3D reinforced stitched carbonepoxy laminates made by tailored ﬁbre placement

3D reinforced stitched carbon/epoxy laminates made bytailored fibre placement

P. Mattheij, K. Gliesche*, D. Feltin

Institute of Polymer Research Dresden, Hohe Str. 6, D-01069 Dresden, Germany

Received 29 January 1999; received in revised form 11 October 1999; accepted 28 October 1999

Abstract

Tailored fibre placement (TFP) preforms made of carbon fibre were 3D reinforced with aramid, polybenzoxazol (PBO), polyethylene andpolyester fibres and vacuum injected with epoxy resin. The effects of stitch distribution and stitching process parameters on Mode Iinterlaminar fracture toughness were analysed using a statistical approach. Stitch distribution had a minor effect but 3D thread tensionhad to be carefully chosen to gain optimum mechanical properties. PBO fibre provided the most improvement in fracture toughness. 3Dreinforcing with aramid fibre reduced tensile and flexural properties by 3-8%. Low velocity impact damage in TFP was larger than in fabricbut smaller than in tape laminates. Compression-after-impact strength was partly increased by 3D reinforcing in some circumstances but noimprovement was found under other conditions.q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Through-the-thickness reinforcement; E. Preform; D. Ultrasonics

1. Introduction

Tailored fibre placement (TFP) is a novel textile manu-facturing technique for continuous fibre preforms. It is basedupon the embroidery technique, which is used for decorat-ing fabrics. This innovative concept allows the manufactureof preforms tailored for a specific composite component.The local fibre orientation as well as the local fibre contentcan be varied, for instance, according to the results of a finiteelement analysis of the component. The perimeter of thepreform can be made near-net shape. Fig. 1a shows theprinciple of TFP. Using stitching, a roving providing in-plane reinforcement is fixed on a base material as the basematerial is moved in thex,y-direction. The roving is fixedwith zigzag stitches on either side of the roving. Themachine moves the base material automatically accordingto a software pattern developed specifically for TFPpreforms. The highly automated manufacturing process,using multiple heads, makes it attractive for series produc-tion of preforms for advanced composite parts.

Advantages of TFP-preforms:

• Great variety of textile structures◦ Stress field aligned fibre placement

◦ 3D-reinforced preforms (full or partial)◦ deep-drawable preforms

• processing of◦ natural, glass, aramid, carbon and ceramic fibres

• maximum exploitation of reinforcing fibres through◦ uniformly stressed fibres in the composite◦ near-net-shape production (no cutting, low waste)

• low production costs through◦ use of rovings◦ high degree of automation

Fig. 1b and c show typical application of TFP-preforms.More details about the TFP-process, achievable mechan-

ical properties and applications are given elsewhere [1–3].In most cases the needle thread used is a thin polyester yarnwhich is suited for maintaining preform integrity until theconsolidation process has ended. However, when using aneedle thread of reinforcing fibres, e.g. aramid, the preformis reinforced in the out-of-plane direction. Thus, a preformfor a specific component can be 3D reinforced with locallyvarying stitch densities.

3D reinforcing is a way to dramatically improve the lami-nate properties in the out-of-plane direction. The improvedinterlaminar fracture toughness is favourable in many appli-cations where peel stresses are to be expected. For instancepeel stresses occur at the plydrops, free edges, or in buckledsub-laminates of an impact damage area in a panel loaded incompression. But 3D reinforcing is usually balanced by a

Composites: Part A 31 (2000) 571–581

1359-835X/00/$ - see front matterq 2000 Elsevier Science Ltd. All rights reserved.PII: S1359-835X(99)00096-2

www.elsevier.com/locate/compositesa

* Corresponding author. Tel.:149-351-465-8320; fax:149-351-465-8284.

E-mail address:[email protected] (K. Gliesche).

reduction in in-plane properties. Many authors have inves-tigated the advantages and drawbacks of stitched 3D rein-forcements in woven and multi-axial warp knitted fabrics.Reviews are given by Dransfield et al. [4], Brandt et al. [5]and Mouritz et al. [6]. Most studies report that 3D stitchingreduced the in-plane properties to some degree. This isbelieved to be caused mainly by fibre misalignment due tothe space occupied by the through-the-thickness thread andto a lesser extent by the damage done during stitching [6]. Alarge increase in Mode I interlaminar fracture toughness isobserved in many cases and to a lesser extent in the Mode IIfracture toughness depend to 3D reinforcing [7]. Compres-sion-after-impact strength is reported to benefit fromstitching [5] but also no improvement has been found [8].These findings indicate that 3D reinforcements should be

used with care, only at those parts in a structure which needthe increased out-of-plane properties and associated in-plane degradation is acceptable. No information exists forTFP laminates.

The aim of this paper is to show the effect of through-the-thickness reinforcing of TFP preforms on in-plane and out-of-plane mechanical properties. Cross-ply and quasi-isotro-pic carbon/epoxy laminates are investigated. The study isdivided into two parts. In the first series of tests the influ-ences of stitch density, stitch distribution and thread tensionon Mode I interlaminar fracture toughness were determined.A statistical non-linear model was generated to determinethe optimum stitching parameters. These stitchingparameters were used in the second test series. In-planeand out-of-plane mechanical properties for 3D reinforced

P. Mattheij et al. / Composites: Part A 31 (2000) 571–581572

3

1

24

5

X

Y

(a)

1 - Needle 4 - Hold down device2 - Needle thread 5 - Base material3 - Carbon roving X,Y - possible move directions

Principle of tailored fibre placement

(b)

Tension-Compression-Strut, Preform and Componenet

(c)

Maschine part for a loom, Preform and Componenet

Fig. 1. Principle of tailored fibre placement and typical applications.

TFP laminates were determined and compared with lami-nates without 3D reinforcement. The effect of the standardpolyester needle yarn used for tailored fibre placement wasalso investigated. The following tests were carried out:tension, flexural, double-cantilever beam (DCB) andcompression-after-impact (CAI). Fabric laminates wereCAI tested for comparison. Besides aramid, which wasused in all the tests, three other 3D reinforcing fibre materi-als (polybenzoxazol (PBO), polyethylene and polyesterfibres) were used in the DCB tests.

2. Experimental details

2.1. Preform manufacturing and fibre material

TFP preforms were manufactured using a fibre placementmachine with four working units. The preforms were madefrom Tenax HTA 5331 12k carbon fibre. The upper andlower yarn was a polyester (PES) with a linear weight of10 tex. Alternatively some preforms for CAI tests weremade with a 5 tex PES yarn. The needle diameter was0.9 mm. The preforms were manufactured using a zigzagstitch with a machine velocity of 400 stitches per minute.The base material was a thin glass fibre fabric with 100 g/m2

areal weight. The 3D reinforcement was brought in bythrough-the-thickness stitching of two preforms togetheron a second embroidery machine. A modified lock stitchwas used with the lower thread being PES and the upperthread being the reinforcement fibre. Only in case of 3Dreinforcing with polyester fibre were both the upper andlower yarn PE. Table 1 shows details about the thread

materials used for making the preforms and for 3D reinfor-cing. Fig. 2 shows the principle lay-up of a 3D reinforcedTFP-preform. The first test series was carried out withpreforms 3D reinforced with aramid fibre. The upper andlower thread tensions were measured statically before 3Dreinforcing the carbon fibre preforms. The stitch densityranged from 1.8 to 15.4 stitches per cm2 with the stitchesbeing aligned in rows. Three stitching parameters, stitch rowspacing, stitch length, and thread tension, were varied onfive levels giving a total of fifteen preform variants. Thevariations were needed to find the optimum properties inall three directions of the composite.

In the second part of the project the stitching parametersof stitch length and stitch row spacing were held constant at3.5 mm. The upper/lower thread tension ratio was 3.2(210 cN/65 cN) for the aramid, 3.8 (250 cN/65 cN) for thePBO and 4.3 (150 cN/35 cN) for PE. For comparisonpreforms of carbon fibre plain weave fabric style 98 150with 250 g/m2 areal weight (Interglas AG) were also tested.

2.2. Consolidation process

The preforms were consolidated using the vacuum bagresin injection process. During the consolidation processevery preform was covered with a steel plate to create asmooth surface on both specimen sides. Mould temperatureduring injection was 358C. The epoxy resin system Rueta-pox VE 3966 (Bakelite AG) is a low viscosity RTM-resin.After the resin had wetted out the complete preform, themould temperature was raised to 808C for 1 h for curing.Typical carbon fibre content in the composite was approxi-mately 55% by volume. The amount of polyester yarn usedto fix the roving in the placement process was typically 1.5–2% by volume. The fibre volume content of the 3-Dreinforcement thread material ranged from 2 to 1.8%.

2.3. Specimens and mechanical tests

Four different mechanical tests were performed in accor-dance with the standards listed in Table 2. The specimendimensions and laminate lay-ups are also given there.Specimens were cut with a water-cooled diamond saw. CAIspecimens were milled to their final dimensions. All the

P. Mattheij et al. / Composites: Part A 31 (2000) 571–581 573

Table 1Threads for preform manufacturing and 3D reinforcing

Thread material (supplier) Tex Preform manufacturing 3D

PES Serafil 200/2 (Amann) 10 × ×PES Serafil (Amann) 5 ×Aramid Kevlar 75 (Amann) 40 ×PE Dyneema SK 65 (DSM) 5.5 ×PBO Zylon AS 500 (Toyobo) 55 ×

Fig. 2. Principle of lay-up of a 3D reinforced TFP laminate.

specimens (four specimens at each level) were cut with the 3Dreinforcement rows in specimen length direction. The tensile,3-point bending and DCB tests were conducted on cross-plylaminates.. All DCB specimens had a starter film of 25mmthickness up to 50 mm from the loading line. This region wasnot 3-D reinforced. Aluminium end blocks sized 10× 10×20 mm3 were bonded on the DCB specimens for load intro-duction. The corrected beam theory was used to derive theGIc

values. As the available compression test machine was limitedto 100 kN maximum load the cross-ply laminate could not betaken for the CAI tests. Therefore, the lay-up for the CAI testswas quasi-isotropic. An angle of 458 between adjacent laminawas chosen to minimise stiffness mismatch. CAI specimenswere low velocity impacted with energy of 15, 30 J, or, forreference, not impacted at all. The hemisphere diameter of thedrop-weight was 25 mm and the side of impact was at the 3Daramid fibre side.

3. DCB tests to derive a statistical model

In a series of preliminary tests the effects of seven preformproduction parameters on composite mechanical propertieswere examined. The outcome of a significance analysis onthe test results was that three production parameters viz. 3D

stitch length, 3D stitch row spacing and the 3D upper/lowerthread tension ratio, had a significant effect on mode I fracturetoughness. Therefore, a statistical model with a square equa-tion describing the effects of these three parameters on Mode Ifracture toughness was generated. A test plan was set upaccording to Table 3. Stitch length, stitch row spacing andupper/lower thread tension ratio had to be varied on five levelswhich requireda total offifteen preform variants. Details of thestatistical analysis are described in Ref. [9]. The 3D reinforce-ment material was aramid fibre with the upper thread beingaramid and the lower thread being PES. Table 3 shows theoutcome of the DCB tests. The test results were used to derivethe following model in [9]

GIc � 1:702 0:62D 2 52:45

� �2 0:58

W 2 52:45

� �2 0:83

T 2 5:582:38

� �1 0:03

D 2 52:45

� �2

20:73

!

1 0:38W 2 52:45

� �2

20:73

!

2 0:57T 2 5:58

2:38

� �2

20:73

!�1�

with GIc Mode I energy release rate (kJ/m2), D the stitch row


Table 2Test methods and carbon/epoxy laminate lay-ups

Test Standard Specimen dimensions (mm3) Laminate lay-upa

Flexure DIN EN 2562 140× 10× 3:0 TFP [(0/90)2/B]s

Tension DIN EN 61 200× 20× 3:0 TFP [(0/90)2/B]s

DCB ESIS Mode I protocol 160× 20× 5:0 TFP [B/(0/90)3/B]s

TFP [(0/90)3/B]s

CAI EIN EN 6038 150× 100× 3:0 TFP [0/145/90/–45/B]s150× 100× 3:6 14 Fabric layers quasi-isotropic

a B� Base material is glass fibre fabric.

Table 3Test plan and test results for determining the statistical model forGIC

Preform variant Stitch length (mm) Stitch row spacing (mm) Upper/lower threadtension ratio (cN/cN)

GIc (kJ/m2)

1 7.45 7.45 7.96 0.462 2.55 7.45 7.96 0.533 7.45 2.55 7.96 0.514 2.55 2.55 7.96 1.185 7.45 7.45 3.2 1.426 2.55 7.45 3.2 2.757 7.45 2.55 3.2 2.048 2.55 2.55 3.2 4.229 8 5 5.58 0.8810 2 5 5.58 2.9311 5 8 5.58 0.9612 5 2 5.58 3.8813 5 5 8.46 0.4814 5 5 2.69 1.5415 5 5 5.58 1.68

spacing (mm),Wthe stitch length (mm) andT the upper/lowertread tension ratio (cN/cN).

Essentially, the model shows that the distribution of thestitches does not influenceGIc. Changing the stitch rowspacing and stitch length such that the stitch densityremains constant gives the nearly same curve (see Fig.3). A stitch density of approximately 8 stitches/cm2 wasseen as a good compromise between reduced in-plane andincreased out-of-plane properties. Consequently, thestitch pattern 3:5 × 3:5 mm2

; resulting in a density of8.16 stitches/cm2, was chosen for further tests. Also,Fig. 3 reveals that the highestGIc is reached with a 3Dthread tension ratio between 3 and 4. At this ratio the 3Daramid fibre is fixed close to the lower preform surface,penetrating the complete preform thickness. At higherratios the aramid is partly pulled out of the preformagain when moving to the next stitch position. The micro-graph of Fig. 4 shows the 3D reinforcement in the lami-nate of preform variant number 7 with a tension ratio of3.2. It is striking that the aramid fibre lies very straight inthe laminate. The preform is very much compacted atmanufacturing and following 3D stitching. The additionalcompaction during the consolidation process at one barpressure reduces the preform thickness only a little more.Therefore, the 3D thread remains straight. The threadtension ratio of 3.2 was taken as the optimum for allfurther tests with aramid reinforcement. Formula (1)predicts aGIc of 2.96 kJ/m2 for the parameter combinationstitch length and stitch row spacing is 3.5 mm and threadtension ratio is 3.2.


Fig. 3. Effect of stitch distribution and thread tension ratio onGIc according to the model.

1 mm

PES yarn

Resin rich area

Fig. 4. Micrograph of an aramid stitch in a TFP cross-ply laminate.

4. Tests with optimum stitching parameters

4.1. Tension and flexure test results

When considering 3D reinforcement, improved out-of-plane properties are usually balanced by decreased in-plane mechanical properties. To assess the effect of 3Dreinforcing on in-plane properties, tension and 3-point-bending tests were carried out. Table 4 shows the resultsof tension and flexure tests on TFP cross-ply carbon/epoxywith and without 3-D reinforcement. The 3D thread waseither PES or aramid fibre. The optimum stitching para-meters derived in the previous section were used giving astitch density of 8.16 stitches/cm2. The aramid side of the3D reinforced specimens was the compression side in theflexure tests.

Looking first at the non-reinforced laminates there is apositive effect when the thinner 5 tex PES yarn ispreferred to the 10 tex PES yarn for preform manufacture.Maximum tensile stress and flexural stress are higher by16.5 and 12.6%, respectively, for the thinner yarn type.

Part of the increase can be explained by a higher fibrevolume content because the resin rich areas in betweenthe individual layers created by the thinner yarn are smal-ler. The micrograph of Fig. 4 shows some PES yarns inbetween the carbon layers of a TFP laminate 3-D rein-forced with aramid. Misalignment of the carbon fibres isalso reduced with the thinner yarn. The increase in thetensile modulus is smaller than that of the flexural modu-lus but is still more than on account of the higher fibrecontent alone. 3D reinforcing with 10 tex PES has a negli-gible effect on the tensile and bending properties.However, 3D reinforcing with 40 tex aramid reduces thetensile strength and the bending strength by about 7–8%.The modulus is reduced by only 3–4% in case of TFPmade with 10 tex PES yarn. But TFP made with 5 tex PESyarn suffers a higher reduction of 8.3%. Logically thepositive effect of a thin 5 tex PES yarn is largelyfrustrated by bringing in a thick 40 tex aramid 3-D yarn.In summary, the results are in accordance with similarvalues found in the literature for carbon fabric and tapematerial [6,7,10].


Table 4Reduction in tensile and flexural properties of a cross-ply TFP laminate due to 3D reinforcing

Test Thread for preform 3D thread (8.16 St./cm2) Laminate thickness (mm) Strength (MPa) Modulus (GPa)

Tensile 10 tex PES – 3.11 726 60.210 tex PES 10 tex PES 3.09 1 1.7% 1 0.0%10 tex PES 40 tex Aramid 3.14 2 6.9% 2 3.2%5 tex PES – 2.98 846 64.55 tex PES 40 tex Aramid 2.87 2 5.5% 2 3.3%

3-P-Bending 10 tex PES – 3.14 976 70.210 tex PES 10 tex PES 3.07 2 1.2% 1 1.0%10 tex PES 40 tex Aramid 3.12 2 7.8% 2 3.8%5 tex PES 20 tex Aramid 2.96 2 6.7% 2 8.3%

Fig. 5. Mode I fracture toughness of TFP with different 3D thread materials.

4.2. Double-cantilever beam test results

The delamination resistance under Mode I loading ischaracterised by the strain energy release rateGIc. 3D rein-forcement is known to have a very positive effect on inter-laminar fracture toughness since the 3-D thread bridges thecrack. Four 3D thread materials (PES, aramid, PBO and PE)were tested for their ability to improveGIc. Details about thethread materials are given in Table 1. The optimum stitchingparameters derived in Section 3 were used giving a stitchdensity of 8.16 stitches/cm2. Only the thread tension ratiohad to be chosen individually for the different thread mate-rials, as discussed in Section 2.1. TFP cross-ply preformswere made by placing carbon fibre rovings on a thin glassfabric base material. Some of the DCB specimens weremanufactured with these glass fabric layers on the outsideof the specimen, to see whether it gives differences in thesize fromG1c. Others were made with the glass fabric layersat the plane of cleavage. Fig. 5 shows the results of the DCBtests and Fig. 6 the typical load-crack opening displacementcurves.

The three groups of columns on the left of Fig. 5 showsGIc values for TFP with the glass fabric base material on theoutside of the specimen. The plane of cleavage is thereforebetween the top carbon layers of two preforms. The non-3Dreinforced fracture toughness is 0.29 kJ/m2. A distinctimprovement is achieved by 3D reinforcing with the10 tex PES yarn used also for fixing the roving in the fibreplacement process. A TFP laminate is built up with anincreasing number of out-of-plane orientated yarns thecloser the layer is to the base material as shown in Fig. 2.This means that there is a gradient in Mode I fracture

toughness across the thickness of a TFP laminate. 3D rein-forcing with aramid improvedGIc to 2.91 kJ/m2 whichalmost equals the value predicted by model (1) of2.96 kJ/m2. This also agrees well with the value of2.82 kJ/m2 found by Jain and Mai [7] for a composite3D reinforced with 40 tex aramid and a density of8 stitches/cm2. Typical load-displacement curvesrecorded during DCB testing show that there is a‘stick-slip’ crack growth, see Fig. 6. This applies tonon-3D reinforced as well as for PES and aramid rein-forced laminates. The aramid reinforced laminate inparticular shows large load drops when single 3Dthreads fail. A mix of pulled out and failed aramidfibres characterise the crack surface of these specimens.

The three groups of columns on the right in Fig. 5 belongto TFP laminates with glass fabric base material at the planeof cleavage. The non-3D reinforced laminate has a remark-ably highGIc of 2.76 kJ/m2. This is caused by multiple crackgrowth in the resin rich area between the two glass fabriclayers. Extra energy is required to propagate the crack inlength direction because rival cracks above and below themain crack occur. This leads to a very rough crack front, alarger fracture area, and crack tip blunting. Additionally, themany PES yarns in this layer, the lower sides of twopreforms, are bridging the crack. 3D reinforcing with thevery thin PE fibre improvesGIc slightly to 3.02 kJ/m2. Thebest Mode I fracture toughness of 8.54 kJ/m2 is achievedwith PBO thread material. All PBO threads are pulled out ofthe laminate without fracture of the fibre. Even if consider-ing the higher linear weight of PBO and the different lami-nate lay-up, the improved strength and maximum strain ofPBO compared to aramid is advantageous. PBO fibre can be


Fig. 6. Typical load-crack opening curves in DCB tests with different 3D thread types.


a) TFP (10 tex PES yarn) b) prepreg tape c) plain weave fabricno 3-D no 3-D no 3-D

d) TFP ( 10 tex PES yarn) e) TFP (5 tex PES yarn) f) plain weave fabric3-D: aramid 3-D: aramid 3-D: 13 PES

Fig. 7. C-scans of quasi-isotropic carbon/epoxy panels impacted with 30 J.

Fig. 8. CAI strength of quasi-isotropic TFP and plain weave fabric laminates.

considered as an interesting alternative to aramid as 3-Dthread material.

4.3. Compression-after-impact test results

Quasi-isotropic CAI specimens made of TFP, plainweave fabric and prepreg tape were low velocity impactedwith an energy of 15, 30 J, or, for reference, not impacted atall. Some of the TFP specimens were 3D reinforced witharamid or PES fibre. The optimum stitching parametersderived in Section 3 (four specimens at each energy level)were used giving a stitch density of 8.16 stitches/cm2. Theentry side of the aramid fibre into the laminate was also theside of impact. Fabric specimens were non-3D reinforced,reinforced once with PES or up to13 times with PES. Thelast variant was used to simulate the effect stitching has inthe TFP manufacturing process. Prepreg specimens wereused only to compare impact damage size and were notCAI tested. Fig. 7 shows ultrasonic C-scans of TFP, fabricand prepreg tape panels with 30 J impact damage. CAI testresults of TFP and plain weave fabric are given in Fig. 8.

An impact energy of 15 J causes indentation depths of0.17 mm in TFP laminates regardless of through-the-thick-ness reinforcement. This is below.3 mm—the limit for abarely visible impact damage (BVID) according toDINEN6038. Impact of 30 J caused indentations of 0.72,0.82 and 0.92 mm in TFP with no fibre, PES, and aramid

3D fibre, respectively. Fig. 7 shows ultrasonic C-scans ofnon-3D reinforced as well as 3D reinforced laminates. Thedelamination in TFP typically has a rhombic shape. 3Dreinforcing with aramid does not have a great effect onthe damage size in the TFP specimens (see Fig. 7a and d).An improved effect due to the 3D fibre might be reachedwhen the 3D thread is on the back of the panel and not inline with the fibres in the exterior layer as in these tests. Thecarbon rovings then might be better protected against dela-mination. This has to be proved in further tests.

There does seem to be an effect of the PES yarn used forfibre placement. The damage in the TFP laminate made with10 tex PES is a little bit larger than for 5 tex PES (see Fig.7d and e). Because the yarn is situated between the indivi-dual plies, the larger resin pockets in the case of the thickeryarn could play a role in the delamination spreading. Fig. 4shows a PES yarn and the surrounding resin pocket. 3Dreinforcing with PES, imitating the stitching process as inTFP laminates, does not diminish the size of the delamina-tion in the fabric laminate (see Fig. 7c and f).

TFP laminates have a clearly smaller delamination areathan prepreg tape laminates. The damage size in the fabriclaminate is the smallest. Fig. 9 shows the damage area as afunction of impact energy per unit thickness and includesvalues from the literature. The diagram confirms the trendthat fabric shows the smallest delamination area. Thedamage area in TFP is larger and comparable with


Fig. 9. Delamination area after low velocity impact on carbon/epoxy.

thermoplastic tape material APC-2. It should be mentionedthat the layer thickness of the TFP, woven fabric, and tapelaminates tested here is, respectively, 0.34, 0.25 and0.12 mm. A thicker layer results in a larger delaminationarea for the same composite [11].

The left three groups of columns in Fig. 8 show the CAIstrengthofTFPmadewith10 tex PESyarn.Comparisonof thecompression strength values without impact shows that 3Dreinforcing with PES reduces the strength slightly by 2%and reinforcing with the thicker aramid thread reduces thestrength by 5% compared to the reference value of 229 MPa.A lower strength has to be expected because of the damagedone by the stitching needle and the displacement of the loadcarrying carbon fibres by the 3D thread. This effect is also seenin the tension and bending test results.

The CAI strength of TFP 3D reinforced with aramid andimpacted with 15 J is improved by 5% compared with thereference to 202 MPa. At an energy level of 30 J the differ-ence between the reference laminate and the laminate rein-forced with PES or aramid is negligible. The 30 J impactdamage is relatively large and therefore global buckling ofthe panels begins at a lower load. The delamination with theexterior fibres in the load direction acts as a buckled sub-laminate. The aramid 3D reinforcement is not able to retardthe growth of the impact damage enough to reach a higherultimate strength. Here a higher stitch density or a 3D fibrewith a higher tensile modulus per mm2 should be advanta-geous. Therefore, PBO or carbon fibre might be more effec-tive in reducing the delamination size and growth. Also, animproved effect of the 3D fibre might be reached when the3D thread is on the back of the panel and not in line with thefibres in the exterior layer as in these tests. The best way toimprove the CAI strength is to diminish the damage sizeduring impact. It is remarkable that the 3D aramid reinfor-cement in TFP made with 5 tex PES yarn is effective. Thestrength reduction by 30 J impact is only 2%. In this case theslightly smaller damage size is effectively prevented fromgrowing by the through-the-thickness aramid fibre.

Some of the fabric specimens were stitched just as in theTFP manufacturing process to examine whether stitchingaffects CAI strength. The results of the non-impacted fabricspecimens reveal that the more the stitching the lower thecompression strength (see the right three groups of columnsin Fig. 8). The fabric laminates impacted with 30 J show that3D reinforcing with PES improves the CAI strengthslightly; it is not known why. But 3D reinforcing 13 timesas in TFP has a negative effect. The fabric specimens aresomewhat thicker, about 3.6 mm compared to 3.0 mm forthe TFP specimens. Therefore, global buckling of the fabricspecimens occurs at a higher strength than the TFP speci-mens resulting in a higher ultimate strength.

5. Conclusion

Tailored fibre placement can be used for through-the-

thickness stitching with locally varying stitch densities.Different 3D fibres like aramid, PBO, polyethylene andpolyester can be processed. Mode I interlaminar fracturetoughness was increased by 3D reinforcing. The PES yarnfor manufacturing the TFP-preforms has a measurable 3Dreinforcement effect. Aramid fibre increased fracture tough-ness by a factor of 10. Referring to linear weight, PBOshowed the most improvement in Mode I interlaminar frac-ture toughness, followed by aramid and PE. Therefore, PBOfibre can be a serious alternative to aramid as a 3D reinfor-cing material. In-plane tensile and flexural properties werereduced by 3D reinforcing with aramid by 3–8%. Low velo-city impact damage area in TFP was larger than in wovenfabric but smaller than in prepreg tape laminates. 3D rein-forcing with aramid did not have a measurable effect on thedelamination size in TFP under the given circumstances.The stitch density of the PES yarn used for fibre placementdid not diminish the impact damage. The effect of 3D rein-forcing with aramid fibre on CAI strength was ambiguous.At 15 J impact energy the CAI-strength improved by 3Dreinforcing. At 30 J impact the aramid 3-D reinforcementwas effective for TFP made with 5 tex PES yarn but not forTFP with 10 tex PES yarn. Further work has to be done onhow to improve the compression-after-impact strength ofTFP laminates by through-the-thickness stitching.

Acknowledgements

This work was sponsored by the “Deutsche Forschungs-gemeinschaft” DFG. We thank you for this support.

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[10] Mouritz AP, Gallagher J, Goodwin AA. Flexural strength and inter-laminar shear strength of stitched GRP laminates following repeatedimpacts. Compos Sci Technol 1997;57:509–22.

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3D reinforced stitched carbonepoxy laminates made by tailored ﬁbre placement

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