5th International Symposium on NDT in Aerospace, 13-15th November 2013, Singapore Monitoring damage growth in composite materials by FBG sensors Alfredo GÜEMES, Antonio FERNANDEZ-LOPEZ, Borja HERNANDEZ-CRESPO Abstract Department of Aeronautics - Universidad Politécnica de Madrid (UPM), Madrid, Spain Phone: +34 913366327, Fax: +34 913366334; e-mail: alfredo.guemes@upm.es, antonio.fernandez.lopez@upm.es, borja.hernandez.crespo@gmail.com Transient waves generated due to the quick release of energy during the delamination appearance and growth requires high sensitivity and high frequency measurements. These measurements are easily done by PZT sensors, nevertheless these sensors present several disadvantages to be implemented at aircraft operating conditions, such the high perturbation due to electromagnetic noise and the weight penalty due to the electrical wires needed. A new concept to measure elastic waves using Fiber Bragg Grating sensors based on Multi Wavelength Filter is presented and experimental results of Acoustic Emission technique are presented. Carbon Fiber Reinforced Plastic plates are loaded to promote delamination growth. FBG sensor network embedded and bonded are used to monitor AE signals related to delamination growth and transversal crack appearance. Experimental results are presented. Keywords: Acoustic Emission, Fiber Optic Sensor, Carbon Fiber Composite 1. Introduction Acoustic Emission (AE) is a well know NDI that consist in detecting stress waves generated by growth of defects in solids, and permit a continuous structural monitoring during the service life of real structures with a few sensors. Most common damages in composite and metallic structures, like fibre breakage, fibre pull-out, matrix crack, delaminations or crack appearance and growth can be detected using this technique. Nevertheless its advantages, this technique has limited applications in aerospace and wind energy structures by the problems induced by a noisy environment and the difficulties introduced by PZT sensors, as the electromagnetic noise and the weight penalty. For aerospace applications, fiber optic sensors point to be the most suitable to be used for continuous monitoring techniques, as their geometry can easily embedded in composite materials, their immunity to electromagnetic interferences and low weigh. Most of the authors that use fiber optic sensor for AE use interferometric techniques to monitor elastic waves propagated in solid. Mach-Zehnder, Michelson and Fabry-Perot interferometers have been used for this purpose. Even these techniques have demonstrated to be effective measuring small amplitude signals at high sampling rate, they are highly limited due to the noise introduced in the fiber between sensor and photodetector, and the impossibility to multiplexing more sensors in the same fiber. Fiber Bragg Grating sensors provide additional advantages to the traditional characteristic of fiber optic sensor, as the multiplexing capability. This type of sensors is widely used to several sensing applications, as strain and temperature. Nevertheless, the combine requirement of high speed and high accuracy for AE measurements require a new interrogation technique different from the traditional, usually based on swept-laser interrogation. This new technique will be presented in this paper, and AE measurements at CFRP coupons will be presented.
2. High Speed Measurement Equipment description The system employed for measuring strain produced by wave propagation though material require high accuracy and sensitivity due to the slight strain signal promoted by the elastic waves, as well as acquisition rates over 100kHz. Acoustic waves promotes wavelength shifts of 0,01pm in the FBG sensor, a sensitivity level of one order of magnitude under than the provide by the commercial equipment. The system designed for AE monitoring with FBG is composed by a broadband light source (LED), an optic amplifier (EDFA), an optic assembly for the optical filter, a photodetector, a data acquisition device, a computer (see Figure 1). Figure 1. Scheme of the AE FBG system The light is introduced in the optic assembly (see scheme in Figure 2). The objective of this scheme is to measure small variations on the FBG sensor wavelength by measuring the greatest variations of light intensity. To do that, we will use a wavelength filter with a specific spectral shape, according to FBG sensor wavelength. To achieve optimal use of the filter, this has been overlapped with the lateral side of the FBG sensor spectrum. Only the amount of light coincident in both spectra will be measured by the photodetector (striped area in the Figure 3). Figure 2. Scheme of the optical assembly
Figure 3. Relative spectral position between FBG Sensor-Optical Filter Propagating waves change the grating pitch of the sensor which then causes the Bragg wavelength to shift. Thereby, when the FBG sensor changes its Bragg wavelength caused by the wave propagation, the reflected signal by the optical filter increase or decrease, generating a light intensity shift, which can be measured by the photodetector using a data acquisition device. Applying this scheme with a National Instruments USB-6366 and high speed photodetectors (New Focus 2053) it is possible to measure 8 FBG at 2 MS/s. 3. Specimen description and Experimental Set-up and Delamination growth will be promoted in CFRP coupon, with an artificial delamination previously induced during the manufacturing, under three-point bending conditions. Initially, a composite panel of 250x300 were laminated with Hexply AS4/8552 and [0 2,90 2 ] s lay-up sequence. This sequence was selected to promote the appearance of transversal intralaminar cracks in the matrix. A release film was introduced in the laminate between the interface of one 0 and 90 layers, as it can see in the manufacturing plate scheme. Afterwards, coupon was mechanized from the panel with the final dimensions of 265x27x1,70mm. Figure 4. Scheme of the CFRP panel with the artificial defect induced (left). Scheme of experimental set-up for 3-point bending (right) A 2mm FBG sensor was bonded at 25mm from the lateral side, in the upper side of the coupon test. Coupon was tested under 3-point bending conditions to avoid strain perturbations induced in the FBG by the applied force. Sensor is located out of support, as can be observed in Figure 4. Force was applied by a manual mechanical test machine, introducing steps of 1kN until failure.
Figure 5: Picture of the test bench 4. Experimental results Coupons were tested in previous described conditions. As the force was increased, transversal crack appears, and the initial induced delamination growth. Damage appearance was evaluated by visual inspection after every step of the test. AE signals measured by the FBG bonded detect both damage induced during the test, delaminations and transversal cracks. Both types of damage generated an ultrasonic wave that can be detected by the strain FBG sensor. Figure 6: Cupon after bending test: transversal crack and delamination growth Figure 7: FBG measurements of coupon test I
Figure 8: FBG measurements of coupon test II However the signal has similar frequency response, the can be easily discriminated, because the energy liberated by the transversal crack appearance is higher than the induced by the delamination growth.
5. Conclusions A high speed and high accuracy FBG based system for acoustic emission applications has been developed. The system allows multiplexing several FBG in the same fiber, using customized filters to measure variations induced by acoustic waves on the sensors. Delaminations growth and crack appearance have been experimentally detected on CFRP by AE techniques using a FBG sensor bonded during three-point-bending test. Results are comparable to PZT sensors. Potential advantages of this technique for continuous monitoring of the integrity of composite structures in aerospace and wind energy using FBG sensors are key to implement condition based maintenance. References 1. K. Diamanti, C. Soutis, Structural health monitoring techniques for aircraft composite structures, Progress in Aerospace Sciences 46, pp 342 352, 2010 2. R. de Oliveira, O. Frazao, J.L. Santos, A. T. Marques, Optic fibre sensor for realtime damage detection in smart composite Computers and Structures 82 1315 1321, 2004 3. J.-R. Lee, H. Tsuda, A novel fiber Bragg grating acoustic emission sensor head for mechanical tests Scripta Materialia 53 pp1181 1186, 2005 4. R. de Oliveira, A. T. Marques, Health monitoring of FRP using acoustic emission and artificial neural networks Computers and Structures 86 pp 367 373, 2008 5. T. Fu, Y. Liu, Q. Li, J. Leng, Fiber optic acoustic emission sensor and its applications in the structural health monitoring of CFRP materials, Optics and Lasers in Engineering 47 pp 1056 1062, 2009