The Toughness and Residual Stresses of Polymers Produced by Fused Filament Fabrication

Fused filament fabrication (FFF), also known as fused deposition modeling (FDM), is one of the most widely‐used additive manufacturing methods. In FFF, the feedstock material, usually a glassy polymer filament, is fed into the nozzle, heated well above the glass transition temperature, and then squeezed and deposited to form layer‐by‐layer 3D structures. FFF possesses the advantage of low cost, user‐friendliness, ease of customization, and the ability to fabricate complex structures. This technology has gained great popularity in prototyping, but its drawbacks, including poor mechanical strength and dimensional accuracy, have limited its broader applications in batch production. It has been known that the mechanical strength of the printed part is greatly affected by the toughness of welds, and the dimensional accuracy is mainly determined by the non‐uniform thermal expansion and the accompanying residual stresses. This work aims to develop a constitutive model for glassy polymers, numerically investigating the trouser‐tear test used to measure the weld toughness, and establishing a novel method to measure the evolution of the residual stresses in FFF.

Glaucoma is the leading cause of irreversible blindness in the world. Patients suffer from gradual loss of vision due to degeneration of the retinal ganglion cell (RGC) axons at the optic nerve head (ONH). A major risk factor for glaucoma is intraocular pressure (IOP) and the increase in IOP initiates biomechanical events at the ONH prior to axonal damage. However, the exact injury mechanism remains unknown. Lamina cribrosa (LC), as a load bearing structure that spans the ONH, serves to protect the RGC axons from IOP-induced deformation. The LC is made up of a meshwork of collagen beams. Within the LC, the non collagen pore spaces are spanned by a network of astrocyte cells. Together, the LC structure and the astrocytic network provide mechanical support to the RGC axons as the axons exit the eye wall and enter into the optic nerve canal. Inflation tests of the human eyes have shown that IOP-induced strain response in the LC varied regionally. The pattern of the strain variation corresponded to the pattern of axonal damage in glaucoma patients. It is hypothesized that variations of microstructure in the LC contributed to the variations in IOP-induced strains. This may translate to unequal deformation of the astrocytic networks among different LC pores, causing some axons to be more susceptible to glaucomatous injury. Therefore, the objective of this work was to first investigate the effect of structural properties of LC on the regional strain responses, then to compare the structural difference in astrocytic network after IOP elevation, and finally to probe into the effect of astrocytic structure on the strains experienced by the axons using finite element modeling.