Study of flexural performance degradation in neat and flax-fibre-reinforced PLA gyroid sandwich panels under humid/dry aging cycles

This study investigates the flexural durability of FFF-printed gyroid-core sandwich structures made from neat PLA and PLA-flax under accelerated humid/dry aging. Both materials were printed using the same sandwich configuration, with a gyroid core of 10 mm cell size and 20% infill density. Specimens were exposed to 0, 200, 400, 600, 800, and 1000 cycles, alternating between dry conditions at 40 ± 2°C and 10 ± 5% RH and humid conditions at 23 ± 2°C and 85 ± 5% RH. Three-point bending tests showed that both materials degraded with aging, but PLA-flax was more affected. After 1000 cycles, neat PLA showed reductions of 11.6% in apparent flexural modulus and 10.6% in apparent flexural strength, while PLA-flax showed greater losses of 24.1 and 25.0%, respectively. The degradation of PLA-flax remained limited during the initial aging stages, then became more pronounced after 600 cycles. Optical observations showed that neat PLA retained similar damage features after aging, mainly facesheet cracking, interlayer debonding, and local layer deformation. In contrast, PLA-flax showed a change in failure morphology after prolonged exposure, with more evident interlayer debonding, matrix cracking, and facesheet–core delamination. A first-order analytical model based on classical sandwich beam theory was used to predict the midspan displacement in the linear bending range. By introducing a stiffness retention factor and an effective correction factor for non-ideal skin–core interaction, the model reproduced the global deformation trend, with average absolute deviations of 1.55% for neat PLA and 2.62% for PLA-flax.