Characterization of fatigue damage in adhesively bonded lap joints through dynamic, full-spectral interrogation of fiber Bragg grating sensors: 2. Simulations

Abstract

In this paper, we simulate the response of fiber Bragg grating (FBG) sensors embedded in the adhesive layer of a composite lap that is subjected to harmonic excitation. To simulate accumulated fatigue damage at the adhesive layer, two forms of numerical nonlinearities are introduced into the model: (1) progressive plastic deformation of the adhesive and (2) changing the boundary of an interfacial defect at the adhesive layer across the overlap shear area. The simulation results are compared with previous measurements of the dynamic, full-spectral response of such FBG sensors for condition monitoring of the lap joint. Short-time Fourier transforms (STFT) of the locally extracted axial strain time histories reveal a transition to nonlinear behavior of the composite lap joint by means of intermittent frequencies that were observed in the experimental measurements and are not associated with the external excitation. The simulation results verify that the nonlinear changes in measured dynamic FBG responses are due to the progression of damage in the lap joint.

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

“…Instead of directly looking for defects, there are relatively more economical 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], resistance monitoring of adhesive joints made conductive by adding carbon nanotubes [24,25] and optical fiber sensors signal surveillance [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. Strain gages can only be applied to the outer surface, but they will disrupt an otherwise smooth surface and are susceptible to environmental degradation.…”

“…There are different kinds of optical fiber sensors. In adhesive joints, both the distributed sensing [29][30][31] and the discrete fiber Bragg grating (FBG) sensors have been used [8,[32][33][34][35][36][37][38][39][40][41][42][43]. The distributed sensors provide a spatially continuous strain measurement along the whole fiber and can reveal the location of any perturbation in strain caused by damages in the bond.…”

“…Unfortunately, most of the single peak FBG investigations in adhesive joints treated the FBG as a strain gage. Applications making use of their full spectral responses are extremely limited [8,42,43]. Webb et al [42,43] applied a dynamic full-spectrum interrogator to a single peak FBG sensor embedded in the adhesive of a single-lap joint.…”

Smart Inlays for Simultaneous Crack Sensing and Arrest in Multifunctional Bondlines of Composites

“…Strain measurements within the adhesive layer using FBG sensors are an increasingly common method. Various experiments have proven the applicability of the FBG technology [ 26 , 27 ]. Filigree sensors can either be embedded into the composite matrix [ 28 , 29 , 30 ] or directly into the adhesive layer [ 31 , 32 ].…”

A monitoring technique for disbond area in carbon fiber–reinforced polymer bonded joints using embedded fiber Bragg grating sensors: Development and experimental validation

“…Full-spectral response of FBGs is sensitive to the strain distribution of the gage section, that is, the spectrum is distorted by non-uniform strain, 9–11 and this characteristic has been applied to damage detection of the composite materials. 12 Webb et al 13,14 applied a dynamic full-spectrum FBG interrogator to a single FBG sensor embedded in the adhesive of a single-lap joint. They detected a change in the full-spectral response caused by alternation of the strain field and indicated that structural nonlinear behavior due to disbond appears in the dynamic spectral response.…”