To date armour systems are typically homogenous in construction (i.e. a monolithic plate) and the localised deformation associated with projectile penetration does not take advantage of the entire strength of the armour piece. The armour system of this proposal is a composite metallic structure which has been named “plymetal.” As the diagram adjacent shows the structure is composed of ribs of H13 a high strength tool steel and ribs of stainless steel which are adjacent and alternating. The next layer of ribs crosses in a perpendicular orientation, much like the change in the orientation of the grains between the laminates of wood in plywood. As the schematic shows this is repeated for several layers and it is this design/construction that is the secret to the enhanced ballistic performance.
As the finite element modelling (the second image) shows when the structure is in compression at stress levels between the yield stress for H13 tool steel and above the yield stress for 316 Stainless Steel, the stainless steel undergoes plastic deformation, while the H13 tool steel is still elastic. Because the H13 is still elastic and the ribs are crossed in a perpendicular orientation the stress is transferred through a relative small cross-section and as such the stress in the H13 ribs is dissipated as the force is transferred through the composite structure. Being that the stainless steels ribs are plastic in this regime it is absorbing energy through plastic deformation. Therefore the dissipation of the stress through the stronger H13 and the absorption of energy by the softer stainless steel give this composite structure an enhanced ballistic performance by utilizing the entire armour piece.
Reference to the third image shows that preliminary trials of plymetal have shown that it resists impacts as well as stainless steel of almost twice the thickness. This has implications for the light weighting of armour systems for defence, as the ballistic performance for the specific strength (strength /density) has been increased through the composite design of the plymetal. Therefore thinner plates will provide the same ballistic protection with reduced weight. There is also a cost advantage associated with the armour system as steel powder used in the 3D printing process is cheaper than other metallic powders or even other armour materials that are exotic and usually expensive.
It is the 3D printing technology of Direct Metal Deposition (DMD) that makes the construction of the plymetal possible. The DMD system has four powder hoppers that can deliver powders of different composition interchangeably during processing, so it is possible to deposit the alternating alloy compositions in one continuous manner.
The design of plymetal is in its infancy and further investigations into the rib dimensions and the nature of the ballistic forces could be tailored to a specific design as well as enhancing the current ballistic performance.