Optimisation of crush energy absorption of non-crimp fabric laminates by through-thickness stitching

Composites: Part A 42 (2011) 712–722

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Composites: Part A journal homepage: www.elsevier.com/locate/compositesa

Optimisation of crush energy absorption of non-crimp fabric laminates by through-thickness stitching Silvano Cauchi-Savona a, Chi Zhang b, Paul Hogg b,⇑ a b

School of Engineering and Materials Science, Queen Mary, University of London, Mile End Road, London E1 4NS, UK Northwest Composites Centre, School of Materials, The University of Manchester, Grosvenor Street, Manchester M1 7HS, UK

a r t i c l e

i n f o

Article history: Received 25 May 2010 Received in revised form 7 February 2011 Accepted 12 February 2011 Available online 21 February 2011 Keywords: A. Polymer-matrix composites (PMCs) B. Mechanical properties C. Damage mechanics E. Stitching

a b s t r a c t Through-thickness stitching has been used to significantly improve the Mode-I interlaminar fracture toughness properties, however these improvements do not always translate positively to the crushing properties as there are many parameters that could influence the energy absorbed. The current research was therefore undertaken to understand the relationship between stitching of non-crimp fabrics (NCFs) and the resultant effects on energy absorption during crushing. A design of experiments (DOE) technique based upon the Taguchi method was utilised to statistically determine the relationship and assist in optimising the crushing properties. The laminates were flat plates manufactured from carbon- and glass– fibre NCFs; stitching was carried out with carbon and Kevlar threads in different configurations, and the laminates were vacuum infused with vinyl-ester and epoxy resins. The results show that by selecting an optimised stitching configuration, an improvement of 30% in the energy absorption can be achieved. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Composite structures are capable of increased energy absorption over that of metals [1] and are therefore being increasingly exploited within automotive structures. The improvement in energy absorption for well-designed composite structures occurs through a series of processes involving splitting and brittle fracturing, while metals crush by a series of folding/buckling mechanisms [2,3]. The crushing process is initiated through a geometric trigger which is typically a chamfer or ply drop-off. This helps in the formation of a central crack which would limit the initial peak load so that the crushing would initiate before the collapse of the composite either by buckling or compressive failure. Following the formation of this initial crack, the crushing can proceed in a stable manner. The crushing process is controlled by various forms of energy dissipation that are: central crack growth, splitting of fronds, delaminations within the fronds, fibre fracture, friction in the laminate and with the crushing platen as well as bending of fronds and tearing [4]. This therefore implies that changing the interlaminar and intralaminar properties of the composites should also influence the energy absorption; for example, in carbon fibre tubes with thermoplastic matrices the energy absorbed increased with increasing interlaminar fracture toughness properties [5]. This has also been observed in matrix toughened thermoset-resin ⇑ Corresponding author. Tel.: +44 (0)1612758167; fax: +44 (0)1613068877. E-mail address: [email protected] (P. Hogg). 1359-835X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2011.02.008

composite tubes manufactured by RTM [6,7]. While matrix-controlled interlaminar toughness is important, significant benefits can be also obtained by aligning the fibres as close as possible to the crushing direction [8] since the compressive stiffness and strength are improved. The addition of randomly oriented chopped glass–fibres to NCFs [7,9] has also been shown to improve the absorbed energy since the random fibres have the benefit of improving the interlaminar fracture toughness of the composite. Previous work based on the crushing of composite plates has shown that the energy absorption capability of a composite is proportional to the Mode-I and Mode-II fracture toughness values [10] as well as the interlaminar shear strength [9]. An alternative method of improving the interlaminar toughness properties of composites involves through-thickness stitching. This has been shown to dramatically improve the Mode-I delamination resistance of composite materials [11,12], while potentially stabilising Mode-II crack growth [12]. This method of improving interlaminar toughness is ideally suited to engineered fabrics such as non-crimp fabrics (NCFs) [13] that have relatively low fracture toughness properties and crush predominantly through a frondsplaying mechanism. Stitching of NCFs also shows benefits at the initial stages of crushing where the stitches could prevent the formation of a long central crack that could destabilise the composite and cause it to crush in a low-energy splaying mode. Conversely, stitching was not shown to benefit laminates having a sufficiently high interlaminar fracture toughness such as composites based on continuous fibre random mat (CoFRM) since these materials exhibit significant fibre and matrix fragmentation

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and have short fronds [13]. It was thereby concluded that there is a minimum Mode-I fracture toughness required to crush in a highenergy mode and further increasing this does not improve the SSCS. Contrary to the above, earlier investigations in stitching on NCF-based flat plates [14] had shown that stitching did not always improve the energy absorbed in crushing of NCFs. For this reason, it was necessary to investigate different stitching parameters to identify the ideal configurations that would improve the energy absorbed by these laminates. There are several stitching parameters that could influence the crushing properties of composites: these include stitching density, thread type and break strength, stitch type as well as manufacturing variables such as needle size/type, thread tension and sewing machine type [15]. It is also important to note stitching distorts the fibres around the stitch and could introduce crimping resulting in a possible decrease in compressive stiffness and strength of up to 15% and 35% respectively [12]. This is significant in that any increase in the crushing properties through stitching has to be balanced with the reduction in strength induced by the stitching to avoid premature collapse/failure of the composite.

2. Optimisation of stitching parameters Preliminary investigations [16] into the crushing of composite plates have shown that, for a range of materials based on glass– fibre NCFs, there was no significant influence of fabric architecture, resin type or fibre orientation between quadriaxial or triaxial orientations. The SSCS fell between two bounds where triaxial configurations had a higher degree of scatter especially where there were large numbers of 0 to ±45 fibres such as in [±45/03]S layup. This was because a long central crack was formed as a result of their poor fracture toughness properties [10] and these orientations failed principally through frond splaying and little fragmentation. This implies that through-thickness stitching would mostly benefit the triaxial configurations. Warrior et al. [13], identified some possible parameters however, the effect of all stitching parameters on crushing have not been thoroughly investigated. There are a few stitching parameters that can be considered significant: the thread Tex, strength, type and thickness, the stitching method, stitching density, sewing machine foot pressure as well as the direction of stitching. For example, too high a foot pressure would adversely affect the final properties of the laminate [17], while too high a thread tension would result in large resin pockets between the stitched strands [12]. While a trial-and-error technique can be used to identify the influences of stitching on the crushing properties, this would require a large number of experimental runs to minimise the effect of experimental and manufacturing noise within the tests. One manner of overcoming this is to use a design of experiments (DOE) approach. This is a statistical-based approach to model and analyse the influence of process variables over some specific parameter that is an unknown function of these process variables. As a result, the most important stage of a DOE is to ensure that as many control factors as possible are selected for analysis. In this respect, both significant and apparently non-significant variables have to be included so that insignificant variables can be isolated and a meaningful result obtained. The aim of this work was therefore to investigate the effects of stitching on NCF-based composites, measured in terms of specific sustained crushing strength (SSCS) and try to identify an ideal stitching configuration from a selection of parameters through the use of a Taguchi DOE. Flat plates were chosen as the test specimens since these can be easily stitched with a conventional industrial lockstitch sewing machine. Furthermore, previous research

713

has shown that the specific energies absorbed in crushing by stabilised flat plates are comparable to the specific energies absorbed by tubes [18,19].

3. Experimental methods 3.1. Materials and manufacturing The crush test specimens used in this research are manufactured from NCFs held together by stitching yarns and non-woven UD fabrics bonded together with a light polyester thread supplied by Saint-Gobain BTI and Sigmatex respectively. The materials of fabrics chosen for the experiments are E-glass (Gf), Tenax HTA carbon (Cf), and a Tenax HTA carbon/nylon hybrid (Cf/PA). The latter is an experimental hybrid design that has been shown to enhance the energy absorption in impact and this might similarly improve the crushing energy by increasing the tearing energy (Mode-III) as occurs with hybrid Carbon/polyethylene tubes [20]. The dry fabrics were selected to maintain an areal weight ratio of UD to ±45° fabrics (UD/±45 ratio) for each fibre type as close to 0.7 as possible. This was to ensure that for a given orientation between fibre types the ratio of 0 to ±45 was consistent. The carbon Uniweave was a Sigmatex Constructex at 325 g/m2, the E-glass UD a Saint-Gobain BTI (SGBTI) ELpb-412 (412 g/m2), the ±45 NCFs were SG BTI CBX-440 (440 g/m2) for carbon, EBX-602 (602 g/m2) for E-glass and the experimental Cf/PA (RINO) fabric contained 15% by weight of nylon and 440 g/m2 of carbon fibres combined in commingled yarns. This gave a UD/±45 ratio of 0.73 for the carbon and carbon/RINO, and 0.68 for the E-glass–fibre laminates (in all research with RINO, the nylon is considered to be part of the matrix and therefore non-structural [21,22]). The stitching threads were 120 Tex Kevlar 29 thread with a tenacity of 185 cN/Tex (220 N) from Atlantic Thread and Supply, and a 400 Tex stretch-broken Cf/PBO thread having a tenacity of 53 cN/Tex from Schappe Techniques. Stitching was performed on a Juki LU-563 N industrial sewing machine fitted with DP/17 size 160 Groz-Beckert San-5 needles for technical textiles. The dry preform was stitched in parallel lines with 5, 10 and 15 mm separation. The pitch, i.e. the distance between the rows of stitches, was kept constant at 5 mm. This gave a stitch density of 4, 2 and 1.3 stitches/cm2 respectively. Three methods of stitching were used – the standard lockstitch (LS), and a modified lockstitch in two configurations: with the loop formed on the top (LSt) or with the loop at the bottom (LSb) (Fig. 1). The difference is that for the LSb, the needle thread that forms the loop is passed through the fabric and around the bobbin head, and hence could be degraded, while for the LSt, the thread would have simply been pulled from the bobbin through the fabric and hence suffer very little degradation. Furthermore, during manufacture, if the loop is formed at the bottom, this would press into the fabric and increase the distortion of the fibres in that region (Fig. 2). After stitching, the laminates were manufactured using a vacuum resin infusion process to ensure full wet-out of the stitched performs. Three resins were used: Reichhold Dion 9102-500 vinyl-ester (VE) cured with 1.5% Butanox LPT, Epikote 828 cured with Vantico HY-932 amine hardener (Ep828) and Cycom RTM823 (Cy823). The VE is a rubber modified pre-accelerated bisphenol-A epoxy-based system. The Ep828 is an unmodified bisphenol-A epoxy resin that has to be heated to lower the viscosity sufficiently to infuse the panels while the Cy823 is a low-viscosity aerospace-grade epoxy designed for RTM and infusion applications while the. In-house testing has shown that the Ep828 has a 30% higher strength and strain to failure than the Cy823 (85 MPa and 3.2% vs. 66 MPa and 2.4%) but a lower stiffness at 2.8 GPa vs. 3.3 GPa. Both epoxy resins were heated up to 60 °C before infusion,

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Fig. 1. Photos of the different types of stitching in samples of composites: (a) lockstitch with carbon thread and glass–fibre fabric, (b) modified lockstitch with loop formed at the top using carbon thread and carbon/RINO fabric – dashed lines indicate the deformed UD caused by stitching, the circle shows a loop that was pulled through the fabric, and (c) modified lockstitch with loop at bottom using Kevlar thread in carbon fibre fabric. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3.2. Taguchi DOE method

Fig. 2. Photos showing the effect of the carbon thread stitches on the carbon fibre laminate. Forming the loop at the bottom (LSb) forces the plies to kink; the low thread tension required to maintain the stitch type produces peaks of fibres on the bagged surface. Stitch line separation is 15 mm, stitch orientation is in transverse (T) – series 16. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

while for the Ep828, the bagged laminate was also preheated to 60 °C to ensure optimal wet-out. The volume fraction of fibres in all the laminates calculated from the weights of the dry fabric and the cured composite was 50 ± 4.3% for the Gf and Cf fabrics and 43 ± 4.8% for the Cf/PA fabrics. The large scatter was attributed to the resin pockets depending on thread type (see Fig. 1a where the gap between the carbon stitches is only resin vs. Fig. 1c where the Kevlar thread does not show large resin pockets) The density was calculated by the Archimedes principle.

Since multiple factors are to be investigated, the Taguchi DOE method, also called Robust Design, was applied. This technique adapts the mathematical formalism of full-factorial classical design of experiments (DOE) by minimising the amount of experiments that need to be performed while allowing the relative influences of each parameter to be statistically determined. Such a reducedfactorial technique makes it more suited at improving the productivity of research and development activities since only key effects need to be identified. DOEs are generally based on a matrix of experiments called orthogonal arrays (OA) – that are a set of experiments where the factors and levels used as the settings of various parameters are changed according to the OA matrix [23]. The Taguchi approach is particularly useful to determine those parameter values that are least sensitive to noise. Interactions between factors are treated as noise to further emphasise the selection of control factors and since the control factors should be strong enough to overcome the interaction effects. Within the Taguchi approach, noise is defined as uncontrollable factors that cause the functional characteristics of a product to deviate from their target values, such that noise can be considered to be external factors such as manufacturing variability. For a Taguchi DOE, the signal-to-noise (S/N) ratio is preferably used to quantify variation instead of the mean squared (MSD) deviation; from Eqs. (1) and (2), it can be seen that the S/N ratio is a transformation of the raw data to allow quantitative evaluation of the design parameters based on both their mean and deviation. There are three S/N ratio characteristics: nominal-the-best, smaller-the-better and larger-the-better. The larger-the-better characteristic is most suited to the current research and is given by

MSD ¼

n 1X 1 n i¼1 y2i

ð1Þ

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S ¼ 10 log10 ½MSD N

ð2Þ

where MSD is the Mean Squared Deviation, yi is the data obtained from the experiments and n is the number of experiments performed. Analysis of variance (ANOVA) is further used to quantify the contribution of each parameter to the total variation in the experimental data. If a design parameter has a large influence compared to the experimental error, it is a significant factor and would play a fundamental role in determining the optimal solution of the design problem. 4. Experimental design: Factors, levels and orthogonal array The factors that were investigated and the different levels within the factors are presented in Table 1. Lay-up orientations of [±45/ 0]2s and [90/±45/02]s were identified as suitable for investigation while the [±45/03]s lay-up was included to determine if the increase in the Mode-I properties as a result of stitching would stabilise the crushing of triaxial layups with large ratios of 0° fibres. The former two fell within the upper-bound of energy absorption while the latter in the lower-bound. Two stitching orientations were selected: transverse and parallel stitch lines to the direction of crush – these are termed Transverse (T) and Longitudinal (L) (Figs. 2 and 3) respectively. In the OA, transverse stitching was selected as a dummy parameter indicated by T0 in Table 1. The L18(21  37) OA was chosen since the interactions between the factors are uniformly distributed among the columns, allowing the main effects to be studied without the interactions confounding the results [23–25]. The assignment of the factors into the OA is shown in Table 2, the fifth column was left blank. Following the identification of the key parameters, a confirmation experiment is performed to check that the experimental design was successful. As a further confirmation, the results of the stitched composites are compared to the equivalent unstitched material to visualise the significance of the improvement in SSCS by stitching. 4.1. Testing Plate crushing tests were performed using a plate crush rig (Fig. 4) as per previous research [10,16]. To keep the results consistent with previous work [10], a plate aspect ratio (i.e. the inside knife-edge separation to thickness ratio – KES/t) of 16 was chosen.

Description

Level 1

Level 2

A

Sewing thread

Kevlar (kV)

B C

Stitch type Reinforcement

Lockstitch (LS) Carbon (Cf)

Carbon/PBO (C/PBO) LSta Glass (Gf)

D

Stitch separation Fibre orientation Resin type

5 mm

10 mm

[±45/0]2s

[90/±45/ 02]s Epikote828 (Ep828) Transverse (T)

F G H

Stitch orientationc

Reichhold Dion 9102-500 (VE) Longitudinal (L)

A 45 steeple chamfer was cut into the laminate using a water cooled diamond saw to act as a trigger. The crushing speed was set to 20 mm/min and the total crush stroke was 55 mm; further details can be obtained from previous work [10,16]. Four specimens per set were tested. The SSCS is calculated by dividing the average stress over the region of steady-state crushing stress by the density of the material as per Eq. (3). This steady-state crushing is usually reached within 5–10 mm of crushing, though for stitching, this steady-state can be achieved after a crush stroke of only 3 mm. Where specimens fail to crush through a nominal stroke of 55 mm, the SSCS is defined as the ratio of the completed crush stroke relative to the nominal stroke. This provides a better indication of the total energy absorbed by the specimen and is compatible with the specific energy absorption (SEA).

SSCS ¼

average crushing stress r ¼ material density q

ratioed SSCS ¼ SSCS 

Table 1 Control factors and their levels used in the experiment. Factors

Fig. 3. Photo of mould side from series 5 specimen having 10 mm stitch separation, LSt and stitched longitudinally (L) to the crushing direction. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

achieved crush stroke nominal crush stroke

ð3Þ

Level 3

5. Results LSbb Cf & Cf/PA (Cf/RINO) 15 mm [±45/03]s Cycom RTM 823 (Cy823) Transverse0 (T0 )

The details in the brackets refer to the nomenclature used in the paper. a LSt implies modified lockstitch with the loop/lock at the top. b LSb implies modified lockstitch with the loop/lock at the bottom. c The stitch orientation is longitudinal or transverse to the crushing direction.

5.1. Plate crushing Sample crushing charts of the specific stress vs. the crushing stroke as well as sample photos of crushed specimens are shown in Figs. 5–7 while the average results and standard deviations are detailed in Fig. 8 where they are compared to trends obtained for the crushing of glass–fibre composites in previous research [16]. Table 2 shows the results together with the calculated S/N ratios for use in the DOE analysis. In all charts, there appear periodic peaks and troughs corresponding to the location of the stitches. This is more predominant in the transverse stitching especially at the 10 mm and 15 mm separations since the crushing stress needs to overcome an entire row

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Table 2 Orthogonal array L18(21  37) of the experimental runs and results. Experimental array

a

Results n

Expt. No.

Thread

Stitch type

Fabric

Stitch separ (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Unstitched Stitched

kV kV kV kV kV kV kV kV kV C/PBO C/PBO C/PBO C/PBO C/PBO C/PBO C/PBO C/PBO C/PBO

LS LS LS LSt LSt LSt LSb LSb LSb LS LS LS LSt LSt LSt LSb LSb LSb

5 10 15 5 10 15 10 15 5 15 5 10 10 15 5 15 5 10

C/PBO

LSt

Cf Gf Cf/RINO Cf Gf Cf/RINO Cf Gf Cf/RINO Cf Gf Cf/RINO Cf Gf Cf/RINO Cf Gf Cf/RINO Cf Cf

Col 5

10

Fibre orient

n

[±45/0]2s [90/±45/02]s [±45/03]s [90/±45/02]s [±45/03]s [±45/0]2s [±45/03]s [±45/0]2s [90/±45/02]s [90/±45/02]s [±45/03]s [±45/0]2s [±45/0]2s [90/±45/02]s [±45/03]s [±45/03]s [±45/0]2s [90/±45/02]s [±45/03]s [±45/03]s

Resin type

Stitch orient

VE Ep828 Cy823 Cy823 VE Ep828 Ep828 Cy823 VE Ep828 Cy823 VE Cy823 VE Ep828 VE Ep828 Cy823 Ep828 Ep828

L T T0 T0 L T T0 L T L T T0 T T0 L T T0 L L

n

SSCS (kJ/kg)

SDa

S/N ratio (dB)

14.80 68.45 76.32 77.80 83.60 76.75 84.50 74.16 25.38 78.73 72.16 16.58 77.28 83.04 90.55 21.34 70.46 77.22 83.40 109.71

0.59 4.93 3.75 8.63 4.00 1.78 2.19 1.09 2.51 3.55 5.82 0.96 5.78 3.84 4.93 1.48 4.92 6.41 3.52 3.84

23.39 36.66 37.63 37.70 38.42 37.70 38.53 37.40 27.99 37.90 37.10 24.36 37.70 38.36 39.11 26.54 36.91 37.68 38.40 40.79

SD = standard deviation.

Fig. 4. CAD model of the plate crush test rig used in this research.

of stitches before the crush can progress. Once these stitches break, the central split progresses to the next stitch line that obstructs the progress of the crack. Since the longitudinal stitch points are somewhat staggered (Fig. 3) this could contribute to the stability of series 5 and 15 that show a crushing pattern with no significant peak stress caused by the trigger. For the Cf and Cf/RINO with VE resins (Fig. 5 series 1, 9, 12 and 16), the crushing process was aborted after a short crush stroke. The crushing for these materials initiated with a typical trigger stress of 140–180 MPa, but the specimen failed in one of two

modes: (a) compression, where the stitch strength was too high to be overcome by the debris wedge, preventing the specimen from separating into two fronds (Fig. 6a, series 1) or (b) through the shearing off of one of the fronds at a stitch line, a behaviour more commonly noted for transverse stitching (Fig. 6b, series 9). The only common feature between these specimens is the Cf/VE combination indicating an interfacial bonding incompatibility between these two materials. For series 4, 50% of the specimens crushed in a stable manner while the others experienced frond breakage and a resultant decrease in crushing stress similar to series 9 but at a

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Fig. 5. Example sustained crushing charts for all experimental runs. The horizontal lines indicate the average SSCS.

significantly longer crush stroke (Fig. 7c) – this is indicated by the large standard deviation for the series in Table 2. The specimens of series 13 all started to crush properly, and then broke during the crush with most of the energy taking place through tearing and friction as the broken specimen slid over itself. It is possible that the stitch lines were too tight or that they distorted the fibres excessively at certain points. The remaining specimens all crushed properly. It should also be noted that in the graph of series 6 there are peaks that, similar to series 1, are separated by approximately the 15 mm stitch line separation for that particular series. From Fig. 8, it can be seen that the SSCS for all Gf laminates has improved, especially the [±45/03]S stitched with a modified lockstitch that unstitched had an SSCS of 48 kJ/kg and fell within

the lower bound of energy absorption [16]. The stitched carbonbased NCFs did not show any apparent improvement in SSCS after stitching. 5.2. DOE data and analysis The averages for the results from the above experiments are calculated for a larger-is-better S/N ratio criterion. These results are then implemented in the experimental datasets for analysis and plotted in Fig. 9. The results show that the resin type is the main factor that influences the crushing, contributing 34.75% to the results. From Fig. 9 it can be noted that the difference between the epoxies is negligible. The key factors that emerge in order of ranking are:

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Fig. 6. Examples of specimen failures due to: (a) crushing fronds not developing as a result of the strength of the stitch resulting in the squashing of the plate (series 1) and, (b) fronds develop then one fails due to the strength of the stitch (series 9).

 resin type – epoxies are significantly better than the VE resins, especially for carbon-fibre laminates since the carbon fibres are incompatible with the VE resins causing these laminates to fail prematurely;  stitch type – the generic lockstitch is the worst, while the LSt is the best;  fibre type – the glass–fibre laminates appear to benefit from stitching whereas the carbon-fibre laminates seem do not show any improvement to the SSCS, and in some cases are worse than the optimal unstitched laminate (Fig. 8);  fibre orientation – increasing the number of central UD plies seems to result in a more stable or higher SSCS than for [±45/ 0]2s;  stitch separation – tight stitching with 4 stitches/cm2 (5 mm separation) gives the worst results, decreasing this to 2 stitches/cm2 should improve the stability of the crush. The other factors, specifically thread type and stitch orientation do not seem to noticeably influence the crushing energy. Nevertheless, when the stitches are parallel to the applied load, the level of scatter is less than for when the stitches are in transverse. This is further confirmed by the ANOVA results (Table 3), where all the F-ratios of the unpooled factors are all greater than the 95% f-probability distribution. The total pooled error of 14.41% is acceptable as error probably due to some difficult to control manufacturing steps e.g. lack of proper tension control on the C/PBO thread (as exemplified by the non-consistent stitch pattern in Fig. 1b), as well as some experimental errors. It is also possible that there are other key effects that have not been considered such as thread tension and diameter, and possibly the confounding of the interaction between the fibre and resin adds to this error.

Fig. 7. Photos of undersides of specimens crushing in: (a) stable mode with uniform fronds after removal from the rig (series 14), (b) squashing of the fronds into a tight radius of curvature at the end of a crush stroke (series 8) and (c) semi-stable crush where the central split is not uniform across the whole width of the specimen (series 4).

The response graph (Fig. 9) shows that there are clear differences in between the best and worst parameters in the four important factors. This makes it is easy to choose the ideal parameters for crushing within the limitations of the current dataset. For the confirmation experiment, the following factors were selected: [±45/03]s carbon fibre NCF with an Epikote 828 epoxy resin. The stitching was performed with a LSt configuration using a C/ PBO thread, longitudinal stitching and a 10 mm separation as these were considered insignificant parameters and therefore subjective

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Fig. 8. Results from the optimisation of the stitching parameters compared to the trends (horizontal lines) obtained for the crushing of unstitched NCF E-glass plates in [17].

Fig. 9. Taguchi response graph for all factors by S/N ratio. Errors were calculated using the ANOVA method, the ranking for each factor is indicated on the graph.

Table 3 Analysis of variance (ANOVA) for the S/N ratios. Source

Pool

Sq

DOF

Mq

A: Sewing thread B: Stitch type C: Reinforcement D: Stitch separation F: Fibre orientation G: Resin type H: Stitch orientation Error due to H Residual error Pooled error Total (St) Sq due to mean ST (Total Sq)

Y

0.00 92.13 53.16 17.06 41.72 245.08 3.14 8.01 21.77 32.91 482.06 22125.39 22607.45

1 2 2 2 2 2 1 1 6 11 17 1 18

0.00 46.07 26.58 8.53 20.86 122.54 3.14 8.01 3.63 3.66 28.36

Y Y Y

F

Sq0

q (%)

12.6 7.27 2.33 5.70 33.51

84.82 45.85 9.75 34.41 237.77

17.59 9.51 2.02 7.14 49.32

1.00

69.48 482.06

14.41 100.00

Sq = sum of squares; DOF = degrees of freedom; Mq = mean sum of squares; Sq0 = pure sum of squares; q (%) = percentage contribution of the unpooled factors; F-ratio is calculated using the pooled error mean square; confidence interval for C & F >95%, B & G >99%.

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Fig. 10. Comparison of the confirmation experiment vs. the unstitched counterpart.

experience in the crushing behaviour was used to decide between the different factors. Carbon fabrics were chosen to check if carbon-fibre laminates could show an improvement in the SSCS by selecting the most favourable stitching parameters. The confirmation results are very close to the predicted values, indicating that the parameters identified in the DOE were significant parameters that influence stitching. Typical crushing charts of the unstitched and stitched materials are shown in Fig. 10 while the average SSCS for the confirmation experiment compared with the unstitched counterpart are shown in Fig. 8 for comparison with other stitched materials. It is interesting to note that the unstitched material has SSCS values equal to most of the other stitched composites. This implies that stitching (with the parameters selected in this research) is not always beneficial and can actually reduce the crushing performance of materials that crush in high-energy modes. This is especially so if the increase in fracture toughness through stitching inhibits the formation and propagation of a stable crushing zone and introduces defects in the laminate that could cause the compressive/buckling load to be lower than the crushing load. 6. Discussion Previous work [10] had shown that composite materials manufactured from triaxial NCF fabrics normally require a long crush stroke to achieve a steady-state crushing due to the long central crack formed by the collapse of the trigger, their lower fracture toughness properties [16] and since they predominantly crush with a frond-splaying mechanism with little fragmentation. In most cases, stitching has been shown to improve the fracture toughness properties of composite materials [11–13], especially in long fibre composites such as pre-pregs and NCFs. Stitching should therefore increase the SSCS for NCFs [13] but preliminary investigations in stitching on flat plates [14] had shown that, in some cases, stitching did not improve the energy absorbed in crushing; in such cases, the additional stitching step could be seen as an ineffective and unnecessary manufacturing process, especially since stitching can decrease the composite structural properties [12].Of the two threads selected for evaluation; the carbon

thread was much thicker than the Kevlar 29 thread despite having a similar unimpregnated break strength. The extra thickness of the C/PBO thread would therefore form larger loops and larger discontinuities/resin pockets within the laminates and probably reduce the composite properties more than for the Kevlar thread (Fig. 1). The results from the DOE have shown that within the parameters selected, the resin was a significant factor in the stability of the crush. This is highlighted because the VE resin does not bond well to carbon-fibres and for the high stitch densities, the stitches would not break and cause a collapse of the composites. Such a collapse could also be due to the fibre misalignment caused by stitching in a LS or LSb manner (Fig. 1). This is partly confirmed by previous research where glass–fibre/epoxy laminates had a more stable crush than for the polyester-based laminates, even though the results had shown only a slight (and hence insignificant) improvement over polyester laminates [16]. Additionally, this influence of resin type on the fibre type indicates that there is an interaction between the two that has become confounded within the DOE. Although it is possible that the slight increase in fracture strength and the toughness of the Ep828 has contributed to the better results, the effect of the resin seems to be its capability to wet-out and bond to the fibres and the compatibility of the resin and hardener combination to the sizing on the fibres. Since the carbon fibres are unlikely to be sized for compatibility with VE resins, this would explain the poor properties of the carbon laminates having a VE matrix. The stitching mode is also a very significant parameter. When the composite is impregnated through resin infusion, and the loop is formed at the top of the laminate (LSt), it is not forced into the composite and therefore distortions are kept to a minimum. With the loop at the bottom (LSb) or in the middle (LS), large distortions can occur that misalign the fibres and decrease the compressive strength of the laminate. The fibre type is also significant in crushing; for unstitched laminates, carbon outperforms glass as confirmed in Fig. 8 where the optimised unstitched carbon had an SSCS of 84.4 kJ/kg cf. 40–60 kJ/kg for unstitched glass. While all the glass laminates showed an improvement in the SSCS due to stitching, the carbon

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laminates did not seem to show any significant improvement, although the stitched carbon tended to have a higher SSCS than the glass laminates (disregarding the specimens with a VE matrix). The improvement of the glass NCFs with stitching has already been shown by Warrior et al. [13] to be a result of the stitches limiting the splaying mode of these materials and encouraging fragmentation to occur. The improved properties of the glass–fibre composites could be due to the increased diameter of the fibres (15 lm vs. the 7 lm for the Tenax HTA fibres) such that they are less sensitive to misalignment and distortions. Another advantage of the glass–fibres is that they have a higher strain to failure than for carbon fibres and hence the splayed fronds are less likely to collapse during crushing as a result of the stitching lines. This stability of the splayed fronds can be observed in the post-crush photos in Fig. 7a. The implication is that by optimised stitching, the SSCS of glass NCF composites with a VE matrix could almost equal the performance of an unstitched carbon laminate but at a much lower cost. For the lay-up of the fibres, the results seem to show little difference between the [90/±45/02]s and the [±45/03]s, while the [±45/0]2s orientation is the worst. It seems that increasing the central 0° plies helps to create a better midplane split and hence better defined fronds. However, Fig. 7c for a [90/±45/02]s orientation shows that with transverse stitching, the fronds can collapse, especially due to the increased energy required to tear the 90° plies. If these results were translated into tube specimens, it is likely that the [90/±45/02]s lay-up should provide a higher energy crush due to the 90° plies. On the other hand, if structural stiffness is also a requirement, [±45/03]s should give higher stiffnesses due to the extra 0° plies. The tight stitch separation of 4 stitches/cm2 can cause the laminates to fail due to the increased force required to break the stitches and the reduced fibre volume fraction due to resin pockets around the stitching loops. At such a stitching density, the stitch points are separated by 5 mm in both the x- and y- orientations, therefore are equidistant when stitched in both longitudinal and transverse direction. In a transverse stitch, even if the stitch line separation were 10 mm or greater, there is effectively a high stitch density at the point where the stitch line needs to be broken. In longitudinal stitching, the stitch density decreases at the fronds and therefore a lower load will be needed to progress the crush. This would make the longitudinal stitching at 10 mm or greater the ideal stitching mode with the sewing machine used (as the limit of the stitch pitch was 5 mm). It is possible that a lighter/thinner thread might allow a high stitch density without destabilising the composites, however a new set of experiments will need to be performed as such inferences from untested parameters in a DOE are not necessarily accurate. Between the different threads, despite the differences in thickness, no difference can be discerned from the results. The confirmation results on a [±45/03]s carbon fibre Ep828 composite stitched longitudinally in a LSt configuration with a 10 mm separation with carbon thread fall within the values predicted by the S/N ratio by the Taguchi. This would confirm that the DOE was a success and that the parameters identified are the key effects. When compared to an equivalent unstitched material (Figs. 9 and 10), it can be seen that an improvement of 30% in the SSCS can be achieved by using the optimal stitching parameters. More importantly, the increase in SSCS is coupled with a higher crushing efficiency, meaning that a larger portion of the crushing stroke absorbs energy. While the stability and higher SSCS can be attributed to the higher Mode-I fracture toughness of the stitched laminate, it is more likely that there is a limit to how much an improvement in Mode-I can stabilise a crush. Too high a load to break the stitches and propagate the crack would cause the fronds to break and neg-

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atively affect the crushing energy; this is confirmed by earlier research on tubes [13].In general, it is unlikely that the extra load required to break the stitches would have a significant impact on the physical crushing stress, and the main benefit of the stitching would be to force the splayed fronds into a tighter radius of curvature that is resisted by the flexural stiffness of the fronds and the energy dissipated in fracturing the fibres.

7. Conclusions The objectives of this research were to try to understand the effect of different stitching parameters on the crushing energy of composite plates. A Taguchi DOE was used to be able to identify the key effects on the SSCS of different composites manufactured from NCFs. The key effects identified within the limits of the selected parameters in order of importance, while disregarding the effect of the resins, are:  stitch type (17.6%) – using a modified lockstitch with the loop formed on the surface of the laminate prevents large distortions from occurring when compared with the loop in the centre (standard lockstitch) or when the loop is at the bottom and forced into the laminate (this is only valid when coupled with a vacuum resin infusion process which would not compact the surface stitches);  reinforcement type (9.5%) – within this dataset, glass–fibre composites show a significant improvement in the SSCS with stitching whereas carbon fibre composites only show marginal improvements unless the stitching is optimised for the layup. Furthermore, glass–fibres are compatible with all the resins tested which would make them more suited for a lower-cost production process;  fibre orientation (lay-up) (7%) – with respect to the orientations investigated, adding more 0° fibres in the centre of the laminate produces a better-defined central split and more stable frond crushing in flat plates;  stitch separation – 10 mm separation provided the optimal balance between fracture toughness properties and improvement in the SSCS. The results further show that non-optimal stitching such as using a very stitch density that could require a high load to break the stitches and propagate the central crack could cause the crushing fronds to fail or the laminate to be destabilised and therefore negatively impact on the laminate’s crushing capability. On the other hand, careful selection of the stitching parameters with respect to a laminate layup can noticeably improve the SSCS over unstitched laminates: the optimised carbon composite showed an improvement of 30% in the SSCS over the equivalent unstitched layup. The results also raise the question of the usefulness of stitching to improve energy absorption. It is not sufficient to simply stitch a laminate to improve the energy absorption without careful selection of all the process parameters.

Acknowledgements The authors would like to acknowledge funding from the Engineering and Physical Science Research Council (EPSRC) and the Overseas Research Scheme (ORS). Gratitude is also owed to Saint Gobain Technical Fabrics, BTI, UK for supplying the NCF fabrics used in this research and Scott Bader and Cytec Fiberite for supplying the resins.

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