The Influences of Residual Stress in Epoxy Carbon-fiber Composites under
High Strain-rate
Hongchueh Lee, Shih-Han Wang, Chia-Chin Chiang, Liren Tsai*
Mechanical Engineering Department, National Kaohsiung University of Applied Sciences
*415 Chien Kung Road, Kaohsiung 807, Taiwan,
liren@cc.kuas.edu.tw
ABSTRACT
Epoxy carbon-fiber composite (FRP) has been widely regarded as premium construction material in fiber composites during curing was monitored using Fiber Bragg Gratings (FBG). Carbon-fiber composites were prepared under steady temperature gradient and the FBGs were embedded during FRP preparation process along axial fiber layout direction. The effect of residual stress to the dynamic tensile stress in these FBG imbedded carbon fiber composites was examined using modified Split Hopkinson Tensile Bar (SHTB). The relationship between residual stress and dynamic tensile stress in the FRP under a high strain rate ranging from 500 to 1000 s-1
was thus studied.
1. Introduction
Carbon fiber composites have been widely considered as the optimal replacement material for various industrial products, such as bicycle, racket, ski, pressure vessel, yacht, aircraft, wind vane, etc.. Despite its high cost, the high strength/weight ratio of carbon fiber composites made it utterly popularly. However, for composite materials, the inherent defects could greatly hamper the reliability and durability of the resultant products. These defects, either form during production or generated by improper handling (drop, indent, impact…etc.) could eventually determine the dynamic strength of the finishing products. In this research, a novel Fiber Bragg Grating (FBG) technology was implanted along with the Split Hopkinson Tensile Bar (SHTB) facility to study the effect of inherent residual strain to the dynamic tensile strength of epoxy carbon fiber composites.
FBG possess great compatibility with Fiber Reinforced Polymer Composites (FRP) [1]. By embedded FBG inside carbon fiber composites, the residual strain of the carbon fiber composites during production could be easily monitored. The wavelength of the embedded FBG changed before and after the FRP curing process, and the residual strain of the FRP could be determined accordingly [2]. To verify the effect of residual stress to the dynamic tensile strength of FRP, a reverse-striking SHTB in Kaohsiung University of Applied Sciences was utilized [3]. The
Proceedings of the SEM Annual Conference June 7-10, 2010 Indianapolis, Indiana USA ©2010 Society for Experimental Mechanics Inc.
automobile and leisure sporting good industries. In this research, the formation of residual strain in epoxy carbon
T. Proulx (ed.), Dynamic Behavior of Materials, Volume 1, Conference Proceedings of the Society for Experimental Mechanics Series, 281
SHTB was able to generate tensile pulse up to 1000 s-1 inside the prepared FRP and ultimately break the
specimens. The results could be used to better understand the role of residual stress to the dynamic tensile strength of epoxy carbon fiber composites.
2. Experimental Configuration and Setup
2.1 Split Hopkinson Tensile Bar (SHTB)
The Split Hopkinson Tensile Bar,Fig. 1, is an adaptation of the device developed by Kolsky [4]. It consists of a gas gun system, an incident bar, a transmitted bar and a specimen assembly. A projectile fired from a gas gun impacts one end of the incident bar and generates a tensile stress pulse propagating down the bar into the specimen. This pulse reverberates within the specimen, sending a transmitted pulse into the transmitted bar and a reflected pulse back into the incident bar. The bars are designed to remain elastic throughout the test so that the complete displacement time and stress-time histories at the interfaces between the specimen and the bars can be determined from measurements of the incident, reflected and transmitted pulses [5]. The incident and transmitted bars of the SHTB were made by 20mm diameter SUS304 stainless steel.
Fig.1 The SHTB facility in KUAS. 2.2 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. To avoid burning the phase mask, the laser power should be <500 mJ/cm2. Along the fiber core, the FBG has a
periodic refractive index modulation with a period of 1.05~1.08 μm,obtained by using phase masks (Lasiris Co. Ltd.) with different periods. 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 [1]. Light source 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 by Eq.(1): 282
evidence for its influence to the dynamic tensile strength. This characteristic at higher strain rates remained to be determined.
Acknowledgements
This work is funded by The National Science Council (grant number NSC-97-2221-E-151-019). The author wants to give special thanks to Mr. Chien-chang Huang. Without his kind help, the experiments won’tbe able to carry on smoothly.
References
[1] Tsai L., Cheng T. C., Lin C. L. and Chiang C. C., Application of the embedded Optical Fiber Bragg Grating sensors in curing monitoring of Gr/Epoxy laminated composites, Proceedings of SPIE, 2009
[2] Mandal J., Bragg grating tuned fiber laser system for measurement of wider range temperature and strain, Optics Communications, 2005
[3] Tsai L., Chiang C. C., Wang S. H., and Lin H. R., Dynamic Response of Low Friction, High Strength Hydrogels, 25th Annual Conference of Chinese Society of Mechanical Engineering, 2009
[4] Bailey J. A., Mechanical Testing and Evaluation, ASM Vol.8
[5] Taniguchi N., Tensile strength of unidirectional CFRP laminate under high strain rate, Adv. Composite Mater., Vol. 16, No. 2, pp. 167–180, 2007
