<p>Additive manufacturing (AM) of complex parts and prototypes using polymeric materials has become a predominant strategy among industries and researchers to achieve rapid manufacturing solutions at a minimal cost.&nbsp; The availability and wide variety of printable polymers make them extremely suitable for producing customized parts in aerospace, automotive, biomedical, dentistry, apparel, and electronics applications. However, polymers are vulnerable to fatigue below the yield stress and can develop microcracks to cause the ultimate failure. The brittle nature of the polymeric materials and their interlayer properties during AM makes it highly important to study the crack propagation and fracture toughness during static and dynamic loading conditions. The interlayer failure of the AM polymers is caused by the combination of three fracture modes, namely, opening (mode I), in-plane shear (mode II), and out-of-plane shear (mode III). Investigating the fracture and fatigue behavior is therefore essential not only to understand their mechanical response but also to predict and prevent their failure. In this study, fracture and fatigue properties of AM polymers are investigated experimentally combining the servohydraulic material testing system (MTS 810) and the three-dimensional (3D) digital image correlation (DIC) technology. The polymers used for the analysis are acrylonitrile styrene acrylate (ASA) and carbon fiber (CF), which are readily available and more prevalent in AM applications. The test specimens are printed in both horizontal and vertical orientations using the Stratasys F170 fused deposition modeling (FDM) 3D printer. Both notched (i.e., pre-cracked) and unnotched specimens are printed by the FDM printer in the shape of the standard ASTM sub-size flat dogbone structure. The MTS 810 machine is operated with a 50 kN load cell and at a load ratio of R = 0.1, to generate results for the fatigue properties. The DIC system, equipped with two FASTCAM MC2 high-speed cube cameras and two tunable LED lights, captures images of the specimen while undergoing the tests to find the strain and displacement. A speckle pattern is created on the specimens’ surface before performing the tests, which acts as the reference point to analyze their respective deformations. The tensile strain is studied ahead of the crack formation, and the crack length measurement is obtained afterward by superimposing strain fields over the photos. Thus, the strain distribution, crack propagation, fatigue process zone, and crack-tip remodeling on the surface of the specimen are identified. The plane-strain fracture toughness, stress intensity factor, and the critical strain energy release rate of the notched specimens are calculated for the three modes of fracture failure. Results show that the horizontally-oriented specimens show better resistance to failure than the vertically-oriented ones. In the unnotched specimens, the crack occurs near the neck due to a high-stress concentration. Results for the high cycle fatigue test indicate that fatigue life decreases as the load increases for all types of specimens. Finally, the fatigue life, crack growth, and fracture toughness of the ASA and CF materials printed in horizontal and vertical orientations are compared to outline their performance metrics to be considered for the appropriate AM applications.</p>

Crack Propagation and Fracture Toughness of Additively Manufactured Polymers

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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