Limitations of composite strength theory for predicting the ultimate strengths of layered 3D printing polymers

Abstract

Composite laminates and 3D printing materials both have layered structures. Although the Tsai–Hill composite strength criterion is useful for predicting the strengths of some 3D printing polymers made with fused filament fabrication (FFF), our experimental and theoretical studies showed that this criterion has some limitations in predicting the strengths of other 3D printing polymers. In this study, a new quadratic strength criterion was employed to predict a conservative lower bound for the strengths of polymers made with FFF and selective laser sintering (SLS). The printing surface angles of the printing specimens ranged from 0° to 90°. Interestingly, the scope of this study unexpectedly widened from strength research to fracture mechanics research because dynamic crack branching was observed in some SLS specimens (printing surface angles ranged from 0° to 75°) under static tension—a novel phenomenon among 3D printing materials. Crack branching not only followed previous crack branching rules, such as rules on the crack speed and energy release but also presented new challenges for dynamic fracture mechanics theory.