The paper at hand focuses on the tensile behavior of ductile cementitious composites reinforced with short, randomly distributed, polymer fibers and a continuous carbon textile under quasi-static and impact loading. Strain-hardening cement-based composites (SHCCs) made of high strength fine-grained matrix with the addition of a 2% volume fraction of 6 mm-long ultra-high molecular weight polyethylene (UHMWPE) fibers and as-spun poly(p-phenylene-2,6-benzobisoxazole) (PBO-AS) fibers, respectively, were reinforced with one layer of carbon textile, which corresponds to a 0.68% volume fraction. The same fine-grained matrix reinforced with carbon textile only served as the reference material. The synergetic action of the two reinforcement types was investigated in uniaxial tension tests on composite specimens, as well as by means of single-yarn pullout tests at displacement rates of 0.05 mm/s in a hydraulic testing machine, and 8 m/s in a tensile split Hopkinson bar. The specimen’s deformations, the formation of cracks, and the fracture processes were monitored optically and subsequently evaluated using digital image correlation (DIC).

The determination of crack propagation velocities can provide valuable information for a better understanding of damage processes of concrete. The spatio-temporal analysis of crack patterns developing at a speed of several hundred meters per second is a rather challenging task. In the paper, a photogrammetric procedure for the determination of crack propagation velocities in concrete specimens using high-speed camera image sequences is presented. A cascaded image sequence processing which starts with the computation of displacement vector fields for a dense pattern of points on the specimen’s surface between consecutive time steps of the image sequence chain has been developed. These surface points are triangulated into a mesh, and as representations of cracks, discontinuities in the displacement vector fields are found by a deformation analysis applied to all triangles of the mesh. Connected components of the deformed triangles are computed using region-growing techniques. Then, the crack tips are determined using the principal component analysis. The tips are tracked in the image sequence and the velocities between the time stamps of the images are derived. A major advantage of this method as compared to the established techniques is in the fact that it allows spatio-temporally resolved, full-field measurements rather than point-wise measurements. Furthermore, information on the crack width can be obtained simultaneously. To validate the experimentation, the authors processed image sequences of tests on four compact-tension specimens performed on a split-Hopkinson tension bar. The images were taken by a high-speed camera at a frame rate of 160,000 images per second. By applying the developed image sequence processing procedure to these datasets, crack propagation velocities of about 800 m/s were determined with a precision in the order of 50 m/s.

The ductile behavior of strain-hardening cement-based composites (SHCC) under direct tensile load makes them promising solutions in applications where high energy dissipation is needed, such as in earthquakes, impacts by projectiles, or blasts. However, the superior tensile ductility of SHCC due to multiple cracking does not necessarily point to compressive and shear ductility. As an effort to characterize the behavior of SHCC under impact compressive and shear loading relevant to the aforementioned high-speed loading scenarios, the paper at hand studies the performance of a particular SHCC and its constituent, cement-based matrices using the split-Hopkinson bar method. For compression experiments, cylindrical specimens with a length-to-diameter ratio (l/d) of 1.6 were used. The selected length of the sample led to similar failure modes under quasi-static and impact loading conditions, necessary to a reliable comparison of the observed compressive strengths. The impact experiments were performed in a split-Hopkinson pressure bar (SHPB) at a strain rate that reached 110 s⁻¹ at the moment of failure. For shear experiments, a special adapter was developed for a split-Hopkinson tension bar (SHTB). The adapter enabled impact shear experiments to be performed on planar specimens using the tensile wave generated in the SHTB. Results showed dynamic increase factors (DIF) of 2.3 and 2.0 for compressive and shear strength of SHCC, respectively. As compared to the non-reinforced constituent matrix, the absolute value of the compressive strength was lower for the SHCC. Contrarily, under shear loading, the SHCC demonstrated higher shear strength than the non-reinforced matrix.

This publication concentrates on the photogrammetric crack width measurement of crack patterns of concrete probes under impact loading in high-speed stereo image sequences. The presented algorithm works for non-planar specimens with deformations that only appear tangential to the surface and the method is based on triangle mesh analysis. Experiments were conducted with cylindrical specimens with an impact load affecting parallel to the main axis of the cylinder.