Evaluation of debonding progress in composite bonded structures by ultrasonic wave sensing with fiber Bragg grating sensors

Abstract

The authors are constructing a damage detection system using ultrasonic waves. In this system, a piezo-ceramic actuator generates ultrasonic waves in a carbon fiber reinforced plastic (CFRP) laminate. After the waves propagate in the laminate, transmitted waves are received by a fiber Bragg grating (FBG) sensor using a newly developed high-speed optical wavelength interrogation system. In this research, this system was applied to the evaluation of debonding progress in CFRP bonded structures. At first, small-diameter FBG sensors, whose cladding diameter is about 1/3 of common optical fibers, were embedded in an adhesive layer of a double-lap type coupon specimen consisting of CFRP quasi-isotropic laminates, and the ultrasonic wave was propagated through the debonded region. After that, the wavelet transform was applied to the received waveforms and the results showed clear difference depending on the debonding length. Hence, a new damage index was proposed, which could be obtained from the difference in the distribution of the wavelet transform coefficient. As a result, the damage index increased with an increase in the debonded area. Furthermore this system was applied to the skin/stringer structural element of airplanes made of CFRP laminates. Both of the waves received by a bonded FBG and by an embedded FBG changed sensitively to the debonding progress. Also, the damage index could evaluate the length of the debonding between the skin and the stringer.

Fibre Optic Sensors for Structural Health Monitoring of Aircraft Composite Structures: Recent Advances and Applications

“…Two parameters were introduced to evaluate qualitatively this waveform change: the damage index DI and the correlation coefficient c. The first was based on the difference between wavelet coefficients of the standard and compared data; the second was equal to the correlation coefficient of the wave envelopes between the two sets of data. More details can be found in [ 153 , 154 ].…”

Strain State Detection in Composite Structures: Review and New Challenges

“…Debonding can be detected with high sensitivity in real time from the recovery in the shape of the reflection spectrum. The development of a damage detection system using ultrasonic waves is described in [57,58] with reference to the skin/stringer debonding of an aeronautical CFRP panel. A novel damage index was investigated that could be acquired on the basis of the difference in the distribution of the wavelet transform coefficient.…”

Damage detection in composites using optical fibre sensors

“…Preinstalled optical connectors were installed The delamination growth can be identified using both parameters. The details can be found in references [64] and [65].…”