ABSTRAC:

Using different impact experiment ways such as the explosion loading system, shooting bar system and dynamic photo elastic apparatus, the experimental investigations of rock impact fracture properties have been done in this paper. The pictures of the dynamic stress fields causing fracture process and the different cracks by different impact rates have been shown in the paper. Through studying the results of the impact experiments the stress- over impulse is proposed to predict the dynamic fracture degree of the given rock under impact load.

I.
INTRODUCTION

The study of the mechanism of rock impact fracture have been paid more attention for recent years. Because of the Inhomogeneous and complicated mechanical properties of the rock it is impossible to describe the whole process of rock dynamic fracture using any unique criterion (Zhu & Wu 1984). The former experimental results (Grady & Kipp 1987) showed that the dynamic strength of the rock increases with loading rate and the rock impact constitutive characteristics are related to the strain rate. But considering the relativity of the impact fracture with the strain rate the theory of rock impact fracture can not completely explain the different forms of the dynamic fractures and their areas (H.L.He & Ahrens 1994). It is one of the basic characters of rock dynamic fracture that the fracture initiation delays the action of stress waves under high impact rate. It means that the dynamic cracking does not depend on instantaneous peak stress. The boundary conditions and whole impacting process play a decisive role in the impact fracture process. The experimental investigation ways will be used to study the mechanism of dynamic fracture in this paper. Using the different impact ways such as the explosion loading system, shooting bar system and dynamic photo-elastic apparatus, the dynamic stress fields causing the fracture process and the different fracture forms by different impact rates and boundary conditions have been shown in the paper. The stress-over impulse is proposed to predict the dynamic fracture degree of the given rock under impact load.

2.
DYNAMIC STRESS FIELDS OF IMPACTING ACTION

The dynamic photo-elastic apparatus was used to get the stress fields of impacting fracture in the epoxy resin plates under the explosion loading. The mould dimension is 30x30x6mm and the explosive is 150mg DDNP. The measured results of the dynamic stress fields showed that the impacting fractures Initiated later than the action of the peak stresses.

2.1
Uniform stress fields of the impact action

The impacting stresses are uniformly propagated firstly in the form of the elastic waves under the explosion loading. The dynamic stress fields caused by the stress waves are uniform and have the peak value of the stresses. It can be seen that the most of the stress streaks are the concentric circles. The spaces of the streak distribution represent the step of the stress variation. Although the stress and its loading rate are very high, the dynamic strain is still in the range of elastic deformation.

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