Final Report Summary - HIPOCRATES (SELF-HEALING POLYMERS FOR CONCEPTS ON SELF-REPAIRED AERONAUTICAL COMPOSITES)
Executive Summary:
Self-healing materials need minimal maintenance and have the ability to repair their own micro-cracks and breaks. They have become a holy grail for the aerospace industry. In the frame of HIPOCRATES, the consortium has made their use a step closer to design epoxy based self-healing composites. They based these new self-healing materials on already widely used resins, so that they could be simply incorporated into current aerospace production methods.
The HIPOCRATES-project has investigated two different self-healing strategies. One involves encapsulating micro-cracks. Microcapsules containing self-healing agents are added to the composites polymer mix, in which a catalyst that starts the reaction has already been dispersed. When a micro-crack occurs, the capsules break and release the healing agent, which comes into contact with the catalyst. The resulting polymerisation reaction closes the crack and prevents further crack growth’. HIPOCRATES has been able to take this strategy one step further in developing an ‘all in one microcapsule” self-healing system which is entirely self-contained (EP16382598.7 TECNALIA). Rather than the catalyst being dispersed through the resin matrix, it is encased in the shell of the capsules in a higher concentration, so the healing reaction can occur more efficiently.
The second method has been to use reversible polymers. These materials contain internal linkages, which will reverse the damage and close a crack upon delivery of an external stimulus such as heat, radiation or electrical induction. This behaviour can be achieved using various reversible polymers and HIPOCRATES synthesized and tested two that are compatible with epoxy systems and can be made from cost effective commercial raw materials.
The materials designed by HIPOCRATES were tested in small-scale demonstrators by mimicking the kinds of high velocity impacts experienced by aircraft due for example to collisions with birds, debris and hail. The effect of compression on the repaired materials were tested to see if they would de-laminate and break apart.
Ultrasound analysis of the tested new materials found that impact and compression damage could be mitigated by using self-healing materials . The incorporation of capsules provided some protection against the initial mechanical impact, however after healing both strategies showed higher resistance to compression than before. The new materials showed 5 to 10% higher resistance to compression forces making the repaired materials resistant to the sorts of impacts that would cause damage before the repair.
The new composites designed in the HIPOCRATES project still need to undergo more testing before they can be used in real applications, but the project team hope that in the next five years these self-healing materials might help reduce aircraft costs by providing an alternative to expensive manual testing and repair.
Project Context and Objectives:
General Objective
Lightweight, high-strength, high-stiffness fibre-reinforced polymer composite materials are leading candidates as component materials to improve the efficiency and sustainability of many forms of transport. Justifying this, advanced structural polymeric composite materials (e.g. CFRPs) are currently integrated and well in use in modern commercial aircrafts, as the examples of Airbus A380, Boeing 787 and forthcoming Airbus 350, even for primary structures. In addition, it is further expected that future aircrafts will have an increased use of composites. For these types of structures, the durability and damage tolerance design of composites is always an emerging need since polymeric matrix systems are generally susceptible to micro-damage induced by a number of events during in-service loading, which can cause macro-damage initiation and structural degradation. Engineering research and design has focused traditionally on either developing new materials with improved properties or developing non-destructive evaluation methods for material inspection, yet all engineering materials eventually fail. In this direction HIPOCRATES project proposes the development of new materials able to heal damage and restore lost or degraded properties, thus contributing to the safety and durability of composites structures through their inherent mechanism to alleviate negative effect on microcraking.
Strategic Objectives
The proposed concept of HIPOCRATES project to develop innovative self-repaired composite materials by transforming widely used resins within aeronautical industry to self-healing materials is expected to offer durability, extend its service life and prolong maintenance protocols leading to lower aircraft operational (maintenance) costs. To this end through these developments, major cost reduction in maintenance activities is ensured contributing thus to the goal of direct operation costs reduction by 50% by 2020. It is also expected that the developments towards self-repaired composite materials will have a significant contribution towards the assurance that safety of aerostructures remains at current high standards regardless of air transport growth. The use of self-healing composite materials shall enhance the safety of aging airframe by substantially decreasing the need for conventional repairing (patch repair, component replacement) and thus reducing the number of operations conducted on the aircraft. This is directly contributing to the two basic objectives of the elimination of and recovery from human error in the maintenance processes as well as the reduction of accident rates by 80% by 2020. Safety of aging airframe is also enhanced by the fact that self-healing composite materials have the ability with no or minimum intervention (i.e. heating) to reverse otherwise irreversible damage such as delamination, fibre/matrix debonding and matrix cracking extending significantly the fatigue life of a component.
In more detail, HIPOCRATES main technical objective is to develop self-repaired composite materials by transforming widely used resins within aeronautical industry to self-healing materials thus facilitating the consequent certification and its related cost. Taking into account the current technological maturity of self-repair, secondary structural composites shall be targeted. The transformation will be done through the epoxy enrichment with appropriate chemical agents, following current state of the art polymer self-healing technologies (encapsulation, hollow fibers, remediable polymers). Moreover the current progress of nanocomposites technology will be utilized towards either better facilitation of self-healing process (e.g. nano carriers) or enhancement of the self-healing performance or integration of other functionalities (e.g. monitoring the self-healing performance, activation of DA reaction). For each of the self-healing approaches, critical parameters will be experimentally studied towards optimum self-healing properties. The study will continue on the effect of the composites self-repair concept with respect to the composite materials properties that will offer durability and prolong the use of advanced aerospace structures (reduction of operational cost) such as mechanical properties (impact resistance, fracture toughness, micro-cracking resistance, and fatigue). Furthermore, feasibility and challenges that arise from incorporating such self-healing thermosetting systems into fibrous composites will be closely investigated at these early stages of development to ensure effective transfer of the properties to large scales required by the industry.
Scientific and technical objectives:
• To provide experimental evidence to meet the State of the Art shortcoming and broaden the understanding of the self-healing mechanisms. Answering to the demand for more global understanding of the healing mechanisms and their performance related to different parameters, such as dynamic loading and long term healing capability, the proposed project will include the durability assessment along with dynamic performance characterization in both polymer and fiber-reinforced composites.
• To develop strategies and respective procedures for enabling self-repairing of composite materials by critically analyzing the established techniques. For making epoxy-based self-healing composite materials possible, novel chemistries will be researched for two healing strategies (encapsulation and reversible polymers. On one hand, compatibility of chemistry for capsule-based healing with epoxy systems will be evaluated. This part will also include hollow glass fibres as carriers. Different scenarios and combinations will be considered for healing agents and catalysts. On the other hand, reversible polymers with epoxide-type systems will be investigated. For this, critical parameters will be their chemical propensity to be able to modify the epoxide systems in an industrially viable way and the resulting thermal (self-healing) behaviour after functionalization. In addition tuning of the activation temperatures of the self-healing action will be attempted through chemical modification. In this way, the temperature window and self-healing efficacies needed for the application will be targeted
• To enhance self-healing activation by the incorporation of nanotechnology (ferromagnetic nanoparticles, CNT). To design and implement activation technologies for self-healing materials.
• To integrate and evaluate the incorporation and compatibility of self-healing technologies with the current processing and manufacturing methods. To investigate the feasibility and challenges arise from the incorporation of such technologies into fibrous composites (pre-preg, infusion/RTM, missing) in order to ensure the effective transfer to large scale required by industry.
• To characterize the healing mechanism performance under aeronautical loading conditions (Fracture, Fatigue, Impact, Compression after impact,...). To develop new protocols and testing methods in order to specifically quantify the healing magnitude.
• To implement the above technologies for the repair of small scale demonstrator, using self-healing materials and activation technologies. Too test and evaluate the developed self-healing features and their impact on the structural integrity of the composite while verifying their compatibility with aeronautic industry basic requirements, constrains and limitations.
Description of work
Within HIPOCRATES proposed project, the effort is divided into 7 work packages according to their specific role within the project. WP1 cover all aspects of requirements & selection of materials, WP2 refers to core development issues on encapsulation strategies in parallel with WP3 that corresponds to the respective core R&D issues on reversible polymers. WP4 is the principal R&D of the project that deals with all issues of integration of self-healing technologies in aeronautical composites. All management and dissemination/exploitation activities are grouped in WP6 and WP7 respectively.
In more detail, WP1 aim is to prepare the R&D strategy that will be followed in the following work-packages. Under this framework key objectives are to set the grounds for common scientific and technological understanding between the self-healing technology providers, the composite materials developers and aeronautical end-users. In parallel to define how composite self-repair concept is translated for composite aero-structures (requirements & limitations) and establish a roadmap towards self-repairing composites. A classification of ideas for implementation in the different phases will be made and a database of candidate materials (already used in aeronautical industry) will be created. The state-of-the-art will be updated with latest developments on self-healing/self-repair concepts. Finally an initial R&D experience on self-healing epoxies transformation with first experimental results will be gained.
WP2 key objectives are to investigate and develop novel chemical routes involved in the field of encapsulation & self-healing and research on procedures for transforming aeronautical grade epoxy systems to self-healing materials for use in composite structures. Any polymer processing issues will be identified and addressed. The developed capsules and respective self-healing epoxy materials will be experimentally tests through self-healing performance. Finally partners with nano expertise will investigate and develop concepts for interactions and synergies between nanotechnologies and micro-encapsulation strategy. WP3 similarly key objectives are to identify and investigate promising chemical routes involved in the field of reversible polymers & self-healing. Moreover directions of establishing new techniques and methods to activate the healing process on demand utilizing different fields (magnetic, electrical etc.) will be explored. As in WP2 any polymer processing issues will be investigated and developed polymer blends will experimentally evaluated. Also in WP3 partners with nano expertise will investigate and develop concepts for interactions and synergies between nanotechnologies and reversible polymers strategy. Both WPs will include development of modelling tools for key parameters of the healing processes.
At this point it should be noted that respective tasks focusing on concepts for interactions and synergies between nanotechnologies and self-healing technologies can be utilized in combination under the framework of a holistic approach. Possible combination on all self-healing technologies and nanotechnology compromise one of the scientific challenges of HIPOCRATES project.
WP4 is the principal R&D of the project that focus on integration of self-healing technologies in aeronautical composite production routes towards the development of composite self-repair concept. Key priority is to optimize current manufacturing processes and propose new/alternative integration strategies and propose technologies in order to answer the need of monitoring of polymer self-healing process in composite material level. Modelling tools will support the optimization process. Furthermore in WP4 technologies shall be developed that will allow the activation of self-healing of the polymer matrix in composite materials. Finally, but most importantly, experimental results to assess the performance of the developed materials in terms of durability and service life (eg. fatigue, impact etc).
WP5 as the final technical work-package of HIPOCRATES project will try to create the initial link between R&D developments and possible aircraft applications by identifying possible scenarios for aeronautical self-repair composites structures. Partners mainly (end users in SME or INDUSTRY) in cooperation with developers (ACADEMIA, Research Organizations, and SME) will propose small scale validation platform taking into consideration potentially interesting aircraft components for self-repair. The small scale validation platform will be designed and manufactured and performance and functional testing protocol will be defined and conducted in order to assess the overall strategic concept of HIPOCRATES project.(self-repairing composite aerostructure)
WP6 will manage and coordinate the project work including communication with the Commission, production of periodic reports, preparation of Consortium Agreement and WP7 aims to perform the technical and commercial evaluation of technological developments obtained and to assess the potentialities to transfer to real applications. Also diffusion and exploitation aspects are undertaken to address future activities to establish a roadmap for the rapid market introduction of the structural concept developed in the project
Project Results:
The main objective of HIPOCRATES project was to develop new materials able to heal damage and restore lost or degraded properties of aeronautic grade epoxy based composites. This objective will be achieved by reducing the negative effect related to the presence of microcraks on composite structures.
To achieve this objective self-healing materials appears as a promising alternative because they counter degradation through the initiation of a repair mechanism that responds to the micro-damage. Self-healing materials have the ability to automatically repair damage to themselves without any external diagnosis of the problem or human intervention.
This project has focused on the development of a self-healing aeronautic grade composites based on two different self-healing technologies extrinsic (microcapsules, vascules) and intrinsic ones (Diels-Alder and supramolecular materials). The former operates by taking advantage of the intentionally embedded healing agent, while the latter enables repeated crack healing by the polymers themselves without the need of additional healing agents. A detailed screening of the technologies has fulfilled (chemistry, compatibility with epoxy based polymers and composites, healing capacity) and the most promising ones were applied in sub structural components (skin-stringer) for the proof of concept at small scale as it is explained in the following paragraphs.
WP1. Requirements and selection of materials
The aim of the first work-package (WP1) of HIPOCRATES project was to prepare the R&D strategy that will be followed in the following work-packages. Under this framework key objectives were:
• Set the grounds for common scientific and technological understanding between the self-healing technology providers, the composite materials developers and aeronautical end-users
• Define how composite self-repair concept is translated for composite aero-structures (requirements & limitations)
• Establish a roadmap for the directions towards self-repairing composites
• Classification of ideas for implementation in the different phases
• Create a database of candidate materials (already used in aeronautical industry)
• Update the state of the art with latest developments on self-healing/self-repair concepts
Built-up an initial R&D experience on self-healing epoxies transformation and produce initial experimental results
After analysing the areas of an aircraft which are made composite materials (Figure 1), the type of damages that these composite can suffer (Figure 2), self-healing technologies (extrinsic and intrinsic), materials use in aeronautic industry and composite manufacturing processes a roadmap (Figure 3) was outline in order to help on defining a work schedule.
The schematic above is depicting the Roadmap from self-healing polymers to self-repaired composites.
• The two main manufacturing processes of composite parts that are used in aeronautical industry and more specifically in Aernnova, end user of that technology, are Autoclave curing Prepreg and Resin Transfer Moulding (RTM). The most usual composite manufacturing process is Autoclave curing Prepreg.
• Type of defects
Taking into account AERNNOVA information in terms of damage tolerance philosophy it was defined that the objective could be to heal class 2 quality (minimum detectable defect size 10 mm diameter) to turn into class 1 (minimum detectable defect size 6 mm diameter).
Three types of damages were identified:
- Holes
- Ply drops
- Drills
• Selection of materials
RTM strategy: Aeronautic grade epoxy MRW444 and a more simple system based on the same chemistry than MRW444 MY721/XB3473 (Hunstman)
Prepeg: Hexcel 8552 AS4
Healing system for encapsulation strategy
Healing agent: DGEBA
Catalyst: Scandium triflate
WP2. Encapsulation strategy (Phase B, C)
The objectives of this work package are:
• To investigate and develop novel chemical routes involved in the field of encapsulation and self-healing.
• To investigate routes for transforming aeronautical grade epoxy systems to self-healing materials for use in composite structures.
• To assess polymer processing issues
• To evaluate the developed capsules and materials through self-healing performance
• To investigate and develop concepts for interactions and synergies between nanotechnologies and encapsulation strategy.
Encapsulation strategy (microcapsules, vascular)
Technically, the healing agent for extrinsic self-healing has to be stored in fragile containers like microcapsules and vascular networks before being embedded in the matrix polymer. In terms of microcapsules strategy the general concept is based on the development of microcapsules which contain a healing agent inside. Being understood that when these microcapsules are distributed and embedded in a polymeric matrix and a crack is made, at least one microcapsule is broken. Therefore, its healing agent is released and mixed in the crack area with a catalyst, allowing the polymerization of the healing agent into a solid polymer that fills the crack by capillary forces and repairs it.
• Healing agent was encapsulated by suspension polymerization process. A maximum of 40wt% of healing agent was encapsulated.
• Microcapsules thermal stability was high enough to support composites manufacturing processes (>250ºC).
• The main achievement is related to the development of “all in one microcapsules” by a novel chemical route (EP16382598.7 TECNALIA). The all in one self-healing system is entirely self-contained. When the capsules are ruptured, the healing agent comes into contact with the catalyst and is then polymerized. Capsules were characterized by TECNALIA (DSC, GC, HPLC, SEM, OM), PATRAS (DSC, SEM, OM, TEM), UOI (Raman spectroscopy).
• Capsules were integrated into epoxy matrix successfully. Their compatibility (capsule/epoxy matrix interface) and dipersability were analyzed by microscopy.
• Healing process was evaluated via various non-destructive techniques (PATRAS, UOI) such as Raman spectroscopy, IR thermography and acoustic emission. In addition mechanical test were performed (lap shear) and positive results were achieved. The incorporation of 20wt% of microcapsules into an epoxy matrix leads to 45wt% of recovery after applying a healing cycle of 2h at 120ºC.
• Nanoparticles to improve heat transfer have been developed. However, in the case of TECNALIA self-healing material, the incorporation of heat conducting nanoparticles seems to be not successful in terms of mechanical properties.
Capsule-based self-healing concepts reveal big advantages since they can be easily embedded and industrialized as the technique of microencapsulation has been investigated since the 1950s Moreover, the encapsulation strategy can fulfill aeronautic processing and manufacturing methods. This strategy shows that it is not possible to repair all types of damages, because the healing answer presents different efficiencies under different type of damages. As with many applications, price and the identification of technological areas of usefulness are now the next step for their application, together with their commercialization and consumer adoption
WP3. Reversible polymers strategy (Phase B, C)
This work package is focused on:
• Identify and investigate promising chemical routes involved in the field of reversible polymers and self-healing.
• To investigate in the direction of establishing new techniques and methods to activate the healing process on demand utilizing different fields (electro-magnetic, electrical, etc.).
• To assess polymer processing issues.
• To evaluate the developed polymer blends.
• To investigate and develop concepts for interactions and synergies between nanotechnologies and reversible polymers strategy.
Material development:
• Development of three different and complementary self-healing systems, 2 matrix materials based on reversible supramolecular chemistry and on reversible covalent chemical bonds and and 1 particle based were developed, tested and successful incorporate into aerospace composite prepregs and test specimen. Both matrix material based systems show multiple healing at elevated temperatures.
• Development of scale-up process and processes of incorporating and testing of the matrix materials as such and as composite matrix
• Testing and demonstration of the self-healing action, development of activation methods and incorporation of nano-particles to accelerate/trigger SH
Activation technologies and methods to activate the healing process on demand utilizing different fields:
• The addition of appropriate magnetic particles is possible to produce the necessary amount of heat needed to activate the mechanism of self-healing. Iron Oxide (Fe2O3) proved to be the more efficient in terms of production sufficiently temperature inside the polymer. The use of hysteresis losses mechanism it is far more efficient than the eddy current mechanism. However and especially seeing the density of the particles and the application area only low concentration application cases seem to practicable.
• The incorporation of heat conducting nano-particles seemed to be less successful in terms of activation functionality.
Non-destructive testing and characterisation
• Evaluation of the healing process via the employment of various non-destructive techniques such as Raman spectroscopy (reveals information on the healing process), IR thermography, Acoustic Emission and stereographic imaging (insight into the failure modes)
• The scattering of the acquired values of Raman spectra by conducting a mapping acquisition in an unstressed and stressed cnt-modified film can provide a tool to map stress accumulation at low stress levels.
• The evaluation of the healing process of the supramolecular polymer supplied by Suprapolix was achieved using Impedance spectroscopy, The self-healing material managed to regain its initial dielectric properties after 5 consecutive healing cycles when the healing process took place at 100 0C.
• A selection of bismaleimide (BMI) cross-liked furan functionalised epoxy resins were investigated for their mechanical performance (Bristol) and recovery of mechanical performance via three repeatable healing cycles, attributed to the thermoplastic behaviour. The 20pph and 30 pph of BMI:100 pph DApp were shown to be the most promising candidates, full recovery observed for the former in bulk material form and the latter in thin film form.
• Building on the previous rheological studies that demonstrated the thermo-reversible nature of the crosslinks allows for substantial flow of the material at elevated temperatures, the self-healing functionality showed great promise for implementation into FRP composites.
• Although a knock-down in mechanical performance was observed for self-healing specimens when compared with the benchmark epoxy resin, it is expected this will be less of an issue for FRP composites that possess a much lower resin volume fraction of ~37%. The potential for a composite that has the capability to repair multiple times has to be appreciated when compared to a marginally higher performing composite that when it becomes damaged needs to be taken out of service and manually repaired at great expense.
WP4. Integration of Technologies in Aeronautical Composites
The work envisioned in this work-package was performed almost in parallel with WP2 & WP3 in order to make better use of the outputs (knowledge and experience) generated by WP2 & 3.
• Focus on integration of self-healing technologies in aeronautical composite production routes towards the development of composite self-repair concept
• Optimize current manufacturing processes and propose new/alternative integration strategies
• Propose technologies in order to answer the need of monitoring of polymer self-healing process in composite material level
• Develop technologies that will allow the activation of self-healing of the polymer matrix in composite materials.
• Provide experimental results to assess the performance.
Moreover, the healing mechanism and performance was also demonstrated under aeronautical loading conditions (compact tension, three point bending, impact..).
Encapsulation strategy
Microencapsulation
• Capsules were successfully integrated within polymers and composites
• CT specimens integrated with 10%wt. Reference 3 capsules achieved 79% recovery of the critical stress intensity factor (KQ) after 12hours at 120oC healing process.
• Τhe knocked down effect was determined for different types of capsules when incorporated within the composite. One part healing system (capsules with catalyst on surface) proved to be more efficient in terms of the knocked down factor. More precisely 10%wt. capsules (reference 2, two parts) combined with 2%wt. Scandium caused a reduction of 4% in flexural modulus and 14% at flexural strength compared to the neat material. By increasing the content to 30%wt. capsules (reference 3, one part) the reduction was limited at 5% and 3% for flexural modulus and strength respectively. It is conceivable that reduction percentage remained at the same level or even reduced further even though the content was increased and this is due to the one part of the healing system and its dispersion only at specific layers.
• Three-point bending tests at specimens with 30%wt. Reference 3 capsules recovered 73% and 69% their flexural modulus and strength respectively after 12h @120oC.
• Mode I, II experiments proved to be inappropriate for capsules to activate the healing process. No properties recovery regarding the critical energy release rate (GIC, GIIC) was observed due to the unstable crack growth.
• Mode I fracture toughness was decreased up to 25% when capsules (30%wt. REF3) were dispersed in the mid place along the crack area.
• On the contrary, Mode II fracture toughness was increased by capsules addition at 30% content up to 131,5 % with regards to the neat material.
• Impact and Compression after impact results for capsules REF3? (Mail Stavros dice que Ref-4) are given in WP5
• Three-point bending tests at specimens containing 10%wt. UF capsules recovered their flexural modulus and strength 85% and 97% respectively. The knocked down factor at this case was higher as the modulus decreased its value 16% and 24% the flexural strength compared to the neat material.
• Mode II experiments did not show any self-healing signs except from the strengthening effect at about 2% more.
• Acoustic emission data were further analysed and were in complete agreement with the mechanical experiments.
• Raman Spectroscopy on samples defined further theexistence or not of the chemical elements of the structure of the capsule at the fracture area.
Vascules
• A detailed feasibility study was conducted regarding the possible materials for vascules formation
• Teflon and steel wires were further used for their implementation within composites
• Three-point bending tests proved not appropriate for vascules activation and the potential healing
• Mode I specimens with two vascules achieved a 126% and 147% recovery in terms of maximum load and fracture toughness respectively.
• Open hole fatigue experiments with two vascules above and below midplane (4 in total) are promising as the manually premixed healing system reduced the damaged area as it was observed through C-Scan
• The introduction of a non-optimised vascular network (in terms of location and orientation) had a marginal influence on the global mechanical properties: a reduction of approximately 15% in initial debonding strength and 10% in final failure load. By reducing the diameter of the vascule, changing its position or locally aligning with the host ply, there is scope to further reduce, or eliminate, the network’s effect on mechanical performance. In terms of property recovery, the initial stiffness was fully restored and the strength, in terms of initial disbonding, was increased due to the localised improvement in fracture toughness.
• Strap lap tension tests with one optimized central vascule recovered almost 140% the stress value after the healing process of the injected healing system.
• Acoustic emission data were further analysed and were in complete agreement with the mechanical experiments.
• IR Thermography visualized the manually injection process of the self-healing system. Also, monitored online the crack tip during entering the self-healing region in Mode I experiments.
Reversible polymers
Supramolecular reversible polymer
• Synthesis of SH-polymers comprising UPy hydrogen bonding units (Supramolecular) in large amounts
• Processing of Supramolecular polymers into films
• Chemical and rheological characterization of Supramolecular polymers to assess thermal stability
• Mode I experiments for CFRPs containing Supramolecular polymer showed a considerable increase for both mode I characteristics (540% and 1550% for the Pmax and GIC values respectively).
• After healing, recoveries of about 30%-60% were observed for both fracture toughness I characteristics.
• Mode II experiments for samples containing Supramolecular polymer revealed a considerable increase for both fracture toughness II characteristics (120% and 100% for the Pmax and GIIC respectively).
• After healing, recoveries of about 80%-100% were observed for both fracture toughness II characteristics.
• Strap lap tension specimens recovered their initial Young’s modulus at 87% after the first healing cycle while after the third healing event the healing efficiency was calculated at 79%.
• Acoustic emission (AE) was employed to identify the acoustic signature of damage and its correlation to healing and was in complete agreement with the mechanical experiments.
• A variety of heating methods were proposed to activate and optimize the healing process, including a heating bolt using resistance heating, an induction heating bolt and ceramic heating plates separately and combined with the heating bolt.
Diels-Alder reversible polymer
• The TNO Diels-Alder prepolymer was evaluated in the form of a self-healing pre-preg material on the central mid-plane of out of autoclave cured DCB, CFRP laminates. The healing performance was evaluated over three heat cycles (each 150 oC/5 min) for the recovery of mechanical performance in composite DCB test specimen coupons.
• Recoveries of about 25-70% were observed.
• For DCB samples containing BMI-prepolymer in powder form in the mid-thickness area, it was observed that the most appropriate amount was that of 120 gsm. These samples exhibited good toughening and the best healing efficiency values (15% for the Pmax value and 2% for the GIC value) and was selected as representative sample for curing in lower temperature (100 oC).
• Utilizing the second curing cycle a degradation of the fracture toughness I characteristics was observed (11% for the Pmax and 3% for the GIC) while the healing efficiency values were strongly increased (26% and 32% for the Pmax and GIC) after the first healing cycle.
• According to mode II experiments it was shown that samples with the higher concentration exhibited the best fracture toughness II characteristics (58% and 56% for the Pmax and GIIC). In addition all material groups exhibited high healing efficiency values from 90-190% for both fracture toughness II characteristics
WP5 Validation Platform for Self-repair Composite Aero structure
Based on the review of WP1 and the monitoring of the developments (WP2, WP3 and WP4) on self-healing polymers towards self-repair composites, this task aims:
• To create the initial link between R&D developments and possible aircraft applications by identifying possible scenarios for aeronautical self-repair composite structures.
• To propose small scale validation platform taking into consideration potentially interesting aircraft components for self-repair
• To design and manufacture the small scale validation platform
• To realize performance and functional tests on the platform
Among possible damage areas skin-stringer component was selected as the configuration to prof self-healing technologies capacities. Material selection (type, sequences) was carried out base on AERNNOVA´s knowledge. All self-healing technologies developers (TECNALIA, TNO, SUPRA and BRISTOL) analyzed if their technologies presented limitations against end user´s typical manufacturing process (curing cycle, processing conditions). In addition, a preselection of the most promising self-healing technologies was done: GIC specimens were manufactured by TECNALIA and UOP and tested by ELEMENT. The results were analyzed by all partners and supramolecular material (SUPO) was selected as the most promising self-healing technology against interlaminar fracture toughness.
The prototype was modelled by FEM (AERNNOVA) in order to define its lay-up, dimensions, geometry to force a failure in the desire area. Moreover, ELEMENT designed the tooling to develop the prototype and release a test plan to demonstrate supramolecular material self-healing capacity.
However, INASCO the partner responsible from the manufacturing of the tool and the prototype was not able to complete these tasks due to a break down on their CNC machine.
In spite of this unfavourable situation, small scale skin-stringer demonstrators were manufactured by UOP. In this case a test with increased interest was defined in order to highlight the enhanced performance characteristics anticipated from the improved developed materials. Rather than tension, compression is of much greater interest in the case of the aeronautical component under investigation. This is due to the fact that the critical buckling load can be decreased by the damage state of the base material, and to the fact that the final failure will occur due to bond line failure under the buckling compressive load. For the introduction of damage, a high velocity impact was decided to be utilised. This was since aircraft vulnerability issues arise from the structural response to HVI from birds, debris, hail etc. Impact response is of high importance, especially in the case of blunt damage that promotes delamination that can grow and reduce the residual strength of the structure. Two self-healing technologies were selected taken into account the work performed by UOP during WP4.
The initial objective of this WP was to manufacture one or more demonstrators (INASCO) with the most promising self-healing technologies. However, due to a break down on CNC machine they were not able to manufacture the mould to obtain the T-stringers demonstrators. In order to be able to prof self-healing concept, PATRAS developed small scale demonstrators and tested them under impact and compression after impact. The technologies selected for these small scale demonstrators were “all in one capsules” developed by TECNALIA and supramolecular reversible polymers developed by SUPO.
Main results:
• Capsules and supramolecular material were successfully integrated with an skin-stringer composite structure
• Supramolecular material was impregnated at carbon fabric and pre-preg was developed
• Capsules were successfully mixed within the adhesive used at the skin-stringer bonding
• Quality control indicated satisfactory results in order to continue with the test campaign
• The damaged area at the modified demonstrator with capsules was slightly reduced after the healing process according to C-Scan figures.
• The presence of capsules reduced the compression after impact load up to 13% but after the healing process a full recovery observed at a rate of 117%.
• The damaged are at the bond line of the Supramolecular prepreg was not significant.
• The presence of the Supramolecular material reduced the Pmax value at the rate of 15% before the impact tests compared to the reference
• The Pmax value for the modified samples with Supramolecular pre-preg was reduced at the rate of 17%
• The Pmax value for the modified samples with Supramolecular pre-preg was increased at the rate of 5% after the healing cycle
WP6 Management and Coordination
The coordinator deals with all the day to day administrative aspects:
• Management of funding
• Distribution of information from the EC to the consortium
• Organization of consortium progress review meetings
• Preparation of periodic report
• Distribution of project reports and further deliverables to the EC
• Resolving of problems occurring between partners during the project
More in detail, management activities are listed:
• Website of the project www.hipocrates-project.eu with the update information of the project. The website is divided in two different areas, one for public dissemination of the project and the second one for projects partners.
• Organization of progress project meetings (KOM, 3months, 12moth, 18month, 24month, 36month)
• Organization of several technical meetings (9 month, 15 month teleconferences)
• Preparation of official reporting documents
• Distribution of the funding among partners
• Coordination of diffusion actions (Aerodays 2015, ECCM17, JEC Composites Show 2017)
WP7 Economical, evaluation, exploitation and dissemination
This WP aims to perform the technical and commercial evaluation of technological developments obtained and to assess the potentialities to transfer to real applications. The diffusion and exploitation aspects are undertaken to address future activities to establish a roadmap for the rapid market introduction of the structural concept developed in the project.
Economical Evaluation
The objectives for the Economical evaluation we established as follows:
• Perform an economical evaluation of the technology developed and demonstrated
• Perform a comprehensive market study in order to quantify the economical benefits from the industrial application of the developed technologies
The activities around this WP were performed after the completion of the project mainly due the reason already given above. Nevertheless a report was prepared (D71- Report on economical Evaluation). An extract of the evaluation is presented hereunder:
An economical evaluation of the developed and demonstrated technology could not be performed within the duration of the project, since the selected technology wasn’t transferred into a demonstrator in order to be fully validated using mechanical testing and applying external healing activation technologies. Although some data on material costs (at a laboratory scale) exists, an economical evaluation requires detailed costs: (1) on incorporating these materials into a composite structural part (i.e. additional labour, new equipment, handling processes and associated costs), (2) on assessing their performance compared to a traditional structure and accounting for potential cost benefits these could bring in new design methodologies and (3) accounting for the effort and cost of equipment to activate the healing mechanism. All the above points can be accounted in detail through the manufacturing, and validation of a physical demonstrator.
Market study
A complete market study is contained within D7.1 where the global composite industry is analysed, including global aerospace MRO, market drivers and industrial trends, and the global composite market forecast.
Exploitation and Dissemination
Section A to this report contains all dissemination activities carried out related to the Project
Potential Impact:
The availability of efficient and cost-effective self-repairable structures is of high technological and economical importance for the European and global aerospace industry. The ageing airframes is becoming a critical problem in most countries around the world increasing significantly the direct maintenance costs of the various airliners. At the same time new aircrafts such as the Airbus 380 or the Boeing 787 Dreamliner incorporate unprecedented usage of composite materials rendering the concept of utilizing self-healing composite materials very attractive. Within this scope, HIPOCRATES is directly addressing current and near future needs of the European Aircraft Industry by developing integrated, safer and “smarter” pan-European transport systems for the benefit of all citizens and society.
HIPOCRATES develops two basic self-healing strategies, the nano-encapsulation and the thermally remediable polymers the focus being at standard aerospace materials. The development of self-healing materials shall lead to self-repairable aerostructures contributing substantially to three major goals listed in the work programme.
- The decrease of direct operational costs by 50% by 2020
- The reduction of accident rate by 80% by 2020
- The achievement of a substantial improvement in the elimination of and recovery from human error
HIPOCRATES meets these goals through the following:
- The service life of the aircrafts is expected to be extended. Self-healing composite materials are expected to have a significantly longer service life as the majority of damage modes (i.e. delamination, debonding, matrix cracking) becomes reversible. Improvement of passenger safety is expected by the development of damage reversible material systems for rehabilitated aircraft structures which is estimated to result in significant accident rate reduction in the near future.
- Minimization of redundant components. The need for components replacement for maintenance reasons shall be reduced.
- Maintenance downtime shortening. By reducing the need for component replacement, downtimes are also expected to be highly shortened affecting positively the attempts for overall maintenance cost reduction.
- Reduction in the total number of operations. A reduction in maintenance will lead to a decrease of the number of operations conducted on the aircraft minimizing the probability for human errors
Moreover the main HIPOCRATES outputs are expected to have essential societal impact. A successful implementation of self-healing composites will lead to the life extension of structural components with the subsequent reduction in scrap. This will make a significant impact by reducing waste, raw material consumption and power consumption in the service life of parts from manufacturing up to recycling or disposal.
In addition, shorting of maintenance periods and downtimes will support passenger friendly airliner operation (reduction of flight delays due to unscheduled aircraft maintenance) and the maximization of airport operating capacity for facing the increasing traffic and will contribute to a more flexible and efficient use of European air-fleet in an expected time scale of 5 years.
The self-healing technologies that will result from this proposal will also lead to an increased role of synergistic industries in the areas of nano-technology, chemicals etc and offer opportunities for the employment of highly skilled professionals. This would contribute in solving rising societal problems interconnected with the high unemployment in Europe in a critical period.
Thus, the strengthening of the competitiveness of European transport industry is expected to be benefited from the implementation of advanced self-healing technologies as described in the HIPOCRATES project will give the European aerospace industry the opportunity to provide better solutions (operational, environmental and technological) than their competitors, to reduce the direct operating costs and thus to increase their market share. Self-healing materials are innovative products that will strengthen the position of European aerospace manufacturers, material providers and end-users (airliners) in the global market.
List of Websites:
www.iapetus-project.eu
Coordinator: Dr Sonia Flórez. TECNALIA
WP1- Requirements&Selection of Materials. Leader: INASCO
WP2- Encapsulation Strategy (phased A, B, C). Leader: TECNALIA
WP3-Reversible polymers strategy (phased A, B, C). Leader: TNO
WP4- INtegration of technologies in aeronautical composites. Leader: University of Patras
WP5- Validation Platform for Self-repair composite aero-structure. Leader: Suprapolix
WP6 - Management and Coordination. Leader: TECNALIA
WP7 - Economical evaluation, exploitation and dissemination. Leader: INASCO
Self-healing materials need minimal maintenance and have the ability to repair their own micro-cracks and breaks. They have become a holy grail for the aerospace industry. In the frame of HIPOCRATES, the consortium has made their use a step closer to design epoxy based self-healing composites. They based these new self-healing materials on already widely used resins, so that they could be simply incorporated into current aerospace production methods.
The HIPOCRATES-project has investigated two different self-healing strategies. One involves encapsulating micro-cracks. Microcapsules containing self-healing agents are added to the composites polymer mix, in which a catalyst that starts the reaction has already been dispersed. When a micro-crack occurs, the capsules break and release the healing agent, which comes into contact with the catalyst. The resulting polymerisation reaction closes the crack and prevents further crack growth’. HIPOCRATES has been able to take this strategy one step further in developing an ‘all in one microcapsule” self-healing system which is entirely self-contained (EP16382598.7 TECNALIA). Rather than the catalyst being dispersed through the resin matrix, it is encased in the shell of the capsules in a higher concentration, so the healing reaction can occur more efficiently.
The second method has been to use reversible polymers. These materials contain internal linkages, which will reverse the damage and close a crack upon delivery of an external stimulus such as heat, radiation or electrical induction. This behaviour can be achieved using various reversible polymers and HIPOCRATES synthesized and tested two that are compatible with epoxy systems and can be made from cost effective commercial raw materials.
The materials designed by HIPOCRATES were tested in small-scale demonstrators by mimicking the kinds of high velocity impacts experienced by aircraft due for example to collisions with birds, debris and hail. The effect of compression on the repaired materials were tested to see if they would de-laminate and break apart.
Ultrasound analysis of the tested new materials found that impact and compression damage could be mitigated by using self-healing materials . The incorporation of capsules provided some protection against the initial mechanical impact, however after healing both strategies showed higher resistance to compression than before. The new materials showed 5 to 10% higher resistance to compression forces making the repaired materials resistant to the sorts of impacts that would cause damage before the repair.
The new composites designed in the HIPOCRATES project still need to undergo more testing before they can be used in real applications, but the project team hope that in the next five years these self-healing materials might help reduce aircraft costs by providing an alternative to expensive manual testing and repair.
Project Context and Objectives:
General Objective
Lightweight, high-strength, high-stiffness fibre-reinforced polymer composite materials are leading candidates as component materials to improve the efficiency and sustainability of many forms of transport. Justifying this, advanced structural polymeric composite materials (e.g. CFRPs) are currently integrated and well in use in modern commercial aircrafts, as the examples of Airbus A380, Boeing 787 and forthcoming Airbus 350, even for primary structures. In addition, it is further expected that future aircrafts will have an increased use of composites. For these types of structures, the durability and damage tolerance design of composites is always an emerging need since polymeric matrix systems are generally susceptible to micro-damage induced by a number of events during in-service loading, which can cause macro-damage initiation and structural degradation. Engineering research and design has focused traditionally on either developing new materials with improved properties or developing non-destructive evaluation methods for material inspection, yet all engineering materials eventually fail. In this direction HIPOCRATES project proposes the development of new materials able to heal damage and restore lost or degraded properties, thus contributing to the safety and durability of composites structures through their inherent mechanism to alleviate negative effect on microcraking.
Strategic Objectives
The proposed concept of HIPOCRATES project to develop innovative self-repaired composite materials by transforming widely used resins within aeronautical industry to self-healing materials is expected to offer durability, extend its service life and prolong maintenance protocols leading to lower aircraft operational (maintenance) costs. To this end through these developments, major cost reduction in maintenance activities is ensured contributing thus to the goal of direct operation costs reduction by 50% by 2020. It is also expected that the developments towards self-repaired composite materials will have a significant contribution towards the assurance that safety of aerostructures remains at current high standards regardless of air transport growth. The use of self-healing composite materials shall enhance the safety of aging airframe by substantially decreasing the need for conventional repairing (patch repair, component replacement) and thus reducing the number of operations conducted on the aircraft. This is directly contributing to the two basic objectives of the elimination of and recovery from human error in the maintenance processes as well as the reduction of accident rates by 80% by 2020. Safety of aging airframe is also enhanced by the fact that self-healing composite materials have the ability with no or minimum intervention (i.e. heating) to reverse otherwise irreversible damage such as delamination, fibre/matrix debonding and matrix cracking extending significantly the fatigue life of a component.
In more detail, HIPOCRATES main technical objective is to develop self-repaired composite materials by transforming widely used resins within aeronautical industry to self-healing materials thus facilitating the consequent certification and its related cost. Taking into account the current technological maturity of self-repair, secondary structural composites shall be targeted. The transformation will be done through the epoxy enrichment with appropriate chemical agents, following current state of the art polymer self-healing technologies (encapsulation, hollow fibers, remediable polymers). Moreover the current progress of nanocomposites technology will be utilized towards either better facilitation of self-healing process (e.g. nano carriers) or enhancement of the self-healing performance or integration of other functionalities (e.g. monitoring the self-healing performance, activation of DA reaction). For each of the self-healing approaches, critical parameters will be experimentally studied towards optimum self-healing properties. The study will continue on the effect of the composites self-repair concept with respect to the composite materials properties that will offer durability and prolong the use of advanced aerospace structures (reduction of operational cost) such as mechanical properties (impact resistance, fracture toughness, micro-cracking resistance, and fatigue). Furthermore, feasibility and challenges that arise from incorporating such self-healing thermosetting systems into fibrous composites will be closely investigated at these early stages of development to ensure effective transfer of the properties to large scales required by the industry.
Scientific and technical objectives:
• To provide experimental evidence to meet the State of the Art shortcoming and broaden the understanding of the self-healing mechanisms. Answering to the demand for more global understanding of the healing mechanisms and their performance related to different parameters, such as dynamic loading and long term healing capability, the proposed project will include the durability assessment along with dynamic performance characterization in both polymer and fiber-reinforced composites.
• To develop strategies and respective procedures for enabling self-repairing of composite materials by critically analyzing the established techniques. For making epoxy-based self-healing composite materials possible, novel chemistries will be researched for two healing strategies (encapsulation and reversible polymers. On one hand, compatibility of chemistry for capsule-based healing with epoxy systems will be evaluated. This part will also include hollow glass fibres as carriers. Different scenarios and combinations will be considered for healing agents and catalysts. On the other hand, reversible polymers with epoxide-type systems will be investigated. For this, critical parameters will be their chemical propensity to be able to modify the epoxide systems in an industrially viable way and the resulting thermal (self-healing) behaviour after functionalization. In addition tuning of the activation temperatures of the self-healing action will be attempted through chemical modification. In this way, the temperature window and self-healing efficacies needed for the application will be targeted
• To enhance self-healing activation by the incorporation of nanotechnology (ferromagnetic nanoparticles, CNT). To design and implement activation technologies for self-healing materials.
• To integrate and evaluate the incorporation and compatibility of self-healing technologies with the current processing and manufacturing methods. To investigate the feasibility and challenges arise from the incorporation of such technologies into fibrous composites (pre-preg, infusion/RTM, missing) in order to ensure the effective transfer to large scale required by industry.
• To characterize the healing mechanism performance under aeronautical loading conditions (Fracture, Fatigue, Impact, Compression after impact,...). To develop new protocols and testing methods in order to specifically quantify the healing magnitude.
• To implement the above technologies for the repair of small scale demonstrator, using self-healing materials and activation technologies. Too test and evaluate the developed self-healing features and their impact on the structural integrity of the composite while verifying their compatibility with aeronautic industry basic requirements, constrains and limitations.
Description of work
Within HIPOCRATES proposed project, the effort is divided into 7 work packages according to their specific role within the project. WP1 cover all aspects of requirements & selection of materials, WP2 refers to core development issues on encapsulation strategies in parallel with WP3 that corresponds to the respective core R&D issues on reversible polymers. WP4 is the principal R&D of the project that deals with all issues of integration of self-healing technologies in aeronautical composites. All management and dissemination/exploitation activities are grouped in WP6 and WP7 respectively.
In more detail, WP1 aim is to prepare the R&D strategy that will be followed in the following work-packages. Under this framework key objectives are to set the grounds for common scientific and technological understanding between the self-healing technology providers, the composite materials developers and aeronautical end-users. In parallel to define how composite self-repair concept is translated for composite aero-structures (requirements & limitations) and establish a roadmap towards self-repairing composites. A classification of ideas for implementation in the different phases will be made and a database of candidate materials (already used in aeronautical industry) will be created. The state-of-the-art will be updated with latest developments on self-healing/self-repair concepts. Finally an initial R&D experience on self-healing epoxies transformation with first experimental results will be gained.
WP2 key objectives are to investigate and develop novel chemical routes involved in the field of encapsulation & self-healing and research on procedures for transforming aeronautical grade epoxy systems to self-healing materials for use in composite structures. Any polymer processing issues will be identified and addressed. The developed capsules and respective self-healing epoxy materials will be experimentally tests through self-healing performance. Finally partners with nano expertise will investigate and develop concepts for interactions and synergies between nanotechnologies and micro-encapsulation strategy. WP3 similarly key objectives are to identify and investigate promising chemical routes involved in the field of reversible polymers & self-healing. Moreover directions of establishing new techniques and methods to activate the healing process on demand utilizing different fields (magnetic, electrical etc.) will be explored. As in WP2 any polymer processing issues will be investigated and developed polymer blends will experimentally evaluated. Also in WP3 partners with nano expertise will investigate and develop concepts for interactions and synergies between nanotechnologies and reversible polymers strategy. Both WPs will include development of modelling tools for key parameters of the healing processes.
At this point it should be noted that respective tasks focusing on concepts for interactions and synergies between nanotechnologies and self-healing technologies can be utilized in combination under the framework of a holistic approach. Possible combination on all self-healing technologies and nanotechnology compromise one of the scientific challenges of HIPOCRATES project.
WP4 is the principal R&D of the project that focus on integration of self-healing technologies in aeronautical composite production routes towards the development of composite self-repair concept. Key priority is to optimize current manufacturing processes and propose new/alternative integration strategies and propose technologies in order to answer the need of monitoring of polymer self-healing process in composite material level. Modelling tools will support the optimization process. Furthermore in WP4 technologies shall be developed that will allow the activation of self-healing of the polymer matrix in composite materials. Finally, but most importantly, experimental results to assess the performance of the developed materials in terms of durability and service life (eg. fatigue, impact etc).
WP5 as the final technical work-package of HIPOCRATES project will try to create the initial link between R&D developments and possible aircraft applications by identifying possible scenarios for aeronautical self-repair composites structures. Partners mainly (end users in SME or INDUSTRY) in cooperation with developers (ACADEMIA, Research Organizations, and SME) will propose small scale validation platform taking into consideration potentially interesting aircraft components for self-repair. The small scale validation platform will be designed and manufactured and performance and functional testing protocol will be defined and conducted in order to assess the overall strategic concept of HIPOCRATES project.(self-repairing composite aerostructure)
WP6 will manage and coordinate the project work including communication with the Commission, production of periodic reports, preparation of Consortium Agreement and WP7 aims to perform the technical and commercial evaluation of technological developments obtained and to assess the potentialities to transfer to real applications. Also diffusion and exploitation aspects are undertaken to address future activities to establish a roadmap for the rapid market introduction of the structural concept developed in the project
Project Results:
The main objective of HIPOCRATES project was to develop new materials able to heal damage and restore lost or degraded properties of aeronautic grade epoxy based composites. This objective will be achieved by reducing the negative effect related to the presence of microcraks on composite structures.
To achieve this objective self-healing materials appears as a promising alternative because they counter degradation through the initiation of a repair mechanism that responds to the micro-damage. Self-healing materials have the ability to automatically repair damage to themselves without any external diagnosis of the problem or human intervention.
This project has focused on the development of a self-healing aeronautic grade composites based on two different self-healing technologies extrinsic (microcapsules, vascules) and intrinsic ones (Diels-Alder and supramolecular materials). The former operates by taking advantage of the intentionally embedded healing agent, while the latter enables repeated crack healing by the polymers themselves without the need of additional healing agents. A detailed screening of the technologies has fulfilled (chemistry, compatibility with epoxy based polymers and composites, healing capacity) and the most promising ones were applied in sub structural components (skin-stringer) for the proof of concept at small scale as it is explained in the following paragraphs.
WP1. Requirements and selection of materials
The aim of the first work-package (WP1) of HIPOCRATES project was to prepare the R&D strategy that will be followed in the following work-packages. Under this framework key objectives were:
• Set the grounds for common scientific and technological understanding between the self-healing technology providers, the composite materials developers and aeronautical end-users
• Define how composite self-repair concept is translated for composite aero-structures (requirements & limitations)
• Establish a roadmap for the directions towards self-repairing composites
• Classification of ideas for implementation in the different phases
• Create a database of candidate materials (already used in aeronautical industry)
• Update the state of the art with latest developments on self-healing/self-repair concepts
Built-up an initial R&D experience on self-healing epoxies transformation and produce initial experimental results
After analysing the areas of an aircraft which are made composite materials (Figure 1), the type of damages that these composite can suffer (Figure 2), self-healing technologies (extrinsic and intrinsic), materials use in aeronautic industry and composite manufacturing processes a roadmap (Figure 3) was outline in order to help on defining a work schedule.
The schematic above is depicting the Roadmap from self-healing polymers to self-repaired composites.
• The two main manufacturing processes of composite parts that are used in aeronautical industry and more specifically in Aernnova, end user of that technology, are Autoclave curing Prepreg and Resin Transfer Moulding (RTM). The most usual composite manufacturing process is Autoclave curing Prepreg.
• Type of defects
Taking into account AERNNOVA information in terms of damage tolerance philosophy it was defined that the objective could be to heal class 2 quality (minimum detectable defect size 10 mm diameter) to turn into class 1 (minimum detectable defect size 6 mm diameter).
Three types of damages were identified:
- Holes
- Ply drops
- Drills
• Selection of materials
RTM strategy: Aeronautic grade epoxy MRW444 and a more simple system based on the same chemistry than MRW444 MY721/XB3473 (Hunstman)
Prepeg: Hexcel 8552 AS4
Healing system for encapsulation strategy
Healing agent: DGEBA
Catalyst: Scandium triflate
WP2. Encapsulation strategy (Phase B, C)
The objectives of this work package are:
• To investigate and develop novel chemical routes involved in the field of encapsulation and self-healing.
• To investigate routes for transforming aeronautical grade epoxy systems to self-healing materials for use in composite structures.
• To assess polymer processing issues
• To evaluate the developed capsules and materials through self-healing performance
• To investigate and develop concepts for interactions and synergies between nanotechnologies and encapsulation strategy.
Encapsulation strategy (microcapsules, vascular)
Technically, the healing agent for extrinsic self-healing has to be stored in fragile containers like microcapsules and vascular networks before being embedded in the matrix polymer. In terms of microcapsules strategy the general concept is based on the development of microcapsules which contain a healing agent inside. Being understood that when these microcapsules are distributed and embedded in a polymeric matrix and a crack is made, at least one microcapsule is broken. Therefore, its healing agent is released and mixed in the crack area with a catalyst, allowing the polymerization of the healing agent into a solid polymer that fills the crack by capillary forces and repairs it.
• Healing agent was encapsulated by suspension polymerization process. A maximum of 40wt% of healing agent was encapsulated.
• Microcapsules thermal stability was high enough to support composites manufacturing processes (>250ºC).
• The main achievement is related to the development of “all in one microcapsules” by a novel chemical route (EP16382598.7 TECNALIA). The all in one self-healing system is entirely self-contained. When the capsules are ruptured, the healing agent comes into contact with the catalyst and is then polymerized. Capsules were characterized by TECNALIA (DSC, GC, HPLC, SEM, OM), PATRAS (DSC, SEM, OM, TEM), UOI (Raman spectroscopy).
• Capsules were integrated into epoxy matrix successfully. Their compatibility (capsule/epoxy matrix interface) and dipersability were analyzed by microscopy.
• Healing process was evaluated via various non-destructive techniques (PATRAS, UOI) such as Raman spectroscopy, IR thermography and acoustic emission. In addition mechanical test were performed (lap shear) and positive results were achieved. The incorporation of 20wt% of microcapsules into an epoxy matrix leads to 45wt% of recovery after applying a healing cycle of 2h at 120ºC.
• Nanoparticles to improve heat transfer have been developed. However, in the case of TECNALIA self-healing material, the incorporation of heat conducting nanoparticles seems to be not successful in terms of mechanical properties.
Capsule-based self-healing concepts reveal big advantages since they can be easily embedded and industrialized as the technique of microencapsulation has been investigated since the 1950s Moreover, the encapsulation strategy can fulfill aeronautic processing and manufacturing methods. This strategy shows that it is not possible to repair all types of damages, because the healing answer presents different efficiencies under different type of damages. As with many applications, price and the identification of technological areas of usefulness are now the next step for their application, together with their commercialization and consumer adoption
WP3. Reversible polymers strategy (Phase B, C)
This work package is focused on:
• Identify and investigate promising chemical routes involved in the field of reversible polymers and self-healing.
• To investigate in the direction of establishing new techniques and methods to activate the healing process on demand utilizing different fields (electro-magnetic, electrical, etc.).
• To assess polymer processing issues.
• To evaluate the developed polymer blends.
• To investigate and develop concepts for interactions and synergies between nanotechnologies and reversible polymers strategy.
Material development:
• Development of three different and complementary self-healing systems, 2 matrix materials based on reversible supramolecular chemistry and on reversible covalent chemical bonds and and 1 particle based were developed, tested and successful incorporate into aerospace composite prepregs and test specimen. Both matrix material based systems show multiple healing at elevated temperatures.
• Development of scale-up process and processes of incorporating and testing of the matrix materials as such and as composite matrix
• Testing and demonstration of the self-healing action, development of activation methods and incorporation of nano-particles to accelerate/trigger SH
Activation technologies and methods to activate the healing process on demand utilizing different fields:
• The addition of appropriate magnetic particles is possible to produce the necessary amount of heat needed to activate the mechanism of self-healing. Iron Oxide (Fe2O3) proved to be the more efficient in terms of production sufficiently temperature inside the polymer. The use of hysteresis losses mechanism it is far more efficient than the eddy current mechanism. However and especially seeing the density of the particles and the application area only low concentration application cases seem to practicable.
• The incorporation of heat conducting nano-particles seemed to be less successful in terms of activation functionality.
Non-destructive testing and characterisation
• Evaluation of the healing process via the employment of various non-destructive techniques such as Raman spectroscopy (reveals information on the healing process), IR thermography, Acoustic Emission and stereographic imaging (insight into the failure modes)
• The scattering of the acquired values of Raman spectra by conducting a mapping acquisition in an unstressed and stressed cnt-modified film can provide a tool to map stress accumulation at low stress levels.
• The evaluation of the healing process of the supramolecular polymer supplied by Suprapolix was achieved using Impedance spectroscopy, The self-healing material managed to regain its initial dielectric properties after 5 consecutive healing cycles when the healing process took place at 100 0C.
• A selection of bismaleimide (BMI) cross-liked furan functionalised epoxy resins were investigated for their mechanical performance (Bristol) and recovery of mechanical performance via three repeatable healing cycles, attributed to the thermoplastic behaviour. The 20pph and 30 pph of BMI:100 pph DApp were shown to be the most promising candidates, full recovery observed for the former in bulk material form and the latter in thin film form.
• Building on the previous rheological studies that demonstrated the thermo-reversible nature of the crosslinks allows for substantial flow of the material at elevated temperatures, the self-healing functionality showed great promise for implementation into FRP composites.
• Although a knock-down in mechanical performance was observed for self-healing specimens when compared with the benchmark epoxy resin, it is expected this will be less of an issue for FRP composites that possess a much lower resin volume fraction of ~37%. The potential for a composite that has the capability to repair multiple times has to be appreciated when compared to a marginally higher performing composite that when it becomes damaged needs to be taken out of service and manually repaired at great expense.
WP4. Integration of Technologies in Aeronautical Composites
The work envisioned in this work-package was performed almost in parallel with WP2 & WP3 in order to make better use of the outputs (knowledge and experience) generated by WP2 & 3.
• Focus on integration of self-healing technologies in aeronautical composite production routes towards the development of composite self-repair concept
• Optimize current manufacturing processes and propose new/alternative integration strategies
• Propose technologies in order to answer the need of monitoring of polymer self-healing process in composite material level
• Develop technologies that will allow the activation of self-healing of the polymer matrix in composite materials.
• Provide experimental results to assess the performance.
Moreover, the healing mechanism and performance was also demonstrated under aeronautical loading conditions (compact tension, three point bending, impact..).
Encapsulation strategy
Microencapsulation
• Capsules were successfully integrated within polymers and composites
• CT specimens integrated with 10%wt. Reference 3 capsules achieved 79% recovery of the critical stress intensity factor (KQ) after 12hours at 120oC healing process.
• Τhe knocked down effect was determined for different types of capsules when incorporated within the composite. One part healing system (capsules with catalyst on surface) proved to be more efficient in terms of the knocked down factor. More precisely 10%wt. capsules (reference 2, two parts) combined with 2%wt. Scandium caused a reduction of 4% in flexural modulus and 14% at flexural strength compared to the neat material. By increasing the content to 30%wt. capsules (reference 3, one part) the reduction was limited at 5% and 3% for flexural modulus and strength respectively. It is conceivable that reduction percentage remained at the same level or even reduced further even though the content was increased and this is due to the one part of the healing system and its dispersion only at specific layers.
• Three-point bending tests at specimens with 30%wt. Reference 3 capsules recovered 73% and 69% their flexural modulus and strength respectively after 12h @120oC.
• Mode I, II experiments proved to be inappropriate for capsules to activate the healing process. No properties recovery regarding the critical energy release rate (GIC, GIIC) was observed due to the unstable crack growth.
• Mode I fracture toughness was decreased up to 25% when capsules (30%wt. REF3) were dispersed in the mid place along the crack area.
• On the contrary, Mode II fracture toughness was increased by capsules addition at 30% content up to 131,5 % with regards to the neat material.
• Impact and Compression after impact results for capsules REF3? (Mail Stavros dice que Ref-4) are given in WP5
• Three-point bending tests at specimens containing 10%wt. UF capsules recovered their flexural modulus and strength 85% and 97% respectively. The knocked down factor at this case was higher as the modulus decreased its value 16% and 24% the flexural strength compared to the neat material.
• Mode II experiments did not show any self-healing signs except from the strengthening effect at about 2% more.
• Acoustic emission data were further analysed and were in complete agreement with the mechanical experiments.
• Raman Spectroscopy on samples defined further theexistence or not of the chemical elements of the structure of the capsule at the fracture area.
Vascules
• A detailed feasibility study was conducted regarding the possible materials for vascules formation
• Teflon and steel wires were further used for their implementation within composites
• Three-point bending tests proved not appropriate for vascules activation and the potential healing
• Mode I specimens with two vascules achieved a 126% and 147% recovery in terms of maximum load and fracture toughness respectively.
• Open hole fatigue experiments with two vascules above and below midplane (4 in total) are promising as the manually premixed healing system reduced the damaged area as it was observed through C-Scan
• The introduction of a non-optimised vascular network (in terms of location and orientation) had a marginal influence on the global mechanical properties: a reduction of approximately 15% in initial debonding strength and 10% in final failure load. By reducing the diameter of the vascule, changing its position or locally aligning with the host ply, there is scope to further reduce, or eliminate, the network’s effect on mechanical performance. In terms of property recovery, the initial stiffness was fully restored and the strength, in terms of initial disbonding, was increased due to the localised improvement in fracture toughness.
• Strap lap tension tests with one optimized central vascule recovered almost 140% the stress value after the healing process of the injected healing system.
• Acoustic emission data were further analysed and were in complete agreement with the mechanical experiments.
• IR Thermography visualized the manually injection process of the self-healing system. Also, monitored online the crack tip during entering the self-healing region in Mode I experiments.
Reversible polymers
Supramolecular reversible polymer
• Synthesis of SH-polymers comprising UPy hydrogen bonding units (Supramolecular) in large amounts
• Processing of Supramolecular polymers into films
• Chemical and rheological characterization of Supramolecular polymers to assess thermal stability
• Mode I experiments for CFRPs containing Supramolecular polymer showed a considerable increase for both mode I characteristics (540% and 1550% for the Pmax and GIC values respectively).
• After healing, recoveries of about 30%-60% were observed for both fracture toughness I characteristics.
• Mode II experiments for samples containing Supramolecular polymer revealed a considerable increase for both fracture toughness II characteristics (120% and 100% for the Pmax and GIIC respectively).
• After healing, recoveries of about 80%-100% were observed for both fracture toughness II characteristics.
• Strap lap tension specimens recovered their initial Young’s modulus at 87% after the first healing cycle while after the third healing event the healing efficiency was calculated at 79%.
• Acoustic emission (AE) was employed to identify the acoustic signature of damage and its correlation to healing and was in complete agreement with the mechanical experiments.
• A variety of heating methods were proposed to activate and optimize the healing process, including a heating bolt using resistance heating, an induction heating bolt and ceramic heating plates separately and combined with the heating bolt.
Diels-Alder reversible polymer
• The TNO Diels-Alder prepolymer was evaluated in the form of a self-healing pre-preg material on the central mid-plane of out of autoclave cured DCB, CFRP laminates. The healing performance was evaluated over three heat cycles (each 150 oC/5 min) for the recovery of mechanical performance in composite DCB test specimen coupons.
• Recoveries of about 25-70% were observed.
• For DCB samples containing BMI-prepolymer in powder form in the mid-thickness area, it was observed that the most appropriate amount was that of 120 gsm. These samples exhibited good toughening and the best healing efficiency values (15% for the Pmax value and 2% for the GIC value) and was selected as representative sample for curing in lower temperature (100 oC).
• Utilizing the second curing cycle a degradation of the fracture toughness I characteristics was observed (11% for the Pmax and 3% for the GIC) while the healing efficiency values were strongly increased (26% and 32% for the Pmax and GIC) after the first healing cycle.
• According to mode II experiments it was shown that samples with the higher concentration exhibited the best fracture toughness II characteristics (58% and 56% for the Pmax and GIIC). In addition all material groups exhibited high healing efficiency values from 90-190% for both fracture toughness II characteristics
WP5 Validation Platform for Self-repair Composite Aero structure
Based on the review of WP1 and the monitoring of the developments (WP2, WP3 and WP4) on self-healing polymers towards self-repair composites, this task aims:
• To create the initial link between R&D developments and possible aircraft applications by identifying possible scenarios for aeronautical self-repair composite structures.
• To propose small scale validation platform taking into consideration potentially interesting aircraft components for self-repair
• To design and manufacture the small scale validation platform
• To realize performance and functional tests on the platform
Among possible damage areas skin-stringer component was selected as the configuration to prof self-healing technologies capacities. Material selection (type, sequences) was carried out base on AERNNOVA´s knowledge. All self-healing technologies developers (TECNALIA, TNO, SUPRA and BRISTOL) analyzed if their technologies presented limitations against end user´s typical manufacturing process (curing cycle, processing conditions). In addition, a preselection of the most promising self-healing technologies was done: GIC specimens were manufactured by TECNALIA and UOP and tested by ELEMENT. The results were analyzed by all partners and supramolecular material (SUPO) was selected as the most promising self-healing technology against interlaminar fracture toughness.
The prototype was modelled by FEM (AERNNOVA) in order to define its lay-up, dimensions, geometry to force a failure in the desire area. Moreover, ELEMENT designed the tooling to develop the prototype and release a test plan to demonstrate supramolecular material self-healing capacity.
However, INASCO the partner responsible from the manufacturing of the tool and the prototype was not able to complete these tasks due to a break down on their CNC machine.
In spite of this unfavourable situation, small scale skin-stringer demonstrators were manufactured by UOP. In this case a test with increased interest was defined in order to highlight the enhanced performance characteristics anticipated from the improved developed materials. Rather than tension, compression is of much greater interest in the case of the aeronautical component under investigation. This is due to the fact that the critical buckling load can be decreased by the damage state of the base material, and to the fact that the final failure will occur due to bond line failure under the buckling compressive load. For the introduction of damage, a high velocity impact was decided to be utilised. This was since aircraft vulnerability issues arise from the structural response to HVI from birds, debris, hail etc. Impact response is of high importance, especially in the case of blunt damage that promotes delamination that can grow and reduce the residual strength of the structure. Two self-healing technologies were selected taken into account the work performed by UOP during WP4.
The initial objective of this WP was to manufacture one or more demonstrators (INASCO) with the most promising self-healing technologies. However, due to a break down on CNC machine they were not able to manufacture the mould to obtain the T-stringers demonstrators. In order to be able to prof self-healing concept, PATRAS developed small scale demonstrators and tested them under impact and compression after impact. The technologies selected for these small scale demonstrators were “all in one capsules” developed by TECNALIA and supramolecular reversible polymers developed by SUPO.
Main results:
• Capsules and supramolecular material were successfully integrated with an skin-stringer composite structure
• Supramolecular material was impregnated at carbon fabric and pre-preg was developed
• Capsules were successfully mixed within the adhesive used at the skin-stringer bonding
• Quality control indicated satisfactory results in order to continue with the test campaign
• The damaged area at the modified demonstrator with capsules was slightly reduced after the healing process according to C-Scan figures.
• The presence of capsules reduced the compression after impact load up to 13% but after the healing process a full recovery observed at a rate of 117%.
• The damaged are at the bond line of the Supramolecular prepreg was not significant.
• The presence of the Supramolecular material reduced the Pmax value at the rate of 15% before the impact tests compared to the reference
• The Pmax value for the modified samples with Supramolecular pre-preg was reduced at the rate of 17%
• The Pmax value for the modified samples with Supramolecular pre-preg was increased at the rate of 5% after the healing cycle
WP6 Management and Coordination
The coordinator deals with all the day to day administrative aspects:
• Management of funding
• Distribution of information from the EC to the consortium
• Organization of consortium progress review meetings
• Preparation of periodic report
• Distribution of project reports and further deliverables to the EC
• Resolving of problems occurring between partners during the project
More in detail, management activities are listed:
• Website of the project www.hipocrates-project.eu with the update information of the project. The website is divided in two different areas, one for public dissemination of the project and the second one for projects partners.
• Organization of progress project meetings (KOM, 3months, 12moth, 18month, 24month, 36month)
• Organization of several technical meetings (9 month, 15 month teleconferences)
• Preparation of official reporting documents
• Distribution of the funding among partners
• Coordination of diffusion actions (Aerodays 2015, ECCM17, JEC Composites Show 2017)
WP7 Economical, evaluation, exploitation and dissemination
This WP aims to perform the technical and commercial evaluation of technological developments obtained and to assess the potentialities to transfer to real applications. The diffusion and exploitation aspects are undertaken to address future activities to establish a roadmap for the rapid market introduction of the structural concept developed in the project.
Economical Evaluation
The objectives for the Economical evaluation we established as follows:
• Perform an economical evaluation of the technology developed and demonstrated
• Perform a comprehensive market study in order to quantify the economical benefits from the industrial application of the developed technologies
The activities around this WP were performed after the completion of the project mainly due the reason already given above. Nevertheless a report was prepared (D71- Report on economical Evaluation). An extract of the evaluation is presented hereunder:
An economical evaluation of the developed and demonstrated technology could not be performed within the duration of the project, since the selected technology wasn’t transferred into a demonstrator in order to be fully validated using mechanical testing and applying external healing activation technologies. Although some data on material costs (at a laboratory scale) exists, an economical evaluation requires detailed costs: (1) on incorporating these materials into a composite structural part (i.e. additional labour, new equipment, handling processes and associated costs), (2) on assessing their performance compared to a traditional structure and accounting for potential cost benefits these could bring in new design methodologies and (3) accounting for the effort and cost of equipment to activate the healing mechanism. All the above points can be accounted in detail through the manufacturing, and validation of a physical demonstrator.
Market study
A complete market study is contained within D7.1 where the global composite industry is analysed, including global aerospace MRO, market drivers and industrial trends, and the global composite market forecast.
Exploitation and Dissemination
Section A to this report contains all dissemination activities carried out related to the Project
Potential Impact:
The availability of efficient and cost-effective self-repairable structures is of high technological and economical importance for the European and global aerospace industry. The ageing airframes is becoming a critical problem in most countries around the world increasing significantly the direct maintenance costs of the various airliners. At the same time new aircrafts such as the Airbus 380 or the Boeing 787 Dreamliner incorporate unprecedented usage of composite materials rendering the concept of utilizing self-healing composite materials very attractive. Within this scope, HIPOCRATES is directly addressing current and near future needs of the European Aircraft Industry by developing integrated, safer and “smarter” pan-European transport systems for the benefit of all citizens and society.
HIPOCRATES develops two basic self-healing strategies, the nano-encapsulation and the thermally remediable polymers the focus being at standard aerospace materials. The development of self-healing materials shall lead to self-repairable aerostructures contributing substantially to three major goals listed in the work programme.
- The decrease of direct operational costs by 50% by 2020
- The reduction of accident rate by 80% by 2020
- The achievement of a substantial improvement in the elimination of and recovery from human error
HIPOCRATES meets these goals through the following:
- The service life of the aircrafts is expected to be extended. Self-healing composite materials are expected to have a significantly longer service life as the majority of damage modes (i.e. delamination, debonding, matrix cracking) becomes reversible. Improvement of passenger safety is expected by the development of damage reversible material systems for rehabilitated aircraft structures which is estimated to result in significant accident rate reduction in the near future.
- Minimization of redundant components. The need for components replacement for maintenance reasons shall be reduced.
- Maintenance downtime shortening. By reducing the need for component replacement, downtimes are also expected to be highly shortened affecting positively the attempts for overall maintenance cost reduction.
- Reduction in the total number of operations. A reduction in maintenance will lead to a decrease of the number of operations conducted on the aircraft minimizing the probability for human errors
Moreover the main HIPOCRATES outputs are expected to have essential societal impact. A successful implementation of self-healing composites will lead to the life extension of structural components with the subsequent reduction in scrap. This will make a significant impact by reducing waste, raw material consumption and power consumption in the service life of parts from manufacturing up to recycling or disposal.
In addition, shorting of maintenance periods and downtimes will support passenger friendly airliner operation (reduction of flight delays due to unscheduled aircraft maintenance) and the maximization of airport operating capacity for facing the increasing traffic and will contribute to a more flexible and efficient use of European air-fleet in an expected time scale of 5 years.
The self-healing technologies that will result from this proposal will also lead to an increased role of synergistic industries in the areas of nano-technology, chemicals etc and offer opportunities for the employment of highly skilled professionals. This would contribute in solving rising societal problems interconnected with the high unemployment in Europe in a critical period.
Thus, the strengthening of the competitiveness of European transport industry is expected to be benefited from the implementation of advanced self-healing technologies as described in the HIPOCRATES project will give the European aerospace industry the opportunity to provide better solutions (operational, environmental and technological) than their competitors, to reduce the direct operating costs and thus to increase their market share. Self-healing materials are innovative products that will strengthen the position of European aerospace manufacturers, material providers and end-users (airliners) in the global market.
List of Websites:
www.iapetus-project.eu
Coordinator: Dr Sonia Flórez. TECNALIA
WP1- Requirements&Selection of Materials. Leader: INASCO
WP2- Encapsulation Strategy (phased A, B, C). Leader: TECNALIA
WP3-Reversible polymers strategy (phased A, B, C). Leader: TNO
WP4- INtegration of technologies in aeronautical composites. Leader: University of Patras
WP5- Validation Platform for Self-repair composite aero-structure. Leader: Suprapolix
WP6 - Management and Coordination. Leader: TECNALIA
WP7 - Economical evaluation, exploitation and dissemination. Leader: INASCO
