| Abstract: | ![]()
The dynamic tensile-stress amplitudes necessary to cause complete spall in unconfined oil-shale samples were experimentally determined in the laboratory for pulse durations typically encountered in the field (60–100 μs). Tensile stresses were generated by free-surface reflection of compressive pulses, and the stress-time history at the free surface was obtained by numerically differentiating the displacement-time history monitored by a fiber-optic proximity sensor. A computer code was written to display the stress profile, obtained experimentally, over the length of the specimen as time increased. In this manner, the amplitude of the resultant stress wave (incident plus reflected) was determined at the measured spall plane. By using many specimens, the fracture thresholds for these pulse durations were determined. Spall thresholds for competent shale of 25-gal/ton yield were found to be approximately 1800 psi (12.4 MPa) for pulses propagating normal to the bedding plane, and nearly 5,000 psi (34.5 MPa) for pulses propagating parallel to the bedding plane. For shale of 34 gal/ton yield, the spall-threshold levels were found to be 1200 psi (8.3 MPa) and 2700 psi (18.6 MPa) for pulses propagating normal and parallel, respectively, to the bedding plane. |
