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A Complex Shaped Reinforced Thermoplastic Composite Part Made of

Commingled Yarns With Integrated Sensor

Article  in  Applied Composite Materials · May 2014

DOI: 10.1007/s10443-014-9400-9

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A Complex Shaped Reinforced Thermoplastic CompositePart Made of Commingled Yarns With Integrated Sensor

Jean-Vincent Risicato & Fern Kelly & Damien Soulat &Xavier Legrand & Wolfgang Trümper &

Cedric Cochrane & Vladan Koncar

Received: 15 April 2014 /Accepted: 14 May 2014# Springer Science+Business Media Dordrecht 2014

Abstract This paper focuses on the design and one shot manufacturing process of complexshaped composite parts based on the overbraiding of commingled yarns. The commingledyarns contain thermoplastic fibres used as the matrix and glass fibres as the reinforcementmaterial. This technology reduces the flow path length for the melted thermoplastic and aimsto improve the impregnation of materials with high viscosity. The tensile strength behaviour ofthe material was firstly investigated in order to evaluate the influence of the manufacturingparameters on flat structured braids that have been consolidated on a heating press. A goodcompatibility between the required geometry and the braiding process was observed. Addi-tionally, piezo-resistive sensor yarns, based on glass yarns coated with PEDOT: PSS, havebeen successfully integrated within the composite structure. The sensor yarns have beeninserted into the braided fabric, before consolidation. The inserted sensors provide the abilityto monitor the structural health of the composite part in a real time. The design andmanufacture of the complete complex shaped part has then been successfully achieved.

Keywords Commingled yarns . GF/PP Thermoplastic . Braiding . Complex shape . Design .

Process . Piezo-resistive sensors

1 Introduction

One shot manufacturing of complex shaped parts can be considered one of the best solutions[1, 2] for the fast production of composite pieces dedicated to the railway and automotiveindustries. This paper outlines the replacement of metallic cross stiffeners with their compositecounterpart, using a one shot manufacturing methodology. Figure 1a represents a crossstiffener made of metallic welded tubes. Figure 1.b and 1.c describe the geometry designed

Appl Compos MaterDOI 10.1007/s10443-014-9400-9

J.<V. Risicato : F. Kelly : D. Soulat :X. Legrand (*) : C. Cochrane :V. KoncarUniv. Lille North of France F-59100, ENSAIT, GEMTEX, Lille, Roubaix 59100, Francee-mail: xavier.legrand@ensait.fr

W. TrümperInstitute of Textile Machinery and Textile High Performance Technology, Technische Universität,Dresden, Germany

to achieve a composite cross stiffener. According to the specifications related to loads, it isnecessary to have reinforcement in multiple directions, not only along the axis of the tubes.Consequently, a process like thermoplastic composite pultrusion, which requires unidirectionalreinforcement, [3] cannot be used. In addition, as the cross stiffener is being applied as astructural part, it is necessary to ensure the continuity of the fibre reinforcement along thebranches during the manufacturing process.

The braiding process of comingled yarns allows for the cost-effective production ofcomposites due to its high degree of automation. Two-dimensional (2 days) biaxial, triaxialand three-dimensional (3 days) braided fabric structures are used as structural elements in anumber of industries, including medical, defence and transportation [4–6]. Complex shapeswith non-regular cross-sections (round, square and flat) can be obtained by this process [6–9].Braided composites undergo large elastic deformation while maintaining high rupture strength[8]. For all of these reasons the over-braiding process [9] associated to comingled yarns hasbeen set up to achieve this complex geometry. One of the key design parameters of braidedcomposites is the angle of the fibres with respect to the longitudinal 0°-axis, defined as thebraiding angle. A lot of studies have shown that the mechanical behaviour of braidedcomposites is dependent on this braid angle [6–8, 10–13]. The angle is closely related toprocess parameters such as the rotational speed of the carriers, the take-up speed but also to thebraid diameter. Consequently the non-regular cross-section generates modification of the braidangle [7] on the different areas of the part and need to be taken into account in the final design.For this reason, influence of the braid angle on the mechanical properties of the composite hasbeen investigated.

The use of composite parts to replace their metal analogues for the structural componentswithin the bodies of transport vehicles offers many positive outcomes. The cost of productionand the weight of the piece is reduced and hence the overall energy consumption is conse-quently reduced. However, because of their inherent differences, metals and compositematerials exhibit very different modes of failure. Figure 2 shows a typical comparison of thefatigue damage of composites in comparison to metals over time. For metals, a predominantsingle crack is the most common failure mechanism. On the other hand, in composite materialsthere are four basic failure mechanisms: fibre breakage, delamination, cracking and interfacial

Fig. 1 Crossing of stiffeners a) metallic problematic b) and c) CAO of assembled composite solution

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debonding [14]. Due to the inherent anisotropies and complex stress fields, the ability tounderstand the true nature of failure in composites is limited. The mechanical performance ofcomposites also needs to be verified with high precision. The ability to measure in real timelocal constraints on reinforcement by a fibrous sensor, is a highly valuable technique fordetecting mechanical damages during all the steps of the process [15]. Piezo-resistive sensorsare well known in the field of smart textiles. Elongation sensors may be found in themonitoring of textile structures used in parachutes canopies [16, 17] or structural composites[18]. This study describes the introduction in the braided preform of mechanical sensorselaborated from conductive polymers applied as liquid dispersion onto the fibers. In this workaqueous dispersions of poly (3,4-ethylenedioxythiophene)-poly (styrene sulfonate) (PEDOT:PSS), currently one of the most commonly used conducting polymers, has been utilized forpiezo-resistive sensor applications [15, 19–21].

This paper describes the different steps required to obtain composite parts prepqred fromglass/polypropylene (G/PP) commingled yarns. First, the yarns have been characterised toensure their compatibly with the braiding manufacturing process. Due to its importance in thetextile process, the effect of winding on yarn parameters has additionally been studied [22].Braiding, and more specifically, overbraiding, are presented and the braiding protocol, with itsresulting parameters, introduced. In initial studies relating to the composite structure, thebraided tubes were cut and flattened prior to consolidation by thermo-compression. Modifi-cations of the braid geometry in both its tubule and flat form structures have been measured.The effect of the braiding angle (35°, 45° and 55°) on the composite part have been tested fortheir tensile behaviour. These studies have been undertaken in order to compare the mechan-ical properties and effect of the load direction (axial or transverse). The 55° angled braids haveadditionally been prepared with integrated flexible piezo-resistive sensors. The sensors haveproven resistant to the high pressure and temperature of the consolidation process and have

Fig. 2 Typical Comparison of Metal and Composite Fatigue Damage [14]

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indicated their potential for structural health monitoring (SHM). Finally, an entire compositeprototype part has been successfully manufactured according to the design shown in Fig. 1.

2 Materials and methods

2.1 Hybrid yarns

Interest in thermoplastic composites has increased in recent years due to their unique propertiessuch as short processing times [23], long term storage, ease of recycling and impact properties[24, 25]. In comparison to high viscous thermoplastic melts, thermoset matrices are fluid andcan be easily processed through a fibrous media. Manufacturing techniques have beendeveloped in order to mix the thermoplastic matrix resin with the textile reinforcement [26].The matrix can be used in the form of a powder or a thin layer applied to a fabric. It can beinserted during stacking, or co-woven to create hybrid fabrics. These developments tend toreduce the flow paths length of thermoplastic resin after melting. Commingled yarns [10, 27,28] have also been developed for this purpose [29, 30]. Spun thermoplastics are intimatelyblended with fibre reinforcements at the filament level in a yarn. The resulting commingledyarn can be used in textile processes such as weaving [31], braiding [32] or knitting [26].Fabrics created with hybrid yarns can then be processed into composite parts by thermo-compression. The combination of glass fibres (G) and polypropylene (PP) is particularlysuitable for low cost and fast production automotive applications: bumpers, front, back andside protections, spoilers [33, 34]..

Commingled yarns are prepared through the air jet mixing of G (Fig. 3a) and PP (Fig. 3b)fibres into a single yarn (Fig. 3c). The mass ratio of G to PP is 70:30, which is equivalent to afibre volume fraction of 44 % of G reinforcement in 56 % of PP matrix. The yarn architectureand heterogeneity of the commingling has been investigated through micrograph imagery of apolished cross section of the sample (Fig. 3c). The distribution of fibres within the yarn isvisible, whereby PP fibres are shown in black and G in grey/bright white. Due to the air jetprocess, it was not possible to define a clear geometry (circular, elliptic …) of the yarns crosssection, only a random dispersion of G and PP is noted.

Tensile tests (NF-EN-ISO 2062) were conducted on 20 commingled yarn specimens inorder to ensure their compatibility with the load tension applied during the braiding process.

ca

b

glass

PP

Fig. 3 Microscopic view of a) GF and b) PP and c) transverse cut of commingled yarn

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The results are given in Table 1. Tensile properties are brought to the composite almostexclusively by the glass reinforcement.

2.2 Winding

Before braiding, the yarns were rewound on bobbins that were to be placed on braiding spools.This step modified the yarn in two ways: 1) breakage or loss of yarn filaments; and 2) slightyarn twisting. Filament breakage is due to yarn friction on several guiding parts in the windingmachine. In the case of the comingled yarn, this phenomenon is clear with visible brokenfibres. The original yarn is kept on a cylindrical bobbin which is fixed during unwinding.Because of this, it creates a twisting that remains on the new bobbins (5 turns per meter). Aslight twist on commingled yarns does not affect the properties of composites and enhances thetextile preforming process [22, 27]. This twisting in fact introduces some advantages on theyarn [22, 35–37], including an increase in yarn strength and a greater resistance to friction.Both of these characteristics are particularly beneficial for braiding. It also influences thetransverse compression behaviour of the composite [38]. When compared to the initial valuesin Table 1, the strength of the comingled yarn has been measured to increase by 5.5 %, and thestrain at breaking has decreased by 9.4 %.

2.3 Braiding

Overbraiding is the covering of a mandrel using the braiding process [4, 9, 11, 39]. Duringoverbraiding, the braid is placed directly onto a mandrel core, which has the inner geometry ofthe desired preform [40]. By repeating the braid, a pre-selected number of layers can bebraided to obtain the desired wall thickness. This technique is highly reproducible andtherefore very effective for the production of net shape cost-effective structural performs [9].The mechanical properties depend on the type and number of yarns on the braiding pattern andcan be adjusted locally by varying braid parameters such as braid angle or mandrel diameter[12, 13]. Before braiding, a winding of all bobbins is necessary. Degradation of the yarn, onsizing, filament or complete yarn, occurs during this step [41, 42]. After winding, dependingon the type of yarn used (roving, twisted, continuous or not) the braiding process canadditionally produce defects at different levels. For fibrous yarns, the braiding process is harshbecause it introduces tension at each individual yarn, yarn/yarn and yarn/tool friction [41].With regard to the commingled yarns, the PP yarns are melted surrounding the G yarnstructure. The G yarns affect the flow of the melted PP media. This step increases the difficultyto predict the behaviour.

Table 1 Yarn propertiesProperties Standard Value

Linear density (Tex = g/km) 636.4

Glass fibre diameter (μm) 17

PP fibre diameter (μm) 47

Strain at break (%) NF-EN-ISO 2062 4.25

Tensile stress (cN/tex) NF-EN-ISO 2062 20.78

Glass content in mass (%) 71.6

Twist No

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The braiding machinery used (Herzog GLH 1/97/96–100), consisted of 96 yarn carriers forthe bias yarn network and 48 for the axial yarns. Bias yarns are divided into two sets followingsinusoidal paths in clockwise and counter clockwise directions that create the interlacing(Fig. 4). The third set of yarns, denoted axial yarns, are added to the middle of each horngear and inserted in the textile architecture along the braid axis. The braid angle θ (Fig. 5a) is aconsequence of the ratio of the take up speed v (m.s−1) and the rotational velocity of the spoolsω (s−1) and can be estimated [43] from the mandrel radius R (mm), the rotational velocity ofyarn carriers on the machinery (s−1) and the take-up speed of the mandrel (mm.s−1). Thepattern used in this study is a regular triaxial braid (Fig. 5b). The prism mandrel geometrymodifies the local mandrel radius and has a strong influence on the braid angle. According tothe mandrel geometry of the final part, the braid angle will vary from 35° to 55° and thisvariation is taken into account for the following material characterization.(Eq 1)

θ ¼ tan−12πRωυ

� �ð1Þ

To evaluate the braided materials properties three specimens, denoted Ref-55, Ref-45 andRef-35, have been investigated. The structure was braided layer by layer upon a tubularmandrel with a 50 mm diameter. Four layers were braided on the same mandrel for eachreference. The spools rotational velocity was held constant during braiding. As the braid wasmade of several layers, the thickness of each layer increases the apparent diameter on whichthe next layer will be braided. For this reason, the take-up speed was adjusted in order to keepthe same braid angle from one layer to the next. According to the yarn count, tubular mandrelgeometry and braiding parameters, the cover factor [45] obtained is close to 100 %.

Fig. 4 Braiding machinery

Fig. 5 a) Triaxial regular braid with braiding angle and b) same modelled with TexGen [44]

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Table 2 gives the braiding parameters for each layer. The braid angles along the tubularstructure were measured in three different locations for each layer denoted A, B, C andrespectively at the beginning, middle and at the end of the braid. The standard―deviationsfor the braid angle given in Table 2 show a good regularity for the braiding process.

In order to achieve the rigid composite form through consolidation by heating press, thefour layered structure was cut along the mandrel direction after braiding, and opened andflattened. Cutting and flattening the braid modified the fibrous architecture and especially thebraid angle as shown in Table 3. The change in the braid angle comes from the modification ofdimensions of samples and consequently the in-plane shear after the cutting, as described inFig. 6. Modification on the braid angle is about 14 % with Ref-55 and Ref-45 because of theirtight interlacing caused by the higher braid angle. Ref-35, with a lower braid angle, is almostnot affected by braiding angle modification from tubular to flat sample because yarns are freerwithin the structure.

2.4 Thermo-compression

The textile architecture made of the braided commingled yarns were heated and pressed inorder to melt the PP matrix and form the composite structure with G reinforcement. Theconsolidation process greatly influences the composite performance [42] but is also dependanton the preform parameters such as the thickness. Processing parameters (pressure and tem-perature) are given in Fig. 7.

Table 2 Manufacturing parameters and braiding angles layer/layer on tubular braid

Specimen Layer Parameters Angles measurements (°) Averageangle/layer

Averageangle

Diameter (mm) Take-up ratio(mm-1)

A B C

Ref-55 1 49,0 150 51,2 52,6 51,7 51,8 48±3,3

2 51,3 165 48,3 48,5 NC 48,4

3 53,5 185 48,2 47,8 NC 48,0

4 55,7 205 42,2 44,9 NC 43,5

Ref-45 1 48,7 215 40,9 41,8 NC 41,3 41±1,3

2 51,0 230 40,0 41,2 40,9 40,7

3 52,8 245 40,3 37,6 40,7 39,5

4 54,5 260 N/A N/A N/A N/A

Ref-35 1 49,0 300 35,7 32,9 31,5 33,4 32±2,0

2 51,2 320 31,9 32,5 30,8 31,7

3 52,5 340 32,2 31,2 28,3 30,6

4 54,4 360 N/A N/A N/A N/A

Table 3 Influence of cutting andopening on braiding angle Specimen Angle on tubular

braid (°)Angle on flatbraid (°)

Modification (%)

Ref-55 48 55 14,0

Ref-45 41 47 14,9

Ref-35 32 32 −0,2

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The composite plates measured approximately 290 × 155 mm in size. The thicknesses weremeasured before and after thermo-compression. For non-consolidated braids, the textilestandard ISO 5084 (Pressure is set to 1 kPa) was used for thickness measurements. Oncomposite plates, a calliper was used. Table 4 compares measurements and gives the com-paction rate Cr for each structure calculated by Equation (2). Nesting, also known as fibrearchitecture reorganisation, occurs when pressure is applied to the braided fabric. Duringcompaction a high braid angle decreases the nesting effect [13]. It is consistent with the braidstructures behaviour whos movement is restricted with higher braid angles.

Cr ¼ 100ebraid � eplate� �

ebraidð2Þ

Due to the manufacturing parameters, the resulting plates differed in terms of their areadensity. The measured area densities are given in Fig. 8 and are compared with theirthicknesses. It is important to clarify that no matrix of the braided preforms is lost duringthe manufacturing process. Consequently, the fibre volume fraction of the samples, is equiv-alent to the fibre volume content in the raw commingled yarn 44 %. The thickness is directlylinked with the area density due to the constant density of the global material.

Fig. 6 Braids on tubular mandrel a) Ref-55 b) Ref-45 c) Ref-35 and after flattening d) Ref-55 e) Ref-45f) Ref-35

Fig. 7 Temperature and pressure profiles for thermo-compression

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2.5 Samples preparation

Plates were cut with a jigsaw in axial (AX) and transverse (TR) directions to prepare thespecimens for testing. Tensile tests were adapted from ISO 527–4 with modified specimensizes (Fig. 9). Greater widths were used than the standard recommendations in order to keep ahigher value than the width of the unit cell. The length for transverse specimens is limited bythe mandrel diameter.

2.6 Testing

Tests were performed on braids in the AX direction and TR direction (Fig. 9), using Instron5900 tensile testing machinery. The testing velocity was set to 2 mm.min−1 and the moduluswas evaluated in 0.05 % and 0.25 % strain intervals. Six specimens were used in the AXdirection and four in the TR direction. A video extensometer was used with 50 mm gauge forAX specimens and 25 mm for TR specimens. According to standards the grip separation is115 mm for AX specimens but was reduced to 50 mm for TR specimens due to lengthrestrictions.

2.7 Mechanical results

For tensile testing of composite plates in the AX direction, a clear breakage is seen (Fig. 10a).This behaviour is the result of the axial reinforcement in triaxial braids. The bias yarns and thethird set of yarns, oriented along the loading direction, are involved to a lesser degree due totheir misalignment. In this direction, tensile strengths are restricted within a relatively smallinterval.

In the TR direction, the tensile properties are determined by the braid angle but thebehaviours of each are different. After the linear response, a yield and a stress loss of about

Table 4 Braids and compositeplates thicknesses and compactionrate

Specimen Thicknesses (mm) Compaction rate (%)

Braid Plate

Ref-55 5,62±0,05 1,98±0,07 65

Ref-45 5,48±0,08 1,81±0,11 67

Ref-[35] 5,48±0,10 1,51±0,03 72

Fig. 8 Comparison of thickness and are density on composite plates

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10 % are observed for 35° and 45° (Fig. 10b). This yield depends on the braid angle and itmodifies the breaking strain from 3 to 10 times higher than the yield strain. For 35°, the strainat breaking is included in 25 % and 55 % intervals. The interval is reduced in the case of 45°:25 % to 30 %. For 55°, breaking occurs rapidly after yield and in a regular way around 9 %. Incomparison to 35 and 45 °, samples with a 55° braid angle have a clear breakage.

The mode of failure encountered in the AX direction is clear and sudden along the braidingangle in +/− directions without variation (Fig. 11). Axial yarns are broken first and the load isthen transferred to bias yarns that tend to be aligned with the load by matrix separation.

The transverse behaviour of glass fibre reinforced thermoplastic braids comes from a visibledeformation of the fibrous architecture. Figure 12 highlights the remaining shrinkage onspecimens after tensile testing. The higher angle of 55° specimens offers a better reinforcementorientation in the TR direction. It also leads to comparable AX and TR stress at breaking forthis specific braid angle. In biaxial braids, the balanced properties are obtained with a 45° braidangle, but in triaxial braiding, axial yarns drastically improve properties in this direction.Finally, experimental results show a clear dependency on braiding angles as mentioned in theliterature (Fig. 13). According to the angle, reinforcement orientation is more or less close to

Fig. 9 Specimen sizes and orientations

Fig. 10 Strain–stress curves for a) axial and b) transverse directions

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the loading orientation. In both AX and TR cases, bias yarns are angled with the loadingdirection.

3 Functionalization: Structural health monitoring

The integration of a sensor system to composite parts was undertaken as follows. The inclusionof a sensor system allows for the structural health of the perform to be monitored,. Thepreparation of piezo-resistive sensor yarns, based on PEDOT: PSS, firstly included thepreparation of the sensor solution. The sensor solution discussed here follows studies under-taken by Trifigny et al. whereby sensor yarns were prepared using a coating of PEDOT: PSSwith PVA [15]. PVA increases the viscosity of the sensor solution allowing easer application.Additionally, the mechanical properties of the dried sensor were noticed to increase. However,because PVA is not stable above 100 °C, such a formulation is not functional for thisapplication which includes a thermal treatment at 200 °C to consolidate the composite. In thisstudy, instead of combining with a secondary solvent, commercially available PEDOT: PSS(Clevios CPP 105 days) was heated at 80 °C to remove the primary solvent at controlledlevels. This acts to increase the viscosity and the conductivity of the solution. The originalsolution of Clevios CPP 105 days contains a 1.2 wt.% solid content of PEDOT: PSS. Throughthe controlled removal of the solvent, solutions/gels containing respective solid contents ofPEDOT: PSS of 1.3, 1.5, 1.8, 2.0 & 2.2 wt.%, were obtained. The sensor solution was appliedto 20 cm lengths of glass yarns by paintbrush, in two separate coats (allowed to dry betweencoatings).

Fig. 11 Specimen a) before and b) after tensile test in AX direction

Fig. 12 Specimen a) before and b) after tensile test in TR direction

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Prepared sensor yarns were then inserted into the 4-layered braided yarn structure (55°braid angle) according to Fig. 14a. In this instance the sensor yarns were inserted manually forproof of concept, however, it is possible to integrate them directly as part of the braidingprocess. Two sensor yarns were inserted from the top face, passing through two layers of braid,continuing along between the two layers of braid in the axial direction and exiting on the sameface as the entry point. The dimensions of the braid structure with the inserted sensor yarns aregiven in Fig. 14b. The prepared structures were consolidated respecting the process parametersoutlined above. Figure 15 shows the 4-layered braided fabric composite, with sensors inserted,both before (Fig. 15a) and after (Fig. 15b) consolidation. The sensors were connected to themeasurement device by means of conductive wires (shown in white) affixed to the sensorsusing silver paint.

Resistivity measurements of sensor coatings were undertaken before and after consolidationin order to estimate the percolation threshold of PEDOT: PSS solutions and consequently toestablish the best dry mass content for sensor development, i.e. that which will give the bestpossible sensitivity. In this case, a 1.5 wt.% loading of was selected for the ongoing studies inorder to benefit from the dramatic change in resistivity to elongation (percolation). Theresistivities of the sensor yarns before and after consolidation are very similar. This indicatesstability of PEDOT: PSS to high temperature and pressure, and confirms its suitability forapplication as a SHM sensor for consolidated composite structures.

Testing of the response of sensors to mechanical stress was undertaken in the axialdirection, following precisely the testing method previously outlined. When a piezo-resistivematerial is strained, it undergoes a change in electrical resistance, and it is this change thatmakes the strain gauge a useful sensorial device. The measure of this resistance change with

Fig. 13 Tensile strength and modulus

Fig. 14 a) Schematic showing insertion of sensor yarn; and b) structural dimensions

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strain is the gauge factor (GF). The GF is defined as the ratio of the fractional change inresistance to the fractional change in length (strain) along the axis of the gauge according to theequation (3):

GF ¼ ΔR=R

ΔL=L¼ ΔR=R

εð3Þ

When the change in resistance (ΔR/R) of the sensor yarn is graphed against the strain (ε orfractional change in length), the GF can be obtained from the slope of the graph (i.e. (ΔR/R)/ε).A consolidated composite part with an inserted sensor yarn, based on a 1.5 wt.% loading ofPEDOT: PSS, shows a GF of 73.5 (Fig. 17). In addition, the sensor covers the GF strain at

Fig. 15 Composite structure with sensor yarns inserted a) before and b) after consolidation

Fig. 16 Calculation of the gauge factor of a PEDOT: PSS-based sensor yarn (1.5 wt.% PEDOT: PSS) insertedand consolidated within a composite plate

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breaking. Metal strain gauges typically exhibit GFs in the range of 0.8 to 3, and single crystalsilicon from 1 to 150, depending on orientation and doping. The larger the GF value, the moresensitive the strain gauge is, hence the PEDOT: PSS sensor shows great potential. It should benoted that the mechanical properties of the composite part were not affected with the insertedsensor. The stress at breaking is 210 MPa and the strain at break is 2.5 %, with the modulus14 GPa. These values are higher than those without sensors because of the addition of two axialyarns (the sensors) within the structure.(Fig.16)

3.1 Complete part manufacturing

The complete solution of the composite cross stiffener preforms (Fig. 1.b) has beenmanufactured according to the steps described above. Step one included the overbraiding ona shaped mandrel. The mandrel (Fig. 17a) was made of sand bonded with polyvinyl alcohol(PVA), a water-soluble binder. The mandrel was covered (Fig. 17b) with non-adhesive tape inorder to reduce yarn friction during the braiding and also to achieve a higher quality surface.The mandrel was then covered (Fig. 17c) with 4 layers of braided reinforcement manufacturedwith a measured braid angle close to 45°.

The braid angles were measured on different faces and locations along the preform for eachbraided layer. Measured braid angles are presented in Fig. 18, allowing a mapping of the angle.Angles tend to increase from one layer to the next and also along the mandrel. The second

Fig. 17 a) sand based mandrel, b) mandrel with non-adhesive fabric surface and c) 4-layer overbraided mandrel

Fig. 18 Measured braid angles on mandrel

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aspect gives the difference of angles from the top, the sides and the bottom of the mandrel. Thetop of the mandrel has the more complex and changing eccentricity that results in a higherrange in the measured angles. The bottom is mainly flat and has a constant eccentricity whichexplains the higher angles values with a reduced range. The average braid angle on the wholepart is 44° with a standard deviation of 3.7°.

After overbraiding, the samples were assembled and consolidated together in order toperform an in situ bonding during this step. As the prepared samples were for proof of conceptand not for mechanical testing, the creation of a complex mould for the consolidation processwould incur a high cost. Therefore, prototypes were instead consolidated under vacuum at200 °C. The consolidation time was set to 60 min heating, 180 min consolidation and 60 mincooling. The resulting consolidated preform is presented in Fig. 19. Wrinkles are visible on thepreforms surface due to the use of the vacuum bag and its low pressure compared to heatingpress. The consequence of the lower consolidation pressure is a lower quality of impregnationcompared to the compressed analogues. Nevertheless, these problems can easily be solved bythe use of an appropriate mould.

Fig. 19 Consolidated crossing of stiffeners made from overbraiding

Fig. 20 Longitudinal section of a stiffener after consolidation

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In order to compare the compaction state of this preform, a consolidated hollow mandrelwas produced and cut along its axis. Points of interest (POI) were selected to measure thethickness of the part. Figure 20 presents a longitudinal cut of a consolidated stiffener and POIpositions. The hollow structure is clearly visible and shows no indication of the sand/PVAmandrel used. Table 5 gives the measured thicknesses along the cutting edge of the mandrel.The regularity of the thicknesses, with regard to the manufacturing method based on a softmould (vacuum bag), are deemed satisfactory. The largest thickness value is located at position5 and is due to the curved part of the edge.

4 Conclusion

This paper investigates the design and manufacturing process of a composite preform partbased on commingled yarns overbraided on a complex shaped mandrel. First, the commingledyarn braiding ability has been verified. The material has then been studied in order to choosethe appropriate fibre architecture. Three constant braiding angles with a regular triaxialbraid were tested: 55°, 45° and 35°. The composite structures were thermo-compacted inorder to create composite plates. These were then cut and tested in AX and TR directionsto identify engineering constants and identify the global behaviour of the materials underloading. Braids with 55° were also tested with integrated piezo-resistive sensors based onPEDOT: PSS, allowing the ability to monitor the structural health of the material.Integrated sensor yarns, which have the ability to monitor the structural health of thecomposite part, have shown resistance to the high temperatures and pressures of theconsolidation process. The mechanical behaviour of the material is not degraded by theinsertion of the sensors and shows compatibility with the textile process for in-lineintegration during the textile manufacturing of the reinforcement. Finally a complete partbased on the studied design has been successfully manufactured and characterized interms of the fibrous architecture. Consolidation was undertaken, demonstrating the feasi-bility of the complete one-shot process.

Acknowledgments The authors thank the EU Commission for the funding that made this researchstudy possible

Table 5 Thickness of compositeaccording to position on longitudinalcut

Position Thickness (mm) Position Thickness (mm)

1 2.12 10 2.00

2 2.30 11 2.66

3 1.94 12 2.30

4 2.42 13 2.32

5 3.70 14 2.18

6 2.34 Mean 2.29

7 2.36 STD 0.22

8 2.40

9 2.70

Mean 2.48

STD 0.48

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Conflict of interest None declared.

Funding This study has received the support from the European Commission through the large-scale integrat-ing collaborative project MAPPIC 3D - number 263159–1 - and entitled: One-shot Manufacturing on large scaleof 3D up graded panels and stiffeners for lightweight thermoplastic textile composite structures.

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