Tidal turbine blade composites - A review on the effects of hygrothermal aging on the properties of CFRP

Accepted Manuscript Tidal turbine blade composites - A review on the effects of hygrothermal aging on the properties of CFRP Parvez Alam, Colin Robert, Conchúr Ó. Brádaigh PII:

S1359-8368(18)30447-5

DOI:

10.1016/j.compositesb.2018.05.003

Reference:

JCOMB 5673

To appear in:

Composites Part B

Received Date: 6 February 2018 Revised Date:

15 April 2018

Accepted Date: 2 May 2018

Please cite this article as: Alam P, Robert C, Brádaigh ConchúÓ, Tidal turbine blade composites - A review on the effects of hygrothermal aging on the properties of CFRP, Composites Part B (2018), doi: 10.1016/j.compositesb.2018.05.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Review Article

Tidal turbine blade composites - a review on the effects of hygrothermal aging on the properties of CFRP ´ Br´adaigha Parvez Alama , Colin Roberta , Conch´ ur O

School of Engineering, Institute for Materials and Processes, The University of Edinburgh, UK

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Abstract

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The aging of polymer-composites is a ubiquitous problem that leads to the degradation of mechanical properties, reducing the service life of an engineered structure, and potentialising premature, catastrophic modes of failure. Polymer-composites used in moist or aqueous environment are subject to moisture influenced changes that affect their physical, chemical and mechanical properties. The coupled problem of polymer-composites aging within an aqueous environment is currently seeing a surge in research efforts. This is partly due to that materials used in renewable energy structures, such as tidal turbine blades, are now high-priority concerns and there is mounting societal pressure for the development of clean energy technology. The coupling of aging and water ingress in polymer-composites is not a trivial subject and is a very slow process, but as a consequence of clean energy technology concerns, there is an ever growing impetus towards the research of exacerbated rates of water aging by the integration of a third influence, heat. Heat is a means by which the rate of aging can be magnified and this combination of heat induced aging with water ingress, termed hygrothermal aging, is the topic of this review. In particular we focus on carbon fibre reinforced plastics (CFRP), as these are composites with superior mechanical properties, a high resistance to corrosion, and are considered to be important materials for the future of clean energy technology. Through this review we aim to elucidate the relevance and applicability of hygrothermal aging to the understanding of CFRP composites in marine structures such as tidal turbine blades.

and yields predictions of a four-fold increase in CFRP use from 2010 to 2020 [7]. The primary reason for the rise is that CFRP have very high mechanical properties when factored against weight. As such, they are increasingly being acknowledged as the only current realistic means to coupling high strength and stiffness to low weight. Other attractive factors of CFRP include a noteworthy resistance to corrosion and to fatigue [8, 9, 10, 11, 12], both of which are critical design considerations for tidal turbine blades [13]. Corrosion resistant CFRP is particularly important in harsh marine environments, as chloride and water are all corrosion promoters [14, 15]. This gives carbon fibres the edge over glass fibres since the latter experiences losses in tensile strength (ca. 8% per week) when left exposed within a 2% NaCl solution by weight [16]. Glass fibres are also subject to microbial attack and corrode on

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1. Introduction

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Keywords: A. Carbon fibre, A. Polymer-matrix composites (PMCs), B. Mechanical properties, B. Fatigue

Tidal turbine devices generate clean, easily predicted, renewable energy, with an expected service-life of 20 years. Turbine blades are the most structurally important components of tidal turbine devices [1]. Upscaling blades to increase energy efficiency, or, reinforcing critically loaded blade sections to improve blade longevity, are both of matters critical of importance in the tidal energy sector [2]. In both cases, higher stiffness and strength composites such as carbon fibre reinforced plastics (CFRP) are potential replacements for glass fibre reinforced plastics (GFRP), which constitute the main material used in turbine blades. The current importance placed on utilising CFRP in tidal turbine blades can be evidenced through the numerous recent patents on the matter [3, 4, 5, 6]. The global demand for CFRP over recent decades has increased exponentially 1

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prolonged exposure to extracellular polymeric substances (EPS), which are secreted by most common water dwelling bacteria [17]. Similarly to glass fibres, basalt fibres also endure stress corrosion cracking when immersed in sea water environments. The double-positive charged elemental iron component of basalt fibres will readily form complexes with free chloride ions from the seawater. These complexes will in turn substitute their chlorides for free hydroxyl ions, which are then free to oxidise in water [19]. When immersed in sea water, ions that catalyse corrosion can leach into fibre reinforced plastics (FRP) and carbon fibres, being relatively inert, are not only mechanically superior [20], but are also superior from the point of view of corrosion resistance. The diffusion of water [21, 22] into hydrophilic materials such as epoxy or polyester may plasticise the composite, reducing the effectiveness of stress transfer between the fibres and matrix [23]. A consequence of the degradation of stress transfer is a reduction of strength [22, 24], which in turn may detrimentally lead to catastrophic failure and a reduced service-life. The detrimental effects of water saturation on turbine blade life can be circumvented by increasing the thickness of composite laminates [25], though doing so will inevitably increase material, manufacturing and transportation costs. There are three main factors that will affect the servicelife of a submerged composite. These include; mechanical loads [24], high-pressure environments [26] and liquid diffusion into the material [23, 27], each of which can directly influence either/or both of the others. Collectively, these parameters can raise salt-water ingress into composites, whilst concurrently imposing mechanical damage. With time, this damage builds up and it has been reported that the accumulation of microcrack density in a composite is closely related to aging [24]. Inconveniently, the progressive build up of microcracks in a composite leads to the exacerbated uptake of water, which in turn further reduces the integrity of composite interfaces and catalyses problems such as premature composite failure, or an inability to meet limit-state conditions after a period of time. Surface erosion on tidal turbine blades, arising from cavitation [28] and silt impact damage [29] can add to the build up of microcracks. Surface erosion based microcracking is more detrimental to composites that have aged, as they are less resistant to mechanical impact damage. Aging-based damage in aqueous environments is nevertheless difficult to research as real-time aging may require several years or even decades before insightful changes in materials properties become noticeable. As such, an increasing number of research groups are using artificial aging methods, typically involving heat. Heat aging is a simple and easy way by which polymers can be made more brittle (in similitude to physical changes through aging). The importance of this process for composites cannot be emphasised enough as it allows age-related structure-properties research to be conducted in a fraction of the time it would ordinarily take for the same composites to be naturally aged. Hygrothermal aging can be viewed as a coupling between polymer aging

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through heat with water ingress that is exacerbated by heat. Hygrothermal aging is therefore a crucial conditioning method when researching the service-life of underwater structures. The consequence of conjoining both heat and water ingress to CFRP is a degradation in the materials properties. A comprehensive review on the effects of water ingress and heat on the physical, chemical and mechanical properties of CFRP is currently of importance as there has been a considerable increase in funding and interest towards renewable energy technology including tidal and wave energy capture devices. Moreover, CFRP demand as an engineering material, as has already been noted, continues to rise and is actively researched for use in tidal and wave energy capture systems. This review will collate key research related to water ingress, heat transfer and hygrothermal aging, and aims to yield insight into the relevance and validity of accelerated aging techniques for real-time aged composites relevant to tidal turbine blade design.

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2. The effects of moisture ingress on CFRP

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2.1. Moisture diffusion in engineering polymers The kinetics of moisture and water ingress into polymeric materials relate invariably to both the molecular microstructures of polymers [30] and to the chemical kinetics [31] that characterise water affinity to the polymer molecules. Free volumes in the polymer bulk arise typically as a function of intermolecular interactions (molecular cohesion) [32, 33], polymer-chain stiffness [34] and curing kinetics [35]. These mediate the extent of crosslinking and entanglements, as well as the final molecular conformations, which in turn determine the bulk polymer microstructures and the availability of free volumes, or holes. Epoxy resins are common matrix materials in CFRP. In epoxy resins, molecular topological factors, such as nanovoids provide free volume access routes to permit water ingress, which results in the interaction of water with polar groups within the epoxy bulk [36]. The volume fractions of such nano-voids only affect the kinetics of diffusion during the initial stages of diffusion, suggesting an adsorptionbased response to moisture transport. Moreover, there is little evidence to show that nano-void sizes truly affect water-diffusivity [37]. Rather, topological factors such as nano-voids encourage water transport through epoxies via a coupled relationship with both molecular polarity and molecular mobility, which are ultimately the rate limiting factors of water transport through epoxy resins [38]. The ingress of water into engineering polymers is known to lead to a degradation of their properties, Figure 1. Gac et al. [39] report polyamide-6 modulus and yield strength reductions of over 70% and 90%, respectively, following sea water immersion at 25◦ C for over 15 days. The glass transition of polyamide-6 decreases in a similar fashion, indicating that water molecules decrease intermolecular friction which in turn raises molecular mobility. Intermolecular bond strength is important to the polymer modulus

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which the model predicts the change in mass will plateau, whereas in fact it decreases in the case of polyester matrix composites while following the model for vinyl ester matrix composites, Figure 2. The continuous decline in the change in mass as a function of immersion time observed for polyester matrix composites after M∞ (refer to Figure 2) is an indication of physical degradation within the composite and/or irreversible chemical degradation [47].

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as in polymers, this property is a function of secondary intermolecular interactions such as electrostatic and van der Waals, and is generally not related to stronger intramolecular covalent bonding. Polymer crystallinity also influences the extent of moisture ingress. Using polyether ether ketone (PEEK) as an example [40], higher levels of polymer crystallinity can be considered as favourable to moisture absorption. This is may be because the ordered structures of polymer crystals are usually ordered through hydrogen bonded networks. Since water ingress enters by substitution into continuum networks of hydrogen bonded molecules [48], it is only logical that crystalline polymers will be more absorptive to water than amorphous polymers, where the networks comprise sparsely distributed non-continuum hydrogen bonds.

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2  −2 h M −M h √ 2 √1 1+ + (1) L w t2 − t1    8 2 Dt M∞ (2) M = 1 − 2 exp −π 2 π h 

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Figure 1. Effect of water immersion on the tensile behaviour of polyamide-6, from [39], reproduced with the permission of Elsevier.

2.2. Moisture diffusion in CFRP

Whereas Fickian diffusion is in most cases, a working model for thermosets like epoxy [31, 30] as well as thermoplastics like polyamide-6 [39], non-Fickian models are generally more appropriate for composite systems [41, 42]. This is due to the inherently more complex microstructural arrangement of composite systems. Moisture uptake, D, from Fick’s second law of diffusion as postulated for composite materials is shown in Equation 1, and the theoretical change in mass, M , of composite materials from water immersion is given in Equation 2 [43, 44, 45]. Here, M1 and M2 are the moisture contents of the CFRP at times t1 and t2 , respectively, h is the composite thickness, L is the composite length, w is the composite width, t is time and M∞ is the maximum change in mass. Kootsookos and Mouritz [46] compare the model predictions of Equation 2 against empirical data for the long-term immersion of CFRP and GFRP in sea water. Fundamentally, the model is accurate up to the maximum change in mass M∞ , after

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Figure 2. Water uptake curves for glass and carbon fibre reinforced plastics (incompletely cured at 88%) for (a) polyester-matrix composites and (b) vinyl ester matrix composites, from [46], reproduced with the permission of Elsevier.

Typically in a bi-component composite, such a CFRP, there are a minimum of four parameters that will affect the rate of diffusion. These are the water-diffusivity rates of the matrix bulk, the interphase region preceding the matrix-fibre interface where polymer chains are pinned, the matrix-fibre interface and fibre, and the fibre itself. 3

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Epichlorohydrin

Phenol formaldehyde resin

Unsaturated polyester resin

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Figure 3. Chemical structure representations of common/standard resin monomers and hardener used in the composite industry.

Further complexities are introduced through the use of fibre sizing, the fibre surface roughness, the microstructural architectures of both matrix and fibre, and the presence of defects. Two fundamental mechanisms can hence be

tied to water ingress into CFRP, diffusion and capillary flow. Whereas capillary flow will persist through voids, defects and from wicking at fibre-matrix interfaces, diffusion relies on molecular arrangements, ultimately forming 4

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sure exerted at the fibre-matrix interface, increasing the frictional resistance during fibre-matrix shear. Nevertheless, any water molecules at the fibre-matrix interface, will reduce frictional resistance and matrix compression can only theoretically heighten fibre-matrix friction provided the percentage of water at the interface is low. The reality of certain engineering structures such as tidal turbine blaes, is that they exist in harsher aqueous environments such as the sea, where the chemistry of the aqueous environment alongside the complex dynamic forces play a more definitive role in material degradation. The primary constituents of sea water include; chloride (22K ppm), sodium (19K ppm), sulphate (3K ppm), magnesium (2K ppm), potassium (750 ppm), calcium (600 ppm) and bicarbonate (200 ppm) ions [52]. Water absorption into a composite is only slightly affected by its composition. In a report comparing the absorption rates of deionised water against sea water into CFRP epoxymatrix tendons, Scott and Lees [53] noted that deionised water absorbed to only a marginally higher extent (1.6% for deionised water, 1.2% for salt water) over a period of 1.6 years. Salt ions from the water diffuse less readily than pure water does into polymeric matrices. This results in a salinity gradient which in turn acts as an osmotic counter pressure, reducing water uptake into composites. Given this understanding, it could be easy to assume that sea water would be less detrimental and corrosive to composite properties than deionised water; however, iterating from Section 1, free ions [16, 19] and microbes [17] may also contribute to material degradation, not just the water. The extent of material degradation depends in turn, on the type of composite in use, CFRP being the better choice for corrosion resistance [8]. Nevertheless, sea water can be more detrimental to CFRP and the equilibirum moisture content can in fact be higher (than for pure water) when mechanical stress is imposed upon a CFRP plate. In such cases, sea water will actually imbibe more effectively than plain water due to blister induced damage by sea water on CFRP plate [18].

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what could be termed a homogenous mixture with the matrix. In a study on water absorption into CFRP, Judd [48] identifies an inversely proportional relationship between the void content of CFRP and the activation energies of absorption. Importantly, Judd deduced that void-presence merely decreases the number of available sites for hydrogen bonding and as such, water ingress into a CFRP composite can be seen as linear function of the sites available for hydrogen bonding. Chemical structures for a few common CFRP matrix polymers are shown in Figure 3. In this figure, epoxide, hydroxyl and ester groups present in thermoset resins such as bisphenol-A-diglycidyl ether (BADGE), epichlorohydrin, phenol formaldehyde, unsaturated polyester and novolac can be considered potential sites for crosslinking. Hardeners such as triethylenetetramine (TETA) contain reactive amine groups crosslinking with the epoxide groups of the resin. Nevertheless, the polarity of these groups also attracts water molecules through adsorption/diffusion phenomena. The crosslinking chemistry of thermosets in general makes them intrinsically hydrophilic compounds and this figure highlights the large concentration of potential hydrogen bonding sites. By this understanding, polymers with larger hydrogen bonding potential will absorb more water than polymers with fewer hydrogen bonding sites. Epoxy matrix CFRP for example, will experience a greater percentage weight gain over vinyl ester, isophthalic polyester and urethane acrylate matrix CFRP [49]. Water uptake into the matrix can detrimentally affect the mechanical performance of CFRP as it will induce swelling of the matrix affecting in turn, the adhesion at carbon fibre-matrix interfaces. The integrity of fibrematrix adhesion is critical to composite mechanical performance and this bonding in a unidirectional (UD) composite is most effective in shear. Water absorption into epoxy-matrix CFRP causes considerable relative shear deformation at the interface between fibre and matrix as evidenced by atomic force microscopy (AFM) [50]. Deforming polymer at fibre-matrix interfaces can have one of two effects. The primary (and perhaps most likely) effect is that interfacial shear deformation will destroy secondary interactions at the fibre-matrix interface, allowing molecules to change their orientations, resulting in either completely, or partially de-bonded composite components. A second possibility is that interfacial interactions at the fibre-matrix interface are sufficiently strong to resist debonding but as a consequence of this, bulk matter close to the interface experiences (a) breaking of cross-links (b) molecular sliding and plastic deformation (c) molecular untangling or (d) a combination of (a-c). While these factors are likely to detrimentally affect the mechanical properties of CFRP, additional factors may, at least in part, balance our the loss in fibre-matrix shear strength. Collings and Stone [51] for example, hypothesise that moisture uptake into CFRP causes compressive straining in the matrix and tensile straining in the fibres in UD-CFRP laminates. The implication here is that matrix compression leads to pres-

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2.3. Effects of moisture uptake on the mechanical performance of CFRP The effects of moisture ingress on the mechanical performance of CFRP has been documented by a plethora of researchers. As has been elucidated in this review, these effects are ultimately a function of physico-chemical changes occurring at the interfaces and within the bulk, which in turn relate to the physico-chemical properties of the polymer and its interactions with the fibre surface (or with the sizing). Selzer and Friedrich [55] performed a comprehensive study into the effects of moisture on the mechanical and fatigue performance of CFRP composites using both PEEK and epoxy resin matrices with high tensile (HT) fibres. Importantly, they report that whereas both the tensile and compressive moduli of UD [0◦ ] CFRP are hardly altered through water ingress using epoxy matrices, [90◦ ] CFRP moduli are reduced as a linear function 5

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of an increased moisture content. Compressive and tensile moduli in both [0◦ ] and [90◦ ] CFRP using PEEK matrices remained almost entirely unaffected by the presence of water. The elastic modulus is a bulk property. As a consequence, when loading a composite in the fibre direction, provided a set volume of matter is present as a continuum, the elastic modulus will not depend on the intimacy of bonds perpendicular to the direction of loading. Contrarily, when loading perpendicular to the fibre direction, any debonding or weakened interfaces will affect the elastic modulus as they will disrupt the solid state continuum that contributes to this material property. Though the viscoelasticity of the matrix will affect the extent and style of debonding at fibre-matrix interfaces, damage in FRPs tends to follow the direction of the fibres [54]. Here, PEEK matrix composites were almost entirely unaffected by moisture ingress because the volume of water that can fully saturate PEEK is low compared to epoxies. Whereas epoxy molecules have numerous potential sites for the electrostatic attachment of water, PEEK has comparatively, only a few. As a result both; the opportunity for interfacial disruption from the presence of water, and for reduced molecular friction within the bulk is much lower in PEEK than in epoxy. This may give thermoplastic matrices like PEEK an edge over epoxy thermoset matrices in aqueous environments. Property maps can be useful indicators in the prediction of how CFRP might fail. Figure 4 from [55], shows a fatigue strength property map for epoxy matrix CFRP as a function of moisture content. The map highlights the different expected failure modes of CFRP under tensiontension cycling and elucidates how moisture content can control the way in which CFRP might fail, which in turn is a significant design consideration for many CFRP-utilising engineered structures. The long-term behaviour of composites has recently been correlated to the rheology of the polymer matrix [56, 57, 58]. The authors of these studies suggest a Burger viscous model as being representative to the conditions of long-term loading. Thermosetting polyamide-6 (PA6) resins, similarly to epoxies, are considerably more absorptive to water than PEEK. Using sea water as the aqueous environment for water ingress tests over a period of 5 months in PA6 and PEEK matrix CFRP, Arhant et al. [59] report CFRP-PA6 composites as being almost twice as absorptive as CFRPPEEK composites. In [90◦ ] tensile tests, a saturation level of 3% was shown to reduce the tensile strength of CFRPPA6 composites by close to 30% [59].

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Figure 4. Fatigue failure map of epoxy matrix CFRP with respect to moisture content, from [55], reproduced with the permission of Elsevier.

3. The effects of hygrothermal aging on CFRP 3.1. Effects of temperature on CFRP Hygrothermal conditioning is essentially a water conditioning of material under elevated temperatures. The temperature ranges used for hygrothermal aging are most typically between ambient temperature and 60◦ C, though the 6

permissible aging temperatures ultimately depend on the glass transition temperature (Tg) of the composite matrix. The general view is that aging temperatures should be below the Tg of the matrix material since above these Tg, chemical oxidation may occur, which alters the chemistry of the material and thus its inherent mechanical properties. It would therefore be beneficial to consider the effects that temperature alone has on the physical, chemical and mechanical properties of composites as a starting point for discussion. Indeed, in metallic turbines heat affects the extent of deformation under the conditions of long-term loading and the same may be true for composites [66]. Carbon fibres start visibly decomposing at temperatures of 300◦ C. Pits and holes form on carbon fibre surfaces at this temperature and these surface defects increase in size as temperature is heightened [67]. Hygrothermal aging cannot be conducted at temperatures as high as this and thus, the effect of temperature on the polymeric and interfacial components of the composite are more relevant to discuss. Temperature increases molecular mobility and reduces the load carrying capacity of polymers. This can be easily evidenced with a stress-strain curve, Figure 5 [39], where an exponential decrease in strength as a function of temperature can be observed. Stress-strain properties of polymers are usually only mildly influenced at low temperatures. Nonetheless, increasing the ambient temperature causes glassy or semi-crystalline domains within the polymer become more amorphous, favouring thus a viscous response to load. Eventually a rubbery state is reached as the glass transition temperature,Tg, is surpassed. At this stage, the ability of the polymer to store energy decreases rapidly while its flow characteristics (and energy losses) increase [60]. In composites, heat affects polymers and their interfaces,

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of thermal studies conducted on CFRP, Tg is recognised as of significance as it is the determining temperature for stress transfer at the matrix-fibre interfaces. When the Tg is surpassed, the interfacial fibre-matrix strengths tend to be hidden as the matrix flows in a more rubbery manner, which then dominates composite behaviour. Several different failure modes have been reported both below and above the Tg [60, 61, 64, 65]. At low temperatures, the probability of interply failure predominates, whereas at higher temperatures interfacial debonding failure is more likely [62, 63, 64]. In contrast to moisture loading CFRP, heating the composite can cause tilting of the carbon fibres. This phenomenon is deemed to be a consequence of severe weakening of the fibre-matrix interface [50], which in turn is due to the heat-excitation of molecules increasing molecular mobility and thus, the destabilisation of secondary interactions at the fibre-matrix interface. The interfacial strength is also to some extent, governed by the difference in thermal expansion between fibre and matrix. While the thermal expansion of the carbon fibres is low, it is high in the polymer matrix. The consequent difference of heatinduced expansion between fibre and matrix is causative to interfacial stresses and weakening [62, 63, 64]. Temperature has to be moderated during thermal treatment as excessive temperatures in CFRP composites such as carbon fibre/biamaleimide, have been shown to cause thermally induced oxidation via a benzene conjugated carbonyl (C=O-) on the bismaleimide molecular structure [80]. Neverthelss, this occurs at treatment temperatures exceeding 150◦ but at the lower treatment temperatures of 80◦ the bismaleimide molecules avoid being oxidised.

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Figure 5. Effect of temperature on the tensile behaviour of polyamide-6, from [39], reproduced with the permission of Elsevier.

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which in turn guides the mechanical properties of composite materials. Cao et al. researched the effects of temperature on the mechanical properties of CFRP, as well as hybrid carbon/glass fibre reinforced polymer and carbon/basalt fibre reinforced polymer composites at temperatures ranging from 16◦ C to 200 ◦ C [60]. They reported that the tensile strengths of CFRP sheets are dramatically reduced as a function of increasing temperature, up to the Tg. At temperatures above the Tg, they noted that there was little further variation in strength. Kim et al. studied the bond properties of epoxy matrix CFRP after thermal exposures ranging from 25◦ C to 200◦ C over periods lasting up to 28 days [61]. They reported that the interfacial responses of the epoxy resin with the fibre is strongly interlinked to the Tg of the polymer. Miyano et al. predicted the creep behaviour of UD CFRP as a function of temperature [62, 63]. Importantly, they revealed that a time-temperature energy shift factor could give rise to dissociated stress transfer behaviour both under and over the Tg. Peters et al. correlated matrix fracture strains to interfacial strengths in composites between the temperature range of -100◦ C to +100◦ C using 0/904 /0 samples [64]. They found that as the matrix fracture strain increase, the interfacial strength is reduced through the input of temperature, clarifying a heightened probability of interfacial failure as opposed to interlaminar (matrix dominated) failure as temperature is increased. Under extreme temperatures fibres can oxidise and combust which affects the mechanical behaviour of CFRP in an atypically observed manner. Wang et al. [65] studied the tensile mechanics of pultruded CFRP up to temperatures of 700◦ C and noted a first reduction in tensile strength occurring between 20-150◦ C, which was related to matrix degradation, and a second reduction in tensile strength between 450◦ C and 706◦ C where epoxy is burnt off and the carbon fibres themselves begin to oxidise and combust. In the majority

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3.2. Hygrothermal based water ingress into CFRP Since both water and temperature individually affect the chemical physics and mechanical properties of CFRP, coupling the two parameters to hygrothermally condition CFRP will be indubitably more complex. Researching hygrothermal aging on high volume fraction (69%) CFRP, Karbhari and Xian [76] suggest a double-tier model for moisture uptake. The first level is considered to be a Fickian response and occurs at a relatively rapid rate, while the second level response is less rapid and is a combined result of the filling of voids within the composite and well as wicking. Both first and second level responses can be exacerbated by raised hydrostatic pressures, Figure 6 [68], which are relevant in higher pressure aqueous environments such as e.g. tidal turbine blades. Importantly, in hygrothermally conditioned composites, the initial Fickian response is thermally activated and the rate of water ingress into CFRP increases exponentially with temperature. This observation is critical, as it evidences the physical alteration to CFRP through thermally induced molecular excitations as being definitive factors in the flux of water, which in turn alludes to that the more heavily crosslinked a composite is, the less easily water may be able to penetrate. How closely water diffusion through a 7

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arrangement loses strength more rapidly than UD-CFRP in tension [70], which is most likely due to a transition from fibre dominated material resistance towards a more matrix controlled resistance to loading. Tensile UD-CFRP properties are intimately related to the strength of bonding between fibre and matrix because a well bonded fibrematrix system is one that maximises stress transfer between composite components on loading, and which therefore maximises load sharing within the composite. An exemplary study by Zafar et al. [23] compared global composite straining in CFRP against fibre straining (using Raman spectroscopic methods). This method of comparison permits an inside view of the effectiveness of fibres as reinforcements to the matrix (in this case epoxy). Zafar et al. report not only that fibre fractures occur at smaller strains in more extreme cases of hygrothermal conditioning, but also that hygrothermal conditioning in sea water is more detrimental to the strain-to-fracture of the fibres within composites. Water content affects the ratio of transverse to longitudinal straining (Poisson’s ratio) in polyamide-6 matrix UD-CFRP [72], such that the rate of strain increase as a function of water content is higher in the transverseto-fibre axis than in the fibre axis. Poisson ratio shifts from ca. 0.36 to 0.43 were noted in [72], which is a substantial difference and elucidates the understanding that increased moisture content concurrently reduces the extent of volumetric change in CFRP when subjected to deformation (i.e. it increases material incompressibility). Contrary to the tensile modulus, which is unaffected by a rising CFRP moisture content, the compressive modulus does reduce as a function of an increased CFRP moisture content [72]. The difference is possibly attributable to the increased elastic microbuckling of carbon fibres as moisture induces the plasticisation, and hence, softening of the matrix. Figure 7 shows a schematic model of elastic microbuckling and its transition to an inelastic (failed) microbuckle, which can be visualised clearly in the optical microscope image for Figure 8 [73]. Unsurprising therefore, both the compressive strength and the strain to failure are also reduced from the introduction of more water within the CFRP composite.

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composite matrix follows Fick’s law in the first level is also in itself guided by temperature since non-Fickian diffusion is the norm for most composite resins above a temperature of 60◦ C [51, 69].

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Figure 6. Pressure influence on water diffusion, C, in dried carbon/epoxy specimens (red) and in previously saurated specimens (blue), from [68], reproduced with the permission of Elsevier.

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3.3. Hygrothermal effects on the uniaxial mechanical properties of CFRP The degradation of CFRP mechanical properties through accelerated hygrothermal aging is of paramount importance. Tual et al. [22], reported the effects of hygrothermal treatments on the tensile properties of epoxymatrix UD-CFRP composites. Whereas the tensile modulus does not appear to vary as a function of hygrothermal immersion time, the tensile strength values notably decrease. This tendency is also true for acrylic matrix UD-CFRP immersed in sea water [70]. While the tensile modulus is a bulk property, the tensile strength relies on the intimacy of attachment between the carbon fibres and the matrix they reinforce. The reduction of strength implies that hygrothermal conditioning decreases the quality of the interfaces in CFRP and the tensile strength will resultantly decline as a function of aging time. The reduction in tensile strength is more drammatic as the moisture content of CFRP increases when the hygrothermal immersion temperature is increased. Comparing between ±45 CFRP laminates, Joshi [71] reports a tensile strength reduction from ca. 220 MPa to 140 MPa over a temperature increase from 20◦ C to 130◦ C, respectively. Additionally, the ±45

3.4. Hygrothermal effects on the flexure of CFRP Assessing flexure can sometimes be beneficial over uniaxial testing as it subjects the sample to bending, tension, compression and shear, all within the same test specimen. 4-point flexure is preferable to 3-point flexure as, unlike 3-point flexure, it experiences pure-bending between the central two rollers. Kootsookos and Mouritz [46] 4-point bend work on polyester and vinyl ester CFRP indicate that in flexure, whereas the strength does degrade as a function of immersion time, there is no statistically significant variation in the flexural modulus. This relationship is very similar that of the tensile modulus and strength values of hygrothermally treated CFRP reported by Tual et al. [22]. An implication here is that during CFRP flexure, the 8

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The influence of hygrothermal aging on the peak fatigue flexural stresses plotted against the number of cycles to failure (S-N curves) is marginal in UD-CFRP epoxymatrix composites [74]. This most likely relates to factors described in Section 2 and which have been relayed in previous hygrothermal studies using temperatures of 70◦ C and 100◦ C [55]. Nevertheless, geometrical factors such as stitching spacing distances have profound effects on the flexural (4-point) fatigue performance of CFRP [74], with longer spacings found to yield superior S-N behaviour. Though the rate of decline in an S-N curve is much higher in wet UD-GFRP as compared to wet UD-CFRP, after hygrothermal aging (60◦ /3 months) the S-N rate of decline has been shown to be very similar for both UD-GFRP and UD-CFRP. This noted, hygrothermal aging affects the peak flexural stress of epoxy matrix UD-GFRP more detrimentally than it does epoxy matrix UD-CFRP [75], indicating that the fibres in a uni-directional arrangement play a critical role in the durability of the overall composite, even though both matrix and interface degradation may occur concurrently. One important effect that CFRP flexure under water immersed conditions has nevertheless, is to decrease water ingress into the material [18]. Water has reduced opportunities for imbibition into flexed CFRP composites because pre-stressing the material in this way actually reduces the sizes of free volumes within the polymer matrix. The consequence of free volume reduction is of course a reduced volumetric presence of water within the matrix and a consecutive reduction in Fickian diffusion. As mentioned in previous sections, heat-only effects on CFRP causes residual tensile straining in the matrix, whereas moisture-only effects on CFRP causes residual compressive straining in the matrix. The two seemingly dichotomous effects in a hygrothermal system are not linearly interchangeable and as such, hygrothermally conditioned CFRP will exhibit a non-linear balance between the two. This is reflected in the overall matrix strain, which favours residual strains in compression when both heat and moisture are present [51], indicating that moisture has the predominating effect during combined heat/moisture hygrothermal conditioning.

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Figure 7. A schematic diagram showing the formation of kinking failure mode and its geometry: (a) in-plane buckling of 0◦ fibres with an initial fibre misalignment φ◦ , (b) deformation of 0◦ fibres via fibre microbuckling mechanism when it is loaded in compression σ ∞ and (c) fibres kinking phenomena causing catastrophic fracture of the UD laminate. The kink band geometry: w = kink band width, β = boundary orientation and φ = φ◦ + γ = inclination angle, from [73], reproduced with the permission of Elsevier.

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Figure 8. Optical microscopic across width view (at 200 × magnification) of UD HTS40/977-2 CFRP composite laminate. Kink band width, w = 60100 µm (or ca. 8-15 fibre diameters) and kink band inclination angle, β = 10-25◦ , from [73], reproduced with the permission of Elsevier.

3.5. Hygrothermal effects on the ILSS of CFRP In UD-CFRP strength and stiffness are fibre dominated properties and as such it is difficult to infer understanding about the matrix effect on the composite performance. The interlaminar shear strength (ILSS) is contrarily a matrix dominated property in UD-CFRP and can be used to better understand the role of the matrix in defining the properties of the material under hygrothermal conditions. In their work on the ILSS of hygrothermally aged carbon fibre/bismaleimide composites (fibre fraction 62%), Sun et al. [77] observe clear differences in the fracture surfaces of dry and wet composites. Dried CFRP fracture surfaces evidenced large-scale fibre debonding from the matrix material such that clean carbon fibre surfaces were easily visible, and where matrix was present the fracture surface

flexural modulus is characterised by the tensile face stiffness, as opposed to compressive or shear stiffness. This implication seems valid as there was no observed change in the modulus for either the polyester CFRP or the vinyl ester CFRP and clarifies that the fibres are able to function as long-fibre reinforcements even after hygrothermal treatment. Had there been sufficient loosening of fibres from the matrix materials, we would expect fibre buckling to dominate on the compressive faces of the CFRP coupons, and we would expect the flexural moduli to drop with increased immersion time. 9

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viable alternative to epoxy resin. Nevertheless, focusing on the percentage reduction can be somewhat deceptive since the top performing epoxy did exhibit a greater percentage reduction than the vinyl ester based CFRP, yet, in both dry and hygrothermally aged states, this epoxy based CFRP also retained an overall higher ILSS than the vinyl ester based CFRP. Thus we may ask, are we after an overall higher mechanical performance in an aged state, or do we prefer that the difference between unaged and aged materials be minimimised? These tests conducted in [49] involved short-term immersions. Longer-term (> 2 years) sea water immersion tests conducted at 30◦ C [46] reveal slightly more about how the uptake of water in vinyl-ester matrix CFRP is a function of curing. According to this study, CFRP mass gain by immersion is actually found to be more rapid in a 100% cured composite as compared to an 80% cured composite. Additionally, over a duration of 3 years, the 80% cured CFRP never as much in mass as does the 100% cured sample. A reason for this is that partially uncured vinyl ester will contain a higher percentage of unreacted chemical species that are more rapidly released in sea water, which in turn results in a reduced rate of mass gain from water ingress. Sun et al. [77] considered the double-tier model of Gautier et al. [79] to be a good means of predicting the ILSS with respect to the effects of heat on water immersed CFRP, represented by Equation 3. In this equation, σ(t) is the ILSS at time, t, σ0 is the ILSS at an initial time, σ∞ is the ILSS after an infinite time (assuming an asymptotic relationship of ILSS on the approach towards an infinite immersion time), and τ is a characteristic time that is a function of temperature.

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appeared cleaved and analogous to brittle fracture. Contrarily, ILSS fracture surfaces from hygrothermally aged CFRP exhibited patchy matrix regions across the fracture surfaces with step-like bands visible where the matrix was present. These step-like bands can be physically interpreted as being homonymous to necking failure in polymers, except that the neck-like bands have arisen from shear deformations. The necking bands themselves are evidence to increased ductility which occurs through the plasticisation of polymer under the conjoint influences of heat and water (unlike the brittle cleaved matrix surfaces in dry CFRP ILSS fracture surfaces). Sun et al. also noted that the ILSS as a function of immersion time always drops and plateaus at approximately 14 days, regardless of the hygrothermal conditioning temperature. Sun et al. used a high temperature range (23-150◦ C) and reported the magnitude of ILSS was exacerbated by temperature, but not the pattern of ILSS variation with immersion time. The the clear indication from these results is that though temperature in a hygrothermal age will indubitably contribute to loss in ILSS [71], water ingress dominates the pattern of change over time. Indeed, Collings and Stone demonstrated that transverse residual strains caused by thermal treatment are reversed by moisture ingress and a complete reversal of thermally induced transverse straining was evidenced at an equilbrium moisture content of 1.7% in the CFRP [51]. The work of Botelho et al. [78] elucidates the importance of composite laminate orientations in the fracture mechanical behaviour of ILSS specimens. Their work involved the Iosipescu ILSS testing of carbon fibre/epoxy composites in both 0/0 and 0/90 layups (with fibre fractions of 60%). SEM imaging revealed that 0/0 samples fractured as expected of a laminated UD-CFRP ILSS specimen, with fracture paths following the fibre axis causing delamination. Contrarily, the 0/90 ILSS specimens fractured, not only between the laminates, but also perpendicularly to interlaminar fracture such that cracks would also propagate through the thickness of the laminates. These micrographs are shown in Figure 9. A reduction of ILSS test strength and stiffness are noted in both [78] and [77] with a reduction of 10/20% being cited as fairly normal in both 0/0 and 0/90 cases. ILSS results published by Tual et al. [22] in fact, clarify that after a percentage reduction in the ILSS, the value then remains constant regardless of hygrothermal immersion times. Since ILSS is a matrix dominated property, the choice of matrix can be critical if the detrimental effects of hygrothermal conditioning are to be avoided. In a comparative study of hygrothermally aged CFRP using different matrices, epoxies were found to not only absorb a greater volume fraction of water upon aging than alternative plastics, but also to experience significantly higher percentage reductions in ILSS tests [49]. The aforementioned alternative plastics included isophthalic polyester, vinyl ester and urethane ester, perhaps the most promising of which is vinyl ester since it exhibited the least ILSS reduction due to hygrothermal conditioning and is also an economically

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σ(t) = (σ0 − σ∞ ) exp(−t/τ ) + σ∞

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3.6. Fibre volume fractions and damping Fibre volume fraction has a profound effect on ratio of longitudinal to transverse moisture flux [76]. In high fibre volume fraction CFRP, an increased initial water immersion temperature gives rise to reduced ratios of longitudinal to transverse desorption diffusion. The fundamental reason for this is down to increased capillarity from close range fibre-to-fibre contacts, resulting from the high fibre volume fractions. When fractions are overly high, we can plausibly suggest that insufficient wetting of fibres, and thus inadequate consolidation, might encourage long thin air gaps between fibres, which would presumably be conducive porous-architectures to capillary driven flow in the fibre axis. A possible means of circumventing non-wetting problems is to use lower viscosity resins, but this may of course have the coupled effect of reducing the mechanical properties of the composite. The hygrothermal effects of damping in a material is sometimes approximately understood to have an inverse relationship with its mechanical properties under the same hygrothermal conditions [81]. Equation 4, following Chamis et al. [82], relates the hygrothermomechanical response of polymer composite matrices to their glass transition temperatures. Here, PM 10

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ature, Tgw is the wet glass transition temperature, Tgd is a dry glass transition temperature and T0 is the reference temperature (corresponding to the reference property P0 ). The inverse of this, relating damping to hygrothermal conditioning is then represented by Equation 5 [81], where PD is a damping property. Equation 5 is a useful, simplistic equation that can be used to predict the specific damping capacity of CFRP, since it is based specifically on the glass transition (Tg) data. Fundamentally there are three important aspects of damping to note in relation to the carbon fibre volume fraction of UD-CFRP composites. First as would be expected, the specific damping capacity decreases as the fibre volume fraction increases. This is a function of a decrease in the volume of damping material (i.e. the matrix component of the composite). Second, thermal treatment (without moisture ingress) heightens CFRP damping as a function of an increasing fibre volume fraction. The third point of note is that under hygrothermal conditioning (i.e. thermal treatment of water immersed CFRP), there is an exponential increase in the damping of CFRP from a thermal-only treatment. This indicates that water ingress exacerbates the effect of heat on damping (and thus on mechanical performance assuming a directly inverse relationship).  0.5 Tgw − T PM = (4) P0 Tgd − T0 0.5  PD Tgd − T0 = P0 Tgw − T

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Figure 9. Failure modes of (a) 0◦ nad (b) 0/90◦ ILSS specimens for carbon/epoxy composite materials, from [78], reproduced with the permission of Springer.

Figure 10. ILSS predictions for CFRP using Equation 3 from [77], reproduced with the permission of Elsevier.

represents a mechanical property, P0 represents a reference property, T is a hygrothermal conditioning temper11

(5)

3.7. Hygrothermal effects on the Tg of CFRP Hygrothermal conditioning gives rise to a non-linear change in the glass transition temperature (Tg) of CFRP. A prevalent view of epoxy aging through hygrothermal treatment is that water ingress reduces Tg by means of plasticisation. This effect is essentially due to a waterbased disruption of intermolecular secondary interactions such as hydrogen bonding and Van der Waals interactions [83, 84], thereby increasing molecular chain mobility. In CFRP composites the situation is somewhat more complex as there are a plethora of interconnected mechanisms that govern the effects on Tg including; fibre effects on chain mobility, polymer deterioration at interfaces, polymer heterogeneity (from the presence of reinforcement), and regions within the matrix that are either polymer rich or polymer poor. In similitude with straight epoxies, CFRP experiences an initial decrease in Tg as a function of immersion time however, with continued immersion the Tg plateaus and at higher immersion times still, the Tg does in fact rise once again [76]. The rise in Tg has been ascribed at least, to bound-water within fully saturated systems causing pseudo-crosslinks within the epoxy limiting molecular movement. Furthermore, when all free volumes are filled there are further physically imposed limits on the molecular chain mobility, resulting in a Tg increase. Though molecular mobility is increased by temperature,

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ical properties of the materials that are currently used to build turbine blades. Glass fibre reinforced plastics (GFRP) are examples of structural composites that are unable to the requirements needed to build longer tidal turbine blades. At present there is no full-scale blade fatigue testing equipment though a novel centre, the Structural Composites Research Facility (SCRF), is under construction at the University of Edinburgh in the UK, with the specific aim of fatigue testing full-scale turbine blades at up to 1 Hz [87]. A facility of this scale will enable high rate testing of complete turbine blades to failure within a matter of months, improving the speed at which such structures can be analysed. Nevertheless, turbine blades are salt water immersed, and are concurrently and continuously subjected to flexural loads, causing blade materials to experience deform and stress cyclically. The scale as well as distribution of loads in tidal turbine blades differs from wind turbine blades in that forces exerted upon the blade underwater come from ambient pressurisation and de-pressurisation, from cavitation, turbulence and seabed boundary layer flow [88, 89, 90]. The SCRF will not be able to test full blades in immersed conditions and as such, smaller scale tests can still provide valuable information about the materials the blade is made up of. Cyclic deformations will decrease water penetrability into the materials of blades, such as CFRP, since there will be a regular squeezing-out of water from the free volumes, and indeed this will concurrently reduce Fickian diffusion. Cyclic loading in this sense, works against water penetration into blade materials, which ordinarily decreases the mechanical properties. Though this phenomenon could be interpreted as one that can improve the longevity of blade materials by reducing water ingress and hence material degradation, the reality is that cyclic loading is itself detrimental to the longevity of the very same materials. At present, the extent to which a material can be saved from water degradation as a function of loading is unknown, however, higher loads result in lower levels of water ingress and pre-stressed water-soaked CFRP do exhibit a degradation matrix dominated properties (ILSS) while fibre dominated properties (strength and modulus) are less affected [18]. It is important therefore, that test conditions reflect as closely as possible, the environmental conditions under which the material is loaded. Uni-axial test standards such as the BS-EN-ISO527-5:2009 [91], alongside flexural text standards such as the ASTM-D-790-02 (for 3-point flexure) [92] and the ASTM-D-6272-02 (for 4-point flexure) [93] are commonly accepted as applicable to the static and fatigue testing of FRP composites. Davies et al. [75] however, have considered dog-bone coupons as an alternative, superior method by which means FRP testing may be conducted. Their primary contention with straight edged samples in flexure relates to roller embedment, giving rise to premature failure in the FRP at the embedding site. This noted, making angular cuts to generate dog-bone shaped CFRP may not be ideal since composite flaying and delamination is an oft-cited problem [94, 95, 96, 97, 98].

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water immersion at elevated temperatures does not seem to depress the Tg further by any level of statistical significance during its initial stage of decrease [76]. The reasons for this are still unclear, however, it might simply be that the intermolecular secondary interactions disrupted by water ingress, would in fact be the same interactions that would otherwise be disrupted by thermal activity. If the maximum number of interactions is disrupted by water ingress, then there are no further bonds available for disruption by temperature. To validate this hypothesis nevertheless, would require some information on the rate at which heat and water individually disrupt intermolecular interactions, which as far as we are aware, is currently unavailable. Indications to fibre-matrix debonding and matrix microfractures in hygrothermally aged CFRP can be inferred from observed changes in the peak height and breadth of a tan-δ curve [76]. In the case of CFRP, both increased immersion times and increased temperatures of immersion yield broader tan-δ curve bases.

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3.8. Hygrothermal effects on microbond measurements Microbond measurements are sometimes used to develop an understanding of the fibre bond strength within a matrix material. Fibre bond strengths are definitive properties of fibre reinforced composites as they are ultimately responsible for the shear resistance between fibre and matrix, and hence, stress transfer between the individual composite components. Biro et al. [85] assessed the effects of hygrothermal treatment to microbond measurements using two different fibre/epoxy composites (T300/Epon-828 and AS4/Epon-828). Though they found that the extent of strength loss was dependent on the composite system tested, the generic trend was the same, this being an initial sharp drop in bond strength followed by a curve that was approximately asymptotic with the horizontal axis (treatment time). Biro et al. suggested three primary factors affecting the fracture mechanics of fibre-matrix microbond tests (in a pull-out mode). These include (a) chemical and secondary interactions at the interface (b) frictional resistance and (c) mechanical interlocking of adhesion. Physical alterations in the composite from its original state will cause a level of interface damage in each case. The choice of surface sizing on carbon fibres are key to optimising fibre-matrix compatibility and is a means to improving mechanical performance. Much of the effect of sizing is due to surface energy, which affects the affinity of the fibre to a wetting matrix material. Increased height differentials of fibre surface topography is a concurrent benefit with sizing fibres [86] since topographical effects relate invariably to interlocking of adhesion, which has been shown in many fibre reinforced composites to be a critical adhesive parameter for achieving superior mechanical performance. 4. Discussion The majority of tidal turbine blades are less than 10 m in length. This can be in part, attributed to the mechan-

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Ultimately, poor quality FRP cutting and manufacturing methods can have a profoundly adverse effect on mechanical and fatigue performance with low quality manufacture being reported to reduce the peak composite strength in fatigue by up to ca. 20% [75]. Fatigue testing of CFRP coupons is predominantly conducted outside of an aqueous environment after a certain period of water immersion. The validity of this approach is questionable since the presence of water during the test may profoundly affect its failure mechanism, and indeed the quickness of failure. An important paper by Siriruk and Penumadu [99] highlights the need for the improved standardisation of fatigue test methods for composites that are submerged. their work focused on carbon fibre-vinyl ester based composites. As noted earlier in this review, vinyl ester based composites are less absorptive to water than epoxies, though their mechanical properties are typically also inferior. Nevertheless, using vinyl-ester matrices in CFRP, Siriruk and Penumadu note that when fatigue tests were conducted on the composites while remaining immersed, the composites failed at much lower cycle numbers than when the same tests were conducted on composites that had been water conditioned and fatigue tested in air. It would seem that the inability of a composite to drain water from its bulk during fatigue loading allows the water trapped in pores to increase internal stresses within the bulk. This is because the water is incompressible and as the water deforms with the composite bulk, the water volume remains constant in the bulk causing pressure build up within the pores. This in turn leads to an exacerbated rate at which micro-fractures accumulate within the bulk of the composite, resulting in coupon failure. From this review, it is clear that hygrothermal conditioning is used as an accelerated aging method. But is it truly representative of long-term water immersion and the aging that occurs in composites under ambient conditions? Brown communicates in his 1991 paper that, “the majority of users do not believe that they properly simulate or predict service” [100]. Correlating hygrothermal aging to equivalent ambient aging is by no means trivial and as such, Brown’s concerns are entirely justifiable. While scholarly reports such as that of Feller [101] attempt to dispel skepticism towards hygrothermal aging as a representative approach to accelerating and mimicking the effects of real-time aging, there is still little information on the validity of the accelerated aging of composites. Comparative studies highlighting differences between hygrothermally aged composites against water immersed composites are available in number [102] however there have been very few attempts to correlate hygrothermally aged moisture absorption and properties against long-term water treatments for composites. This is something that has been done for epoxies such as Shell Epon epoxy resin [103] who correlated the accelerated aging of these polymers to equivalent real-time aging by assuming that the activation energy, E, follows an Arrhenius-like behaviour according to Equation 6, where kT is the aging rate at

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temperature T (in Kelvin), A is a constant and R is the ideal gas constant. CFRP composites are more complex than polymers as they are made up of carbon fibres and the polymer matrix. In composites, polymer activation energies are affected by molecular pinning at interfaces with the fibres and as such it is harder to accurately predict the equivalent real-time aging of hygrothermally aged composites. Experimental work can be undertaken by which means such predictions can be made, however, it can take decades to accomplish this. Davies et al. have done exactly this and the conclusion we draw from their work is that the accuracy of hygrothermal aging is very much dependent upon the polymer matrix that is used [104, 105]. The development of models that can predict real-time water aging years to hygrothermally aged composites will be important if hygrothermal aging is to become more practicable. kT = Ae−E/RT

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5. Conclusions

Hygrothermal aging of composites combines water ingress with the heat-accelerated aging. Water ingress and heat are reviewed and are noted to have detrimental effects on the mechanical performance of CFRP. The extent to which water and heat will affect CFRP performance will depend on (a) the volume of water absorbed into the composite (b) the temperature and heat exposure time (c) the type of polymer used as a matrix material (d) the concentrations and types of bulk and interfacial defects and (e) the mechanism by which means water enters the composite (i.e. wicking and diffusion). Though the hygrothermal approach is understood to be of considerable benefit in the accelerated design of aged and submerged composites, there is still no clear-cut means by which engineers can correlate hygrothermal aging temperatures and times to real-time submerged years. The approach is still invaluable as the understanding and design of composites submerged in water for long time periods can only realistically be undertaken if engineers are allowed to design both qualitatively and within short time-scales. Future directions should nevertheless focus on developing predictive models by which means engineers can more effectively correlate hygrothermally aged composites to time-aged submerged composites. 6. Acknowledgments The authors would like to acknowledge the European Union for funding this research through the following projects: MARINCOMP, Novel Composite Materials and Processes for Marine Renewable Energy, Funded under: FP7-People, Industry Academia Partnerships and Pathways (IAPP), Project reference: 612531. POWDERBLADE, Commercialisation of Advanced Composite Material Technology: Carbon-Glass Hybrid in Powder Epoxy for Large Wind Turbine Blades, Funded under:

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Horizon 2020, Fast Tract to Innovation Pilot, Project reference: 730747. References

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