The research presented in this study focuses on understanding fundamental mechanisms that drive material response under dynamic loading conditions. The objectives of the research were to: (1) to understand damage initiation and propagation in the bulk geomaterial under a variety of loading conditions and (2) to systematically investigate the strain rate effects on the triaxial compressive response of cementitious materials through the development of an innovative, first of its kind large-diameter (50 mm) triaxial Kolsky bar system.
The triaxial compressive response of high-strength concrete is needed to understand pressure-dependent material behavior, which is important for modeling extreme loading events. However, non-destructive damage analysis and dynamic triaxial experiments require specimens that are smaller than those typically used for model calibration. Reducing the specimen diameter from 50 mm to 25 mm showed negligible differences in the material response of a high-strength concrete (no coarse aggregate). However, a scalar correction factor is proposed to account for reductions in length-to-diameter ratio (L/D). By isolating size effects, results from experiments with scaled specimens can be implemented for model calibration efforts.
This study also investigates how cracking and pore collapse in high-strength concrete develops under hydrostatic loading and triaxial loading with confinement pressures up to 200 MPa. The impact of changes in specimen length-to-diameter ratio on damage mode were also evaluated. For brittle failure modes, three-dimensional crack networks were segmented to determine damage distribution and the angles of primary failure planes. High-strength concrete specimens were scanned using X-ray microtomography in both the pristine and damaged conditions to quantify changes in porosity size distributions as a result of pore collapse and crushing. Additionally, damaged specimens were then evaluated for residual compression strength. It was observed that although peak stresses increase with reduced length-to-diameter ratios, the dominant failure modes are not substantially influenced.
Lastly, a triaxial Kolsky bar technique is provided to simultaneously investigate strain rate and pressure dependencies. A cylindrical specimen with diameter and length of 25.4 mm was investigated at quasi-static and dynamic strain rates with confining pressures up to 200 MPa. Annular pulse shapers were incorporated to ensure stress equilibrium under constant strain rate deformations. Furthermore, dynamic pressure variations were theoretically approximated and determined to be negligible. The dynamic increase factor was found to decrease as confining pressures increased. Additionally, a shift in the brittle-to-ductile transition point was also observed to show a more brittle failure mode under dynamic strain rates. Lastly, a dynamic failure surface is presented to illustrate the strain-rate and pressure dependencies of high-strength concrete.
Mechanical Engineering, Civil Engineering, Materials Science
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Williams, Brett, "Damage Evolution and High-Rate Response of High-Strength Concrete under Triaxial Loading" (2021). Mechanical Engineering Research Theses and Dissertations. 35.