where I is the identity tensor, and tr ⁡ ⁡ ( ε ε ) is the trace of the rate-of-strain tensor. So this decomposition can be explicitly defined as: σ σ = − − p I + λ λ ( ∇ ∇ ⋅ ⋅ u ) I + μ μ ( ∇ ∇ u + ( ∇ ∇ u ) T ) .