PSI - Issue 25

Mikhail Eremin / Procedia Structural Integrity 25 (2020) 465–469

467

Mikhail Eremin / Structural Integrity Procedia 00 (2019) 000–000

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4. Results and discussion

As a result of samples loading, the diagrams of axial stress–axial strain were obtained for samples with di ff erent porosity. Fig. 1 illustrates obtained diagrams. Detail consideration of diagrams suggests that initial nonlinearity of loading curves is successfully described in the framework of the model applied. It is generally related to the closure of microcracks and micropores as reported by Martin and Chandler (1994); Hoek and Martin (2014). The onset of the second stage of samples’ deformation is characterized by the establishment of a linear relationship between stress and strain – behavior of samples obeys the Hooke’s law.

Fig. 1. The loading diagrams for samples with di ff erent porosity.

The third stage of samples’ deformation is related to the initiation of cracks and is characterized by the violation of the Hooke’s law. Gradual deviation from the linear dependence between stress and strain can be observed. Finally, the peak stress is immediately followed by the descending portion of the loading diagram which is characterized by unstable propagation of cracks from multiple stress concentration-prone regions. Turn to residual state occurs when the macroscopic bearing capacity of samples is exhausted. Magnified view of Fig. 1 highlights that the higher the porosity the lower the UCS of the sample. Let us also consider the features of AE related to damage accumulation by samples and matched with corresponding stages of the loading diagram. The number of AE impulses was calculated directly as the number of particles where the fracture criterion was met (damage parameter D equal to unity). As an example of modelling, Fig. 3 illustrates an obtained diagram of AE combined with loading diagram of the sample with a porosity of ≈ 6%. It can be concluded that the first impulses of AE are observed at axial stress of ≈ 1.5 MPa. The latter means that the ratio of crack initiation stress σ ci to peak stress σ p is equal to 0.25. Diagram of AE also suggests that intensification onset of AE occurs at axial stress ≈ 4.95 MPa. Hence, the ratio of crack damage stress σ cd to peak stress σ p is equal to 0.84. These ratios find a satisfactory agreement with data reported by Martin and Chandler (1994); Hoek and Martin (2014). Comparison of the results obtained by numerical modelling with experimental data collected by Olkhovatenko (2014) gives a fairly satisfactory agreement. Fig. 3 illustrates the comparison. It can be concluded that the applied model slightly overestimates the strength of samples at porosity exceeding 15%.

5. Summary

The features of deformation and failure of sandstone samples with explicit consideration of pore space were in vestigated. It is found that the model applied finds a satisfactory agreement with experimental observations reported