A review of factors that influence the fracture toughness of extrusion-based additively manufactured polymer and polymer composites

Abstract

Fracture toughness, is a critical aspect of a component’s strength and structural integrity. In additive manufacturing (AM) of polymers and polymer composites, the fracture toughness of the manufactured part is influenced by the bonding between the layers which is dependent on the material, geometric and processing parameters. Known factors that influence the bond strength and interfacial adhesion in 3D printed parts can be broadly divided into three main categories; 1) deposition related conditions (direction, nozzle size and geometry, temperature), 2) solidification related conditions (heat transfer, molecular movement, crystallinity and void formation) which also play a major role in defining properties and 3) composition and rheological behavior of the resins and the fiber microstructure (content, orientation and surface treatment). In this review, we identify factors that have been reported to influence interface bonding in 3D printed polymer and polymer composite parts. We discuss best practices, as recommended in the reported literature, to address fracture behavior and strategies to improve fracture toughness and structural integrity. Due to the layer-by-layer nature of 3D printed parts, which makes them orthotropic, most AM studies are mainly focused on application of Mode I and mixed mode loading to characterize interlaminar fracture toughness. Intralaminar and translaminar toughness behavior have mostly remained untouched or scarcely discussed in the AM literature. Our focus is to specifically address fracture toughness of 3D printed polymer composites made using the fused filament fabrication (FFF) additive manufacturing method. The material, process and geometric parameters used in FFF will impact the adhesion between layers and hence the fracture toughness and structural integrity. The review identifies the parameters that play key roles and discusses the mechanisms put forth by various researchers that influence the fracture toughness behavior under various modes of fracture. Important gaps in the literature are identified and the future outlook of AM using the FFF process is discussed.