Making headway in military helmet padding with open-cell lattice structure

Materials World magazine
1 Apr 2020

An open-cell lattice architecture for helmet padding could reduce head injuries in combat. Shardell Joseph speaks to the USA Army researchers behind the development.  

A highly-tuned open-cell lattice structure made from a photopolymer was found to improve the compression characteristics of helmet padding. This is said to reduce the acceleration impact to a soldier’s head during collisions or blunt impact events, such as falling to the ground or being targeted by a projectile.   

The team from the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory (CCDC ARL) reported a 27% increase in energy attenuation efficiency when these lattice structures – produced using 3D printing – were inserted into combat helmets, compared to current best-performing foam pads. 

‘We could achieve this by increasing the thickness of the current suspension system inside the helmet interior padding, but that would lead to a larger, heavier helmet,’ CCDC ARL Materials Engineer, Dr Thomas Plaisted, told Materials World.    

‘So, the main purpose of the new design is to create a more efficient suspension system by attenuating greater amounts of impact energy within the same space that the current design occupies. To achieve the higher efficiency, we are using a highly architected material that buckles in a specific manner. ‘The research has demonstrated the utility of architected lattice structures to improve the performance of the helmet, particularly  in locations where the head experiences more concentrated load during impact.’  

Opening up the lattice   

To make the prototype padding they used a photopolymer known as thiol-ene urethane acrylate, with computational modelling determining the optimal lattice design padding to achieve desired characteristics. Initially in a liquid state, the photopolymer is selectively cured to form the lattice geometry through an additive manufacturing (AM) process that applies UV light to polymerise the liquid into an elastomer.

The cellular structure is fabricated using architected lattice topology, which showed improved results over the collapse process in elastomeric lattices. According to the study, Elastomeric microlattice impact attenuators, published in Science Direct, microlattices with ordered architectures can exhibit higher densification strain and strength, and cellular architectures can better tailor their mechanical response to impact compared with foams with stochastic architecture. 

‘Unlike a traditional foam, which consists of a porous microstructure made up of tiny cells, the lattice structures are much more open,’ Plaisted said. ‘These lattices can be uniform throughout or be designed to have a gradation in properties through the thickness. ‘As elastomers, the microlattices have the ability to deform and rebound to their original shape. This property is important for applications where the helmet must protect the head over multiple impact events.’ The additional degrees of freedom in designing the cellular architecture are therefore leveraged to enhance contact area during deformation.

During compression, Plaisted explained that the lattice initially bears the load with little deflection. Upon reaching a critical load, the lattice elements will begin to buckle, folding sideways as the application of load continues. ‘With careful design, extended buckling can be accommodated by achieving a high packing efficiency’, Plaisted said. ‘The force levels that control the collapse process are tuned by changing the thickness of the lattice member, the angle of each member, and the number of intersections through the thickness of the pad, in addition to the polymer from which they are made.’   

Into the field  

Plaisted believes that the process can be scaled up to create large sheets of these microlattices rapidly. ‘There is additional work to be done to verify the blunt impact performance of the padding at hot and cold temperature extremes, as well as performing comfort assessments of this suspension system relative to traditional padding materials,’ he said. ‘We are transitioning these results to our partners at CCDC Soldier Center for further evaluation and testing.’