Additively-manufactured personal protection for blunt force trauma in sport

Protective equipment in sport is used to reduce the risk of incurring injury while competing, with many of these items focused on reducing the impact forces and loading rates experienced by the wearer during blunt force trauma. Cellular materials, such as foams are the most common materials used for these applications as their cellular nature produces a long stress plateau in the loading response. This enables foams to absorb large quantities of energy at a low peak stress. While foams are effective at offering protection from impact, disadvantages of their use with respect to protective clothing, include poor thermal regulation, excessive weight and bulk, and any associated fatigue.

With the increasing utilisation of Additive Manufacturing for end use products, interest has grown in the adoption of Additively Manufactured Cellular Structures within bespoke sports equipment such as helmet liners and running shoe midsoles. The design freedom offered by Additive Manufacturing means that periodic lattice structures can be tuned on a cell-by-cell basis to specific energy requirements. However, in order to implement this design process, a clear understanding of how the specific design features of a cellular structure effect its compressive performance was required.

The objective of this body of work was to identify which geometric performance factors had the greatest effect on the compressive performance of a cellular lattice in order inform the development of a design tool to make this knowledge and design process accessible to a wider audience.

Eight materials from four Additive Manufacturing processes were screened for their suitability. Of these, Nylon specimens manufactured using the Material Extrusion process were deemed the most suitable process and material combination for this investigation. Five design factors, identified through a thorough literature review, were screened for their effect on the energy capacity and plateau stress of the lattice structures using the process of Design of Experiments. Cell width, strut cross sectional area, fillet percentage, strut shape, and lattice orientation were all found to have a significant effect (p<0.01) on energy capacity and plateau stress. Cell width and strut cross sectional area were revealed to have the largest effect sizes and further analysis revealed that cell width displayed a curved response with both energy capacity and plateau stress.

A design tool was developed in Grasshopper, a generative plugin for Rhinoceros 3D, using these results. A drive equation was experimentally derived between impact energy, and design factors cell width and strut cross sectional area. This was used to guide the development of a Graphical User Interface to enable users to design a lattice structure tailored to a specific protective requirement.

A verification study was run on the Graphical User Interface using a series of samples designed for three different energy levels. Results were analysed using two one-sided tests and successfully indicated protective performance over 87.5% of their target value.