Bead geometry–induced stress concentration factors in material extrusion polymer additive manufacturing

Abstract

Purpose Material extrusion additive manufacturing processes inevitably produce bead-shaped surface patterns on the walls of parts, which create stress concentrations under load. This study aims to investigate the influence of such stress concentrations on the strength along the build direction (“Z-strength”). Design/methodology/approach This work consists of two main parts – an experimental demonstration to show the significance of stress concentrations on the Z-strength, followed by numerical modeling to evaluate the theoretical stress concentration factors (kt) for such shapes. Meso-scale finite element analysis (FEA) was performed to evaluate kt at the roots of the intersecting bead shapes. The critical bead shape parameters influencing kt were identified, and parametric FEA studies were performed on different bead shapes by varying the normalized parameters. Findings The experimental results showed that up to a 40% reduction in the effective Z-strength could be attributed only to the presence of surface bead shapes. Bead overhang and root radius were identified as critical shape parameters influencing kt. The results of the parametric FEA studies were used to generate a single empirical equation to determine kt for any bead shape. Originality/value Predictive models for Z-strength often focus on crystallization kinetics and polymer chain interdiffusion to predict interlayer adhesion strength. The authors propose that the results of such studies must be combined with surface bead-shape induced stress concentration factors to obtain the combined, “effective” Z-strength.

Interrelationships between process parameters, cross‐sectional geometry, fracture behavior, and mechanical properties in material extrusion additive manufacturing

“…Despite FFF's popularity, parts fabricated with FFF tend to be weaker and more brittle than those fabricated with traditional thermoplastic formative manufacturing (e.g., injection molding, extrusion) or other forms of thermoplastic AM (e.g., powder bed fusion/selective laser sintering) 1–3 . Worse properties may be caused by poor interlayer welding, 4,5 the presence of voids, 6,7 extrudate shape, 8,9 compositional variation across the extruded road, 10 or a combination of factors.…”

“…Allum et al reported that the surface topography of FFF parts, with the characteristic stacked layers of extrudate, leads to stress concentration in the notches that reduces the strength and fracture toughness even when conditions are appropriate for full welding 8 . Kundurthi et al also showed that the stress concentration factor, which is due to the geometry of stacked layers, reduces the effective z‐strength 9 …”