Energy Absorbing Lattice Structures

July / September 2017

Individual Research Project under the supervision of Dr. Mazdak Ghajari

This project was carried out as part of an Undergraduate Research Opportunity Project (UROP) at Imperial College London. I worked in a multidisciplinary group, producing a body of work which could go on to improve personal protective equipment. I worked in a lab to design, manufacture and evaluate the structures.

Currently, helmets are predominantly manufactured from stochastic foams with a density between 20mg/3 and 30 mg/cm3 . This project focused on working to develop a hollow metal lattice structure, which could potentially absorb more energy than foam per unit mass.

Some details of this project have been eliminated for confidentiality.

Design

Firstly, Finite Element Analysis was used to predict the optimum shape of unit cell and strut dimensions for maximum energy absorption and minimum mass.

Manufacture

In order to create the hollow design, an electroplating process was used in which the solid support is later removed, leaving the thin walled part.

ABS was 3D printed to create 'templates' which could be electroplated. These were smoothed using an acetone vapour before being sprayed with silver to make them conductive. Afterwards, they were electroplated in either copper or nickel, running a chronoamperometry sequence on a potentiostat. Finally, when a sufficiently thick layer had been deposited, the ABS template was dissolved using acetone.

Nickel electroplating was time consuming and unsuccessful

Copper electroplating was found to be more successful

Electroplating was first attempted without acetone smoothing the templates, however, the result was a poorly adhered, patchy layer as shown here.

Acetone smoothing

Results

I had many attempts at creating small samples of the hollow structures, however, due to the timescale of the project, a lattice was never produced. The final attempt shows a layer of copper was deposited thick enough that when the acetone removed the template, it was self supporting.

The results from the chronoamperometry can be seen here.

This is a typical chronoamperometry result. At first the current builds up as it deposits the first layer of copper. This happens at an exponential rate until one full layer has been applied. At this point, the current plateaus until the oxide layer on the anode is so large that no more current can flow.

Future

This project was only an introduction to what could potentially be taken much further as a PhD. the manufactured method needs to be refined to consistently reproduce the samples, and then they need to be impact tested. If the concepts presented here were validated, they could be applied in a range of settings including helmets and body armour, similar to the Vicis helmet. This is a NFL helmet that employs the principles of Euler buckling to absorb impact energy.

Source: vicis.co

© Anna Bernbaum