Non-coordinated degradation of cohesive traction and fracture energy in hygrothermally aged epoxy adhesive bonds

The long-term performance of adhesively bonded joints is governed by the durability of the adhesive layer under environmental exposure. For amine-cured epoxy systems, hygrothermal ageing has been associated with degradation of interfacial cohesive-zone parameters—the peak traction and the fracture energy—at systematically different rates, through radical-mediated C–O bond scission that decreases cross-link density and glass transition temperature. The proposed cohesive-zone-level framework is evaluated across multiple epoxy adhesive formulations (brittle/structural and toughened), while quantitative transferability to a given formulation remains parameterisation-dependent. Here we identify a derived cohesive-zone quantity, the failure separation (the ratio of twice the fracture energy to the peak traction), as the key diagnostic quantity of non-coordination and develop a polymer-physics interpretation linking the shortening to ageing-induced network damage. For brittle and structural epoxy formulations, the failure separation shortens to 33–98% of its unaged value across 11 independently reported hygrothermal degradation series. A force-sharing cascade model extended to chain scission predicts that the intrinsic chain-scission fracture energy changes by only approximately 3.2% at φ ≈ 0.10. Within the proposed cascade extension, the shortening is interpreted as arising from defect-induced premature failure of bridging chains rather than from a loss of intrinsic fracture resistance. A three-quantity mechanistic description—near-constant chain-scission energy, a moderately degraded interface strength for which a glass-transition–yield–stress relation is considered as an interpretive pathway, and a strongly shortened failure separation—is proposed and evaluated against 11 independently reported brittle/structural epoxy degradation series, one experimentally observed toughened-epoxy counter-example, and one constructed equal-degradation reference case. The framework provides a step towards connecting molecular-level characterisation of epoxy network degradation and the time-dependent evolution of cohesive-zone parameters.