Gets the Wallace Tensor for determining yield strength
        criteria.

        Args:
            tau (3x3 array-like): stress at which to evaluate
                the wallace tensor

def get_wallace_tensor(self, tau):
        """
        Gets the Wallace Tensor for determining yield strength
        criteria.

        Args:
            tau (3x3 array-like): stress at which to evaluate
                the wallace tensor
        """
        b = 0.5 * (np.einsum("ml,kn->klmn", tau, np.eye(3)) +
                   np.einsum("km,ln->klmn", tau, np.eye(3)) +
                   np.einsum("nl,km->klmn", tau, np.eye(3)) +
                   np.einsum("kn,lm->klmn", tau, np.eye(3)) +
                   -2*np.einsum("kl,mn->klmn", tau, np.eye(3)))
        strain = self.get_strain_from_stress(tau)
        b += self.get_effective_ecs(strain)
        return b