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SMA Cytec Multilayer Prepreg

Final Report Summary - SMACMP (SMA Cytec Multilayer Prepreg)

Executive Summary:
This document represents the final report of the research activities “SMA Cytec Multilayer
Prepreg” performed within the framework JTI-CS-2009-01-GRA-01-00. It includes the description
of all the main phases of the project, a results summary and a benefit assessment including some
socioeconomic considerations. In particular, the report describes the SMA material selection and
preliminary tests carried out to verify the possibility of impregnate carbon fibers and metallic wires
exhibiting a martensitic transition with a structural epoxy resin.

The Resin Film Infusion process proved to be an excellent manufacturing option, given the cost
and commercial availability of the selected “dry” products. The process can be easily controlled
and scaled-up to satisfy the aerospace market material quantities and quality standards.
NiTiNol knitted fabrics and hybrid Carbon/NiTiNol woven textiles were successfully impregnated
and tested in CEM laboratories. The resulting material showed controlled resin contents and
excellent levels of impregnation, visual quality and handle-ability.
Hybrid composite structures were characterized to assess the impact of NiTiNol wires in different
product forms, contents and positions on key laminates mechanical performance. In addition shape
memory materials potential to enhance composite structures impact resistance and tolerance
properties was evaluated. Finally a complete microscopic (SEM) evaluation was carried out to
study the fiber/matrix interface of the selected products. A technology assessment study was
completed specifically focusing on the balance between SMA materials driven impact
resistance/tolerance improvements versus weight penalty for each investigated material and
Project Context and Objectives:
Over the past years the use of composite materials in modern aircraft has been progressively extended to important primary structures starting from the empennages to the entire fuselage of the new large commercial transports [1]. The composites share has reached 50 % in the last civil aircraft programmes like Boeing 787 and Airbus A350 and goes up to 80% in some military applications.
FRP composites have the capabilities to meet all the most important design drivers like as low mass, reduced cost and improved damage tolerance making an aircraft more attractive for the airlines by reducing its operative costs and increasing its reliability. One of the most important benefits coming from extensive use of CFRP is the high level of integration that leads to strongly optimised structural concepts.

However, unlike conventional structures fabricated by aluminium using mechanical fastening techniques, composite integral structures do not contain redundant structural members that could act as crack stoppers or retarders and thus offer an intrinsic lack of fail-safety capability. International regulations often penalise such structures by imposing extra design safety factors [2].
Therefore, it is clear that the biggest challenge remains a relative poor impact and damage tolerance capability of composites. Hail, bird impacts and runway debris represents a big threat for aircraft structures and their effects (abrasion, dents and/or perforations) shall be taken into account in the design phase. This approach often leads to a redundant design reducing the advantages coming from the use of CFRP and slowing down the progress related to the introduction of new materials, since they are unable to offer the level of damage resistance required to safely operate civil aircrafts.

Several interesting progresses have been made in the last years with the introduction of a new generation of toughened epoxy resins and the development of out-of-plane reinforcements as Z-pins, 3D waiving, braided preforms and stitched fabrics. All these techniques increase the impact resistance of CFRP and reduce problems related to delamination but don’t give a definitive solution to the problem and often do not justify the certification effort needed to use them in aircrafts production.

The alternative approach investigated in this research activity is to increase the impact resistance of laminates by developing a novel material technology based on Shape Memory Alloys (SMA’s).
SMA’s are not new to aeronautics, but their use has been often related to the wing shape morphing and to the active control of aeroelastic phenomena (the concept of morphing airfoils as aerodynamic control surfaces has been deeply explored since it was used by the Wright brothers in their first successful flight in 1903 [3]).Thanks to their characteristics SM alloys such as NiTiNol (Nickel Titanium Naval Ordnance Laboratory) can be used for planform alternation, out-of-plane transformation, and airfoil adjustment with excellent results.
These interesting properties are the result of some intrinsic properties of this class of materials: shape memory effect and pseudo-elasticity.
These properties are made possible through a molecular rearrangement similar to the one occurring during a phase transformation. SMA’s molecules remain closely packed during the transformation allowing the substance remains solid. The different phases involved in the rearrangement are Martensite and Austenite, two crystalline structures of the alloys.
The shape memory effect is observed when the material is cooled up to a temperature below the critical temperature at which the Austenite phase transforms into Martensite and vice versa. At this stage the alloy is completely composed of Martensite which can be easily deformed. After distorting the SMA the original shape can be recovered simply by heating the alloy above the critical temperature [4].

Pseudo-elasticity occurs in shape memory alloys when the alloy is completely composed of Austenite i.e. the temperature is above the critical value. Unlike the shape memory effect, pseudo-elasticity occurs without a change in temperature. The load on the shape memory alloy is increased until the Austenite is transformed into Martensite simply due to the loading.

The load is absorbed by the Martensite, but when the load is removed it begins to transform back to Austenite and the alloy springs back to its original shape.
Positive effects of SMA’s on the buckling behaviour of composites have been reported by Kuo et al. while their positive influence on dynamic stability of beams is described by Tsai.
Lau et al. demonstrated that the excellent damping properties of the SMA wires could greatly improve the overall structural damping behavior of composites.

Concerning the impact behaviour, many studies have shown that shape memory alloy wires can absorb a lot of the energy during the impact due to their superelastic and hysteretic behavior. Thus, embedding superelastic shape memory alloy fibres in a laminate could make the composites tougher and increase their damage impact resistance, especially for the low impact velocity typically associated to the aircraft structures [5].
The SM alloys have been used to stitch both carbon and glass composite plates [6], to realise cross-ply and multi-angle ply reinforced composites [7], as fillers [8] and to manufacture hybrid fabrics.
All these applications have shown that by optimizing the form, the size and the position of SMA’s in the composite structures a significant improvement can be obtained in terms of impact behavior. However, it appears clear that to successfully use SMA’s in the production of aircraft structures ton a large scale, a product form suitable to be easily handled and included in a conventional layered structure shall be developed.

[1] Jens Hinrichsen, Cesar Bautista - The Challenge of Reducing both Airframe Weight and Manufacturing Cost - AIR & SPACE EUROPE VOL. 3 • No 3/4 2001
[2] Zhang X et al., Fail-safe design of integral metallic aircraft structures reinforced by bonded crack retarders - Engineering Fracture Mechanics (2008)
[3] T.K.Barlas G.A.M.vanKuik Review of state of the art in smart rotor control research for wind turbines - Progress in Aerospace Sciences 46(2010) 1–27
[4] SMA/MEMS Research Group University of Alberta internet website
[5] M. Meo, E. Antonucci, P. Duclaux, M. Giordano, Finite element simulation of low velocity impact on shape memory alloy composite plates, Composite Structures, Volume 71, Issues 3-4, Fifth International Conference on Composite Science and Technology - ICCST/5, December 2005, Pages 337-342
[6] Kin-tak Lau, Vibration characteristics of SMA composite beams with different boundary conditions, Materials & Design, Volume 23, Issue 8, December 2002, Pages 741-749
[7] Paine JSN, Rogers CA. J Intell Mater Syst Struct 1994; 5:530.
[8] G.A. Lópeza, M. Barrado et Al., Mechanical spectroscopy measurements on SMA high-damping composites - Materials Science and Engineering A 521–522 (2009) 359–362

Project Results:
Task 1.1 and 1.2 - Feasibility study, test plan and material selection

Several commercially available metal alloys showing martensitic effects were considered as potential candidates for the project activities.

Nickel Titanium alloys were eventually selected as baseline materials for the study due to their optimum compromise between cost, processability, availability and mechanical performance (deliverable 1.1). In particular, NITiNol is a flexible alloy which can be finely tuned in terms of mechanical and pseudoelastic properties allowing a better control of the product form and temperature efficiency ranges.

The chemical composition of the intermetallic compound has been proved to be very critical to control the transformation temperatures. The whole range of commercially available alloys is covered by a composition range of less than 2 w/w% of Nickel content changes. With this composition window, NiTinol alloys cover as much as -100°C up to +100°C for the As temperatures.

Two product forms were identified as potential candidates for the project activities:
1. A NiTiNol woven product
2. A Carbon/NiTiNol hybrid woven fabric

1) A manufacturing feasibility study has been successfully completed with the support of a company specialized in the manufacture of metal fibers based textiles. SMA superelastic fibers were woven in specific conditions, although still in limited quantities, using a standard textile process known as weft knitting.
The material showed a good visual quality and handle ability. Only limited signs of distortion were spotted at the edge of the fabric. The product was easily cut in selected shapes. Preliminary trials showed its suitability for standard composite impregnation processes.

2) A hybrid NiTiNol/Carbon woven fabric has been identified as second possible material for impregnation.
The material showed good visual quality, handle ability and process ability and proved to be a potential candidate for conventional prepregging/infusion manufacturing processes.
Customised grades with specific fiber types, volume fractions, A/W can be potentially produced to further improve the product performance.

Task 1.3 and 1.4 - Evaluation of the manufacturing processes and prepreg realization

Different prepregging processes including Resin Film infusion (RFI), Solvent and Hot-Melt impregnation have been evaluated (milestone 1). A Cytec proprietary structural system (CYCOM® 977-2) has been selected for the study due to its thermo-mechanical performance and minor restrictions in terms of curing profiles and possibility to be qualified and commercialized.
Leveraging on the small scale process trials larger quantities of high quality prepregged hybrid or metal fabrics have been manufactured to support the project activities (deliverable 1.2).

After manufacturing, the material is uniform in quality and condition, does not exhibit any characteristics detrimental to handling, lay-up and cure processes and is available for panel manufacturing (milestone 2).

Task 2.1 – Realization of coupons and small panels

Four different multilayer configurations have been studied to achieve a better control the volume fraction and location of the SMA alloy components in the composite structure. Moreover, hybrid materials were laid-up in specific locations of the stacking sequence as described in the figure below to evaluate the effect of SMA distribution and content on laminates impact resistance.

CFRP multilayer panels have been then cured according to standard aerospace cycles (milestone 3). Several coupons have been manufactured to support a lab test campaign and to evaluate the damage tolerance and fatigue properties of hybrid Carbon/NiTiNol composite structures (deliverable 2.1).

Task 2.2 - Multilayer CFRP characterization

A [+/-, 90/0]3s lay-up and approximately 4.5 mm specimens were used for OHC, CAI and Bearing tests. Compression strength after impact at defined energies was measured according to EN 6038. The dent depth after impact was measured and the resulting damage area determined by ultrasonic characterization (C-SCAN). OHC coupons were tested according to EN6036. Photomicrographs of SMA modified laminates were made using a Nikon LV100 microscope to detect fiber/resin interfacial issues.

SMA wires present in both hybrid materials showed an almost cylindrical shape, good distribution in the laminate and excellent adhesion to the epoxy system.

The introduction of hybrid plies in the panels stacking sequence has been evaluated and compared to the unmodified baseline. All the results were reported in a detailed confidential technical document (deliverable 2.2) sent to Alenia Aeronautica.

Task 2.3 – Assessment and benefit

A technical report summarizing the results obtained from the mechanical characterization and the techniques used to impregnate the hybrid plies has been prepared (deliverable 2.3).
Standard methods as NDI and CAI tests were used to evaluate the potential of the material solutions on composites; the tests demonstrated that even if SMA wires have an influence on the impact behaviour of hybrid laminates, the resulting damage area reduction and the increase of strength are less than expected and still far from making this solution competitive in terms of cost and weight when compared to conventional CFRP or Al.
Time and costs constraints imposed by this research and the fact that only few SMA products are available on the market have not allowed a thorough investigation of the hybrid solutions that has still an unexploited potential and remain a possible solution for future aircraft applications.

Potential Impact:
Socioeconomic impact

The activities carried out in this proposal are aligned with those set by ACARE - Advisory Council for Aeronautics Research in Europe - the European Technology Platform for Aeronautics & Air Transport and to be reached in 2020 [9]. The introduction of innovative materials like SMA meshes in the aircraft composite structures could provide innovative solutions able to achieve the following 2020 objectives:

➢ to meet society’s needs for a more efficient, safer and environmentally friendly air transport
➢ to achieve global leadership for European aeronautics, with a competitive supply chain, including small and medium sized enterprises.

This research was specifically targeted to increase the level of safety through the use of cost effective enhanced SMA technology on aircraft primary structures.
As a matter of fact, the use of advanced composite materials brings to the aircraft design the benefits coming from a reduction in part counts and a simplification in the inspection and maintenance procedures that affect the operation of the aircraft making it more attractive for the airlines. However, composite integral structures do not offer the intrinsic capabilities required to safely operate in environments where the impact threats shall considered probable.

In Europe, the regime for civil aviation is rigorously regulated by objective requirements even if the safety factors are for the most part implied rather than specified. As example, the rules clearly state that “the effects of cyclic loading, environmental degradation, accidental and discrete source damage must not reduce the structural integrity below an acceptable residual strength level. All necessary instructions for ensuring continued airworthiness in this regard must be promulgated” [10].
Hail, bird impacts and runway debris represents a big threat for aircraft structures and their effects (abrasion, dents and/or perforations) shall be taken into account in the design phase. This approach often leads to a redundant design reducing the advantages coming from the use of CFRP and slowing down the progress related to the introduction of new materials, since they are unable to offer the level of damage resistance required to safely operate civil aircrafts. This problem affects in particular the regional aircrafts that have reduced thickness fuselage and then match with difficulty the peculiar design requirements coming mainly from impact without paying a high duty to the introduction of composites.
For these reasons the GRA platform is the ideal background to explore the possibilities offered by the multilayer composite concept; a CFRP panel reinforced with a structure capable to increase significantly its impact-resistance and damage tolerance characteristics could solve most of the problems coming from the use of composite materials on sensible aircraft parts like wing leading edges, control surfaces, fuselage sections and radomes. In particular, the adoption of a metallic mesh embedded into a CFRP panel seems to be one of the most promising solutions to improve the overall performance;
On these bases, the outputs of this proposal could have a major impact on the composite manufacturing process research and could enable the full exploitation of these new materials and technologies in the global aerospace and high performance industrial market. The key results of the project will provide aircraft and component manufacturers with the ability to acquire the benefits that SMA reinforced multilayer CFRP offer to their products, and to enhance the potential of these materials to be applied to other industrial sectors, for instance automotive and rail transport, wind turbine blades.

Dissemination and/or exploitation of project results

The project can influence and provide benefits to all aircraft sectors: large commercial transport, business jet, regional transport, helicopters and military aircrafts.
The results of the activities have been shared with Alenia Aeronautica through the preparation of the technical reports scheduled in the Annex I and through several presentations, meetings and teleconferences which allowed also tuning the research on the real needs of the final user of the technology.
Cytec is still considering dissemination of the project results through the participation to European Composite Technology conferences such as JEC/SAMPE and/or publication on a specialized press ensuring an adequate protection of results generated by the project.

[10] John W. Bristow, P.E. Irving - Safety factors in civil aircraft design requirements - Engineering Failure Analysis 14 (2007) 459–470

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