Physics
Scientific paper
Jan 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003icbg.conf....8a&link_type=abstract
Impact Cratering: Bridging the Gap Between Modeling and Observations, February 7-9, 2003. LPI Contribution No. 1155. Houston,
Physics
Dynamic Models, Tensile Stress, Crystal Defects, Cracks, Impact Velocity, Rocks, Aluminum, Bedding Equipment, Craters, Data Reduction, Deformation, Flat Plates, Fragmentation, Fracturing, Meteorite Craters
Scientific paper
Dynamic tensile strengths of two crustal rocks, San Marcos gabbro and Coconino sandstone (Meteor Crater, Arizona), were determined by carrying out flat plate impact experiments. Porosity of San Marcos gabbro is very low, and the reported porosity for Coconino sandstone is approx. 25%. Aluminum flyer plates were used for gabbro with impact velocities of 13 to 50 m/s, which produce tensile stresses in the range of 120 to 450 MPa. PMMA flyer plates were used for sandstone with impact velocities of 5 to 25 m/s, resulting tensile stresses in the range of approx. 13 to 55 MPa. Impact was normal to the bedding of sandstone. Tensile duration times for two cases were approx. 1 and approx. 2.3 microns, respectively. Pre-shot and post-shot ultrasonic P and S wave velocities were measured for the targets. Velocity reduction for gabbro occurred at approx. 150 MPa, very close to the earlier result determined by microscopic examination. The reduction of S wave is slightly higher than that of P wave. This indicates that the impact-induced cracks were either aligned, or there were residual fluids within cracks, or both. Data for sandstone velocity reduction was few and scattered caused by its high porosity. The range of dynamic tensile strength of Coconino sandstone is within 25 and 30 MPa. Obvious radial cracks at certain stresses indicate that deformation was not restricted to one dimensional strain as being assumed. Spall fragmentation occurred above 40 MPa. The combination of impact velocities, U (km/s), and impactor radii, a0)(m, are constrained by Meteor Crater fracture depth, approx. 850 m, and the dynamic tensile fracture strength from our experiments, 40 MPa. Volume of the crater for each impact was calculated using V = 0.009mU1.65, where V is crater volume (cu m), m is the mass of the impactor (kg). Volume of impact with U = 28 km/s, a0 = 10 m is close to the real Meteor Crater volume, 7.6e7 cu m. Impact energy for this case is 3.08 Mt., which agrees well with theoretical calculation (3.3 to 7.4 Mt.).(1 Mt. = 4.18e 15J).
Ahrens Thomas J.
Ai H.
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