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# Bounded deformation

In mathematics, a function of bounded deformation is a function whose distributional derivatives are not quite well-behaved-enough to qualify as functions of bounded variation, although the symmetric part of the derivative matrix does meet that condition. Thought of as deformations of elasto-plastic bodies, functions of bounded deformation play a major role in the mathematical study of materials, e.g. the Francfort-Marigo model of brittle crack evolution.

More precisely, given an open subset Ω of Rn, a function u : Ω → Rn is said to be of bounded deformation if the symmetrized gradient ε(u) of u,

$\varepsilon(u) = \frac{\nabla u + \nabla u^{\top}}{2}$

is a bounded, symmetric n × n matrix-valued Radon measure. The collection of all functions of bounded deformation is denoted BD(Ω; Rn), or simply BD. BD is a strictly larger space than the space BV of functions of bounded variation.

One can show that if u is of bounded deformation then the measure ε(u) can be decomposed into three parts: one absolutely continuous with respect to Lebesgue measure, denoted e(u) dx; a jump part, supported on a rectifiable (n − 1)-dimensional set Ju of points where u has two different approximate limits u+ and u, together with a normal vector νu; and a "Cantor part", which vanishes on Borel sets of finite Hn−1-measure (where Hk denotes k-dimensional Hausdorff measure).

A function u is said to be of special bounded deformation if the Cantor part of ε(u) vanishes, so that the measure can be written as

$\varepsilon u = e(u) \, \mathrm{d} x + \big( u_{+}(x) + u_{-}(x) \big) \odot \nu_{u} (x) H^{n - 1} | J_{u},$

where H n−1 | Ju denotes H n−1 on the jump set Ju and $\odot$ denotes the symmetrized dyadic product:

$a \odot b = \frac{a \otimes b + b \otimes a}{2}.$

The collection of all functions of bounded deformation is denoted SBD(Ω; Rn), or simply SBD.

## References

• Francfort, G. A. and Marigo, J.-J. (1998). "Revisiting brittle fracture as an energy minimization problem". J. Mech. Phys. Solids 46 (8): 1319–1342.
• Francfort, G. A. and Marigo, J.-J. (1999). in Variations of domain and free-boundary problems in solid mechanics (Paris, 1997): Cracks in fracture mechanics: a time indexed family of energy minimizers, Solid Mech. Appl.. Kluwer Acad. Publ., 197–202.