WO2024160329A1

WO2024160329A1 - Method of assessing constituent components of a composite structure

Info

Publication number: WO2024160329A1
Application number: PCT/DK2024/050020
Authority: WO
Inventors: Benjamin Green; Payam JAVADIAN
Original assignee: Vestas Wind Systems A/S
Priority date: 2023-01-31
Filing date: 2024-01-31
Publication date: 2024-08-08

Abstract

According to the present invention there is provided a method of assessing constituent components of a composite structure (10). The composite structure (10) comprises a plurality of layers of fibre material (12) embedded in a thermoset epoxy matrix having a cross-linked network structure. The method comprises exposing the composite structure to a treatment fluid (22) to at least partially degrade the thermoset epoxy matrix. The treatment fluid (22) comprises formic acid and/or acetic acid. The method further comprises temporarily restraining the composite structure during exposure to the treatment fluid to maintain the order and/ or orientation and/or number of the layers of fibre material. Further, the method includes separating one or more layers of fibre material from one or more other layers of fibre material to thereby assess the constituent components of the composite structure (10).

Description

METHOD OF ASSESSING CONSTITUENT

COMPONENTS OF A COMPOSITE STRUCTURE

Technical field

The present invention relates to composite structures comprising a plurality of layers of fibre material embedded in a thermoset epoxy matrix, and more specifically to a method of assessing constituent components of such a composite structure.

Background

Composite materials, such as fibre reinforced plastics, are used in many applications for their advantageous strength and weight properties. By way of a typical example, a composite structure may comprise one or more layers of reinforcing material embedded in a polymer matrix. For example, a composite body such as a composite wind turbine blade shell may include a plurality of layers of fibre material embedded in a thermoset epoxy matrix. In many examples, following manufacture of such composite structures or bodies, only exterior and/or interior surfaces of the structure may be visible. As such, from a visual inspection it may not be possible to assess the constituent components of the composite structure.

Assessment of the constituent components of a composite structure may be useful in a number of different situations. For example, an assessment may be beneficial for quality control purposes or to investigate the cause of a fault or flaw. Further, when repairing a composite body, such as a wind turbine blade, it may be advantageous to arrange substantially equivalent repair materials in substantially the same order and orientation as the existing materials of the composite body.

In existing methods, an assessment of the constituent components of a composite structure typically involves significant damage to the composite structure to mechanically remove each layer, for example by grinding, to then assess each subsequent layer in the structure. In addition to damaging the composite structure, such a method is also timeconsuming and difficult to control. Further, because the layers of fibre material are necessarily ground off to uncover the subsequent, underlying layer of material, it is difficult to accurately assess the individual layers and their respective composition in detail, because the fibre material is damaged or destroyed in the investigation process. As such, an improved method for assessing components of a composite structure is required. It is against this background that the present invention has been developed.

Summary

According to the present invention there is provided a method of assessing constituent components of a composite structure. The composite structure comprises a plurality of layers of fibre material embedded in a thermoset epoxy matrix having a cross-linked network structure. The method comprises exposing the composite structure to a treatment fluid to at least partially degrade the thermoset epoxy matrix. The treatment fluid comprises formic acid and/or acetic acid. The method further comprises temporarily restraining the composite structure during exposure to the treatment fluid to maintain the order and/or orientation and/or number of the layers of fibre material. Further, the method includes separating one or more layers of fibre material from one or more other layers of fibre material to thereby assess the constituent components of the composite structure.

For example, assessing the constituent components of the composite structure may involve assessing at least one of the orientation of the fibre material, the type of fibre material, the thickness of the fibre material, the number of layers of the fibre material, and/or the sequence of the layers of the fibre material.

In some preferred examples, the fibre material may comprise carbon fibres, glass fibres, aramid fibres, metal fibres and/or polyethylene fibres. Accordingly, the composite structure, comprising fibre material embedded in a thermoset epoxy matrix, may therefore be described as comprising fibre-reinforced polymer in some examples. In particular, in some examples the fibre material may comprise one or more manufactured sheets, plies or mats of fibrous reinforcing material. For example, the composite structure may comprise any of one or more plies of woven reinforcing fibres, one or more plies of fibre fabric material, one or more plies of unidirectional fibrous material (i.e. with reinforcing fibres extending in one direction), one or more plies of multi-axial fibrous material (i.e. with reinforcing fibres extending in multiple selected directions, such as biaxial or triaxial fibrous material), or one or more cut or chopped strand mats such as felts or veils, to name some possible non-limited examples.

In some examples, the fibre material may comprise fibres selected from one or more of synthetic fibre semi-synthetic fibre, regenerated fibre, plant fibre, carbon fibre, rock fibre, glass fibre, and/or metal fibre. In some preferred examples, the fibres may be in the form of at least one sheet comprising fibres, for example at least one sheet comprising fibres embedded in a polymer which is different from the thermoset epoxy matrix.

The composite structure may comprise metal-containing components in some examples. In examples wherein the composite structure comprises metal, such metal material may preferably be in the form of a grid, mesh, wire or a sensor. The method may facilitate an examination of such metal-containing components in the composite structure by at least partially degrading the thermoset epoxy matrix such that the or each metal-containing component may be released from the thermoset epoxy matrix.

The thermoset epoxy matrix of the composite structure may comprise an epoxy resin based polymer and at least one reactant hardener agent or catalyst. The epoxy resin may have been cured by conventional heating or irradiation (e.g. ionization, IR-radiation, e- beam etc.). Additionally or alternatively the epoxy resin may have been cured by being subjected to at least one hardener, such as one or more anhydride curing agent, and/or one or more thiol curing agent, and/or one or more amine curing agent. In some preferred examples the thermoset epoxy matrix may not include a disulphide bridge moiety. In some preferred examples, the thermoset epoxy matrix is an amine cured thermoset epoxy matrix. Examples of amine cured thermoset epoxy resins are Olin Airstone 760, Hexion RIMR 035C infusion epoxy, and Aditya Birla Recyclamine system. Such epoxy resin systems allow for separation of layers of fibre material via swelling and/or dissolving in relatively mild conditions in acidic solutions comprising formic acid and/or acetic acid. For formic acid, a solution of at least 50 wt-% formic acid at ambient temperature and pressure was found to be highly effective. However, increased temperature, pressure and concentration increases reaction speed. In some examples, the composite structure may be described as comprising a composite laminate structure because the plurality of layers of fibre material are laminated together by the thermoset epoxy matrix. Further, in some examples, the composite structure may comprise a sandwich structure, with one or more components such as lightweight core material or composite layers sandwiched between an inner skin and an outer skin of the composite structure.

It will be appreciated that separating one or more layers of fibre material from one or more other layers of fibre material may be described as delaminating the plurality of layers of fibre material of the composite structure. Accordingly, through exposing the composite structure to the treatment fluid, the composite structure may exhibit delamination, i.e. dissociation, into separate layers or separate constituent components. As noted above, exposing the composite structure to the treatment fluid at least partially degrades the thermoset epoxy matrix. As used herein, unless otherwise stated, degrading the thermoset epoxy matrix comprises breaking down the cross-linked network structure of the thermoset epoxy matrix such that the epoxy loses its structural integrity and therefore does not bond the constituent components of the composite structure, i.e. the layers of fibre material, together. For example, degrading the thermoset epoxy matrix may comprise dissolving the epoxy matrix in some examples to release, i.e. dissociate, the layers of fibre material from the epoxy. Alternatively, in some examples degrading the thermoset epoxy matrix may comprise swelling the epoxy matrix to release, i.e. dissociate, the layers of fibre material from the epoxy, as described later in more detail.

It should be understood that, as used herein, references to “swelling” and equivalent terms refer to the entrance, i.e. penetration, of a swelling fluid into the thermoset epoxy matrix, without complete dissolution of the thermoset epoxy matrix, to open up or expand the crosslinked network structure of the thermoset epoxy matrix in a spatial sense, causing an increase in size and/or mass of the composite structure. Further, as used herein, the term “swelling fluid” should be understood to mean a fluid comprising formic acid and being capable of swelling and/or decomposing the thermoset epoxy matrix.

Exposing the composite structure to a treatment fluid comprising formic acid and/or acetic acid to swell the thermoset epoxy matrix advantageously facilitates penetration of the treatment fluid into the network structure of the thermoset epoxy matrix. Overtime, swelling the thermoset epoxy matrix mechanically breaks up the network structure of the thermoset epoxy matrix to form a multitude of thermoset epoxy fractions liberated from the layers of fibre material, thereby releasing the layers of fibre material.

The method may therefore comprise allowing the treatment fluid to soak into the composite structure for a sufficient time period to allow the treatment fluid to mechanically break up the network structure of the thermoset epoxy matrix. The time period may be dependent on the concentration of the treatment fluid, i.e. the concentration of the formic and/or acetic acid, the size of the composite structure, and the temperature and pressure in the environment in which the composite structure is exposed to the treatment fluid. In some examples, the method may comprise allowing the treatment fluid to soak into the composite structure for up to 144 hours, preferably for between 1 hour and 130 hours, more preferably for between 10 hours and 100 hours, more preferably for between 24 hours and 96 hours, more preferably for up to 72 hours, more preferably for up to 48 hours. In some preferred examples, the treatment fluid may be at least partially liquid, i.e. in a liquid state, when the composite structure is exposed to the treatment fluid. In some examples, the treatment fluid may comprise a mixture of gaseous fluid and liquid fluid when the composite structure is exposed thereto. However, in some particularly advantageous examples the treatment fluid may be liquid when the composite structure is exposed thereto.

In some preferred examples, the composite structure may be exposed to the treatment fluid at a treatment fluid temperature of up to 100°C, such as from 8°C to 75°C, preferably from 10°C to 50°C, more preferably from 20°C to 35°C, and still more preferably from 22°C to 27°C. The composite structure may be exposed to the treatment fluid at atmospheric pressure or at elevated pressure conditions. For example, the composite structure may be exposed to treatment fluid at a pressure of up to 3 bars, preferably a pressure of up to 2 bars, more preferably a pressure of up to 1.5 bars.

In some examples, the composite structure may be a whole composite body, i.e. a whole composite part. Accordingly, the method may comprise exposing a composite body to the treatment fluid. However, in some other examples the composite structure may be a specimen part. As such, the method may further comprise extracting the specimen part from a larger composite body.

A specimen part facilitates an assessment of the constituent components of the composite body without necessitating undue damage to the composite body. Further, the smaller specimen part may be examined and assessed under more closely controllable conditions, such that a more accurate and reliable assessment is possible. Compared to previous assessment methods, such as grinding away layers of a composite body to establish the constituent components thereof as described by way of background, the use of a specimen part in accordance with examples of the method described herein is less damaging to the existing composite body. Further, the method facilitates assessment of the individual constituent components of the specimen part, and the layers of fibre material of the specimen part are not damaged. Accordingly, the layers can be assessed and measured in greater detail to obtain more reliable and accurate information than is possible using previous methods.

It will be appreciated that the “larger composite body” in such an example is defined in relation to the specimen part, i.e. the composite body is larger than the specimen part. The specimen part may also be referred to as a sample part. Notably, the function of the specimen I sample part is to facilitate an assessment or investigation of the constituent components of the composite body without necessitating testing and/or analysis of the entire composite body. As such it will be appreciated that the terms “specimen part” and “sample part” may be used interchangeably herein.

In some examples, the composite body may be at least part of a means of transport, i.e. a vehicle, such as a floating vessel, an aircraft or a road vehicle for example. In some other examples, the composite body may be an item of sporting equipment, such as a ski, a bicycle, a helmet or a tennis racket for example. However, the method may be particularly advantageous in examples wherein the composite body is a wind turbine component.

For example, in some examples the composite body may be a nose cone of a wind turbine rotor, a spinner, or part of a hub. Alternatively, the composite body may be a nacelle cover or housing, part of a tower of a wind turbine, such as a tower wall, a tower platform, or a hatch. In some particularly advantageous examples the wind turbine component may be a wind turbine blade component.

For example, the wind turbine blade component may be a composite reinforcing structural component of the wind turbine blade. In some particularly advantageous examples, the wind turbine blade component may be a composite shell of a wind turbine blade. As will be described in more detail later, the method may be particularly advantageous for assessing constituent components of a composite shell of a wind turbine blade since the majority of blade repairs are conducted on blade shells. The assessment may for example to establish the correct fibre material, and/or orientation of such fibre material, for repairing a damaged region of a composite wind turbine blade shell.

In addition to facilitating an assessment of the fibre material content, type and orientation, the method may also facilitate an assessment of other features of the composite body. For example, the composite body and/or composite structure may include further constituent components in addition to the fibre material, in some examples. For example, the wind turbine blade may comprise lightning protection components, such as lightning conductors and/or collectors. For example, the composite body may include non-composite parts which may be in the form of a metallic mesh. Further, in some examples the constituent components may include sensors, heating elements, non-epoxy coatings, core elements or other non-fibre elements in addition to the fibre material. Further, the blade may contain carbon fibre in different forms, such as pre-cured pultrusions with a different polymer resin matrix, or spun carbon fibres cured in the same epoxy matrix as other parts of the blade or component. Separating the layers of the composite structure may enable a more thorough analysis of the carbon material. It should be understood that the method may facilitate an analysis of such constituent component of the composite structure - either as part of the initial assessment or as offline further analysis of components not fully accessible initially, such as carbon spar caps or sensors not fully in-dept investigated in the initial assessment.

In some examples, extracting the specimen part from the larger composite body may comprise using a mechanical cutting tool to extract the specimen part from the composite body. Such an extraction method may advantageously reduce the risk of damaging or displacing the constituent components such as the fibre material of the composite structure, i.e. the specimen part, during extraction of the specimen part. Further, such a method may advantageously minimise damage to the composite body during the extraction process. Particularly when the extraction of the specimen part is combined with follow-up repair of composite body, then the method allows for investigation without permanently reducing structural integrity of the composite body.

In some examples, the specimen part may be extracted from the composite body using a jigsaw or a multitool. In some examples, extracting the specimen part from the larger composite body may comprise extracting the specimen part from the composite body using a rotating cutter. In some examples such a rotating cutter may be a disk cutter or a saw blade. In some preferred examples the rotating cutter may comprise a core drill, or a hole saw. As such, the specimen part may be a core sample. Additionally or alternatively, in some preferred examples the rotating cutter may comprise a diamond-tipped, and/or carbide-tipped, cutting edge or cutting teeth. Again, such cutting apparatus advantageously minimises damage to the specimen part and composite body during the extraction process.

In some examples, after extracting the specimen part from the larger composite body the method may comprise a step of repairing the larger composite body where the specimen part was extracted. This may for example be by applying an adhesive or resin, such as a filling or expanding adhesive to where the specimen part was extracted. Optionally, the adhesive or resin may be reinforced by glass fibres sheets or chopped fibres.

In some examples, the larger composite body may comprise a damaged region. Accordingly, the method may comprise extracting the specimen part from a location on the composite body distanced no further than 1 m, preferably no further than 0.5 m, from the damaged region. The method may therefore be implemented in particularly advantageous examples to assess the constituent components of the composite body near to a region requiring repair. Accordingly, the method facilitates an investigation to establish the correct materials for repairing the damaged region of the composite body. Further still, in some advantageous examples the method may beneficially assist in determining the orientation of fibre materials near to the damaged region. This helps to ensure that the correct repair materials, i.e. substantially equivalent materials to those present in the existing composite body; are arranged in the correct orientation, i.e. in a substantially equivalent orientation to the orientation of existing materials in the composite body, when repairing the composite body.

For example, as described previously the composite body may be a composite shell or shear web or another part of a wind turbine blade. Such a wind turbine blade may be damaged in use, for example by erosion, lightning strikes, or impact damage such as bird strikes. Accordingly, the composite body, for example the composite wind turbine blade, may comprise a damaged region and the method may comprise extracting the specimen part from a location on the composite body distanced no further than 1 m, and preferably no further than 0.5 m, from the damaged region. As previously described with general reference to a composite body, the method described herein advantageously facilitates investigation and assessment of the constituent components of the composite wind turbine blade such that the blade may be repaired using substantially equivalent materials arranged in a substantially equivalent orientation to the existing materials of the composite wind turbine blade shell. This helps to ensure that load paths are accurately replaced, reestablished or - if required - reinforced when repairing the blade.

Further, unnecessary additional damage to the blade composite body can be avoided by extracting the specimen part, i.e. the composite structure, from the wind turbine blade shell, to investigate the constituent components of the shell. This means that, following assessment of the constituent components of the composite structure, the blade shell can be repaired in a relatively simple manner. Conversely, previous repair methods required extensive grinding operations to remove material around the damaged region, typically forming a large, chamfered recess, to ascertain the type and orientation of fibre materials required for the repair. Accordingly, it is evident that the method described herein minimises such additional damage to the blade, thereby reducing cost and time required for repairs, whilst also improving the structural integrity and performance of any such repair. In some examples, the specimen part may have a width of 0.010 m to 0.100 m, preferably 0.025 m to 0.075 m, more preferably 0.04 m to 0.06 m. This helps to ensure that the specimen part provides an accurate representation of the constituent components of the composite body. For example, a specimen part with a width as specified by the minimum values above advantageously ensures the specimen is sufficiently large enough to account for any potential variations in the arrangement of fibre material in the composite body, such as variations in the type and orientation of any fibre material, density and/or areal weight of fibre material, or any other variations resultant from manufacture of the composite body, to provide an accurate representation of the constituent components of the composite body. Further, a specimen part with a width as specified by the maximum values above helps to ensure that the composite body is not unnecessarily damaged, i.e. limiting the size of the specimen part extracted from the composite body helps to ensure that the extraction of the specimen does not compromise the structural integrity of the composite body. In some examples the specimen part may have a width of 0.050 m. It will be appreciated that a width of the specimen part may also be referred to as a diameter of the specimen part in some examples.

It is preferred that the size of the specimen part is relatively small compared to the size of the composite body being investigated. It is preferred that the area of the specimen part is less than 2% of the area of the composite body, preferably it is preferred that the area of the specimen part is less than 1 % of the area of the composite body, and more preferably it is preferred that the area of the specimen part is less than 0.5% of the area of the composite body.

In some examples, the specimen part may have an aspect ratio of less than 1 :2, preferably less than 1 :1.75, more preferably less than 1 :1.5, where aspect ratio is defined as the ratio of specimen part width to specimen part height. This helps to ensure that the composite structure, i.e. the specimen part, and the constituent components thereof remains stable when exposed to the treatment fluid and when the thermoset epoxy matrix is subsequently degraded to facilitate accurate assessment of the order and/or orientation of the constituent components, i.e. the layers of fibre material.

It will be appreciated than in examples wherein the specimen part is substantially cylindrical, such as examples where the specimen part is extracted from the composite body using a rotating mechanical cutting tool, such as a hole saw or a core drill, the specimen part width is the diameter of the specimen part. As such, any references herein to specimen part width should be understood to refer equally to specimen part diameter. In some examples the treatment fluid may comprise at least 20 wt-% formic acid, preferably at least 50 wt-% formic acid, more preferably at least 80 wt-% formic acid. Formic acid may also be referred to as methanoic acid. At 20°C and 1 atm the formic acid is in a liquid state. The concentration of formic acid in the treatment fluid may be selected based on the composition and/or crosslinking density of the thermoset epoxy matrix.

Alternatively, in some other examples the treatment fluid may be a dissolution fluid. For example, the treatment fluid may comprise acetic acid and/or formic acid. Accordingly, exposing the composite structure to the treatment fluid dissolve the thermoset epoxy matrix in the composite structure. For example, the thermoset epoxy matrix may be a chemically disassemblable epoxy type resin, such as Recyclamine® for example, which upon exposure to a treatment fluid containing formic acid and/or acetic acid may be broken into dissolvable epoxy fractions. Following dissolution of the thermoset epoxy matrix, the constituent components of the composite structure may be released for simple assessment.

In some examples, the treatment fluid may comprise one or more additional components, in addition to formic acid and/or acetic acid. As such, the treatment fluid may be an aqueous solution comprising formic acid and/or acetic acid. Additional components of the treatment fluid may include at least one additional organic acid, trifluoroacetic acid, trichloroacetic acid, propionic acid, methanesulfonic acid, trifluoromethanesulfonic acid, performic acid or an anhydride of any of these organic acids. Additionally or alternatively, the one or more additional components may include at least one inorganic acid, such as hydrochloric acid. Further, in some examples the one or more additional components may comprise at least one alcohol, such as methanol, ethanol, propanol, isopropanol, butanol, t-butanol, or amyl alcohol.

With reference still to examples wherein the treatment fluid may comprise one or more additional components, in some examples the one or more additional components may comprise an additional solvent. For example, the treatment fluid may comprise one or more of tetrahydrofuran (THF), dimethylformamide (DMF), N-Methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), dichloromethane, chloroform, acetone, acetonitrile, chlorobenzene, diethylether, dioxane, ethylene glycol, polyethylene glycol (PEG), glycerin, hexamethylphosphoramide (HMPA), nitromethane, pyridine, trimethylamine, toluene, xylene, benzen, dimethylacetamide (DMAc), dimethoxyethane (DME), diglyme or dichloroethan. A treatment fluid comprising an additional solvent may be a single phase or a phase fluid system. In some examples the one or more additional components in the treatment fluid may comprise at least one dissolved salt, such as NaCI, KCI, CsCI, NaHCOs, KHCO3, CsHCOs, Na₂COs, K2CO3, CS2CO3, any salt comprising a quaternary ammonium cation, or any salt comprising either a tetrafluoroborate anion or hexafluorophosphate anion. Further, in some examples the treatment fluid may comprise an additional component that comprises at least one surfactant, preferably selected from anionic and/or non-ionic surfactant, such as sulfates, sulfonates, gluconate, cocamide, ethoxylates, and/or alkoxylates.

It will be appreciated that in some examples, the method may comprise selecting the type and/or amount of the at least one additional component based on the constituent components of the composite structure, such as the layers of fibre material, to ensure that exposing the composite structure to the treatment fluid does not cause any substantial solvation or disintegration of the constituent components.

It will further be appreciated that in other examples, the method may comprise selecting the type and/or amount of the at least one additional component based on the constituent components of the composite structure, such as a coating of the composite structure, to ensure that exposing the composite structure to the treatment fluid will lead to solvation or disintegration of the constituent components, particularly where the such constituent component may prevent or slow down the degradation of the epoxy matrix by the treatment fluid.

In some preferred examples the combination of components and any reaction components does not include a disulphide bridge moiety. In some preferred examples the treatment fluid may have a pH value of at least 2, for example the treatment fluid may have a pH value between 2.5 and 4.

In some examples, exposing the composite structure to the treatment fluid may comprise discrete application of the treatment fluid to the composite structure, such as by spraying, sprinkling, splashing, or painting the fluid onto one or more exposed surfaces of the composite structure. It will be appreciated that discrete application of the treatment fluid onto one or more exposed surfaces of the composite structure is different compared to submerging the composite structure in the treatment fluid. Discrete application comprises a controlled, in some examples measured, application of treatment fluid to a specific portion of the composite structure, without necessarily exposing the entire structure to the treatment fluid. Such an application may facilitate a greater control of the degradation process. Accordingly, such an application may facilitate more control when separating the one or more layers of fibre material from one or more other layers of fibre material of the composite structure.

In some examples, the method may include the steps of: a) discrete application of the treatment fluid to a single exposed surface of the composite structure to at least partially degrade the thermoset epoxy matrix and thereby release a single layer of fibre material from the composite structure; b) subsequently removing said single layer of fibre material to present a new exposed surface of the composite structure; and c) repeating steps a) and b) at least once such that a plurality of layers of fibre material in the composite structure are individually exposed to the treatment fluid and subsequently removed from the composite structure, one layer at a time.

In some examples, the method may additionally include an optional step of determining at least one of the orientation of the layer of the single layer of fibre material, the type of fibre material of the single layer, the thickness of the fibre material in the single layer. For example, such a determination and/or analysis may be conducted prior to step b) in some examples. Further, step c) may be repeated to determine the number of layers of fibre material, and/or the sequence of layers of fibre material present in the composite structure.

In some other examples, exposing the composite structure to the treatment fluid may comprise submerging the composite structure in the treatment fluid. For example, the method may comprise submerging the composite structure in a beaker or other suitable container containing the treatment fluid. In some other examples, the method may comprise arranging the composite structure in a beaker or other suitable container, temporarily restraining the composite structure, and subsequently exposing the composite structure to the treatment fluid by introducing the treatment fluid to the beaker or container. In some preferred examples, submerging the composite structure in the treatment fluid may comprise fully submerging the composite structure such that a depth of the treatment fluid in the beaker is greater than or equal to a height (or thickness) of the composite structure. In some other examples it may only be desired to examine a portion of the composite structure. Accordingly, the method may comprise partially submerging the composite structure in the treatment fluid in some examples.

In some preferred examples, the treatment fluid may not be agitated after the composite structure is submerged therein. This helps to ensure that the order and/or orientation of the layers of fibre material in the composite structure are maintained throughout exposure of the composite structure to the treatment fluid to facilitate accurate assessment of the constituent components of the composite structure.

In some examples, temporarily restraining the composite structure may comprise applying a restraining force to an upper surface of the composite structure during exposure to the treatment fluid. For example, in some examples the treatment fluid may have a density that is greater than the density of one or more constituent components of the composite structure. Accordingly, when submerged in the treatment fluid, if not restrained, there may exist a risk that one or more of the constituent components may float or otherwise move such that the order and/or orientation of the layers of fibre material is not maintained. In examples comprising submerging the composite structure, it may therefore be advantageous to apply a restraining force to the upper surface of the composite structure to ensure that the order and/or orientation of the layers of fibre material is maintained whilst the thermoset epoxy matrix is degraded, i.e. swelled or dissolved, to release the constituent components.

In some preferred examples, the restraining force may be applied via one or more pins. This may advantageously facilitate access to the composite structure for exposure to the treatment fluid. Additionally or alternatively, in some examples the composite structure may be exposed to the treatment fluid in a treatment container or beaker. Such a treatment container or beaker may be sized and configured to apply a restraining force, for example via the sides of the container, to support the composite structure during exposure to maintain the order and/or orientation and/or number of the layers of fibre material for analysis.

In some preferred examples, the restraining force may be a variable restraining force, such that the layers of the composite structure are compressed together during degradation of the epoxy matrix between the layers. For example, the restraining force may be applied by one or more elastic restraints, or by moveable pins, for example.

In some examples, after separating the one or more layers of fibre material from one or more other layers of fibre material, the method may further comprise rinsing said separated layer(s) of fibre material to remove any treatment fluid from said separated layer(s). The or each separated layer of fibre material may be rinsed using a washing fluid. In some examples the washing fluid may comprise a surfactant, which in some examples may comprise an anionic surfactant and/or an amphiphilic surfactant, such as a quaternary ammonium surfactant. In some examples the washing fluid may comprise a neutralising agent to neutralise the pH of the treatment fluid.

In some examples, the composite structure may further comprise one or more support elements. The method may further comprise separating the or each support element from the plurality of layers of fibre material in the composite structure after exposure to the treatment fluid.

As used herein, it will be understood that the term “support element” refers to any element or feature of the composite structure, and/or composite body, that does not provide a reinforcing function in use. For example, such support elements may include any of a filler material, a shaping aid, an electrical component such as a lightning protection component, paint, glue and/or core elements such as foams and/or woods.

In some examples, the support elements may comprise wood such as balsa wood. Additionally or alternatively, in some examples the support elements may comprise foamed plastic. Foamed plastic may comprise at least one of polystyrene (PS), polyurethane (Pll), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyolefins (polyethylene (PE) and polypropylene (PP)) or ABS foams. In some preferred examples the foamed plastic may be rigid.

In some examples, the fibre material and/or the support elements may comprise at least one additional thermoset matrix comprising cross-linked polyester, polyurethane, vulcanized rubber, cross-linked polyvinylester, cross-linked polyimides, cross-linked phenol-formaldehyde, cross-linked polybenzoxazine, cured amino resin, cured furan resin, cured maleimide resin, cured silicone or any combinations comprising at least one of the aforementioned thermoset matrix materials.

The composite structure is preferably exposed to the treatment fluid in a treatment container. The treatment container is preferably closed or closeable. In some examples, exposing the composite structure to the treatment fluid may comprise arranging the composite structure on a conveyor belt and transporting the composite structure through a treatment container where the treatment fluid is applied to the composite structure, for example by spraying or sprinkling. Brief of the

Examples of the present invention will now be described by way of non-limiting example(s) only, with reference to the accompanying figures, in which:

Figure 1 is a schematic cross-sectional view of a composite structure comprising a plurality of layers of fibre material embedded in a thermoset epoxy matrix;

Figure 2 is a schematic cross-sectional view of the composite structure submerged in a treatment fluid to degrade the epoxy matrix;

Figures 3a and 3b are schematic cross-sectional views of an alternative method for exposing the composite structure to the treatment fluid by discrete application of the fluid;

Figures 4a to 4d are schematic cross-sectional views of examples of different methods for temporarily restraining the composite structure during exposure to the treatment fluid;

Figure 5a is a schematic perspective view of a composite body;

Figure 5b is a schematic cross-sectional view of a step in a method of extracting a specimen part from the composite body; and

Figure 5c is a schematic perspective view of the specimen part.

Detailed

Figure 1 is a schematic cross-sectional view of a composite structure 10 comprising a plurality of constituent components. For example, the composite structure 10 comprises a plurality of layers of fibre material 12 embedded in a thermoset epoxy matrix 14 having a cross-linked network structure. The fibre material 12 may comprise reinforcing fibres such as glass reinforcing fibres and/or carbon reinforcing fibres, in some examples. Further, as shown in Figure 1 , in some examples the composite structure 10 may include one or more support elements 16, such as balsa wood or foamed plastic, between the layers of fibre material 12. The composite structure 10 may therefore have a sandwich structure in some examples. As explained by way of background, whilst visible in the schematic cross- sectional view of Figure 1 , the constituent components of a composite structure 10 are typically not identifiable from a visual inspection of an inner surface 18 or an outer surface 20 of the composite structure 10. Accordingly, examples of a new method of assessing the constituent components of the composite structure 10 will now be described with reference to the remaining figures.

Byway of an initial overview, in each example the method involves exposing the composite structure 10 to a treatment fluid 22 to at least partially degrade the thermoset epoxy matrix 14. Degrading the thermoset epoxy matrix 14 releases, i.e. dissociates, the constituent components of the composite structure 10, such as the fibre material 12, from the epoxy 14 binding the components together. Accordingly, the method additionally involves separating one or more layers of fibre material 12 from one or more other layers of fibre material 12. This facilitates an assessment of each constituent component of the composite structure 10, without damaging the respective component, i.e. without damaging the layers of fibre material 12.

Referring initially to Figure 2, which shows the composite structure 10 after exposure to the treatment fluid 22, in some examples the composite structure 10 may be exposed to the treatment fluid 22 by submerging the composite structure 10 in the treatment fluid 22. As explained previously, exposure to the treatment fluid 22 degrades the thermoset epoxy matrix 14 such that the layers of fibre material 12 dissociate from the epoxy 14 and from one another, and can then be separately removed for assessment.

Alternatively, as shown in Figures 3a and 3b, in some other examples the treatment fluid 22 may be applied to the composite structure 10 discretely, such as by spraying, sprinkling, splashing, or painting the fluid onto an exposed surface 24 of the composite structure 10. Accordingly, the method may involve releasing a single layer of fiber material 12 from the composite structure by applying the treatment fluid 22 to the exposed surface 24 to degrade the thermoset epoxy matrix 14 binding that layer 12 to the other layers 12 of the composite structure 10. The single layer 12 may then be removed, presenting a new exposed surface 24 of the composite structure 10 to which the treatment fluid 22 may then be applied to repeat the process, releasing and removing a single layer of fibre material 12 at a time.

As previously described, in each example the epoxy matrix 14 is at least partially degraded by exposure to the treatment fluid 22. The treatment fluid 22 comprises formic acid and/or acetic acid to degrade the epoxy matrix 14 by swelling or dissolving the epoxy 14. For example, the thermoset epoxy matrix 14 may be a traditional type epoxy resin which upon exposure to a treatment fluid 22 comprising formic acid may swell such that the epoxy 14 disintegrates into swelled epoxy particles. In some other examples the thermoset epoxy matrix 14 may be a chemically disassemblable epoxy type resin, such as Recyclamine® for example, which upon exposure to the treatment fluid 22 may be broken into dissolvable epoxy fractions. Accordingly, the treatment fluid 22 may comprise acetic acid and/or formic acid to dissolve the epoxy 14 in such an example. In some preferred examples the treatment fluid 22 may comprise at least 50 wt-% formic acid.

In order to facilitate an accurate assessment of the order and/or orientation of the fibre material 12 and/or other constituent components of the composite structure 10, the method also involves temporarily restraining the composite structure 10 during exposure to the treatment fluid 22. Figures 4a to 4d show different examples of methods for temporarily restraining the composite structure 10 during exposure to the treatment fluid 22. It is preferred that the size of the container is less than 2x the size of the composite structure. For example, for a composite structure with a circular cross section the diameter of the container should preferably be less than 2x the diameter of the composite structure. More preferably, the size of the container is 1.2x to 1.8x the size of the composite structure to ensure stable temporary retaining and sufficient volume of treatment fluid.

For example, as shown in Figure 4a in some examples the composite structure 10 may be arranged in a container 26 or beaker shaped to match the shape of the composite structure 10. As such, the container 26 itself, i.e. side surfaces 28 of the container 26, may help to restrain the composite structure 10 and constituent components thereof during exposure to the treatment fluid 22.

Particularly in examples wherein the composite structure 10 is submerged in the treatment fluid 22, it may be advantageous to apply a restraining force to an upper surface 30 of the composite structure 10 during exposure to the treatment fluid 22. This helps to maintain the order and/or orientation of the constituent components of the composite structure 10, even if such constituent components are less dense than the treatment fluid 22 and would otherwise float or move when submerged in the treatment fluid 22 and released from the epoxy 14 of the composite structure 10. By way of example, Figure 4b shows an example wherein the restraining force is applied to the upper surface 30 by one or more pins 32 engaging the surface 30, and Figure 4c shows an example where the method involves strapping the composite structure 10 down, for example using one or more elastic restraints 34. Figure 4d shows an example where the composite structure 10 has an opening through the layers 12, resin 14, and support elements 16. In this case, the composite structure may for example be prepared by a hole saw with a steering bore and the hole from the steering bore creates the opening. The sample is restrained by a pin 35 extending through the opening of the composite structure for example from the bottom of the treatment container 26. The pin 35 prevents sidewards displacement and mix up of the individual layers while maintaining good access for the treatment fluid 22. Furthermore, it is preferred that the diameter of the pin is close to the diameter of the opening, so rotation of the individual layers 12 and support elements 16 will be limited or prevented. Alternatively, two or more sets of corresponding steering bores and pins 35 separated by a distance may prevent rotation of the individual layers 12.

In an alternative example the temporary restraining the composite structure 10 is realized by introducing an orientation feature to the composite structure 10. This may for example be a non-centrally arranged smaller through bore or provide a recognizable mark to the composite structure 10 so each layer 12 before exposing to the treatment fluid 22. Such a recognizable mark may for example be a (wedge) cut into the side of an otherwise rotation symmetrical cylinder. After separation, the relative rotational orientation of the individual layers 12 may now be established by aligning the recognizable marks or the individual layers 12 so the structure of the composite is temporarily restrained and readily identified.

In each example, temporarily restraining the composite structure 10 whilst the thermoset epoxy matrix 14 is degraded helps to ensure that despite the layers of fibre material 12 being released from the binding epoxy matrix 14, the orientation and/or order of such layers 12 is maintained or readily identified. As such, an examination and assessment of the layers of fibre material 12 after degrading the epoxy 14 provides a clear and accurate indication of the type and arrangement of fibre material 12 in the composite structure 10. It will be appreciated that in examples wherein the composite structure 10 comprises a support element 16, such as the balsa wood or foamed plastic core material, the method may also involve separating the support element 16 from the layers of fibre material 12 after exposure to the treatment fluid 22. As such the method may also facilitate an assessment of such support elements 16.

In each of the examples of the method described herein, after separating a layer of fibre material 12 from the or each other layer of fibre material 12 the method may also involve rinsing or washing the separated layer 12 (not shown). For example, such a rinsing step may advantageously remove any residual treatment fluid 22 from the separated layer 12, facilitating a more thorough and accurate assessment of the separated layer 12.

Whilst in some examples the previously-described method may be performed on a whole composite body to assess the constituent components thereof, in some examples such a method may not be practical. For example, for larger composite bodies, such as composite wind turbine blades, it may not be practical to submerge the entire blade shell in the treatment fluid 22, or to apply treatment fluid 22 to the entire composite body, to assess the constituent components. Instead, in some examples it will be preferred that the method may involve extracting one or more specimen parts 36 from a larger composite body 38, and assessing the specimen parts 36 in accordance with examples of the method described herein. As such, it will be appreciated that previous references to a composite structure 10 are equally applicable to a specimen part 36 of a composite body 38, i.e. the composite structure 10 may be a specimen part 36.

Non-limiting examples of a specimen part 36 and extraction of the specimen part 36 from a larger composite body 38 will now be described with reference to Figures 5a to 5c.

Reference is made initially to Figure 5a, which shows a schematic perspective view of a composite body 38. In this example the composite body 38 is a composite wind turbine blade shell. However, it will be appreciated that a composite wind turbine blade shell is only one advantageous example of a composite body 38 for which the methods described herein may be applicable. Accordingly, the method may be equally applicable to other composite bodies 38 such as kayaks, bicycle frames, and other sporting equipment as well as wind turbine components such as nacelle covers and platforms, for example. It will be appreciated that, as indicated in Figures 5a, the specimen part 36 is smaller than the larger composite body 38, and the larger composite body 38 is therefore defined as “larger” in relation to the specimen part 36.

In some examples the composite body 38, for example a composite wind turbine blade shell 38, may comprise a damaged region 40. With reference to a composite blade shell 38, the damaged region 40 may result from a lightning strike, impact damage, or erosion, for example. Examples of the method described herein may be particularly advantageous for assessing the constituent components of the composite body 38 as part of a repair or reconditioning method. For example, an accurate assessment of the constituent components and their order and/or orientation in accordance with examples of the method described herein may enable the arrangement of substantially equivalent fibre material in a substantially equivalent orientation to the existing fibre material 12 of the composite body 38 when repairing or reconditioning the damaged region 40 of the composite body 38. To provide an accurate indication of the constituent components of the composite body 10, their order and/or orientation, in some preferred examples the specimen part 36 may be extracted from a location 42 on the composite body 38 distanced no further than 1 m from the damaged region 40.

Referring now to Figure 5b, in some examples the specimen part 36 may be extracted from the composite body 38 using a mechanical cutting tool 44, such as a core drill. The schematic cross-sectional view in the example of Figure 5b shows a core drill 44 part-way through extracting a specimen part 36 from the composite body 38. The method may also comprise marking an exposed surface of the composite body 38 at the extraction location 42 prior to extracting the specimen part 36 such that, in the event of any movement of the specimen part 36 (e.g. rotation of the specimen part 36 after a core drill has released the specimen part 36 from the composite body 38), reference can be made to the marking to verify the orientation of the specimen part 36 relative to the composite body 38. Marking may for example be a written or carved mark in the surface of the specimen part or a smaller drilled hole off the centre of the core drill.

Figure 5c shows an example of a specimen part 36 following extraction from the composite body 38. The specimen part 36 has a carved mark 46 and a smaller drilled through hole 48 as examples of markings to verify the orientation of the specimen part 36. The specimen part 36 preferably has a width or diameter D of between 0.010 m and 0.100 m to keep the specimen part 36 stable and facilitate temporary restraint of the specimen part 36 during exposure to the treatment fluid 22 as described previously. For similar reasons, the specimen part 36 preferably has an aspect ratio (width : height) of less than 1 :2.

It will be appreciated that following extraction of the specimen part 36 from the composite body 38, any of the previously described examples exposing the composite structure 10 to the treatment fluid 22 may be performed on the specimen part 36. As such, with brief reference to Figure 2, the specimen part 36 may be submerged in treatment fluid 22, or, with reference to Figures 3a and 3b, the treatment fluid 22 may be applied discretely to a single exposed surface 24 of the specimen part 36. Similarly, the specimen part 36 may be temporarily restrained as described previously with reference to the examples of Figures 4a to 4d. Alternatively, the temporarily restrain may be effected by the smaller through hole 48 allowing for reestablishing the relative orientation of the individual layers relative to each other.

It will be appreciated that the description provided above serves to demonstrate a plurality of possible examples of the present invention. Features described in relation to any of the examples above may be readily combined with any other features described with reference to different examples without departing from the scope of the invention as defined in the appended claims.

Claims

1. A method of assessing constituent components of a composite structure (10), the composite structure comprising a plurality of layers of fibre material (12) embedded in a thermoset epoxy matrix (14) having a cross-linked network structure, and the method comprising; exposing the composite structure (10) to a treatment fluid (22) to at least partially degrade the thermoset epoxy matrix (14), the treatment fluid (22) comprising formic acid and/or acetic acid; temporarily restraining the composite structure (10) during exposure to the treatment fluid (22) to maintain the order and/or orientation and/or number of the layers of fibre material (12); and separating one or more layers of fibre material (12) from one or more other layers of fibre material (12) to thereby assess the constituent components of the composite structure (10).

2. The method of Claim 1 , wherein the composite structure (10) is a specimen part (36), and wherein the method further comprises extracting the specimen part (36) from a larger composite body (38).

3. The method of Claim 2, wherein the composite body (38) is a wind turbine component.

4. The method of Claim 3, wherein the wind turbine component is a wind turbine blade component.

5. The method of any preceding claim, wherein the thermoset epoxy matrix is an amine cured thermoset epoxy matrix.

6. The method of any of Claims 2 to 5, wherein extracting the specimen part (36) from the larger composite body (38) comprises using a mechanical cutting tool (44) to extract the specimen part (36) from the composite body (38).

7. The method of any of Claims 2 to 6, wherein the larger composite body (38) comprises a damaged region (40), and wherein the method comprises extracting the specimen part (36) from a location (42) on the composite body (38) distanced no further than 1 m, preferably no further than 0.5 m, from the damaged region (40).

8. The method of any of Claims 2 to 7, wherein the specimen part (36) has a width of 0.010 m to 0.100 m, preferably 0.025 m to 0.075 m, more preferably 0.04 m to 0.06 m.

9. The method of any of Claims 2 to 8, wherein the specimen part (36) has an aspect ratio of less than 1:2, preferably less than 1:1.75, more preferably less than 1:1.5, where the aspect ratio is defined as the ratio of specimen part width to specimen part height.

10. The method of any preceding claim, wherein the treatment fluid (22) comprises at least 20 wt-% formic acid, preferably at least 50 wt-% formic acid, more preferably at least 80 wt-% formic acid.

11. The method of any preceding claim, wherein exposing the composite structure (10) to the treatment fluid (22) comprises discrete application of the treatment fluid (22) to the composite structure (10), such as by spraying, sprinkling, splashing, or painting the fluid (22) onto one or more exposed surfaces (24) of the composite structure (10).

12. The method of Claim 11 , comprising: a) discrete application of the treatment fluid (22) to a single exposed surface (24) of the composite structure (10) to at least partially degrade the thermoset epoxy matrix (14) and thereby release a single layer of fibre material (12) from the composite structure (10); b) subsequently removing said single layer of fibre material (12) to present a new exposed surface (24) of the composite structure (10); and c) repeating steps a) and b) at least once such that a plurality of layers of fibre material (12) in the composite structure (10) are individually exposed to the treatment fluid (22) and subsequently removed from the composite structure (10), one layer at a time.

13. The method of any of Claims 1 to 10, wherein exposing the composite structure (10) to the treatment fluid (22) comprises submerging the composite structure (10) in the treatment fluid (22).

14. The method of Claim 13, wherein temporarily restraining the composite structure (10) comprises applying a restraining force to an upper surface (30) of the composite structure (10) during exposure to the treatment fluid (22).

15. The method of any preceding claim, wherein after separating the one or more layers of fibre material (12) from one or more other layers of fibre material (12), the method further comprises rinsing said separated layer(s) of fibre material (12) to remove any treatment fluid (22) from said separated layer(s) (12).

16. The method of any preceding claim, wherein the composite structure (10) further comprises one or more support elements (16), and wherein the method further comprises separating the or each support element (16) from the plurality of layers of fibre material (12) in the composite structure (10) after exposure to the treatment fluid (22).