The effects of stoichiometry on the atomic structure and the related mechanical properties of boron carbide (B(4)C) have been studied using density functional theory and quantum molecular dynamics simulations. Computational cells of boron carbide containing up to 960 atoms and spanning compositions ranging from 6.7% to 26.7% carbon were used to determine the effects of stoichiometry on the atomic structure, elastic properties, and stress-strain response as a function of hydrostatic, uniaxial, and shear loading paths. It was found that different stoichiometries, as well as variable atomic arrangements within a fixed stoichiometry, can have a significant impact on the yield stress of boron carbide when compressed uniaxially (by as much as 70% in some cases); the significantly reduced strength of boron carbide under shear loading is also demonstrated.

There are various observations and experiments showing that, in addition to standard shock-wave fronts, which propagate with high trans-sonic velocities, some other much slower wave fronts can propagate within substance undergoing intensive damage. These moving fronts propagate within intact substance leaving behind them intensively damaged substance. These fronts were coined as failure waves. The failure waves can be modeled differently—in this letter they are modeled as sharp interfaces separating two states: the intact and comminuted states. Several penetration experiments with transparent glasses and ceramics have shown that failure fronts have an extremely rough morphology. We suggest a simple thermodynamic theory which allows interpreting appearance of the roughness as a manifestation of morphological instability of failure fronts. For the case of isotropic phases the instability criterion is presented in explicit form.

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

We propose a thermodynamic approach, allowing one to determine the shape of the intensively fractured zones (IFZs), simultaneously with the distribution of stress and strain within the IFZ appearing in brittle fracture. The approach combines Gibbs' variational paradigm of the theory of heterogeneous systems with Griffith's variational paradigm of the theory of brittle fracture. We suggest some simple constitutive models for solid substances undergoing brittle fracture and solve some boundary value problems on nucleation of an isolated IFZ within an isotropic elastic material.

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.