Characterization of Porous Rock at High Strain Rates, 18-9344

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Principal Investigators
Kathryn A. Dannemann
Sidney Chocron
Ali Minachi
Amitava Ghosh
James D. Walker

Inclusive Dates:  08/15/02 - Current

Background - Understanding the response of rocks to large seismic induced loads or various impulsive loading situations is a topic of interest. During such rapid loading events, the response of the rock and its failure differ from that observed in static load experiments. Dynamic laboratory measurements using compressive Hopkinson bar techniques have shown that rock fracture is strain rate sensitive. There are two important deficiencies in studies performed to date: 1) most data for split Hopkinson pressure bar (SHPB) compressive behavior of rocks are for small samples; and 2) few studies relate the observed dynamic mechanical behavior with the microstructure, flaw population, or void content. The small sample size is an issue because voids can be relatively large with heterogeneous distributions. Improved characterization of the void distribution in a tested sample is needed to better understand the fracture phenomena and associated breakage of the material.

Approach - The overall objective of this research program is to characterize the high strain rate compressive behavior of rock. Two aspects of rock behavior are of particular interest: 1) the effect of pores and strain rate on the failure of rock, and 2) subsequent post-failure response. High strain rate data were obtained using the SwRI SHPB system with 3.8-cm diameter steel bars. A new approach was employed for confining rock samples to obtain data on the post-failure response of the rock. Nondestructive evaluation techniques, i.e., computed tomography (CT), were used to construct a picture of the interior of each rock sample. This information, along with diagnostics during the test and post-test evaluation, will aid in understanding the damage development process during dynamic testing of the rock. Such diagnostics allow insight into the failure mechanisms, which then can be correlated with damage accumulation. Given an understanding of the failure mechanisms, a constitutive model can then be developed that includes microstructural information (e.g., void content, flaw population) for the rock.

Accomplishments - Apache Leap Tuff rock was chosen for investigation. Rock samples were tested at strain rates ranging from 20 to approximately 500 s-1, using a 3.8-centimeter diameter SHPB system. A nondispersive ramp loading pulse was obtained during the SHPB experiments by applying an Al pulse shaper. High-speed imaging techniques were utilized to monitor damage development during dynamic compression loading of select samples. The SHPB experimental results were confirmed through numerical simulations of the test using LS-DYNA; good correlation was observed between the experimental and numerical simulation results. Several low strain rate (10-4 s-1) compression tests were also conducted for comparison. The test results show some strain rate sensitivity for this rock. Prior to testing, all samples were characterized with CT to determine void size and distribution. Sample faces were also evaluated with stereomicroscopy; phase distribution differences were photo documented. The original intent of this investigation was to correlate dynamic compressive behavior with porosity and incorporate the findings into a constitutive model. Because void size proved too small to differentiate, phase/density differences were captured using the CT technique. Currently, CT results and stereomicroscopy images are being correlated with dynamic test results.

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