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Towards improved motorcycle helmet test methods for head impact protection: Using experimental and numerical methods
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Biomedical Engineering and Health Systems, Neuronic Engineering.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Head injury is the leading cause of death and major trauma for users of powered two-wheelers (PTWs). Helmet use can prevent and reduce head injuries when a crash is inevitable. However, today’s motorcycle helmets do not protect equally against all types of head injuries. All helmets available on the market today are designed, manufactured and tested to meet certain standards. Current test standards evaluate helmet performance by dropping a helmeted headform perpendicular to an impact surface, and passing or failing a helmet based on the peak linear acceleration between 250-400G. Yet, real-world head impacts, being either linear (perpendicular) or oblique, impart both linear and angular head acceleration. Oblique impacts, which are known to be more common than linear impacts from in-depth analysis of motorcycle accidents, can transmit the tangential impulse to the head and hence cause the head to rotate. Head rotation has been hypothesised to be the main cause of traumatic brain injury (TBI) ranging from mild injuries such as concussions to more severe injuries such as acute subdural haematomas and diffuse axonal injuries. Therefore, there is a great need to develop test methods that replicate real-world accidents and reproduce realistic head impact responses. A number of potential test methods that subject the head to rotational insults are available today. However, there are still several questions that need to be answered: At what speed and angle should the helmet be tested? How boundary conditions of the head in the test methods, i.e. free, partially constrained or a surrogate neck, affect the kinematics of the head?

To answer these research questions, both experimental and numerical methods, such as finite element (FE) methods were used. Experimental tests for the helmet were performed using multiple test methods, providing a comparison between the test methods and data for subsequent validation of the FE helmet model coupled with anthropomorphic test devices (ATDs). FE Human body models (HBMs) with accurate anatomical structures and material properties were employed to evaluate the biofidelity of current test methods. Brain tissue strain of a head model resulting from direct impacts or inertial loadings were used to provide a direct causal link between the mechanical insult and the brain injury.

The first study in the thesis showed that both the US and European helmet standards lacked consideration for head rotation in linear impact tests. The US helmet standards use a partially constrained headform, which does not permit head rotation and hence not rotation induced TBI. European standards, on the other hand, adopt a free headform but the head rotation is not measured or assessed. The brain tissue strain resulting from the European standard tests at which rotation is allowed was up to 6.3 times higher than that in the US standards. In the second study, 300 simulations of possible motorcycle accidents were performed to understand the effect of impact velocity angle on impact severity. The results indicated that a 30o or 45o impact angle produced greater brain tissue strain than other impact angles, i.e., 15o, 60o and 75o. In the third study, it was found that when the helmeted head impacted the ground from low to high tangential velocities, i.e., 0-216 km/h, the motion of the helmet exhibited rolling and sliding phenomena. Since the helmet rolling and sliding phenomena govern impulses transmitted to the head-helmet system, and consequently the brain tissue strain, it is desirable to test helmets at speeds covering both the rolling and sliding regime. The tangential velocity at which motion transitioned from rolling to sliding was identified to be 10.8 m/s (38.9 km/h), given that the normal velocity is 5.66 m/s (20.4 km/h) and the coefficient of friction between the helmet outer shell and the impact surface is 0.45. In the final study, simulations with and without the experimental neck (Hybrid III) were compared to the HBMs. The results showed that the Hybrid III head-neck ATD used in the laboratory setting proved to correlate less with the head responses of the HBMs than the free headform. In particular, the Hybrid III head-neck ATD correlated poorly with the HBMs in axial (inferior-superior) acceleration and over-predicted the maximum angular velocity by up to 75%. However, the free headform was also limited in replicating the chin-neck and helmet-torso interactions. The need for a more biofidelic surrogate neck, especially under axial compression, is evident.

In summary, this thesis demonstrates methodologies for a reason and objective based decision making process and provides important information in the design of future helmet test methods and standards. Some of the major findings in this thesis, despite focusing on motorcycle helmets, can also be applied to other types of helmets.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. , p. 50
Series
TRITA-CBH-FOU ; 2019:58
Keywords [en]
Head Injuries; Helmet; Injury Prevention; Motorcycle; Test Standard, Finite Element Method
National Category
Engineering and Technology Medical and Health Sciences
Research subject
Applied Medical Technology
Identifiers
URN: urn:nbn:se:kth:diva-262734ISBN: 978-91-7873-343-9 (print)OAI: oai:DiVA.org:kth-262734DiVA, id: diva2:1362393
Public defence
2019-11-11, T2, Hälsovägen 11, Huddinge, 10:00 (English)
Opponent
Supervisors
Note

QC 2019-10-21

Available from: 2019-10-21 Created: 2019-10-18 Last updated: 2019-10-21Bibliographically approved
List of papers
1. The biomechanical differences of shock absorption test methods in the US and European helmet standards
Open this publication in new window or tab >>The biomechanical differences of shock absorption test methods in the US and European helmet standards
2019 (English)In: International Journal of Crashworthiness, ISSN 1358-8265, E-ISSN 1754-2111, Vol. 24, no 4, p. 399-412Article in journal (Refereed) Published
Abstract [en]

Nowadays crash helmets are tested by dropping a free or unrestrained headform in Europe but a guided or restrained headform in the United States. It remains unclear whether the free fall and the guided fall produce similar impact kinematics that cause head injury. A ?nite element helmet model is developed and compared with experimental tests. The resulting head kinematics from virtual tests are input for a ?nite element head model to compute the brain tissue strain. The guided fall produces higher peak force and linear acceleration than the free fall. Eccentric impact in the free fall test induces angular head motion which directs some of the impact energy into rotational kinetic energy. Consequently, the brain tissue strain in the free fall test is up to 6.3 times more than that in the guided fall. This study recommends a supplemental procedure that records angular head motion in the free fall test.

Place, publisher, year, edition, pages
Taylor & Francis Group, 2019
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-252954 (URN)10.1080/13588265.2018.1464545 (DOI)000468457900004 ()2-s2.0-85046630058 (Scopus ID)
Note

QC 20190802. QC 20191021

Available from: 2019-08-02 Created: 2019-08-02 Last updated: 2019-10-21Bibliographically approved
2. The effect of impact velocity angle on helmeted head impact severity: A rationale for motorcycle helmet impact test design
Open this publication in new window or tab >>The effect of impact velocity angle on helmeted head impact severity: A rationale for motorcycle helmet impact test design
2018 (English)In: Conference proceedings International Research Council on the Biomechanics of Injury, IRCOBI, International Research Council on the Biomechanics of Injury , 2018, p. 454-469Conference paper, Published paper (Refereed)
Abstract [en]

The impact velocity angle determined by the normal and tangential velocity has been shown to be an important description of head impact conditions but can vary in real-world accidents. The objective of this paper was to investigate the effect of impact velocity angle on helmeted head impact severity indicated by the brain tissue strain. The human body model coupled with a validated motorcycle helmet model was propelled at a constant resultant velocity but varying angle relative to a rigid surface. Different body angles, impact directions and helmet designs have also been incorporated in the simulation matrix (n=300). The results show an influence of impact velocity angle on brain tissue strain response. By aggregating all simulation cases into different impact velocity angle groups, i.e., 15, 30, 45, 60 and 75 degrees, a 30- or 45-degree angle group give the highest median and inter-quartile range of the peak brain tissue strain. Comparisons of strain pattern and its peak value between individual cases give consistent results. The brain tissue strain is less sensitive to the body angle than to the velocity angle. The study suggests that UN/ECE 22.05 can be improved by increasing the current 'oblique' angle, i.e. 15 degrees inclined to vertical axis, to a level that can produce sufficient normal velocity component and hence angular head motion. This study also underline the importance of understanding post-impact head kinematics, and the need for further evaluation of human body models.

Place, publisher, year, edition, pages
International Research Council on the Biomechanics of Injury, 2018
Keywords
finite element method, Head impact conditions, Impact severity, Motorcycle helmet, Test method
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:kth:diva-246506 (URN)2-s2.0-85061089583 (Scopus ID)
Conference
2018 International Research Council on the Biomechanics of Injury, IRCOBI 2018, 12-14 September 2018, Athens, Greece
Note

QC 20190329. QC 20191021

Available from: 2019-03-29 Created: 2019-03-29 Last updated: 2019-10-21Bibliographically approved
3. High-speed helmeted head impacts in motorcycling: A computational study
Open this publication in new window or tab >>High-speed helmeted head impacts in motorcycling: A computational study
2019 (English)In: Accident Analysis and Prevention, ISSN 0001-4575, E-ISSN 1879-2057Article in journal (Refereed) Accepted
Abstract [en]

The motorcyclist is exposed to the risk of falling and impacting ground head-first at a wide range of travellingspeeds – from a speed limit of less than 50km/h on the urban road to the race circuit where speed can reach well above 200km/h. However, motorcycle helmets today are tested at a single and much lower impact speed, i.e. 30km/h. There is a knowledge gap in understanding the dynamics and head impact responses at high travelling speeds due to the limitation of existing laboratory rigs. This study used a finite element head model coupled with a motorcycle helmet model to simulate head-first falls at travelling speed (or tangential velocity at impact) from 0 to 216km/h. The effect of different falling heights (1.6m and 0.25m) and coefficient of frictions (0.20and 0.45) between the helmet outer shell and ground were also examined. The simulation results were analysed together with the analytical model to better comprehend rolling and/or sliding phenomena that are often observedin helmet oblique impacts. Three types of helmet-to-ground interactions are found when the helmet impacts ground from low to high tangential velocities: (1) helmet rolling without slipping; (2) a combination of sliding and rolling; and (3) continuous sliding. The tangential impulse transmitted to the head-helmet system, peak angular head kinematics and brain strain increase almost linearly with the tangential velocity when the helmet rolls but plateaus when the helmet slides. The critical tangential velocity at which the motion transit from the rolling regime to the sliding regime depends on both the falling height and friction coefficient. Typically, for a fall height of 1.63m and a friction coefficient of 0.45, the rolling/sliding transition occurs at a tangential velocity of 10.8m/s (38.9 km/h). Low sliding resistance in helmet design, i.e. by the means of a lower friction coefficient between the helmet outer shell and ground, has shown a higher reduction of brain tissue strain in the sliding regime than in the rolling regime. This study uncovers the underlying dynamics of rolling and sliding phenomena in high-speed oblique impacts, which largely affect head impact biomechanics. Besides, the study highlights the importance of testing helmets at speeds covering both the rolling and sliding regime since potential designs for improved head protection at high-speed impacts can be more distinguishable in the sliding regime than in the rolling regime.

Place, publisher, year, edition, pages
Elsevier, 2019
National Category
Engineering and Technology Medical and Health Sciences
Identifiers
urn:nbn:se:kth:diva-262731 (URN)10.1016/j.aap.2019.105297 (DOI)
Note

QC 20191021

Available from: 2019-10-18 Created: 2019-10-18 Last updated: 2019-10-21Bibliographically approved
4. Head impact responses in simulated motorcycle accidents and laboratory reconstructions
Open this publication in new window or tab >>Head impact responses in simulated motorcycle accidents and laboratory reconstructions
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology Medical and Health Sciences
Identifiers
urn:nbn:se:kth:diva-262732 (URN)
Note

QC 20191021

Available from: 2019-10-18 Created: 2019-10-18 Last updated: 2019-10-21Bibliographically approved

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