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

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Principal Investigator
Kathryn A. Dannemann
Co-Investigators
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 current interest. During such rapid loading events, the response of the rock and its failure differs 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 since 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. The two aspects of rock behavior that are of particular interest include the effect of pores and strain rate on the failure of rock and the 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. NDE 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 investigated in compression at high (20 to 500 s-1) and low strain (10-4 s-1) rates. The high strain rate compression tests were conducted using a 3.8-cm diameter SHPB system. An Al pulse shaper was used during the SHPB experiments to obtain a nondispersive ramp loading pulse. Higher strain rate data were obtained than previously available for the Apache tuff rock. Some strain rate strengthening was observed. Numerical simulations of the SHPB tests using LS-DYNA showed good correlation with the experimental results, thus confirming the experimental results. High-speed imaging techniques were utilized to monitor damage development during dynamic compression loading of select samples. In addition, CT provided for internal examination of samples prior to testing to correlate other structural features (e.g., phase content and density differences) with mechanical test results; the voids in Apache tuff rock proved too small to differentiate by CT. The structural features were quantified for comparisons using histograms created based on varying grayscales on the CT images. Pre-test CT evaluation of samples, high speed imaging during test, and post-test evaluations were applied to relate the observed mechanical behavior to the rock structure. 

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