Process-microstructure-anisotropy relationships in FFF-printed PLA/wood flour composites: Multiaxial ultrasonic determination of the orthotropic elastic tensor

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Abstract

Fused filament fabrication (FFF) produces strong, direction-dependent elastic anisotropy that conventional uniaxial testing cannot fully capture. This study introduces a multiaxial ultrasonic methodology that identifies the complete orthotropic stiffness tensor of FFF-printed polylactic acid (PLA) and PLA/wood-flour (PLA/WF) composites and uses it to quantify how layer thickness, infill density and infill pattern control their directional elastic response. Chamfered cubic specimens (72 in total, 24 processing configurations, three replicates each) were tested along three principal axes and three face-diagonal directions to obtain the nine independent stiffness coefficients, with a within-configuration repeatability of 1.2–6.2 %. Infill pattern is the principal driver of elastic anisotropy: the in-plane stiffness ratio E2/E1 reaches 2.9 for the Line pattern at low infill density, against near-unity values for the Combined pattern. Wood flour redistributes stiffness preferentially transverse to the filaments at low infill density, recovering E₁ by up to 110 % and reducing in-plane anisotropy by up to 41 %, while the build direction remains intrinsically softer than the in-plane directions regardless of raster orientation. Compliance-matrix inversion further suggests apparent near-zero or negative Poisson’s ratios (ν₁₂ as low as −0.06) for one Line-pattern configuration at low infill density; this auxetic-like response is reported as a working hypothesis requiring direct strain-field validation. A four-way ANOVA on the raw replicate measurements confirms these trends quantitatively, with highly significant effects of infill density and material on all nine stiffness coefficients (p < 0.0001) and significant material × density interactions for eight of the nine coefficients (p < 0.05). Acoustic microscopy supports these findings by revealing inter-raster voiding, wood-flour-mediated bridging and layer-thickness-dependent interlayer features. The proposed framework offers a non-destructive route to the complete elastic characterization of FFF-printed biobased composites and quantitative guidance for the design of structurally anisotropic components.

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