Simultaneous measurement of temperature and strain in glass fiber/epoxy composites by embedded fiber optic sensors: I. Cure monitoring

Abstract

In this paper (Part I) the use of fiber optic sensors for real-time monitoring of the cure kinetics of GFRP composites is explored. The proposed sensing system allows the simultaneous measurement of both temperature and strain by monitoring the change in reflected wavelength from two coupled fiber Bragg grating (FBG) sensors that have been embedded into the composite laminate. Instrumented GFRP laminates with 12, 18 and 24 reinforcing plies, respectively, were prepared by means of the vacuum bagging technique. Samples were cured in a thermally controlled oven at 80 °C and 30 kPa for 240 min (isothermal stage) and then cooled down to ambient temperature by turning off the heating source (cooling stage). The obtained results, combined with proper data post-processing, have proven the effectiveness and potentiality of the proposed sensing system to measure the progression of the composite cure kinetics. It was shown that temperature within the specimen can differ significantly from the set-point temperature inside the oven because of the heat released during the exothermal reticulation of the epoxy resin. The combined sensing system also allowed the residual strain accumulated within the composite during the cooling stage to be accurately measured. Once the laminate had been cured, the embedded optical sensing system reveals itself purposeful for real-time structural health monitoring and damage assessment of the finished component. This aspect is discussed with more detail in the accompanying paper (Part II).

FBG-based monitoring of solidification strain development in a microstereolithography photocurable resin

“…Expanding Eq. (3) into a more generalized form (Montanini and D'Acquisto, 2007) to account for resin shrinkage contributions, the FBG data can be used to calculate the degree of photopolymer conversion during photocuring. In such an investigation the FBG-based calculated degree of curing can be supported (to better understand and characterize the FBG response) by differential scanning photo-calorimeter (DSPC) tests to measure the heat generated during the photo-curing process of the photopolymer and to indicate its degree of conversion.…”

“…Many scientific works utilizing FBG sensors for in situ material/structure cure monitoring have been reported. For example, fiber Bragg sensors (FBG) have been used for simultaneous monitoring of the fabrication strain and the cure temperature in fiber reinforced composites (Chehura et al, 2005;Kang et al, 2003;Montanini and D'Acquisto, 2007;O'Dwyer et al, 1998). A single embedded FBG sensor within an epoxy resin, placed into a rectangular mold, was implemented to detect on-line the resin gelification onset and the strain build evolution during the whole non-isothermal cure cycle (Antonucci et al, 2006).…”

Monitoring of solidification induced strains in two resins used in photofabrication

“…The fact that the mechanical behaviour of the glass fibre that contains the grating is not affected, leads to the conclusion that the technique is well suited for the determination of non-uniform axial stains development due to the shrinkage of the resin during polymerisation. Scientific works have been reported that use Fibre Bragg grating (FBG) sensors for simultaneous monitoring of the fabrication strain and the cure temperature in epoxy resins and fibre reinforced composites [16][17][18].…”

In-situ monitoring of the curing of epoxy resins by Raman spectroscopy

“…However, it becomes more complex as soon as the reactive medium contains several monomers because the bands of interest from the different monomers overlap [6][7].…”