Hygrothermal Damage Monitoring of Composite Adhesive Joint Using the Full Spectral Response of Fiber Bragg Grating Sensors

Abstract

Adhesive joints in composite structures are subject to degradation by elevated temperature and moisture. Moisture absorption leads to swelling, plasticization, weakening of the interface, interfacial defects/cracking and reduction in strength. Moisture and material degradation before the formation of defects are not readily revealed by conventional non-destructive examination techniques. Embedded fiber Bragg grating (FBG) sensors can reflect the swelling strain in adhesive joints and offer an economical alternative for on-line monitoring of moisture absorption under hygrothermal aging. Most of the available works relied on the peak shifting phenomenon for sensing. Degradation of adhesive and interfacial defects will lead to non-uniform strain that may chirp the FBG spectrum, causing complications in the peak shifting measurement. It is reasoned that the full spectral responses may be more revealing regarding the joint’s integrity. Studies on this aspect are still lacking. In this work, single-lap joint composite specimens with embedded FBGs are soaked in 60 °C water for 30 days. Spectrum evolution during this period and subsequent tensile and fatigue failure has been studied to shed some light on the possible use of the full spectral response to monitor the development of hygrothermal degradation.

Damage Monitoring of Composite Adhesive Joint Integrity Using Conductivity and Fiber Bragg Grating

“…Instead of directly looking for defects, there are techniques suitable for real-time monitoring of the integrity degradation of adhesive joints. These include strain/stiffness monitoring using back face strain gages [ 21 , 22 , 23 ], optical fiber sensors signal surveillance [ 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 ], and resistance monitoring of conductive adhesive joints [ 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 ]. Strain gages can only be applied to the outer surface.…”

“…There are different kinds of optical fiber sensors. In adhesive joints, both distributed sensing [ 29 , 30 , 31 ] and discrete fiber Bragg grating (FBG) sensors have been used [ 6 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 ]. The distributed sensors measure strain along the fiber and are equivalent to a train of strain gages, with the best available spatial resolution going down to the millimeter range.…”

“…This requires the FBG spectrum to be single-peaked. FBGs embedded in adhesive joints sometimes showed spectrum splitting on adhesive curing [ 24 , 44 , 47 , 48 , 49 ]. Moreover, applied loading and damages may cause heavy chirping [ 6 , 46 , 47 ].…”

“…FBGs embedded in adhesive joints sometimes showed spectrum splitting on adhesive curing [ 24 , 44 , 47 , 48 , 49 ]. Moreover, applied loading and damages may cause heavy chirping [ 6 , 46 , 47 ]. It has been shown that if the deformation of the spectral shape instead of a single peak of the FBG is used, information regarding the initiation and development of degradation and damages may be gained [ 6 , 46 , 47 ].…”

“…Moreover, applied loading and damages may cause heavy chirping [ 6 , 46 , 47 ]. It has been shown that if the deformation of the spectral shape instead of a single peak of the FBG is used, information regarding the initiation and development of degradation and damages may be gained [ 6 , 46 , 47 ]. However, in these works, changes in spectral shape relied on subjective visual judgment.…”