Novel Enclosed Delamination Injection Repair Process for Strength Restoration

Abstract

Delaminations are a common damage mechanism that occurs on composite structures due to in-service impacts. Enclosed damages less than 50 mm in diameter are of particular interest for repair. Current practices for repair of delaminations include: cut out of the damage and applying a patch or doubler to the cutout site, or injection using a repair resin. The use of a bonded doubler repair is more invasive and requires a cutout that acts as a stress concentration in the area of the repair. Injection repairs have traditionally been a faster, less expensive way to repair composite delaminations. However, the aerospace industry has not accepted injection repair as a strength restorative process. Questions about quantifying delamination internal contamination and ensuring percentage of repair resin fill within a delamination have been primary reasons why injection repairs have not been credited with strength restoration. Development of a new quantitative internal surface cleaning, preparation and repair process for delaminated composite materials is required to provide justification for strength restoration using injection repair. Fabricating representative local delaminations was performed with a modified end-notch flexure (ENF) test. A unidirectional 24-ply panel was fabricated to simulate damage to laminates. Intentional contamination was introduced into the induced delamination of each panel type. Contamination removal and surface preparation using solvent and atmospheric plasma cleaning were completed on the modified ENF coupon and verified utilizing in-line monitoring with Quantitative Gas Analysis (QGA) and post processing inspection with Fourier Transform Infrared Spectroscopy (FTIR). A modified low-viscosity injection resin was developed and utilized to repair the post-tested ENF coupons. The modified epoxy exhibited comparable shear strength characteristics to un-modified neat resin while achieving a >97% reduction in viscosity. Each ENF panel type was assessed for Mode II fracture toughness (GIIc) of the parent material and repair of fractured coupons. Fractured coupons simulating an enclosed delamination were repaired: without contamination, with contamination, and after the novel contamination removal processes were completed. Mode II interlaminar fracture toughness was restored or increased when compared to control test groups for all coupons that were repaired or cleaned and repaired.