SAFE A HEAD. Structural analysis and Finite Element simulation of an innovative ski helmet. Prof. Petrone Nicola Eng.

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2 SAFE A HEAD Structural analysis and Finite Element simulation of an innovative ski helmet Prof. Petrone Nicola Eng. Cherubina Enrico

3 Goal Development of an innovative ski helmet on the basis of analyses and tests performed on commercial helmets. Approach Development of a theoretical methodology to define the helmet s geometry from a mechanical point of view and subsequent virtual testing of the resulting model.

4 Steps of the research Impact and penetration tests on commercial helmets; Characterization of constitutive materials; Development and validation of a theoretical method for evaluating the necessary padding thickness; Creation of thickness-distribution maps referring to a standard headform for impact tests; Finite Elements simulation of impact and penetration tests on the helmet s virtual model; Analysis of results on the basis of the safety standards (UNI-EN 177); Geometry optimization for the preparation of a prototype.

5 Tests on helmets height = 1.5 m speed = 5.4 Impacts on flat anvil: m/s mass = 5.5 kg + helmet 12 1 A C 8 force [N] 6 helmet A 4 helmet B 2 B displacement [m] D

6 Tests on helmets height =.8 m speed 4 m/s mass = 3 kg Penetrating striker: 25 helmet A 2 A helmet B helmet C 15 helmet D C force [N] 1 5 B,1 time [s],1,1,1 D

7 Tests on materials On PC and ABS: On EPS: 1 2, ,5 Helmet Helmet Helmet Helmet Helmet Helmet Stress [MPa] 1,5,1,2,3,4,5 Strain,6,7,8 A B D E F G 6.8 s1.15 s-1.1 s s-1 5 stress [MPa] ,1,2 strain,3,4,5

8 Theoretical method for thicknesses Based on Hertz s theory for impacts V (spherical padding against flat anvil); Considers the EPS s stress-strain curve; Incremental deformation controlled by energy absorption; Iterations stop for: E absorbed = Ekinetic.

9 Theoretical method for thicknesses Input: EPS s-ε curve Input: kinetic energy to be absorbed 1% 2,5 1%,14 9% 2,19 8%,43 7% 1,5 Stress [MPa] 1 5% E Shell,99,86,9,57 2%,52 1% % Input: headform curvature E Padding,81 3%,1,2,3,4,5,6,7,8 Strain,48 6% 4%,5,1 F A G B D E Output: Thickness necessary in one point

10 Method validation Comparison with a real impact test (padding 7 g/l, thickness: 2mm, shell removed) Impact test results Theoretical method results 12 Peak force [N] 1644, ,9 16,36 16 Absorbed energy [J] 11499,9 Maximum displacement x [mm] Error: 7% Error: 4% Time [s],1,1

11 Thickness distribution Higher head curvature means thicker EPS layer; Different solutions are tested, related to the employment of different shell materials ( energy to be absorbed by the padding);

12 First helmet model Shell s profile following the thinnest thickness distribution (42 J absorption)

13 Finite Elements simulations Employment of the software LS-DYNA for impact analyses; Preparation of simple models to understand the mechanisms of the program; Replication of tests physically performed on foams in order to compare the results and verify the reliability of the output; Simulation of impact and penetration tests on the helmet model; Analysis of the results and modification of the parts.

14 Preliminary phase: base impacts Sphere on deformable plane

15 Test virtual replication (I) Compression of an EPS cylinder 2,5 2 1,5 Stress [MPa] 1,5,1,2,3,4 Strain,5,6,7

16 Test virtual replication (II) Top impact of the standard headform on an EPS plate

17 Test virtual replication (III) Penetration tests EPS plate EPS plate + ABS plate

18 Impact tests on first model Falling mass: Headform + EPS padding + outer shell 5.7 kg Impact velocity 5.4 m/s TOP Kinetic energy 83.5 J LATERAL FRONTAL

19 Impact tests first results Focus on headform s acceleration over the impact time Necessity of containing the acceleration peaks ACCELERATION LIMIT UNIEN 177

20 Helmet modifications Increased padding thickness (RED distribution); Previous shell position New shell position Smoothing of edges to avoid stress concentrations.

21 Impact tests on final model Back impact test added All the peaks are within the limit.

22 Penetration test TOP strike (smallest thickness, critical area) The striker stops before reaching the headform; Force displacement graph comparable to experimental ones

23 Final prototype model Remaining parts scaled on the new versions of padding and shell; Positioning pins are added to facilitate the assembly phase

24 Final prototype model

25 THANK YOU FOR YOUR ATTENTION!