4.c. Status of NHTSA Research: Preliminary PMHS Injury Risk Curves and Potential Injury Criteria in Rear Impact

Yun-Seok Kang (YSK) presented the status of NHTSA research (WCWID-01-06).

The study is divided in two phases. In Phase 1 tests are performed using a laboratory seat and in Phase 2 one using a production car seats.

Aims of Phase 1 (laboratory seat) were:

• Evaluate the biofidelity of candidate dummies
• Investigate the mechanism of injury
• Relate injury to measured PMHS variables

The seat included load cells in the base, back and head restraint. Springs controlled the initial response, with dampers to control rebound. Tests were performed at 17 km/h, 8.5 g and 24 km/h, 10.5 g – two tests with each of seven PMHS (plus four low-speed tests with one additional PMHS).

A new technique for mounting instrumentation to each cervical body without disrupting the main musculature of the neck was developed. This gave the rotation and displacement of each vertebral body. It was found that intervertebral flexion dominated the response.

Disc ruptures, subluxations (representing WAD) and ligament lacerations were identified upon necropsy.

Correlated intervertebral kinematics (linear/angular acceleration, velocity and displacement) and injury; used this to suggest criteria and correlate with global kinematics in order to identify potential injury criteria on the dummy. Rotation about the y-axis had the best correlation, with angular velocity and facet joint slide reasonable. However, each intervertebral level may have a different threshold, so the data was normalised using the physiological range of motion.

JD commented that the data is being treated as though the measurements at each intervertebral level are independent, but they may not be – if you damage at one level it may release other levels. The rupture and laceration only occurred in the lab seat tests, due to over-riding the head restraint resulting in hyper-extension. In the car seat tests (to be presented later), hyper-extension did not occur and there were none of these more serious injuries – only subluxations.

IV-NIC rotation was the best injury predictor, and rotation measures were better correlated than displacements or strains
Potential ‘global’ PMHS injury criteria were NDCrot and NDCx (and NIC).

However, this test condition was designed for evaluating biofidelity, repeatability etc. – not to represent a real seat; rigid head restraint affects loads; upper and lower neck loads not accurate after head restraint contact due to ramping-up motion; and seat back rotation was more uniform that real seats. The lab seat induces flexion, which will left-censor the risk functions considerably – therefore the values from this study should not be used directly.

This, with other limitations, meant that tests were performed with production car seats.

There was some discussion regarding the lack of muscle tone in PMHS subjects. The muscle tone in vivo (once the head is upright) to maintain position is 1-5% of maximal exertion, and reaction to loading occurs after the period of interest, so no significant limitation to using PMHS.

Phase 2: Production car seats

• Paired BioRID II and PMHS tests
• Verify measures from experimental seats highly correlated to injury

Toyota Camry and Chevy Cruze chosen – one with good IIHS and Euro NCAP ratings, one with poor. Three pulses: FMVSS 202a, JNCAP, 24 km/h. Seven PMHS, with the combination of seat and pulse altering the dose. Aimed for 50 mm backset, which was achieved for five PMHS.

Global motion was rearward (extension), but at each cervical vertebral level, flexion again dominated. BioRID also showed flexion (lower magnitude at the higher, 24 km/h severity).

Again, injuries at each vertebral level were compared with intervertebral kinematic measurements. Rotation about the y-axis had the best correlation. NDCrot had the best correlation for these seats.

In conclusion:

• IV-NICrot was the best PMHS injury risk predictor.
• The most promising BioRID injury criteria were IV rotation and NDCrot.

KM noted that the IIHS and Euro NCAP assessments seem to be doing the right sort of thing, so could just add the metric(s) recommended from the GTR-7 work. JD noted that the metrics used in the consumer information tests are focussed on preventing hyper-extension, so an additional metric to prevent injurious flexion would be welcome.

KM noted that the subluxations cannot be observed on a live patient, and we are currently working with a neurosurgeon to determine whether the subluxations would be associated with WAD, rather than the more severe, conventionally AIS-coded injuries.

Presentation from KM on the alignment of the US and Japan studies:

Best injury predictor for a dummy:

• US – iV-NICrot
• Japan – iV-NICrot (well correlated with strain and strain rate)

Potential ‘global’ injury criteria

• IV-NICrot → NDCrot, NDC, NIC
• IV-NICrot → NIC, UNFx, UN My, LN Fx, LN My

Common ground

• NIC
• US: Investigated forces and moments in PMHS. May need to consider direct correlation with the forces and moments in the BioRID
• Japan: Investigated NDCrot and NDCx (well correlated to WAD2+)

Sensitivity analysis showed that the PMHS neck forces and moments were very sensitive to the position of the centre of the contact, which was not well characterised. Also, the BioRID skull cap force did not match the head restraint forces in 5 out of 7 tests.

Future work:

• Ensure the dummies represent the final regulatory tool
• Re-run 5 injury criteria sled tests using (both BioRIDs) with Camry and Cruze seats
• Conduct paired BioRID/Hybrid III sled tests
  • To get extension metrics to supplement the flexion metric
  • 5 seats (Cruze, Camry, Toyota Matrix, Ford F150, Honda Odyssey), all three
    pulses
  • Check BioRID metrics deliver at least the protection level of 202a with Hybrid III head angle metric – which is the minimum requirement for the US to be able to adopt BioRID, because the current safety level cannot be degraded.