Experimental Investigation on Mode I/II/III Fracture Behaviour of Additively Manufactured PLA Materials: Effects of Build Orientation

Abstract

Polylactic acid (PLA) is a bio-based material that can potentially replace environmentally harmful petroleum-based polymers in engineering applications due to its high strength. The fused filament fabrication (FFF) additive manufacturing (AM) technique has recently been widely used to manufacture PLA-based components. However, the build orientation in the FFF technique influences the fracture properties in different fracture modes due to the stacking of the material layers. Although there are studies done on the mode I and II fracture behaviour of the PLA manufactured by FFF, no study is available to investigate the effect of build orientation on all three fracture modes, including Modes I, II and III. This study aims to experimentally investigate the effect of build orientation on fracture behaviour (e.g., fracture toughness and crack propagation) of PLA fabricated by FFF for three pure fracture modes (Modes I, II and III). Mode I and II fracture toughness values were extracted using the symmetric and asymmetric four-point bending tests, respectively. A Mode III testing procedure is developed to apply appropriate loading in a tensile testing machine. Specimens were fabricated by FFF in three build orientations and tested. Fracture toughness and corresponding fracture energy values were calculated for different build orientations. Crack propagation in the samples after fracture was investigated through the images taken with a camera and scanning electron microscope. Based on fracture surfaces, it was observed that the failure could be of interlayer, intralayer and combined types, considering the build orientations. The differences in the fracture surfaces govern the fracture behaviour of PLA materials fabricated in different build orientations. Build orientation was observed to be a dominant factor in all three fracture modes of FFF-printed PLA. The testing procedures and the fracture properties provided in this study can be used to develop FFF-printed PLA-based engineering applications.