Structural integrity and quality of short fiber composite parts produced by Big Area Additive Manufacturing (BAAM) are largely affected by inherent bead microstructural features such as voids. Unfortunately, our understanding of void nucleation and evolution during polymer deposition process is lacking. Flow modeling focused on the associated microstructural formation provides a means for better understanding the process-structure-properties relations in large area extrusion deposition additive manufacturing of fiber reinforced composites. Our prior computational effort that investigated mechanisms that may promote micro-void formation was based on 2-dimensional planar models of a single ellipsoidal fiber motion in purely viscous polymer extrusion/deposition flow through a BAAM nozzle. Here we present a 3D finite element modelling approach to simulate single fiber out-of-plane rotations utilizing velocity and velocity gradient values computed along streamlines obtained from a 3D extrusion/deposition simulation of the BAAM polymer deposition process. The pressure distribution on the fiber’s surface along the flow path provides new insight into potential micro-void nucleation mechanism. Results show low pressure regions occur near the fiber’s surface which varies across the printed bead and through its thickness.
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MODELING CARBON FIBER SUSPENSION DYNAMICS FOR ADDITIVE MANUFACTURING POLYMER MELT FLOWS
The addition of short carbon fibers to the feedstock of large-scale polymer extrusion/deposition additive manufacturing results in significant increases in mechanical properties dependent on the fiber distribution and orientation in the beads. In order to analyze those factors, a coupled computational fluid dynamics (CFD) and discrete element modeling (DEM) approach is developed to simulate the behavior of fibers in an extrusion/deposition nozzle flow after calibrations in simple shear flows. The DEM model uses bonded discrete particles to make up flexible and breakable fibers that are first calibrated to match Jeffery’s orbit and to produce interactions that are consistent with Advani-Tucker orientation tensor predictions. The DEM/CFD model is then used to simulate the processing of fiber suspensions in the variable flow and geometries present in extrusion/deposition nozzles. The computed results provide enhanced insight into the evolution of fiber orientation and distribution during extrusion/deposition as compared to existing models through individual fiber tracking over time and space on multiple parameters of interest such as orientation, flexure, and contact forces.
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- Award ID(s):
- 2055628
- PAR ID:
- 10507586
- Publisher / Repository:
- https://utw10945.utweb.utexas.edu/2023-table-contents
- Date Published:
- Journal Name:
- 2023 International Solid Freeform Fabrication Symposium
- Format(s):
- Medium: X Size: 1.728MB
- Size(s):
- 1.728MB
- Location:
- Austin, Texas, USA
- Sponsoring Org:
- National Science Foundation
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The process-structure-property relationship in Large Area Additive Manufacturing (LAAM) technology is an ongoing area of research as the inherent microstructural properties (chiefly fibers and voids) affect the performance of printed parts. Unfortunately, we currently lack adequate understanding of micro void nucleation and evolution during the LAAM and fused deposition modelling (FDM) processes. Modeling of the polymer melt flow during the extrusion process is important in understanding the underlying microstructural formation and associated properties of the print, that determines the part performance in service conditions. In this paper we compute fiber-induced local pressure fluctuations which may promote void formation in the bead’s microstructure. On a macro-scale, we determine flow fields of a purely viscous, Newtonian planar polymer deposition flow through a LAAM nozzle which are utilized on a micro-scale model where we simulate the evolution of a single ellipsoidal fiber along streamlines obtained from the macro-model. On the micro-scale, we determine instantaneous values of the translational and rotational velocities of the rigid ellipsoidal fiber that satisfies a balance of hydrodynamic forces and couples on the fiber’s surface based on a Newton Raphson algorithm and we track the fiber’s motion along the flow path via an explicit numerical iterative algorithm. Model verification is achieved by benchmarking results with solutions from well-known Jeffery’s equation of motion of a particle in homogeneous simple shear flow. We account for rotary diffusivity due to short-range fiber-fiber interaction in the FEA simulation by determining an effective fluid domain size representative of the interaction coefficient of the melt flow through a correlation analysis that yields an equivalent steady state orientation based on the Advani- Tucker equation. We also consider different possible motions of the fiber along individual LAAM flow paths from a given set of random initial fiber conditions to determine pressure bounds on the fiber surface along each streamline. For improved computational efficiency, calculations are carried out with respect to the fiber’s local coordinate axes to overcome the rigor of adaptive remeshing during the quasi-transient analysis. Results show low pressure extremes near the fiber’s surface which varies across the printed bead as well as through its thickness. Discussion is provided to gain insight into the effect of low-pressure extremes on micro void formation, particularly at the nozzle exit and during die swell/expansion.more » « less
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Abstract Fiber‐filled composite materials offer a unique pathway to enable new functionalities for systems built via extrusion‐based additive manufacturing (or “3D printing”); however, challenges remain in controlling the fiber orientations that govern ultimate performance. In this work, a multi‐material, shape‐changing nozzle—constructed by means of PolyJet 3D printing—is presented that allows for the spatial distribution of short fibers embedded in polymer matrices to be modulated on demand throughout extrusion‐based deposition processes. Specifically, the nozzle comprises flexible bladders that can be inflated pneumatically to alter the geometry of the material extrusion channel from a straight to a converging–diverging configuration, and in turn, the directional orientation of fibers within printed filaments. Experimental results for printing carbon microfiber‐hydrogel composites reveal that increasing the nozzle actuation pressure from 0 to 100 kPa reduced the proportion of aligned fibers, and notably, prompted a transition from anisotropic to isotropic water‐induced swelling properties (i.e., the ratio of transverse to longitudinal swelling strain decreased from 1.73
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The formation and development of micro-voids within the bead microstructure of a polymer composite during the extrusion/deposition additive manufacturing process continues to be of interest given the adverse effect these features have on part quality. A computational method is employed here to investigate potential volatile-induced micro-void nucleation mechanism which simulates the evolution of a single rigid ellipsoidal fiber in purely viscous polymer extrusion/deposition flow through a Large Area Additive Manufacturing (LAAM) nozzle. Our previous studies on potential micro-void nucleation mechanisms have assumed a Newtonian fluid property definition for the polymer melt flow, the current study assesses the effect of assuming a generalized Newtonian fluid (GNF) model on the fiber’s response. Preliminary findings based on Jeffery’s flow assumption reveal the fiber’s orientation kinetics are unaffected by the shear thinning fluid behavior, however there is a reduction in the pressure distribution on the fiber’s surface as the power law index is decreased which is expected to reduce the likelihood for microvoid nucleation.more » « less
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