Vol. 5, No. 1 March 2004 pp. 18-22

Monitoring Failure Behaviour of Pultruded CFRP Compositesby Electrical Resistance Measurement

Yaqin Mao, Yunhua Yu, Dezhen Wu and Xiaoping Yang♠

College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China♠ e-mail: yxp@public.bta.net.cn

(Received December 6, 2003; Accepted March 8, 2004)

Abstract

The failure behaviours of unidirectional pultruded carbon fiber reinforced polymer (CFRP) composites were monitored bythe electrical resistance measurement during tensile loading, three-point-bending, interlaminar shear loading. The tensilefailure behaviour of carbon fiber tows was also investigated by the electrical resistance measurement. Infrared thermographynon-destructive evaluation was performed in real time during tensile test of CFRP composites to validate the change ofmicrodamage in the materials. Experiment results demonstrated that the CFRP composites and carbon fiber tows weredamaged by different damage mechinsms during tensile loading, for the CFRP composites, mainly being in the forms ofmatrix damage and the debonding between matrix and fibers, while for the carbon fiber tows, mainly being in the forms offiber fracture. The correlation between the infrared thermographs and the change in the electrical resistance could be regardedas an evidence of the damage mechanisms of the CFRP composites. During three-point-bending loading, the main damageforms were the simultaneity fracture of matrix and fibers firstly, then matrix cracking and the debonding between matrix andfiber were carried out. This results can be shown in Fig. 9(a) and (b). During interlaminar shear loading, the change in theelectrical resistance was related to the damage degree of interlaminar structure. Electrical resistance measurement was moresensitive to the damage behaviour of the CFRP composites than the stress/time curve.

Keywords : CFRP (carbon fiber reinforced polymer), electrical resistance measurement, damage mechanism, matrix cracking,debonding

1. Introduction

CFRP composites are important structural materials due totheir high tensile strength, high tensile modulus and low den-sity, and therefore attractive for aircraft applications. Withincreasing use in primary parts, it becomes urgently necessaryto detect, evaluate and understand the development of damagein CFRP components in order to guarantee safe use overtheir entire lifetime. Structure health monitoring refers to themonitoring of the integrity of a structure for the purpose ofhazard mitigation. Recent attention on structure health monitor-ing has been centered on the use of embedded or attachedsensors, such as optical fiber, piezoelectrical, microelectro-mechanical, acoustic, dynamic response, phase transformation,and other sensors.

The CFRP composites have conductivity due to containingconductive carbon fibers. The conductivity of CFRP compositesdepends on the directions, the orientation and the volumefraction of conducting carbon fibers. As for unidirectionalCFRP composites, with the increasing of volume fraction ofcarbon fiber, the conductivity of the materials increases.When the fiber volume fraction reaches to a threshold (about40%~60%), a strongly connected electrical network withanisotropy forms. if the unidirectional CFRP composites

subject to damage, the conductivity of the materials willchange. So through monitoring the variation of the electricalresistance, the damage degree in the unidirectional CFRPcomposites can be detected so as to avoid fateful mar. Theelectrical resistance measurement is a particularly effectivemethod for detecting small and subtle defects in CFRPcomposite materials. Previous work has focused on the setupof electrical resistance experiments and probing into damagemechanism of CFRP laminates [1-5].

In this work, the changes in the electrical resistance of theunidirectional pultruded CFRP composites during tensile,three-pointbending, interlaminar shear loadings were monitoredin order to analyze damage mechanisms of the CFRP com-posites.

2. Experimental

The unidirectional pultruded CFRP composites used inthis work were manufactured via the continuous pultrusionprocess as reported in reference [6]. The tensile test speci-mens, with a cross section of 6×2 mm2, were cut from thepultruded CFRP composites. The specimens for three-pointbending and interlaminar shear testing had the same cross

CarbonScience

Monitoring Failure Behaviour of Pultruded CFRP Composites by Electrical Resistance Measurement 19

section of 25×2 mm2. In order to perform the electricalresistance measurement, copper electrodes were adhered tothe polished specimen surfaces as shown in Fig. 1(a), withthe type SY-73 conductive silver adhesive [7] from BeijingInstitute of Aeronautical Materials. The silver adhesive,cured at 120oC for 2 hours, has an electrical resistance of0.06 Ω(≤0.1 Ω), so the contact electrical resistance betweenthe copper electrodes and carbon fibers can be neglected,and the electrical resistance values measured in this experi-ment are stable.

The electrical resistance was measured by four-wiretechnique with digital multimeter (Agilent 34401A) to ensurea quasi DC resistance value. An infrared thermographyapparatus was used in the tensile test for non-destructiveevaluation. The Schematics of the experimental setup wereshown in Fig 2. Three-point bending and interlaminar sheartests were carried out with Instron-1185 universal materialstesting machine according to ASTM 02344-84 and ASTM-790-00.

3. Results and Discussion

3.1. Tensile loading testFig. 3(a) and Fig. 3(b) illustrated the results of the tensile

tests on the unidirectional pultruded CFRP composite sample(how about to change to specimen ?? they are two testsrespectively.) and carbon fiber tow. The stress-time and elec-

Fig. 1. Polarized photographys of polished surfaces of theCFRP composites (10 × 50) (a) profile and (b) cross section.

Fig. 2. Schematics of the experimental setup; the electricalresistance measurement during (a) tensile test, (b) three-pointbending and interlaminar shear test, and (c) Infrared thermgro-phy tracking during tensile test.

20 Y. Mao et al. / Carbon Science Vol. 5, No. 1 (2004) 18-22

trical resistance-time curves of the samples were simultane-ously measured.

The longitudinal electrical resistance was constant asincreasing the tensile stress up to the 0.7 Gpa as shown inFig. 3(a). This was due to the less elongation quantity ofsample. However, the tensile stress was over the 0.7 GPa, theelectrical resistance suddenly dropped, which was related tothe increasing contact degree of 00 fiber alignment in theunidirectional composites. The improved 00 fiber contactdegree can be explained as below: above 0.7 GPa of a stress,the fact that the elastical modulus (Ef) of carbon fibers is farhigher than that of the matrix (Em), caused the shear stressbetween matrix and fibers, resulting in matrix damage anddebonding between matrix and fibers, as shown in Fig. 4 [8].Having some spaces to move in, the carbon fibers surround-ed by insulating matrix were unwinded, resulting in thecompactness alignment of 00 fibers and the subsequentdecrease in the volume resistance along the fiber directions.With the more increasing in tensile stress, some filaments

with defects fractured, causing a series of electrical resistancepeaks. The fracture fiber ends touched to the neighborhoodfibers, froming a conductive network as shown in Fig. 5,therefore, the whole change trend of the electrical resistancewas ever decreasing with a series of electrical resistancepeaks. Once residual intact fibers couldnt support the increasingstress, the sample completely ruptured. The electrical resistanceincreased in a stepwise manner to a higher value due to thedestroying of the conductive network.

In order to analyze the damage mechanism of the CFRPcomposites by the electrical resistance method, a tensile testat 2 mm/min was performed on a single fiber tow (T-300, 3K). It was found from the result, as shown in Fig. 3(b), thatbefore rupture, the electrical resistanceof the fiber towalways increased with increasing stress, with a stress above2.3 GPa, the electrical resistances rose suddenly to a highervalue because of fiber rupture, accompanying with a much

Fig. 3. Longitudinal electrical resistance and tension stress vs time curve (a) CFRP specimen with 62 vol.% of carbon fibers and (b) T-300 carbon fiber tow (3 K).

Fig. 4. Matrix damage and the debonding between matrix andfiber caused by straightening of fibers under tension.

Fig. 5. Conductive network form.

Monitoring Failure Behaviour of Pultruded CFRP Composites by Electrical Resistance Measurement 21

obvious decline in the stress/time curve.

3.2. Infrared thermography non-destructive evaluation Infrared thermography non-destructive evaluation was used

to validate the damage in the materials. Fig. 6 shows thesimultaneously obtained curves of the electrical resistance,stress, and temperature of the middle section of specimensduring tensile testing at 1mm/min (tensile speed). From A toB, the electrical resistance and the temperature of thespecimens were hardly change with increasing the stress,indicating that there was no clear microstructure changeoccurred in the materials. From B to C, the electricalresistance decreased, while the temperature of the specimenrose with increasing the stress and remarkable microstructurechanges were occurred in the materials. From C to D, theelectrical resistance reduced to the lowest, a peak at M

moment appeared in the temperature vs time curve, with anindicator of great changes in the microstructure. At D point,the electrical resistance of the specimens was suddenly risento the highest value and the corresponding temperature wasdropped from the peak value [the peak point is correspond-ing to the M point ?? yes], due to the specimen breakage andrelease the accumulated heat into surroundings. Infraredthermograph at M moment was shown in Fig. 7. There arenumerous mechanisms in the CFRP composites which producethe temperature changes of sample as shown in Fig. 6, e.g:matrix deformation and cracking, fiber fracture, especiallythe debonding and delamination of composites. The correlationbetween the infrared thermographs and the change in the

Fig. 6. Curves of electrical resistance, stress, and temperature ofspecimens.

Fig. 7. Infrared thermographs at M moment.

Fig. 8. Stress and transverse resistance vs time curve (a) three-point bending test (2 mm/min) (b) interlaminar shear test (1 mm/min).

22 Y. Mao et al. / Carbon Science Vol. 5, No. 1 (2004) 18-22

electrical resistance is useful for analyzing the damagemechanisms of the CFRP composites.

3.3. Three-point bending and interlaminar shear loading testFig. 8 shows the changes of transverse electrical resistance

on the CFRP composite during three-point bending andinterlaminar shear loadings.

Two transitions of the electrical resistance were occurredin Fig. 8(a). According to the bending damage theory forCFRP composites [9], a premature failure appeared at thecompressive side beneath the loading nose. The main damageforms were the simultaneous fracture of matrix and fibers.Meanwhile, matrix cracking and debonding between matrixand fibers were followed, as shown in Fig. 9(a), whichleaded to the first transition of the electrical resistance. Thenthe crack propagated from the compressive layer to thetensile layer, causing progressive debonding, and resultantdelaminationin in the tensile layer. The fact that the currentmay flow in every laminar resulted in the increasing in theelectrical resistance. While the tensile layer completelyfractured, the broken fibers pulled out from the matrix, asshown in Fig. 9(b), leading to the second transition of theelectrical resistance. The eletrical resistance change resultedfrom the microstructure changes in the materials.

Fig. 8(b) shown that the transverse electrical resistancedecreased with increasing shear stress. While the specimenwas subjected to completely shear destroy, the transverseelectrical resistance dropped to the lowest. The shear stressdestroyed the interface structure between matrix and fibers,causing the carbon fibers surrounded by insulating matrixhave some spaces to move in, and touch to adjacent carbonfibers. The imprived transverse contacts between carbonfibers resulted in the dropping in the transverse electricalresistance. The change in the transverse electrical resistancecan obviously indicate the damage appearance, but with amuch lower resolution in stress/time curve.

4. Conclusions

The electrical resistance measurement was sensitive tomonitoring the early damages in the tensile loading test ofunidirectional CFRP composites. The different damage mech-anisms were demonstrated by the decreasing in the electricalresistance of the CFRP composites and the increasing in theone of pure fiber tow. The correlation between the infraredthermographs and the changes in the electrical resistancecould be regard as an evidence of the damage mechanisms ofCFRP composites. During three-pointbending loading test,the main damage forms were the simultaneous fracture ofmatrix and fibers firstly, then matrix cracking and thedebonding between matrix and fibers occurred, all this damagesresulted in two transitions of the electricla resistance. Duringinterlaminar shear loading test, the dropping in the electricalresistance relected the damage degree of the interlaminarstructure. The electrical resistance measurement was moresensitive to damage behaviours of CFRP composites than thestress/time curve.

Acknowledgements

The authors wish to thank to the Center of Instron testmachine lab., Beijing University of Chemical Engineeringand Non-destructive detecting lab, China Research Instituteof Architecture Science, Beijing for supporting this work.This work was supported by Hi-Tech Research andDevelopment Program of China (2001A335030).

References

[1] Schulte, K.; Baron, C. Composite Science and Technology1989, 36, 63.

[2] Chung, D. D. L.; Materials Science Engineering 1998, R22,57.

[3] Wittich, H.; Schulte, K.; Kupbe, M.; Kloiem, H.; Bauhofer,W. In: Proc, 2nd ECCMCTS, Hamburg, 1994, 447.

[4] Wang, X.; Chung, D. D. L. Materials. Research 1999, 14, 4224.[5] Thiagaran, C. Ph.D dissertation, Cranfield University,

U.K., 1996.[6] Wang, C. Z.; Yu, Y. H.; Yang, X. P. Acta Materiale Comop-

ositae Sinica. 2003, 20, 28.[7] Abray, J. C.; Bochard, S.; Chateauminois, A. Composites

Science and Technology 1999, 59, 925.[8] Piggott, M. R. Composite Science and Technology 1995,

53, 201.[9] Zhao, Q. S. “Composites” (in Chinese). National Defence

Industry Press (1st ed.), Beijing, 1979, 178.

Fig. 9. SEM photographs of flexural fracture surfaces.