PSI - Issue 6

A.M. Bragov et al. / Procedia Structural Integrity 6 (2017) 161–167 Author name / Structural Integrity Procedia 00 (2017) 000–000

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Based on the time history of stress obtained in each splitting test, values of the following mechanical properties of materials were determined: maximal tensile stress and time to failure corresponding to the maximal stress for different stress rates. Stress increase rate was taken to be equal to the angular coefficient of the line approximating the rising (practically linear) part of the tensile stress time history. The diagrams of the above mechanical characteristics as a function of the value of stress increase rate, constructed based on the experiments, showed that, when the latter increases, the materials harden and time to failure decreases, similar to compression. To model the dynamics of failure of the tested materials, the incubation failure time theory was used, as developed in the works by N.F. Моrozov and Yu.V. Petrov (Petrov and Utkin (1989), Petrov and Morozov (1994), Petrov et al. (2003)). Response of the tested material to applied dynamic loading, which is characterized by its amplitude and rate, can be described using parameters of the incubation time criterion. In particular cases of failure, the incubation time criterion has the following form: • compression failure (average compression stress in the specimen as a function of time is integrated) ( ) comp cr t t t dt σ σ τ τ ′ ′ ≤ ∫ − 1 • tensile failure (tensile stress in the center of the specimen as a function of time is integrated) ( ) tens cr t t t dt σ σ τ τ ′ ′ ≤ ∫ − 1 In these formulas, σ cr comp and σ cr tens are static compression and tensile strengths, respectively; τ is incubation failure time. The values of incubation failure time determined for fine-grain concretes and rocks loaded in tension and compression are summarized in Table 2 3. Theoretical investigations

Table 2. Values of incubation failure time for the tested materials Compression

Tension 67 mcs 15 mcs 62 mcs 40 mcs

Fine-grain concrete

5 mcs 2 mcs

Fiber-reinforced concrete

Gabbro-diabase

-

Limestone

47

4. Conclusion

The experimental results revealed an increase in dynamic strength of the materials tested as compared with the static one. Besides, time to failure decreased with the increasing strain rate and stress increase rate. For the implemented testing schemes, simple relations for determining incubation time and for constructing relations between strength and rate were derived, based on the linear relation between stresses and strains up to the time of failure. At the same time, the possibility of predicting ultimate impact loading values and using incubation time of failure as a measure of dynamic strength of the material was proved.