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IMPACT MECHANICS OF HELMET COMPONENTS

Hazel, B. van den (2015) IMPACT MECHANICS OF HELMET COMPONENTS.

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Abstract:In horse riding head protection of the jockey is important, therefore this research is performed in collaboration with Albion. Albion is specialized in many sports protection gear. The aim of this research is to test, model and optimize impact absorbing material to develop a selection guide and a complete set of data that describes the performances and properties of different liners for jockey helmets. Further the padding should facilitate a good fit, thermal comfort and be lightweight. In general, solid polymeric foams can be divided into closed-cell and open-cell foams. Closed-cell foams consist of closed cells which are full of gas. Open-cell foams are softer and air fills all of the spaces in the material. Three phases of deformation are commonly observed during compression of polymeric foams, namely quasi-static linear response, collapse plateau and densification region. An important independent parameter in designing an impact layer is the shoulder point, which is defined as the optimal ratio W/σ, where W is energy density [J/m3] and σ is stress [MPa]. At this point the stress-strain curve switches from collapse plateau to densification region. The most efficient foam is the one that absorbs the most energy up to the lowest possible maximum stress during deformation, thus the one with the highest shoulder point. Foams exhibit strain rate dependency as they are made of visco-elastic materials. The faster the strain rate, the stiffer the material. Therefore two load cases are observed, quasi-static and dynamic compression. Strain rates up to 1000 s−1 show quasi-static response. Various materials are tested to find the optimal material for impact absorbing. This includes 24 different foams and 21 different structures. All uniaxial quasi-static compression experiments are done with an Instron 5569 Material Testing Machine, using a 50 kN load cell at a maximum speed of 500 mm/min. All uniaxial dynamic compression experiments are performed with a striker drop test with a drop mass of 5.04 kg from a maximum height of 3 meter. The acceleration is measured by a Kistler 8715A acceleration sensor. In addition to this sensor, a piezoresistive sensor is designed, made and used. From the quasi-static compression can be concluded that cardboard has the best performances. All investigated materials show strain rate dependency, which means that they can absorb more energy at higher strain rates. Combined performance of an impact layer in combination with a soft comfort layer is always worse, because equivalent stiffness is less than both separate stiffness’s. On top of this padding a thin soft layer can be used to increase the comfort for the jockey, but this is out of the scope of this research. Quasi-static results give a good insight in the impact absorbing characteristics of all materials. In addition, simulations of higher impact energies from quasi-static compression data showed expected characteristics. In case of dynamic compression experiments lower shoulder points are found, as a result of high peak moduli. Moreover, acceleration data extracted from the impact tester could not be integrated twice to get the travel distance. To avoid double integration the impact energy density is divided by the peak stress to investigate impact characteristics of each material. From many experiments can be concluded that D3O Aero never reach densification, because of shear thickening. Other experiments show the linear elastic, collapse plateau and densification. It can be concluded that more dynamic compression experiments and research has to be done to finalize and optimize the design guide for jockey helmets. The safety of jockeys has high priority, so therefore the padding must be designed on HIC less than 1000 and acceleration of a human head less than 300g, where g is the standard gravity constant. The design guide is based on quasi-static results to select and design jockey helmets. It shows the six best solutions sorted on increasing density to design lightweight, where the required layer thickness is less than the allowable layer thickness.
Item Type:Internship Report (Master)
Clients:
RMIT University, Australia
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
Keywords:Impact Mechanics, Polymeric foams, Shoulder point, Helmet padding
Link to this item:http://purl.utwente.nl/essays/69908
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