In this lecture, we will cover:

• The unrestrained drilling degrees of freedomin our model,
• Stiffness matrix rank and condition number,
• We will prepare for drilling stabilisation as the corrective strategy.

We begin by returning to the shell structure that behaved unexpectedly after a small parameter change. We then inspect the stiffness matrix more directly by considering its rank and condition number. These checks reveal that the matrix has become rank deficient and badly conditioned, which is consistent with the presence of zero-energy modes. We connect this deficiency back to unrestrained drilling rotations in the model, giving you a concrete explanation for why the solver can fail under certain configurations.

Next up

In the next lecture, we will move from diagnosis to remedy by deriving a practical strategy for avoiding a singular stiffness matrix.