Composite Processing and Repairs

The main potential for expansion of UV/EB into aerospace and certain commercial applications is by developing radiation curing of polymeric fiber-reinforced composites. The initial work on composite skin repairs involve applying the UV curing technology with bisacryl phosphine oxide to ensure the cure of relatively thick layers. A total of ten layers were used at a time. The UV cured composites closely matched those produced by heating.  

An alternative is a cationically initiated epoxy resin system cured by electron beam. The experiments with this process showed the following advantages  

EB curable adhesives aluminum to aluminum, graphite to aluminum, and graphite to graphite were used successfully. 

The repairs were performed on lower- and upper-wing skin assemblies and on a composite liquid hydrogen tank. On average, they took 50-70% of the time required for a normal thermal repair. The only process where EB carmot be used is resin transfer molding (RTM) because the electrons cannot penetrate the thick steel tooling required for it. The typical dose for this process was found to be 100 kGy.  

On-aircraft repairs of composite using a rapid-cure resin system of composite component with UV light irradiation based on TRI patenP have been developed by the U.S. Air Force Research Laboratory. Alternating layers of the acrylate-based resin system and woven fiberglass (the widely used wet la)mp procedure) are applied to fill the hole and form a UV curable composition. The width of the patch can be up to 2 ft (0.6 m) and the depth as much as 0.2 in. (5 mm). The cure time using a 400 W UVA lamp is reported to be 20 min. Although it is essentially a depot repair, it can be done field when necessary to return an aircraft to service. Because of the necessity to cure relatively thick repair patches, Us-acylphosphine oxide was used as a photoinitiator. An example of the patented UV curable resin system used for the repairs is in Table 11.1. 

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