Dynamic compressive response of 3D printed thermoplastic polyurethane honeycombs with graded densities

Fused filament fabrication (FFF) 3D printing of thermoplastic polyurethanes (TPUs) offers a unique capability to manufacture tailorable, flexible cellular structures which can be designed and optimised for specific energy absorbing applications. This paper describes the manufacture and dynamic compressive analysis of 3D printed cellular structures with graded densities. An optimised 3D printing procedure was developed which allowed the manufacture of high quality structures, with low internal voidage, which were capable of undergoing repeated cyclic loading to densification without failure. The honeycombs produced all had an average relative density of 0.38±0.01 and were graded in density either in a continuous manner or with discrete step changes in density within the structure. After analysing their quasi-static compressive response, all arrays were subject to sinusoidal compression over a range of amplitudes (5%-45% strain peak-peak) at a frequency of 0.5Hz. It was found that by grading the structural density in different ways, mechanical damping may be tailored. Cyclic compressive testing also showed how strain softening of the TPU parent material could lead to reduced energy absorption and reduced damping over the course of 50 cycles; to what degree of this behaviour occurred was found to be dependent on the strain history. All samples were also subject to impact loading with a flat steel plate at strain rates of up to 45s^-1 and specific impact energies of up to 270mJ/cm³ using a drop weight tower. Under high energy impact loading which caused significant densification in the uniform density structure, all samples with an element of grading transferred lower peak loads than the uniform density structure revealing the potential of density grading of TPU structures to provide superior impact protection in extreme environmental conditions.