Experimental analysis of heterogeneous shape recovery in 4d printed honeycomb structures

Additive manufacturing of complex structures with functional and programmable attributes (i.e., 4D printing) enables complex and innovative designs with tunable, adaptable or shape memory properties. To capitalize on the potential of 4D printing, it is instrumental to establish the knowledge base needed to predict the behavior of 4D printed structures. Accordingly, this work is dedicated to study the stability and shape recovery properties of 4D printed honeycomb structures using thermoplastic shape memory polymer (SMP). In particular, the dimensional stability of samples subjected to heat at temperatures around the glass transition temperature of the SMP material was assessed. In addition, the local shape recovery properties were investigated in samples programed into various temporary shapes with highly heterogeneous stored strain fields. The as-printed samples experienced dimensional instability manifested through a time dependent and heat induced strain accumulation when subjected to temperatures of ±10 °C around the SMP glass transition temperature. The rate of strain accumulation was temperature dependent and exhibited a high strain accumulation rate initially, followed by a relatively saturated response after about 10 min of temperature exposure. The total strain accumulation in the honeycomb samples was heterogeneous due to local variation in the deposition direction during printing. In the recovery experiments, despite the dissimilar levels of stored strain during programing, full local recovery was achieved independent of magnitude or nature of the stored strains. However, the local strain recovery rates varied in different regions and was attributed to different levels of stored strain and variation in the deposition direction. Overall, the work sheds important quantitative insight into the heterogeneous material response, dimensional stability and shape recovery of 4D printed structures manufactured using fused deposition and thermoplastic SMP.