Publications

To establish mechanical properties and failure criteria of silicon carbide (SiC-N) ceramics, a series of quasi-static compression tests has been completed using a high-pressure vessel and a unique sample alignment jig. This report summarizes the test methods, set-up, relevant observations, and results from the constitutive experimental efforts. Results from the uniaxial and triaxial compression tests established the failure threshold for the SiC-N ceramics in terms of stress invariants (I{sub 1} and J{sub 2}) over the range 1246 < I{sub 1} < 2405. In this range, results are fitted to the following limit function (Fossum and Brannon, 2004) {radical}J{sub 2}(MPa) = a{sub 1} - a{sub 3}e -a{sub 2}(I{sub 1}/3) + a{sub 4} I{sub 1}/3, where a{sub 1} = 10181 MPa, a{sub 2} = 4.2 x 10{sup -4}, a{sub 3} = 11372 MPa, and a{sub 4} = 1.046. Combining these quasistatic triaxial compression strength measurements with existing data at higher pressures naturally results in different values for the least-squares fit to this function, appropriate over a broader pressure range. These triaxial compression tests are significant because they constitute the first successful measurements of SiC-N compressive strength under quasistatic conditions. Having an unconfined compressive strength of {approx}3800 MPa, SiC-N has been heretofore tested only under dynamic conditions to achieve a sufficiently large load to induce failure. Obtaining reliable quasi-static strength measurements has required design of a special alignment jig and load-spreader assembly, as well as redundant gages to ensure alignment. When considered in combination with existing dynamic strength measurements, these data significantly advance the characterization of pressure-dependence of strength, which is important for penetration simulations where failed regions are often at lower pressures than intact regions.

The purpose of the present work is to increase our understanding of which properties of geomaterials most influence the penetration process with a goal of improving our predictive ability. Two primary approaches were followed: development of a realistic, constitutive model for geomaterials and designing an experimental approach to study penetration from the target's point of view. A realistic constitutive model, with parameters based on measurable properties, can be used for sensitivity analysis to determine the properties that are most important in influencing the penetration process. An immense literature exists that is devoted to the problem of predicting penetration into geomaterials or similar man-made materials such as concrete. Various formulations have been developed that use an analytic or more commonly, numerical, solution for the spherical or cylindrical cavity expansion as a sort of Green's function to establish the forces acting on a penetrator. This approach has had considerable success in modeling the behavior of penetrators, both as to path and depth of penetration. However the approach is not well adapted to the problem of understanding what is happening to the material being penetrated. Without a picture of the stress and strain state imposed on the highly deformed target material, it is not easy to determine what properties of the target are important in influencing the penetration process. We developed an experimental arrangement that allows greater control of the deformation than is possible in actual penetrator tests, yet approximates the deformation processes imposed by a penetrator. Using explosive line charges placed in a central borehole, we loaded cylindrical specimens in a manner equivalent to an increment of penetration, allowing the measurement of the associated strains and accelerations and the retrieval of specimens from the more-or-less intact cylinder. Results show clearly that the deformation zone is highly concentrated near the borehole, with almost no damage occurring beyond 1/2 a borehole diameter. This implies penetration is not strongly influenced by anything but the material within a diameter or so of the penetration. For penetrator tests, target size should not matter strongly once target diameters exceed some small multiple of the penetrator diameter. Penetration into jointed rock should not be much affected unless a discontinuity is within a similar range. Accelerations measured at several points along a radius from the borehole are consistent with highly-concentrated damage and energy absorption; At the borehole wall, accelerations were an order of magnitude higher than at 1/2 a diameter, but at the outer surface, 8 diameters away, accelerations were as expected for propagation through an elastic medium. Accelerations measured at the outer surface of the cylinders increased significantly with cure time for the concrete. As strength increased, less damage was observed near the explosively-driven borehole wall consistent with the lower energy absorption expected and observed for stronger concrete. As it is the energy absorbing properties of a target that ultimately stop a penetrator, we believe this may point the way to a more readily determined equivalent of the S number.