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Lode Coordinates : ウィキペディア英語版
Lode Coordinates

Lode coordinates (z,r,\theta) or Haigh-Westergaard coordinates (\xi,\rho,\theta).〔Menetrey, P.H., Willam, K.J., 1995, ''Triaxial Failure Criterion for Concrete and Its Generalization'', ACI Structural Journal〕 are a set of tensor invariants that span the space of real, symmetric, second-order, 3-dimensional tensors and are isomorphic with respect to principal stress space. This right-handed orthogonal coordinate system is named in honor of the German scientist Dr. Walter Lode because of his seminal paper written in 1926 describing the effect of the middle principal stress on metal plasticity.〔Lode, W. (1926). '' Versuche über den Einfuss der mittleren Hauptspannung auf das Fliessen der Metalle Eisen Kupfer und Nickel''. Zeitung Phys., vol. 36, pp. 913–939.〕 Other examples of sets of tensor invariants are the set of principal stresses (\sigma_1, \sigma_2, \sigma_3) or the set of mechanics invariants (I_1, J_2, J_3). The Lode coordinate system can be described as a cylindrical coordinate system within principal stress space with a coincident origin and the z-axis parallel to the vector (\sigma_1,\sigma_2,\sigma_3)=(1,1,1).
==Mechanics Invariants==
The Lode coordinates are most easily computed using the mechanics invariants. These invariants are a mixture of the invariants of the Cauchy stress tensor, \boldsymbol, and the stress deviator, \boldsymbol, and are given by〔Asaro, R.J., Lubarda, V.A., 2006, ''Mechanics of Solids and Materials'', Cambridge University Press〕
: I_1 = \mathrm(\boldsymbol)
: J_2 = \frac\left(- \frac\text(\boldsymbol)^2\right ) = \frac\mathrm\left(\boldsymbol\cdot\boldsymbol\right) = \frac\lVert \boldsymbol \rVert^2
: J_3 = \mathrm(\boldsymbol) = \frac\mathrm\left(\boldsymbol\cdot\boldsymbol\cdot\boldsymbol\right)
which can be written equivalently in Einstein notation
: I_1 = \sigma_
: J_2 = \frac\left(- \frac\text(\boldsymbol)^2\right ) = \fracs_s_ = \fracs_s_
: J_3 = \frac\epsilon_\epsilon_\sigma_\sigma_\sigma_ = \fracs_s_s_
where \epsilon is the Levi-Civita symbol (or permutation symbol) and the last two forms for J_2 are equivalent because \boldsymbol is symmetric (s_=s_).
The gradients of these invariants〔Brannon, R.M., 2009, ''KAYENTA: Theory and User's Guide'', Sandia National Laboratories, Albuquerque, New Mexico.〕 can be calculated by
: \frac
: \frac = \boldsymbol - \frac\right)}\boldsymbol
: \frac = \boldsymbol\cdot\boldsymbol - \frac\boldsymbol
where \boldsymbol is the 3x3 identity matrix and \boldsymbol is called the Hill tensor.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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