The propagation speed of multiple dynamic cracks in fiber-reinforced cement-based composites measured using DIC

Abstract

The primary purpose of this work is to explore the propagation speed and spreading direction of multiple dynamic cracks in steel fiber-reinforced cement-based composites using Digital Image Correlation (DIC). Three-point bending beams with an off-centered notch were employed to facilitate the initiation of mixed-mode crack. Specifically, beams with different fiber contents, 0% , 0.4% and 0.8% in volume fraction, were manufactured. Tests were carried out at low loading-line displacement rates (2.2 μm/s and 22 mm/s) using a servo-hydraulic machine, under impact loading (1.77 m/s and 3.55 m/s) using a drop-weight impact device. A high-speed video camera was used to record the process of (multiple) crack propagation. Besides the shear crack started from the notch tip, the flexural cracks initiated from the central bottom surface were also formed in all beams except those with a fiber content of 60 kg/m3 (0.8% in volume ratio) impacted at 1.77 m/s. The flexural cracks propagated with a maximum speed of 1297 m/s, whereas the shear cracks reached up to 1149 m/s. Such high speeds of multiple-crack propagation, more than 50% of the Rayleigh wave speed, have been rarely measured in the lab. In addition, the evolution of simultaneous crack formation was recorded; the influence of both fiber content and loading rate is examined. Furthermore, all three types of material were previously characterized through independent tests, the flexural strength, the toughness indices as well as the residual strength factors are reported.