4. THUMS Skeletal Kinematics: Rib Cage Deformation Under a Lateral Impactor Load
Objective and model setup
This case builds a THUMS (Total Human Model for Safety) thorax impact scenario in LS-DYNA and extracts rib cage deformation time histories under a lateral impactor. The goal is not a full injury-risk assessment but a reproducible pipeline: place the impactor, define contact and loading, run the solver, and post-process rib deflection at defined measurement locations. The output is a set of displacement/deflection curves that let you judge localized skeletal response (e.g., struck-side rib inward deflection versus sternum and contralateral ribs).
Assume a THUMS v4/v5 occupant model in LS-DYNA keyword format, seated upright, with the thorax centered near the global origin. Confirm units before anything else: THUMS is typically distributed in mm-ton-s (mm, tonne, s, N, MPa). If your deck is in m-kg-s, all lengths, masses, and derived stiffnesses change by orders of magnitude and the impactor velocity must be converted accordingly. Verify with a quick check: occupant mass should read roughly 75–80 kg equivalent (7.5e-2 to 8.0e-2 tonne in mm-ton-s), and thorax width should be on the order of 300 mm.
Impactor definition and lateral loading
A lateral impactor is modeled as a rigid body (or a rigid shell backed by a deformable pad) driven into the struck-side thorax. For a controlled rib-deformation study, a cylindrical or hemispherical rigid impactor of ~150 mm diameter is a reasonable teaching choice. Define the impactor as *MAT_RIGID with a realistic mass (e.g., 10–23 kg depending on the loading device you are emulating) and constrain all translational/rotational DOFs except the lateral (Y or X, depending on your model's lateral axis) direction.
Drive the impactor with a prescribed velocity or a prescribed motion. A clean, reproducible approach is a constant-velocity impactor using *INITIAL_VELOCITY_RIGID_BODY plus a short *BOUNDARY_PRESCRIBED_MOTION_RIGID ramp, or a displacement-controlled *BOUNDARY_PRESCRIBED_MOTION_RIGID with a defined *DEFINE_CURVE. For a lateral thorax study, a peak impactor velocity of 3–6 m/s over a 20–40 ms pulse is a common teaching range; state your chosen value explicitly and keep it fixed across runs so deformation differences reflect model response, not loading changes.
*MAT_RIGID
$ MID RO E PR N COUPLE M ALIAS
1 7.85e-9 2.10e+5 0.3 0.0 0.0 0.0
$ CMO CON1 CON2
1.0 7.0 7.0
*BOUNDARY_PRESCRIBED_MOTION_RIGID
$ PID DOF VAD LCID SF VID
1 2 2 101 1.0 0
*DEFINE_CURVE
$ LCID SIDR SFA SFO OFF TID
101 0 1.0 1.0 0.0 0
$ A1 O1
0.0 0.0
5.0e-3 -3.0e+3
25.0e-3 -3.0e+3
30.0e-3 0.0
Here DOF 2 corresponds to the lateral axis in this example; confirm the actual lateral axis of your THUMS deck (some decks use X as lateral). The velocity curve is in mm/s in mm-ton-s units, so −3.0e+3 mm/s = −3 m/s. The negative sign drives the impactor toward the struck side; keep the sign convention consistent with your coordinate check from the previous alignment step.
Contact and thorax interaction
Define contact between the impactor and the thorax surface. Use *CONTACT_AUTOMATIC_SURFACE_TO_SURFACE with the impactor as slave and the thorax skin/rib surface as master, or a single-surface contact if the impactor is included in a broader contact set. Set friction (e.g., 0.2–0.3) and a reasonable penalty scale. Avoid overly stiff contact that produces spurious rib penetration or hourglassing; check the contact energy in glstat and sleout.
Rib deformation measurement locations
Rib cage deformation is best captured as relative displacement between a struck-side rib node and a stable reference (sternum or contralateral rib). Define measurement nodes on the struck-side ribs at mid-axillary line (e.g., ribs 4–8), and a reference node on the sternum or the contralateral rib at the same level. Use *DATABASE_HISTORY_NODE to record nodal coordinates over time, then compute deflection as the change in the struck-to-reference distance relative to the initial distance.
*DATABASE_HISTORY_NODE
$ ID NODE1 NODE2
1 10045 0
2 10046 0
3 10047 0
4 10048 0
5 10049 0
Record the initial coordinates at t = 0 from the nodout or history output, then compute per-rib deflection:
deflection(t) = d(t) - d(0)
d(t) = |r_struck(t) - r_ref(t)|
where r_struck and r_ref are the struck-side rib node and reference node positions. A positive deflection indicates inward (compressive) rib motion toward the reference; report the sign convention explicitly so the curves are interpretable.
Run control and output
Set a termination time that covers the impact pulse plus a short unloading window (e.g., 60–80 ms total). Request glstat, matsum, sleout, and nodout at a fine interval (e.g., every 0.1 ms) so the deflection time history is well resolved. Check the energy balance: hourglass energy should stay below ~5% of internal energy, and sliding energy should not dominate. If hourglassing is high in the ribs, review element formulation and contact stiffness before trusting the deflection curves.
Interpreting the rib deformation time histories
Plot the per-rib deflection curves against time. A typical lateral impact response shows: an early elastic rise as the impactor contacts the struck-side ribs, a peak deflection near the end of the impactor stroke, and a rebound as the thorax unloads. Compare struck-side ribs at different levels (e.g., rib 4 vs. rib 7) to see how deflection is distributed vertically, and compare struck-side versus contralateral ribs to confirm the response is localized rather than a rigid-body shift. If all ribs deflect nearly identically, suspect that the reference node is moving with the struck side or that the impactor is loading the whole thorax rather than a localized region.
Verification checklist
| Check | Expected / pass condition | Failure symptom |
|---|---|---|
| Units consistent | Mass ~7.5e-2 tonne; thorax width ~300 mm | Deflections of 1e-3 mm or 1e3 mm; solver instability |
| Lateral axis correct | Impactor moves along the model's lateral axis | Impactor loads front/back; rib deflection near zero |
| Contact energy | Sliding energy < ~5% of internal energy | Spurious rib penetration; noisy deflection curves |
| Hourglass energy | < ~5% of internal energy | Rib deflection dominated by hourglass modes |
| Reference node stable | Sternum/contralateral node moves < struck-side node | All ribs deflect identically → reference is moving |
| Deflection sign | Positive = inward (compressive) toward reference | Curves inverted; misinterpreted injury direction |
Deliverable: rib deflection extraction record
Record the following for each run so the result is reproducible and comparable across impactor velocities or model versions:
- THUMS version, unit system, and lateral axis definition.
- Impactor mass, diameter, velocity curve (peak and pulse duration), and contact definition.
- Measurement node IDs for struck-side ribs 4–8 and the reference node ID.
- Initial distance d(0) per rib and peak deflection per rib with time of peak.
- Energy summary: internal, sliding, hourglass at end of run.
- Any deviation from the checklist above and its resolution.
With this record, a second analyst can rerun the deck, reproduce the deflection curves, and judge whether the localized skeletal response is consistent with the intended lateral loading.