Role of Print Architecture on Fracture Behavior of Additively Manufactured ABS: Opto-Mechanical Investigations
Type of DegreePhD Dissertation
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Acrylonitrile Butadiene Styrene or ABS is a popular and inexpensive isotropic, amorphous thermoplastic widely used for Additive Manufacturing (AM) of engineering parts. An AM process called Fused Filament Fabrication (FFF) that involves layer-by-layer deposition of melted thermoplastic wire through a heated nozzle along predetermined paths is common for printing thermoplastics such as ABS. Individual layers printed during AM can be configured differently and could introduce anisotropy into the part due to weaker planes between individual beads even when the feedstock is isotropic. The feasibility of tailoring these individual layers in different directions and designing print architectures introduces uncertainty in the mechanical behavior of AM parts. Hence, the role of the print architecture on mechanical failure and fracture properties must be critically evaluated. In this work, AM ABS parts with three different in-plane print architectures, namely [0º/90º]n, [45º/-45º]n, and [0º/45º/90º/-45º]n, are considered and their elastic and fracture properties have been evaluated under quasi-static and high strain rate loading using full-field optical techniques used in conjunction with a hybrid experimental-numerical method. The full-field measurement of local in-plane displacements is performed optically up to crack initiation and during crack growth using Digital Image Correlation (DIC) method in quasistatically and dynamically loaded AM ABS specimens. The early part of the dissertation details the challenges associated with the prevailing approaches of extracting fracture properties from full-field displacement data obtained from DIC. To overcome the limitations, a method of analyzing DIC data by transferring it to a corresponding Finite Element (FE) model for computing the energy release rate as the J-integral and then partitioning it into individual stress intensity factors is developed. Details of this “Hybrid DIC-FE” methodology are presented before undertaking experimental work. In the next part of the work, the tensile and fracture behaviors of three in-plane print architectures, namely [0º/90º]n, [45º/-45º]n, and [0º/45º/90º/-45º]n orientations, under quasi-static loading conditions are examined. Uniaxial tension experiments are performed on dog-bone-shaped AM ABS specimens and quasi-static fracture experiments on edge-notched symmetric three-point bend specimens using a universal testing machine. Even though the printed architectures show macroscopic elastic isotropy, significant differences in the failure strain, crack initiation and growth parameters, and failure modes among the three architectures are observed. These differences are explained using tests performed on comparable unidirectional prints. The results suggest that [0º/45º/90º/-45º]n is preferable to the other two more common configurations for a relatively gradual failure behavior and higher resistance to crack growth. The next part examines the high strain rate fracture behaviors of three different print architectures, namely [0º/90º]n, [45º/-45º]n, and [0º/45º/90º/-45º]n in-plane orientations, under stress-wave loading conditions and compares the results with the quasi-static counterparts. Elastic properties under high strain rate loading are measured on printed cubes using ultrasonic transducers. The high strain rate fracture experiments are carried out on V-notched AM ABS specimens using a modified-Hopkinson pressure bar apparatus. Distinct crack initiation and growth behaviors with different failure modes are observed in the three architectures under quasi-static and high strain rate loading conditions despite macroscale elastic isotropy. The results favor [0º/45º/90º/-45º]n architecture due to a better crack growth behavior relative to the other two print architectures, suggesting that the fracture performance can be enhanced via print architecture. The final section of this dissertation details the effect of print architecture on the mixed-mode fracture behavior of AM ABS specimens. An Arcan loading apparatus that allows for direct optical measurements in the crack tip vicinity is developed and mixed-mode (I/II) experiments under quasi-static loading are performed for the three architectures studied in this work. Distinct failure loads, load-point displacement at failure, and failure modes are observed under different loading conditions ranging from mode-I, mixed-mode (I/II), and mode-II conditions. The optical measurements from DIC are used with the hybrid DIC-FE methodology to extract energy release rates and stress intensity factors. The critical values at crack initiation are identified and the fracture envelopes are plotted to evaluate the mixed-mode (I/II) performance for all three architectures. Significant differences in the behaviors are observed, with the [0º/45º/90º/-45º]n architecture having a better fracture profile among the three architectures studied. Fracture mechanisms at play are further explained via fractography using images of crack paths and fractured cross-sections.