Stretch broken carbon fiber for primary wind turbine structure

Stretch broken carbon fiber (SBCF) composites offer significant benefits over conventional continuous fiber composites. These include the greatly enhanced formability of unidirectional tape form products prior to cure without any compromise on the mechanical performance after curing. The recent work performed under the US Army funded Stretch Broken Carbon Fiber for Primary Aircraft Structure program by Montana State University (MSU) is reviewed, along with the results achieved by previous efforts to develop this technology. The achievement of shorter stretch break lengths to improve formability is a principal objective of the work and a successful achievement of the recent efforts. In addition, alternate means of manufacturing SBCF prepregs are discussed, including conventional spooled fiber presentation suitable for running on current prepregging equipment, as well as a newly developed direct to prepreg approach, where the stretch breaking of the fibers is done in-line with the prepregging process. While aerospace primary structure has been the focus of current research efforts, applications in other industries such as wind turbine blades are also obvious areas of applications for SBCF technology.


Introduction
Carbon fiber composite materials are used extensively in aerospace and other structures because of the combination of strength, stiffness and low density which they offer.For conventional carbon fiber composites, made with continuous fibers, the materials are however limited in terms of formability to produce complex shapes.This has led to many structures being designed as 'hybrids' with composite skins of relatively simple shapes combined with interior structures of more complex shape such as frames, stringers and spars being made from metallic materials such as aluminum alloys.Aside from the weight penalty which is incurred because of the use of metals, the combination of carbon and aluminum has led to significant corrosion issues where the aluminum becomes the anode in contact with the carbon fiber cathode.Attempts to mitigate this by the use of e.g.glass fiber isolation plies, have been only partially successful and fleets of aircraft such as the early F18 models are now experiencing problems sufficient to require them being taken out of service to be repaired at costs measured into the $US billions.There is thus significant impetus to produce carbon fiber composite materials which are more conformable, which would allow the metallic substructure to be replaced by composites in a cost effective manner.Stretch broken carbon fiber (SBCF) is a technology which offers this potential and is the subject of this paper.

. SBCF basic principles
Stretch breaking of fibers using sets of nip rollers driven at differential speeds is not a new technology and has been employed with various textiles for many years.For carbon fibers, the same technology is used to generate random breaks at natural flaws already present such that when combined with a resin matrix, the material can be formed into complex shapes in a similar manner to the traditional stretch forming of aluminum sheet.Breaking of carbon fibers is governed by three flaw types, surface flaws, internal microstructure flaws and the basic Carbon-Carbon theoretical cohesive strength as shown in Figure 1.It is noted that the primary difference between high strength aerospace grade carbon fibers and lower industrial strength carbon fibers are the flaw distributions of Figure 1.Strength is a flaw driven property, Stiffness (Young's modulus) is a bulk property governed by the fiber microstructure.The random breaking at "natural flaws" is an important concept which warrants some discussion compared to other aligned, discontinuous fiber material forms.The tensile strength of carbon fiber composites along the axial fiber direction is governed primarily by fiber flaw size and distribution.During loading, individual fibers break, and the load is distributed to the surrounding filaments.Once a critical number of breaks occur, ultimate tensile strength is achieved.This was identified and analyzed in the classic paper by Rosen as shown in Figure 2. [1].Once cured however, the strength and stiffness properties of the material are the same as those of the equivalent continuous fiber product as will be discussed in 1.2 below.A fiber breaks.At its broken end, it carries no load, but it "shear lags" load back into the surrounding region the matrix.Ineffective length δ, is defined as the length

How fiber reinforced composites work
Sometimes called the "band-aid effect."

Previous SBCF Development Work
Previous programs have developed methodologies for the production and evaluation of SBCF materials, references [2] [3] [4] [5] with some degree of success.Figure 5 shows some of the parts produced under these prior programs.The reasons that the materials have yet to be widely adopted include the fact that the stretch break length of the legacy materials was insufficiently short to allow the formability of more complex geometries and the fact that the economics and cost of production of the materials was prohibitive in lieu of investment in scale-up.As described in section 2, progress has been made in addressing these issues.Nevertheless, the previous work has demonstrated the potential for SBCF plus also illustrated the lack of impact on strength properties once the materials are cured, as shown in Figure 6.The retention of the strength of the continuous fiber material is an expected result given that the ineffective shear lag length is of the order of ten filament diameters, which for IM7 fiber are of the order of 5 microns, such that unless multiple break points align in a local region, shear transfer into adjacent filaments ensures continuity of the load path.There is also a theory that by pre-breaking at such natural flaws before applying and curing a matrix, there could be property enhancements since less damage will occur compared to when such defects fail surrounded by a cured resin matrix.It is noteworthy to state that most discontinuous fiber composite materials forms where the fibers have been cut or chopped to achieve the formable shorter fibers have a 20% or more reduction in tensile strength properties compared to continuous because the natural flaws are still present, combined with the flaws from the chopping or cutting.

SBCF recent developments
The recent and ongoing work on SBCF at MSU has a number of objectives including: • Reducing the stretch break length from those achieved in previous programs • Developing an alternative manufacturing technique called direct to prepreg (DTP) as well as the more conventional spooled fiber presentation used on previous programs • Scaling up both manufacturing techniques to achieve a cost effective product • Demonstrating that either technique can produce more complex parts than achieved previously both by lab scale forming tests and full sized demonstration parts • Generating mechanical property data to confirm that the SBCF materials are equivalent to the baseline continuous fiber products

Materials
The current work has utilized IM7 12K fiber from Hexcel, both sized and unsized, 8552 resin from Hexcel and Cycom 977-3 resin from Solvay.The combination of the fiber and the two resins covers many applications in military aerospace primary structure and extensive databases exist.The SBCF is made on two stretch breaking machines at MSU, known respectively as Bobcat 2 and SB2, and converted to unidirectional prepreg on a pilot line to produce material for testing and evaluation.In terms of the ultimate product forms, the DTP process produces unidirectional tape with the prepregging achieved at the same time as stretch breaking occurs.The more traditional spooled single tow presentation can also produce unidirectional tape using a more conventional prepregging process but can also be used for 2D or 3D weaving, braiding or stitched multiaxial reinforcements.Figure 7 shows the 'reel' 'ribbon' or 'pancake' presentation of the DTP material.A much larger machine called Bobcat 3 is under construction which will allow much wider DTP prepreg to be made, plus also be capable of simultaneously stretch breaking significantly more tows for the traditional spooled presentation.It should be emphasized that the stretch breaking technology under development is not limited to the aforementioned fiber and resins, nor is it intended that it be limited to aerospace primary structure applications only.Other markets such as automotive are clearly potential users but will require different materials such as rapid cure resin systems or thermoplastic matrices.

Stretch break length improvement
Figure 8 shows the fiber length distribution of MSU SBCF materials compared with those of legacy SBCF materials from previous programs.It can be seen that not only is the mean stretch break length of the MSU SBCF significantly less than those of the legacy materials, but the distribution is tighter and the upper tail representing a number of long fibers has been eliminated.It is believed that the presence of this small percentage of long fibers has been detrimental to the formability of the legacy materials as much as the higher mean lengths.The improvement in stretch break length has been achieved by the development of techniques different from those of previous programs and are the subject of US provisional patent application number 63/070,151, entitled 'Techniques, Method and Device for the Manufacture of Stretch Broken Carbon Fiber'.

Tow tenacity issues with short break length SBCF
Experience from previous programs has shown that SBCF tow is initially fragile and requires special techniques to apply sizing in order to produce a handleable product.The shorter stretch break length of the MSU SBCF, while improving formability, has presented additional challenges which have however been overcome using procedures different from those employed on past programs.Figure 9 shows the tensile tenacity of MSU single tow SBCF as a function of temperature.The tow has tenacity up to around 60°C, allowing it to be woven or prepregged, but at higher temperatures loses tenacity in order for stretch forming to take place during part manufacture.

Laboratory scale evaluation testing for formability and other characteristics
A variety of test methods have been developed by MSU as there are no standard test methods for the formability of composites.These include a formability 'plunger' test, ply on ply and ply on tool friction test, a dome formability test based on the metals Ericksen test plus creep and mechanical true stress strain curve measurement.This work is described in detail in a series of papers to be presented at CAMX 2022, references [5], [6] [7] [8] [9] and will therefore not be covered explicitly in this paper.

Processes for part forming and manufacturing
The emphasis of the current program in terms of part processing is autoclave curing with process cycles similar to those used for the production of parts containing continuous carbon fiber prepregs.It is however intended to explore other processes such as press molding later in the program, but this is not the focus of the current work.Figure 10 shows an outline of the intended geometries to be used in the short to medium term.All of these features and sub-structures could be utilized in wind turbine blades structures.The tapered sine wave spar example of Complex structure is particularly interesting to optimize single spar wind turbine blade design.With shorter fiber lengths in the present SBCF materials, it is also expected that it will prove possible to attempt forming into more aggressive geometries than demonstrated in the previous work, including beaded panels with narrower bead widths, representative of what would be envisaged to prevent buckling in the shear webs of spars, ribs and frames, without the need to use separate mechanically or adhesively attached stiffeners.Designed experiments are in the process of being conducted to confirm optimum cure cycles for different resin systems, which will likely include intermediate temperature 'forming' holds below the point at which the thermoset resins begin to advance, before completing the cure in a conventional fashion.There are three basic mechanisms which permit forming for uncured prepregoverall layup slippage against the tool (drawing in metals terminology), interlayer slippage and membrane stretching.Only SBCF permits the latter.It is intended to utilize all three mechanisms and employ only 'natural clamping' as provided by the autoclave pressure.The end results will be evaluated in terms of parameters such as edge wrinkling and thinning of the complex features such as beads.The metrics and information gained will be ultimately employed in producing complex full scale parts.The results of this ongoing work will be the subject of a future paper.Figure 12 shows variations in complex geometry along the length of a modern wind turbine blade with complex geometry along the length.The shapes and substructures shown in Figure 10-12 are applicable to modern composite wind turbine blades to reduce the cost of manufacturing, durability and damage tolerance, and operation costs compared to continuous fiber composites.

Conclusions and future work
Advances have been made in the development of a more conformable and potentially more cost effective stretch broken carbon fiber which can be used for unidirectional prepregs, woven prepregs plus other reinforcement forms.The advances include the achievement of a shorter break length with a tighter distribution than previous efforts, together with the development of processes suitable for the production of either a conventional spooled single tow presentation or a direct to prepreg 'ribbon' format.A great benefit for SBCF could be realized if, by stretch breaking industry fibers, the natural flaws shown in Figure 1 could be mitigated via the stretch breaking process, such that the properties of industrial or commercial grade carbon fiber products could approach aerospace carbon fibers.Additional equipment is in the process of being manufactured which will further scale up production capability and improve cost effectiveness.The production of large demonstration articles representative of aerospace primary structure will be the subject of a future paper, and these result should be directly applicable to wind composite wind turbine blade structures.In addition to the production of lighter, more cost effective structures, these developments address a serious corrosion issue arising from the use of composite skins over aluminum substructure which has now become apparent for older aircraft but will also be manifest for both later and future composite primary structures.
a. "Nick", flaw on surface (most deleterious to filament strength) b.Internal microstructural flaw (highly oriented carbon structure in the fiber) c.Carbon-Carbon Cohesive Strength (Very High) Fiber flaws a., b., and c. are governed by individual probability distributions with respect to flaw size and frequency.

Figures 1
Figures 1 and 2 illustrate the principle and an actual end result respectively.

Figure 3 .
Figure 3. Schematic of a stretch broken carbon fiber material.

Figure 4 .
Figure 4. SEM of a stretch broken carbon fiber material showing break locations.

Figure 6 .
Figure 6.Mechanical property comparison of SBCF and continuous fiber materials.

8 Figure 9 .
Figure 9. Tensile tenacity of single tow MSU SBCF as a function of temperature.

Figure 10 .
Figure 10.Outline of proposed basic formability and manufacturing study geometries.

Figure 11 .
Figure 11.External and internal complex structure of modern wind turbine blade (cross section).

10 Figure 12 .
Figure 12.External and internal complex structure of modern wind turbine blade[11] (along the length).