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Supersonic fractures are
fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displ ...
s where the fracture propagation velocity is higher than the
speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as we ...
in the material. This phenomenon was first discovered by scientists from the Max Planck Institute for Metals Research in Stuttgart (
Markus J. Buehler Markus J. Buehler is an American materials scientist and engineer at the Massachusetts Institute of Technology (MIT), where he holds the endowed McAfee Professorship of Engineering chair. He is a member of the faculty at MIT's Department of Civi ...
and Huajian Gao) and IBM Almaden Research Center in San Jose, California (
Farid F. Abraham Farid F. Abraham (born May 5, 1937) is an American scientist. He has pioneered new methods of using computer modeling in the fields of fracture mechanics, membrane dynamics and phase transformation behavior of matter. He has written two textboo ...
). The issues of intersonic and supersonic fracture become the frontier of dynamic
fracture mechanics Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics ...
. The work of Burridge initiated the exploration for intersonic crack growth (when the crack tip velocity V is between the shear in wave speed C^8 and the longitudinal wave speed C^1.Brittle fracture mechanism
Eurekalert.org. Accessed May 19, 2012. Supersonic fracture was a phenomenon totally unexplained by the classical theories of fracture.
Molecular dynamics Molecular dynamics (MD) is a computer simulation method for analyzing the physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giving a view of the dynamic "evolution" of th ...
simulations by the group around Abraham and Gao have shown the existence of intersonic mode I and supersonic mode II cracks. This motivated a continuum mechanics analysis of supersonic mode III cracks by Yang. Recent progress in the theoretical understanding of
hyperelasticity A hyperelastic or Green elastic materialR.W. Ogden, 1984, ''Non-Linear Elastic Deformations'', , Dover. is a type of constitutive model for ideally elastic material for which the stress–strain relationship derives from a strain energy density ...
in dynamic fracture has shown that supersonic crack propagation can only be understood by introducing a new length scale, called χ; which governs the process of energy transport near a crack tip. The crack dynamics is completely dominated by material properties inside a zone surrounding the crack tip with characteristic size equal to χ. When the material inside this characteristic zone is stiffened due to hyperelastic properties, cracks propagate faster than the longitudinal wave speed. The research group of Gao has used this concept to simulate the Broberg problem of crack propagation inside a stiff strip embedded in a soft elastic matrix. These simulations confirmed the existence of an energy characteristic length. This study also had implications for dynamic crack propagation in composite materials. If the characteristic size of the composite microstructure is larger than the energy characteristic length, χ; models that homogenize the materials into an effective continuum would be in significant error. The challenge arises of designing experiments and interpretative simulations to verify the energy characteristic length. Confirmation of the concept must be sought in the comparison of experiments on supersonic cracks and the predictions of the simulations and analysis. While much excitement rightly centres on the relatively new activity related to intersonic cracking, an old but interesting possibility remains to be incorporated in the modern work: for an interface between elastically dissimilar materials, crack propagation that is subsonic but exceeds the Rayleigh wave speed has been predicted for at least some combinations of the elastic properties of the two materials.


See also

*
Characteristic energy length scale The characteristic energy length scale \chi describes the size of the region from which energy flows to a rapidly moving crack. If material properties change within the characteristic energy length scale, local wave speeds can dominate crack dyna ...


References

Fracture mechanics {{classicalmechanics-stub