Simplified Scenario-Based Global Warming Potential Assessment of Carbon/Flax Fiber Hybrid Composites for Bicycle-Frame Applications

Carbon fiber-reinforced composites provide exceptional specific strength for lightweight structures but are associated with high global warming potential (GWP) during production. This study evaluates the GWP reduction potential of carbon/flax fiber hybrid composites for a one-piece main frame of a children’s bicycle. A simplified, scenario-based assessment was performed using a 3.0 kg reference mass and a fixed 70:30 reinforcement-to-resin mass ratio. The analysis varied flax substitution levels (0–40 wt.%), resin systems (epoxy and unsaturated polyester), manufacturing processes (resin transfer molding and prepreg/press), and end-of-life (EoL) treatments (incineration and landfill). Increasing flax content consistently reduced GWP across all scenarios. At 40 wt.% flax substitution, resin transfer molding achieved a 29.6–32.7% GWP reduction, whereas prepreg/press processing yielded a 21.5–23.0% GWP reduction. Material production dominated the GWP footprint in molding processes, whereas higher manufacturing burdens in prepreg/press routes partially offset the substitution benefits. GWP break-even mass ratios ranged from 1.273 to 1.486, indicating that hybrid frames can accommodate moderate mass increases without compromising environmental advantages. These results demonstrate that flax hybridization effectively reduces GWP, though the magnitude of reduction depends critically on manufacturing and EoL management choices. The proposed screening framework provides a practical tool for early-stage material and process selection in lightweight composite design.