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The fracture toughness of a rich grade of oil shale (high kerogen content) was determined as a function of loading rate (˙K1) and orientation of the crack with respect to the bedding planes. Several independent test methods were employed in order to attain a wide range of loading rates and to examine the effect of specimen size and configuration on the test results. The test methods used in this series of experiments include ASTM Test for Plane-Strain Fracture Toughness of Metallic Materials (E 399 - 78a), the J-resistance curve method, the short rod method, and the instrumented Charpy impact test. The static tests were conducted at a nominal loading rate of 10−2 MPa·m1/2/s using each of the aforementioned first three methods. The dynamic tests were conducted at loading rates in the range 100 to 104 MPa·m1/2/s using both servo-controlled and impact loading of three-point bend specimens. The results from the various static test methods are in good agreement. However, there is a clear effect of both specimen size and crack orientation on the measured static fracture toughness. The results from the dynamic tests show some evidence of a rate effect on fracture toughness as well as the same orientation dependence as the results from the static tests, but no specimen size dependence was evident.
oil shale, fracture properties, experimental techniques, strain rate, ceramics, crack propagation, composite materials
Member of technical staff, Geomechanics Division 5532, Sandia National Laboratories, Albuquerque, N. Mex.