3D-printed polymers exhibit a strength size effect

This study is an investigation on the size dependence of strength of a 3D printed acrylic polymer. 3D printed beams are used in three-point bend fracture experiments. Three print modes of the PolyJet process are used to manufacture beams of dimensions commonly considered in 3D printed structures (1-5 mm). It is found that for that range of dimensions, the fracture response is in the nonlinear size-strength domain and specimens neither follow the limiting linear elastic fracture mechanics nor the strength criterion. Consequently, strength and toughness are size dependent. Moreover, a strong interaction between specimen dimensions and print layer thickness was found. A size threshold exists below which there appears to be an interaction between specimen dimensions and print layer thickness, and for specimens of dimension below that threshold exhibit a declining strength with size. From the present experiments, the size threshold is estimated to be 50 times the print layer thickness. The finding of a maximum strength relative to geometric dimensions should be accounted for in designing with 3D printed materials.