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1. Cauchy Stress Tensor

   Linked via "invariants"

   where $\sigma_p$ represents a principal stress and $\mathbf{I}$ is the identity matrix.
   The fundamental description of the stress state can be equivalently summarized by the three principal stresses, as the transformation of the tensor into its principal axes form is an orthogonal transformation that preserves certain intrinsic mathematical properties known as invariants):
   **First Invariant (Bulk Modulus Indicato…