IN-SITU OBSERVATION OF EN ECHELON INTERMITTENT CRACKS OF SHALE IN MICRO-NANO SCALE

In macroscopic scale, en echelon intermittent fractures are common in nature and in engineering rocks. Nevertheless, do these en echelon intermittent fractures still occur in micro-nano scale? What is the specific process of crack growth and evolution? This is a basic scientific problem for rock mechanics. Using the arch specimens of shale with a notch on the arch boundary, in-situ tension and crack growth observation are conducted under a scanning electron microscope(SEM). from the in-situ SEM observations, we learn that:(1)En echelon intermittent cracks still can be observed in micro-nano scale with a "S" or reversed "S" shape. They are induced by combined actions of complex tension-shear stress field ahead of the crack tip and the mineral anisotropy.(2)the arrangements, growth and evolution process of these en echelon intermittent cracks generally follow a certain regularity. the adjacent intermittent cracks are approximately parallel to each other. for each single crack, the intermediate tensile crack is parallel to the maximum tension stress, and the wing cracks at both ends tend to grow towards the maximum shear stress.(3)En echelon intermittent cracks in micro-nano scale only represent a transient state of shearing deformation or an intermediate growth stage of the main crack.(4)There exists a hierarchical structure among some en echelon intermittent cracks in micro-nano scale. the en echelon intermittent cracks in relatively large scale are included in those in relatively small scale. This paper provides observation evidence for en echelon intermittent cracks, which contributes for analysis on the evolution process of multi-scale cracks.