Composites for high energy absorption

Polymers-filled lattice structures

This special composite is made by filling a metal lattice structure with polymer. Lattice structures are engineered materials made by the repetition of cells in the space. The mechanical properties of lattices are controllable by changing the inner microstructures. Recently, the spread of additive manufacturing (AM) supported the creation of high-resolution metal lattice structures, especially thanks to the DMLS (direct metal laser sintering) process. 

The polymers-filled lattices are composite materials with great potentialities, especially for energy absorption applications in aeronautic, space and high speed vehicles.

The fabrication of hybrid materials based on lattice structures filled with polymers has been introduced very recently. The benefits expected from these materials are the improvement of the mechanical properties and the increasing of specific energy absorption (SEA) respect to the traditional lattices with air among struts. 

Fabrication

The refinement of fabrication methods and the detailed design of the materials allow us to catch the full potential of lattice-polymer hybrid structures. For sure, by considering other preliminary studies reported in the literature, we have been motivated to develop a stable and reliable fabrication process, to get repeatable and reliable composite materials. The AM process based on powder-bed technology and 316L steel material is used to fabricate the lattice structures. Stereolithography (SLA) and vacuum casting process are then used to fill the lattice with polymer resins with different compositions and hardness. 

1) lattice structures building process by SLM, 2) temporary core building by SLA, 3) silicone mold fabrication, 4) silicone mold refinement, 5) vacuum casting of the polymer, 6) lattice filled with polymer release. 

Composite samples made with internal 316L steel lattice with variouos cells shape and morphology and filling polymer. Compression tests are used to know mechanical properties and to estimate the energy absorption per unit volume. These information are used to design and produce the prototypes and final components.

Energetic properties

The comparison of the hybrid samples compression curves with the curves obtained for the constituent materials (lattice structure and polymer) shows higher strength and stiffness. 

As expected, the polymer filling causes the translation of the force-displacement curve towards higher values. However, the relative increase of the mechanical strength between the hybrid material and the sum of constituents is generally much higher than expected. 

The most relevant property of polymer-filled composites is that the energy absorbed by the hybrid materials is much higher (up to 70%) than the sum of the energies absorbed by their constituent materials. 

Increment of the strain energy of the hybrid materials with respect to the sum of energies associated with their constituents.