (b) of ~2.5 mm, referring to ASTM D5045-14 [8]. The height (h) of the beam was ~5 mm. The experimental setup is schematically described in Fig. 11.1. A notched composite beam specimen was mounted at the bar end through a designed fixture. The beam contacted with the fixture at two cylinders with a distance of 15 mm. During the experiment, the bar was impacted by a striker, driving the specimen to the indenter fixed ahead of a load cell. The indenter transversely loaded the specimen until the material failed. The synchrotron X-ray penetrated the specimen from the side and captured the crack geometry in real time, which was transferred as the optical signal by a scintillator, then magnified via a 5X optical lens, and finally sent to a high-speed camera for recording. 11.3 Results and Discussions Figures 11.2 and 11.3 compare dynamic failure of the pre-notched GFRC beam captured via the high-speed X-ray PCI technique and common optical imaging technique. Both of the two composites were stacked in the manner of [0◦/90◦]12. Obviously, X-ray penetrated the specimen and enabled to identify individual layers of the composite, while the optical method was only able to observe the surface morphology of the specimen. Once loading, the high-speed X-ray PCI technique captured in accuracy the notch opening, matrix cracking, fiber bridging, fiber/matrix debonding, fiber breakage, and delamination around the notch tip inside the composite. In comparison, the optical imaging technique can merely recognize roughly the opened notch and delamination around the notch tip. However, due to the limited size of the beamline, damage evolution inside the composite was only observed within an area of 2.56 mm×1.6 mm around the notch tip. In contrast, by changing the objective lens, the optical imaging technique was capable of revealing the delamination away from the notch tip.
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