Interlaminar mechanical performance of latent-cure epoxy joints

Because of uncertainty in bond strength of adhesively bonded composite structures for air flight, redundant load paths, such as mechanical fasteners or other crack arresting features, are required for certification. However, these redundant load paths add extra weight to the structure, induce significantly greater fabrication time and cost, and introduce local stress concentrations (in the case of mechanical fasteners) from which damage can more readily initiate. Structures with co-cured joints are more predictable, require much fewer redundant load paths, and are therefore more desirable. A secondary bonding technique for polymer-matrix composites that aims to produce co-cure-like joints with a continuous interface is briefly presented. A brief progression of mechanical testing results is reported and compared to actual co-cured joint performance. The purpose of the work is to determine the feasibility of this technology to produce a secondary bond equivalent in performance and pre-dictability to that of a co-cured material. Joint performance was measured using six ASTM standard tests to measure interlaminar tensile and shear strength as well as mode-I and mode-II interlaminar fracture toughness at ambient temperature. After 50 process iterations, the final overall average joint performance exceeded 80% of the co-cured performance and in some tests had outperformed the co-cured specimens. The results suggest a high degree of feasibility of the technology to be pursued for future development with the intent to increase per-formance as well as reliability.