For example, solid.SXY is the material frame XY-plane shear stress, also known as a second Piola-Kirchhoff stress, while solid.sxy is the corresponding spatial frame stress, or Cauchy stress. There are also a few mixed tensors, most notably the deformation gradient solid.FdxY, which has one spatial and one material index because it is used in converting quantities between the material and spatial frames.
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