Final Project “Investigation of Head Impact during an Inverted Drop Test”

1. Final Project“Investigation of Head Impact during an Inverted Drop Test” ME 272 12/18/06 Luke Gibbons

2. Problem Statement • Inverted 8 foot drop test of 2000+ lb auto with 250+ lb dummy • Focus our attention upon determining the pressure experienced by the skull during impact • We will use Nastran 4-d’s Finite Element Analysis (FEA) to determine the von Mises stress (psi) experienced during impact • We will also use Nastran to determine the location and direction of the force experienced by the skull • Finally, we will compare the results with documented tolerance limits of the human brain and the Severity Index (SI)

3. Problem Setup [1] • A lifelike dummy will be modeled using standard dummy dimensions • The head and body will be modeled with material representing cortical, or dense, skull bone • The use of cortical bone to represent the entire body will act as a safety factor because the body will now weigh more than an average human for the given volume

4. Problem Setup • The dummy’s body will be joined in Nastran 4-d using revolute joints for all joints except the head-neck connection • The dummy’s head will be connected to the neck by means of a spherical joint • We assume the Coefficient of Restitution is 0.3 • We will mesh the head using a 0.5 inch mesh size

5. Problem Setup • A restraint system was added to model a lap and chest seatbelt • The restraint system was modeled as two spring/damper systems anchored to the auto in the same location and attached to the dummy at the center of the upper and lower back • The standard value of 9.81 m/s² for gravity was implemented

6. Problem Setup • We will also look at the Gadd Severity Index (SI) • According to Gadd, a SI value above 1,000 is considered “dangerous to life” • The Severity Index is calculated by: where a is the acceleration of the head and t is the impact duration [2]

7. Results [1] • The maximum von Mises stress the brain can withstand before neurological lesions occur is 18 kPa • At the present height above the ground, the car takes 0.61 seconds to contact the ground while the dummy’s head takes 0.62 seconds to initially contact the roof of the auto and 0.63 seconds until the maximum impact point • The green arrows show pressure applied to the skull

8. Results: 0.60 seconds • Before Impact

9. Results: 0.62 seconds • Initial Impact

10. Results: 0.63 seconds • Full Impact

11. Results: 0.64 seconds • Whiplash Effect

12. Results: Stress vs. Time

13. Results: Stress During Impact

14. Results: Stress Distribution at Maximum Impact

15. Conclusion • The brain cannot withstand the fall with the present restraining system • The head experiences von Misses stress of over 152 MPa, which is significantly larger than the critical von Mises stress the brain can withstand (18 kPa) [1] • The point where the head connects to the neck receives a tremendous force at maximum impact even after the head absorbs a portion of the force • The head experiences the greatest acceleration during whiplash • The whiplash effect is prevalent, and according to Gadd [2], with a head acceleration of 386 m/s² and assuming an impact duration of 0.001 seconds, the Severity Index value is almost 3,000, 3 times greater than Gadd’s “dangerous to life value” of 1,000

16. Reference • [1] Raul, J., Baumgarter, D., Willinger, R., Ludes, B. “Finite Element Modeling of Human Head Injuries caused by a Fall”. International Journal of Legal Medicine. Published online 03 July, 2005 • [2] Gadd, C.M. “Use of a Weighted Impulse Criterion for Estimating Injury Hazard”. Proceedings of the 10th Stapp Car Crash Confrence, Society of Automotive Engineers, New York NY, pp. 164-174, 1966