Multiscale analysis of elastoplastic-to-cracking mechanisms in PLLA

Poly(L-lactic acid) (PLLA) is a semicrystalline polymer of central importance in biomedical engineering, yet accurately predicting its complex failure evolution from plastic yielding to crazing remains a significant challenge. This study establishes a novel multiscale physical-mechanical framework to investigate the elastoplastic-to-cracking transition mechanisms in PLLA specimens across different molecular weights ((1.4–5.0) × 10⁵ g/mol). Surface characterization via atomic force microscopy (AFM) and macroscopic uniaxial tensile testing revealed a distinct three-stage failure process—comprising elastic deformation, plastic hardening, and craze-mediated cracking—that is heavily influenced by molecular weight-dependent lateral intermolecular linkages. To computationally model this sequence, an Extended Finite Element Method (XFEM) platform was developed, employing 3D Voronoi tessellations to map the semicrystalline aggregates and a cohesive element insertion method to simulate the fibrillated plastic process zone. The plastic hardening response was captured using the Johnson-Cook model, wherein the hardening modulus increased from 9.8 to 141 MPa with rising molecular weight. Concurrently, craze damage evolution was governed by a traction-separation law, demonstrating a critical energy release rate increase from 1.43 to 4.63 N/m. The numerical framework demonstrated strong quantitative agreement with experimental stress-strain trajectories and fracture morphologies, accurately capturing localized stress concentrations (e.g., ~55.2 MPa at crack initiation) and the transition toward a crystalline-like fracture mode in higher molecular weight variants. This multiscale approach provides a fundamental theoretical framework for understanding the micro-mechanical failure of PLLA, serving as a preliminary baseline for the future design of load-bearing implants under complex physiological conditions.