The presence in the 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 nonfunctional, 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. This work addresses a short review of the space debris “challenge” and 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 a home-made implosion-driven hypervelocity launcher. The different self-healing capabilities were determined and the SWNT contribution was discussed with respect to the experimental parameters.
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 macroscale 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.
Self-healing materials are conceived as having the potential to heal and restore their mechanical properties when damaged, thus enhancing the lifetime of materials and structures. Typical examples of self-healing materials can be found in polymers, metals, ceramics, and their composites which are subjected to a wide variety of healing principles. Healing can be initiated by means of an external source of energy as was shown in the case of a bullet penetration [
On the other hand, with their well-known excellent mechanical and electrical properties, carbon nanotubes (CNTs) are inherently multifunctional and can serve as an ideal structural reinforcement. Considerable interest has focused on using CNTs as a passive reinforcement to tailor their mechanical properties [
In this work, we successfully developed self healing materials composites, consisting of different blends of microcapsules containing various combinations of a 5-ethylidene-2-norbornene (5E2N) and dicyclopentadiene (DCPD) monomers, reacted with the ruthenium Grubbs’ catalyst. The self healing materials were then successfully mixed with an Epon 828 based resin epoxy and single-walled carbon nanotubes (SWNTs) materials and infused into the layers of woven carbon, fibers reinforced polymer (CFRP). The CFRP specimens structures were then subjected to hypervelocity impact conditions—prevailing in the space environment—using a home-made implosion-driven—hypervelocity launcher. Finally, the impacted CFRP specimens were systematically characterized with the three-point bending tests for flexural strengths evaluation, where both the self-healing efficiency and the CNT contribution were discussed.
The optic fibres are cylindrical silica waveguides. They consist of a core surrounded by a concentric cladding, with different refraction index, guaranteeing the light propagation (Figure
(a) Description of the optical fibre. (b) Fibre Bragg grating concept.
If broadband light is travelling through an optical fibre containing such a periodic structure, its diffractive properties promote that only a very narrow wavelength band is reflected back (Figure
Typical optical power spectrum (reflectance in dBm) of our embedded FBG.
At constant temperature for a longitudinal strain variation,
When an axial stress is applied to the optical fiber, the reflected spectrum wavelength shifts. This shift is to make wavelengths for axial tension higher and to lower wavelengths for axial compression. The axial strain applied to the optical fiber at the location of FBG can be calculated from the shift in the peak wavelength as follows:
Schematic of FBG sensor reflected spectrum under various strain states.
Space debris coming from human devices set in space mainly occurs in Low Earth Orbital (LEO) below 2000 km and around the geostationary orbit (GEO) altitude. Meteoroids, which are natural phenomena, are found everywhere in space. However, impact effects from meteoroids and debris are similar, average impact velocities in LEO are 10 km/s for space debris and 20 km/s for meteoroids:
average material density of meteoroids is lower than that of the space debris, in LEO, meteoroids dominate for sizes between 5 and 500 microns (0.5 mm), space debris dominates for larger sizes.
Micrometeoroids are small particles from an asteroid or comet orbiting the sun (called meteoroids) that survives their passage through the Earth’s atmosphere and impacts the Earth or the satellite surface [
Hypervelocity impact events may modify the original chemical composition of an impactor, fractionating volatile from refractory elements. Thus micrometeoroid residues may not necessarily retain the stoichiometric chemical signature of their parent mineral; in this case analytical results are not easily compared to those of mineral standards. Notwithstanding such difficulties, Energy Dispersive (EDS) spectra and X-ray elemental maps of residues that contain the following elements can be used as indicators of a micrometeoroid origin [ Mg + Si + Fe (mafic silicates, e.g., olivine or orthopyroxene), Mg, Ca, Na, Fe, Al, Ti + Si (clinopyroxene), Fe + S (Fe-sulfides), Fe + Ni (minor or trace) + S (Fe-Ni sulfides), Fe + Ni concentration at meteoritic levels (metal), Si + C (silicon carbide), Fe, Mg, Al + Si (phyllosilicates, e.g., serpentine), Ca, C, O (calcite), K, Cl, P, and Cr have also been individually identified in meteoritic samples; therefore, may be indicative of a micrometeoroid origin under some circumstances.
Because of the complexity of any original micrometeoroid (polymineralic composition), it is possible that a single impactor could be any one of many combinations from the above list.
The remnants of space debris material may be identified from the residue chemistry of the EDS spectra and X-ray elemental maps that contain the following elements [ mainly Ti + possible minor C, N, O, and Zn (paint fragment), mainly Fe + variable Cr, Mn + possible trace Ni (specialised steels), mainly Al + minor Cl, O, and C (rocket propellant), mainly Sn + Cu (computer or electronic components), enrichments in Mg, Si, Ce, Ca, K, Al, and Zn (glass impactor, possibly from other solar cells).
The presence of the Ti/Al layer within the solar cell complicates the identification of artificial impacts since Ti has been traditionally used as an indicator of paint fragment impact. In the Hubble Space Telescope, solar cells containing Ti, Al, and Ag, were ascribed to artificial debris particles, such as paint fragments. Thus, whilst Ti on its own state is probably a good indicator of paint fragments, when observed along with Al and Ag, it is more likely to represent a melt from the host solar cell.
The classification of impact residues in terms of either space debris or micrometeoroid in origin is extremely complex, and often it is not possible to give a totally unambiguous answer. For example, although it is highly likely that a residue composed of Al and O is the remnant of solid rocket motor debris (Al2O3), it could conceivably also be corundum (Al2O3) which has been identified in primitive meteorites, although it is extremely rare.
Apart from the classification criteria given for the residual material of either micrometeoroid or space debris origin, there is a strong possibility that spacecraft and satellites surfaces may also be subject to contamination. There are several different possible sources of contamination, arising from laboratory handling, to ground exposure, to space environment itself whereby contaminants are effectively encountered at low velocity and are thus only loosely bound.
Hypervelocity impacts create a shock wave in the material and lead to very high pressures (>100 GPa) and temperatures superior to 10000°K. Further detailed supplementary information is given, for example, in the the impact process lasts only a few microseconds, the impactor and target material are fragmented, often molten and/or vaporised, depending on the impact velocity and materials, most of the impact energy ends up in the the ejected mass can be much larger than the mass of the impactor, a small fraction (less than 1%) of the ejected material is ionized; this latter phenomenon is function of the impactor velocity.
On the other hand, collision damage (Table kinetic energy of the particle (speed), design of the spacecraft (bumpers, external exposure points), collision geometry (especially the angle of collision),
Summary of a number of objects orbiting in space.
Category (and origin) | Size | Numbers in orbit | Probability of |
---|---|---|---|
Large debris | >10 cm | 1–5 × 104 (Low) |
1/1000 |
(Satellites, rocket bodies, fragmentation material). | (collision results in total breakup and loss of capability) | ||
| |||
Medium | 1 mm–10 cm | 1–5 × 106 (Medium) |
1/100 |
Fragmentation debris, explosion debris, leaking coolant. | (collision could cause significant damage and possible failure) | ||
| |||
Small | <1 mm | >1012 (High) |
Almost 1/1 |
Aluminum oxide particles, paint chips, exhaust products, bolts, caps, meteoric dust. | collision should cause insignificant damage |
The impact ranges are of about: 1 cm (medium) at 10 km/sec: this can fatally damage a spacecraft, 1 mm and less: which erode thermal surfaces, damage optics, and puncture fuel lines.
The near space environment is actually very polluted by significant traces of recent human space history. All the space vehicles that have left the earth participated to this evolution of collision risks in space for active spacecraft. The population of space debris is composed of a very large variety of parts from the smallest (less than a millimetre) up to complete vehicles (up to several tons for lost spacecraft) through all debris issued from vehicles (rocket stages for example) explosions (Figure
Spread of collision debris orbital planes. Adapted from [
The use of CFRP in space structures has largely spread, it can be noticed through the number of papers dedicated to study its reliability, health monitoring in space, its response to debris.
Typical satellite service modules are square or octagonal boxes with a central cone/cylinder and shear panels. The cone cylinder and shear panels (SP) are generally constructed from a sandwich panel with CFRP face sheets and an aluminium honeycomb (HC) core (CFRP/Al HC SP). Similarly, the upper and lower platforms are also CFRP/Al HC SPs. The lateral panels of the service module are, due to thermal reasons, generally sandwich panels with aluminium face sheets and aluminium honeycomb cores. These panels are also generally wrapped with multilayer insulation blankets (MLIBs).
Lateral panels may be made with CFRP/Al HC SP. Other payloads include telescopes, which require a specific design quite often constructed predominantly of CFRP (for stability and pointing requirements). Truss-type structures are often used for supporting antennas, solar arrays, and so forth, typically made of CFRP.
Single-walled carbon nanotubes (SWCNTs) materials have been synthesized by using the developed plasma torch technology (detailed process can be found in [
Morphology of the grown SWCNTs: (a) representative TEM images of the SWNT materials. The inset is aHRTEM closeup showing nanotubes of 1.2 nm-diam. (b) Typical Raman spectrum of the nanotube materials, where the various RBM, D, and G bands are clearly identified. The inset shows a close-up of the RBM band located at 185 cm−1, corresponding to an individual SWCNT having a mean diameter of 1.2 nm in total agreement with the TEM analysis in (a).
All the chemicals (Grubbs, catalyst first generation, monomers, etc.) were used as received.
Due to the relatively high freezing temperature of dicyclopentadiene (DCPD), another monomer with larger functional temperature range was necessary. A number of candidates were evaluated, as shown in Table
Comparison between different monomers [
Compound | Toxicity |
|
|
CAD$/L |
---|---|---|---|---|
Dicyclopentadiene (DCPD) | Flammable, harmful | 32.5 | 170 | ~70 |
| ||||
5-Ethylidene-2-norbornene (5E2N) | Harmful | −80 | 148 | ~85 |
| ||||
1,5-Cyclooctadiene (COD) | Harmful | −69 | 150 | ~75 |
| ||||
Methylcyclopentadiene dimer (MCPD) | Poisonous | −51 | 70–80 | ~60 |
| ||||
5-Vinyl-2-norbornene (5V2N) | Harmful | −80 | 141 | ~160 |
|
While 5E2N has a wider liquid temperature range than DCPD, it is important to investigate its stability. The self healing technique was tested in vacuum and over a long time. To permit simple curing, the selected monomers for the self-healing have to comply with the following requirements: (i) must be air stable, (ii) undergo a romp opening metathesis polymerisation (ROMP) reaction.
The study permitted a preliminary selection of the following compounds summarized in Table dicyclopentadiene, DCPD, di(methylcyclopentadiene), DMCP, 5-ethylidene-2-norbornene, 5E2N, 5-vinyl-2-norbornene, 5V2N, 1,5-cyclooctadiene, COD.
Hence, as mentioned above, the 5E2N was selected as the optimal monomer with very wide range of temperature functionality [−80 to +148°C] and is less expensive and better relative to safety and environment (toxicity and flammability).
For the fabrication stage, the first issue is to achieve a controlled microencapsulation synthesized with desired characteristics (e.g., size, shell thickness, healing content, etc.) and desired properties (e.g., microcapsules having a rough surface morphology that aids in the adhesion of the microcapsules with the polymer matrix during composite processing). One should note here that all the involved materials must be carefully engineered. For example, the encapsulation procedure must be chemically compatible with the reactive healing agent, and the liquid healing agent must not diffuse out of the capsule shell during its potentially long shelflife. At the same time, the microcapsule walls must be enough resistant to processing conditions of the host composite, while maintaining excellent adhesion with the cured polymer matrix to ensure that the capsules rupture upon a composite fracture. The second issue is hence to disperse the microcapsules as well as the catalysts, appropriately into the matrix materials (Epon 828 with Epicure 3046), and finally making the sample. The samples are then tested by creating certain damages into it, and healing performance is observed.
The encapsulation of the 5-ethylidene-2-norbornene 5E2N in poly(melamine-urea-formaldehyde) microcapsules was achieved following the protocol here below. The drying of the microcapsules was improved by rinsing them successively with deionised water and acetone.
Microcapsules are synthesized according to the flowchart in Figure
Flowchart for preparing 5E2N/poly(melamine-urea-formaldehyde) (PMUF) microcapsules.
5E2N monomer encapsulation (a) typical optical photo, followed by (b) scanning electron microscope (SEM) image of the small microcapsule showing the core-shell structure of the encapsulation.
The detailed process of the 5E2N encapsulation and the effect of the whole involved synthesis parameters on the capsule size and their corresponding healing efficiency will be the subject of a separate publication.
The self-healing demonstrator consisted of woven CFRP samples, containing three main constituents. Host matrix: an epoxy prepolymer (Epon 828) and a curing agent (Epicure 3046); this epoxy is used in space for internal structures Microcapsules: the monomer healing agents (5E2N) prepared as small microcapsules (diameter less than 15 microns); the monomer is homogeneously spread within the epoxy and forms about 10% of the structure weight. Different concentrations of single-walled carbon nanotubes materials. Catalyst: Grubbs catalyst first generation (ruthenium metal catalyst); In order to polymerize, the healing agent must come into contact with a catalyst. A patented catalyst, called Grubbs’ catalyst, is used for this self-healing material. It is important that the catalyst and healing agent remain separated until they are needed to seal a crack. The healing chemical process that permits the reaction between the monomer and the catalyst is called ROMP—Ring Opening Metathesis Polymerization.
The diffusion of the monomer from the pure microcapsules in vacuumwas measured by placing a weighted amount of capsules in a Schlenk tube under vacuum (10−2 mbar) for an extended period of time. We see from Figure
Microcapsules % weight loss under vacuum as a function of time.
The 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 (namly, 5E2N, DCPD) prepared as small microcapsules (diameter less than 15 microns) kept within thin shells of polymelamine (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.
After the hypervelocity impact tests, 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 the combination of the two monomers as will be detailed below) in the crack. Once the monomer and catalyst enter in contact, the self-healing reaction is triggered (i.e., polymerization between the healing agent (monomer) and matrix-embedded catalyst particles) [
The use of the 5E2N monomer can be considered as an innovative healing method, since none of the chemical products proposed previously would be functional, in space environment conditions, because the DCPD monomer melting temperature is 33°C (i.e., it is in the solid phase at room temperature), and the chemical activities are somehow relatively slow, with the risk of evaporation during the reaction. However, the drawback to using this monomer is that the resulting polymer is linear and thus it has inferior mechanical properties as compared to DCPD. We propose a route towards the combination of these two monomers thereby obtaining simultaneously a fast autonomic self-repair composite with excellent mechanical properties. In a second step, we aim at combining the known higher mechanical properties of CNT materials with these monomers to evaluate the self healing capability.
The fabricated microcapsules containing the healing agent monomer were embedded into a woven carbon fiber reinforced polymer (4 woven layer were used and the average size of the microcapsules is equal or less than 15
Fabrication procedure of the manufacturing of CFRP with self healing and FBG sensors.
(a) Integration of 4–8 FBGs sensors embedded between 2nd and 3rd CFRP layer and concentrated inside a circle surface of 5 cm-diam. corresponding to the exposed area to debris during the hypervelocity impact experiment. (b) Final prototype of the sample.
Impact tests were performed with the implosion-driven hypervelocity launcher using the CFRP composite samples described above. The sample (i.e., CFRP+FBG) was mounted in the target chamber (see schematic of Figure
(a) Schematic of the launcher and the FBGs. Some FBGs were installed on the explosion tube (launcher), and some at the end of the tube within the CFRP sample. (b)Typical example of the CFRP samples containing FBG sensors after shooting under the hypervelocity impact.
All the fabricated samples are tested under the same conditions for a comparison purpose. The impact resulted in the complete penetration of the sample and a significant amount of delamination on both sides of the sample (Figure
Since the impact is supposed to produce delaminations within the CFRP sample, we first attempted to quantify these delaminations through their nominal thickness. This represents the ratio of the thickness after/before impact experiment.
The thicker zone should then correspond to where the delamination occurred. To do so, the CFRP sample is cut into slices, and the matrix thicknesses were built by the estimation of the thickness through the cross-sectional optical photos (thickness is estimated each 3 mm step in the 2 dimensions of the square sample (12 cm × 12 cm), and matrix of more than 650 values is obtained for each case and each sample under study.) We have then used an appropriate software code to trace two dimensions color graphic, where the color indicates the width (/thickness) of the zone (wider zone is where the delamination is larger). Figure
(a) Optical photo of the CFRP structure after impact tests, (b) representative cross-sectional view optical photos of the CFRP sample. Color code diagram simulation of the impacted sample in cases (c) with and (d) without self healing materials.
Two main conclusions were drawn. The color code diagram indicates that the thicker portion of the sample containing delaminations is more localised around the impact zone, especially in the case of the self healing samples. The thickness distribution of the CFRP samples containing self healing material seems to be much more homogenized comparatively to the pristine one (i.e., pure Epon 828), which indicates a clear healing effect (i.e., less delamination propagation after the healing process).
Impacted CFRP samples have been measured under the flexural “3-point bending test” after the healing process (48 hours and 40°C) to investigate their mechanical properties. Each sample was cut into 7 to 8 slices as shown in Figure
Flexural tests were performed twice: one measurement after the hypervelocity impact and healing process (48 hours, 40°C) and a second measurement after a second healing process (48 hours, 40°C).
The mechanical measurement performed on the slice where the impact crater is located is taken as a reference to compare samples between them in terms of mechanical recovery.
As mentioned above, seven sets of CFRP samples specimens (
Summary of the impacted CFRP structures. All the self-healing materials are mixed with 1 wt.% of ruthenium Grubbs, catalyst and 10 wt.% of microcapsules.
Number of woven CFRP layer | Number of embedded FBG sensors | Embedded material | Mechanical strength (MPa) | Healing efficiency | |
---|---|---|---|---|---|
Set no. 1 | 4 | 4 | Virgin Epon 828 | 245 | 45 |
Set no. 2 | 4 | 4 | Epon 828 + microcapsules 5E2N | 276 | 58 |
Set no. 3 | 4 | 4 | Epon 828 + microcapsules DCPD | 290 | 63 |
Set no. 4 | 4 | 4 | Epon 828 + microcapsules (5E2N + DCPD) | 281 | 61 |
Set no. 5 | 4 | 4 | Epon 828 + microcapsules (5E2N + DCPD) + 0.5 wt.% CNT | 297 | 68 |
Set no. 6 | 4 | 4 | Epon 828 + microcapsules (5E2N + DCPD) + 1 wt.% CNT | 310 | 74 |
Set no. 7 | 4 | 4 | Epon 828 + microcapsules (5E2N + DCPD) + 2 wt.% CNT | 326 | 83 |
Three kinds of self healing materials were employed: one based on encapsulated 5E2N monomer, the second based on the encapsulated DCPD monomer, a third class based on a 50/50 wt. % mixture of these two monomers.
All the self healing materials are blended with a 1 wt. % of ruthenium Grubbs catalyst (RGC) and 10 wt. % of microcapsules.
The third class of the self-healing material was then mixed with different concentrations of SWNTs. The epoxy matrix is kept the same for all samples (namely, Epon 828 with the epicure 3046).
Figure
Example of a graphic of force as a function of the displacement with the mechanical properties we can deduce (right panel) from the (ASTM D2344 standard).
(a) Comparison of the deduced mechanical strengths, as indicator of the self healing efficiency and the CNT effect. (b) The corresponding healing efficiencies and (c) a schematic of the CFRP sample cut into different slices for the three-point bending flexural tests.
When comparing to the pristine samples (i.e., containing only epoxy material), we can extract the recovery due exclusively to the healing materials [ 31 MPa are due exclusively to the self healing material based 5E2N, which represents an enhancement in terms of the mechanical strength of about 13%. When using the DCPD based healing agent, a better healing is obtained (improvement up to 18% of the mechanical strength). 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.) A clear improvement is obtained when integrating the SWNT material, even with a concentration as low as 0.5 wt. %. Then, an enhancement up to 81 MPa in terms of the mechanical recovery is due to the healing materials containing 2 wt. % of SWNT (comparison of the specimen #7 and #1), which represents an improvement in the mechanical strength of about 33%.
Finally, Figure
It is worth to mention at this level that the carbon nanotubes present in the crater zone area are supposed to disappear with the ejected mass (
Two main issues are presently in progress:
Figure
(a) FBG-center wavelength (CWL) signals with respect to the time of the impacted CFRP structures having different CNTs loads. (b) FBG-center wavelength position as a function of the CNT loads.
We noted that samples with 1% and 2% CNT showed high damping effects due to the presence of a higher load of CNT. Work is actually under progress to correlate the mechanical contribution of the CNT materials to their morphological and structural properties.
The first effects of the pellet hitting the CFRP material structures occur in the few first
Temporal response of the FBG sensors to the pellet at hypervelocity of 4.3 km/s and a frequency acquisition of 2 MHz.
Bore pellet: 3 mm diameter, Hypervelocity: 4.3 km/s, Acquisition frequency: 2 MHz,
We successfully synthesized self-healing materials to be used mainly in space environment. The autorepair composite was blends of microcapsules containing various combinations of a 5-ethylidene-2-norbornene (5E2N) and dicyclopentadiene (DCPD) monomers, reacted with ruthenium Grubbs’ catalyst (RGC). Both monomers were encapsulated into a (polyMelamine-urea-formaldehyde) (PMUF) shell material, where the capsules average size was less than 15 microns. Then, the self healing materials were mixed with a resin epoxy based Epon 828 and single-walled carbon nanotubes (SWNTs) by means of a 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 a home-made implosion-driven-hypervelocity launcher. After the hypervelocity event, the three-point bending tests show clearly that the optimum self healing material having the best mechanical strength was an equal-weight blend of 5E2N and DCPD monomers, while a huge improvement in terms of the autorepair efficiency was obtained when adding small quantities of SWNT (of 2 wt. % and less). These results establish well that 5E2N/DCPD/CNT/RGC system is a realistic possibility which possess tremendous potential for space applications. However, additional experimental investigation, especially a systematic cryomicrotome analysis as a function of the carbon nanotube loads, is needed to demonstrate clearly the damping effect of the SWNT material and their role as a cross-links in the formed polymer.
The authors would like to acknowledge the financial assistance of the Canadian Space Agency for this work. They are also grateful to Dr. Stéphane Gendron and Dr. Darius Nikanpour (CSA) for their constructive advice and support during this work.