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Study of layer orientation on the fracture behavior of two additively manufactured thermoset resins, A
Brunstad, Nicholas John
Brunstad, Nicholas John
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2021
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This work examines the fracture behavior of additively manufactured (AM) thermosetting resins to enable rapid production of lightweight and functional polymeric and composite materials that can meet a wide range of applications. Specifically, the role of print orientation on the quasi-static and dynamic fracture response of two distinct AM polymer materials, DA-3 and PM-EM828, is presented. While predictive simulations often leverage quasi-static fracture criterion, impulsively loaded cracks can have substantially different resistance to growth. To study dynamic fracture, a unique long-bar apparatus is used to fire a striker at the opposite end of notched and pre-cracked specimens to create a dominantly dynamic Mode-I (opening) load. Digital Image Correlation (DIC) is used in conjunction with ultra-high-speed imaging to capture the evolving displacement fields ahead of the crack tip. The elastodynamic solution for a stationary crack is optimized using a least square fit to extract the evolving critical stress intensity factor (SIF) leading to fracture initiation. These results are compared to quasi-static experiments of the same material and similar geometries on a standard load frame. Findings suggest that the print orientation does slightly affect the quasi-static and dynamic fracture response of DA-3, however the PM-EM828 does not show statistically significant orientation dependencies on fracture behavior. The DA-3 exhibited between 46% to 60% higher quasi-static fracture toughness values than dynamic, on average, but the trends mirrored the printed orientation dependency of dynamic loading. Conversely, PM-EM828 exhibited approximately 20% lower quasi-static fracture values than dynamic values and had little to no orientation dependency. The overall toughness of the PM-EM828 layers are on a similar level to their interlayer adhesion zones, suggesting why there may be little print orientation dependence. Conversely, the DA-3 layers are less brittle than their interlayer adhesion zones, and so the orientation of the adhesion zones with respect to the print orientation seems to allow for a greater resistance to fracture.
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