Tu3G. 2.pdf
The International Conference on Fiber Optics and Photonics 2016 © OSA 2016
Riding the Feature - Feature Guided Wave-Based Defect Identification using Fiber Bragg Grating Sensors B. Srinivasan1, P. Ray1,2, P. Rajagopal2, K. Balasubramaniam2
2
1 Department of Electrical Engineering, IIT Madras, Chennai, India Centre for Non-destructive Evaluation, Department of Mechanical Engineering, IIT Madras, Chennai, India Author e-mail address:
[email protected]
OCIS codes: (060.2370) Fiber optics sensors, (060.3735) Fiber Bragg gratings, (310.2790) Guided waves
Harnessing of ultrasonic guided waves confined in local features such as bends and welds, known as featureguided waves (FGW) has emerged as a promising technique for non-destructive testing and structural health monitoring of industrial and aerospace structures [1,2]. Conventional techniques to detect such waves such as piezoelectric transducers and Doppler vibrometers are not practical for field applications [2]. In this paper, we introduce a fiber Bragg grating (FBG)–based technique which uses FGW to detect anomalies or defects in plate structures with transverse bends. We demonstrate that we are able to obtain good consistency between simulation and experimental results, both in the case of defect-free bent plates and those with transverse defects. Experiments were conducted on a 90̊ bent aluminum plate (60 mm × 50 mm × 3 mm) with a shear excitation provided at the edge of bent region to generate shear-horizontal feature guided (SHB) modes as shown in Fig. 1. SHB modes are generated on this sample using a wide band of commercially available shear wave PZT transducer (Panametric NDT V151). In order to limit the frequency bandwidth of excitation, the transducer is driven using a RITEC pulser receiver (RPR 4000 Ritec Inc., USA) generating a 5 cycle Hanning windowed tone burst. To preferentially pick these modes using FBG, the sensor is oriented transverse to the incoming waves [3]. To detect any defect on the bent region using the propagating SHB modes, a FBG sensor is pasted across the bend as shown in the schematic below. These modes propagate along the bend and a part of its energy gets reflected back in presence of a defect such as cracks. The FBG is interrogated using a tunable laser source (TLS), which is tuned to the middle portion of one slope in the FBG reflection spectrum. The reflected optical intensity modulated signal, corresponding to the dynamic strain [4] is extracted using a circulator and fed into an APD-TIA based optical receiver. The output is then fed to a digital storage oscilloscope (Agilent DSO 7012B) operating at a sample frequency of 2 GHz.
Circulator TLS
3
1
Analog Circuit Board APD - TIA Optical Receiver
Band Pass filter 5 kHz œ1.25 MHz
2 Defect
PZT Probe
RITEC PulserReceiver
Sync
DSO
90ᵬbent plate
Figure 1. Schematic of the experimental setup showing the FBG-based detection of a transverse defect on a bent plate structure
Initial experiments were conducted on a defect free bent plate structure with FBG sensor placed 200 mm away from the shear wave excitation of 600 kHz. Fig. 2(a) shows the time trace of the signal captured from the FBG sensor for this case. Further experiments were carried out on a similar plate having a transverse defect, 50 mm away from the sensor.
Tu3G. 2.pdf
(a)
The International Conference on Fiber Optics and Photonics 2016 © OSA 2016
(b)
Figure 2. Time domain trace of SHB mode captured using FBG pasted 200 mm away from the point of excitation for (a) nodefect case (b) Transverse defect 50 mm away from sensor, at an excitation frequency of 600 kHz.
In case of a plate with defect, the presence of reflected wave packet can be clearly seen from the above time trace and its location can be estimated using the time of flight difference between the initial and reflected wave packet. To estimate the size of the defect, we have monitored the reflection amplitude as a function of different excitation frequencies (different ratios of defect size versus wavelength) [5]. Our experimental results are in excellent agreement with the results reported previously in literature. We have also extended this work to elongated pipe structures recently, which will be presented. Such results establish fiber Bragg gratings as a viable alternative to conventional techniques for structural health monitoring of bent plates.
References [1] Fan, Z. and Lowe, M. J. S., “Elastic waves guided by a welded joint in a plate”, Proceedings of the Royal Society London A: Mathematical, Physical and Engineering, 465, 2053-2068 (2009) [2] Manogharan P, Yu X, Fan Z, et al., “Interaction of shear horizontal bend (SHB) guided mode with defects”, NDT & E International, 75, 39– 47 [3] A.V. Harish, P. Ray, P. Rajagopal, K. Balasubramaniam and B. Srinivasan, “Detection of fundamental shear horizontal mode in plates using fibre Bragg gratings”, Journal of Intelligent Material Systems and Structures 27(16), 2229-2236 (2016) [4] Lissak, B., Arie, A. and Tur, M., “Highly sensitive dynamic strain measurements by locking lasers to fiber Bragg gratings,” Opt. Lett., 23 (24), 1930-1932 (1998) [5] P. Ray, P. Rajagopal, B. Srinivasan and K. Balasubramaniam, “Feature-guided wave based health monitoring of bent plates using fiber Bragg gratings”, Journal of Intelligent Materials Systems and Structures, DOI: 10.1177/1045389X16667554 (2016)
