H. O. K. Kirchner

Bio: H. O. K. Kirchner is an academic researcher from University of Paris-Sud. The author has contributed to research in topics: Tensor & Dislocation. The author has an hindex of 14, co-authored 22 publications receiving 649 citations.

Evidence for an elementary process in bone plasticity with an activation enthalpy of 1 eV

Abstract: The molecular mechanisms for plastic deformation of bone tissue are not well understood. We analysed temperature and strain-rate dependence of the tensile deformation behaviour in fibrolamellar bone, using a technique originally developed for studying plastic deformation in metals. We show that, beyond the elastic regime, bone is highly strain-rate sensitive, with an activation volume of ca 0.6 nm 3 . We find an activation energy of 1.1 eV associated with the basic step involved in the plastic deformation of bone at the molecular level. This is much higher than the energy of hydrogen bonds, but it is lower than the energy required for breaking covalent bonds inside the collagen fibrils. Based on the magnitude of these quantities, we speculate that disruption of electrostatic bonds between polyelectrolyte molecules in the extrafibrillar matrix of bone, perhaps mediated by polyvalent ions such as calcium, may be the rate-limiting elementary step in bone plasticity.

Plastic flow stress of b.c.c. transition metals and the Peierls potential

Abstract: Analysis of the temperature dependence of the flow stress of the b.c.c. metals α-Fe, Nb, Mo and Ta reveals their Peierls potentials. They are dam-like with a flat maximum or even have an intermediate minimum, but they are never sinusoidal. The elastic energy of the activated bulges in the dislocations is fully taken into account in a quasi-anisotropic calculation by making it trapezoidal. In the thermally activated configuration the amplitude of the bulge (the height of a kink pair) is the lattice periodicity a, double activation to 2a is excluded.

Kink pair nucleation and critical shear stress

Abstract: The activation energy ΔH ∗ for forming a rectangular kink pair in a dislocation on a Peierls potential is calculated. If the potential is smooth and has only one minimum, the energy ΔH ∗ (τ) decreases monotonously with the applied stress τ. If the potential has the shape of a camel-hump, with an intermediate minimum, discontinuities appear in ΔH ∗ (τ) . When the top of the potential is nearly flat or has a shallow minimum, the ΔH ∗ (τ) curve has a hump, causing a hump in the temperature dependence of the critical shear stress.

Elastically anisotropic angularly inhomogeneous media II. The Green's function for piezoelectric, piezomagnetic and magnetoelectric media

Abstract: A wedge in which the anisotropic elastic, piezoelectric, piezomagnetic and magnetoelectric constants show an angular variation is subjected to various boundary conditions along its flanks, as well as to elastic and electric line sources such as dislocations, line forces, line charges and/or line currents in its interior. Fourier transforms are used to obtain the fields in the wedge. Completeness of the eigenfunctions is proven.

Angularly inhomogeneous piezoelectric piezomagnetic magnetoelectric anisotropic media

Abstract: The field around a line defect along the axis of an angularly inhomogeneous, elastically anisotropic medium with piezoelectric, piezomagnetic and magnetoelectric coupling has a surprisingly simple form.

Quantum Theory of Fields

Abstract: To say that this is the best book on the quantum theory of fields is no praise, since to my knowledge it is the only book on this subject But it is a very good and most useful book The original was written in German and appeared in 1942 This is a translation with some minor changes A few remarks have been added, concerning meson theory and nuclear forces, also footnotes referring to modern work in this field, and finally an appendix on the symmetrization of the energy momentum tensor according to Belinfante Quantum Theory of Fields Prof Gregor Wentzel Translated from the German by Charlotte Houtermans and J M Jauch Pp ix + 224, (New York and London: Interscience Publishers, Inc, 1949) 36s

Snow avalanche formation

Abstract: [1] Snow avalanches are a major natural hazard, endangering human life and infrastructure in mountainous areas throughout the world. In many countries with seasonally snow-covered mountains, avalanche-forecasting services reliably warn the public by issuing occurrence probabilities for a certain region. However, at present, a single avalanche event cannot be predicted in time and space. Much about the release process remains unknown, mainly because of the highly variable, layered character of the snowpack, a highly porous material that exists close to its melting point. The complex interaction between terrain, snowpack, and meteorological conditions leading to avalanche release is commonly described as avalanche formation. It is relevant to hazard mapping and essential to short-term forecasting, which involves weighting many contributory factors. Alternatively, the release process can be studied and modeled. This approach relies heavily on snow mechanics and snow properties, including texture. While the effect of meteorological conditions or changes on the deformational behavior of snow is known in qualitative or semiquantitative manner, the knowledge of the quantitative relation between snow texture and mechanical properties is limited, but promising developments are under way. Fracture mechanical models have been applied to explain the fracture propagation, and micromechanical models including the two competing processes (damage and sintering) have been applied to explain snow failure. There are knowledge gaps between the sequence of processes that lead to the release of the snow slab: snow deformation and failure, damage accumulation, fracture initiation, and fracture propagation. Simultaneously, the spatial variability that affects damage, fracture initiation, and fracture propagation has to be considered. This review focuses on dry snow slab avalanches and shows that dealing with a highly porous media close to its melting point and processes covering several orders of scale, from the size of a bond between snow grains to the size of a mountain slope, will continue to be very challenging.

Biomimetic materials research: what can we really learn from nature's structural materials?

Abstract: Nature provides a wide range of materials with different functions and which may serve as a source of bio-inspiration for the materials scientist. The article takes the point of view that a successful translation of these ideas into the technical world requires more than the observation of nature. A thorough analysis of structure-function relations in natural tissues must precede the engineering of new bio-inspired materials. There are, indeed, many opportunities for lessons from the biological world: on growth and functional adaptation, about hierarchical structuring, on damage repair and self-healing. Biomimetic materials research is becoming a rapidly growing and enormously promising field. Serendipitous discovery from the observation of nature will be gradually replaced by a systematic approach involving the study of natural tissues in materials laboratories, the application of engineering principles to the further development of bio-inspired ideas and the generation of specific databases.