Cure monitoring of smart composites using Fiber Bragg Grating based embedded sensors

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Abstract

A sensor embedded in the composite laminate can act as a temperature transducer during the composite cure mechanism. Once the composite is cured, the same sensor can be used to provide the information about the mechanical changes that influence the performance of the material. Fiber Bragg Grating (FBG) sensor is one such sensor which one can use for the composite cure monitoring. We present here the results obtained with an associated FBG sensor system for the cure monitoring of smart composites. The performance of the embedded FBG sensor smart composite specimens under 3- and 4-point bending conditions are also being investigated. Finally, the performance analysis has been extended to cantilever specimens.

Introduction

Recent trends in fiber optic smart sensors have developed tremendous potential for the research and developments of such systems for various industrial applications. Simultaneous measurement of multiple perturbations is important in situations where different parameters have to be monitored over the life time of the sensor or in applications where the output signal need to be modified or analysed properly to compensate for the sensitivity to more than one external perturbation. Incorporation of fiber-optic sensors into the Fiber-reinforced Composite Materials enables monitoring from the curing stage at conception, through its working life to impending failure. Fiber Bragg Grating (FBG) sensors (fibers with intra-core gratings) are very well suited for measuring temperature and strain variations. The significant information is directly related to the absolute wavelength change 1, 2, 3, 4, 5. Thus, these sensors can be made independent of source intensity variations and losses. Furthermore, due to their narrow-band wavelength reflection, they are also conveniently multiplexed in a fiber-optic network that uses wavelength-domain multiplexing. This technology and its concept are very well suited for applications in aircraft structures which will lead to a reduction in maintenance, repair, and downtime of future aircraft and could also find broad application in ships, submarines, and pressure vessels, where composite materials are penetrating. The present research aims at exploring efficient and reliable ways of fixing the fibers into composite structures and on-line monitoring of composite cure process, which has wide significance in the context of aircraft structure applications.

As far as the fiber sensors for cure monitoring methods are concerned, most of the reported research were based on transmission spectroscopy, evanescent wave spectroscopy and refractive index monitoring. Few works have been reported with the FBG strain sensors for composite cure monitoring. In this paper, we are presenting the results obtained with an associated FBG sensor system for the cure monitoring of smart composites. Also, a comprehensive study of the sensor system for the analysis of smart CFRP (Carbon Fiber Reinforced Plastics) and GFRP (Glass Fiber Reinforced Plastics) specimens under 3- and 4-point bending conditions are incorporated. The study has been extended to cantilever specimens.

Section snippets

Sensor principle

The FBG has a longitudinal periodic variation of the refractive index in the single mode fiber core. Normally, such intra-core gratings are written using high intensity UV laser source. Wavelength of the light reflected at this fiber depends on the gratings spacing, i.e., spacing between the refractive index variations. Light travelling inside the fiber core will be scattered in the regions of varying refractive indices. The scattered light will generally be out of phase and thus, tend to

Experimental configuration

The schematic of the system configuration used for the study is shown in Fig. 1. The light from the LED is coupled into the optical fiber which is divided into two using the 2/2 fiber coupler. The interrogating arm of the 2/2 coupler is equipped with the intra-core FBG of specific wavelength selection. The reflected wavelength is analyzed using the PicoWave reference system as shown in the figure. Most of the studies mentioned in this paper are carried out using this system configuration, save

Cure monitoring of composite specimens (the FBG sensor based smart structure as a self-monitoring NDT device)

Cure monitoring of thermoset composites with embedded FBG sensors have been carried out using the experimental configuration of Fig. 1. We have used the intracore Bragg grating fibers of different wavelength specifications embedded within multi layer CFRP and GFRP specimens. Fig. 4 shows the results obtained with embedded FBG sensors for composite cure monitoring with specimens of different specifications. The FBG is normally embedded between the adjacent layers of the composite plies. The

Conclusions

Smart materials and structures are envisioned to integrate internally the functions of environmental sensing, structural control and mechanical actuation. We have presented here the applicability of using FBG sensors for monitoring the cure process of such smart composite materials. Also, a comprehensive study of the sensor system for the analysis of smart CFRP and GFRP specimens under 3- and 4-point bending conditions are incorporated. The slope values of the calibration curves for the

Acknowledgements

The authors acknowledge the financial support (AcRF 10/96) from NTU.

Dr. V.M. Murukeshan obtained his MSc and MPhil courses in Physics with specialization in Quantum Electronics from Cochin University (CUSAT), India in 1988 and 1991, respectively. Then he joined the Applied Optics Lab of the IIT, Madras for his doctoral work from where he received the PhD degree in 1998. He was awarded the DAAD fellowship of Germany in 1993 and carried out his further doctoral studies at the Universitaet Oldenburg, Germany. He is a Life Member of the Optical Society of India.

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Dr. V.M. Murukeshan obtained his MSc and MPhil courses in Physics with specialization in Quantum Electronics from Cochin University (CUSAT), India in 1988 and 1991, respectively. Then he joined the Applied Optics Lab of the IIT, Madras for his doctoral work from where he received the PhD degree in 1998. He was awarded the DAAD fellowship of Germany in 1993 and carried out his further doctoral studies at the Universitaet Oldenburg, Germany. He is a Life Member of the Optical Society of India. Presently, he is with the school of Mechanical and production Engineering, Nanyang Technological University, Singapore where he is working as a Research Fellow in the Sensors and Actuators SRP. His research interests include Fiber-optic sensors for smart structure applications, Optical MEMS and Applied Optics.
Dr. Ong Lin Seng obtained his BSc (Eng) in 1982 and PhD degree in 1985 from the University of Strathclyde, UK. He was awarded the prestigious British Gas Research Scholarship for his postgraduate studies and has won the best PhD student prize for his research work. He returned to Singapore in 1986 and immediately joined the Nanyang Technological University to pursue his teaching and research career. He is at present an Associate Professor at the Division of Engineering Mechanics of the School of Mechanical and Production Engineering. His research interests include structural stress analysis, structural failure analysis, and since the last 3 years, development of optical fiber sensors for smart material and structure applications. He has published over 50 referred journals and conference papers and is active in Singapore Codes and Standards committees. He has organized a few local and regional conferences. He is a Chartered Engineer of UK and a member of the Institution of Mechanical Engineers.
Mr. Chan Peng Yang graduated with Bachelor of Engineering with Honours Class II in Mechanical Engineering from Nanyang Technological University in 1997. Presently, he is pursuing his Masters degree in Engineering. His research interests are on the R&D of fiber optic sensors for smart structure applications.
Dr. Seah Long Key obtained his BSc in Mechanical Engineering and PhD in Applied Mechanics from the University of Strathclyde, UK in 1985 and 1989, respectively. He is at present an Associate Professor at the Division of Engineering Mechanics of the School of Mechanical and Production Engineering, NTU. His current research is involving the design and buckling behaviour of cold-formed steel structures, experimental stress analysis, fibre optic strain sensor. Current research projects are in the area of distributed fibre optic strain sensor and distributed fibre optic chemical and gases sensor.

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