Jody W C Pang

Kevin S Fancey
email: k.s.fancey@hull.ac.uk

Analysis of the tensile behaviour of viscoelastically prestressed polymeric matrix composites

Keywords: A. Polymer-matrix composites (PMCs), B. Mechanical properties, C. Residual stress, Viscoelasticity

A novel composite material is reported, in which tension, applied to polymeric fibres, is released prior to moulding them into a matrix. Following matrix solidification, compressive stresses imparted by the viscoelastically strained fibres impede crack propagation. Previous Charpy impact studies had demonstrated that these

viscoelastically prestressed composites could absorb typically 25-30% more energy than control (unstressed) counterparts and the current study focuses on their tensile behaviour as a function of fibre volume fraction, V f . Tensile testing was performed on continuous unidirectional nylon 6,6 fibre -epoxy resin samples. Compared with control counterparts, the results showed that viscoelastic prestressing improved tensile properties, the effects being V f -dependent. Increases in tensile strength, modulus and energy absorbed (to 0.25 strain) exceeded 15%, 30% and 40% respectively at an optimum V f , this being ~35-40%. Strain-to-failure was reduced by 10-20%, thereby lowering any improvement in tensile toughness (energy absorbed to fracture) to <10%.

Mechanical properties of the fibres themselves were not significantly influenced by the treatment used for generating composite prestress, and we propose that the observed improvements to tensile properties may be attributed to: (i) direct contribution from compressive stress, (ii) attenuation of the dynamic overstress effect on fibre fracture and

Introduction

In contrast with the concept of prestressed concrete structures, the principle of exploiting purposely induced (compressive) prestress in polymeric matrix composites (PMCs) is relatively recent. Conventionally, prestressed PMCs (PPMCs) can be produced by applying and maintaining tension on glass fibres as the surrounding resin matrix cures. For beam-shaped geometries, this elastic prestressing method has been found to increase impact resistance by up to 33%, when compared with unstressed (control) counterparts [START_REF] Motahari | Impact strength of fibre pre-stressed composites[END_REF]. A similar increase in flexural stiffness and strength is also reported [START_REF] Motahari | Fibre prestressed composites: improvement of flexural properties through fibre prestressing[END_REF]. Another study [START_REF] Hadi | On the influence of pre-stress on the mechanical properties of a unidirectional GRE composite[END_REF] indicates that increases of ~25% for tensile strength and ~50% for (tensile) elastic modulus can be achieved. These improvements, using E-glass fibre in epoxy resin, have been attributed to mechanisms involving compressive stresses within the matrix which (i) impede crack propagation and (ii) reduce composite strain resulting from external tensile and bending loads [START_REF] Motahari | Impact strength of fibre pre-stressed composites[END_REF][START_REF] Motahari | Fibre prestressed composites: improvement of flexural properties through fibre prestressing[END_REF][START_REF] Hadi | On the influence of pre-stress on the mechanical properties of a unidirectional GRE composite[END_REF].

This paper focuses on further investigations into the mechanical properties of a novel form of PPMC. The novelty involves stretching polymeric fibres under a load, for a period of time, to induce creep; the load is subsequently released prior to moulding the fibres into a resin matrix. A significant proportion of the fibre deformation is viscoelastic and, when the matrix solidifies, compressive stresses are imparted by the viscoelastically strained fibres as they attempt recovery against the surrounding (solid) matrix material. This viscoelastically generated compressive prestress improves impact toughness: using nylon 6,6 fibres in a polyester resin matrix, Charpy tests have demonstrated that viscoelastically prestressed samples can absorb typically 25-30% more impact energy than their control (unstressed) counterparts, with some tests showing up to 50% improvement [START_REF] Fancey | Investigation into the feasibility of viscoelastically generated prestress in polymeric matrix composites[END_REF][START_REF] Fancey | Prestressed polymeric composites produced by viscoelastically strained nylon 6,6 fibre reinforcement[END_REF][START_REF] Fancey | Fibre-reinforced polymeric composites with viscoelastically induced prestress[END_REF][START_REF] Pang | An investigation into the long-term viscoelastic recovery of Nylon 6,6 fibres through accelerated ageing[END_REF]. In contrast with (conventional) elastic prestressing, the viscoelastic prestress technique offers the following benefits. First, the fibre stretching and moulding operations are de-coupled, enabling more flexible fabrication procedures and facilitating the production of complex component geometries. Second, should there be a tendency for the prestress effect to deteriorate due to localised matrix creep effects near the fibre-matrix interface, longer term recovery mechanisms within the polymeric fibres will be expected to counteract this [START_REF] Fancey | Fibre-reinforced polymeric composites with viscoelastically induced prestress[END_REF].

To demonstrate viability of the viscoelastic prestressing concept, research has focused on nylon 6,6 fibres (a common fibre material with good mechanical properties), as a precursor to research with other fibres. Subjecting nylon 6,6 fibres to a 24 h tensile creep stress of ~350 MPa produces ~12% strain (there are no fibre fractures). Releasing the stress removes the contribution from elastic strain, leaving a residual recovery strain that decreases only very slowly with time, i.e. ~3% (initial), ~2.5% (2 h) and ~2% (100 h) [START_REF] Fancey | Investigation into the feasibility of viscoelastically generated prestress in polymeric matrix composites[END_REF][START_REF] Fancey | Prestressed polymeric composites produced by viscoelastically strained nylon 6,6 fibre reinforcement[END_REF][START_REF] Fancey | Fibre-reinforced polymeric composites with viscoelastically induced prestress[END_REF][START_REF] Pang | An investigation into the long-term viscoelastic recovery of Nylon 6,6 fibres through accelerated ageing[END_REF]. Consequently, there is no significant difference between the diameters of recovering fibres and unstressed (control) fibres [START_REF] Fancey | Investigation into the feasibility of viscoelastically generated prestress in polymeric matrix composites[END_REF]. Accelerated ageing (by timetemperature superposition) to an equivalent of 100 years at 20°C, has shown that viscoelastic recovery mechanisms within the fibres remain active, the strain contribution from viscous flow being negligible (<10 -4 %) [START_REF] Pang | An investigation into the long-term viscoelastic recovery of Nylon 6,6 fibres through accelerated ageing[END_REF]. This is further supported by observing the long-term force generated from viscoelastic recovery, the mechanisms being associated with thermally activated slippage of molecular segments controlling the release rate of stored elastic energy [START_REF] Pang | Force measurement from viscoelastically recovering Nylon 6,6 fibres[END_REF]. By subjecting composite samples to accelerated ageing, the influence of these long-term viscoelastic recovery mechanisms on mechanical performance has also been demonstrated, in that the improvement in impact toughness resulting from viscoelastic prestressing is not observed to deteriorate over a 100 year timescale [START_REF] Pang | An investigation into the long-term viscoelastic recovery of Nylon 6,6 fibres through accelerated ageing[END_REF].

To date, evaluating the mechanical performance of viscoelastic PPMCs has been limited to the impact testing of composite samples with very low fibre volume fraction (V f ). Further to an initial study [START_REF] Pang | An evaluation of viscoelastically prestressed polymeric matrix composite materials[END_REF], this paper investigates the tensile properties of these PPMCs over a range of V f values, using epoxy resin as matrix material. An investigation is also made to determine whether the tensile properties of the fibres themselves are changed as a result of the stretching treatment. Clearly, if such changes (e.g. work-hardening) are unknown, erroneous inferences could be made with regard to the influence of viscoelastic prestressing effects in composite samples.

Experimental

Production of samples

Batches of composite samples required two sets of rectangular specimens, one set being viscoelastically prestressed 'test' samples, the other comprising unstressed, but otherwise identical, 'control' samples. Fibre reinforcement consisted of continuous multi-filament nylon 6,6 yarn (Goodfellow Cambridge Ltd) with 140 filaments, 27.5 m filament diameter. Two identical lengths of as-received yarn (one 'test' the other 'control') were simultaneously annealed at 150°C for 0.5 h to remove previous thermal/stress history [START_REF] Fancey | Prestressed polymeric composites produced by viscoelastically strained nylon 6,6 fibre reinforcement[END_REF]. The designated test yarn was then subjected to 349 MPa tensile stress for 24 h, using a bespoke fibre stretching facility, whilst the control yarn was positioned in close proximity for exposure to the same ambient conditions (20-21.5°C, 30-40% RH). Prior to moulding, test and control yarns were each folded and cut into multiple lengths and then combed into flat ribbons.

A bisphenol-A based, low viscosity epoxy resin with good optical transparency (ABL-Stevens Resin & Glass, U.K.) was used with polyoxyalkyleneamine base hardener catalyst. This was a room temperature-curing resin, the gel time being ~15 h at 20°C. Unidirectional continuous fibre -resin samples were prepared using a leaky mould method (Figure 1), based on principles from Ladizesky and Ward [START_REF] Ladizesky | Ultra-high-modulus polyethylene fibre composites: I-The preparation and properties of conventional epoxy resin composites[END_REF]. In addition, the mould was designed so that sections could be removed to facilitate the demoulding process. Two identical moulds were used for each batch, to enable simultaneous preparation of test and control samples from the same resin mix.

Immediately following mould assembly, weights were positioned on top of the moulds; this enabled excess resin to flow from each (open) mould end to obtain the correct sample thickness (as determined by the spacers in Figure 1). The moulding procedure was completed within 0.5 h of the fibre stretching process. Following demoulding, the resulting test and control composite strip lengths were each cut into two samples.

Photographic studies

Scanning electron microscopy (SEM) and optical microscopy (OM) were used to obtain photographic evidence of effects that could adversely influence composite sample characteristics. The principal concern was to ensure that there would be no differences between test and control samples, other than those from prestress effects.

SEM was used to investigate possible topographical changes in test yarn fibres following the applied creep stress. Composite sample cross sections (cut from the centre of the moulded strip, between the two samples designated for tensile testing)

were studed by OM, for matrix features and fibre spatial distribution. 7

Fibre tensile tests

A TA Instruments Q800 Dynamic Mechanical Analyser (DMA) with software was used for this purpose. This enabled a single filament, extracted from the nylon yarn, to be tensile tested at a controlled ambient temperature (20°C). The DMA was operated in force-controlled tension mode, the ramp force rate being 0.25 Nmin -1 .

Monofilaments were selected at random; the nominal fibre length was 35 mm and a 15 mm gauge length was used. Tensile tests were performed on sets of filaments at 24, 120, 168 and 1344 h after the stretching process, each set comprising five test and five control filaments. Tensile strength and strain-to-failure (STF) values were measured.

Tensile tests on composite samples

Each moulded batch comprised two test and two control samples, prepared according to CRAG [START_REF] Curtis | CRAG test methods for the measurement of engineering properties of fibre reinforced plastics[END_REF]. Sample dimensions were 200 x 10 x 1 mm. To determine the influence of fibre content on composite tensile properties, batches had V f values of 16%, 28%, 41% and 53%, and typically, three batches were tested at each V f value. All batches were tested at 20-21°C, 430 h after moulding. Testing was performed using a Lloyd LR100K machine (with analysis software) at a loading rate of 5mm/min.

Stress/strain curves provided data on tensile strength, STF, modulus, strain-limited toughness and tensile toughness. Here, strain-limited toughness is defined as the energy/unit volume absorbed to a fixed strain (0.25) and is calculated from the area under the stress-strain curve. This is a more relevant performance parameter for deflection-limited design than the tensile toughness, the latter being calculated to total composite tensile failure.

Results and discussion

Photographic analysis

Figure 2 shows the topographical characteristics typical of the test and control fibres. The longitudinal features observed in both fibre groups are considered to be from the original fibre manufacturing process. Most importantly, there appears to be no difference in surface features between the test and control groups that could influence fibre-matrix bonding, and this supports earlier findings [START_REF] Fancey | Investigation into the feasibility of viscoelastically generated prestress in polymeric matrix composites[END_REF].

Composite cross sections, from 28% V f samples, are shown in Figure 3. These sections have characteristics representing all samples, as they show areas with no fibres and also regions of fibre clustering, in common with samples of lower and higher V f values respectively. There are no discernible differences between test and control sections, either in terms of matrix features or fibre spatial distribution.

Fibre tensile tests

Table 1 summarises the results. between test and control fibres (Figure 2). Thus it can be inferred that any improvements to the mechanical properties observed from a composite test sample, compared with its control counterpart, must be attributed to compressive prestress effects within the composite rather than changes to the mechanical properties of the fibres themselves. Tensile strength data as a function of V f are shown in Figure 5 and Table 2.

Composite tensile tests

Clearly strength values increase with V f ; however, although test and control samples are similar for V f at 16% and 53%, the prestressed samples have higher strengths at the intermediate V f values. As indicated by the curve in Figure 6, there appears to be an optimum value for V f (~35%) at which the increase in strength from prestressing is maximised. We suggest that this can be attributed to the competing roles of fibres and matrix, which are determined by their respective cross-sectional areas: too few fibres will result in less compressive stress within the matrix; conversely, too many fibres will reduce the cross-sectional area over which the compressive stress can function.

From Table 2, the increase in strength for the test samples at 28% and 41% V f is 30 MPa and 49 MPa respectively, and it can be inferred from this information and the curve in Figure 6 that the maximum increase in tensile strength could be ~50 MPa.

Improved strength may simply be attributed to the need for a tensile load to overcome compressive forces within the matrix, thereby impeding tensile failure mechanisms, such as matrix cracking and fibre fracture. From viscoelastic force measurement studies [START_REF] Pang | Force measurement from viscoelastically recovering Nylon 6,6 fibres[END_REF] however, we estimate that the axial stress exerted by the fibres on the matrix would not exceed ~10 MPa at the age the samples were tested (430 h). Thus the observed increases in strength are substantially greater than any compressive stress imparted to the matrix that would have to be overcome during tensile testing. Clearly, there must be contributions to composite strength in addition to the direct effects of matrix compression. From Sections 3.1 and 3.2, it would be difficult to attribute the additional strength increase to changes in tensile properties or surface topography of the nylon fibres, and it is interesting to note that the effect can also be observed in data [START_REF] Hadi | On the influence of pre-stress on the mechanical properties of a unidirectional GRE composite[END_REF] from elastic prestressing using glass fibres. For values of composite prestress up to 50 MPa, samples in Ref. [START_REF] Hadi | On the influence of pre-stress on the mechanical properties of a unidirectional GRE composite[END_REF] with 35-45% V f can be seen to exhibit increases in tensile strength (relative to equivalent samples with zero prestress) that are substantially greater than the applied prestress. This suggests that the phenomenon is not limited to a particular fibrematrix system or method of prestressing.

For brevity, we will define the contribution from direct effects of matrix compression as mechanism (i). We suggest that additional contributions to composite strength may arise from two further mechanisms, i.e. (ii) and (iii). Mechanism (ii) relates to the phenomenon described by Manders and Chou [START_REF] Manders | Enhancement of strength in composites reinforced with previously stressed fibres[END_REF], which can be summarised as follows. The failure of a fibre within a composite of aligned fibres causes a stress wave to propagate outwards, and this subjects neighbouring fibres to a dynamic (oscillatory) overstress. The dynamic contribution decays with time to a static stress concentration. Since the overstress generally exceeds the static effect, the probability of adjacent fibres also failing is increased, thereby weakening the composite.

For mechanism (ii), we propose that the stress fields created by viscoelastically strained fibres (that impart compression to the matrix) may attenuate the stress wave effect, thereby reducing the probability of collective fibre failure. Thus in addition to mechanism (i), there could be a contribution from mechanism (ii) to the observed increases in tensile strength.

Mechanism (iii) relates to an effect proposed by Motahhari and Cameron [START_REF] Motahari | Fibre prestressed composites: improvement of flexural properties through fibre prestressing[END_REF], in that fibres in a prestressed composite can contribute much more effectively to load support. This arises from the ability of taut and straightened fibres to respond instantaneously and more collectively to an applied load. Therefore, as the load increases, variations in the levels to which individual fibres are deformed will be reduced, so that subsequent fibre fractures should proceed less progressively. This, relative to a composite with no prestress, will provide higher strength and less sample displacement during the fibre fracture process. The mechanism was proposed for elastically prestressed composites subjected to flexural loads in Ref. [START_REF] Motahari | Fibre prestressed composites: improvement of flexural properties through fibre prestressing[END_REF], and, although a viscoelastically prestressed composite may exhibit some waviness in fibre lay, the fibres are taut and would therefore respond quickly and more collectively to tensile loads, improving mechanical integrity. It should be noted that no significant deviations in fibre alignment (along sample length) could be observed in our composite batches.

Thus mechanism (iii) could apply to the current situation and therefore contribute to increased tensile strength.

The modulus, toughness and STF data in Figures 678and Tables 2 and3 may also be explained through the mechanisms discussed above. The trend in the modulus data of Figures 6 and7 has similarities with the strength data, the increase from prestressed samples exceeding 30% at 41% V f . Although mechanism (i) must have a significant role, some contribution from mechanism (iii) would also be expected, since improved mechanical integrity (through a more collective response from fibres to tensile loads) will increase composite resistance to deformation.

The strain-limited toughness data in Figures 6 and8 show similar characteristics to the modulus results, and the increase exceeds 40% at 28% and 41% V f . This is not surprising, since the toughness, defined here to a fixed strain (0.25), will return a large value from a steep stress-strain curve. Therefore, the explanation for increased toughness can, as with the modulus data, be related to mechanisms (i) and (iii); i.e. more work is required from tensile loads to counteract increased composite resistance to deformation, which arises from compressive stress and the more collective response of fibres.

The STF values for test samples are consistently lower than the control counterparts in Table 2 by 10-20%. This may be attributed to the less progressive extension and fracture of fibres in the test samples, i.e. mechanism (iii). In contrast with the strain-limited toughness data, tensile toughness (to failure) will be sensitive to STF values. Table 3 indicates that tensile toughness is actually reduced in the test samples at 16% and 53% V f . At these V f values, viscoelastically induced prestressing provides no increase in strength, hence lower STF values will reduce the area under the stress-strain curve, resulting in lower tensile toughness. The effect of reduced STF also limits the increase in tensile toughness at the intermediate V f values, these being 8% (28% V f ) and 2% (41% V f ) from data in Table 3.

Conclusions

By performing tensile tests on viscoelastically prestressed composites and individual nylon 6,6 fibres, we have found (relative to control counterparts) the following:

1. Mechanical properties of the fibres themselves, and their topographical characteristics, are not significantly influenced by the stretching treatment used for generating prestress in the composite material.

2. Viscoelastically induced prestressing increases tensile strength, modulus and toughness, the effects being V f -dependent. The optimum V f value for maximum increase in these properties was indicated to be ~35-40%.

3. Maximum increases in tensile strength, modulus and strain-limited toughness, resulting from prestressing, were found to exceed 15%, 30% and 40% respectively.

4. The strain-to-failure for prestressed samples was reduced by 10-20%, thereby lowering any improvement in tensile toughness (energy absorbed to fracture) to <10%.

We suggest that the V f dependency leading to an optimum value can be attributed to the competing roles of the fibres (that generate the compressive stress) the matrix (over which the compressive stress can function). The increase in tensile strength is substantially higher than the value that would be expected from the direct action of compressive stress alone, and additional indirect contributions resulting from prestress are proposed; these involve attenuation of the dynamic overstress effect on fibre fracture and improved mechanical integrity (through a more collective response from fibres to tensile loads). Such mechanisms may also be used to explain changes observed in the other tensile properties.

This work demonstrates that viscoelastically prestressed composites can, in addition to increased impact toughness, provide the opportunity for improved tensile properties, without the need to increase mass or section dimensions.

useful discussions, Dr Mike Fagan for facilitating the DMA work, and to Mrs Sue Taft and Mr Garry Robinson for their technical support.

Figure captions

  Fibre clamping problems led to the premature failure of one filament in each of the 168 and 1344 h control groups, thus sample number was reduced to four in these cases. There are slight variations in tensile strength and STF at different age values; also both test and control STF values indicate a possible increase with time. It is well known that moisture can have a plasticising effect on nylon fibres, and local variations in humidity at the time of testing and/or progressive moisture absorption during storage (in sealed polyethylene bags) may have contributed to these variations.The overall means from test and control groups in Table1are similar for both strength and STF data, suggesting that the effect of the stretching process on fibre tensile properties could be insignificant. A more rigorous approach using hypothesis tests (two-tailed) at each age value has also been performed. These have revealed no difference between the test and control population means (strength and STF) to a significance level of 2%. At 5% significance level, only the tensile strength data at 24 h indicate a possible difference between the test and control population means. Therefore, subjecting test fibres to the stretching treatment does not cause any significant change in the tensile properties (such as work-hardening effects), relative to corresponding control (unstretched) fibres. Moreover, no changes have been observed in the surface features

Figure 4

 4 Figure 4 represents typical stress-strain curves from a batch of test and control

Fig. 1 .

 1 Fig. 1. Schematic of the leaky mould principles for sample preparation with dimensions

Fig. 2 .

 2 Fig. 2. SEM micrographs of test and control nylon 6,6 fibres, subjected to the annealing

Fig. 3 .Fig. 4 .Fig. 5 .Fig. 6 .

 3456 Fig. 3. Optical micrograph (polished) sections of test and control composite samples at

Fig. 7 .Fig. 8 .

 78 Fig. 7. Influence of fibre volume fraction on tensile modulus of test samples compared

Table 3

 3 Mean energy absorbed to 25% strain (strain-limited toughness) and energy absorbed to fracture (tensile toughness) from tensile tests on composite samples; S.E is the standard error of the mean.

	V f (%)	Energy absorbed to 25% strain ± S.E (kJm -3 )	Energy absorbed to fracture ± S.E (kJm -3 )
		Test	Control	Test	Control
	16	16590 ± 209	13374 ± 190	21595 ± 897	26677 ± 1414
	28	27223 ± 747	19360 ± 373	39076 ± 886	36182 ± 2467
	41	40129 ± 149	28183 ± 651	48395 ± 2390	47385 ± 416
	53	42556 ± 352	32085 ± 981	57014 ± 905	63931 ± 1550

Acknowledgements

The authors would like to thank The Leverhulme Trust for financial support (Grant Ref. F/00181/K). Special thanks to Professor Ken Swift and Dr Daxue Sun for

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Benjamin Bax

Jörg Müssig
email: joerg.muessig@hs-bremen.de

Impact and tensile properties of PLA/Cordenka and PLA/flax composites

Keywords: A. Short-fibre composites, B. Impact behaviour, D. Scanning electron microscopy (SEM), E. Injection moulding, Natural fibre

The interest for biodegradable polymers and natural fibre reinforced polymers has recently grown because of increasing environmental concerns. But the impact properties of bast fibres reinforced polymers cannot reach the levels of traditional fibre reinforced polymers.

PLA (polylactic acid) was reinforced with Cordenka rayon fibres and flax fibres, respectively. The mechanical properties of these composites which are examples for completely biodegradable composites were tested and compared. The samples were produced using injection moulding. The highest impact strength (72 kJ/m 2 ) and tensile strength (58 MPa) were found for Cordenka reinforced PLA at a fibre-mass proportion of 30%. The highest Young's modulus (6.31 GPa) was found for the composite made of PLA and flax. A poor adhesion between the matrix and the fibres was shown for both composites using SEM. The promising impact properties of the presented PLA/Cordenka composites show their potential as an alternative to traditional composites.

Introduction

The use of traditional composites made of glass, aramid or carbon fibre reinforced plastics have recently been discussed critically because of increasing environmental consciousness [START_REF] Mohanty | Biofibers, biodegradable polymers and biocomposites: An overview[END_REF]. Thus, the recent research and development efforts have led to new products based on natural resources. Some of these are biodegradable polymers like PLA (polylactic acid), cellulose esters, polyhydoxyalkanoates and starch polymers.

Furthermore, natural fibre-reinforced polymers made with natural fibres like flax, hemp, kenaf, jute or cotton fibres are important research and development (R&D) achievements. Composites made of natural fibres and biopolymers are completely biodegradable and are called "green composites" because of their environmentally beneficial properties [START_REF] Mohanty | Natural Fibres, Biopolymers, and Biocomposites: An Introduction[END_REF].

The lower price in manufacturing as well as the better performance of traditional, petrochemical plastics compared to polymers based on natural resources are the outcome of the R&D efforts of several decades, whereas only recently scientists have been trying to improve biopolymers which came to public only a few years ago so that they can become cheaper and more competitive [START_REF] Mohanty | Biofibers, biodegradable polymers and biocomposites: An overview[END_REF]. Natural fibres as reinforcement for petrochemical polymers like PP (polypropylene) have already been established in the automotive industry [START_REF] Karus | All Natural on the Inside? Natural fibres in Automotive Interiors[END_REF]. Natural fibres themselves are fibre-reinforced structures in which the cellulose content and the angle of microfibrils influence their mechanical properties [START_REF] Sanadi | Sunhemp fibre-reinforced polyester[END_REF].

The only thermoplastic polymer based on natural resources which can be produced with a capacity of over 140,000 tons per year is PLA. Because of this and because of its properties it is one of the most promising biopolymers [START_REF] Heinemann | Polylactid -Struktur, Eigenschaften und Anwendungen[END_REF]. The low emission of greenhouse gases and the low amount of energy used for PLA production are also interesting factors [START_REF] Vink | Applications of life cycle assessment to NatureWorks tm polylactide (PLA) production[END_REF]. Only its high price (1.80 -4.00 Euro/kg) compared to the price of petrochemical thermoplasts lowers its competitiveness [START_REF] Heinemann | Polylactid -Struktur, Eigenschaften und Anwendungen[END_REF]. But a decrease of the price is expected with increasing demand and increasing R&D achievements in the next years [START_REF] Müssig | Formpressen mit PLA und Hanffasern. 4. N-FibreBase Kongress Naturfaserverstärkte Kunststoffe (NFK) -Wood-Plastic-Composites[END_REF].

PLA is a thermoplast which can either be synthesised by condensation of lactic acid or ring opening polymerisation of lactide which is the diester of lactic acid. Lactic acid is a chiral organic acid which mainly occurs in the L-form. It is produced by fermentation of dextrose which itself is gained from annually renewable resources like corn [START_REF] Henton | Polylactic Acid Technology[END_REF].

Mohanty et al. [START_REF] Mohanty | Biofibers, biodegradable polymers and biocomposites: An overview[END_REF] give a good overview on the research on green composites up to the year 2000. But PLA has become an important part of R&D in the recent years because of the benefits listed above. Mechanical tests on natural fibre reinforced PLA have been performed with different types of fibres like kenaf [START_REF] Nishino | Kenaf reinforced biodegradable composite[END_REF][START_REF] Anonym | Complete mobile phone housing made of PLA[END_REF], jute [START_REF] Plackett | Biodegradable composites based on L-polylactide and jute fibres[END_REF], flax [START_REF] Oksman | Natural fibres as reinforcement in polylactic acid (PLA) composites[END_REF] abaca [START_REF] Shibata | Biocomposites made from short abaca fibre and biodegradable Polyesters[END_REF], hemp [START_REF] Müssig | Formpressen mit PLA und Hanffasern. 4. N-FibreBase Kongress Naturfaserverstärkte Kunststoffe (NFK) -Wood-Plastic-Composites[END_REF] and Cordenka rayon fibres [START_REF] Ganster | Novel cellulose fibre reinforced thermoplastic materials[END_REF].

Not much literature on the adhesion between the matrix and the fibres in "green composites" has been published, although it determines their mechanical properties significantly. Most publications are based on the interaction between fibres and polyoelfins. It is known that natural fibres have a poor adhesion to hydrophobic matrices like PP because of their hydrophilic nature [START_REF] Franco | Fiber-Matrix Adhesion in Natural Fiber Composites[END_REF]. Because of the hydrophilic nature of PLA, Heinemann and Fritz assumed a better interaction between PLA and natural fibres than between PP and natural fibres [START_REF] Heinemann | Polylactid -Struktur, Eigenschaften und Anwendungen[END_REF]. Nevertheless, Oksman et al. found long fibre pull-outs and clean fibre surfaces in the fracture surface of the PLA/flax fibre composite which proves a poor adhesion between the fibres and the matrix [START_REF] Oksman | Natural fibres as reinforcement in polylactic acid (PLA) composites[END_REF].

The insufficient impact strength of biodegradable composites, which plays an important practical role for example in housings of portable electronic devices, prevents a broader field of application. Thus, this work deals with the mechanical properties of PLA/flax and PLA/Cordenka (a rayon fibre that is usually used to reinforce tires) composites. The testing methods are the Charpy impact test and the tensile test. A relation between the fibre-mass proportion and the mechanical properties of the composites is analysed.

Furthermore, the fracture surfaces of the composites are investigated using SEM.

Materials and methods

Materials

Polymer / Polylactide

NatureWorks TM PLA Polymer 6202D by the company Cargil Dow LLC was used as the matrix material. This thermoplast has a density of 1.24 g/cm 3 , a melt temperature of 160-170 °C and a glass transition temperature of 60-65 °C. A tensile strength of 44.46 MPa, a Young's modulus of 3.11 GPa and an impact strength of 16.13 kJ/m 2 was measured for this material in a former publication [START_REF] Müssig | Formpressen mit PLA und Hanffasern. 4. N-FibreBase Kongress Naturfaserverstärkte Kunststoffe (NFK) -Wood-Plastic-Composites[END_REF]. The used PLA was in fibre form.

Ingeo fibre type SLN2660D (company: Far Eastern Textile Ltd., Taipei, Taiwan) is a medium denier (0.67 tex) round cross-section polylactic acid (PLA) staple fibre (64 mm) for nonwoven applications (strength 37 cN/tex; elongation at break 55 %) [START_REF]Nonwovens Fibre -Ingeo Fiber SLN2660D[END_REF].

Reinforcement Fibre / Flax (Linum usitastissimum L., Linacea)

Flax of the type Holstein flax HO-0401b, planted in 2003 was used. For this type the fibre strength is 874 N/mm² (59.9 cN/tex), Young's modulus is 14583 N/mm² and the strain is 6.6 %. Bundle width mean value is 31 µm [START_REF] Müssig | Le management de la qualité appliqué à l'emploi des fibres naturelles dans l'industrie[END_REF].

Reinforcement Fibre / Cordenka

Cordenka 700 rayon fibres (company: Cordenka, Obernburg, Germany) was used as reinforcement for the different composites. Fibres out of yarn type RT700 1840/1000 were used. In this connection RT700 specifies the type, 1840 the polyfil fineness in dtex and 1000 the number of fibres in the yarn [START_REF] Cordenka | Cordenka 700 Super 3[END_REF].

According to Ganster and Fink fineness of Cordenka fibre is 0.18 tex, strength is 56±4 cN/tex (833 N/mm²), elongation is 13±2 %, and Young´s modulus is 1300 cN/tex ( 20GPa) [START_REF] Ganster | Novel cellulose fibre reinforced thermoplastic materials[END_REF].

Composite production

Samples with three different fibre-mass proportions (10%, 20%, 30%) for the PLA/flax composite and samples with four different proportions (10%, 20%, 30%, 40%) for the PLA/Cordenka composite were produced. For each of these seven different composites,

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Scanning electron microscopy

The fracture surfaces which emerged during mechanical testing were examined using a scanning electron microscope (SEM) type CS 24 from Obducat CamScan with an acceleration voltage of 20 kV. For each test series the specimen with an impact strength value closest to the average was chosen for microscopy. The specimens had been coated with platinum iridium over a span of six minutes.

Results and Discussion

Microscopy SEM images of the fracture surfaces of impact specimens can be seen in figures 1-3.

Pulled out fibres and the corresponding holes are visible in both composites.

Furthermore, it can be seen that the surfaces of the pulled out fibres are clean. Both observations can also be found in the literature for the PLA/flax composite [START_REF] Oksman | Natural fibres as reinforcement in polylactic acid (PLA) composites[END_REF] and suggest a poor adhesion between the fibres and the matrix. They are more distinct in the case of the PLA/Cordenka composite: the fibre pull-outs are much longer and the fibre surfaces are cleaner which indicates an even worse adhesion between PLA and Cordenka than between PLA and flax.

There are gaps between the fibres and the PLA which could either occur because of debonding during mechanical testing or because of poor approximation during composite production. Either would also indicate a poor matrix/fibre adhesion.

As a comparison, the composite of flax fibres in a MAPP (maleic anhydride-grafted polypropylene) matrix can be seen as an example for a good matrix/fibre adhesion: in SEM images only very small fibre pull-outs which were coated with matrix material were found [START_REF] Bos | Mechanical properties of short-flaxfibre reinforced compounds[END_REF].

ACCEPTED MANUSCRIPT

-9 -Heinemann and Fritz [START_REF] Heinemann | Polylactid -Struktur, Eigenschaften und Anwendungen[END_REF] predicted a good adhesion between natural fibres and PLA because of its hydrophilic nature. PLA has slightly polar oxygen atoms which could form hydrogen bonds to the hydroxyl groups of the natural fibres. But because of the results of this work and former works it can either be assumed that these hydrogen bonds only have a small influence on the fibre/matrix adhesion or that an approximation between the polar groups cannot occur during processing because of the macromolecular structure of cellulose, hemicellulose and lignin which are the main components of natural fibres.

The stronger adhesion between MAPP and natural fibres in comparison to PLA results

from additional covalent bonds between the maleic acid anhydride and the hydroxyle groups of the natural fibres [START_REF] Lampke | Beitrag zur Charakterisierung naturfaserverstärkter Verbundwerkstoffe mit hochpolymerer Matrix[END_REF].

Impact strength

The results of the Charpy impact test are shown in table 1 and figure 4. All values are normally distributed ( =5%, David-test). It is apparent that the impact strength of the PLA/flax composites increases with increasing fibre-mass proportion. But the highest value is still 31% lower than the value for pure PLA. This result converges well with the result of Oksman et al. [START_REF] Oksman | Natural fibres as reinforcement in polylactic acid (PLA) composites[END_REF]. Small differences can be explained by the use of different types of PLA and different ways of processing.

The impact strength of Cordenka reinforced PLA increases significantly and reaches a maximum value of 72 kJ/m 2 at a fibre-mass-ratio of 30% which is approximately 4.5 times higher than the value for pure PLA. A comparison of the rates of improvement which can be reached using different types of fibres to reinforce PLA can be found in table 2.

As jute reinforcement, the use of flax fibres leads to a decrease of impact strength [START_REF] Plackett | Biodegradable composites based on L-polylactide and jute fibres[END_REF].

Kenaf can also improve the impact properties of PLA but a clear improvement can only be reached by the use of aliphatic copolymers [START_REF] Anonym | Complete mobile phone housing made of PLA[END_REF]. In a publication by Ganster and Fink [START_REF] Ganster | Novel cellulose fibre reinforced thermoplastic materials[END_REF] the impact strength of a PLA/Cordenka composite with a fibre-mass proportion of 25% was examined with a value of approximately 70 kJ/m 2 .

Thus, the impact strength of the PLA/Cordenka composite is more promising than all other biodegradable composites. It is even higher than the impact strength of flax reinforced PP or MAPP whose impact strengths are approximately 30 kJ/m 2 [START_REF] Bos | Mechanical properties of short-flaxfibre reinforced compounds[END_REF] and which are two of the most important natural fibre reinforced polymers in the automotive industry [START_REF] Karus | All Natural on the Inside? Natural fibres in Automotive Interiors[END_REF].

Debonding, pull-out and fracture of the fibres are the three mechanisms of energy absorption during impact. It is assumed that the strain energy which is released by fibre debonding and fracture is proportional to the debonded length. Consequently, a poor adhesion between matrix and fibres leads to a higher energy absorption [START_REF] Thomason | Influence of fibre length and concentration on the properties of glass fibre-reinforced polypropylene: 4. Impact properties[END_REF]. Thus, the better fibre/matrix adhesion in the PLA/flax composite could be an explanation for the worse impact strength compared to the PLA/Cordenka composite.

The decrease of impact strength between the fibre-mass proportions of 30% and 40% in the PLA/Cordenka can be explained by comparing figure 2 and3: there are many fibres which are not completely surrounded by matrix material and are in contact with other fibres. Consequently, less energy can be absorbed during debonding. Above a certain fibre proportion, which is different for every composite, the mechanical properties cannot be improved anymore. This point appears to be between 30% and 40% for this composite.

Tensile properties

The results of the tensile tests are shown in table 1 and figure 5 and6. All values are normally distributed except for the PLA/Cordenka composite with a fibre-mass proportion of 10% ( =5%, David-test). The specimens of the composite with a fibremass proportion of 40% did not fail in the testing area but in the area where they were fixed to the testing machine and consequently can not be used for further discussion. The Young's modulus as well as the tensile strength of both composites increase with increasing fibre-mass proportion. The flax reinforcement leads to a better improvement of the modulus while the Cordenka reinforcement leads to a better improvement of the tensile strength.

The value of the tensile strength of flax reinforced PLA with a fibre-mass proportion of 30% (54.15 MPa) is very close to the value found in literature (53 MPa) but the modulus (6.31 GPa) is smaller than the value found in the same publication (8.3 GPa) [START_REF] Oksman | Natural fibres as reinforcement in polylactic acid (PLA) composites[END_REF]. To find out if this difference is due to the fact that no extensometer was used in this work, another experiment was done: the Young's moduli of six specimens of the PLA/flax composite with a fibre proportion of 30% were measured using an Instron extensometer. The average value (7.05 GPa; S.D.: 0.09 GPa) was slightly larger than the one found in the first experiment. This leads to the assumption that the different values between literature and experiments presented here are influenced by the way of measuring.

The values for the Young's moduli of the PLA/Cordenka composite are close to those found by Ganster and Fink [START_REF] Ganster | Novel cellulose fibre reinforced thermoplastic materials[END_REF]. But the tensile strength differs significantly to the one found in literature. The tensile strength in that work is given by a value of approximately 110 MPa while the highest value found in this work is only 57.7 MPa.

But the higher values for pure PLA in that work, which was around 70 MPa, has to be taken under consideration when comparing the tensile strengths. It can generally be said that it is hard to compare the values for the tensile strength of the different biodegradable composites because the authors have found completely different values for the pure PLA matrix which reach from values of 21 MPa [START_REF] Nishino | Kenaf reinforced biodegradable composite[END_REF] up to values above 110 MPa [START_REF] Shibata | Biocomposites made from short abaca fibre and biodegradable Polyesters[END_REF]. Nevertheless, the relative improvement caused by the reinforcement fibres can be compared (table 2). The best improvements have been reached using jute fibres [START_REF] Plackett | Biodegradable composites based on L-polylactide and jute fibres[END_REF] and kenaf fibres [START_REF] Nishino | Kenaf reinforced biodegradable composite[END_REF]. The Cordenka fibres in this work caused an increase in tensile strength of 30%.

Conclusion

The composite made of PLA and Cordenka fibres shows promising mechanical properties, especially impact properties (72 kJ/m 2 ). Possible fields of application could be the automotive or the electronic industry. But further research and development of this composite is necessary. Optimised parameter for composite production, fibre length and fibre proportion have to be found. Furthermore, exact tests on the moisture, thermal and chemical resistance have to be made. The ecological benefits must also be investigated in long time tests.

But the PLA/Cordenka composite has the potential to become an ecologically beneficial alternative to natural reinforced composites with petrochemical matrices in the future.

  seven tensile specimens according to DIN EN 61 and seven impact specimens according to DIN EN ISO 179 were produced. Multilayer webs with random in-plane reinforcement fibres were fabricated using a carding machine. For this experiments a lab roller card (manufactured by Anton Guillot, Aachen, Germany) with a working width of 30 cm was used. The rollers are clothed with flexible card clothing. The produced multi layer webs have a web-weight of about 800 g/m². According to NatureWorks PLA Polymer 6202D data sheet these webs were dried for 4 hours at 80°C before they were pressed with a hydraulic press by the company Rucks Maschinenbau GmbH, Glauchau, Germany (type: KW 214.1). The composites were pressed at a temperature of 170°C and at 18 MPa for 5 minutes. Pellets ACCEPTED MANUSCRIPT -7 -were made of the resulting plates using a shredder. These pellets were dried again before injection moulding under the conditions given above. A machine by the company Battenfeld, Bad Oeynhausen, Germany (type: UNILOG 4000) was used for injection moulding. The temperatures in the screw were set to 170°C, 175°C and 180°C; the temperature in the injector was set to 180°C. A dwell pressure of 10 MPa was used for all composites except for the composite with a fibre proportion of 40% for which a pressure of 12 MPa was used. Mechanical testing All mechanical tests took place at 50% relative humidity and 23 °C. The specimens had been conditioned under the same circumstances for at least 24 hours before testing. The cross sectional area of each specimen had been measured with a gauge at 3 locations within the testing area before testing. The Charpy impact strength of the composites was tested according to DIN EN ISO 179. A Thwing-Albert FRANK (Waldbüttelbrunn, Germany) testing machine (type 53302) with a pendulum of 4 J energy was used to measure the unnotched, rectangular specimens (80 mm * 10 mm * 4mm). Seven specimens were tested for each material. The tensile strength and the Young's modulus were determined according to DIN EN 61. A Zwick/Roell (Ulm, Germany) 250 kN test machine operated at a crosshead speed of 10 mm/min was used for testing the dogbone-shaped specimens (testing area: 80 mm * 10 mm * 4 mm; total length 150 mm). The strain of the samples was recorded by the testing machine itself; no extensometer was used. The Young's modulus was calculated within 50-600 N.

Fig. 1 .Fig. 2 .Fig. 3 .

 123 Fig. 1. Overview of the fracture surface of composites with a mass proportion of 10%

Fig. 4 .

 4 Fig. 4. Impact strength of the composites versus their fibre-mass proportion. The

Fig. 5 .

 5 Fig. 5. Tensile strength of the composites versus their fibre-mass proportion. The

Fig. 6 .

 6 Fig. 6. Young's modulus of the composites versus their fibre-mass proportion. The

Table 1 .

 1 The mechanical properties of PLA/flax and PLA/Cordenka composites. 1

	Fibre
	content		Charpy Impact		Tensile Stress		Young' s modulus
	(%)		(kJ/m 2 )			(MPa)			(GPa)
		flax	Cordenka	flax		Cordenka	flax		Cordenka
		average S.D. average S.D. average S.D. average S.D. average S.D. average S.D.
	10	9.97	2.05 43.44	8.11	42.73	1.29	50.40	1.19	3.90	0.18	3.27	0.04
	20	10.45 1.53 62.97	6.01	49.23	1.40	50.75	1.90	5.06	0.07	3.97	0.07
	30	11.13 1.55 72.24	11.79	54.15	4.57	57.97	5.08	6.31	0.12	4.85	0.03
	40		51.34	10.95

Table 2 .

 2 Comparison of the mechanical properties of PLA composites with different reinforcement fibres. Because the values for mechanical properties for pure PLA presented in different papers differ significantly from each other percentage values compared to the value for pure matrix material given in every particular work are presented. Values marked with an asterisk are calculated by using information which was not given in numeric values but in figures.

		Fibre	Charpy	Tensile	Young' s
	Fibre type	proportion	Impact	Strength	modulus	Source
	abaca	20%	-	*104%	*170%	[13]
	Cordenka	25%	*188%	*157%	*146%	[14]
	Cordenka	30%	447%	130%	155%	This study
	flax	30%/40%	*63%	106%	244%	[12]
	flax	30%	69%	121%	202%	This study
	jute	40%	93%	182%	271%	[11]
	kenaf	70%	-	286%	492%	[9]

Acknowledgments

This research was done at the Faserinstitut Bremen FIBRE e.V., Bremen, Germany. The flax fibres were provided by Egon Heger from Hostein Flachs, Mielsdorf, Germany, the Cordenka fibres from Rudolf Einsiedel, Cordenka GmbH Obernburg, Germany and the PLA fibres from Georg Goedecke, NAFGO GmbH, Neerstedt, Germany. The support from Gevork Babaian and Tanja Slootmaker from the Faserinstitut Bremen FIBRE e.V. during this research is also acknowledged.


Lars Böger
email: lars.boeger@tuhh.de

Malte H G Wichmann

Ole Leif

Karl Meyer

Schulte

Load and health monitoring in glass fibre reinforced composites with an electrically conductive nanocomposite epoxy matrix

Fibre reinforced polymers (FRPs) are an important group of materials in lightweight constructions. Most of the parts produced from FRPs, like aircraft wings or wind energy turbine blades are designed for high load levels and a lifetime of 30 years or more, leading to an extremely high number of load cycles to sustain. Consequently, the fatigue life and the degradation of the mechanical properties are aspects to be considered. Therefore, in the last years condition monitoring of FRP-structures has gained importance and different types of sensors for load and damage sensing have been developed.

In this work a new approach for condition monitoring was investigated, which, unlike other attempts, does not require additional sensors, but instead is performed directly by the measurement of a material property of the FRP. An epoxy resin was modified with two different types of carbon nanotubes and with carbon black, in order to achieve an electrical conductivity. Glass fibre reinforced composites (GFRP) were produced with these modified epoxies by resin transfer moulding (RTM). Specimens were cut from the produced materials and tested by incremental tensile tests and fatigue tests and the interlaminar shear strength (ILSS) was measured. During the mechanical tests the electrical conductivity of all specimens was monitored simultaneously, to assess the potential for stress/strain and damage monitoring.

The results presented in this work, show a high potential for both, damage and load detection of FRP structures via electrical conductivity methods, involving a nanocomposite matrix.

Introduction

Fibre reinforced polymers (FRPs) have become the most important material class in the field of lightweight constructions, especially in the aircraft and wind energy industry. Due to their high specific stiffness and strength as well as their outstanding fatigue performance, FRPs are nowadays an irreplaceable material in structural component design. However, their specific discrete build-up makes maintenance as well as damage analysis of these materials a challenging task. As FRPs are especially sensitive to intrinsic damage, such as delamination or matrix cracking, a reliable method for the detection and assessment of such failures is desirable. Furthermore, an in situ monitoring of damage development and/or apparent stresses and strains could be a useful tool to increase the reliability and lifetime as well as to ease the maintenance of structural components exhibited to permanent cyclic loading conditions.

In this context, electrical conductivity methods have been widely investigated for the detection of failure in carbon fibre reinforced polymers (CFRPs). The electrical conductivity of the carbon fibres was first used by Schulte et al. [START_REF] Schulte | Load and failure analyses of CFRP laminates by means of electrical resistivity measurements[END_REF] to monitor damage in CFRPs, which could be related to fibre breakage. AC and DC electrical methods have been extensively studied since then and have been used to study a variety of damage mechanisms, e.g. delamination or matrix cracking, under static and dynamic loading conditions [START_REF] Kaddour | Electrical resistance measurement technique for detecting failure in CFRP materials at high strain rate[END_REF][START_REF] Todoroki | Application of electric potential method to smart composite structures for detecting delamination[END_REF][START_REF] Irving | Fatigue damage characterization in carbon fibre composite materials using an electrical potential technique[END_REF][START_REF] Seo | Damage detection of CFRP laminates using electrical resistance measurement and neural network[END_REF][START_REF] Abry | In situ detection of damage in CFRP laminates by electrical resistance measurements[END_REF][START_REF] Abry | In situ monitoring of damage in CFRP laminates by means of AC and DC measurements[END_REF][START_REF] Kupke | Non-destructive testing of FRP by d.c and a.c. electrical methods[END_REF][START_REF] Todoroki | Delamination identification of cross-ply graphite/epoxy composite beams using electrical resistance change method[END_REF][START_REF] Todoroki | Effects of Delamination shape with a matrix crack on monitoring by electrical resistance method[END_REF][START_REF] Todoroki | Matrix crack detection of CFRP using electrical resistance change with integrated surface probes[END_REF]. The "piezoresistive" effect of carbon fibres also allows a sensing of the applied stress/strain [START_REF] Kupke | Non-destructive testing of FRP by d.c and a.c. electrical methods[END_REF][START_REF] Wang | Piezoresitivity in continuous carbon fibre polymer-matrix composite[END_REF][START_REF] Todoroki | Electric resistance change method for cure/strain/damage monitoring of CFRP laminates[END_REF][START_REF] Todoroki | Electrical resistance change of unidirectional CFRP due to apllied load[END_REF][START_REF] Ogi | Characterization of piezoresitance behaviour in a CFRP unidirectional laminate[END_REF]. Although these methods can offer quite some insight into matrix related failure mechanisms, e.g. delamination, the application of an electrically conductive matrix can provide a much more detailed insight into these mechanisms. Furthermore, the application of an electrically conductive matrix allows the sensing via electrical methods, also in composites reinforced with non conductive fibres, e.g. glass or polymer fibres.

The effects of external strain on the conductivity of insulator/conductor composites based on polymers have been investigated by Pramanik et al. [START_REF] Pramanik | Effect of extensional strain on the resistivity of electrically conductive nitrile-rubber composites filled with carbon filler[END_REF] and Radhakrishnan [START_REF] Radhakrishnan | Structure and electrical properties of polypyrrolethermoplastic elastomer blends[END_REF] et al. These tests were performed above the glass transition temperature (T g ) of the polymer matrix and give a basic insight into the electrical behaviour of polymers containing a conductive phase under external load. Flandin et al. [START_REF] Flandin | Characterization of the damage in nanocomposite materials by a.c. electrical properties: experiment and simulation[END_REF][START_REF] Flandin | AC electrical properties as a sensor of the microstructural evolution in nanocomposite materials : experiment and simulation[END_REF] first used nanoscopic conductive fillers with different aspect ratios in a thermoplastic matrix to monitor the applied macroscopic mechanical strain and the damage evolution during loading. Nanoscaled carbon black particles [START_REF] Kupke | Non-destructive testing of FRP by d.c and a.c. electrical methods[END_REF] as well as microscaled carbon black particles [START_REF] Muto | Hybrid composites with self-diagnostic function for preventing fatal failure[END_REF] have also been used to modify the matrix of glass-fibre reinforced thermosets. In these works, it was shown that external stress as well as apparent mechanical damage can be detected in these multiphase composites via electrical conductivity methods.

The advances in nanotechnology and the increasing commercial availability make carbon nanotubes (CNTs) a very interesting nanofiller. Due to their electrical properties, their high aspect ratio and their tendency to form percolated networks in a viscous matrix, they are excellent candidates for the modification of thermosetting matrix systems. A good review about carbon nanotubes and their composites can be found in [START_REF] Thostenson | Nanocomposites in context[END_REF][START_REF] Fiedler | Fundamental aspects of nano-reinforced composites[END_REF]. Electrical conductivity can be achieved at very low filler contents [START_REF] Sandler | Ultra-low electrical percolation threshold in carbon-nanotube-epoxy composites[END_REF][START_REF] Gojny | Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube epoxy composites[END_REF]. Typical dimensions of multi-walled carbon nanotubes are diameters in the range of 10 -30 nm and lengths of up to several micrometers. Because of their dimensions in the nanometre regime, they are excellent candidates for the modification of FRP matrix systems, since the small particles do not show a filtering effect in liquid composite moulding processes, the particle-doped matrix penetrates the fibre bundles and electrical conductivity in the composite can be provided.. We recently reported about the mechanical and electrical properties of a nanoparticle modified glass fibre reinforced epoxy composite, produced via a vacuum assisted resin transfer moulding process (VARTM) [START_REF] Gojny | Influence of nanomodification on the mechanical and electrical properties of conventional fibrereinforced composites[END_REF][START_REF] Wichmann | Glassfibre-reinforced composites with enhanced mechanical and electrical properties -Benefits and limitations of a nanoparticle modified matrix[END_REF]. Especially the carbon nanotube modified matrix systems provide improved mechanical properties as well as an electrical conductivity in the order of 10 -2 S/m [START_REF] Wichmann | Glassfibre-reinforced composites with enhanced mechanical and electrical properties -Benefits and limitations of a nanoparticle modified matrix[END_REF], which should be sufficient for stress/strain and damage sensing via electrical conductivity methods as the resulting absolute resistance values merasured in the composites are in a measurable range (e.g. several k ) [START_REF] Fiedler | Fundamental aspects of nano-reinforced composites[END_REF][START_REF] Fiedler | Can carbon nanotubes be used to sense damage in composites?[END_REF]. Recently, Thostenson and Chou reported about carbon nanotube modified glass fibre reinforced epoxy composites and their potential for strain and damage sensing. It could be shown that with a weight fraction of 0.5% CNTs in the epoxy matrix, mechanical stresses/strains, as well as matrix failure can be detected via electrical conductivity measurements [START_REF] Thostenson | Carbon nanotube networks: Sensing of distributed strain and damage for life prediction and self healing[END_REF].

Damage sensing in larger GFRP parts is usually done with fibre-optical sensors. The optical glass fibres are embedded in the fabrics used for the fabrication. In case of damage in the part the strain in the fibre is changed locally, which results in changes in the coefficient of refraction. This leads to a characteristic light pattern that can be detected [START_REF] Takeda | Delamination detection in CFRP laminates with embedded small-diameter fibre Bragg grating sensors[END_REF][START_REF] Park | Comparison of nondestructive microfailure evaluation of fibre-optic Bragg grating and acoustic emission piezoelectric sensors using fragmentation test[END_REF][START_REF] Leng | Structural health monitoring of smart composite materials by using EFPI and FBG sensors[END_REF][START_REF] Hofer | Fibre optic damage detection in composite structures[END_REF][START_REF] Waite | Use of optical fibre for damage and strain detection in composite materials[END_REF].

The reliability of fibre-optical sensors for damage detection in large composite parts is insufficient for different reasons. With respect to the production costs it is not possible to apply a dense network of optical glass fibres to large parts and if a crack is propagating without crossing one of the fibre sensors this damage will not be detected. The resolution of the sensors is also limited and is connected with a minimum size of the defect, because a small defect will not cause a change in fibre strain that can be detected.

Not to be neglected is the fact that fibre-optical sensors may also induce damage to composite parts. Due to their diameter, which is 5-10 times the diameter of the reinforcement fibres, they can act as a notch in the composite material and therefore reduce the threshold for crack initiation [START_REF] Lee | The mechanical characteristics of smart composite structures with embedded optical fibre sensors[END_REF]35].

The detection of damage in composite parts by measuring variations in an intrinsic material property, as the matrix conductivity, therefore offers benefits compared to fibre-optical sensors.

In this paper, we report about glass fibre reinforced epoxy laminates, modified with very small amounts (0.1 and 0.3 wt.%) of conductive carbon nanofillers (carbon black and MWCNTs). Combined mechanical/electrical tests were performed in order to evaluate the sensing potential of these multiphase composites. Static tests (interlaminar shear strength tests, stepped tensile tests) as well as dynamic fatigue tests (cyclic tensile tests) were performed and the electrical conductivity was measured in situ. It could be shown that all conductive nanocomposite matrix systems exhibit a potential for stress/strain and damage sensing, however exhibiting significantly different sensor characteristics.

Materials

The epoxy matrix in this study consists of the epoxy resin Araldite LY 556, the anhydride hardener Aradur 917 and the imidazole accelerator DY070, supplied by Huntsman Advanced Materials, Bergkamen / Germany. The LY556 system is a hot-curing epoxy system used in filament winding, pultrusion or pressure-moulding processes, for example.

To achieve an electrical conductivity, the epoxy matrix was modified by the addition of 0.3 wt.% of nanoscaled carbon particles. Three different types of nanoparticles were used in this study: Double Wall Carbon Nanotubes (DWCNT) from Nanocyl, Belgium, Multi Wall Carbon Nanotubes (MWCNT) from Arkema, France, and carbon black XE2 from Degussa, Germany. The modified epoxy systems were used to manufacture glass-fibre laminates, using the VARTM technique.

In order to disperse the nanoparticles in the epoxy matrix, the particles were manually mixed with the epoxy resin (without hardener and accelerator). This pre-mixture of nanoparticles and resin was then added batch-wise to a three roller mill. Details of the dispersion process are described elsewhere [START_REF] Gojny | Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube epoxy composites[END_REF]. The blended epoxy resin was then collected, bottled and stored at low temperatures to avoid the re-formation of nanoparticle-agglomerates.

Prior to the VARTM manufacturing of the material, the suspension of epoxy and nanoparticles was mixed with the hardener and accelerator, using a laboratory vacuum stirrer to avoid mixing in air bubbles and to apply high shear rates to the compound. The mixing pot was then removed from the vacuum stirrer and then inserted in the resin-source container of the VARTM-system. The epoxy matrix was finally degassed and slightly heated up to 45°C to compensate the increased viscosity, resulting from the addition of the nanoparticles. As fibrereinforcement, two layers of glass-fibre non-crimp fabrics (Saertex Wagener GmbH, Germany) were used to finally achieve a laminate with a [0°, +45°, 90°, -45°, +45°,90°,-45,0°] stacking sequence. The materials produced had a fibre-volume-content of 35%. The mould was slightly heated up to 45°C to keep the viscosity of the epoxy matrix low during the whole injection process.

For each material, additionally, a second laminate was produced. During its processing, an electrical field with the field strength of 333V/cm was applied to the RTM-mould in zdirection, in order to induce a preferred orientation of the nanoscaled carbon particles in zdirection and to stimulate the formation of a conductive network [START_REF] Wichmann | Glassfibre-reinforced composites with enhanced mechanical and electrical properties -Benefits and limitations of a nanoparticle modified matrix[END_REF]. The electrical resistance of the specimens under mechanical loading was measured in situ using a Keithley 2602 sourcemeter instrument. A constant voltage was applied to the specimens, the current was measured and the resistance was calculated from these values. The sourcemeter-instrument provides sampling rates up to 50s -1 for electrical measurements.

Results and discussion

Interlaminar shear strength

The ILSS test was performed to introduce a well-defined, spontaneously occurring damage (delamination) to the specimens while the electrical conductivity was continuously measured.

In an earlier work, the interlaminar shear strength of a GFRP system was enhanced by addition of nanoparticles [START_REF] Wichmann | Glassfibre-reinforced composites with enhanced mechanical and electrical properties -Benefits and limitations of a nanoparticle modified matrix[END_REF]. Therefore, the mechanical results of the tests were also investigated. Fig. 2 shows the interlaminar shear strength of the nanoparticle-modified composites. It can be seen that the ILSS values are slightly increased by the addition of carbon nanoparticles, although the changes are not significant. It also seems that the application of an electrical field leads to a reduction in improvement. All in all, the influence of the nanoparticles on the ILSS is not comparable to the results found in [START_REF] Wichmann | Glassfibre-reinforced composites with enhanced mechanical and electrical properties -Benefits and limitations of a nanoparticle modified matrix[END_REF].

Under mechanical load, the specimens showed a typical progression of force and resistance change with testing time. Figure 3, as an examplary testing result, shows that the throughthickness-(Z-direction)-resistance of the specimens is fairly unaffected by the introduction of shear forces. A similar course of resistance over time can be observed for the resistance in 0° direction of the specimen. At the time, when the sudden delamination of the laminates occurs, the resistance is rising abruptly. This behaviour is representative for all specimens investigated in this study. For each specimen, the variation of resistance was measured, and normalised to the initial resistance. Figure 4a shows the normalised values for the resistance changes at break of the specimens, measured in longitudinal (0°) direction of the specimens. It can be clearly seen that for all kinds of matrix modification a significant resistance change of at least 10% was obtained. The carbon-black modified epoxy laminates exhibited values around 25% to 45%. The values for the resistance change measured in z-direction (Fig. 4b) are about 10%-15% for the nanotube-modified laminates and around 20%-40% for the carbon black modified laminates. This abrupt increase in the electrical resistance can be easily explained with the interruption of conductive pathways in the materials by the delamination.

In this context it is important to point out, that the delamination, which occurs in the fibre plane orthogonal to the z-direction, can be electrically detected in both measurement directions (0°-direction and z-direction). Two reasons can be figured out for this phenomenon.

First, the delamination is never a purely two-dimensional failure, but is also propagating in thickness direction (translaminar crack). Second, the electrical pathways of the nanoparticles are randomly three-dimensionally oriented in the laminates and therefore, a resistivity measurement in 0°-direction is also affected by a delamination in the fibre plane, since the electrical pathways are also partly oriented in z-direction. This is confirmed by the results shown in Fig. 4a. It can be seen that the orientation of the nanoparticles in z-direction, introduced by the electrical field during curing, leads to an increase in failure sensitivity when measuring in 0°-direction. The conductive pathways in the laminates which were subjected to the electrical field are statistically preferentially oriented in the z-direction and thus an interlaminar failure will have a stronger impact on 0°-conductivity.

Incremental tensile tests

Incremental tensile tests proved to be very suitable to identify the potential of the nanocomposite matrix systems for the sensing of stresses/strains and occurring damage.

Figure 5a shows exemplary courses of stress and strain over time for the chosen testing procedure. The example shows the measurement on a specimen containing 0.3 wt.% MWCNTs.

The sample was subjected to tensile loading cycles, with increasing maximum strain per cycle. At the end of each cycle, the sample was unloaded to = 0. In Figure 5a, it can be seen that after the fifth loading cycle, some residual strain occurs when the specimen is unloaded to zero stress. This residual strain is increasing with each subsequent loading cycle and can be attributed to plastic matrix deformation and damage occurring, such as matrix cracking in the 90°-layers [36]. Figure 5b shows a corresponding resistance change curve, in this case measured in z-direction. It can be clearly seen that the change in resistance can be directly correlated with the applied tensile load. The resistance increases and decreases monotonically with the cyclic stress/strain. Furthermore, it can be observed that after the fifth loading cycle an irreversible resistance change occurs when the specimen is unloaded. This directly correlates with the emerging residual strain which could be observed after the fifth loading cycle (Figure 5a). This result also underlines the high capability of conductive polymer matrix systems for damage sensing applications. Figure 6 shows the correlation of the emerging residual strain after each cycle with the corresponding irreversible resistance change.

It can be seen that already very small residual strains in the order of 0.05 % can be detected with the electrical resistivity measurements. The corresponding irreversible resistance change was 0.02 % in this case. In the subsequent loading cycles, the residual strain increases gradually to a value of 0.52% after the 11 th cycle, where the corresponding irreversible resistance change was found to be 1.4%.

We found a very similar behaviour for all the nanocomposite matrix laminates. The occurring residual strains could always be correlated to irreversible resistance changes. For the carbon nanotube modified matrix systems, a final residual strain of ~ 0.5% was found to correlate with irreversible resistance changes between 1 and 5%. As already observed in the ILSS measurements, the carbon black modified epoxy matrix system exhibited a higher sensitivity, with irreversible resistance changes between 10 and 20%. However, the absolute resistance of these specimens was measured in the range of several tens of M and was two orders of magnitude higher than the resistance of the carbon nanotube modified laminate specimens, which confirms the results measured on the ILSS specimens.

The setup of the incremental tensile test is also especially suited for the evaluation of the stress/strain sensing capabilities of the tested materials. As shown in Figure 5, the resistance changes reversible with the applied load. In order to evaluate the strain sensing properties and to compare the behaviour of the different nanocomposites, the resistance-strain behaviour was investigated. In this paper we just give a first introduction into the stress/strain sensing characteristics. A detailed analysis of this very complex behaviour is in preparation and will be published soon.

Figure 7 shows the resistance change vs. strain curves of the 10 th loading cycle from the stepped tensile tests. In Figure 7a, the curve for the 0.3 wt.% MWCNT composite is displayed; in Figure 7b, the same curve is shown for the 0.3 wt.% carbon black composite.

The resistivity was measured in z-direction in both cases and the maximum strain applied was 2.5%.

The different characteristics of these exemplary curves are representative for the behaviour of the respective nanocomposite matrix. The general resistance-strain behaviour was found to follow an exponential power law in the form of

ε β α ⋅ ⋅ + = ∆ e r R R n 0 ,
where R is the absolute resistance change, R 0 the initial resistivity of the specimen, r n the residual resistance change at the beginning of the load cycle and is the applied strain.

However, the resistance-strain behaviour of the CNT modified laminates can be approximated very well with a linear approximation. In contrast, the carbon black modified laminates exhibited as pronounced exponential resistance-strain behaviour. Here again, the higher sensitivity of the resistance versus mechanical loading can be observed for the carbon black.

A value of 2.5% mechanical strain corresponds to a resistance change of 6.3% for the MWCNT modified laminates, while for the carbon black modified laminates a resistance change of 36.2% was observed. Furthermore, an initial gauge factor (IGF) was evaluated by calculating the slope of the tangent, applied between 0.05 and 0.25% of reversible strain. The IGF was found to be 1.54 for the MWCNT containing laminate and 3.39 for the carbon black containing laminate.

It is obvious that the different behaviour of the carbon nanotube modified matrix system and the carbon black modified matrix system has to be attributed to the intrinsic structure of the percolated conductive paths in the composite. Two main factors will influence the sensitivity for stresses/strains and occurring damage. The first factor is the redundancy of the network structure. In this context, the higher sensitivity of the carbon black modified composites can be explained by the lower density of conductive pathways, as the concentration of 0.3 wt.% used in our experiments is quite low, regarding the percolation threshold of this material [START_REF] Gojny | Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube epoxy composites[END_REF].

Concerning the measured specific conductivity of the carbon black composites, a value of around 10 -4 S/m is a hint for the concentration being close to the percolation region. For the MWCNT modified laminates a specific conductivity of some 10 -2 S/m was measured. The percolation threshold of this MWCNT/epoxy system was found to be below 0.1 wt.%. In summary, the carbon black modified laminate with a concentration of 0.3 wt.% is close to the percolation region and therefore much more sensitive to applied stress/strain and occurring damage. The laminates containing 0.3 wt.% of MWCNTs are far above the percolation threshold and therefore exhibit a much more redundant conductive network structure. Thus, the sensitivity of this system versus damage is lower, compared to the carbon black modified matrix.

Regarding the resistance-strain behaviour, the different characteristics can furthermore be explained by the different filler geometries. As the carbon black particles are of spherical nature, the conductive pathways through the composites consist of spherical particles, which are connected by point-point contacts. In the case of carbon nanotubes, the network structure is very different as the aspect ratio of the CNTs is very high. The CNTs can be up to several micrometers long. Each CNT can possess multiple contacts to other CNTs and the CNTs are three-dimensionally randomly oriented within the matrix and can be geometrically entangled.

Such a network will behave completely different, when exhibited to mechanical strain.

Because of the higher amount of contacts and the different orientation with respect to the loading direction, the CNT networks are electrically less sensitive to mechanical strain than networks consisting of spherical carbon black particles. Furthermore, it is noteworthy that the sensing of strain applied in the 0° direction can be well detected via electrical resistance measurements performed in through-thickness-direction, meaning perpendicular to the loading direction. This can be explained by the three dimensional network structures of the conductive nanoparticles. As the glass-fibre fabrics are inhibiting a direct electrical path between the two specimen surfaces, the resulting electrical pathways will be partly oriented in 0°-direction.

Dynamic tensile tests

Dynamic measurements were performed to show the resistance change during fatigue loading of the materials. It could be observed that a decrease of the stiffness of the specimens is accompanied by an increasing electrical resistance. In Figure 8, the graphs for the stiffness of the material, expressed over the dynamic modulus, which is measured in terms of the slope between the maxima and the minima in the hysteresis loop of each cycle, are shown.

Basically, three facts are remarkable concerning the resistance development during fatigue life: i. a stiffness decrease is accompanied by a resistance increase, ii. the difference between the resistance at maximum and minimum load is increasing during fatigue life, iii. major defects, such as delamination of single 0° rovings on the specimen surface, for example, could be detected by an abrupt increase of the electrical resistance. These effects could be observed for both, resistance measured in length-and thickness direction of the specimen, whereas the detection of major defects was easier by resistance measurements in thickness direction as shown in Figure 9. A 100% change in resistance was observed when a delamination of a major part of the specimen occurred.

Gagel [36] showed the correlation between stiffness degradation and matrix crack density during loading of a GFRP with the same lay-up. In this work a correlation between resistance change and stiffness was observed for a similar material. Hence, it can be concluded that the change in resistance correlates with the crack density. To sum up, electrical resistivity measurements seem to be a promising tool for the detection of damage in nanoparticle modified GFRPs. Both matrix crack density and single major defects show special effects on the electrical resistance of the materials. A detailed investigation of the electrical conductivity development in GFRP laminates during fatigue life is in preparation.

Conclusions

The results of the experiments presented here demonstrate the high potential of electrical conductivity methods for stress/strain and damage sensing in FRPs with carbon nanoparticlemodified matrix systems. This method works only by the measurement of the electrical resistance and without additional sensors that have to be applied to the materials and is therefore a good example for "self-sensing" materials.

The sensitivity of this new method seems to be much higher compared to other damage sensing methods. During incremental tensile tests and fatigue tests it was possible to clearly measure resistance changes in the materials that were related to microscale damage, such as inter-fibre failure. This kind of damage can not be detected by other damage sensing methods.

Later in the fatigue tests, when macroscopic damage (delamination or rupture of fibre bundles) occurred, these events were leading to explicit signals in the electrical conductivity measurement of the materials. Therefore, this new method offers the possibility for an in-situ measurement of the damage accumulation during the entire fatigue life of large composite structures, beginning from microscale damage and ending in rupture of fibre bundles or final failure.

Interestingly, also a dependency of the electrical resistance on the load applied to the composite was found. Therefore, not only the accumulation of damage can be detected by these measurements. It is also possible to directly measure the strain state of a composite structure.

The interlaminar shear strength of the laminates was not significantly improved as shown in earlier studies [START_REF] Wichmann | Glassfibre-reinforced composites with enhanced mechanical and electrical properties -Benefits and limitations of a nanoparticle modified matrix[END_REF]. It can be concluded that the significance of the effect of nanoparticles on the mechanical properties of an epoxy resin is different depending on the type of resin. However, in the context of functional matrix properties, such as health monitoring capabilities, it is of high importance to demonstrate that the mechanical properties are not affected negatively by the nanoparticle modification. Future studies will deal with a detailed investigation of the stress/strain -conductivity characteristics and the development of conductivity during fatigue life.

After 4 h

 4 curing at 80°C, the material was removed from the mould. Specimens according to DIN EN ISO 527-4 with the dimensions 25 x 3 x 250 mm and modified specimens acc. to ISO 14130 with the dimensions 21 x 3 x 10 mm were cut and edge-polished. The tensile-testspecimens acc. to DIN 527-4 were furnished with tabs of 2 mm thickness. Each tab consists of two 1 mm thick layers of [+/-45°] GFRP on the specimens side, aluminium on the fixtures side, respectively. The specimens were post-cured at 140°C for 8 hours and contacted with conductive silver paint. For resistance-measurements in 0°-direction of the specimens the two opposite front and back sides of the specimens were painted with conductive silver paint. For the measurements in z-direction of the ILSS-specimen the upper and lower surface was fully painted with conductive silver and the remaining edges were edge polished to prevent shortcircuiting the resistance measurement. The contacting for the tensile test-specimen was carried out similar, however here only a 15mm wide electrode was applied to both sides in the middle of the specimen. The specimens and the experimental setup for both ILSS and tensile tests are shown in figure 1. "z" and "0°" indicate the direction of the in-situ resistance measurement during the mechanical testing.

  Three different mechanical testing methods were conducted in our investigations and the potential for electrical conductivity methods for condition monitoring was evaluated: the interlaminar shear strength (ILSS) test, incremental tensile tests and dynamic tensile tests. The interlaminar shear strength of the composites was measured according to ASTM D-2344 with the short beam method. A Zwick Z010 universal testing machine was used to determine the ILSS of at least 15 specimens per material. The stepped and the dynamic tensile tests were performed on a servo-hydraulic Instron/Schenck testing machine with a maximum load capacity of 100 kN.

  Figures:

Figure 1 :

 1 Figure 1: schematic visualization of specimen for in-situ resistance measurements during ILSS test (left) and tensile-test (right).

Figure 2 :

 2 Figure 2: Interlaminar shear stresses of the modified GF-NCF-EP materials (EF = electrical field applied during curing).

Figure 3 :Figure 4 :

 34 Figure 3: Progression of force and resistance change during ILSS test.

Figure 5 .

 5 Figure 5.: a) Measured stress and strain and b) electrical resistance change during a stepped tensile test. The specimen contained 0.3 wt.% MWCNTs in the matrix and the resistivity was measured in z-direction.

Figure 6 :

 6 Figure 6: Apparent residual strain at the end of each loading cycle and corresponding irreversible resistance change, measured during the cyclic tensile test.

Figure 7 :

 7 Figure 7: Electrical resistance change vs. mechanical strain, measured during the 10 th loading cycle of a cyclic tensile test (resistivity measured in z-direction) for a) the composite containing 0.3 wt.% of MWCNTs and b) the composite containing 0.3 wt.% of carbon black.

Figure 8 :

 8 Figure 8: Stiffness and resistance change for a specimen under dynamic tensile load. Resistance measured in longitudinal (0°)-direction.

Figure 9 :

 9 Figure 9: Stiffness and resistance change for a specimen under dynamic tensile load.Resistance measured in thickness (Z)-direction.

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	36	Gagel A, Lange D, Schulte K. On the relation between crack densities, stiffness
		degradation, and surface temperature distribution of tensile fatigue loaded glass-fibre
		non-crimp-fabric reinforced epoxy. Composites A 2006;37:222-228.

Acknowledgements

The authors gratefully acknowledge the support of the European Commision Framework 6 Programme, Priority 4, Aeronautics and Space (Aerospace Nanotube Hybrid Composite Structures with Sensing and Actuating Capabilities, NOESIS), Contract No. AST4-CT-2005-516150.