# Strain Rate Dependencies and Competitive Effects of Dynamic Strength of Some Engineering Materials
## 2 Materials and Methods
### Experimental Methods  
Uniaxial compression or splitting (indirect tension) tests of cylindrical specimens were considered in this study (Figure 1). Splitting tests are applicable only to brittle and quasi-brittle behaviors of materials. Therefore, both schemes were used only for quasi-brittle materials. Metallic materials were subjected only to compression tests. Tests of bitumen were considered only for the case of compression at room temperature and splitting at a negative temperature (-10 \({}^{\circ}\)C), because this is a typical loading of asphalt concrete under appropriate weather conditions.  
The purpose of the experiments was to determine the critical stress leading to macro-failure for each loading rate. The measured loads and displacements made it possible to establish stresses in the cylindrical specimens according to known formulas for mechanics of materials [34; 35] and fracture mechanics [36].  
A standard testing machine consisting of a temperature chamber was used for tests under slow strain rates (10\({}^{-3}\)-10\({}^{-2}\) 1/s). At least three specimens were tested in the case of quasi-brittle materials  
Figure 1: Test schemes for cylindrical specimens: (**a**) compression; (**b**) splitting (\(\sigma_{t}\) is the tensile stress).  
for each loading scheme. Two samples of each metallic material were used in the compression tests. Quasi-static testing of bitumen was not considered in this paper.  
For high strain rate testing, a split Hopkinson pressure bar (SHPB) setup and a drop tower impact tester machine (DTIM) (CEAST 9350, Instron, Turin, Italy) were used.  
The DTIM allowed testing specimens at strain rates of 10-10\({}^{3}\) 1/s. A flat-tipped impactor with a diameter of 50 mm was used for compression and splitting tests.  
The SHPB allowed testing specimens at strain rates of 10\({}^{2}\)-10\({}^{4}\) 1/s. In the setup, aluminum rods (D16) with a diameter of 20 mm were used (SHPB-20). Details on material testing using the SHPB, requirements for specimens, data acquisition and processing are given in [2; 37; 38; 39]. Mandatory requirements for testing and specimens to eliminate the influence of inertial effects in a specimen, lateral confinement and friction effect in the process of radial distribution of a specimen were met. The sizes of the specimens and the load pulse shape were selected to ensure an equilibrium state at both contacts specimen-bars and to minimize inertial effects.  
Note that both dynamic test units provide continuous load growth, which is the same as a standard quasi-static test machine. Therefore, in this work we consider only cases when failure or plastic flow begins at the stage of load growth. Pulse loads, for example, as in the spall test procedure, should be considered in a separate study.  
The loading/stress rate under dynamic loads varied as a result of a change in impactor speed. There are no generally accepted requirements for the number of specimens in dynamic tests; however, for these experiments, three specimens of a quasi-brittle material and two specimens of a metallic material were tested for each loading rate. Due to the technical features (acceleration of impactor, implementation of contact between the specimen and impactor, formation of a load wave, etc.) of the dynamic test equipment, it is difficult to ensure accurate repetition of a load profile. For this reason, along with the failure features of quasi-brittle materials, the loading or strain rate may differ for the same stipulated impactor speed.