Strain-Rate-Dependent Behavior and Constitutive Modeling of Stereolithography-Printed Polylactic Acid and Hydroxyapatite Composites

Polylactic acid (PLA) reinforced with hydroxyapatite (HA) has attracted significant attention as a candidate biomaterial composite due to its biodegradability, bioactivity, and tunable mechanical properties. However, the strain-rate-dependent mechanical response of Stereolithography (SLA) 3D printed PLA+HA composites and its constitutive representation remain insufficiently explored, limiting predictive modeling for load-bearing applications. A simplified Zhu–Wang–Tang framework was employed to construct a constitutive model. This study presents an integrated experimental and constitutive modeling approach that combines quasistatic strain-rate characterization, simplified ZWT constitutive modeling, and complementary surface and microstructural characterization. PLA + HA composites containing 1, 5, 10, and 15 wt.% HA were fabricated by SLA and tested under compressive loading at strain rates of 10^-4 s^-1 and 10^-3 s^-1. The experimental true stress–true strain data were used to identify the nonlinear elastic and Maxwell I parameters using a sequential fitting procedure. The simplified ZWT model successfully reproduced the strain-rate-dependent mechanical response of the investigated composites with a high agreement with the experimental results. Surface wettability measurements demonstrated improved hydrophilic behavior with increasing HA content, while scanning electron microscopy (SEM) revealed progressively increased particle clustering and localized micro-void formation at higher HA contents, providing direct microstructural evidence supporting the observed mechanical behavior. This study establishes a constitutive basis linking HA content, strain-rate-dependent mechanical response, and microstructural characteristics of SLA-printed PLA+HA composites, providing a foundation for future numerical simulations of polymer-based biomedical components.

Polylactic Acid + Hydroxyapatite composites; Simplified Zhu–Wang–Tang constitutive model; Stereolithography 3D printing; Strain rate sensitivity; Viscoelasticity

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