Theoretical modeling and experimental characterization of stress and crack development in parts manufactured through large area maskless photopolymerization
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Large Area Maskless Photopolymerization (LAMP) is a disruptive additive manufacturing technology developed in the Direct Digital Manufacturing Laboratory at Georgia Tech. Due to polymerization shrinkage during the layer-by-layer curing process, stresses are accumulated that can give rise to cracks and delaminations along the interfaces between adjacent layers. The objective of this doctoral dissertation is to investigate the mechanisms of stress evolution and cracking/delamination during the LAMP manufacturing process through theoretical modeling and experimental characterization methods. The evolving conversion degree in a layer was characterized through Fourier Transform Infrared Spectroscopy and this leads to a so-called print-through curve. The polymerization shrinkage strain in each exposed layer was calculated on the basis of the theoretical relationship between the volumetric shrinkage and the degree of conversion. Furthermore, the material’s elastic modulus, which also evolves with the degree of conversion, was characterized by three-point bending tests. With the degree of conversion, cure-dependent modulus and shrinkage strain as the three primary inputs, finite element modeling was conducted to dynamically simulate the layer-by-layer manufacturing process and to predict the process-induced stresses. To investigate the fracture process, Mode I and Mode II interlaminar fracture toughness of the LAMP-built laminates was characterized, using the double cantilever beam (DCB) test and the end notched flexure (ENF) test, respectively. In order to predict the crack initiation and propagation occurring in a LAMP-built part, a mixed-mode cohesive element model was developed. The Mode I and Mode II cohesive parameters, which are used to describe the bilinear constitutive behavior of the cohesive elements, were determined by matching the numerical load-deflection curves to the experimental ones obtained from the DCB tests and the ENF tests, respectively. Using this model, the fracture of a hollow-cylinder part was analyzed and the simulation results were compared with experiments. Finally, several possible strategies for mitigating the shrinkage related defects were investigated. Reducing the overall polymerization shrinkage, optimizing the print-through curve and delaying the gel point of resin composite were demonstrated to be effective in reducing stresses and cracks.