sensors Article

Monitoring Pre-Stressed Composites Using Optical Fibre Sensors Sriram Krishnamurthy 1 , Rodney A. Badcock 2 , Venkata R. Machavaram 2 and Gerard F. Fernando 2, * 1 2

*

Engineering System Department, Royal Military College of Science, Shrivenham SN6 8LA, UK; [email protected] School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK; [email protected] (R.A.B.); [email protected] (V.R.M.) Correspondence: [email protected]; Tel.: +44-121-414-8244

Academic Editor: Jandro L. Abot Received: 19 March 2016; Accepted: 20 May 2016; Published: 28 May 2016

Abstract: Residual stresses in fibre reinforced composites can give rise to a number of undesired effects such as loss of dimensional stability and premature fracture. Hence, there is significant merit in developing processing techniques to mitigate the development of residual stresses. However, tracking and quantifying the development of these fabrication-induced stresses in real-time using conventional non-destructive techniques is not straightforward. This article reports on the design and evaluation of a technique for manufacturing pre-stressed composite panels from unidirectional E-glass/epoxy prepregs. Here, the magnitude of the applied pre-stress was monitored using an integrated load-cell. The pre-stressing rig was based on a flat-bed design which enabled autoclave-based processing. A method was developed to end-tab the laminated prepregs prior to pre-stressing. The development of process-induced residual strain was monitored in-situ using embedded optical fibre sensors. Surface-mounted electrical resistance strain gauges were used to measure the strain when the composite was unloaded from the pre-stressing rig at room temperature. Four pre-stress levels were applied prior to processing the laminated preforms in an autoclave. The results showed that the application of a pre-stress of 108 MPa to a unidirectional [0]16 E-glass/913 epoxy preform, reduced the residual strain in the composite from ´600 µε (conventional processing without pre-stress) to approximately zero. A good correlation was observed between the data obtained from the surface-mounted electrical resistance strain gauge and the embedded optical fibre sensors. In addition to “neutralising” the residual stresses, superior axial orientation of the reinforcement can be obtained from pre-stressed composites. A subsequent publication will highlight the consequences of pres-stressing on fibre alignment, the tensile, flexural, compressive and fatigue performance of unidirectional E-glass composites. Keywords: pre-stress; composites; autoclave; extrinsic fibre Fabry-Perot; fibre Bragg gratings; Strain; temperature; residual stresses

1. Introduction Advanced fibre reinforced organic matrix composites (AFRCs) are used extensively in applications where weight is at a premium, for example aerospace, automotive and wind energy. A number of techniques are available for manufacturing AFRCs, including filament winding, pultrusion, autoclaving, resin transfer molding (and variants), spray layup and hand layup [1]. The general production process involves the impregnation of the reinforcement or preform with a thermoplastic or thermosetting resin system. Depending on the manufacturing technique in question, the impregnated fibres or preforms are either stored until required (for example, prepregs), or used immediately

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(filament winding, pultrusion). In the case where semi-crystalline thermoplastic matrices are used, the polymer is heated above its glass transition temperature (amorphous polymers) or melting point (semi-crystalline polymers), shaped, consolidated and then cooled to ambient temperature. Thermoset-based preforms are generally heated to between 80 and 180 ˝ C to initiate the cross-linking of the resin and hardener; a catalyst is used sometimes to accelerate the cross-linking reactions. These reactions result in the conversion of the liquid resin system to a highly cross-linked solid polymer that is insoluble. After a predefined processing schedule, the composite is cooled to ambient temperature. Since the preforms associated with the production of composites are processed at elevated temperatures, residual stresses can develop in the composite due to a number of reasons [2–6]. Twigg et al. [7] proposed that the sources of residual stresses in composites can be classified as extrinsic or intrinsic. Examples of intrinsic sources of residual stresses include the following. (i)

(ii)

Resin shrinkage: The crystallisation process in semi-crystalline thermoplastics is accompanied by significant shrinkage. The rate of cooling of the thermoplastic matrix and the presence of nucleating agents will influence the degree of crystallinity and hence, the magnitude of the observed shrinkage. In a thermosetting resin, the shrinkage is due to the formation of covalent bonds during the cross-linking (polymerisation) of the functional groups in the resin and hardener. Mismatch in the coefficients of thermal expansion: Since there is generally a significant mismatch in the thermal expansions of the fibre and matrix, and adjacent plies of different fibre orientation, residual stresses will develop in the composite when it is cooled from the processing temperature.

Twigg et al. [8] reported that extrinsic factors such as an interaction between the preform and the processing tool, and the shape of the component can also contribute to the development of residual stresses. Other notable papers that addressed issues relating to residual stresses in organic-matrix composites can be found in references [9–12]. An overview of residual stresses in other classes of materials and the techniques for quantifying them can be found in reference [13]. The presence of residual stresses can result in the loss of dimensional stability and moreover, these stresses can be high enough to cause cracking within the matrix, even before the application of a mechanical load [14,15]. Hence, there has been significant research activity in the development of techniques to minimise or mitigate the influence of process-induced residual stresses in fibre reinforced composites. For example: (i) modification of the cure schedule [16–20]; (ii) electron beam curing [21]; (iii) modifying the interphase properties [22–25]; (iv) deployment of shape memory alloys [26,27]; (v) using expandable monomers [28,29] and (vi) pre-stressing the reinforcement or preform during the production of fibre reinforced composites [30]. Other processing related issues that can influence the performance of AFRCs include, fibre misalignment, waviness and voids [31–33]. The rationale for the current work was to develop a practical production method to reduce the magnitude of the residual fabrication strain in a unidirectional glass fibre reinforced composite. This was achieved by pre-stressing the preforms to a predetermined value prior to autoclave-based processing. The primary areas of novelty in the current work include the following: (i)

The pre-stressing rig was based on a flat-bed design thus enabling conventional vacuum-bagging and autoclave-based processing to be used. This enabled the production of void-free composites thus giving greater confidence in assessing and understanding the consequence of pre-stressing on the resultant physical and mechanical properties of the composite. (ii) A load-cell was integrated into the rig to enable the magnitude of the applied pre-stress to be monitored and quantified. (iii) An end-tabbing procedure was developed to ensure that only the prepregs within the end-tab regions were cross-linked or cured. This was achieved by cooling the prepregs outside the end-tab regions. This meant that the applied pre-stress was distributed uniformly over the preform. Furthermore, it enabled the relative orientation of the embedded optical fibre sensors to be maintained during processing. It is known that the displacement of optical fibres from their intended orientation can result in significant uncertainty in interpreting the output data.

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to be maintained during processing. It is known that the displacement of optical fibres from their intended orientation can result in significant uncertainty in interpreting the output data. Two classesof ofoptical opticalfibre fibresensors sensorswere wereintegrated integratedinto intothe theprepregs prepregsduring duringthe thelamination lamination Two classes stages—fibreBragg Bragggratings gratingsand andextrinsic extrinsicfibre fibreFabry-Perot Fabry-Perotinterferometric interferometricstrain strainsensors. sensors.Optical Optical stages—fibre fibre embedment procedures and protocols were developed to ensure their survival whilst fibre embedment procedures and protocols were developed to ensure their survival whilst pre-stressing and subsequent vacuum bag-based autoclave processing. pre-stressing and subsequent vacuum bag-based autoclave processing. The embedded embedded optical optical fibre fibre sensors sensors enabled enabled the theevolution evolution of ofstrain strainwithin withinthe theprepregs prepregstotobe be The monitored in real-time during the autoclave cure cycle, and also during the subsequent cooling monitored in real-time during the autoclave cure cycle, and also during the subsequent cooling ofthe thecomposite compositeto toambient ambienttemperature. temperature. of Surface-mounted electrical resistance Surface-mounted electrical resistance strain strain gauges gaugesand andoptical opticalfibre fibrestrain strainsensors sensorswere wereused usedto quantify the magnitude of the strain released when the composite was unclamped from the to quantify the magnitude of the strain released when the composite was unclamped from the pre-stressingrig. rig. pre-stressing

The above-mentioned attributes represent a significant advancement over the techniques that above-mentioned attributes represent a preforms significantand advancement the techniques that haveThe been reported previously for pre-stressing monitoringover the evolution of residual have been reported previously for pre-stressing preforms and monitoring the evolution of residual fabrication strains during the production of fibre reinforced composites. fabrication strains during the production of fibre reinforced composites. 2. Review of Techniques for Manufacturing Pre-Stressed Composites 2. Review of Techniques for Manufacturing Pre-Stressed Composites Pre-stressed composites are manufactured by applying and maintaining a pre-load on the Pre-stressed composites are manufactured by applying and maintaining a pre-load on the reinforcing fibres or laminated preforms during processing. Once the processing schedule has been reinforcing fibres or laminated preforms during processing. Once the processing schedule has been completed, the composite is cooled to room temperature and the previously applied pre-load is released. completed, the composite is cooled to room temperature and the previously applied pre-load is The following section presents a brief review of selected papers that have reported on techniques to released. The following section presents a brief review of selected papers that have reported on manufacture pre-stressed composites. techniques to manufacture pre-stressed composites. 2.1.Dead-Weight Dead-WeightLoading Loading 2.1. Withreference referencetotoFigure Figure1,1,Jorge Jorgeetetal. al.[34] [34]used usedaadead-weight dead-weighttechnique techniquetotoapply applythe therequired required With loadtotoE-glass E-glassfibre fibrebundles bundleswhich whichwere weretraversed traversedaround arounda aseries seriesofofsteel steelpins. pins.They Theyprepared prepared load E-glass/polyester resin composites using a wet-layup process. The composite was cured room E-glass/polyester resin composites using a wet-layup process. The composite was cured atatroom temperature for twenty four hours, followed by three hours of post-curing at 80 °C. The applied temperature for twenty four hours, followed by three hours of post-curing at 80 ˝ C. The applied pre-loadwas wasininthe therange range0–100 0–100N. N. pre-load

Figure illustration of of the the pre-stressing pre-stressingmethod methoddeveloped developedbybyJorge Jorge [34] where Figure1.1. Schematic Schematic illustration et et al. al. [34] where the the reinforcing fibres were wound around a series of pins; the pre-load was applied using weights. reinforcing fibres were wound around a series of pins; the pre-load was applied using weights. “P” “P” represents load fibre bundle. represents thethe load onon thethe fibre bundle.

2.2.V-Slot-Based V-Slot-BasedPre-Stressing Pre-Stressing 2.2. Schulteand and Marissen used aluminium withV-shaped two V-shaped slots and a pair of Schulte Marissen [35][35] used aluminium platesplates with two slots and a pair of matched matched loading-pins to preforms. pre-stress preforms. A schematic of their pre-stressing riginis loading-pins to pre-stress A schematic illustrationillustration of their pre-stressing rig is shown ˝ ˝ Figure 2. They prepared hybrid (0 -Kevlar/90 -carbon epoxy) cross-ply composites with a [0/90/90/0]

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shown in Figure 2. They prepared hybrid (0°-Kevlar/90°-carbon epoxy) cross-ply composites with a shown in Figure 2. They prepared hybrid (0°-Kevlar/90°-carbon epoxy) cross-ply composites with a [0/90/90/0] lay-up sequence. The maximum pre-strain achieved using their pre-stressing rig was [0/90/90/0] lay-up sequence. The maximum pre-strain achieved using their pre-stressing rig was lay-up sequence. The maximum achieved using their pre-stressing rig was 1.1%. The preform 1.1%. The preform was cured inpre-strain a hot-press at 125 °C. 1.1%. The preform was cured in a hot-press at 125 °C. was cured in a hot-press at 125 ˝ C.

Figure 2. 2. Schematic illustration illustration of the the V-slot mechanical mechanical pre-stressing method method used by by Schulte and and Figure Figure 2. Schematic Schematic illustration of of the V-slot V-slot mechanical pre-stressing pre-stressing method used used by Schulte Schulte and Marissen [35]. Marissen Marissen [35]. [35].

2.3. Filament Filament Winding 2.3. 2.3. Filament Winding Winding With reference to Figure Figure 3, Hadi Hadi and Ashton Ashton [36] demonstrated demonstrated that pre-stressed pre-stressed composites With With reference reference to to Figure 3, 3, Hadi and and Ashton [36] [36] demonstrated that that pre-stressed composites composites can be be manufactured manufactured via filament winding. Here the tension on the fibre bundles was controlled as a can via filament winding. Here the tension on the fibre bundles was controlled can be manufactured via filament winding. Here the tension on the fibre bundles was controlled asasa means of applying the required pre-tension. In the filament winding process, resin-impregnated ameans meansofofapplying applyingthe therequired required pre-tension. pre-tension. In In the the filament filament winding winding process, process, resin-impregnated resin-impregnated fibre bundles bundles are are traversed traversed left left and and right right continuously continuously as as they they are are wound wound around around aa rotating rotating mandrel fibre fibre bundles are traversed left and right continuously as they are wound around a rotating mandrel mandrel to produce produce the preform. preform. The relative relative traverse and and rotation speeds speeds of the the impregnated fibres fibres and the the to to produce the the preform. The The relative traverse traverse and rotation rotation speeds of of the impregnated impregnated fibres and and the mandrel respectively dictate the winding angle of the reinforcing fibres. Hadi and Ashton [36] mandrel thethe winding angle of theofreinforcing fibres. Hadi Ashton prepared mandrel respectively respectivelydictate dictate winding angle the reinforcing fibres.and Hadi and[36] Ashton [36] prepared unidirectional pre-stressed composites using a flatmandrel. square-sided mandrel. Rose and unidirectional pre-stressed composites using a flat square-sided Rose and Whitney [37] also prepared unidirectional pre-stressed composites using a flat square-sided mandrel. Rose and Whitney cross-ply [37] also prepared cross-ply preforms using filament winding and the prepared preformspre-stressed using filament winding and the preforms were cured in Whitney [37] also pre-stressed prepared cross-ply pre-stressed preforms using filament winding and the preforms were cured in an autoclave. an autoclave. preforms were cured in an autoclave.

Figure 3.3.Schematic Schematic illustration the production of pre-stressed using filament illustration of the of production of pre-stressed compositescomposites using filament winding [36]. Figure 3. Schematic illustration of the production of pre-stressed composites using filament winding [36]. winding [36].

2.4. Hydraulic Pre-Stress Rig 2.4. Hydraulic Pre-Stress Rig 2.4. Hydraulic Pre-Stress Rig This is a variation on the dead-weight loading method illustrated in Figure 1 where Tuttle et al. [38] This is a variation on the dead-weight loading method illustrated in Figure 1 where Tuttle et al. [38] is a variation on to thepre-stress dead-weight loadingThe method illustrated in Figure 1 where Tuttle et al. on [38]a usedThis a hydraulic device prepregs. processing of the preform was carried out used a hydraulic device to pre-stress prepregs. The processing of the preform was carried out on a used a hydraulic device to pre-stress prepregs. The processing of the preform was carried out on a hot-press as illustrated in Figure 4. The hydraulic loading fixture was mounted on a horizontal hot-press as illustrated in Figure 4. The hydraulic loading fixture was mounted on a horizontal hot-press as illustrated in Figure 4.plies The remained hydraulic parallel loading fixture was mounted horizontal frame, frame, such that the pre-stressed to the heated platenson of athe hot-press. One frame, such that the pre-stressed plies remained parallel to the heated platens of the hot-press. One such that pre-stressed plies remained parallel to the heated platens of theend hot-press. One end to of the end of thethe preform was attached to a fixed loading-rod, and the opposite was connected end of the preform was attached to a fixed loading-rod, and the opposite end was connected to the preform a fixed opposite end was connected to the that hydraulic hydraulicwas ramattached throughto which theloading-rod, desired loadand wasthe applied to the prepregs. They reported in the hydraulic ram through which the desired load was applied to the prepregs. They reported that in the ram through which the desiredoccurred load wasbetween applied to prepregs. They reported thatatin high the initial initial experiments, slippage thethe ply and loading-rod surface fibre initial experiments, slippage occurred between the ply and loading-rod surface at high fibre experiments, slippage occurred between the ply and loading-rod surface at high fibre pre-stress levels; pre-stress levels; the slippage was eliminated by knurling the surface of the loading rods. The load pre-stress levels; the slippage was eliminated by knurling the surface of the loading rods. The load the slippage was eliminated by knurling theasurface of gauge the loading rods.attached The loadonapplied to the fibres applied to the fibres was monitored using pressure that was the hydraulic ram applied to the fibres was monitored using a pressure gauge that was attached on the hydraulic ram was line.monitored using a pressure gauge that was attached on the hydraulic ram line. line.

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Figure 4. Schematic illustration of the hydraulic cylinder-based pre-stressing rig used by Figure4.4.Schematic Schematicillustration illustration ofthe the hydrauliccylinder-based cylinder-based pre-stressingrig rig usedby by Tuttleetetal. al. [38]. Figure Figure 4. al. Schematic illustrationof of thehydraulic hydraulic cylinder-basedpre-stressing pre-stressing rigused used byTuttle Tuttle et al.[38]. [38]. Tuttle et [38].

2.5.Horizontal HorizontalTensile TensileLoading Loading Machine 2.5. 2.5. Loading Machine Machine 2.5. Horizontal Horizontal Tensile Tensile Loading Machine Motahhari and Cameron [39]produced produced pre-stressedcomposites compositeswith withthe theaid aidofofaahorizontal horizontal Motahhari and Cameron [39] Motahhari and Cameron [39] produced pre-stressed pre-stressed composites with the aid of a horizontal horizontal Motahhari and Cameron [39] produced pre-stressed composites with the aid of a tensiletesting testingmachine machineas asillustrated illustratedininFigure Figure5.5.In Intheir theirdesign, design, theends endsofofthe thefibres fibreswere were secured tensile tensile testing as in 5. design, the the of were secured secured tensile testing machine machine as illustrated illustrated in Figure Figure 5. In In their their the ends ends of the the fibres fibres secured usingclamps clamps andthe theapplied applied loadwas was monitored viaaadesign, load-cell. A U-shaped moldwere wasused used to using and load monitored via load-cell. A U-shaped mold was using clamps and the applied applied load load was was monitored monitored via via aa load-cell. load-cell. A U-shaped mold was used to to using clamps and the A U-shaped mold was used to manufacturethe thecomposites. composites.AAsimilar similarmethod methodwas wasused usedby byLee Lee[40] [40]and andAli Ali[41] [41]totopre-stress pre-stressprepreg. prepreg. manufacture manufacture the composites. A similar method was used by Lee [40] and Ali [41] to pre-stress prepreg. manufacture the composites. similar method was usedthe by magnitude Lee [40] andofAli to pre-stress This pre-stressing technique A was capable ofmeasuring measuring the[41] applied pre-loadprepreg. during This capable magnitude the This pre-stressing pre-stressing technique technique was was capable of of measuring the the magnitude of of the applied applied pre-load pre-load during during This pre-stressing technique was capable of measuring the magnitude of the applied pre-load during the curing process. the curing process. the process. the curing curing process.

Figure5.5. 5. Schematic Schematic illustrationof thehorizontal horizontaltensometer-based tensometer-basedpre-stressing pre-stressingrig rigused usedby byMotahhari Motahhari Figure Figure ofthe the horizontal tensometer-based pre-stressing rig used by Motahhari Figure 5.Schematic Schematicillustration illustration of the horizontal tensometer-based pre-stressing rig used by Motahhari andCameron Cameron[39]. [39]. and Cameron [39]. and and Cameron [39].

2.6.Fibre FibrePre-Stressing Pre-StressingRig Rig 2.6. 2.6. Fibre Pre-Stressing Rig 2.6. Fibre Pre-Stressing Rig Thepre-stressing pre-stressingrig rigdesign designdeveloped developed byZhao Zhao and Cameron [42] is illustrated in Figure The Cameron is in 666 The pre-stressing byby Zhao andand Cameron [42] [42] is illustrated in Figure 6 where The pre-stressingrig rigdesign designdeveloped developed by Zhao and Cameron [42] is illustrated illustrated in Figure Figure where thefibres fibreswere were first wound onto steel frame andthen then was attached to a secondary frame. where the first steel and was to frame. the fibres wound onto aonto steelaaaframe and then wasitititattached to a secondary frame. The where the were fibresfirst were first wound wound onto steel frame frame andit then was attached attached to aa secondary secondary frame. The pre-stress loading of the fibres on the frame was achieved via a tensile test machine. Once the The pre-stress loading of the fibres on the frame was achieved via a tensile test machine. Once the pre-stress loading of theof fibres on theon frame achieved via a tensile machine. Once theOnce required The pre-stress loading the fibres the was frame was achieved via atest tensile test machine. the required load was reached, locking-bolts were used to maintain the fibres in the stressed state. The required was reached, used maintain in stressed state. load wasload reached, were usedwere to maintain fibres inthe thefibres stressed state. The pre-stressed required load waslocking-bolts reached, locking-bolts locking-bolts were used to tothe maintain the fibres in the the stressed state. The The pre-stressed preform wasacured cured usingaahot-press. hot-press. pre-stressed preform was using preform was cured using hot-press. pre-stressed preform was cured using a hot-press.

Figure Schematic pre-stressing rigrig developed by by Zhao andand Cameron; this this rig Figure6.6.Schematic Schematic illustrationof ofthe thefibre fibre pre-stressing developed Zhao Cameron; Figure Figure 6. 6. Schematic illustration illustration of of the the fibre fibre pre-stressing pre-stressing rig rig developed developed by by Zhao Zhao and and Cameron; Cameron; this this was used in combination with a hot-press [42]. rig was used in combination with a hot-press [42]. rig rigwas wasused used in incombination combinationwith withaahot-press hot-press[42]. [42].

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2.7.Biaxial BiaxialLoading LoadingFrame Frame 2.7. Jevonsetetal.al. adapted the design originally proposed and [42] Cameron [42] and Jevons [43][43] adapted the design originally proposed by Zhaoby andZhao Cameron and developed a pre-stressing method for pre-stressing cross-ply laminates could be processed in an autoclave. adeveloped method for cross-ply laminates that could bethat processed in an autoclave. Their design Their design consisted of C-channel sections with four clamps linked to the frame by bolts consisted of C-channel sections with four clamps linked to the frame by bolts as illustrated in Figure as 7. illustrated in end-tab Figure 7.regions, The intended end-tab regions, at the ends of were the laminated The intended at the ends of the laminated prepregs, cured in aprepregs, hot-press were and cured in a hot-press Several mm diameter holes were drilled through the end-tabbed. Several 10and mmend-tabbed. diameter holes were10drilled through the end-tabs and this assembly end-tabs and this assembly was bolted on to the loading frame. The required pre-stress was applied was bolted on to the loading frame. The required pre-stress was applied through the loading pins through the loading using the a mechanical test machine; locking-bolts clamps using a mechanical testpins machine; locking-bolts between the the clamps were thenbetween tightenedthe when the were then tightened when theapplied desiredtolevel of pre-load was applied thepre-stressed prepregs. The frame was with desired level of pre-load was the prepregs. The frame withtothe prepregs the pre-stressed and prepregs was vacuum-bagged vacuum-bagged processed in an autoclave. and processed in an autoclave.

Figure Figure7.7.Schematic Schematicillustration illustrationof ofthe thebiaxial biaxialloading loadingframe-based frame-basedprepreg prepregpre-stressing pre-stressingmethod method[43]. [43].

Inconclusion, conclusion, aanumber numberof ofingenious ingeniousdevices deviceshave havebeen beenused usedfor formanufacturing manufacturingpre-stressed pre-stressed In composites. The following following sections sections report report on on the the design design and and development development of of aa fibre fibrepre-stressing pre-stressing composites. technique that that permitted permitted the the preforms preforms to tobe beprocessed processedininan anautoclave. autoclave. The The development development of of the the technique residualstrain strainininthe the composite during processing monitored in-situ embedded optical residual composite during processing waswas monitored in-situ usingusing embedded optical fibre fibre sensors sensors [44]. [44]. 3.3. Materials and Methods 3.1. 3.1.Pre-Stressing Pre-StressingRig RigDesign Design The Theprimary primarydesign designrequirements requirementsfor forthe thepre-stressing pre-stressingrig rigwere wereas asfollows: follows:(i) (i)aameans meansto toapply apply and monitor the magnitude of the pre-stress on the preform during and after processing; and monitor the magnitude of the pre-stress on the preform during and after processing; (ii) (ii) aa facility to enable optical fibres to be integrated into the preform and for the entry/exit points to facility to enable optical fibres to be integrated into the preform and for the entry/exit points tobe be protected; protected;and and(iii) (iii)processing processingthe thepre-stressed pre-stressedpreforms preformsin inan anautoclave autoclaveusing usingconventional conventionalvacuum vacuum bagging baggingprocedures. procedures. Schematic of the theflat-bed flat-bedpre-stressing pre-stressingrigrig design presented in Figure Schematic illustrations illustrations of design areare presented in Figure 8a,b.8a,b. The The basic mode of operation of the rig is the controlled separation of two sections of the flat-bed basic mode of operation of the rig is the controlled separation of two sections of the flat-bed with the with theload-screws. aid of load-screws. With reference to 8a, Figure two blocks serve the flat-beds of which aid of With reference to Figure two8a, blocks serve as the as flat-beds of which one is one is fixed to a base-plate and the other is able to slide on guide bars. The guide bars are attached to fixed to a base-plate and the other is able to slide on guide bars. The guide bars are attached to the the fixed block shown Figure 8b.These Theseguide guidebars barsalso alsoaid aidin inthe the alignment alignment of of the fixed block as as shown in in Figure 8b. the clamps clamps of of the the movable and fixed blocks. The applied load on the preform (when it is clamped in the rig) is monitored movable and fixed blocks. The applied load on the preform (when it is clamped in the rig) is via a load-cell is housed a recess onathe fixed as illustrated in Figure 8b. The end monitored viathat a load-cell thatwithin is housed within recess onblock the fixed block as illustrated in Figure 8b. of the load-screw is positioned in contact with the load-cell such that it is capable of transmitting the The end of the load-screw is positioned in contact with the load-cell such that it is capable of applied pre-stress on the laminated The locking bolts illustrated in Figure are used transmitting the applied pre-stressprepregs. on the laminated prepregs. The locking bolts 8a,b illustrated in to lock the position of the movable block and this is achieved by clamping it to the base-plate after Figures 8a,b are used to lock the position of the movable block and this is achieved by clamping it to applying the required pre-stress the end-tabbed prepregs. the base-plate after applying thetorequired pre-stress to the end-tabbed prepregs.

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(a)

(b) Figure8. 8. (a) (a) Side-elevation Side-elevation of of the the flatbed flatbed rig rig for for applying applying the the desired desired pre-stress pre-stress on on laminated laminated prepregs; prepregs; Figure (b) Top-elevation of the flat-bed pre-stressing rig. (b) Top-elevation of the flat-bed pre-stressing rig.

The load-cell used in this study was Model No: 060-4771-01-08, RDP Electronics Ltd, The load-cell used in this study was Model No: 060-4771-01-08, RDP Electronics Ltd, (Wolverhampton, UK). The capacity of the load cell was 44.5 kN in compression and it was rated for (Wolverhampton, UK). The capacity of the load cell was 44.5 kN in compression and it was rated for operations up to 121 °C. The load-cell was calibrated by applying a static compressive load using a operations up to 121 ˝ C. The load-cell was calibrated by applying a static compressive load using pre-calibrated Instron mechanical test machine, model 8501 (Instron, High Wycombe, UK). The a pre-calibrated Instron mechanical test machine, model 8501 (Instron, High Wycombe, UK). The calibration test was conducted at room temperature and the rate of loading was 0.125 kN/s. Static calibration test was conducted at room temperature and the rate of loading was 0.125 kN/s. Static compression tests were conducted at 70, 100 and 120 °C and these tests were carried out using a compression tests were conducted at 70, 100 and 120 ˝ C and these tests were carried out using a mechanical test machine (Instron model 1195) equipped with an air-circulating oven. mechanical test machine (Instron model 1195) equipped with an air-circulating oven. 3.2. Evaluation of the Pre-Stressing Rig 3.2. Evaluation of the Pre-Stressing Rig The pre-stressing rig was evaluated initially by clamping a steel plate and loading it in tension. The pre-stressing rig was evaluated initially by clamping a steel plate and loading it in tension. The steel plate was instrumented with an array of surface-mounted electrical resistance strain The steel plate was instrumented with an array of surface-mounted electrical resistance strain gauges gauges as shown in Figure 9. Mechanical loading was applied to the steel plate via the load-screw as as shown in Figure 9. Mechanical loading was applied to the steel plate via the load-screw as described described previously. The magnitude of the applied load and strain were monitored using the previously. The magnitude of the applied load and strain were monitored using the load-cell on the load-cell on the rig and the surface-mounted electrical resistance strain gauges respectively. rig and the surface-mounted electrical resistance strain gauges respectively.

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9. Schematic illustrationof of the the location of the electrical resistance strain gauges strain Figure 9.Figure Schematic illustration location ofsurface-mounted the surface-mounted electrical resistance that were used to monitor the strain on a steel plate when it was loaded on the flat-bed pre-stressing rig. gauges that were used to monitor the strain on a steel plate when it was loaded on the flat-bed pre-stressing rig. Sensors 3.3. Fibre Optic

The optical fibre sensors used in this study were (i) extrinsic fibre Fabry-Perot interferometric 3.3. Fibre Optic Sensors

(EFPI) strain sensors and (ii) fibre Bragg grating (FBG) strain and temperature sensors. Single-mode fibres use in at 800 and were 1500 nm (SM 800 and SMFabry-Perot 1500, Fibercore Ltd., Theoptical optical fibredesigned sensorsforused thisnm study (i) extrinsic fibre interferometric Southampton, UK) were used to fabricate the sensors. The EFPI sensors with cavity lengths in (EFPI) strain sensors and (ii) fibre Bragg grating (FBG) strain and temperature sensors. Single-mode the range 50 to 100 µm were fabricated in-house using SM 800 (5.6/125 µm) optical fibres and 128 µm optical fibres designed for use at 800 nm and 1500 nm (SM 800 and SM 1500, Fibercore Ltd., precision bore fused silica capillaries. The details of the fabrication process and the mode of operation Southampton, UK) were used to fabricate the[44–46]. sensors. sensors on with cavity lengths of the sensors have been published previously TheThe FBGsEFPI were inscribed SM 1550 (9/125 µm) in the range 50optical to 100fibres µmusing werethefabricated using SM 800 µm) optical fibres phase maskin-house technique in conjunction with(5.6/125 an excimer laser operating at 248and nm 128 µm (Bragg Star). precision bore fused silica capillaries. The details of the fabrication process and the mode of

operation of Interrogation the sensors ofhave been published 3.3.1. the EFPI Strain Sensor previously [44–46]. The FBGs were inscribed on SM 1550 (9/125 µm) optical fibres using the phase mask technique in conjunction with an excimer laser A super-luminescent diode (SLD) operating at a center wavelength of 850 nm was used operating at 248 nm (Bragg Star). to illuminate the EFPI sensor and the resulting interference fringes were detected using a CCD spectrometer (HR 2000, Ocean Optics, Oxford, UK). The absolute cavity length was measured from the

3.3.1. Interrogation of the EFPI Sensorequation: interference spectrum usingStrain the following

A super-luminescent diode (SLD) operatingλ1at λ2 a center wavelength of 850 nm was used to d“ (1) 2.∆λ illuminate the EFPI sensor and the resulting interference fringes were detected using a CCD spectrometer Optics, corresponding Oxford, UK).toThe absoluteintensities cavity length was measured from where (HR λ1 and2000, λ2 areOcean the wavelengths the maximum of two adjacent bright fringes and ∆λ is the free spectral range. The longitudinal strain on the sensor is expressed as the ratio the interference spectrum using the following equation: of the change in cavity length to the gauge length of the sensor. 3.3.2. FBG Strain and Temperature Sensor

d

12 2.

(1)

The FBG sensors were monitored using a Bragg interrogation unit (Fiberpro Ltd., Dae-Jeon, Korea) wavelength sweep laser andto a detector. The influence of strainof(ε)two andadjacent where λequipped 1 and λ2 with are an theinbuilt wavelengths corresponding the maximum intensities temperature (T) on a fibre Bragg grating can be expressed as [47]: bright fringes and Δλ is the free spectral range. The longitudinal strain on the sensor is expressed as ´ ¯ the ratio of the change in cavity length ∆λ Bto the gauge length of the sensor. λB

“ α f ibre ` η ∆T ` p1 ´ pe qε

3.3.2. FBG Strain and Temperature Sensor

λB p“ 2Λneff q

(2)

The FBG sensors were monitored using a Bragg interrogation unit (Fiberpro Ltd., Dae-Jeon, Korea) equipped with an inbuilt wavelength sweep laser and a detector. The influence of strain (ε) and temperature (T) on a fibre Bragg grating can be expressed as [47]:

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constant of the fibre; and η is the thermo-optic coefficient of the core. The response of the Sensors sensing 2016, 16, 777FBGs can be expressed as: 9 of 24 temperature



Bα where λB (=2Λneff ) is the Bragg resonance T wavelength;  fibre is the coefficient of thermal expansion (3) B and ( fibre  )grating period; pe is the photo-elastic of the fibre; neff is the effective index of the core Λ isthe constant of the fibre; and η is the thermo-optic coefficient of the core. The response of the temperature Thesensing FBGs FBGs usedcan inbe this work as: were produced using the same phase mask and laser irradiation expressed ∆λ B conditions. The temperature sensor was produced by encapsulating a FBG inside a fused capillary in ∆T “ (3) λ B pα f ibre ` ηq

a strain-free condition whilst an unprotected FBG was used to monitor the strain and temperature. The FBGs this work were produced the same mask and the laserwavelength irradiation The axial strain on used the in unprotected FBG was using calculated byphase decoupling shifts conditions. The temperature sensor was produced by encapsulating a FBG inside a fused capillary in a obtained from the FBG temperature sensor. strain-free condition whilst an unprotected FBG was used to monitor the strain and temperature. The axial strain on the unprotected FBG was calculated by decoupling the wavelength shifts obtained from 3.4. Preparation of Pre-Stressed Composites the FBG temperature sensor.

16-plys of unidirectional prepregs were laminated using conventional procedures. In order to 3.4. Preparation of Pre-Stressed Composites apply a pre-stress to the laminated prepregs, it was necessary to attach end-tabs. This was carried 16-plys of unidirectional prepregs were laminated using conventional procedures. In order to out by apply co-curing aluminum end-tabs ontoit the designated areas in the preform. Aout schematic a pre-stress to the laminated prepregs, was necessary to attach end-tabs. This was carried illustration of the sequence operations associated preparation of the end-tabbed by co-curing aluminumof end-tabs onto the designatedwith areasthe in the preform. A schematic illustration preform of the sequence of operations associated with the preparation of the end-tabbed preform is shown in is shown in Figure 10. Figure 10.

Figure 10. Schematic illustration depicting depicting thethe sequence of operations that were carried out to co-cure Figure 10. Schematic illustration sequence of operations that were carried out to the aluminium end-tabs on to the prepregs (within the end-tab region). co-cure the aluminium end-tabs on to the prepregs (within the end-tab region).

It was necessary to cool the section of the preform in the immediate vicinity of the end-tab during

It was necessary to cool the section of the preform in the immediate vicinity of the end-tab the co-curing process; this was done to ensure that the curing only occurred within the end-tabbed during the co-curing thisof this wasjigdone thethecuring only occurred within the regions. Schematicprocess; illustrations to coolto theensure preformthat outside end-tabbed region are shown end-tabbed regions. of this jig to cool preform outside the end-tabbed in Figure 11a,b. Schematic Water from a illustrations mains-tap was circulated through the the rig and the output was directed to a sink. region are shown in Figure 11a,b. Water from a mains-tap was circulated through the rig and the output was directed to a sink.

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(a)

(b) Figure Figure 11. 11. (a) (a) Plan-view Plan-view of of the the cartridge-based cartridge-based heating heating of of the the end-tabbed end-tabbed region region and and the the water-based water-based cooling of the preform in the vicinity of the heated region; (b) Side-view of the heating/cooling cooling of the preform in the vicinity of the heated region; (b) Side-view of the heating/cooling systems systems used in the production of the end-tabbed used in the production of the end-tabbed preform.preform.

Aluminium alloy (5251-H2, Metalfast Ltd., Swindon, UK) end-tabs were cut to size (30 (l) mm × Aluminium alloy (5251-H2, Metalfast Ltd., Swindon, UK) end-tabs were cut to size (30 (l) mm ˆ 200 (w) mm × 1.5 (t) mm) and abraded on both sides. They were then degreased with acetone and 200 (w) mm ˆ 1.5 (t) mm) and abraded on both sides. They were then degreased with acetone and dried. The end-tabs were aligned with the ends of the preform and cured in a hot-press at 120 ˝°C for dried. The end-tabs were aligned with the ends of the preform and cured in a hot-press at 120 C for 1 h. After the end-tabs were co-cured on the ends of the preform, 10 mm diameter holes were drilled 1 h. After the end-tabs were co-cured on the ends of the preform, 10 mm diameter holes were drilled at at regular intervals to enable it to be clamped to the pre-stressing rig. Subsequent to drilling, the regular intervals to enable it to be clamped to the pre-stressing rig. Subsequent to drilling, the holes holes were cleaned using a de-burring tool to avoid puncturing the vacuum bag. Prior to clamping the were cleaned using a de-burring tool to avoid puncturing the vacuum bag. Prior to clamping the end-tabbed preform to the pre-stressing rig, the rig was degreased and a coating of a silicone release end-tabbed preform to the pre-stressing rig, the rig was degreased and a coating of a silicone release agent (Rocol, PR, RS Components Ltd., Bristol, UK) was applied. The prepregs that were previously agent (Rocol, PR, RS Components Ltd., Bristol, UK) was applied. The prepregs that were previously co-cured with the aluminium end-tabs were placed between the serrated clamp plates and bolted to co-cured with the aluminium end-tabs were placed between the serrated clamp plates and bolted to the pre-stressing rig using M12 bolts. A torque of 20 Nm was applied to each of the bolts using a the pre-stressing rig using M12 bolts. A torque of 20 Nm was applied to each of the bolts using a torque wrench. torque wrench. The desired mechanical pre-stress was applied to the end-tabbed prepregs by means of the The desired mechanical pre-stress was applied to the end-tabbed prepregs by means of the load-screw and the applied load was monitored via the load-cell. Once the required pre-stress was load-screw and the applied load was monitored via the load-cell. Once the required pre-stress was applied, the locking bolts were tightened to 20 Nm; this clamped the moving block to the base plate. applied, the locking bolts were tightened to 20 Nm; this clamped the moving block to the base plate. The prepreg assembly was then vacuum bagged using conventional procedures and cured in the The prepreg assembly was then vacuum bagged using conventional procedures and cured in the autoclave as outlined in Section 3.5. autoclave as outlined in Section 3.5. During curing, the strain development in the prepregs/composite was monitored in-situ using During curing, the strain development in the prepregs/composite was monitored in-situ using the the EFPI and FBG sensors. After curing and cooling to room temperature, the vacuum bag was EFPI and FBG sensors. After curing and cooling to room temperature, the vacuum bag was removed. removed. ERSG sensors were bonded on the surface of the composite in such a manner that that they ERSG sensors were bonded on the surface of the composite in such a manner that that they were were above the embedded optical fibres. A schematic illustration of the relative position of the ERSG above the embedded optical fibres. A schematic illustration of the relative position of the ERSG is is shown in Figure 12. The ERSG were bonded using cyanoacrylate adhesive (TML Ltd., Doncaster, shown in Figure 12. The ERSG were bonded using cyanoacrylate adhesive (TML Ltd., Doncaster, UK). The previously induced fabrication strain in the cured composite was released by slackening UK). The previously induced fabrication strain in the cured composite was released by slackening the the load-screw, locking bolts and the clamp bolts on the pre-stressing rig. The magnitude of the load-screw, locking bolts and the clamp bolts on the pre-stressing rig. The magnitude of the strain strain released upon unclamping the cured composite from the pre-stressing rig was recorded using released upon unclamping the cured composite from the pre-stressing rig was recorded using the the surface-mounted electrical and the embedded optical sensors. surface-mounted electrical and the embedded optical sensors.

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Figure 12. Side and plan-views of the relative locations of the surface-mounted electrical resistance Figure 12. Side and plan-views of the relative locations of the surface-mounted electrical resistance strain and theplan-views embeddedof optical fibre sensors. Figuregauges 12. Side and the relative locations of the surface-mounted electrical resistance strain gauges and the embedded optical fibre sensors.

strain gauges and the embedded optical fibre sensors.

3.5. Autoclave Processing of the Laminated Prepregs and the Pre-Stressing Rig Assembly 3.5. Autoclave Processing of the Laminated Prepregs and the Pre-Stressing Rig Assembly 3.5. Autoclave Processing of the Laminated Prepregs and the Pre-Stressing Rig Assembly A schematic illustration of the vacuum bagging assembly that was used for the production of A schematic illustration of the vacuum bagging assembly that was used for the production of the the conventional composites in Figure A similar assembly wasfor used processing A schematiccomposites illustration ofpresented the vacuum bagging assembly that was for thefor production of conventional is is presented in Figure 13. A13. similar assembly was used used processing the the prepregs pre-stressing rig. 13. A similar assembly was used for processing the pre-stressed conventional composites is presented in Figure pre-stressed prepregs in in thethe pre-stressing rig. the pre-stressed prepregs in the pre-stressing rig.

Figure Schematic illustration illustration ofof thethe keykey components of the materials contained within the vacuum Figure 13.13. Schematic components of the materials contained within the bag in the autoclave (the pre-stressing rig is not shown). vacuum bagSchematic in the autoclave (the pre-stressing is not shown). Figure 13. illustration of the key rig components of the materials contained within the vacuum bag in the autoclave (the pre-stressing rig is not shown). autoclave used thisstudy study was was custom-modified Ltd., Poole, UK)UK) to include TheThe autoclave used ininthis custom-modified(Aeroform (Aeroform Ltd., Poole, to include vacuum and pressure-rated input and output ports for the optical fibres to enable real-time strain vacuum and pressure-rated input and output ports for the optical fibres to enable real-time strain The autoclave used in this study was custom-modified (Aeroform Ltd., Poole, UK) toand include temperature monitoring. Prior to vacuum bagging the preform and the pre-stressing rig assembly, and temperature monitoring. Prior vacuum bagging thethe preform and the pre-stressing rig assembly, vacuum and pressure-rated inputto and output ports for optical fibres to enable real-time strain pre-shaped silicone blocks were packed around the rig to prevent puncturing of the vacuum bag when pre-shaped silicone blocks were around the rig prevent puncturing of the rig vacuum bag and temperature monitoring. Priorpacked to vacuum bagging theto preform and the pre-stressing assembly, the vacuum was applied. The vacuum-bagged preform and the pre-stressing rig assembly were placed when the vacuum was applied. The vacuum-bagged preform and the pre-stressing rig assembly pre-shaped silicone blocks were packed around the rig to prevent puncturing of the vacuum bag inside the autoclave and a vacuum (´0.084 MPa (´850 millibars)) was applied. The manufacturer’s were placed inside the autoclave andvacuum-bagged a vacuum (−0.084 MPa (−850 millibars)) was rig applied. The when the vacuum was applied. The preform pre-stressing assembly recommended cure cycle for the E-glass/913 epoxy prepreg systemand was the 1 h at 120 ˝ C. The autoclave manufacturer’s recommended cure cycle the E-glass/913 epoxy prepreg was 1applied. h at 120 The °C. ´1 , with were placed inside the autoclave and afor vacuum (−0.084 (−850 millibars)) was programmed to heat up the vacuum bagged assembly at 2MPa K¨ min asystem pressure of 0.69 MPa −1 The autoclave was programmed heat vacuum epoxy baggedprepreg assembly at 2was K·min with°C. a manufacturer’s recommended cureto cycle forup thethe E-glass/913 system 1 h at, 120 (101 psi).

pressure of 0.69was MPaprogrammed (101 psi). The autoclave to heat up the vacuum bagged assembly at 2 K·min−1, with a pressure of 0.69 MPa (101 psi).

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3.6. Characterising the Response of the Embedded Optical Fibre Sensors 3.6. Characterising the Response of the Embedded Optical Fibre Sensors A 16-ply UD E-glass/epoxy composite with embedded optical fibre sensors was used to assess A 16-ply UD E-glass/epoxy composite with embedded optical fibre sensors was used to assess the response of the embedded sensors in the autoclave to temperature, vacuum, pressure and the the response of the embedded sensors in the autoclave to temperature, vacuum, pressure and the cure cycle. These tests were conducted using a laminate of dimensions 290 mm ˆ 200 mm ˆ 2 mm. cure cycle. These tests were conducted using a laminate of dimensions 290 mm × 200 mm × 2 mm. The EFPI and FBG sensors were embedded between the 8th and 9th ply. After the laminated prepreg The EFPI and FBG sensors were embedded between the 8th and 9th ply. After the laminated prepreg was processed, reference (unbonded) EFPI and FBG sensors were also located on the top-surface of the was processed, reference (unbonded) EFPI and FBG sensors were also located on the top-surface of composite. The movement of the sensors was restricted by attaching the optical fibres (away from the the composite. The movement of the sensors was restricted by attaching the optical fibres (away sensing regions) to the surface of the composite using a high-temperature polyimide adhesive tape. from the sensing regions) to the surface of the composite using a high-temperature polyimide A K-type thermocouple was also surface-mounted on the laminate/composite. The prepregs with the adhesive tape. A K-type thermocouple was also surface-mounted on the laminate/composite. The embedded optical fibre sensors were processed as described previously. prepregs with the embedded optical fibre sensors were processed as described previously. 3.7. In-Situ Residual Strain Monitoring in Composites 3.7. In-Situ Residual Strain Monitoring in Composites The process-induced residual strain in unidirectional [0]16 E-glass epoxy composites, with and Thepre-stress, process-induced residual in-situ strain using in unidirectional [0]16 E-glass composites, with and without was monitored embedded EFPI and FBGepoxy sensors. without pre-stress, was monitored in-situ using embedded EFPI and FBG sensors. 3.8. Coding of the Composites 3.8. Coding of the Composites In order to identify the composites with different levels of pre-stress, the following coding system In order to identify the composites with different levels of pre-stress, the following coding was used: system was used: (i) URX where U = unidirectional, R = reference (without pre-stress), X = panel number. (i) URX where U = unidirectional, R = reference (without pre-stress), X = panel number. For example, example, UR6 UR6 represents For represents unidirectional unidirectional reference reference panel panel number number 6. 6.

(ii) UPTX UPTX where where PP ==pre-stressed pre-stressed and and T= T=pre-stressing pre-stressingrig, rig,thermal thermalexpansion-induced expansion-induced pre-stress. pre-stress. (ii) UPX_FkN (iii) UPX_FkN where where PP ==pre-stressed, pre-stressed, FkN FkN ==pre-load pre-loadin inkN. kN. For example, UP6_14kN represents unidirectional pre-stressed panel number 6 with a 14 kN For example, UP6_14kN represents unidirectional pre-stressed panel number 6 with a pre-load. 14 kN pre-load. 4. 4. Results Resultsand andDiscussion Discussion 4.1. Calibration of the Load-Cell With reference referencetoto Figure it is that seenthethat load recorded by the on the Figure 14a,14a, it is seen loadthe recorded by the load-cell onload-cell the pre-stressing pre-stressing is in goodwith agreement with thatfrom measured from the pre-calibrated test rig is in goodrigagreement that measured the pre-calibrated mechanicalmechanical test machine. machine. Figure 14b shows that the response from the load-cell is linear during the static tests Figure 14b shows that the response from the load-cell is linear during the static tests conducted ˝ C.and 121 °C. conducted 27, 121 70, 99 at 27, 70, 99atand

(a) Figure 14. Cont. Figure 14. Cont.

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(b) (b) Figure 14. (a) Comparison the load recorded load-cell (used pre-stressing Figure 14. 14. (a) (a)Comparison Comparisonofof ofthe theload loadrecorded recorded by the load-cell (used on the pre-stressing rig) and Figure byby thethe load-cell (used on on thethe pre-stressing rig) rig) andand the the output from the pre-calibrated mechanical test machine; (b) Comparison of the load measured by the output the pre-calibrated mechanical machine; (b) Comparison of load the load measured by output fromfrom the pre-calibrated mechanical test test machine; (b) Comparison of the measured by the the the via the test at temperatures. the load-cell load-cell and the applied applied load via mechanical the mechanical mechanical test machine machine at specified specified temperatures. load-cell and and the applied loadload via the test machine at specified temperatures.

4.2. Evaluation of the Pre-Stressing Rig 4.2. Evaluation Evaluation of of the the Pre-Stressing Pre-Stressing Rig Rig 4.2. The loading efficiency of the pre-stressing rig was evaluated by clamping steel plate and The loading loading efficiency efficiency of of the the pre-stressing pre-stressing rig rig was was evaluated evaluated by by clamping clamping aaa steel steel plate plate and and The applying the required static tensile load. The resultant strain in the steel plate was measured using applying the the required required static static tensile tensile load. load. The The resultant resultant strain strain in in the the steel steel plate plate was was measured measured using using applying surface-bonded electrical resistance strain gauges (ESRGs). With reference to Figure 15, it can be surface-bonded electrical resistance strain gauges (ESRGs). With reference to Figure 15, it can be surface-bonded electrical resistance strain gauges (ESRGs). With reference to Figure 15, it can be seen seen that the strains measured at different positions on the steel plate, for an applied load, are in seenthe thatstrains the strains measured at different positions the plate, steel plate, an applied load, in that measured at different positions on theon steel for anfor applied load, are in are good good agreement. This demonstrates that the clamping and loading mechanisms on the pre-stressing good agreement. This demonstrates that the clamping and loading mechanisms on the pre-stressing agreement. This demonstrates that the clamping and loading mechanisms on the pre-stressing rig rig capable of aa uniform load to the plate. rig were were capable of applying applying uniform to steel the steel steel plate. were capable of applying a uniform loadload to the plate.

Figure 15. Strain measured on the steel plate via the ERSGs as of Figure 15. 15.Strain Strainmeasured measured steel plate viasurface-bonded the surface-bonded surface-bonded as aa function function of the the Figure onon thethe steel plate via the ERSGsERSGs as a function of the applied applied load measured by the load-cell of the pre-stressing rig. applied load measured by the load-cell of the pre-stressing rig. load measured by the load-cell of the pre-stressing rig.

4.3. Effect of the Autoclave Processing Parameters on the Sensors 4.3. Effect Effect of of the the Autoclave Autoclave Processing Processing Parameters Parameters on on the the Sensors Sensors 4.3. The effect of the applied vacuum (826 mbar (−0.084 via vacuum bag, on an FBG that The effect effect of of the the applied applied vacuum vacuum (826 (826mbar mbar(´0.084 (−0.084 MPa)), MPa)), via the the vacuum vacuum bag, bag, on on an an FBG FBG that that The MPa)), via the was embedded in aa unidirectional laminate is shown in Figure 16a. A compressive strain of 80 µε was embedded in unidirectional laminate is shown in Figure 16a. A compressive strain of 80 µε was embedded in a unidirectional laminate is shown in Figure 16a. A compressive strain of 80 µε was was recorded for this applied vacuum. The interesting point to note is that the strain recorded by the was recorded forapplied this applied vacuum. The interesting to is note thatstrain the strain recorded the recorded for this vacuum. The interesting pointpoint to note thatisthe recorded by theby FBG, FBG, when the vacuum bag was returned to atmospheric conditions, was approximately −35 µε. FBG, when the vacuum bag was returned to atmospheric conditions, was approximately −35 µε. when the vacuum bag was returned to atmospheric conditions, was approximately ´35 µε.

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(a)

(b) 120 Theoretical Prediction Embedded EFPI Strain

100

Embedded FBG Strain Reference EFPI

700

80

Strain ()

Reference FBG Strain Autoclave Temperature

500

60

Embedded FBG Temperature

40

300

Temperature rise (oC)

900

20

100

0

-100

-20

0

1

2

3

4

5

6

7

8

9

Time (hours)

(c) Figure 16. (a) Figure 16. (a) The The effect effect of of the the applied applied autoclave autoclave vacuum, vacuum, via via the the vacuum vacuum bag, bag, on on an an embedded embedded FBG FBG strain sensor; (b) Typical outputs from the surface-located reference EFPI and FBG sensors strain sensor; (b) Typical outputs from the surface-located reference EFPI and FBG sensors and and the the cure cure cycle parameters in the autoclave. The step in the FBG optical fibre sensor traces after approximately 4-h cycle parameters in the autoclave. The step in the FBG optical fibre sensor traces after approximately coincides with the time when the door of the autoclave was opened and the vacuum bag was 4-h coincides with the time when the door of the autoclave was opened and the vacuum bag was detached.; the embedded embedded and and surface-located surface-located EFPI FBG detached.; (c) (c) A A comparison comparison of of the the outputs outputs from from the EFPI and and FBG sensors, including the data from the thermocouple, as a function of the cure schedule for a 16-ply sensors, including the data from the thermocouple, as a function of the cure schedule for a 16-ply unidirectional composite. The indicate the the relevant relevant y-axis y-axis for for each each dataset. dataset. unidirectional E-glass E-glass composite. The arrows arrows indicate

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The implication of the data presented in Figure 16a is that due care and attention needs to be paid to maintaining the relative orientation of the embedded optical fibre sensor in specified substrates (resin, prepreg, fabric, concrete, etc.). In the current paper, the alignment of the FBG and EFPI was controlled and assured because sections of the optical fibres, within the end-tabs, were pre-aligned and bonded to the matrix. The act of tensioning the end-tabbed preform, to apply the required pre-tension, ensured the alignment and the retention of the orientation of the optical fibre sensors during processing. The issues associated with the retention of the relative orientation of embedded sensors was highlighted previously [48]. The feasibility of measuring the residual strain using optical EFPI sensors was first demonstrated by Liu et al. [48] using a cross-ply carbon/epoxy composite. They embedded two temperature-compensated EFPI strain sensors between plies numbers 2 and 3 (sensor 1), and 8 and 9 (sensor 2) in a cross-ply (0, 902 , 02 , 90, 0, 90)S carbon/epoxy laminate. They measured the cavity length of EFPI sensors before and after processing and reported the residual strain to be 90 µm and 550 µm in sensors 1 and 2 respectively. The authors proposed that the observed discrepancy may have been due to the relative orientations of the two sensors. The effect of autoclave processing parameters on the output of the FBG and EFPI sensors (placed on the surface of the composite and embedded) was investigated using a pre-cured composite panel of dimensions 290 mm ˆ 200 mm ˆ 2 mm. The composite with the sensors was vacuum-bagged and subjected to a typical autoclave cure schedule. Figure 16b shows the observed relationship between the cure cycle parameters on the output from the reference EFPI and FBG sensors (located on the surface of the cured composite panel but within the vacuum bag). The FBG sensor was influenced by temperature, pressure and the force exerted by the vacuum bag. The initial fluctuations in the FBG signal, reaching a maximum compressive strain of approximately 40 µε, is likely to have been caused by the lateral compression caused by the application of the vacuum in the vacuum bag and the applied external pressure. At around 50 ˝ C, the strain recorded by the FBG decreases from approximately +50 µε to a minimum of ´50 µε. However, the signal is noisy since the mode of operation of the autoclave pressure is cyclic once the target value has been reached. It is difficult to account for the change in magnitude of the FBG signal from +50 to ´50 µm. However, it is possible that some form of relaxation occurring in the lateral forces experienced by the fibre along its length. A possible reason for this observation is the glass transition temperature of the acrylate coating on the optical fibre being approached and/or being exceeded. In the current experiment, the acrylate coating was retained on the optical fibre apart from the sensing regions in the EFPI and FBG sensors. The compressive strain is reversed at approximately 80 ˝ C and the FBG signal is seen to oscillate with the applied autoclave pressure to around +20 and ´20 µε. After 2-h of processing, the heater controller was turned off and the strain recorded by the FBG is seen to decay in tandem with the temperature. The EFPI sensor is relatively insensitive to temperature because the coefficients of thermal expansion for the optical fibre and the capillary are similar [45,46]. However, the action of the vacuum bag will impress the EFPI sensor on to the surface of the pre-cured composite as the vacuum and pressure is applied. This may account for the initial rise in the measured strain (~15 µε) after approximately 15 min and it could be attributed to the bending induced by the vacuum bag acting on the ends of the capillary and the optical fibre. A decrease is observed in the recorded strain from approximately 15 µε to 25 µε at 85 ˝ C. This coincides with the autoclave pressure reaching its set value. Since this small but detectable decrease in the stain was observed for the FBG and EFPI sensors, it is speculated that this may be attributed to the glass transition temperature of the acrylate coatings being exceeded on the two types of optical fibres. After the autoclave door was opened, a small increase in the strain was observed from the reference EFPI sensor. This may be attributed to the relaxation of the materials within the vacuum bagging materials. However, this ceases after approximately seven hours as the autoclave was permitted to cool naturally. The thermal expansion of a pre-cured unidirectional composite was measured using the embedded optic fibre sensors. The cured composite with the surface-located and embedded sensors were subjected to a typical autoclave cure cycle. The composite was heated from ambient to 120 ˝ C at 2 K¨ min´1 with

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a dwell of one hour. The pressure was ramped from atmospheric to 0.69 MPa at 0.015 MPa per minute and held at this pressure for one hour. The thermal expansion of a unidirectional composite (αc ), in the fibre direction is given by: αc “

αm νm Em ` α f ν f E f Em νm ` E f ν f

(4)

where α, ν and E are the thermal expansion coefficient, volume fraction and modulus respectively. The subscript m and f denote the matrix and fibre respectively. Figure 16c shows the output from the embedded, surface-located EFPI and FBG sensors and the thermocouple data as a function of the processing time and the autoclave temperature. In this instance, the strains recorded by the embedded EFPI and FBG sensors show a similar trend and magnitude during the heating and isothermal periods. During the cooling phase, a small divergence is seen between the EFPI and the FBG sensors. Excellent correlation is seen in Figure 16c between the embedded FBG temperature sensor and the output from the thermocouple. The outputs from the reference EFPI and FBG sensors that were located on the surface of the composite were discussed previously. A good correlation is observed between the outputs from the embedded EFPI and FBG sensors and that predicted using Equation (4). A summary of the relevant properties that were used for computing the thermal expansion of the unidirectional 16-ply composite is presented in Table 1. The measured and the predicted values for the thermal expansions are presented in Table 2. Table 1. A summary of relevant properties that were used for computing the thermal expansion of the unidirectional 16-ply E-glass/epoxy resin composite. Properties

E-Glass

Epoxy Resin

Young‘s Modulus (Mpa) Poisson‘s ratio Thermal expansion coefficient (˝ C´1 ) Volume fraction

72,500 0.22 5.00 ˆ 10´6 0.6

4000 0.4 60.0 ˆ 10´6 0.4

Table 2. A summary of the measured and predicted thermal expansions for the unidirectional 16-ply E-glass/epoxy resin composite. Method

Thermal expansion (ˆ10´6 K´1 )

Theoretical prediction (Equation (4)) Experimental measurement (EFPI and FBG)

6.38 6.45

4.4. Residual Strain Monitoring in Composites without Pre-Stress Figure 17 shows the output from an embedded EFPI sensor in a 16-ply UD E-glass/epoxy reference composite (manufactured without any pre-stress) during autoclave processing. A summary of the residual strains measured at specified stages of the cure cycle is presented in Table 3. The general trends observed in Figure 17 can be grouped into the following regions: (a) after embedding the EFPI sensor but before curing the laminated prepregs; (b) during the temperature ramp-up period; (c) during the isothermal dwell; and (d) cooling phase. Table 3. A summary of the strain data measured by the embedded EFPI sensors at different stages of processing for three reference composite panels where no pre-stress was applied. Reference Composite Panels

Strain Before Embedding (µε)

After Embedding (µε)

At 120 ˝ C (µε)

Final Residual Strain at Room Temperature (µε)

UR1 UR8 UR9

0 0 0

´71 ´63 ´101

40 42 24

´587 ´640 ´586

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Region a: Subsequent to embedding the EFPI sensor, a compressive strain of approximately 50 µεRegion was recorded. Thetoorigin of this may bestrain attributed to the manual a: Subsequent embedding thecompressive EFPI sensor, astrain compressive of approximately 50 µε procedures thatThe were used laminate and consolidate theattributed prepregs to with aid of a roller. This was recorded. origin of to this compressive strain may be the the manual procedures that action of consolidating the consolidate plies, layer-by-layer, is likely to have theaction EFPI with a capillary were used to laminate and the prepregs with the aid ofresulted a roller.in This of consolidating diameter 300 µm, beingis impressed into resulted the prepregs resulting in the observed compressive the plies,of layer-by-layer, likely to have in thethus EFPI with a capillary diameter of 300 µm, strain. being impressed into the prepregs thus resulting in the observed compressive strain. Region Region b: b: During Duringthe the heating heating cycle, cycle, no no significant significant changes changes were were observed observed in in the the output output of of the the ˝ C when EFPI EFPI sensor sensor until until approximately approximately 60 60 °C when aa positive positive strain strain was was recorded. recorded. This This increase increase may may be be attributed to a combination of the relaxation of the previously induced compressive strain due to the attributed to a combination of the relaxation of the previously induced compressive strain due to the lamination lamination process process and and thermal thermal expansion expansion of of the the various various materials. materials. During During the the heating heating phase, phase, the the viscosity of the theresin resininitially initially decreases. Hence, is reasonable to assume that any constraint or viscosity of decreases. Hence, it is it reasonable to assume that any constraint or bending bending introduced on thefibre optical andduring sensorlamination during lamination relax from its constrained introduced on the optical andfibre sensor will relaxwill from its constrained position. position. The viscosity/temperature for the913 Fiberdux 913 resin system reported by the The viscosity/temperature profile forprofile the Fiberdux resin system reported by the manufacturer manufacturer thatviscosity the minimum viscosity is 120–130 reached ˝between 120–130 °C.the After this indicated that indicated the minimum is reached between C. After this period, viscosity period, the viscosity resin increases as the cross-linking reactions become prominent. This of the resin increases of as the cross-linking reactions become prominent. This cross-linking process is cross-linking is accompanied byHowever, shrinkageasofthe thetemperature resin. However, the temperature in the accompaniedprocess by shrinkage of the resin. in theas autoclave is increasing in autoclave is increasing region-b, and therefore, constitutive in the composite and the region-b, and therefore,inthe constitutive materials the in the compositematerials and the pre-stressing rig undergo pre-stressing rig undergo thermal expansion. thermal expansion. Region at at thethe end of of thethe temperature ramp is Region c:c:The Theobserved observedexcursion excursionofofapproximately approximately7 7°C˝ C end temperature ramp possibly duedue to to two exothermic ring-opening ring-opening is possibly twofactors. factors.Firstly, Firstly,it itcould couldbebeaa consequence consequence of of the exothermic cross-linking cross-linkingreaction reactionbetween betweenthe theepoxy epoxyresin resinand andamine aminehardener. hardener.Secondly, Secondly,ititcould couldbe bedue due to to the the temperature-controller in the autoclave overshooting the set isothermal value. On inspecting region-c temperature-controller in the autoclave overshooting the set isothermal value. On inspecting region-c in in Figure Figure 17, 17, ititisisapparent apparentthat thatpreforms preforms were were above above the the set setisothermal isothermaltemperature temperaturefor forapproximately approximately 20 fromthe theend end thermal ramp. exothermic excursion was observed forprepregs all the 20 min from of of thethe thermal ramp. This This exothermic excursion was observed for all the prepregs processed in this study. processed in this study. Region Region d: d: Here Here the the heating heating was was turned turned off off and and the the composite composite was was permitted to cool naturally. With residual strain reference With reference reference to to Table Table 3, the average residual strain measured measured from from three three UD reference composites, at ambient temperature, was –604 µε. composites, at ambient temperature, was –604 µε.

Figure laminate (manufactured Figure17. 17.Strain Strainmeasured measuredusing usingan anembedded embedded EFPI EFPI sensor sensor in in aa reference reference [0] [0]16 16 laminate (manufactured without any pre-stress) during autoclave processing. The coded regions (a–d) without any pre-stress) during autoclave processing. The coded regions (a–d)represent representthe thefollows: follows: (a) (a) before before curing; curing; (b) (b) heating heating cycle; cycle; (c) (c) dwell dwell period; period; and and (d) (d) cooling cooling cycle. cycle.

4.5. 4.5. Residual ResidualStrain Strain Development Development in in aa UD UD Pre-Stressed Pre-Stressed Composite Composite This unidirectional [0] [0]16 E-glass epoxy This section section reports reports on on the the use use of of embedded embedded EFPI EFPI sensors sensors in in unidirectional 16 E-glass epoxy composites where the preform was pre-stressed. The EFPI sensors were used to monitor composites where the preform was pre-stressed. The EFPI sensors were used to monitor the the strain strain during and after processing. A limited number of experiments were also undertaken using during and after processing. A limited number of experiments were also undertaken using embedded embedded FBG Inelectrical addition,resistance electrical strain resistance strain gauges were surface-bonded on the FBG sensors. Insensors. addition, gauges were surface-bonded on the composite composite after processing but before they were unclamped from the pre-stressing rig. after processing but before they were unclamped from the pre-stressing rig. The magnitude ofThe the

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strain released, when the released, compositewhen was unclamped fromwas the pre-stressing rig, was usingrig, the magnitude of the strain the composite unclamped from therecorded pre-stressing embedded optical andoptical the surface-bonded ERSGs. was recorded usingfibre the sensors embedded fibre sensors and the surface-bonded ERSGs. Figure 18 18 shows shows the the outputs outputs from the various sensors sensors when when the composite, composite, with with aa previously previously Figure applied pre-stress was unclamped from the pre-stressing rig. Itrig. can Itbecan seenbethat thethat applied applied pre-stressofof150 150MPa, MPa, was unclamped from the pre-stressing seen the pre-load was maintained throughout the curing process to within 1 kN. During the heating cycle, applied pre-load was maintained throughout the curing process to within 1 kN. During the heating the load-cell did not the pre-stress induced in theinpreform due due to thermal expansion of the cycle, the load-cell didrecord not record the pre-stress induced the preform to thermal expansion of rig. rig. ThisThis is because in the current set-up, there is no of reference for the load-cell to record the the is because in the current set-up, there is frame no frame of reference for the load-cell to record thermally-induced pre-stress experienced by the by preform. During the heatingthe phase, the pre-stressing the thermally-induced pre-stress experienced the preform. During heating phase, the rig assembly rig expands and expands this will induce a pre-stress onathe preform.onHowever, the load appliedthe to pre-stressing assembly and this will induce pre-stress the preform. However, the preform to preform thermal expansion of theexpansion rig will not by be therecorded load-cell.by the load-cell. load applied due to the due to thermal of be therecorded rig will not

Figure Figure 18. 18. Strain Strain development development throughout throughout the the processing processing of of aa 16-ply 16-ply UD UD prepreg prepreg system system that that was was subjected to a pre-stress of 150 MPa. subjected to a pre-stress of 150 MPa.

On inspecting the output from the EPPI sensor in Figure 18, it is seen that a compressive strain On inspecting the output from the EPPI sensor in Figure 18, it is seen that a compressive strain of of approximately 60 µε is induced in the preform prior to the application of the pre-load. As approximately 60 µε is induced in the preform prior to the application of the pre-load. As expected, the expected, the application of a pre-stress 150 MPa resulted in an increase in the strain recorded by the application of a pre-stress 150 MPa resulted in an increase in the strain recorded by the EFPI sensor. The EFPI sensor. The residual strain and the magnitude of the strain that was released when the composite residual strain and the magnitude of the strain that was released when the composite was unloaded was unloaded from the pre-stressing rig, for four pre-stress levels, are summarised in Tables 4 and 5 from the pre-stressing rig, for four pre-stress levels, are summarised in Tables 4 and 5 respectively. respectively. Table 4. EFPI and FBG-based strain data from unidirectional composites that were pre-stressed via Table 4. EFPI and FBG-based strain data from unidirectional composites that were pre-stressed via thermal expansion of the rig, and applied pre-load values on the preforms corresponding to 7, 14 and thermal expansion of the rig, and applied pre-load values on the preforms corresponding to 7, 14 and 24 kN. The strain data were acquired at room temperature. 24 kN. The strain data were acquired at room temperature.

Composite Panel Code Composite Panel Code UPT8 UPT8 UPT11 UPT11 UP1_7kN UP1_7kN UP2_7kN UP2_7kN UP2_14kN UP2_14kN UP3_14kN UP3_14kN UP4_14kN UP4_14kN UP5_14kN UP5_14kN UP6_14kN UP6_14kN UP7_14kN UP7_14kN UP9_14kN UP9_14kN UP12_14kN UP12_14kN UP1_24 kN UP1_24 kN

Residual ResidualStrain Strain(µε) (µε) Sensor Type and Position onthe thePanel Panel Sensor Type and Position on EFPI (Middle) FBG (Middle) EFPI (40 mm from the Edge) EFPI (Middle) FBG (Middle) EFPI (40 mm from the Edge) −566 ´566 -- −581 ´581 −586 ´586 −574 ´574 −564 −643 ´564 ´643 −503 −653 ´503 -´653 −573 ´573 - ´553 - −553 ´520 - −520 ´605 -- −605 ´529 ´742 - −529 −742 ´714 −714 ´592 −592 -

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Table 5. 5. Strain Strain recorded recorded from from the the EFPI, EFPI, FBG FBG and surface-mounted ERSG sensors when the pre-stress Table released by byunclamping unclampingthe thecomposite compositefrom fromthe thepre-stressing pre-stressing rig.The The strain data were acquired was released rig. strain data were acquired at at room temperature. room temperature. Composite Composite Panel Code Panel Code UPT8 UPT11 UPT8 UP1_7kN UPT11 UP1_7kN UP2_7kN UP2_7kN UP2_14kN UP2_14kN UP3_14kN UP3_14kN UP4_14kN UP4_14kN UP5_14kN UP5_14kN UP6_14kN UP6_14kN UP7_14kN UP7_14kN UP9_14kN UP9_14kN UP12_14kN UP12_14kN UP1_24 kN kN UP1_24

Pre-Stress Pre-Stress Released (MPa) Released (MPa)

Strain Released Upon Unloading the Composite from the Pre-Stressing Rig (µε) Strain Released Upon Unloading the Composite from the Pre-Stressing Rig (µε) Sensor Type and Position on the Panel Sensor Type and on the Panel EFPI FBG EFPI (40Position mm ERSG ERSG (40 mm (Middle) (Middle) from the (40 Edge) (Middle) from the EFPI FBG EFPI mm ERSG ERSG (40Edge) mm (Middle) (Middle) from-the Edge) (Middle) from −160 the Edge) −109 −108 -- −194 −221 ´109 ´160 −184 -- ´108 ´194 ´221 ´184 - −190 -−223´190 ´223 -−630−656 −696 ´630 ´656 ´696 −637 -−685 −602 −763 ´637 ´685 ´602 ´763 −579 −628 ´579 ´628 −537 −548 -´537 ´548 −552 -- −613 -´552 ´613 −600 -- −639 -´600 ´639 −551 −684 - −677 ´551 ´684 ´677 -´608 ´600 −608 - −600 ´944 ´958 ´1113 −944 -- −958 −1113

51.21 * 51.21** 51.21 79.95* 51.21 79.95 80.36 80.36 108.50 108.50 107.22 107.22 108.62 108.62 108.41 108.41 112.89 112.89 108.45 108.45 113.57 113.57 107.61 107.61 150.07 150.07 *: The thermal expansion-induced pre-stress pre-stress calculated using classical mechanics. *: The thermal expansion-induced calculated using classical mechanics.

Table 5, 5, the the EFPI EFPI strain strain data data from from the the repeat repeat experiments, experiments, at pre-loads With reference to Table 14 kN, kN, were were in in the therange range´537 −537 to to ´637. −637. A corresponding to 14 A reasonable reasonable agreement is observed between the datasets for the EFPIs and the ERSGs when they were embedded and surface-mounted the centre centre of of the the composite. composite. The ERSGs, when located 40 mm from the edge of the respectively, at the composite, recorded a higher value when compared to those located at the centre; this is presumably were discussed discussed previously. previously. due to edge effects. Reasons for the variation in the FBGs were Figure 19 19shows showsthe thestrains strains recorded EFPI, and ERSG sensors a composite Figure recorded by by the the EFPI, FBGFBG and ERSG sensors when when a composite (Panel (Panel code UP9_14kN), with a previously applied pre-stress of 108 MPa, was unloaded from the code UP9_14kN), with a previously applied pre-stress of 108 MPa, was unloaded from the pre-stressing pre-stressing rig manner. in a stepwise manner. Theby data bydid thenot FBG sensor not follow the rig in a stepwise The data recorded the recorded FBG sensor follow thedid stepwise pre-load stepwise as EFPI observed with the EFPI and sensors. Thisof is data because the ratefor of release aspre-load observedrelease with the and ERSG sensors. ThisERSG is because the rate acquisition data sensors acquisition FBG sensors was per two minute, measurements minute, that for the EFPI was and FBG wasfor two measurements whereasper that for thewhereas EFPI and ERSG sensors sensors was 1 measurement perstrains second. Initially,from the strains measured EFPI,are FBG 1ERSG measurement per second. Initially, the measured the EFPI, FBG andfrom ERSGthe sensors in and ERSG sensors areasin as the composite was unloaded further, thelower EFPI agreement. However, theagreement. composite However, was unloaded further, the EFPI sensor records a slightly sensor records a slightly compressive strain when compared thethe FBG andhand, ERSGthe sensors. On compressive strain whenlower compared to the FBG and ERSG sensors.to On other FBG and the other hand,are theinFBG ERSG sensors are in good agreement. ERSG sensors goodand agreement.

Figure 19. 19.Strain Straindata data from the EFPI, FBGERSG and sensors ERSG when sensors when a composite code Figure from the EFPI, FBG and a composite (Panel code(Panel UP9_14kN), UP9_14kN), with a applied previously appliedofpre-stress of was 108 MPa, was unloaded the pre-stressing with a previously pre-stress 108 MPa, unloaded from the from pre-stressing rig in a rig in a stepwise stepwise manner.manner.

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Table 6 shows the average strain-release measured via the EFPI, FBG and ERSG sensors when the preform that was previously subjected to a pre-stress 108 MPa, was cured and then unloaded from the Table 6 shows the average strain-release measured via the EFPI, FBG and ERSG sensors when pre-stressing rig. The average strain-release measured with the FBG sensor is slightly higher than that the preform that was previously subjected to a pre-stress 108 MPa, was cured and then unloaded obtained from the EFPI sensor. However, by considering the standard deviations and the number of from the pre-stressing rig. The average strain-release measured with the FBG sensor is slightly samples that were evaluated, it can be concluded that the strain recorded from the optical fibre (EFPI higher than that obtained from the EFPI sensor. However, by considering the standard deviations and FBG) and ERSG sensors show a reasonable agreement. After releasing the pre-stress, the final and the number of samples that were evaluated, it can be concluded that the strain recorded from residual strain in the composite, εfinal , can be expressed as: the optical fibre (EFPI and FBG) and ERSG sensors show a reasonable agreement. After releasing the pre-stress, the final residual strain in the composite, final, can be expressed as: ε “ ε ε´ ε (5) f inal

r

p

= (5) where εr is the residual strain measured before releasing the composite from the pre-stressing rig residual strain measured before releasing theand composite pre-stressing rig where εr is the cure (this represents and thermally-induced residual stresses) εp is the from strainthe measured when the (this represents cure and thermally-induced residual stresses) and ε p is the strain measured when the pre-load is released. pre-load is released. Table 6. The average strain released when the composites were un-loaded from the pre-stressing ring. Table 6. The were average strain released when the composites The preforms previously subjected to a pre-stress of 108 were MPa. un-loaded from the pre-stressing ring. The preforms were previously subjected to a pre-stress of 108 MPa. Sensor Type Number of Samples Sensor Type Number of Samples EFPIEFPI FBGFBG ERSG ERSG

6 2 7

6 2 7

Average Strain Released Average Strain Released Upon Unloading (µε) Upon Unloading (µε) ´592.8 −592.8 ´646.0 −646.0 ´616.2 −616.2

Standard Deviation Standard (µε) Deviation (µε) 5656 5454 3535

Figure 20 thethe finalfinal residual strain measured for all thefor pre-stressed composites manufactured Figure 20shows shows residual strain measured all the pre-stressed composites in this study. in The compressive residual strain in thestrain composites reduces asreduces the previously applied manufactured this study. The compressive residual in the composites as the previously pre-stress on the on preforms increases. However, the application of a of pre-stress to the preform of applied pre-stress the preforms increases. However, the application a pre-stress to the preform approximately 108108 MPa results of approximately MPa resultsininthe theproduction productionofofcomposites compositeswith withaa near-zero near-zero residual strain. Above an anapplied appliedpre-stress pre-stressofof108 108 MPa, final residual strain is tensile. demonstrates Above MPa, thethe final residual strain is tensile. ThisThis demonstrates that that the the pre-stressing method reported canused be used to control the final residual strain in composites. pre-stressing method reported herehere can be to control the final residual strain in composites.

Figure the composites produced in this study as aasfunction of the Figure 20. 20. The Themeasured measuredresidual residualstrains strainsinin the composites produced in this study a function of pre-stress levels that were applied to the prepregs prior to curing. the pre-stress levels that were applied to the prepregs prior to curing.

In Figure Figure 20, 20, itit is is apparent apparent that that the the strain strain gradient gradient across across the the panel panel (middle (middle and and 40 40 mm mm from from In edge) increases with an increase in pre-stress. This variation in strain reaches a maximum of 14% at aa edge) increases with an increase in pre-stress. This variation in strain reaches a maximum of 14% at 150 MPa MPapre-stress. pre-stress.ItItisisproposed proposedthat thatthis thisdifference difference measured strain-release may be due to 150 inin thethe measured strain-release may be due to an an edge effect caused by the variation in Poisson’s contraction at the unconstrained edge. edge effect caused by the variation in Poisson’s contraction at the unconstrained edge. 5. General Discussion This study had demonstrated conclusively that the application of a pre-stress to the preform, and maintain this stress during autoclave-based processing, is a viable method for counteracting the

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5. General Discussion This study had demonstrated conclusively that the application of a pre-stress to the preform, and maintain this stress during autoclave-based processing, is a viable method for counteracting the fabrication-induced residual strain in unidirectional composites. In addition to the parameters discussed in the introduction that can give rise to residual fabrication strain in composites, attention also needs to be paid to non-uniform temperature and cure gradients during the processing of preforms [49,50]. With reference to strain metrology using optical fibres, due consideration needs to be given to the nature of the interface between the surface of the optical fibre/sensor and the matrix [1,51,52]. Appropriate packaging and sensor protection systems are also important if the longevity of the sensor system is to be ensured [53,54]. Although the optical fibre sensors were effective in quantifying the magnitude of the residual strains in pre-stressed composites, for the first time, the reasons for the variations in the strain data need to be appreciated. Optical fibres can be embedded with significant ease in prepregs, however, due attention needs to be paid to the orientation of the sensor in relation to the reinforcing fibres [1]. Any bending of the optical fibres, in the vertical or horizontal planes to the reinforcing fibres, will give rise to induced strain in the sensor. In general, in-plane movement of the sensor is possible when there is significant movement of the resin during processing. This in-plane movement can be induced by excessive resin-flow during processing or if excessive pressure is applied. This movement of the sensor can be restricted by securing the input and output ends of the optical fibres under a pre-defined tension. The thermal expansion of the substrate that the optical fibres are attached to could also have an influence on the measured strain via the sensors. Out-of-plane movement of the optical fibre sensor has been observed for unidirectional composites [1]. However, a detailed study needs to be undertaken to assess the effect of processing conditions on the relative movement of the optical fibre sensors; due attention also needs to be paid to the effect of the optical fibres on the orientation of the reinforcing fibres. The issue here is that the localised strain measured by the optical fibre sensor may not give an accurate description of the strain. Instrumental drift can be a problem especially if the laboratory or environment housing the interrogation equipment experiences a significant change in temperature. The effect of the autoclave processing parameters (pressure, vacuum) needs to be quantified for a specific prepreg system. The FBG is more sensitive to lateral loading than the EFPI sensor. This current study has shown that more reliable data can be obtained from the FBG sensor when reflection spectra are recorded as opposed to analyses where only the peak-reflection is monitored. The latter can give rise to erroneous conclusions especially where the sensing region experiences off-axis loading. 6. Conclusions This study has demonstrated the feasibility of manufacturing unidirectional composites where the magnitude of the residual stress can be managed by applying a pre-stress to the preform prior to processing. This was achieved using a custom-designed pre-stressing rig that was based on a two-part flat-bed design. This rig permitted the pre-stressed prepregs to be processed in an autoclave using conventional procedures. The feasibility of using conventional EFPI and FBG sensors for monitoring the development of residual strain was also demonstrated. The evaluation of the pre-stressing technique was carried out in four stages. Firstly, the load-cell was calibrated at specified applied loads and temperatures. Subsequent to this, the clamping/loading efficiency was demonstrated by loading a steel plate in the rig. The resultant strain was logged at specified positions using surface-bonded ERSGs. Secondly, an end-tab rig was designed to enable aluminium end-tabs to be co-cured to the ends of the prepregs but without curing the regions away from the end-tabs; this was achieved by using a water-cooling system. Thirdly, the effect of the autoclave processing parameters on the outputs from the sensors used in this investigation was studied. Finally, composites were manufactured with predefined levels of pre-load and the effect of pre-stress on the residual stress in composites was determined.

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The residual strain in unidirectional E-glass glass fibre/epoxy composite panels was measured using embedded EFPI and FBG sensors. This study has shown conclusively that the magnitude of the residual stress in a composite can be managed by the application of pre-stress during processing. For example, it was demonstrated that applying a pre-stress of 108 MPa to a unidirectional E-glass epoxy preform, resulted in a composite panel with a negligible residual strain. Acknowledgments: The authors wish to acknowledge funding from EPSRC for the custom-modified autoclave (GR/M83605/01). Funding from the Department of Engineering Systems, Cranfield University (Shrivenham Campus) for S.K. is acknowledged. Financial assistance provided for R.B. and V.R.M. by the University of Birmingham is duly acknowledged. The authors also wish to thank Professor Brian Ralph, Mike Teagle, Guillaume Kister, Roshan Degamber, Amer Hameed, Mark Paget and Maggie Keats for all the help and assistance given to the authors over the course of this research. The EFPI sensors used in this program were fabricated and calibrated by the late John Tetlow. Author Contributions: S.K. fabricated the composites and sensors. R.A.B. was responsible for day-to-day supervision of the project. V.R.M. assisted with the fibre optic sensors with S.K. performing the measurements and associated experimental work. The flat-bed pre-stressing rig was proposed by G.F.F. and design was executed by S.K., R.A.B. and G.F.F. G.F.F. was the principal investigator on the EPSRC-funded grant GR/M83605/01 to instrument the autoclave with fibre optic sensors and was the originator of the pre-stressed composites project. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript: AFRCs CCD EFPI ERSG FBG ILSS SLD SMA

Advanced Fibre Reinforced Composites Charged coupling device Extrinsic Fibre Fabry-Perot interferometer Electrical resistance strain gauges Fibre Bragg grating Interlaminar shear strength Super-luminescence diode Shape memory alloy

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