The improvement of ceramic armor materials and efficient design of armor structures requires well-validated constitutive laws for dynamic deformation of ceramics. For this purpose, a constitutive model for the response of ceramics was developed by Deshpande and Evans. The objective of this dissertation is to critically evaluate the capabilities of the Deshpande-Evans (DE) model in predicting the response of a typical armor ceramic (notably, a fine-grained alumina). The key deformation processes active during impact are probed via two simplified model problems that highlight different aspects of the material response. Experimental results are compared with numerical simulations utilizing the DE model in order to assess the fidelity of the model predictions. In addition, parameter studies are performed to probe the role of various mechanical properties in the material response during an impact event.
Ceramic behavior in the initial stages of impact was probed by: (i) quasistatic indentation, to allow careful instrumentation and the exclusion of dynamic effects, and (ii) confined impact, to incorporate the dynamic aspects of the deformation while preventing spallation and disintegration of the target. Comparisons of the sizes of damage zones obtained experimentally and by finite element (FE) analysis in these tests allowed calibration of the material properties in the DE model. It also provided evidence that the DE model correctly predicts the fracture behavior of the alumina in the initial stages of impact and the granular flow response of the freshly-comminuted material. The fracture toughness and flaw size exert the most significant influence on the simulation results in this regime.
Penetration and ceramic deformation during impact were further probed by impact of lightly-constrained monolithic alumina tiles and steel-alumina trilayers. The DE model correctly captures the behavior at low velocities. As the severity of the impact increases, the model fails to capture the localization of deformation in the ceramic and the steel back sheet and under-predicts the maximum displacement of the back sheet. This is attributable to an evolution in the friction angle of the comminuted material during the early stages of granular deformation. A modification to the granular flow law in the DE model to account for this evolution was implemented. It was subsequently used in FE analyses to demonstrate the importance of an evolving friction angle with granular strain on the efficacy of the ceramic in preventing penetration of the structure.
|Advisor:||Zok, Frank W.|
|Commitee:||Beltz, Glenn E., McMeeking, Robert M., Pollock, Tresa M.|
|School:||University of California, Santa Barbara|
|School Location:||United States -- California|
|Source:||DAI-B 74/04(E), Dissertation Abstracts International|
|Keywords:||Armor alumina, Ballistic impact, Ceramic, Constitutive laws, Damage, Deformation, Finite element analysis|
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