Stiffness and Failure of Carbon/Epoxy Sandwich Beams with Body- and Face-Centred Z-Strut (BFCZ) Lattice Cores at Elevated Temperature and Under One-Sided Heat Flux: A Numerical Study with Validation on Pyramidal-Core Beams

Lattice-core sandwich structures made of carbon fibre-reinforced polymer (CFRP) are attractive for lightweight aerospace and naval structures, but their epoxy matrix softens near its glass transition temperature (Tg). This study predicts the stiffness and failure of CFRP sandwich beams with a body- and face-centred Z-strut (BFCZ) lattice core at elevated temperature and under one-sided heat flux, using a three-dimensional finite element (FE) model coupled to a verified unit-cell thermal model. Validation was possible only for pyramidal cores: against published bending tests at 20–200 °C, the FE model, calibrated once at 20 °C, reproduced the stiffness to within 10% and, using node strengths from independent shear tests, the failure load to within 13% up to 180 °C. The BFCZ results are therefore predictions that require experimental verification. At equal strut diameter, the BFCZ beam is predicted to be 1.85 times stiffer and 2.05 times stronger than the pyramidal beam, but it contains 4.6 times more strut material; at equal mass or equal core density (2–15%), the pyramidal core is predicted to be at least as stiff and at least as strong. Under heat fluxes of 2.5–10 kW·m−2, the exposed face reaches 90% of its steady temperature rise within 2.5–5 min, when most of the stiffness and strength is lost; assuming uniform heating to the exposed-face temperature underestimates the residual stiffness by about 28% at 5 kW·m−2.