Interlaminar fracture, or delamination, is a common failure mode which often occurs in composites as a result of low energy impact or manufacturing defects.  Polymer matrix composites (PMCs) are especially susceptible to matrix microcracking when subjected to repeated thermomechanical loading. Often these matrix microcracks coalesce and lead to other damage modes including fiber/matrix debonding and ply delamination. Long term degradation of material properties results and much effort is directed towards reliable damage prediction and property degradation models. Low velocity impact from events such as a tool drop or a glancing bird strike on an aircraft is another common cause of delamination damage in fiber reinforced PMCs. In many cases, this damage is difficult to detect and even more difficult to repair because it often forms deep within the structure. Once this damage has developed, the integrity of the structure is greatly compromised.

The objective of this project was to design and evaluate a new class of environmentally benign, low viscosity resins reinforced with nanosize alumina and silica particles for the repair of advanced composites.

This project designed and evaluated a new class of environmentally benign, low viscosity cyanate ester resins reinforced with nanosize reinforcement for the repair of advanced composites. The use temperature limit for this repair technology is high because of the polymer’s high glass transition temperature of greater than 500°F (260°C) and onset of decomposition above 750°F (400°C).

The prepolymer also has near infinite room temperature stability, facilitating reduced wastes due to spoilage compared to traditional thermosets. In demonstrations of the technology on bismaleimide (BMI)/carbon composite panels, the strength of damaged composites that were repaired with the technology through a resin injection process exceeded the strength of the virgin undamaged composite material.

The repair systems evaluated in this project were rheologically engineered for optimum crack filling and stability for repairment to withstand high loadings, environmental extremes and wide service temperatures. The repair resins and associated repair techniques developed through this research program were designed to meet the following technical objectives:

• Repair system will have a low viscosity to completely fill the damage zone
• No or minimal volatile organic compounds, hazardous air pollutants, and hazardous waste will be generated by the repair process
• A high cured glass transition temperature will be obtained (over 260°C) in the adhesive to allow the repair of composites for extreme temperature environments
• Repair system will be mechanically superior to current repair processes.

BECy was observed to be an effective resin for the repair of BMI-cf composite panels commonly used for high temperature applications. A mixed mode fracture of interface and substrate failure was observed in BECy bonded lap shear specimens. This failure was attributed to the formation of a strong interfacial bond between BECy and the BMI-cf substrate. In short beam shear bending tests, the low viscosity and high toughness of BECy at room temperature was observed to play a vital role in enhancing the structural strength of the specimen. The double cantilever beam (DCB) tests clearly demonstrate the enhancement in interlaminar debonding strength as a result of BECy repair. The crack resistance curves that quantify the crack initiation and propagation behaviors in delaminated composite specimens reveal that BECy repaired composites have high resistance to crack initiation and propagation. From scanning electron microscopy analysis of the fracture surfaces, fiber bridging was observed for BECy repaired DCB specimens. The results from lap shear, SBSB and DCB tests reveals that 100% repair efficiency can be accomplished by repairing the delaminated BMI-cf composites with BECy resin. Qualitative evaluation of repair efficiency from C-Scan images and florescence dye penetration tests reveal complete infiltration of BECy resin into the microscopic cracks in the delamination zone. Furthermore, a repair efficiency of 155% in HPS after HPS test and 100 % in CAI test were observed.

Different types of nanoreinforeced resins were prepared by mixing alumina nanoparticles, silica nanoparticles, and multi walled carbon nanotubes in BECy. The adhesive bond strength screened by lap shear testing revealed that alumina filled BECy is best for injection repair. However, Al filled nano resin had no significant effect on the repair strength versus the neat BECy system.

In demonstrations of the technology on BMI/carbon composite panels, the strength of damaged composites that were repaired with the technology through a resin injection process exceeded the strength of the virgin undamaged composite material. This technology will reduce the environmental hazards associated with current composite repairs and open up new repair opportunities specifically for high temperature composites, such as BMI matrix composites.