PSI - Issue 13

N.S. Selyutina et al. / Procedia Structural Integrity 13 (2018) 705–709 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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3. Rocks

The behavior of the ultimate stress of rocks depends on the saturation ratio. Consider the behavior of ultimate stress of granite and tuff subjected to tensile in split Hopkinson pressure bar tests (Ogata et al. (1994)) at 10 0 – 10 1 s -1 , spalling tests (Lou (1994)) and static experiments (Okubo et al. (1992), Lou (1994)) of order 10 -3 s -1 . Dependences of limiting tensile stress for dry and saturated tuff (Fig.1a) and granite (Fig.1b) on strain rate calculated by the fracture criterion (1) are shown in Fig. 1. Compare ultimate stress values of tuff and granite in different the range of strain rate depending on the saturation ratio (Fig.1). The great static strength of dry tuff is as big as dry granite same. In the range of strain rate 10 0 – 10 1 s -1 , the limiting tensile stress of dry tuff is a bit higher than saturated tuff and the ultimate stresses of dry and wet granite overlap. Decrease of static strength of the water saturated materials can be partly explained by the assumption that in the presence of water the process of microcracking in rocks and concrete is sharply intensified, and the fracture processes proceed considerably faster comparing to failure of materials with normal saturation ratio. This well-known effect related with an action of hydrostatic pressure in specimen subject to static loading (10 -3 s -1 ). In the range of strain rate 10 1 – 10 2 s -1 , the ultimate stress of tuff is same as in the range of 10 0 – 10 1 s -1 but wet granite is a lot bigger than dry granite. The opposite effect of behaviour the ultimate stress of dry and saturated granite can explain a presence of temporal peculiarities of fracture process, characterized by parameters of incubation time criterion. The incubation time of saturated tuff ( τ =0.39 ms; α =1) is a bit bigger than dry tuff ( τ =0.37 ms; α =1). The great rupture stress of dry tuff at strain rate 10 -3 – 10 1 s -1 is taken notes. The incubation time of wet granite ( τ =21.5 μ s; α =1) is a bit bigger than dry granite ( τ =11.8 μ s; α =1). The biggest incubation time points out the great dominant influence of incubation process in the material subjected to dynamic loading. From the fact that an order of incubation times of granite is less than tuff follows that the dominant value of incubation process in specimens of tuff should observe at strain rate more 10 2 s -1 . Thus, the behaviour of temporal characteristics can predict minimal loading rate with which a beginning of the dominance of incubation process on the ultimate stress in comparison with hydrostatic pressure influence. Opposite behaviour of the ultimate stress between dry and wet of granite (Fig.1b) under static (10 -3 s -1 ) and dynamic (10 1 – 10 2 s -1 ) loading often knows as a competitive inversion effect.

Fig. 1. Effect of water saturation of the ultimate stress of: (a) tuff (Ogata et al. (2004), Okubo et al. (1992)); (b) granite (Ogata et al. (2004), Lou (1994)).

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