Integrating elevated-temperature nanoindentation and microstructure-informed simulations to construct temperature-dependent yield surfaces of closed-cell PMI foam

Polymethacrylimide (PMI) foam is a widely used core material in lightweight sandwich structures, yet its mechanical behavior under elevated temperatures (typical co-cure molding conditions up to 120 °C) remains insufficiently understood from a microstructure-informed perspective. This study establishes a temperature-dependent, microstructure-informed experimental-computational framework to quantitatively link cell-wall properties to macroscopic yield surfaces across temperatures from 25 °C to 120 °C. Elevated-temperature nanoindentation reveals that the cell-wall elastic modulus decreases from 5752 to 3562 MPa and yield stress from 97.2 to 61.6 MPa, while the hardening exponent increases from 0.10 to 0.55, evidencing a brittle-to-ductile transition. High-fidelity finite element simulations based on μ-CT reconstructed microstructures capture the evolution of deformation mechanisms: at near-ambient temperatures (25-40 °C), plastic hinges and shear bands dominate; at elevated temperatures (80-120 °C), uniform plastic flow emerges and shear band formation is suppressed. Intra-cell gas pressure, modeled as fluid cavities, reduces the peak equivalent plastic strain by 3-4%, acting as a microscopic passive damper that synergizes with thermal softening to promote uniform deformation. Using a statistically equivalent Voronoi model validated against uniaxial experiments, multiaxial simulations (over 100 loading paths per temperature) are performed to construct initial yield surfaces in the σ m - σ e plane. The elliptical yield surfaces, centered on the hydrostatic axis due to initial isotropy, exhibit tension-compression asymmetry and contract toward the origin with increasing temperature. This physically based framework quantitatively bridges temperature-dependent microstructural mechanisms to macroscopic yielding, providing predictive capability for thermal-mechanical processing of PMI foam sandwich structure.