MechaniCS: for safe helmet use

"Today's motorcycle and bicycle helmets are based on old safety standards that have become obsolete," explains Rémy Willinger, Professor of Biomechanics at the ICube laboratory at the University of Strasbourg. Current helmet safety standards are based on the "Head Injury Criterion" and date back to the 1970s. They do not incorporate the knowledge accumulated over the last few decades in fields such as solid mechanics, fluid mechanics, biomechanics and medicine.

"Consumers' choice of helmet is based on aesthetics or comfort, but the safety of one helmet compared to another is not part of the choice criteria, because manufacturers do not yet put enough emphasis on information", explains Laurent Santucci, partner at Assurance Mutuelle des Motards (AMDM).

Oblique shocks and traumatic brain injury

Before helmets are marketed, their protective capacity is tested by dropping a helmeted artificial head onto a horizontal surface. The height at which the helmet is dropped gives it a given velocity on impact, resulting in a deceleration of the head that is characteristic of the helmet.

To pass current safety standards, helmets are tested only for the protection they provide to the head in a so-called linear impact, i.e. a fall perpendicular to the impacted surface.

These tests have a number of limitations. Firstly, most real-life impacts are caused by oblique trajectories. For example, when a motorcyclist is thrown onto the road following a collision, his or her trajectory includes a vertical component, as well as a horizontal one, at the moment of impact. These oblique impacts induce translational and rotational forces on the skull, resulting in shearing effects in the brain matter. Today's helmets offer fairly good protection against skull fractures, but are not optimized for this type of brain trauma.

To better understand the effect of impacts in real-life conditions, Rémy Willinger's team has been involved in the analysis of 125 head injuries that have occurred over the last fifteen years. " During these analyses, we identified the kinematics of the victims, then simulated the head trauma to calculate the shear stresses in the brain matter, which we correlated with the occurrence of comas", explains the researcher. This step enabled us to show that the elongation of nerve connections resulting from rotational forces on the cranium could lead to irreversible brain damage. By analyzing these traumas, the team was able to define tolerance limits for brain damage, enabling them to evaluate helmets in terms of their level of protection. The results of these studies have been transferred to the research laboratories of automobile companies, so that they too can optimize their protection systems.

MechaniCS: a coupled experimental and numerical approach

To determine which helmets best protect brain matter during oblique impacts, the ICube laboratory's MechaniCS (Mechanics of Fluids, Materials, Biomechanics, Design and Simulation) platform has developed more comprehensive helmet testing methods than those usually carried out. In addition to testing the impact of helmets on horizontal surfaces, they are also tested on inclined surfaces, simulating impacts from oblique trajectories. The helmeted artificial heads feature sensors located at their center of gravity, which record linear and rotational accelerations to give an overall view of the helmets' protective capacity. Each helmet model is tested under six impact conditions, three linear and three oblique: two tests for each zone of the head (frontal, lateral and occipital). " We reproduce each test three times to eliminate any manufacturing defects in a particular helmet", explains Rémy Willinger.

The six acceleration curves (linear and angular) collected during the tests are then fed into a digital head model to calculate the brain's response to shocks and compare them with previously defined tolerance limits.

To this end, the head is digitally modeled using the finite element method: the skull and brain are "sliced" into small geometric elements to which tissue mechanical properties are associated. " This approach accounts for complicated geometries and enables us to calculate the internal stresses and deformation of the material when faced with a given loading", explains the researcher. By recording the various accelerations of the artificial head, it is therefore possible to estimate the effects on the brain via the digital model.

This approach provides an estimate of the protection afforded by different helmet models in the face of different shocks. The test results give an overall rating ranging from one to five.

Certimoov: an initiative that lists laboratory evaluations of motorcycle and bicycle helmets

The Certimoov website was created in 2018. It is accessible to all and has a dual function. Firstly, the scores associated with tests on different helmet models by the MechaniCS platform are accessible to consumers. "As part of Certimoov, we test helmets that already exist on the market in order to communicate the results to the general public," explains Laurent Santucci.

Secondly, helmet test protocols are made available online to manufacturers so that they can optimize their helmets according to the objective methods developed by the scientists.

This research could lead to new helmet models. " To better respond to oblique impacts, the general idea is to decouple the outer part of the helmet from the inner part in contact with the head, in order to limit the effect of rotational forces," explains Rémy Willinger. Numerous research and development projects are currently underway on these helmet models. "We don't look for solutions, that's up to the manufacturers, we just carry out objective tests to give them the tools and knowledge. However, we can also assist manufacturers in optimizing their helmets if they so wish," adds Laurent Santucci.

Certimoov is therefore a database that helps consumers make the right choices, and manufacturers choose the right models, to bring safety criteria back to the fore. Eventually, this database will include other test protocols and results on different types of protective equipment, such as back protectors.

Partnerships and multidisciplinarity at the root of major advances in the field

To achieve these results, the MechaniCS platform required a high degree of multidisciplinary collaboration, notably in artificial intelligence, mechanics of materials (solid, fluid and gelatinous), and biomechanical aspects open to neurosurgery and medicine.

This work was initially co-financed by the MAIF Foundation, which supported the development of test protocols by the ICube laboratory. Since 2019, Assurance Mutuelle des Motards has joined in the funding of the project to ensure its sustainability and to communicate its results to the public and to manufacturers.

Thanks in part to this initiative, new safety standards incorporating the protection provided by helmets against oblique impacts will become mandatory from 2023.