Safety First: Designing the Next Generation Bicycle Helmet
Academia focus - UCL
University College London (UCL) has numerous long established relations with the commercial world which enables its students to gain practical, hands-on experience beyond the classroom environment. One such relationship exists between UCL Mechanical Engineering and leading engineering software solutions provider Desktop Engineering (DTE).
UCL Mechanical Engineering uses 200 student and 35 academic licences of CATIA V5 from Dassault Systèmes. Components commonly used by the students include surface design, assembly design, mechanical design, FE analysis and modelling. CATIA was chosen by Dr Baker as he sees it as the ‘industry standard’. The emphasis of the course is geared towards teaching practical engineering, with design and CATIA at the heart of that focus.
All third year students are tasked with designing a product in industry leading software, such as CATIA, which will have commercial value. For this particular case study, we will examine the work of James Cook and how he is in the process of designing a new bicycle helmet that looks to raise the bar in terms of safety.Designing the Bicycle Helmet
Bicycle-related head injuries in the United States resulted in an estimated 81,000 emergency room visits in 2011. It has been shown that 70% of bicycle-related fatalities are due to head injuries. The number of bicycle-related traumatic brain injuries (TBI) has increased steadily over the past 15 years, in spite of increased rates of helmet use among cyclists.
In 2013, a study into bicycle accident reconstructions, suggested that the mean impact angle between the helmet direction and the horizontal was 28 degrees, suggesting that in a typical fall the horizontal velocity component is three times the vertical velocity component. Mandatory helmet test standards asses linear head acceleration but fail to take into account angular head acceleration. Enduring research has demonstrated that angular accelerations to the head are a major cause of TBI, including concussion, diffuse axonal injury (DAI) and acute subdural hematoma (SDH) even in the absence of a direct impact to the head. The mechanisms for these injuries have been further investigated through physical models, cadaver studies and computational simulations, which have demonstrated that the brain is highly susceptible to the shear strain induced by angular head rotations.
Further complicating the problem, it was concluded that SDH was generated by a short duration and high amplitude angular head acceleration, while DAI was generated by a longer duration in combination with lower head acceleration.
There are two successful and novel solutions that have proved to mitigate head injury by absorbing both linear and rotational head acceleration:
Multi-Impact Protection Systems (MIPS)
Angular Impact Mechanism (AIM)
MIPS – The MIPS helmet is similar to a free helmet except for a low friction Teflon film between the shell and the liner. When there is an oblique impact, the low friction layer allows the shell to rotate relative to the liner. The flexible joint at the front and rear absorbs energy, and prevents the shell rotating so far that it hits the rear of the neck. A MIPS helmet has been proven to reduce the peak angular acceleration by 30% compared to a conventional Expanded Polystyrene (EPS) helmet.
AIM – The AIM system employs an elastically suspended aluminium honeycomb liner. For mitigation of linear acceleration, the honeycomb serves as a non-elastic crumple zone to absorb the normal component of the impact force. The honeycomb acts as an elastic spherical bearing between the outer shell and inner liner. To enable elastic translation between the outer shell and inner liner the honeycomb was attached at discrete fixation points. The AIM helmet proved to reduce maximum linear acceleration of the head form by 14%, whilst also reducing the peak angular acceleration by 34%.
A design matrix was created to differentiate the various mechanisms used to mitigate oblique forces. The design criterion used in the matrix was based on the BS EN 1078:2012 ‘Helmets for pedal cyclists and for users of skateboards and roller skates’ and on up-to-date head injury thresholds.
The solution chosen by James to best mitigate head injuries received the highest rating from the weighted design matrix. The solution is ‘Oblique Mitigation System’ (OMS).
Oblique Mitigation System
This mechanism combines both the MIPS and AIM technology, where two honeycomb liners are able to rotate relative to a low friction separation layer. The elastically suspended honeycomb inner and outer shells coupled with a low friction intermediate liner allows maximum potential to absorb shear forces. This composite structure provides an innovative suspension system that mitigates oblique impact.
CAD Modelling and 3D Printing
Modelling was undertaken using CATIA, which provided a perfect engineering platform to design and adapt the model. The finite element analysis package in CATIA was applied to make an initial judgement on the structural rigidity of a particular component.
The designs in CATIA were used to create the flat honeycomb core/sandwich specimen for the normal drop tests. From evaluation of the drop test, the geometry was altered in CATIA. In addition, CATIA was used for creating a prototype OMS helmet.
Once modelled in CATIA, the parts were manufactured in UCL’s Institute of Making on a 3D printer. The choice of material determined from testing results. The ability to model, manufacture, test and evaluate within UCL allowed a quick iterative design process.Find out more
Design engineering was traditionally taught in an antiquated way using drawing boards. At UCL Mechanical Engineering we endeavour to push our students to design something that results in an end product and to do this we need the latest technology. This means we have to invest, but in terms of the quality of student we produce it is worth it. We encourage hands-on, problem solving design engineering and solutions like CATIA help us deliver graduates that are more capable of handling the day-to-day challenges of the work environment.
Dr Tim Baker, Lecturer - UCL Mechanical Engineering
Designing an internal honeycomb structure for the helmet presented James with a number of modelling challenges. The honeycomb geometry itself is non-manifold which can be problematic in certain situations. The honeycomb wireframe needed to the drawn onto the internal helmet surface rather than a simple, flat surface and then the honeycomb needed to be “extruded” as a solid but in a direction that was normal to the surface at every point.
DTE provided CATIA support by outlining and testing a repeatable modelling workflow that James could use to work on different iterations of the design. This process might have been relatively easy for a simple piece of wireframe, but one important part of the process was not just obtaining the end result but doing so with a method that did not require huge numbers of clicks and interactions by the user. It was important that any changes and redesigns could be done with minimal effort.
Once the theoretical modelling process was understood, DTE then looked at importing a 3D scan of a real head and using that geometry to create a custom-fit honeycomb model. Again, DTE provided CATIA support to illustrate the reverse engineering path, from importing the 3D scanned data, to creating a smoothed surface model from this scan which could then be used to fully test the previous honeycomb modelling process around a more real-world shape.
By the end of process, James had a solid model which he could create a first-off 3D printed sample and a working process to create further custom-fit physical models within a very short space of time.
Dr Tim Baker, a lecturer at UCL Mechanical Engineering, explains how the relationship with DTE evolved:
During the working week I wear both an academic and a commercial motorsport hat and know DTE from previous work that I have done in the industry. Back in 2002, I was looking for a CAD solution and came across DTE due to a colleague’s recommendation. Since that day I have worked with DTE on a regular basis and have always received a great service. I always find they’re willing to go the extra mile in terms of helping our students with their design projects.
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