An FBG-Based Impact Event Detection System for Structural Health Monitoring

C. S. Shin,1 B. L. Chen,1 and S. K. Liaw1,2
1Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan
2Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan

Received 1 September 2009; Revised 5 December 2009; Accepted 26 January 2010

Academic Editor: Yi Qing Ni

C. S. Shin, B. L. Chen, and S. K. Liaw, “An FBG-Based Impact Event Detection System for Structural Health Monitoring,” Advances in Civil Engineering, vol. 2010, Article ID 253274, 8 pages, 2010. doi:10.1155/2010/253274

Abstract

Some structures are vulnerable to localized internal damages incurred by impact of small objects. An impact monitoring system using fiber Bragg grating (FBG) sensors has been established. Its ability to detect very low to medium energy impacts has been demonstrated on an aluminum plate and a 22 m long wind turbine blade. Previous analysis of this technique showed that the accuracy by which an impact position can be located is limited by equipment noises and angular insensitivity of the FBG. By employing two intensity demodulation schemes with different demodulation sensitivities and ranges, we try to differentiate the relative importance of the above limiting effects. Based on the results, directions for further improvement on impact source locating accuracy will be discussed and the implication of applying such systems on large-scale structures will be examined.

1. Introduction

Impacts due to bird-strike and hailstorm may do much harm to structures such as aircraft and wind turbine blades. The structure will be especially vulnerable if it is made of polymeric composite as impacts may induce localized small internal damages. On actingupon byfluctuating service loading, these insidious defects may grow and eventuallylead to catastrophic failures. Although nondestructive examination techniques for the detection of internal damages are available, they are limited in resolution.Moreover, to stage a thorough examination over the entire structure can be highly time and resource consuming for a large-scale structure. The problem can be much alleviated if one knows where to look at and what to look for.

The current work investigates the possibility of establishing an impact event monitoring system using fiber Bragg grating (FBG). FBG has found increasing applications as sensors in aerospace, structural, medical, and chemical applications for vibration, temperature, strain, impact, and general structural health monitoring [1–5]. It is chosen in the current task because of its good long-term durability and stability. In case of composite structures, optical fiber sensors possess the additional advantage of being compatible with common polymeric materials, making them easily embeddable inside a structure without inducing significant weakening of the material.

Previous works on impact location identification mostly used piezoelectric sensors (e.g., [6, 7]). A three-sensor-based scheme was used to locate the impact position of a projectile on a target screen by measuring acoustic waves with microphones [8, 9]. However, this scheme requires a priori knowledge of the wave speed. For practical applications, calibration of the wave speed in advance may not be feasible. Our own work showed that the whereabouts of an impact event can be located by analyzing the differential time-of-flight among signals picked up by four different FBG sensors [10]. Changes in the measurand are reflected as shift in the characteristic Bragg wavelength of an FBG. Light intensity demodulation techniques [11–13] have been shown to possess the required dynamic response for logging impact events [10, 14, 15]. In this work, two different light intensity demodulation schemes have been compared. The possibility of logging and locating impact will be tested on a flat aluminum plate and a wind turbine blade.

2. Experimental and Analysis Procedures

2.1. Impact Testing
2.2. FBG Interrogating Schemes
2.3. Impact Source Locating
3. Results and Discussion
3.1. Detection of an Impact Event
3.2. Impact Source Location

4. Conclusions

References

  1. Y. J. Rao, “Recent progress in applications of in-fibre Bragg grating sensors,” Optics and Lasers in Engineering, vol. 31, no. 4, pp. 297–324, 1999.
  2. F. G. Tomasel and P. A. A. Laura, “Assessing the healing of mechanical structures through changes in their vibrational characteristics as detected by fiber optic Bragg gratings,” Journal of Sound and Vibration, vol. 253, no. 2, pp. 523–527, 2002.
  3. J. S. Leng and A. Asundi, “Non-destructive evaluation of smart materials by using extrinsic Fabry-Perot interferometric and fiber Bragg grating sensors,” NDT and E International, vol. 35, no. 4, pp. 273–276, 2002.
  4. J. Leng and A. Asundi, “Structural health monitoring of smart composite materials by using EFPI and FBG sensors,” Sensors and Actuators A, vol. 103, no. 3, pp. 330–340, 2003.
  5. H.-Y. Ling, K.-T. Lau, L. Cheng, and W. Jin, “Fibre optic sensors for delamination identification in composite beams using a genetic algorithm,” Smart Materials and Structures, vol. 14, no. 1, pp. 287–295, 2005.
  6. P. T. Coverley and W. J. Staszewski, “Impact damage location in composite structures using optimized sensor triangulation procedure,” Smart Materials and Structures, vol. 12, no. 5, pp. 795–803, 2003.
  7. M. Meo, G. Zumpano, M. Piggott, and G. Marengo, “Impact identification on a sandwich plate from wave propagation responses,” Composite Structures, vol. 71, no. 3-4, pp. 302–306, 2005.
  8. A. Tobias, “Acoustic-emission source location in two dimensions by an array of three sensors,” Non-Destructive Testing, vol. 9, no. 1, pp. 9–12, 1976.
  9. H. A. Canistraro and E. H. Jordan, “Projectile-impact-location determination: an acoustic triangulation method,” Measurement Science and Technology, vol. 7, no. 12, pp. 1755–1760, 1996.
  10. B. L. Chen and C. S. Shin, “Fiber Bragg gratings array for structural health monitoring,” Materials and Manufacturing Processeses, vol. 25, no. 1-4, 2010.
  11. L. Zhang, R. Fallon, L. A. Everall, J. A. R. Williams, and I. Bennion, “Large-dynamic-range and high-resolution from a strain sensing system using long-period grating interrogating FBG strain sensor,” in Proceedings of the 24th European Conference on Optical Communication (ECOC '98), vol. 1, pp. 609–610, Madrid, Spain, September 1998.
  12. R. W. Fallon, L. Zhang, A. Gloag, and I. Bennion, “Multiplexed identical broad-band-chirped grating interrogation system for large-strain sensing applications,” IEEE Photonics Technology Letters, vol. 9, no. 12, pp. 1616–1618, 1997.
  13. R. W. Fallon, L. Zhang, A. Gloag, and I. Bennion, “Identical broadband chirped grating interrogation technique for temperature and strain sensing,” Electronics Letters, vol. 33, no. 8, pp. 705–707, 1997.
  14. C. S. Shin, B. L. Chen, and C. C. Chiang, “A dynamic strain measurement system using fiber grating sensors and its application in structural health monitoring,” in Proceedings of the World Forum on Smart Materials and Smart Structures Technology (SMSST '07), p. 335, Nanjing, China, May 2007.
  15. C. S. Shin, B. L. Chen, J. R. Cheng, and S. K. Liaw, “Impact response of a wind turbine blade measured by distributed FBG sensors,” Materials and Manufacturing Processes, vol. 25, no. 1-4, 2010.