Evaluation of the Energy Absorption and Piezoresistive Characteristics of Multifunctional Carbon Fiber-Reinforced Peek Lattice Structures Processed via Additive Manufacturing

We study experimentally the mechanical and piezoresistive characteristics of carbon fiber (CF)/poly ether ether ketone (PEEK) lattice structures subject to quasi-static compression and low-velocity impact loading. The lattice structures were fabricated via fused filament fabrication (FFF) using composite filaments with 30 wt. % chopped carbon fibers included in a matrix of PEEK. Three different 2.5D cell geometries with equal relative density (33%) were considered, an irregular hexagonal honeycomb as well as a chiral and re-entrant lattice. Quasi-static compression tests are performed to examine the energy absorption capacity of the 2.5D lattice structures and to identify their failure modes, while low-velocity drop weight tests are carried out to study the compressive behavior of the lattice structures under impact conditions with strain rates up to 340 s^-1. We also measured the in-plane and out-of-plane piezoresistive properties of the 2.5D lattices under quasi-static compression.

The tests conducted under quasi-static in-plane compression reveal that the inclusion of CF in the PEEK cell structure results in a significant stiffening effect for all cell geometries considered here. Furthermore, the chiral and re-entrant CF/PEEK lattices showed higher specific energy absorption than those made of neat PEEK, while the opposite trend was observed for the hexagonal honeycomb due to the formation of a shear band in the CF/PEEK honeycomb lattice structure associated with a significant drop in load-carrying capacity. The highest specific energy absorption under in-plane quasi-static compression was observed for the neat PEEK honeycomb lattice which allowed for uniform and stable folding of the cell walls enabled by viscoplastic flow of the neat PEEK. However, under in-plane low-velocity impact loading, the CF/PEEK honeycomb lattices showed a sharp increase in both peak load and energy absorption when the strain rate was increased beyond 61 s^-1. The latter trend was not observed in the neat PEEK honeycomb structures which showed nearly constant energy absorption and peak load at higher rates of strain. For a strain rate of 106 s^-1, we report an increase of almost 300% in the absorbed impact energy when the CF/PEEK was used instead of the neat PEEK in the honeycomb lattice. On the other hand, when the impact load was applied in the out-of-plane direction, only minor differences in the absorbed energies were observed between the PEEK and CF/PEEK honeycomb lattice.

The CF/PEEK lattice structures also exhibited pronounced piezoresistivity under both in-plane and out-of-plane compression with initial gauge factors ranging between 0.73-2.05, making them suitable candidates for structural systems that can self-monitor the degree of deformation and/or damage induced by operational or accidental loads. They appear to be particularly suitable for the development of smart orthopaedic prosthesis, such as self-sensing hip or knee implants.