Spall experiments for the measurement of the tensile strength and fracture energy of concrete at high strain rates

To study the behavior of concrete under dynamic loads, a Hopkinson-Bar was set up and used. Cylindrical concrete specimens were positioned at the end of the incident bar and the spall event was studied. The purpose of this contribution is to explain the measurement of the tensile strength and the specific fracture energy. To determine the tensile strength, the measured free surface velocity at the end of the specimen is used. The method is known from plate impact experiments and was adapted to Hopkinson-Bar experiments. The measurement of the specific fracture energy is more difficult in spall experiments. It cannot be measured directly as it can be done in direct tension tests. A method is proposed where the fracture energy is calculated from the change of the fragment velocities while cracking takes place. The experimental results of the investigation complete the data of the literature in regard to higher strain rates. In former investigations conducted by Weerheijm (PhD thesis. Delft University of Technology: Delft University Press; 1992), an increase of the specific fracture energy with the strain rate or the crack opening velocity was not seen. The experiments performed within this contribution consider the fracture behavior at higher strain rates. A sharp increase in the specific fracture energy at this strain rates was measured. The following paper describes the method and the experiments to measure the tensile strength and the specific fracture energy in spall experiments.