Fig. 3: Progressive weight change in GRE composites under freeze–thaw treatment.

Weight change (%) in glass fiber-reinforced epoxy (GRE) composites is shown over multiple freeze–thaw cycles (a), highlighting four distinct phases: Phase 1, water uptake; Phase 2, crack initiation and epoxy removal; Phase 3, crack propagation; and Phase 4, ongoing degradation. The pre-thaw (blue line) and post-thaw (orange line) curves illustrate a pattern of cumulative weight change, while the inset symbols illustrate mechanisms within each phase, such as water ingress, crack development, and epoxy loss. The detailed observation of each phase is shown from (b–e), where Phase 1 (b) shows water uptake, with larger error bars reflecting specimen variability; saturated and partially filled voids are depicted in the inset. Phase 2 (c) illustrates crack initiation as pre- and post-thaw curves intersect, indicating equilibrium between water uptake and epoxy removal. In Phase 3 (d), crack propagation is evident, with increased water ingress leading to overall weight gain; the inset shows crack extension reaching adjacent voids. Phase 4 (e) reveals ongoing epoxy degradation, with schematic diagrams indicating progressive resin loss. Pre-thaw refers to the weight measurement taken after a 30-min thaw following freezingPost-thaw refers to the weight measurement taken after an additional 8-h soak following that initial thaw.