Short communication

Electrical resistance curing of carbon-®bre/epoxy composites

Christopher Joseph, Christopher Viney*

Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK

Received 21 August 1998; received in revised form 5 July 1999; accepted 4 August 1999

Abstract

Carbon-®bre/epoxy resin composites were cured by the ohmic heating that results from passing an electrical current through the®bres. Additional cured composite was prepared conventionally. All samples consisted of eight-layer stacks of pre-preg, containing38% ®bre by volume. Vickers hardness measurements demonstrate that both processes lead to the same level of cross-linking. Inthree-point bend tests, the properties of resistance cured composite compare favourably with those of oven-cured samples. Sig-

ni®cantly less energy is used in the process of resistance curing small samples, relative to curing in an oven or autoclave. # 2000Elsevier Science Ltd. All rights reserved.

Keywords: A. Carbon ®bres; A. Polymer-matrix composites; B. Curing; B. Mechanical properties; Electrical resistance heating

1. Introduction

The method of curing carbon-®bre/epoxy resin compo-site has remained largely unchanged over the past 30 yearsor more. Heating an autoclave to the curing temperaturetakes signi®cant amounts of energy and time. Additionaltime is taken to cool the autoclave after curing. Processingis, therefore, more costly and less continuous than wouldbe optimal for mass-production. Along with the percep-tion among many manufacturers that carbon-®bre-con-taining materials are intrinsically expensive [1], thisconcern limits the more widespread adoption of CFRP.Carbon ®bres have a high electrical conductivity relative

to cured epoxy. Ohmic heating of copper-coated singlecarbon ®bres has been used in assessing the suitability ofsuch ®bres as reinforcement for metal-matrix composites[2]. The feasibility of producing durable electrodes fromcarbon ®bre [3] has been considered also. However, thepossibilities of exploiting the conductivity of carbon®bre during composite processing have received scantattention. The notable exception is work by Gillespie etal. [4±7], who passed current through a layer of ®bres

surrounded by thermoplastic polymer; the latter can bemade to ¯ow and produce a weld between two existingpieces of composite. The properties of the weld approachedthose of a normal CF/PEEK composite, demonstratingthat robust joins can be achieved.The present work investigates whether resistive heating

can also be used to cure a stack of CF/epoxy pre-pregsheets. Several considerations motivated this study: (1)the possibility of saving energy, since heat would bedelivered immediately and directly to the workpiece; (2)the possibility of heating the matrix more evenly than inan autoclave Ð leading to more uniform shrinkage andinternal stress distribution; (3) the possibility of re®ningthe method to achieve a cross-link gradient in the vicinityof each ®bre, with the matrix being sti�er close to the®bres and more compliant away from them; and (4) thepossibility of evolving electrical resistance curing into amore continuous process.

2. Experimental

2.1. Samples cured by electrical resistance heating

Samples consisted of eight layers of 914c TS(6K) pre-preg sheeting produced by Hexcel Composites (Duxford,UK). The ®bre content of this pre-preg is 38% by

0266-3538/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0266-3538(99 )00112-8

Composites Science and Technology 60 (2000) 315±319

* Corresponding author at Department of Chemistry, Heriot-Watt

University, Edinburgh EH14 4AS, UK. Tel.: +44-131-451-3759; fax:

+44-131-451-3180.

E-mail address: c.viney@hw.ac.uk (C. Viney).

volume. Layers were separated at each end of the sam-ple, and were held in position relative to one another, bya set of copper blocks (Fig. 1). The blocks were alignedby locking pins passing through the entire height of thestack (Fig. 2). Aluminium foil was used to wrap theends of each layer before clamping, to promote goodelectrical connection but prevent the epoxy from stick-ing to the copper. Alternate blocks at each end of theapparatus were connected to the power supply, ensuringa direct connection to each layer (Figs. 1 and 2). Thecentral section of electrically cured samples was held inan open-ended support (Fig. 3), the insulating surfacesof which were wrapped in PTFE tape to facilitate samplerelease after curing. Pressure of up to 35 kPa was

applied using a set of metal weights; the contact areawas 60�30 mm.Using a variable power supply, the voltage was set to

maintain a constant current of 15 A. A control experi-ment, with a thermocouple placed in the middle of thestack of layers, con®rmed that the pre-preg manufac-turer's recommended cure temperature of 185�C isreached within 10 min, and that the temperature is sub-sequently maintained within �3�C of this target. Smalladjustments to the voltage were necessary to compen-sate for changes in sample resistivity as curing tookplace; typically the potential had to be increased from�6.0 V to �6.4 V during the ®rst 10 min of each cure.No discontinuous changes in sample conductivity wereregistered, suggesting that the supply potential was toolow to cause dielectric breakdown of the matrix. Sam-ples were cured for a total of 60 min as recommended bythe pre-preg manufacturer. An alternating current sup-ply was used, to prevent ion migration within the sam-ple during heating and to minimise equipment cost.

2.2. Oven cured samples

Eight-layer stacks of the same pre-preg material werewrapped in a PTFE sheet (domestic baking tray liner)and placed under the same set of weights in a pre-heatedoven (Philip Harris Ltd, Shenstone, UK; model N18C) atthe recommended curing temperature of 185�C. A curingtime of 60 min was used, to allow straightforward com-parison with samples cured by electrical resistance heat-ing. Vacuum bags were not used in conjunction witheither process; the aim was not to obtain the best possiblemechanical properties from cured samples, but simply tocompare the e�ects of electrical versus oven curing underan otherwise constant set of conditions.

2.3. Mechanical property characterisation

Impact tests on composite materials are of limitedvalue [8], and each test provides only a single measuredquantity for each sample. Tensile tests on CF compositematerials require a complex sample shape Ð waisted intwo dimensions Ð to ensure the sample does not disin-tegrate into its constituent laminates before ®bre frac-ture can occur [9,10]. A three-point bending system(constructed for small specimens [11]; separation ofouter support points is 1 cm) was therefore chosen. Thisapparatus ®ts into an Instron tensile testing machine(Model 8561). Four test specimens, each approximately20 mm long by 5 mm wide, were cut from the centralregion of each cured sample. Conversion of Instroncrosshead displacement to sample strain was achievedby using standard formulae [12].Hardness measurements were carried out on a Vickers

hardness tester, using a pyramidal diamond indenterand a 2.5 kg load.

Fig. 1. Schematic representation of pre-preg layer stacking and elec-

trical connectivity.

Fig. 2. Schematic details of clamp geometry and dimensions.

Fig. 3. Schematic illustration of method used to support and compact

the stack of pre-preg layers while it is being cured by electrical resis-

tance heating.

316 C. Joseph, C. Viney / Composites Science and Technology 60 (2000) 315±319

2.4. Light microscopy

The cut surfaces of specimens prepared for mechan-ical testing were photographed in re¯ected light, using ametallurgical microscope (Meiji Techno Co, Japan) anddigital camera (Panasonic Solid State Colour, modelWV-CD110AE). Testing in three-point bend did notbreak specimens all the way through, so external frac-ture surfaces were not available for examination.

3. Results and discussion

3.1. Extent of cure

Vickers hardness measurements (4 readings in eachcase) yielded values of 77�1 and 75�1 for electricallycured and oven cured composite respectively. The simi-larity of these values demonstrates that both processescure the samples to a comparable extent.Fig. 4(a) and (b) show cut surfaces of the two types of

material. It is easier to distinguish the original pre-preglayers in oven cured composite. The qualitative fracturebehaviour of specimens during three-point bending isconsistent with this structural di�erence: resistance curedsamples showed little or no tendency to spall, but ovencured samples invariably produced numerous splinters.

3.2. Mechanical properties

Load versus de¯ection data collected in three-pointbend tests allow easy determination of: (1) peak stresswithstood by the material; (2) strain at which material

ceases to carry useful load; and (3) amount of energyrequired to break the specimens. Data are collated inTable 1. The spread in data is characterised by the dif-ference between maximum and minimum readings, aswell as by the standard error in the mean.Values of peak stress recorded for resistance cured

material are consistently lower than those obtained afteroven curing.With a three-point bend test, it is not possible to

completely break a su�ciently ¯exible material. A `bro-ken' specimen was designated as one which could nolonger carry a 100 MPa nominal stress in the later stagesof deformation, i.e. after the peak stress had beenencountered. The chosen limit of 100 MPa is highenough to ensure that no specimen was deformed to astage where its centre impinged on the base of the testrig, but low enough in comparison to the peak stress toensure that the material is no longer useful. In this way,a ®xed basis of comparison was maintained. Resistancecured composite can deform to approximately twice thebreaking strain of oven cured material (Table 1).Total energy absorbed was calculated from the load

and de¯ection data, using the Trapezium Rule for inte-gration. Energy absorption is greater in the case ofresistance cured material (Table 1); the higher strain tobreaking more than compensates for the lower nominalpeak stress.

3.3. Errors

Potentially signi®cant misalignment can occur: (1)during layup; and (2) in the bend test rig, where the ®bremay not lie perpendicular to the supports. The ®rst

Fig. 4. Optical micrographs of (a) resistance cured and (b) oven cured samples. The ®bre direction is vertical.

C. Joseph, C. Viney / Composites Science and Technology 60 (2000) 315±319 317

instance is relatively easy to minimise, as the dimensionsof the pre-preg strips (30mm�300mm) are large enoughfor any error greater than 0.2� to be noticed and recti®ed.Misalignment in the test rig could be kept to within 0.5mm (10% of the specimen width) along the 20 mm spe-cimen length. Together with the cumulative error due tomisalignment during lay-up, this leads to a maximumerror of 2�, equivalent to a strength reduction of around20% [13]. This magnitude of error is comparable to thedi�erence between the maximum and average nominalpeak stresses for both sample types, suggesting that thehigher measured values are a reliable estimate of theattainable sample strength.

3.4. Energy use

Electrical resistance curing heats the compositeimmediately and directly. The small oven used hererequired 500 W of power, compared to a maximum of96 W for resistance curing, and the oven needed to beswitched on for 25 min before reaching the correcttemperature. Even taking account of the fact that theoven only draws power for approximately 2/3 of thetime after pre-heating, the energy requirement for resis-tance curing was less than 18% of that used for ovencuring. While the provision of heat is not the onlyenergy cost in the manufacture of CF/epoxy composite, asaving of over 82% in this respect, with no correspondingincreases elsewhere, must be regarded as signi®cant.

4. Conclusions

1. Stacks of CF/epoxy pre-preg layers can be curedinto functional, monolithic composite by electricalresistance heating.

2. Electrical resistance curing of small samples isachieved with a signi®cantly lower overall energyconsumption, compared to oven curing.

3. In three-point bend tests, oven cured specimenscan support a higher nominal peak stress, butresistance cured specimens can sustain approxi-

mately twice the breaking strain and exhibitgreater energy absorption.

Acknowledgements

Weare grateful toDr. JohnHarding,Dr. Steve Roberts,Mr. Silas Denyer (University of Oxford) and ProfessorJ.W. Gillespie (University of Delaware) for helpful dis-cussions. In addition, we thank Silas Denyer for providinga supply of the Hexcel pre-preg, and Professor Gillespiefor providing copies of relevant publications and reports.

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Table 1

Mechanical property comparison for resistance cured and oven cured composite

Curing procedure No. of specimens Property Units Mean Range Sample SD

Resistance 7 Nominal peak stress MPa 520 438±637 70

Oven 9 Nominal peak stress MPa 773 685±926 74

Resistance 7 Breaking strain (%) Ð 4.1 3.5±5.5 0.6

Oven 9 Breaking strain (%) Ð 2.2 1.7±2.6 0.3

Resistance 7 Energy to break MJ mÿ3 0.57 0.50±0.63 0.05

Oven 9 Energy to break MJ mÿ3 0.43 0.37±0.54 0.06

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