(Dr. Min Zhou, advisor)
"An Experimental Study of the Constitutive and Failure Behavior of Concrete and Mortar under Impact Loading"
The need to understand the dynamic behavior of concrete and mortar at high strain rates is of critical importance in a range of applications including airport runways and structures subject to blast or penetration. Under dynamic conditions, the strain-rate dependence of material response and high levels of hydrostatic pressure cause the material behavior to be significantly different from what is observed under quasistatic conditions. This thesis focuses on two aspects of the behavior of concrete and mortar: (1) the dynamic stress-carrying capacity at strain rates above 104 s-1 and pressure above 1GPa and (2) the dynamic failure behavior under conditions of uniaxial strain generated by planar impact.
The experimental analyses use plate impact and split Hopkinson bar. Normal impact experiments involving impact velocities up to 482 ms-1 are conducted. Experiments show that the stress-carrying capacity of the materials depends strongly on loading rate and hydrostatic pressure. The marked increase in stress is attributed to the effect of higher strain rates which are on the order of 104 s-1 and to the effect of lateral confining stresses, which are on the order of 1 GPa. The effect of material inhomogeneity on the measurements is also analyzed experimentally.
The analysis of failure behavior focuses on the possible existence of clearly defined failure waves in mortar similar to those in brittle materials, which involve the complete loss of tensile strength under conditions of uniaxial compressive strains during planar plate impact. Experiments conducted do not provide evidence to support the theory of failure wave occurrence in mortar. However, a gradual failure process commencing with initial loading wave is encountered.