The dynamic strength, complete stress-strain and strain rate relationship of the Bukit Timah granite measured on a split Hopkinson pressure bar (SHPB) using a modified loading method have been obtained. The half-sine loading waveform is generated By a shaped projectile and is specially designed for the 75 mm diameter SHPB. The test samples are 70 mm in diameter and they were cored from a granite block obtained from an underground cavern site. The cut surfaces of every sample were ground to a smooth finish and checked to ensure that it is perpendicular to the length. Results of this test series arc compared with data obtained from a conventional loading method. The experimental results reinforce the notion that the half-sine wave generated by a shaped projectile is ideal Incident wave in the determination of the dynamic strength and complete stress-strain and strain rate relationship of granite an other brittle engineering materials in a large diameter SHPB.
In the design of underground protective structures that are likely To be exposed to impact or shock loads, it is essential to accurately measure the dynamic mechanical properties and to obtain the complete stress-strain relationship of the surrounding rocks. This can be achieved by conducting rock tests in a large diameter split Hopkinson pressure bar (SHPB). Although the dynamic properties of rocks can be measured by using a large diameter SHPB, there are at least three disadvantages when sucha conventional facility is used (Li & Gu, 1994), namely:
Strong oscillation and dispersion of the incident wave in a large diameter bar will result in serious and detrimental oscillation of the resultant test curves.
Conventional loading method generates a sudden rise in the loading waveform, and the consequence is that it will cause serious non-uniform the stress or strain state in the rock specimen.
Variation of strain rate during the loading regime is extremely erratic and unstable, and the resulting data is open to interpretation that could seriously jeopardise the validity of the test data.
Although the Fast Fourier Transform (FFT) technique or the dispersion correction method can reduce wave dispersion and oscillation (Follansbee & Frantz. 1983; Gong et al 1990; Zhao &Gary, 1996; Lifshitx & Leber 1994), there are other problems that cannot be resolved. These are the serious non-uniform stressor strain state within the specimen and the violent variation of strain rate during the entire loading process. To overcome the disadvantages that are inherent in a conventional SHPB in measuring the dynamic rock properties, a novel loading method has been proposed (Li & Gu 1994. Li et ai, 2000). This invol ves the design of a shaped projectile to control the loading so that a half-sine wave can be initiated on impact on the input bar. To verify the feasibility of the proposed new method and the accuracy of the data. Results form the tests arc compared with data obtained from a conventional solid circular bar projectile.
