UC San Diego

A common damage mechanism that occurs on polymer matrix composite (PMC) aircraft structures are delaminations which are interlaminar defects between composite plies. A current practice for repair of delaminations include injection using a repair resin. Injection repairs have traditionally been a faster, less expensive way to repair composite delaminations. However, the aerospace industry and safety regulating authorities consider injection repair cosmetic and have not accepted it as a strength restorative process. Questions about quantifying the removal of, internal contamination, how to characterize restored strength, and ensuring desired percentage of repair resin fill within the delaminations have been primary reasons why injection repairs have not been credited with strength restoration. Therefore, a new quantitative internal surface cleaning, preparation and repair process was developed for delaminated composite materials including the development and utilization of a new test coupon and configuration.

First, laboratory manufactured fractures were successfully compared to delaminations taken from actual aircraft structures using advanced surface metrology. Laboratory manufactured delaminations methods include a novel modified end notch flexure coupon (ENF) loaded in Mode II for fracture propagation and an out of plane impact specimen. Multiple lay-up configurations were characterized and surface metrology results indicate that the ENF fracture surfaces was most closely related to the in-service damage due to statistical comparison of arithmetic mean (Pa) and root mean square (Pq) surface profilometry and arithmetic mean roughness (Ra) results.

Materials and process development was also conducted to create a novel injection repair procedure. First, intentional contamination procedures were successfully developed, verified, and introduced into simulated delaminations. Contamination removal and internal fracture surface preparation using solvent and atmospheric plasma cleaning procedures were remarkably successful in removing hydrocarbon contaminant archetypes from contaminated fractures. Real-time mass spectrometry providing quantifiable cleaning verification of hydrocarbon contamination removal was successfully implemented into the novel procedure. Destructive post-processing verification of surface cleaning by infrared spectrometry, and goniometry was successfully completed. In addition, a modified low-viscosity injection resin was also developed for delamination infiltration and was found to exhibit shear strength characteristics comparable to un-modified structural adhesive while achieving a >84% reduction in viscosity. Following the development of a simulated enclosed delamination modified ENF coupon, application of the surface cleaning and modified repair resin to the previously fractured and contaminated coupons were tested. After re-pair of the modified ENF coupons, Mode II interlaminar fracture toughness (GIIc) and calculated stiffness was quantitatively restored or increased for all coupon configurations. Fracture analysis was completed to verify desired cohesive repair resin or adherend matrix brittle fracture failure modes of repaired test specimens. Therefore application of the novel injection repair procedure provided quantifiable local mechanical property restoration that contributes to the overall repair of aerospace PMC structures to carry designed loads.