Acoustic Resonance Testing of Additively Manufactured Mechanical Metamaterials

Acoustic Resonance Testing (ART) is a well established, volumetric, non-destructive technique for quantitative assessment of solid parts. In the present work, the suitability of ART in the investigation of complex metallic structures, in this case so-called mechanical metamaterials, was explored. Two types of lattice structures and one pantographic structure were included in the study. Lattices with different rod diameter (e.g. 150 µm and 260 µm) and number of missing rods (e.g. 1 or 5) as well as material variation (NiTi, Fe-based alloy and Ti6Al4V) in pantographic structures were considered.
Firstly mathematical approximations were used to estimate the vibration frequency range of the structures’ constituent elements like a single rod. Secondly a simulation-based natural frequency study was carried out for each type of structure and compared with the experimentally acquired data from the ART. The combination of simulation and experiments allowed interpretation of the measured resonant peaks and their changes caused by variations in sample geometry, material or presence of defects.
A distinction between the different rod thicknesses and materials was observed. The detection of missing rods was also possible, but no explicit correlation between the number of missing rods and the change in resonant frequency could be established.
Summing up the present work provided encouraging results about the suitability of the acoustic resonance testing in the evaluation of complex additively manufactured structures. The method has the potential to be scaled and integrated in production lines in the future. However, further in-depth investigations and adaptation are required to be able to implement defect classification. The transfer of the gained knowledge to new structures should be addressed in further research projects.