RESEARCH OF ACOUSTIC EMISSIONS FROM THE SYSTEM OF COMPLANARY CRACKS

The dynamic problem of the displacement field in an elastic half-space caused by the time-steady displacement of the surfaces of the system of disc-shaped coplanar cracks is solved. The solutions are obtained by the method of boundary integral equations. The dependences of elastic displacements on the surface of the half-space on the wave number, the number of defects and the depths of their occurrence are constructed.

Attenuation and dispersion of P-waves in fluid-saturated porous rocks with a distribution of coplanar cracks - scattering approach

Wave-induced fluid flow (WIFF) between cracks and micro-pores is one of the major mechanisms in causing attenuation and dispersion within seismic frequency ranges. Previous non-interaction-approximation (NIA) models often assume the distribution of cracks is dilute, neglecting the influences of interacting cracks on dispersion and attenuation. To overcome this restriction, we investigate the interaction between coplanar cracks and their influences on seismic dispersion and attenuation. First, a scattering problem for a longitudinal (P) wave normally impinging on a plane with equally distributed coplanar cracks in a porous medium is solved using integral transform approach. Then, based on the solution, an effective wavenumber is derived for P-wave propagation in a porous material with coplanar cracks. It is found that the magnitude of dispersion and attenuation can significantly increase when the spacing between adjacent cracks decreases even if the crack density is unchanged. Moreover, frequency-dependent asymptotic behavior of inverse quality factor is also different from that of the NIA models at frequencies lower than the WIFF relaxation frequency. Specifically, the inverse quality factor scales with the square root of frequency at low frequencies. When the spacing between adjacent cracks is large, an additional frequency-dependent scale occurs at relatively higher frequencies (but still lower than the WIFF relaxation frequency) with inverse quality factor scales with the first power of frequency. When the spacing becomes much larger so that the interaction between the adjacent cracks is negligible, the present model exactly reduces to a NIA model for a distribution of aligned slit cracks and the first power scale can prevail attenuation within low frequencies.

The Influence of Bridge Length on Failure Patterns of Specimens Containing Two Coplanar Cracks under Uniaxial Compression Load

Unixial compression tests have been conducted to investigate the influence of bridge length between two collinear cracks on the specimen strength and fracture patterns. Based on the local connectivity rate of cracks, the evolution of the fracture patterns of the bridge is illustrated. It can be concluded that the sensitivity of the strength to the bridge length is reduced as the connectivity rate, L, is higher than 0.8. The fracture pattern composed of tensile and shear fracture will be induced in rock bridges when the bridges are short. Wing cracks develop well as the distance between prefabricated cracks increases, however, the fracture events caused by the conjunction of the secondary cracks are restrained.