CIJ.jl

What is CIJ.jl?

A Julia package for dealing with linear elastic constants, with particular applicability to geophysics problems.

How to install

Although not registered as an official package, CIJ.jl can be added to your Julia install like so:

```julia julia> ] # Type ']' to enter pkg mode

(v1.6) pkg> add https://github.com/anowacki/CIJ.jl ```

You then need only do

julia julia> import CIJ

and if that works, you're ready to go.

How to use

CIJ.jl, to make your life as easy as possible, does not insist on your using a special type for elastic constants, but relies on them being AbstractArrays of an elastic tensor's Voigt matrix. The Voigt matrix is a 6 × 6 array representing the full 3 × 3 × 3 × 3 × tensor, subject to symmetries present for linear elasticity.

Throughout CIJ.jl, we make no assumptions about the units of a matrix C, but when it does matter (for instance, calculating phase velocities with phase_vels()), then it is assumed you are dealing with density-normalised constants (i.e., the units are m<sup>2</sup> s<sup>-2</sup>), sometimes called A<sub>ij</sub> instead. (This is the same as Pa/(kg m<sup>-3</sup>)),

Let's try out a simple example of what we can do:

```julia julia> C = zeros(6, 6);

julia> CIJ.is_stable(C) false ```

Here, the is_stable() function tells one whether a set of elastic constants C is dynamically possible. It turns out, a material where all constants are zero is not. Olivine, however, should be, so

```julia julia> C, rho = CIJ.ol(); # Return density-normalised constants, and density, for olivine

julia> is_stable(C) true ```

is not a surprise.

EC type

Whilst you can deal with plain Arrays, CIJ exports the EC type which is a wrapper around a StaticArrays.MMatrix.

Calculations using this type are quicker. For instance, finding the compliance matrix S from the stiffness matrix C is found by matrix inversion, and this is about twice as fast when using ECs rather than plain Arrays.

ECs are mutable and can be treated just like any other 6 × 6 matrix, including accessing and setting elements like C[i,j].

Construct an EC object by calling the EC() constructor:

julia julia> EC([10i+j for i in 1:6, j in 1:6]) 6×6 EC{Float64}: 11.0 12.0 13.0 14.0 15.0 16.0 12.0 22.0 23.0 24.0 25.0 26.0 13.0 23.0 33.0 34.0 35.0 36.0 14.0 24.0 34.0 44.0 45.0 46.0 15.0 25.0 35.0 45.0 55.0 56.0 16.0 26.0 36.0 46.0 56.0 66.0

Note that ECs are enforced to be symmetric, and will copy the upper half of the input matrix into the bottom half. Subsequent modification of any element will be reflected in both upper and lower halves automatically, so ECs cannot be non-symmetric.

To specify the type of the elements T, use the parametric constructor, EC{T}().

Calculating phase velocities

One of the common uses for the package is to compute the phase velocities in a given direction through some elastic constants. This is done using the phase_vels() function, which returns a NamedTuple, like so:

```julia julia> C, rho = CIJ.ol();

julia> az, inc = 20, 45; # Directions

julia> vp, vs1, vs2, pol, avs = CIJ.phase_vels(C, az, inc) (vp = 8590.639816324323, vs1 = 5422.968116295959, vs2 = 4602.70348828534, pol = -20.682503753509465, avs = 16.363285381017125) ```

vp, vs1 and vs2 and so on, are velocities in m s<sup>-1</sup>, assuming C is in m<sup>2</sup> s<sup>-2</sup>, pol is the orientation of the fast shear wave in degrees, and avs is the percentage shear wave anisotropy along this direction.

Converting to Voigt notation

If you have an 81-component tensor c, how do you get the Voigt matrix C?

julia C = CIJ.cij(c)

And the other way?

julia c = CIJ.cijkl(C)

Plotting

If you are using Julia v1.9 or later, and you have also loaded a backend from the Makie.jl ecosystem (e.g., by doing import GLMakie, import CairoMakie, etc.), then you can create plots of phase velocity surfaces by calling one of the following functions:

• CIJ.plot_hemisphere for a set of upper-hemisphere phase velocity surfaces;
• CIJ.plot_hemisphere! for a single surface into an existing Makie.PolarAxis;
• CIJ.plot_sphere for a 3D view of the phase velocity surface; and
• CIJ.plot_sphere! for a 3D view into an existing Makie.Axis3.

For example:

```julia julia> import GLMakie

julia> CIJ.plot_hemisphere(CIJ.ol()[1]) ```

julia julia> CIJ.plot_sphere(CIJ.ol()[1], :avs)

Getting help

Functions are documented, so at the REPL type ? to get a help?> prompt, and type the name of the function:

```julia help?> CIJ.phase_vels

phase_vels(C, az, inc) -> vp, vs1, vs2, pol, avs

Calculate the phase velocities for the 6x6 elasticity matrix C along the direction (az, inc), in degrees, and return P-wave velocity vp, the fast and slow shear wave velocities, vs1 and vs2, the polarisation of the fast shear wave pol, and the shear wave velocity anisotropy, avs. Velocities are in m/s if the tensor C is in m^2/s^2 (i.e., is a density-normalised tensor, sometimes called A).

az is the azimuth in degrees measured from the x1 towards to -x2 axis.

inc is the inclination in degrees from the x1-x2 plane towards the x3 axis. ```

Other software

• If you use MATLAB, then you should use MSAT.
• If you use Fortran, then you should investigate the module anisotropy_ajn which is in the seismo-fortran repo.

Why the name?

Linear elastic constants are a fourth-rank tensor, relating the stress σ in a material to the strain ε with the relationship

σ<sub>ij</sub> = c<sub>ijkl</sub> ε<sub>kl</sub>,

where i, j, k and l are indices taking values 1 to 3 and representing the three cartesian directions in space. This leads to there being 3 × 3× 3 × 3 = 81 numbers in the tensor c, which is somewhat unwieldy.

However, certain symmetries mean one can reduce this to 21, and represent the 4-tensor with a symmetric 2-tensor or matrix instead; the so-called 'Voigt notation'. Typically, the lowercase 4-tensor c<sub>ijkl</sub> becomes the uppercase matrix C<sub>ij</sub>, and thus the package is born.

Acknowledgments

Credit goes to James Wookey and Mike Kendall for the original set of Fortran routines on which the code is based, and which lives on in the seismo-fortran repo.