PSI - Issue 61

Bahman Paygozar et al. / Procedia Structural Integrity 61 (2024) 232–240 Paygozar et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Additive manufacturing (AM) of polymers has recently grown in the automotive, aeronautics, and marine industries (Zeng et al.,). This is because AM techniques, in a layer-wise manner, provide them with specific properties and shape complexity while reducing material waste as much as possible (Ngo et al. 2018). Due to the layer-wise fabrication fashion in AM, there is no need for special tools or molds to give a specified shape to the products; the manufacturing procedure just depends on the data introduced by the geometry of the part loaded to the printing machine (Labonnote et al. 2016). Considering the versatility of additively manufactured polymeric materials and the potential industries employing them, several studies have investigated different material properties like fracture toughness (Campilho et al. 2011). The material extrusion AM technique, called fused filament fabrication (FFF), is widely used to manufacture polymeric parts additively due to its low cost and ease of use. However, FFF-fabricated parts generally show anisotropic, locally orthotropic response and have less elastic modulus, strain to failure, strength, and fracture resistance when compared with parts fabricated by traditional methods like thermoforming and injection modeling (Ivanova et al. 2013). In this regard, taking account of the benefits of AM and its potential drawbacks, the study of its mechanical properties, particularly fracture behavior, is of high importance. Fracture toughness is a crucial property regarding a component's strength and structural integrity. Generally, pre notched specimens are utilized to extract fracture properties of materials under various loading modes. Considering the loading direction, three different modes of loading can be introduced: Mode I (tensile opening), Mode II (sliding mode), and Mode III (tearing mode). Taking account of the present methods used to determine the fracture behavior of AM parts, the analyses of static fracture mechanics- excluding the fatigue and impact fracture types- are grouped based on the specimen shape and loading mode. Generally, FFF-fabricated materials tend to fail under mode I or mode II fracture, in which the interlayer fracture mechanism outweighs the intralayer fracture (Sedighi et al. 2020). Several studies have recently investigated the Mode I fracture behavior of FFF-fabricated polymeric materials. For example, Patterson et al. (2018) studied crack propagation in the specimens fabricated by FFF with Polylactic acid (PLA) material. Different fracture modes were investigated experimentally through three-point bending tests of the single-edge notch bending (SENB) specimens. Another study investigated the influence of crack insertion in the SENB specimens manufactured through FFF fabricated PLA material ( Vălean et al. 2020 ). They determined the Mode I and Mode II fracture toughness under symmetric four-point bending loading. They revealed that the effect of notch insertion is more evident in mode I than the other modes. Moreover, the effects of the crystallinity and loading rate on mode I fracture properties of PLA were studied by Park et al. (2006). They showed that the quasi-static fracture toughness of PLA declines with the rise of crystallinity, whereas the impact fracture toughness is inclined to enhance with crystallinity. Ayatollahi et al. (2020) researched the effects of in-plane raster angle on the fracture strength of FFF-fabricated PLA specimens. They revealed that the highest fracture strength was observed when the specimen was 3D printed with ±45 ° raster angle. It was also observed that crack propagation was dependent on the raster angle. Similarly, it was found that the aforementioned raster angle reflects the highest mechanical properties of PLA specimens fabricated by FFF (Zhang et al. 2019). Moreover, McLouth et al. (2017) investigated the impacts of print orientation on the fracture toughness of additively manufactured specimens. They showed that the print orientation, which leads to the use of more filament along the crack tip, results in a larger fracture toughness. Regarding the simulation of fracture behavior of AM polymers, a few studies have been done to date because of the complexity of predicting the crack propagation patterns affected by interface bonding and AM microstructures. To overcome this issue, the extended finite element method (XFEM) (Mubashar et al. 2014), which was introduced recently and allows for crack propagation through finite elements (FEs), can be successful in this domain of study. In this regard, fracture simulations by the XFEM were performed for FFF-fabricated ABS (Akhavan-Safar et al. 2020). The thermoplastic polymer PLA is currently used in various FFF machines, ranging from commercial to desktop machines. As a result of this ubiquitous application, the Mode-I fracture behavior of the PLA fabricated by FFF is investigated in this research both experimentally and numerically using the XFEM. For this purpose, three-point (3P)