Monitoring of cure‐induced strain of an epoxy resin by fiber Bragg grating sensor

Abstract

In this study, we present a method to detect cure-induced strain in an epoxy resin (EP) with a Fiber Bragg Grating (FBG) sensor. By embedding the optical fiber into the EP resin the characteristics during isothermal cure (gel point, vitrification) could be precisely detected due to changes in the fiber strain. In a follow up dynamic temperature scan the coefficient of thermal expansion and the glass transition temperature (T g ) of the fully cured EP were determined by the FBG sensor technique. All results obtained by the fiber optical method showed a very good agreement with those deduced by independent techniques, viz. rheological measurements, differential scanning calorimetry, and thermomechanical analysis.

Strain development in a filled epoxy resin curing under constrained and unconstrained conditions as assessed by Fibre Bragg Grating sensors

“…Strain development (strain-time curves) for the curing EP under constrained and unconstrained condition along the cylinder axis (vertical) was measured and shown together with the actual temperature in Figure 4. As already discussed in our earlier works [8,9], at the beginning no strain signal could be detected because the curing EP was in liquid form and the cure shrinkage was relieved by liquid flow. After about 170 minutes the FBG sensor detects a strain signal for the first time.…”

“…After about 380 minutes the first deviation of the linear decreasing strain line was found to represent the vitrification of the curing EP. This was proven by differential scanning calorimetry measurements, as disclosed in our earlier reports [8,9]. Vitrification indicates the transformation from a rubbery to a glassy material and it occurs when the glass transition temperature T g of the curing material equals the isothermal curing temperature.…”

“…Many authors applied this method for the 'health monitoring' of composite structures [1][2][3][4][5][6][7]. Further studies aim at the use of fibre optical technique to characterize cure of resins [8][9][10][11][12][13][14][15][16][17][18][19]. In our earlier works [8,9] we reported about the use of FBG sensors to monitor the cure-induced strain development in epoxy resins (EP).…”

“…Further studies aim at the use of fibre optical technique to characterize cure of resins [8][9][10][11][12][13][14][15][16][17][18][19]. In our earlier works [8,9] we reported about the use of FBG sensors to monitor the cure-induced strain development in epoxy resins (EP). Beside the detection of characteristic phenomena like gelation and vitrification, the coefficient of thermal expansion was determined and parameters of the cure regime on the released strain were investigated.…”

“…This shift can be detected by an optical spectrum analyzer with an accuracy of 1 pm. Further information about the theoretical background of the FBG theory as well as the measuring setup of the Bragg wavelength shift, is given in the literature [8][9]20]. During thermoset cure the material solidifies and contracts due to volume shrinkage.…”