To investigate the mechanical properties of tunnel lining concrete under different moderate-low strain rates after high temperatures, uniaxial compression tests in association with ultrasonic tests were performed. Test results show that the ultrasonic wave velocity and mass loss of concrete specimen begin to sharply drop after high temperatures of 600°C and 400°C, respectively, at the strain rates of 10‒5s‒1 to 10‒2s‒1. The compressive strength and elastic modulus of specimen increase with increasing strain rate after the same temperature, but it is difficult to obtain an evident change law of peak strain with increasing strain rate. The compressive strength of concrete specimen decreases first, and then increases, but decreases again in the temperatures ranging from room temperature to 800°C at the strain rates of 10‒5s‒1 to 10‒2s‒1. It can be observed that the strain-rate sensitivity of compressive strength of specimen increases with increasing temperature. In addition, the peak strain also increases but the elastic modulus decreases substantially with increasing temperature under the same strain rate.
An optical measurement method of radial displacement of a ring sample during its expansion with velocity of the order 172 m/s and estimation technique of plastic flow stress of a ring material on basis of the obtained experimental data are presented in the work. To measure the ring motion during the expansion process, the Phantom v12 digital high-speed camera was applied, whereas the specialized TEMA Automotive software was used to analyze the obtained movies. Application of the above-mentioned tools and the developed measuring procedure of the ring motion recording allowed to obtain reliable experimental data and calculation results of plastic flow stress of a copper ring with satisfactory accuracy.
The article presents the results of investigation of ultra-strength nanostructured bainitic steel Fe-0.6%C-1.9%Mn-1.8%Si-1.3%Cr-0.7%Mo (in wt. %) subjected to shear and uniaxial compression under high strain rate loading. Steel of microstructure consisted of carbide-free bainite and carbon enriched retained austenite presents a perfect balance of mechanical properties especially strength to toughness ratio. Two retained austenite morphologies exist which controlled ductility of the steel: film between bainite laths and separated blocks. It is well established that the strain induced transformation of carbon enriched retained austenite to martensite takes place during deformation. Shear localisation has been found to be an important and often dominant deformation and fracture mode in high-strength steels at high strain rate. Deformation tests were carried out using Gleeble simulator and Split Hopkinson Pressure Bar. Shear and compression strength were determined and toughness and crack resistance were assessed. Susceptibility of nanostructured bainitic steel to the formation of adiabatic shear bands (ASBs) and conditions of the bands formation were analysed. The results suggest that the main mechanism of hardening and failure at the dynamic shearing is local retained austenite transformation to high-carbon martensite which preceded ASBs formation. In the area of strain localization retained austenite transformed to fresh martensite and then steel capability to deformation and strengthening decreases.