Strength and Fatigue Analysis of FDM-fabricated Non-assembly Bi-material Compliant Mechanisms

Abstract

Multi-material compliant mechanisms have shown the potential to be utilized in the upper-limb prosthesis. The mechanisms consist of flexural and rigid parts, where the flexural components can serve as flexible joints between rigid bodies in the device (e.g., finger and finger segments). This configuration is feasible to be fabricated using a type of additive manufacturing called Fused Deposition Modelling (FDM) process, without the need of further assembly and extensive post-processing. Knowledge of the mechanical characteristics of such mechanisms, however, is still limited. Therefore, this study presents the strength and fatigue characteristics of bi-material compliant mechanisms to determine the feasibility of applying the mechanisms in the upper-limb prosthesis for long-term use. The basis of the mechanisms was a configuration of two rigid clamps and a flexible beam that were automatically assembled during manufacturing. Two materials selections (PLA-TPU and Tough PLA-TPU) and three geometries (rectangular, cylindrical, and tapered-shaped interface) were used to create six groups of samples. These groups were subjected to tensile testing and fatigue testing to assess their strength and fatigue behavior. The results of mechanical testing were also verified with the results of finite element simulation. It was found that four groups fulfilled the strength requirement, which were mechanisms in both material configurations with cylindrical and tapered-shaped interface. These groups, however, failed to demonstrate their durability during fatigue testing. Finally, the proposed method of fabrication and mechanical testing as well as the obtained mechanical characteristics of the mechanisms were analyzed to give insights for future development.