International Journal of Rock Mechanics and Mining Sciences
Effects of loading rate on rock fracture
Section snippets
Nomenclature
- COD
crack-open-displacement;
speed of crack-open-displacement;
- ISRM
International Society for Rock Mechanics;
- SHPB
split Hopkinson pressure bar;
- SR
short rod specimen;
- D
specimen diameter;
- E
Young's modulus;
- Fc
critical tensile load;
- Pc
critical compressive load;
- KIc
static fracture toughness;
- KId
dynamic fracture toughness;
- tc
time interval from the start of loading to the point when the critical state of the crack is achieved;
- tm
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 tc 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 ≈102 MPa m1/2 s−1, KIc increases at first, and then decreases with . The other is that at ≈104 MPa m1/2 s−1, firstly KId 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 (<106 MPa m1/2 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 =10−2−106 MPa m1/2 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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