Army model provides new insight into mysteries of failures in ceramic armor

February 08, 2018

By ARL Public Affairs

ADELPHI, Md. (Feb. 8, 2018) – Recent work by U.S. Army Research Laboratory scientists has led to a model that can predict how ceramic compositions behave in different military situations, a challenge the research and defense communities have been trying to overcome for decades.

Armor plate inserts used in current Soldier body armor is made of boron carbide, a light-weight ceramic material that is very hard and stiff. These properties make the material attractive for use in other Army protection systems such as structural armor in aircraft and land vehicles, said Dr. John Clayton, Mechanical Engineer and laboratory fellow.

"Unfortunately, on the other hand, boron carbide also suffers from intrinsic brittleness and a tendency sometimes to undergo a structural transformation that weakens the material when subjected to blast or impact loading," he explained. "These drawbacks inhibit its performance in armor applications, especially those involving multiple impacts, for example repeated hits by multiple bullets, fragments, or other projectiles."

The material may break or separate into fragments, often preceded by or in conjunction with deformation twinning, or a shearing mechanism of the crystal lattice and amorphization, or a change from an ordered crystal structure to a disordered glassy structure, Clayton said.

The high-fidelity model Clayton and colleague Jaroslaw Knap created consists of a set of mathematical relations which included theory and equations, and a method of their solution using high performance parallel computing. Predictions of this new model have clarified effects of activity of mechanisms and granular microstructures on failure resistance of boron carbide crystals. Information obtained from the model will facilitate design and optimization of the material during processing for enhanced ductility and strength, ultimately leading to more effective lightweight ceramic armor systems.

Before their design, model descriptions of the combined behaviors of fracture, twinning and amorphization in multiple crystals comprising the material, for example, a sample of micrometer dimensions or larger didn't exist.

Clayton said a continuum mechanical theory is used to model physical mechanisms of twinning, solid-solid phase transformations, and failure by cavitation and shear fracture. This sequence of mechanisms has been observed in atomic simulations and/or high pressure deformation or impact experiments on boron carbide. In the new modeling approach, geometric quantities such as the metric tensor and connection coef?cients can depend on one or more director vectors, also called internal state vectors.

After development of the general nonlinear theory, a ?rst problem class considers simple shear deformation of a single crystal of the ceramic material. For homogeneous ?elds or stress-free states, algebraic systems or ordinary differential equations are obtained that can be solved by numerical iteration. Results are in general agreement with atomic simulation, without introduction of ?tted parameters.

The second class of problems addresses the more complex mechanics of heterogeneous deformation and stress states involved in deformation and failure of polycrystals that comprise the armor material. Finite element calculations, in which individual grains in a three-dimensional polycrystalline microstructure are fully resolved invoke a partially linearized version of the theory. Results provide new insight into effects of crystal morphology, activity or inactivity of different inelasticity mechanisms, and imposed deformation histories on strength and failure of the ceramic aggregate under compression and shear loading protocols.

The importance of incorporation of inelastic shear deformation, in addition to density change, in realistic models of amorphization of boron carbide is discovered, as is a greater reduction in overall strength of polycrystals containing one or a few dominant ?aws rather than many diffusely distributed micro-cracks. Perhaps most important in the context of design of boron-based ceramics for improved failure resistance, processing or alloying steps that would increase surface energy and decrease the inelastic shear accommodation in twinned or amorphous regions should lead to more diffuse damage patterns and an increase in overall strength of the polycrystal, reinforcing a tentative conclusion drawn from semi-analytical results reported for shock loading of single crystals in published prior work [J.D. Clayton, "Finsler Geometric Continuum Dynamics and Shock Compression", Int. J. Fract. 208, 53-78, 2017].

Their research is published in the Springer journal Continuum Mechanics and Thermodynamics [J.D. Clayton and J. Knap, "Continuum Modeling of Twinning, Amorphization, and Fracture: Theory and Numerical Simulations", Continuum Mech. Thermodyn., in press, Dec. 2017].


The U.S. Army Research Laboratory is part of the U.S. Army Research, Development and Engineering Command, which has the mission to ensure decisive overmatch for unified land operations to empower the Army, the joint warfighter and our nation. RDECOM is a major subordinate command of the U.S. Army Materiel Command.

 

Last Update / Reviewed: February 8, 2018