3. Troubleshooting Negative Volume Errors in a Dummy Pelvis Model

Failure Signature and Termination Path

A negative volume error in a pelvis flesh mesh is not a random solver failure — it is a deterministic consequence of one or more elements inverting under belt loading. LS-DYNA terminates with a message of the form Error negative volume in solid element #XXXXX (or shell element for a shell-based flesh representation), and the reported element ID is the single most valuable diagnostic clue. The task in this case is to move from that termination message to a corrected model that runs to normal termination without introducing nonphysical stiffness or mass scaling artifacts.

Two mechanisms dominate in dummy pelvis flesh meshes under lap-belt or belt-path loading:

• Geometric distortion — a hexahedral element with poor aspect ratio or high skew collapses as the belt compresses the flesh against the iliac wing. The Jacobian determinant crosses zero before the material reaches its failure strain.
• Material instability — an overly soft or under-damped flesh material (for example a low-shear-modulus *MAT_SIMPLIFIED_RUBBER or *MAT_VISCOELASTIC with insufficient bulk stiffness) allows excessive element distortion at realistic belt forces, again driving the Jacobian negative.

Distinguishing these two causes determines whether the fix is a mesh-quality correction or a material-parameter correction — or both.

Locating the Distorted Element

Start from the termination message and confirm the element's state at the last written state. A practical workflow:

1. Read the d3hsp file and record the reported element ID and the cycle/time of termination.
2. In d3plot, use the fringe plot of effective plastic strain (or pressure for a rubber-like flesh) at the last frame before termination. The offending element typically shows an extreme local value or a visible inversion.
3. Cross-reference the element ID against the mesh to identify its node connectivity and its position relative to the belt contact patch and the iliac crest.
4. Check the element's initial quality metrics (Jacobian, aspect ratio, warpage, skew) in the pre-processor. A pre-existing marginal element is the usual culprit.

The following SVG summarizes the diagnostic decision path from termination message to root cause and corrective action.

Mesh-Quality Correction

When the offending element has a marginal initial Jacobian (below roughly 0.3) or a high aspect ratio in the belt contact region, the fix is geometric. Typical corrective actions for a pelvis flesh hex mesh:

• Refine the element size locally in the belt path over the iliac crest, where the flesh is thinnest and compression is highest.
• Replace collapsed or highly skewed hexes with a transition to tetrahedral elements, or use *SECTION_SOLID with an appropriate element formulation (ELFORM 1 for reduced integration with hourglass control, or ELFORM -1 / -2 for fully integrated variants if distortion is severe).
• Re-run the pre-processor quality check and confirm minimum Jacobian above 0.4 in the contact patch before re-submitting.

Note that switching to fully integrated solids removes hourglassing but increases stiffness and cost; verify that the pelvis flesh compression response remains within the baseline corridor established in the reference model.

Material-Parameter Correction

If the mesh is clean but the element still inverts, the flesh material is too compliant or under-damped for the applied belt load. For a simplified rubber or viscoelastic flesh:

• Increase the shear modulus G (or the Mu term in the Ogden/Mooney-Rivlin form) so the flesh resists compression at realistic belt forces.
• Ensure the bulk modulus is at least two to three orders of magnitude above the shear modulus to prevent volumetric collapse; a common error is setting bulk stiffness too low, allowing near-incompressible flesh to compress unphysically.
• Add or increase damping (for *MAT_VISCOELASTIC, the decay constant and bulk/shear relaxation) to suppress high-frequency element oscillation that precedes inversion.

Apply one change at a time and re-run, so the effect on the termination and on the injury metrics is attributable.

Verification Criteria

A corrected model is accepted only when all of the following hold: