A self-healing advanced composite system was designed and optimized using minimum self-healing (SH) agent (~0.02%) deposited in microscopically ordered arrays through inkjet printing, to arrest cracks along interfaces between plies (see figure). The approach consisted of depositing thermoplastic, low-viscosity microdroplets with chemically and mechanically comparable properties to epoxy matrix in aerospace-grade composites onto fiber-reinforced epoxy prepregs before curing. The SH agents remained arrested and encapsulated between epoxy plies without direct contact with neighboring microdroplets. This ensured consistent integrity of the composite while preserving the SH capability.
The spatial control offered by inkjet printing allowed study of compositional variation in SH agents to optimize SH efficiency. A comparison was made between the systems with discretely printed SH agents and fully covered plies using the same printing method. The efficiency of discrete (hexagonal) patterns was higher compared to the fully printed surfaces, as the former method enabled the adjacent composite plies to cure without the loss in chemical adhesion.
Poly(methyl methacrylate) (PMMA) was printed using toughened carbon fiber epoxy prepreg Cycom977-2 as the substrate. An appropriate amount of damage was introduced into specimens to demonstrate SH. Double cantilever beam (DCB) and short beam shear (SBS) tests were adopted to evaluate the SH efficiency.
Carefully selected, printed self-healing agents increased both shear modulus and fracture toughness of CFRP simultaneously, without imparting any parasitic weight, and restored the properties of the damaged and self-healed composite to a large degree, following the post-damage heat treatment. The specimens with printed PMMA exhibited the highest mode I interlaminar fracture toughness (GIC), and the shear modulus both before and after healing cycles.
Following the deposition of inkjet-printed patterns, the composites were cured and manufactured. Damage process was carried out using a tensometer to perform the SBS test to induce the equivalent amount of damage to all specimens. The healing cycle imitated the curing cycle in order to investigate the harshest temperature environment that the composite may be subjected to at this stage.
A coffee-staining effect was evident after the full cure cycle was completed; however, the droplets preserved their geometry and the surface aspect ratio. A similar investigation was carried out on the printed carbon fiber prepreg. In this case, the droplets of PMMA were not clearly observed following the curing cycle, indicating the reaction with epoxy during the curing process. Fluorescein was added to PMMA and PEG during the printing process to help identify the deposited droplets during the thermal cycle. Printed fluorescein within PEG and PMMA dots disappeared after heating. There could be two explanations: the polymer penetrated into the epoxy during the thermal cycle, or the fluorescein reacted and evaporated during the process. An interferometer was used to image PMMA droplets printed on a glass slide and prepreg.
The interferometry results suggested that the printed PMMA droplets reacted with the epoxy during the curing cycle, and their visibility was not as evident as on the glass slides. Their distinct chemical composition guaranteed the preservation of thermoplastic islands in the material after the heating process.
Two standard mechanical tests were used to evaluate the structural integrity and SH efficiency of the inkjet-printed engineered composite material. Samples with printed SH agent possessed higher shear modulus, with printed PMMA samples showing the highest shear modulus, indicating a synergistic mechanism between the printed droplets and the base material. There is no significant difference among the virgin and printed samples with regard to the average maximum load and the average maximum interlaminar shear strength, showing that the structural integrity of the composite has been preserved with printed additives.
The samples with printed SH agent possessed higher shear modulus than that of the virgin samples after the secondary heating cycle — again, the highest stiffness associated with PMMA printed composites. The SH agents can impart beneficial properties throughout the service life of the composite.
Considering that the composite system is already toughened and hardened, the imparted toughening, stiffening, and SH properties through the method of PMMA discrete deposition at ~0.02% weight fraction represent a very successful achievement. Furthermore, manufacturing of these composites can be automatically scaled up without causing any disruption to the existing composites supply chain, and without a need to develop new equipment.
This work was done by Alma Hodzic and Patrick Smith of the University of Sheffield for the Air Force European Office of Aerospace Research & Development. AFRL-0228