Adhesive Joint Integrity Monitoring Using the Full Spectral Response of Fiber Bragg Grating Sensors

Abstract

Although adhesive joining has many advantages over traditional joining techniques, their integrity is more difficult to examine and monitor. Serious structural failures might follow if adhesive joint degradation goes undetected. Available non-destructive examination (NDE) methods to detect defects are helpful in discovering defective joints during fabrication. For long-term monitoring of joint integrity, many of these NDE techniques are prohibitively expensive and time-consuming to carry out. Recently, fiber Bragg grating (FBG) sensors have been shown to be able to reflect strain in adhesive joints and offer an economical alternative for on-line monitoring. Most of the available works relied on the peak shifting phenomenon for sensing and studies on the use of full spectral responses for joint integrity monitoring are still lacking. Damage and disbonding inside an adhesive joint will give rise to non-uniform strain field that may chirp the FBG spectrum. It is reasoned that the full spectral responses may reveal the damage status inside the adhesive joints. In this work, FBGs are embedded in composite-to-composite single lap joints. Tensile and fatigue loading to joint failure have been applied, and the peak splitting and broadening of the full spectral responses from the embedded FBGs are shown to reflect the onset and development of damages. A parameter to quantify the change in the spectral responses has been proposed and independent assessment of the damage monitoring capability has been verified with post-damage fatigue tests.

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

“…In a previous paper [ 82 ], we have shown that tensile loading on virgin specimens progressively shifted the FBG spectra towards longer wavelengths without changing the spectral shape initially. As loading increased, some secondary peaks emerged and were enhanced with further increase in loading.…”

“…Reference [ 82 ] suggested that for damage monitoring, the effect of loading should be precluded and the spectra to be measured at the load-free condition. Occurrence of damages perturbs the residual stress field, leading to change in spectral responses.…”

“…The average fatigue lives of three different batches of virgin specimens tested under a cyclic load range between 4.5% and 45% of their average batch failure strengths is 184,200 cycles, with a standard deviation of 43,273 cycles. Figure 12 shows two typical unload spectra evolution during the course of fatigue life reported in reference [ 82 ]. These are from FBGs embedded on the left side (FBGL) and in the centerline (FBGM) from the same specimen.…”

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.…”