Analytical and Experimental Study of Residual Stresses in CFRP

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Hindawi Publishing Corporation Journal of Nanomaterials

Volume 2013, Article ID 671469,4pages http://dx.doi.org/10.1155/2013/671469

Research Article

Analytical and Experimental Study of Residual Stresses in CFRP

Chia-Chin Chiang,

1

Vu Van Thuyet,

1

Shih-Han Wang,

2

and Liren Tsai

1

1_{Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road,}

Kaohsiung 807, Taiwan

2_{Department of Chemical Engineering, I-Shou University, No. 1, Sec. 1, Syuecheng Rd., Kaohsiung 84001, Taiwan}

Correspondence should be addressed to Liren Tsai; liren@cc.kuas.edu.tw Received 15 September 2013; Accepted 7 November 2013

Academic Editor: Liang-Wen Ji

Copyright © 2013 Chia-Chin Chiang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Fiber Bragg Grating sensors (FBGs) have been utilized in various engineering and photoelectric fields because of their good environment tolerance. In this research, residual stresses of carbon fiber reinforced polymer composites (CFRP) were studied using both experimental and analytical approach. The FBGs were embedded inside middle layers of CFRP to study the formation of residual stress during curing process. Finite element analysis was performed using ABAQUS software to simulate the CFRP curing process. Both experimental and simulation results showed that the residual stress appeared during cooling process and the residual stresses could be released when the CFRP was machined to a different shape.

1. Introduction

Carbon fiber composites have been widely considered as the optimal replacement material for various industrial products. For composite materials, the inherent defects could greatly hamper the reliability and durability of the resultant products. There have been many studies of embedded stress sensors, particularly for damage detection in composite materials [1,

2]. FBGs possess great compatibility with CFRP [3], and by embedding FBG inside carbon fiber composites, the residual strain of the carbon fiber composites during production could be easily monitored. Because of thermal expansion, coeffi-cient of the elements in the composite materials is different, and the shape of the optical spectra changed during curing process. These changes induced a shift in wavelengths of the optical fiber sensors, which could be converted to residual stress [4]. In this research, residual stresses of carbon fiber reinforced polymer composites (CFRP) were studied using both experimental and analytical approach. The simulation of residual stress during cooling process was carried out using ABAQUS. Through comparison between experimental and simulation results, the formation of residual stress in the CFRP was confirmed.

2. Materials and Methods

2.1. Fiber Bragg Grating Sensors (FBG). The FBG involved

was fabricated from single cladding photosensitive fiber using the side writing method. The photosensitive fiber was produced by Fibercore Co. Ltd. (PS1250/1550). The FBGs are photoimprinted in photosensitive optical fiber by 248 nm UV radiation from a KrF Excimer laser. The impulse frequency of laser is 10 Hz. Along the fiber core, the FBG has periodic refractive index modulation with a period of 1.05∼1.08 𝜇m. This resulted in a peak Bragg reflecting wavelength of 1540∼ 1564 nm. The reflectivity of the resulting FBG was about 99% and the FWHM (Full width Half Maximum) of the FBG is about 0.175 nm. Light sources export energy to the carbon fiber composite with FBG by coupler, and the energy change was then recorded and analyzed by oscilloscope. The residual strain of imbedded carbon fiber composites could be determined by comparing the wavelength difference in the FBG before and after curing process using

Δ𝜆

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4 Journal of Nanomaterials 0 4000 8000 12000 16000 20000 Time (s) −0.003 −0.002 −0.001 0 0.001 St re ss S11 (MP a)

Figure 7: Stress S11 at core of FBG with cooling rate 1.92∘C/min.

The residual strain was able to be monitored during curing and the residual strain was recorded for each of specimens. The residual stresses of FRP specimens were measured before and after machining. When the specimens were machined to dog-bone shape, clear residual stress drop was observed. The averaged reduction of residual stress after machining was about 40%.

The simulation of FBG sensor during curing process was performed using coupled temperature and displacement. The influence of the cooling process on residual stress in carbon fiber reinforced polymer was evident. The thermal residual stress is strongly affected by the cooling rate. It is concluded that proper temperature control of the curing process can reduce considerable residual stress in CFRP.

Acknowledgment

This work is supported by the National Science Council, Taiwan (Grant no. NSC 101-2221-E-151-014).

References

[1] S. Ogihara, N. Takeda, and A. Kobayashi, “Experimental char-acterization of microscopic failure process under quasi-static tension in interleaved and toughness-improved CFRP cross-ply laminates,” Composites Science and Technology, vol. 57, no. 3, pp. 267–275, 1997.

[2] Y. Okabe, S. Yashiro, R. Tsuji, T. Mizutani, and N. Takeda, “Effect of thermal residual stress on the reflection spectrum from fiber Bragg grating sensors embedded in CFRP laminates,” Composites A, vol. 33, no. 7, pp. 991–999, 2002.

[3] L. Tsai, T. C. Cheng, C. L. Lin, and C. C. Chiang, “Application of the embedded optical fiber bragg grating sensors in curing monitoring of Gr/Epoxy laminated composites,” in Smart Sen-sor Phenomena, Technology, Networks, and Systems 2009, vol. 7293 of Proceedings of SPIE, March 2009.

[4] L. Sorensen, T. Gm¨ur, and J. Botsis, “Residual strain develop-ment in an AS4/PPS thermoplastic composite measured using fibre Bragg grating sensors,” Composites A, vol. 37, no. 2, pp. 270– 281, 2006.

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