Impact Engineering

Attached file


The Working Group 3 within COST TU1101 action was aimed to integrate biomechanics investigation
in the context of bicycle helmet optimisation in terms of head protection. The initial plans
were to better understand the impact kinematics for bicyclists via real world accident simulation
and to develop an advanced helmet test methods which includes realistic head impact conditions
and biomechanical based pass fail criteria. A final task was to propose improvements of head
protection by investigating new material and design.

The work has been very focused on the question on how to design a better test method for bike
helmets. This work should be based on real accident situations. The reason why this is essential is
that the current test methods for bicycle helmets are not based on real accident situations. The
current test methods for bike helmets include a linear shock absorption test where the helmet is
dropped vertically to a flat surface. This COST action identified this as a problem as earlier accident
reports suggested that an angled impact is more common than a pure radial impact to the ground
(Verschueren 2009, Bourdet et al. 2013).

If angled impacts are more common the impact force to the ground will be a combination of a
normal force and a tangential force between the ground and the helmet. The tangential force is
due to the coefficient of friction which for the bicycle helmet is relative high when falling to the
road. A tangential force if high enough could cause the helmet and the head to rotate. It has in
earlier studies been shown that a rotational motion to the head could cause both concussion and
more severe brain injuries as subdural hematoma and diffuse axonal injury (Holborn 1943, Genarelli
et al. 1982, 1983, Deck et al 2007 and Kleiven 2007). It is therefore believed that helmets shoudl be
tested for angled impacts and to measure the rotational kinematics transfered through the helmet
to the head.

Further focus was also on the pass fail criteria as currently no head injury criteria exist for
complex head impact configurations. Based on advanced head FE modelling and a number of real
world head trauma simulations, a first attempt of model based head injury criteria as well as its
implementation into a novel helmet test method was evaluated.
Due to the initial project proposal for TU1101, WG3 has delivered what could have been
expected. Then at the beginning of the project we were a bit too enthusiastic making a too
ambitious project plan. WG3 planned to improve helmets by addressing new material and design.
That objective was not achieved.

The present report first focusses on the kinematic analysis of bicyclists in case of real world
accident. A review of accident analysis is reported and three accident cases are exposed in
details. The synthesis of this accident analysis is introduced in chapter 3 where new helmet
impact conditions are addressed in terms of initial velocity vector and in terms of head boundary
conditions. This chapter end out with a proposal of a new helmet test conditions including
tangential tests. Chapter 4 presents an extensive review of existing head injury criteria, based  both on global kinetic parameters and FE head modelling. Also a synthesis of existing injury criteria
evaluation is reported before concluding with a proposal for pass fail criteria. In the very last
chapters a synthesis of a new helmet test method proposal is presented followed by a section on
further research.