PSI - Issue 28

A.M. Bragov et al. / Procedia Structural Integrity 28 (2020) 2174–2180 Author name / Structural Integrity Procedia 00 (2019) 000–000

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estimates, and at this level of knowledge about materials there are several points of view regarding the explanation of this effect. There is an opinion that dynamic hardening of brittle media is not a feature of the material, but is caused by inertia, which distorts the real conception about the resistance of material to force influence (Li and Meng (2003), Zhang et al. (2009), Li et al. (2009)). Therefore, the presented report reveals and analyzes the phenomena that occur in the process of testing concrete using split Hopkinson pressure bar method that allows one to achieve strain rates from 10 1 s –1 to 10 4 s –1 .

Nomenclature c

sound speed

Young's modulus of the measuring bars cross-sectional area of the measuring bars the initial cross-sectional area of the specimen

E A

A 0 L 0

the length of the specimen the incident compressive pulse the reflected tensile pulse the transmitted compressive pulse

 I ( t )  R ( t )  T ( t )

σ s ( t ) stress in the specimen ε s ( t ) strain in the specimen � � ���

strain rate in the specimen

2. Theoretical basics of the test method The split Hopkinson pressure bar set-up consists of two bars and a very short bar-insert (specimen) between them. In first of the bars, after the action of a striker, a one-dimensional elastic compression wave is generated, which propagates through the bars at a sound speed с . Upon reaching the specimens, this wave splits, due to the difference in cross-sectional areas and acoustic impedance of the bar and the sample material: part of it is reflected back and part passes through the specimen into the second bar. Moreover, due to the small length of the specimen, the wave propagation time along it is short compared with the duration of the loading pulse, which leads to multiple reflection of the waves and, therefore, a uniaxial stress state with a uniform distribution of stresses and strains along its length is realized in the test. This is a main assumption of the split Hopkinson pressure bar method. During loading, the specimen can be deformed up to fracture, while the bars are deformed elastically. By registering elastic waves in cross sections of measuring bars with strain gauges (a incident compressive pulse  I (t) , reflected tensile pulse  R (t) and a transmitted compressive pulse  T (t) ), it is possible to determine the processes of change in time of stress, strain and strain rate in the sspecimen using the formulas of the split Hopkinson pressure bar method.

Fig. 1. Scheme of dynamic specimen loading.