Warpage of thin additively manufactured continuous fiber thermoset composites

Additive manufacturing (AM) of thin continuous fiber thermoset composites can have large and anisotropic residual stresses that result in out of plane shape distortion, or warpage, and limits applicability to thin structures. In this study, the warpage of thin AM laminates was investigated as a function of deposited fiber direction and microstructure heterogeneity. The experimental results were compared with predictions made by Classical Lamination Theory (CLT), which assumed homogeneous deposited layers, and microstructure-informed CLT, which accounted for heterogeneity in the thickness direction. For thin unidirectional laminates, a curvature was observed in the transverse direction upon slow (0.3 °C/min) unconstrained cooling from the maximum curing temperature (140°C). Magnitudes of the observed curvatures ($\kappa$) decreased as the number of deposited layers (n) increased from n = 1 ($\kappa$ = 8 m⁻¹) to n = 8 ($\kappa$ = 1 m⁻¹) and did not match the CLT prediction of zero curvature. The variation in through-thickness fiber volume fraction was quantified by optical cross-section micrographs and used to make microstructure-informed CLT predictions which ranged from $\kappa$ = ∼0.6 m⁻¹ (n = 8) to $\kappa$ = 26 m⁻¹ (n = 1). While the curvature of the symmetrically stacked cross-ply laminates (0° and 90°) had near zero curvature and agreed with CLT predictions, the asymmetrically stacked cross-ply laminates predicted a large saddle-like curvature that was not fully observed due to delamination between orthogonally aligned layers. For thin (≤4 layers, ∼0.24 mm/layer) AM thermosetting laminates without delamination, the experimentally observed curvature values were roughly bounded between predictions made by CLT and by microstructure-informed CLT.