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On a finite element based approach to the design and optimisation of elastomeric additively manufactured cellular structures for impact mitigation in helmets

Rhosslyn, Adams 2022. On a finite element based approach to the design and optimisation of elastomeric additively manufactured cellular structures for impact mitigation in helmets. PhD Thesis, Cardiff University.
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Physical activity that includes elevation or speed, such as cycling, carries the risk of head injury. The risk of injury is mitigated by wearing safety helmets. Whilst cycling is adopted as an exemplar when evaluating new helmets, garnering notable research interest in recent years, head injury remains a notable cause of mortality and morbidity in cycling accidents. Indeed, head injury still causes 69–93% of fatal bicycle accidents; hence, new helmet technologies have significant social importance. A critical strategy in head injury mitigation in cycling accidents is the use of a protective helmet and is strongly advocated by The World Health Organisation. It has been reported that the likelihood of sustaining a traumatic brain injury (TBI) is reduced (18%) when wearing a helmet compared to non-helmet cyclists (48%). Current bicycle helmets are designed to protect the user by deforming to mitigate the impact energy and reduce the resultant acceleration. Due to the onset of permanent deformation within the helmet following an impact, its protective capacity is diminished. Consequently, they do not provide adequate performance when subject to a history of loading, such as multiple or consecutive impacts. Indeed, it is common to wear a previously damaged helmet despite contrary advice. Current multi-hit solutions are derived from elastomeric foams which have been applied in several other sporting helmets. These materials, however, suffer from a lack of development in recent years. Furthermore, they have limited geometric freedom, which precludes optimisation. This thesis describes four related investigations which present a finite-element based optimisation approach to the development of new helmet liners leveraging the mechanical benefits of cellular structures and elastomeric materials realised through additive manufacturing. A laser sintered elastomer was characterised by performing low, intermediate and high rate uniaxial tension tests manufactured under three build orientations.Furthermore, planar, equibiaxial tension and stress relaxation tests were carried out. These data demonstrated notable anisotropy, as well as significantly different behaviour across strain rates and deformation modes, necessitating fit of an augmented hyperelastic and linear viscoelastic model. FE software was then used to calibrate material model coefficients, with their validity evaluated by comparing the simulated and experimental behaviour of the material in isolated (uniaxial tensile) and mixed modal (honeycomb compression) deformation states. Close correlation demonstrated that the material model coefficients were valid, removing a barrier to adopting exclusive finite-element based in future investigations. Further, laser sintering was used to manufacture different structural variations of a novel pre-buckled circular honeycomb. The mechanical behaviour of these structures was examined under both quasi-static and dynamic impact loading. Pre-buckled circular honeycombs with aspect ratios, defined as the ratio of minor to major axis of the honeycomb elliptical profile, e = 0.8 and e = 0.6 were compared to a traditional, straight-walled honeycomb, e = 1.0. It was found that the mechanical behaviour of the honeycomb can be tailored to yield different mechanical responses. Principally, decreasing the aspect ratio reduced the stress at yield, as well as the total energy absorbed until densification, however, this alleviated the characteristic stress-softening response of traditional honeycombs under static and impact conditions. When subjected to multiple cycles of loading, a stabilised response was observed. Finite element simulation closely agreed with the experimental results. A simplified, periodic boundary condition model was also investigated, which closely represented the experimental results whilst alleviating computational run time by nominally 75%. The numerical full factorial parameter design sweep identified a broad range of mechanical behaviour, enabling identification of geometric bounds to be used in future optimisation studies. Finite element simulation was used to analyse the response of an elastomeric prebuckled honeycomb structure under impact loading, to establish its suitability for use in helmet liners. Finite element-based optimisation was performed using a search algorithm that uses a radial basis function. This approach identified optimal configurations of the pre-buckled honeycomb structure, based on structural bounds identified from previous investigations, subject to impact loading conditions. Furthermore, the influence of objective function, peak acceleration and head injury criterion was analysed with respect to the resultant mechanical behaviour of the structure. Numerical results demonstrate that this class of structure can exceed the performance threshold of a common helmet design standard and minimise the resultant injury index. Experimental testing, facilitated through laser sintering, validated the output of the numerical optimisation. When subject to initial impact loading, the fabricated samples satisfied their objective functions. Successive impact loading was performed to assess the performance and degradation. Samples optimised for peak acceleration demonstrated superior performance after stabilisation, relative to their initial response. The finite-element based optimisation sequence was repeated using boundary conditions (contact area, mass and velocity) associated with the helmet design envelope and standard. Two optimal configurations, based on different objective functions, peak translational acceleration and head injury criterion, were then proliferated throughout a helmet liner before being subjected to typical frontal head impacts under direct and oblique conditions. Comparison was drawn relative to two densities of a common multi-hit material used in helmet liners, vinyl nitrile foam. Kinematic-based injury criteria were calculated, as well as tissue-based injury criteria, using a validated finite element model of the human head for each impact. Results demonstrated that the optimal pre-buckled honeycomb liners had the best performance, yielding reductions in both kinematic and tissue-based injury criteria. Under direct impacts, values for head injury criterion, maximum principal strain and cumulative strain damage measure were reduced by 34%, 8.6% and 23.7%. Under oblique impact, values for head injury criterion, rotational injury criterion, generalized model for brain injury threshold, head impact power, maximum principal strain and cumulative strain damage measure were reduced by 49.9%, 56%, 29.6%, 40.8%, 14.9% and 66.7%. Honeycombs optimised using head injury criteria as the objective function yielded a reduction in all severity metrics relative to honeycombs optimised using peak translational acceleration, though design standards mandate an acceptable threshold for peak translational acceleration yet not for other injury metrics. The work reported in this thesis identifies a successful design and optimisation strategy to aid and inform the development of next generation helmet liners.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Helmets , Additive Manufacturing , Impact mitigation , Finite Element Analysis , Traumatic Brain Injury , Elastomers , Honeycomb ,
Date of First Compliant Deposit: 16 August 2022
Last Modified: 18 Aug 2022 11:06

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