Dynamic failure behavior of ceramics under multiaxial compression
An experimental technique has been developed that is capable of (1) dynamically loading the specimen in multiaxial compression; (2) controlling the stress state in the specimen in the range from uniaxial stress to uniaxial strain; and (3) allowing the recovery of the sample after loaded by a single,...
| Summary: | An experimental technique has been developed that is capable of (1) dynamically loading the specimen in multiaxial compression; (2) controlling the stress state in the specimen in the range from uniaxial stress to uniaxial strain; and (3) allowing the recovery of the sample after loaded by a single, well defined pulse for the characterization of the failure mode. In this technique, cylindrical ceramic specimens were loaded in the axial direction using a split Hopkinson pressure bar modified to apply a single loading pulse, and were confined laterally either by shrink fit sleeves, or by eletro-magnetic force.
Quasi-static and dynamic multiaxial compression experiments have been performed on a machinable glass ceramic, Macor, and a monolithic engineering ceramic, sintered aluminum nitride (A1N). The cylindrical ceramic specimens were confned laterally by shrink fit sleeves: the amount of confining pressure (0-230 MPa) was varied by using different sleeve materials. The quasi-static axial load was applied by a hydraulic driven Material Test System (MTS), whereas the dynamic axial load was provided by a modified split Hopkinson (Kolsky) pressure bar (SHPB). Under both quasi-static and dynamic loading conditions, the experimental results for both materials showed that the failure mode changed from fragmentation by axial splitting under conditions of uniaxial stress (without lateral confinement) to localized deformation on faults under moderate lateral confinement. The fault initiation process was studied experimentally in detail. Based on the experimental results, a compressive brittle failure process was summarized. A transition from brittle to ductile behavior was observed in Macor under high confinement pressure which was achieved using a second sleeve around the inner sleeve. The compressive failure strengths of both materials increased with increasing confinement pressure under both quasi-static and dynamic loading conditions. The highest dynamic compressive strengths of Macor and A1N measured in the experiments were 1.35 GPa and 5.40 GPa, respectively, whereas their quasi-static compressive strength were measured to be 0.43 GPa and 2.5 GPa, respectively.
Based on the experimental results on A1N together with available data in the literature, a failure/flow criterion was developed for ceramic materials under multiaxial loading. A Mohr-Coulomb criterion and an improved Johnson-Holmquist model were found to fit the experimental data for brittle failure, whereas the materials exhibited pressure insensitive plastic flow at high pressures. Observations made in other types of dynamic experiments (e.g., shock wave loading) were rationalized based on the postulated failure mechanisms and the possibility of plastic flow beyond the Hugoniot elastic limit (HEL). The effect of various material properties on the failure behavior was investigated using the proposed failure criterion. The applicability of the present model to a range of ceramics was also explored and the limitations of the model were outlined.
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