Delamination Strain Energy Release Rate in Carbon Fiber/Epoxy Resin Composites

Size: px

Start display at page:

Download "Delamination Strain Energy Release Rate in Carbon Fiber/Epoxy Resin Composites"

Horace Greene
6 years ago
Views:

1 Materials Science Forum Vol. 555 (2007) pp online at (2007) Trans Tech Publications, Switzerland Delamination Strain Energy Release Rate in Carbon Fiber/Epoxy Resin Composites M.V. Gordić 1, I.M. Djordjević 2, D.R. Sekulić 2, Z.S. Petrović 3 and M.M. Stevanović 2,a 1 Milutin Milanković General High School, Belgrade, Serbia, 2 Vinča Institute of Nuclear Sciences, POB 522, Belgrade, Serbia, 3 Kansas Polymer Research Center, Pittsburg State University, USA a stem@sezampro.yu Keywords: Carbon/epoxy composites, Fracture toughness, Gamma irradiation, Interlaminar strain energy, R-Curves, Release rate. Abstract. The paper reports on an experimental study of the Mode I interlaminar fracture of unidirectional carbon fibers/epoxy resin composites. Mode I delamination strain energy release rate G IC was determined in double cantilever beam (DCB) test, before and after gamma irradiation at various doses. Glass transition temperature, T g of epoxy matrix was determined from dynamic mechanical measurements. The delamination surfaces of tested coupons were observed by scanning electron microscopy. The variations in G IC values were correlated with irradiation doses, T g values and the features of delamination microfractographs, as well as with the variation under irradiation of matrix or fibre/matrix dominated mechanical properties. Introduction The delamination is one of the major fracture modes of many advanced laminated composite structures. Therefore, better understanding of the interlaminar fracture resistance of laminates is very useful for the structural design and development of materials. In continuous fiber- reinforced plastics the value of delamination critical strain energy release rate, G IC, is used as a measure of fracture toughness. The mode-i double cantilever beam (DCB) test is commonly used for the determination of G IC values. The results, expressed in terms of the critical strain energy release rate, G IC, are used mostly for comparative purposes. Many researchers have investigated delamination fracture toughness of composite laminates for mode-i loading [1-7], although delamination is not only the result of pure mode-i loading. However, the data on the effect of irradiation on the interlaminar fracture toughness are scarce [6]. This paper describes the study of effects of gamma irradiation at different doses up to 25 MGy on Mode I delamination strain energy release rate in unidirectional carbon fibers/epoxy resin composites. The interlaminar strain energy release rate before and after irradiations is determined in the mode-i double cantilever beam (DCB) test and the effects of gamma irradiation at different doses on this property are correlated with irradiation doses, glass transition temperatures, features of delamination microfractographs, as well as with effects of irradiation on matrix or fiber/matrix dominated properties of composites. Experimental The laminated plates were prepared from a unidirectional high strength carbon fiber (ENCA HTS)/epoxy prepreg, commercially available under the trade name HexPly 6376-NCHR), supplied by Hexcel. Epoxy resin was based of the tetraglycidyl-p-aminophenol derivative of metilenedianiline. The plates were prepared by hot (175 o C) platen pressing, according to the processing conditions recommended by the supplier. The double cantilever specimens (DBC) (Fig. 1) were cut from the plates with nominal dimensions (3.6x25x 125 mm). A 13 µm thick PTFE film All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, (ID: /03/07,16:09:26)

2 516 Research Trends in Contemporary Materials Science was used to generate the starter crack. The precrack length, a o, was approximately 5 mm. Piano hinges were glued to the specimens for load transmission. The procedures followed current ISO guidelines [8]. Both lateral edges of specimens were coated with white paint (the typewriter correction fluid) and several marks were made to facilitate the monitoring of crack position. The specimens were dried at 75 C for 24 h and then stored in desiccators for 24 h before testing. The DBC test was performed in a Universal Testing Machine INSTRON M 1185, at a constant crosshead speed of 1 mm/min under normal conditions (Fig.1). The force and the displacement signals of the testing machine were recorded on a chart recorder. The delamination length was measured visually on the specimen edge. The point, at which the onset of delamination movement from the starter film occurred on the edge of the specimen, was marked on the force-displacement curve. The precrack loading was stopped at a delamination length increment of 5 mm. The position of the tip of the precrack was marked on both edges of the specimen after unloading. Both of the specimen lateral edges were coated with typewriter correction fluid and several marks were made to facilitate the monitoring of crack position. Fig. 1 Geometry for the double cantilever beam (DCB) specimen with PTFE insert as a starter of delamination and with piano hinges for load introduction b specimen width 2h specimen thickness a o initial delamination length a total delamination length A insert length L specimen length l 1 distance from centre of piano hinge axis to midplane of specimen. Data processing was carried out according to the Method B of ISO standard documents [8]. Critical energy release rate G IC is calculated by equation 2 3m P G IC = ( δc) 2/3 (1) 2(2 h) b where P is the load, δ the opening displacement, b the specimen width, 2h specimen depth. The 1 3 compliance C is equal to δ/p and m represents the slope of ( b C ) / versus a/ 2 h plot. The laminate plates were irradiated, by 60 Co gamma source, under flux of 12 kgy/h, up to the final doses: 5.0, 11.7, 16.7, 20.0, and 25.0 MGy. The G IC values for nonirradiated and irradiated coupons were the mean values with standard deviation from 10 different propagation distances, varying from rate initiating point up to 50 mm of crack propagation. The G IC value of every 10 propagation points was a mean value obtained on 5 tested coupons. The TA Instrument 2980 DMA was used in the temperature range from -100 to 250 o C at a heating rate of 5 o C/min and frequency of 10 Hz for testing dynamic mechanical properties of nonirradiated and irradiated to different doses unidirectional CF/ER. The tests were carried out in the single cantilever mode. Storage modulus, loss modulus and tan delta were recorded. Matrix glass transition temperature (T g ) was deduced from the position of the α peak (the highest temperature peak) on the loss modulus curve (Fig. 2).

3 Materials Science Forum Vol Storage and Loss Modulus, MPa N Temperature, o C tan delta Fig. 2 Characteristic DMA diagram of tested coupons. Storage modulus upper line, loss modulus middle line and tan delta lower line. 0,25 0,2 0,15 0,1 0,05 0 Disscussion Many researchers have investigated the delamination fracture toughness of composite laminates for Mode-I loading [1-7], although delamination is not only the result of pure Mode-I loading; it can be the result of pure Mode-II and of their mixed-mode loadings as well. However, the data on the effect of irradiation on interlaminar fracture toughness are scarce. Humer et al. [9] reported that both specific energies released in the crack opening and in interlaminar shear mode, measured at 196 o C, show degradation after the gamma irradiation up to 0.18 MGy at 196 o C. The degradation was slightly intensified when the irradiated samples were subjected to warm-up cycles at room temperature before testing at 196 o C [9]. Figure 3 shows the effect of gamma irradiation dose on G IC values. G IC decreases at low doses up to 5 MGy, followed by an increase up to dose of 11.7 MGy and a decrease with further irradiation dose increase. The measured T g values follow the same trend with rising irradiation dose resulting in a proportionality between G IC and T g values of irradiated coupons (Fig. 4). G IC /G ICo 1,3 1,2 1,1 1,0 0,9 0, dose [MGy] G IC /G ICo 1,3 1,2 1,1 1,0 0,9 0, Tg [ o C] Fig. 3 G / G IC ICo value as a function of irradiation dose. Fig. 4 G / G versus IC ICo T g values.

$5 Microfractographs of delaminated DCB coupons a. nonirradiated, b. irradiated up to 11.7 MGy and c. irradiated up to 20 MGy.$ $to 20 MGy specimens have smooth fracture surfaces, which are dominated by matrix failure (Figs 5a and 5c), as a result of pure Mode I loading and created by Mode I delamination.$

4 518 Research Trends in Contemporary Materials Science In our previous study [10] the rise after irradiation dose of 11.7 MGy of in-plane shear strength values was followed by a decrease of the same property, measured at 25, 70 and 130 oc when coupons were irradiated to 20 MGy. a. b. c. Fig. 5 Microfractographs of delaminated DCB coupons a. nonirradiated, b. irradiated up to 11.7 MGy and c. irradiated up to 20 MGy. SEM images of fracture surfaces of nonirradiated specimens and of specimens irradiated up to doses of 11.7 and 20 MGy are shown in Fig. 5. Both nonirradiated and irradiated.to 20 MGy specimens have smooth fracture surfaces, which are dominated by matrix failure (Figs 5a and 5c), as a result of pure Mode I loading and created by Mode I delamination. However, the fracture of specimens irradiated up to dose of 11.7 MGy shows the failure due to mixed Mode I and II loading (Fig. 5b), induced by the tensile crack opening through the matrix, but also a shear failure propagating through the fiber-matrix interface can be observed. Improving the resistance to crack propagation through the matrix, irradiation of coupons to 11.7 MGy (Fig/ 3) causes the crack to propagate through the fiber/matrix interface. As a result, the shape of R-curve (Fig. 6) of coupon irradiated to 11.7 MGy is atypical (according to criteria given in ISO [1]), the delamination resistance curve displaying the decreasing resistance with the growth of delamination length. R-curves for coupons irradiated to 5 and 25 MGy, with GIC / GICo < 1, are typical delamination resistance curves [1] showing increasing delamination resistance with delamination length. 0 MGy 11.7 MGy 5 MGy 25.0 MGy 2 GIC (J/m ) a (mm) Fig. 6 R-curves of nonirradiated and irradiated DCB coupons.

5 Materials Science Forum Vol Conclusions A study of the effect of gamma irradiation on the Mode I interlaminar strain energy release rate of carbon/epoxy unidirectional laminates has shown that G IC decreases at low doses (5 MGy), followed by an increase with increasing dose up to 11.7 MGy, and a decrease with further increase of the irradiation dose. Glass transition temperature values follow the same trend with rising irradiation dose, resulting in proportionality between G IC and T g values of irradiated coupons. The nonirradiated and irradiated up to 20 MGy specimens have smooth fracture surfaces characteristic of the matrix failure, as a result of pure mode-i loading created by Mode I delamination, while the fracture of specimens irradiated up to dose of 11.7 MGy is attributed to the failure due to mixed Mode I and Mode II delamination. The shape of the R-curve of coupon with positive irradiation effect on the G IC value is atypical, showing decreasing resistance with delamination length, while the R-curves for coupons with negative irradiation effect on the G IC value represent typical delamination resistance curves, showing increasing delamination resistance with delamination length. Acknowledgement The authors are grateful to the Ministry of Science and Environmental Protection of the Republic of Serbia for the financial support. References [1] I.A. Ashcroft, D.J. Hughes and S.J. Shaw: Int. J. Adhes. Vol. 21 (2001), p. 87. [2] I.A. Ashcroft and S.J. Shaw: Int. J. Adhes. Vol. 22 (2002), p. 151 [3] A.B. de Morais, M.F. de Moura, A.T. Marques and P.T. de Castro: Comp. Sci. Technol. Vol. 62 (2002), p [4] K.Y. Kim and L. Ye: Composites Part A: Appl. Sci. Manufact. Vol. 35(4) (2004), p [5] J. Schön, T. Nyman, A. Blom and H. Ansell: Comp. Sci. Technol. Vol. 60(2) (2000), p [6] A.J. Brunner, B.R.K. Blackman and J.G. Williams: Comp. Sci. Technol. 66(6) (2006), p [7] A.B. Pereira and A.B. de Morais: Comp. Sci. Technol. Vol. 64 (13-14) (2004), p [8] Fiber Reinforced Plastic Composites. Determination of Mode I Interlaminar Fracture Toughness G IC for Unidirectional Reinforced Materials, Standard ISO (2001). [9] K. Humer, H.W. Weber, E.K. Tschegg, H. Gerstenberg and B.N. Goshchitskiit: Cryogenics Vol. 35 (1995), p [10] D.R. Sekulic, I.M. Djordjevic, M.V. Gordic, Z.H. Burzic and M.M. Stevanovic: Mat. Sci. Forum Vol. 512 (2006), p. 549.

INTERLAMINAR REINFORCEMENT TO COMPOSITE LAMINATES BY DISTRIBUTING WHISKERS ALONG THE INTERFACE

INTERLAMINAR REINFORCEMENT TO COMPOSITE LAMINATES BY DISTRIBUTING WHISKERS ALONG THE INTERFACE Wen-Xue Wang 1, Yoshihiro Takao 1, Terutake Matsubara 1 and Hyoung Soo Kim 1 Research Institute for Applied