Voigt notation
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In mathematics, Voigt notation or Voigt form in multilinear algebra is a way to represent a symmetric tensor by reducing its order.^{[1]} There are a few variants and associated names for this idea: Mandel notation, Mandel–Voigt notation and Nye notation are others found. Kelvin notation is a revival by Helbig^{[2]} of old ideas of Lord Kelvin. The differences here lie in certain weights attached to the selected entries of the tensor. Nomenclature may vary according to what is traditional in the field of application.
For example, a 2×2 symmetric tensor X has only three distinct elements, the two on the diagonal and the other being offdiagonal. Thus it can be expressed as the vector
 .
As another example:
The stress tensor (in matrix notation) is given as
In Voigt notation it is simplified to a 6dimensional vector:
The strain tensor, similar in nature to the stress tensor—both are symmetric secondorder tensors , is given in matrix form as
Its representation in Voigt notation is
where , , and are engineering shear strains.
The benefit of using different representations for stress and strain is that the scalar invariance
is preserved.
Likewise, a threedimensional symmetric fourthorder tensor can be reduced to a 6×6 matrix.
Mnemonic rule
A simple mnemonic rule for memorizing Voigt notation is as follows:
 Write down the second order tensor in matrix form (in the example, the stress tensor)
 Strike out the diagonal
 Continue on the third column
 Go back to the first element along the first row.
Voigt indexes are numbered consecutively from the starting point to the end (in the example, the numbers in blue).
Mandel notation
For a symmetric tensor of second rank
only six components are distinct, the three on the diagonal and the others being offdiagonal. Thus it can be expressed, in Mandel notation, as the vector
The main advantage of Mandel notation is to allow the use of the same conventional operations used with vectors, for example:
A symmetric tensor of rank four satisfying and has 81 components in threedimensional space, but only 36 components are distinct. Thus, in Mandel notation, it can be expressed as
Applications
The notation is named after physicist Woldemar Voigt. It is useful, for example, in calculations involving constitutive models to simulate materials, such as the generalized Hooke's law, as well as finite element analysis,^{[3]} and Diffusion MRI.^{[4]}
Hooke's law has a symmetric fourthorder stiffness tensor with 81 components (3×3×3×3), but because the application of such a rank4 tensor to a symmetric rank2 tensor must yield another symmetric rank2 tensor, not all of the 81 elements are independent. Voigt notation enables such a rank4 tensor to be represented by a 6×6 matrix. However, Voigt's form does not preserve the sum of the squares, which in the case of Hooke's law has geometric significance. This explains why weights are introduced (to make the mapping an isometry).
A discussion of invariance of Voigt's notation and Mandel's notation be found in Helnwein (2001).^{[5]}
References
 ^ Woldemar Voigt (1910). Lehrbuch der kristallphysik. Teubner, Leipzig. Retrieved November 29, 2016.
 ^ Klaus Helbig (1994). Foundations of anisotropy for exploration seismics. Pergamon. ISBN 0080372244.
 ^ O.C. Zienkiewicz; R.L. Taylor; J.Z. Zhu (2005). The Finite Element Method: Its Basis and Fundamentals (6 ed.). Elsevier Butterworth—Heinemann. ISBN 9780750664318.
 ^ Maher Moakher (2009). "The Algebra of FourthOrder Tensors with Application to Diffusion MRI". Visualization and Processing of Tensor Fields. Springer Berlin Heidelberg. pp. 57–80. doi:10.1007/9783540883784_4.
 ^ Peter Helnwein (February 16, 2001). "Some Remarks on the Compressed Matrix Representation of Symmetric SecondOrder and FourthOrder Tensors". Computer Methods in Applied Mechanics and Engineering. 190 (22–23): 2753–2770.