Effects of loading rate on rock fracture

doi:10.1016/S0148-9062(99)00031-5

International Journal of Rock Mechanics and Mining Sciences

Volume 36, Issue 5, July 1999, Pages 597-611

https://doi.org/10.1016/S0148-9062(99)00031-5 Get rights and content

Abstract

By means of a wedge loading applied to a short-rod rock fracture specimen tested with the MTS 810 or SHPB (split Hopkinson pressure bar), the fracture toughness of Fangshan gabbro and Fangshan marble was measured over a wide range of loading rates, $k ̇$ =10⁻²–10⁶ MPa m^1/2 s⁻¹. In order to determine the dynamic fracture toughness of the rock as exactly as possible, the dynamic Moiré method and strain–gauge method were used in determining the critical time of dynamic fracture. The testing results indicated that the critical time was generally shorter than the transmitted wave peak time, and the differences between the two times had a weak increasing tendency with loading rates. The experimental results for rock fracture showed that the static fracture toughness K_Ic of the rock was nearly a constant, but the dynamic fracture toughness K_Id of the rock ( $k ̇$ ≥10⁴ MPa m^1/2 s⁻¹) increased with the loading rate, i.e. log(K_Id)=a log $k ̇$ +b. Macroobservations for fractured rock specimens indicated that, in the section (which was perpendicular to the fracture surface) of a specimen loaded by a dynamic load, there was clear crack branching or bifurcation, and the higher the loading rate was, the more branching cracks occurred. Furthermore, at very high loading rates ( $k ̇$ ≥10⁶ MPa m^1/2 s⁻¹) the rock specimen was broken into several fragments rather than only two halves. However, for a statically fractured specimen there was hardly any crack branching. Finally, some applications of this investigation in engineering practice are discussed.

Section snippets

Nomenclature

COD

crack-open-displacement;

C O ̇ D

speed of crack-open-displacement;

ISRM

International Society for Rock Mechanics;

SHPB

split Hopkinson pressure bar;

short rod specimen;

specimen diameter;

Young's modulus;

F_c

critical tensile load;

P_c

critical compressive load;

K_Ic

static fracture toughness;

K_Id

dynamic fracture toughness;

t_c

time interval from the start of loading to the point when the critical state of the crack is achieved;

t_m

time for the peak value of the load in static fracture or of the strain in the

Rock specimens

The tested rocks are Fangshan gabbro and Fangshan marble. All specimens of each rock were drilled from one large block. The mineralogical composition of the two rocks is given in Table 1.

The type of rock specimen used for measuring the fracture toughness of the rock employed in this study is the short rod (SR) specimen, which was proposed as a method for determining the fracture toughness of rock by ISRM in 1988 [28]. The SR specimen has a notch cut parallel to the core axis. A tensile load is

Fracture toughness

The values of the fracture toughness of the two rocks are given in Table 4, Table 5. In the tables ‘N’ means that a specimen was separated into two equal parts from the main crack surface and ‘F’ denotes that a specimen was broken into three or more fragments. Specimens C01–C04 and No.41–No.50 were tested on the MTS 810. Specimens gp01–gp16 were tested on the SHPB with slow impact. The t_c values of gg01–gg13 and No.02–No.40 in Table 4 were determined by the dynamic Moiré method and the strain

Static and dynamic fracture characteristics of rocks

As described above, the fracture toughness of rock tested at low loading rates (static or slow impact loading) did not change much. However, there are two unusual changes in the fracture toughness of gabbro in Fig. 5. One is that, at the loading rate $k ̇$ ≈10² MPa m^1/2 s⁻¹, K_Ic increases at first, and then decreases with $k ̇$ . The other is that at $k ̇$ ≈10⁴ MPa m^1/2 s⁻¹, firstly K_Id is small, then it increases with the loading rate, see Fig. 9 (the data from Fig. 5). This is an interesting phenomenon,

Conclusions

1.
The experimental method for determining dynamic rock fracture toughness introduced in this paper is a feasible way of both measuring dynamic rock fracture toughness and studying the loading rate effect on rock materials under the condition of not very high loading rates ( $k ̇$ <10⁶ MPa m^1/2 s⁻¹). For other similar brittle materials this method should be also effective.
2.
For gabbro and marble, both relationships of fracture toughness and loading rate at the loading rates $k ̇$ =10⁻²−10⁶ MPa m^1/2 s⁻¹ are

Acknowledgements

The experimental study in this investigation was completed at the University of Science and Technology Beijing, China. The analytical work was accomplished at the Luleå University of Technology. The financial support from the National Committee of Science and Technology of China, the Ministry of Metallurgical Industry of China, and SKB in Sweden is greatly appreciated. The authors would like to thank Dr. Finn Ouchterlony for his valuable comments and two reviewers for their helpful suggestions

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