MITIGATING THE EFFECTS OF SPACE DEBRIS ON COMPOSITE
STRUCTURES EMBEDDING SELF HEALING AND CARBON
NANOTUBE NANOCOMPOSITE MATERIALS
B. Aïssa 1,2, D. Therriault 3, E. Haddad 1, W. Jamroz 1, K. Tagziria 1, J. Loiseau 4, A. Higgins 4, M. A. Khan 5 and S. V. Hoa5
1
MPB Communications Inc., Department of smart materials and sensors for space mission. Pointe Claire, Quebec, H9R 1E9, Canada.
2
Institut National de la Recherche Scientifique, Énergie, Matériaux et Télécommunications, 1650 Boulevard Lionel Boulet, Montreal metropolitain, J3X 1S2, Canada.
3
Center for Applied Research on Polymers (CREPEC). Mechanical Engineering Department, Ecole Polytechnique de Montréal, Montreal, Quebec, Canada;
4
Shock waves physics Laboratory. Department of Mechanical Engineering. McGill University, Montreal, Quebec, H3A 2T5, Canada.
5
Concordia Center for Composites. Department of Mechanical and Industrial Engineering. Concordia University. Montreal, Quebec, H3G 2M8, Canada.
Keywords: Self healing, space debris, composite structure, carbon nanotubes, hypervelocity impact tests.
ABSTRACT
The presence in space of micrometeoroids and orbital debris, particularly in the lower earth orbit, presents a continuous hazard to orbiting satellites, spacecrafts and the international space station. Space debris includes all non-functional, man-made objects and fragments. As the population of debris continues to grow, the probability of collisions that could lead to potential damage will consequently increase. We report on our recent results obtained on the application of self healing composite materials on impacted composite structures used in space. Self healing materials were blends of microcapsules containing mainly various combinations of a 5-Ethylidene-2- Norbornene (5E2N) and dicyclopentadiene (DCPD) monomers, reacted with ruthenium Grubbs’ catalyst.
The self healing materials were then mixed with a resin epoxy and single-walled carbon nanotubes (SWNTs) using vacuum centrifuging technique. The obtained nanocomposites were infused into the layers of woven carbon fibers reinforced polymer (CFRP). The CFRP specimens were then subjected to hypervelocity impact conditions by using an advanced implosion driven-hypervelocity launcher - to simulate the space debris impact- with projectiles of about 4 mm in diameter and velocities up to 9 km/s. Although the microencapsulated self healing materials would not heal the impact's crater zone, we focused mainly on the reparation of potential delaminations developed around the impact-crater over distances much larger than the crater diameter. The different self-healing capabilities were determined and the SWNTs contribution was discussed with respect to the experimental parameters. 1. INTRODUCTION
A major challenge for space missions is that all materials degrade over time and are subject to wear, especially under extreme environments and external solicitations. Impact events are inevitable during the lifetime of a space composite structure, and
once they are damaged they are hardly repairable. More specifically, polymeric composites are susceptible to cracks that may either form on the surface or deep within the material where inspection/detection is often impossible. Materials failure normally starts at the nanoscale level and is then amplified to the micro up to the macro-scale until catastrophic failure occurs. The ideal solution would be to block and eliminate damage as it occurs at the nano/microscale and restore the original material properties.
This work reports on our recent results obtained on the application of self healing composite materials on impacted composite structures used in space. Self healing materials were blends of microcapsules containing mainly various combinations of a 5- Ethylidene-2-Norbornene (5E2N) and dicyclopentadiene (DCPD) monomers, reacted with ruthenium Grubbs’ catalyst. The self healing materials were then mixed with a resin epoxy and single-walled carbon nanotubes (SWNTs) using vacuum centrifuging technique.
The obtained nanocomposites were infused into the layers of woven carbon fibers reinforced polymer (CFRP). The CFRP specimens were then subjected to hypervelocity impact conditions -prevailing in the space environment- using an advanced implosion driven-hypervelocity launcher. The fiber Bragg Grating (FBG) sensors were embedded in the composite material providing real-time information about the impact event and the healing process. The different self-healing capabilities were determined and the SWNTs contribution was discussed with respect to the experimental parameters.
2. MATERIALS
Single walled carbon nanotubes (SWNTs) materials have been synthesized by using the developed plasma torch technology (detailed process can be found in the Ref. [1]). In this approach, a carbon containing ethylene (C2H4) substance combined with
gaseous catalyst based ferrocene (Fe (C5H5)2) vapour are injected in an inert gas
plasma jet. Figure 1 shows the morphology of the grown SWNTs where the transmission electron microscopy micrographs (TEM) show single nanotubes of 1.2 nm in diameter. The encapsulation of the (5E2N): 5-Ethylidene-2- Norbornene and (DCPD): dicyclopentadiene in poly (melamine urea formaldehyde) microcapsules was achieved following the protocol described in our Ref. [2]. Several batches of microcapsules were produced following the reported process. The size of the microcapsules was mainly controlled by varying the stirring-speed during the synthesis process.
Some samples of the smallest microcapsules synthesized from different batches are shown in Fig. 2. All samples were dried in air for at least 24 hours after their final washing and filtering. The woven CFRP samples containing self-healing demonstrator consists of epoxy used in space for internal structures (Epon 828 resin, with the Epicure 3046 curing agent), and 2 different healing agents (namely, 5E2N, DCPD) prepared as small microcapsules (diameter less than 15 microns) kept within thin shells of poly melamine (urea formaldehyde).
The monomer is homogeneously spread within the epoxy and forms about 10% of the total weight. The Grubbs catalyst was then distributed within the epoxy structure (1 to 2 % of the total weight). Different series of samples specimens were prepared, with and without CNTs.
Figure 1: TEM of the as grown SWNTs
Figure 2: (up): optical image and; (down): scanning electron microscopy (SEM) image of the 5E2N monomer microcapsules.
3. METHODS
After the hypervelocity impact tests (Impact tests were performed with the implosion-Driven Hypervelocity launcher), the crack formed on the CFRP samples reaches a microcapsule and causes its wall rupture, which releases the healing agent monomer (5E2N or DCPD or combination of the two monomers). The FBGs sensors were embedded between the 2nd and 3rd CFRP layer, but concentrated inside a circle surface of 5 cm in diameter (see Fig. 3).
All the fabricated samples are tested under hypervelocity impact test at McGill University (Prof. A. Higgins Lab.) under the same conditions for comparison. To simulate the orbital space debris, small projectiles (3-4 mm-diameters) and velocities up to 9 km/s were employed. The set up of the test is shown in figure 4.
Impacted CFRP samples have been then measured under the flexural “3 point bending test” after the healing process (48 hours and 40o C) to investigate their mechanical properties and to evaluate the self healing capability after impacts event.
Figure 3: Integration of 4-8 FBGs sensors embedded between 2nd and 3rd CFRP layer and concentrated inside a circle surface of 5 cm-diam.
Figure 4: (a) Hypervelocity impact tests Launcher built at McGill University. Launched projectiles to 9 km/s. (b) Section where the CFRP samples are placed.
4. RESULTS AND CONCLUSIONS
When comparing the mechanical recovery to the pristine samples (i.e., containing only epoxy material), we can extract the healing part due exclusively to the self healing materials [3]. In doing so, we conclude the following:
(i) 31 MPa are due exclusively to the self healing material based on 5E2N, which represents an enhancement in terms of the mechanical strength of about 13 %.
(ii) When using the DCPD based healing agent, a better healing is obtained (improvement up to 18 % of the mechanical strength).
(iii) When using a mixture of 50/50 wt. % of DCPD/5E2N healing agent, a slight decrease occurred (From 18 to ~15 %) in terms of the flexural strength, which is due to the incorporation of the 5E2N part (recall that the 5E2N is a linear polymer having lower mechanical strength, its addition to the DCPD slightly decreases somehow the overall mechanical strength of the mixture).
(iv) A clear improvement is obtained when integrating the SWNT material, even with concentration as low as 0.5 wt. %.
(v) Then, an enhancement up to 81 MPa in terms of the mechanical recovery is due the healing materials containing 2 wt. % of SWNT, which represents an improvement in the mechanical strength as high as 33 %.
ACKNOWLEDGEMENTS
We acknowledge the financial assistance of the Canadian Space Agency and the constructive advise of Dr. D. Nikanpour and Dr. S. Gendron from CSA, for their support during this work.
REFERENCES
[1] O. Smiljanic, B.L. Stansfield, J.P. Dodelet, A. Serventi and S. Désilets, Gas-phase synthesis of SWNT by an atmospheric pressure plasma jet, Chem. Phys. Lett. 356 (2002) 189-193.
[2] B. Aïssa, E. Haddad, K. Tagziria, W. Jamroz, Exploring Self Healing of CFRP Laminates Exposed to Hypervelocity Small Pellets Simulating Space Debris, proceedings of 26th Technical Conference/Second Joint US-Canada Conference on Composites, (2012) paper 1066.
[3] X. Liu, X. Sheng, J.K. Lee, M.R. Kessler, Isothermal cure characterization of dicyclopentadiene, J. Therm. Anal. Calorim. 89 (2007) 453-457.
