Failure Waves In Rock Under Shock Wave Loading

Inhomogeneous brittle materials contain numerous pre-existing microcracks, holes, inclusions, or other defects and under the shock wave compression they could be activated and growth due the local shear stress effect. Compared with the ductile materials, brittle materials exhibit some anomalous mechanical properties under the extreme condition of shock wave compression. In this thesis, recent studies and questions on failure wave in brittle materials were reviewed, and a systematic study was conducted by using AOW (Amphibolized Olivine Websterite) rock as a representative candidate for the inhomogeneous brittle materials. At shock stresses much below the Hugoniot elastic limit (σ_HEL ), the AOW rock specimens have been shock compressed with an impact of thick OFHC copper plate, and the velocity histories at the rear surface of the specimens have been measured. The main work and conclusions in the dissertation are briefly summarized as follows:A delayed failure zone has been first detected clearly in the AOW rock at shock stress of 0.8±0.2GPa and 3.9±0.1GPa, respectively, and the failure is apparently different from the failure wave in the stressed glass. For the three specimen thicknesses (6, 8, and 10mm), the trajectories of the failure fronts are fitted by two straight lines, respectively, at the two different loading stresses. It is apparent that the failure fronts are behind the shock wave front with a certain delay. The relationship between the time (T, in μs) and distance (X, in mm) is written as: t =0.15x +1.2, at σ = 0.8±0.2GPa t = 0.15x + 0.5,at σ=3.9±6.1GPa and t = 0.15x forshockwave front. The failure delay time has been determined as 1.2μs at shock stress of 0.8 ± 0.2GPa, and it decreases with the increasing of shock stress. Up to 3.9±0.1GPa, nearly half of the σ_HEL , it reduces to 0.5μs. The trajectory of the delayed failure front is parallel to that of the shock wave front. Such a fact should imply that the expanding velocity of the failure zone has the same velocity as the shock wave front. This velocity is apparently higher than that of the failure wave observed in the shocked glasses. The threshold to induced delayed failure was resulted from the experiments, and it is about 0.1⁰.6GPa. The value is apparently lower than that of the AOW rock σ_HEL The spall strength of the different distance from impact face in the shocked rock sample was measured by VISAR, and the results confirm the existence of the failure zone in stressed AOW rock sample. Physical mechanism ofthe delayed failure formation has been discussed. A physical explanation considering the "in situ activation and growth" of the pre-existing microcracks by the local shear stress under shock compression is proposed for the present observations.